I;: .I.
Main enay rmdet title: ~ : A ~ F P W & h g M S g w - n d B d i t i a a b
f
Table of Contents
Table of Contents
5. Cooling of Thermoformed Parts. . . . . . . . . Means of cooling the formed part Non-conventional cooling methods
69 72
&moformingMaterials' Chemical Descriptions . . . . . . 135 lics 136 ies 136 k f i n s
137
6. Trimming of Thermoformed Parts. . . . Tools for trimming
74
7. Thermoforming Equipment
. . . . . . . . . . . . . . . . . . . . . . . . . . 79
Single-station thermoformer 80 Shuttle thermoformer 82 Rotary thennoforming equipment 82 Continuous in-line thermoformers 85 In-line thermoformer 97 Linear thermoformers 100 Pneumatic thermofpnners 102 Hydraulically operated thermoformers Mechanically operated thermoformers S k i packaging equipment 104 Blister packaging equipment 104 104 Snap packaging Vacuum packaging 104 Packaging machinery 105 Control mechanisms 109
b ~f plastics &tics
8. Thermoforming-RelatedMaterial Properties. ... . . . . . . . . 111 Glass transition temperature 111 Heat deflection temperature 112 112 Softening range and hot strength Specific heat 115 Thermal conductivity 116 1 16 Thermal expansion Heat of fusion 1 17 Thermal diffusivity 1 18 1 18 Thermal stability
Regrind utili&on
133
n
157
155
Welding 186 Solvent bonding 186 Adhesive bonding 187 Riaigidizing thennoformed p&s , 187 Bonding nwltipk parts 187 Foaming-in-place 1W Fikr-raiq.h~eQstruct%ralslyipoxts 188 Finishing and decorating thennoformed parts
tr
Foreword rui
' I 8 &
1W
1 2 . ~ & b e d a n d C o m p e t b g F ~ P r. .~. .~. .~. . . . . . 193 Forming processes p r f d lower bmperahlres Packaging coa@iw f d a g 196 Litalions for themnoforming 197 I.jection and e x t ~ ~ ~nroIdihg iun 198 Blowmolding and mktmoldhg 199 Plastisol or slush molding 199 I
194
i P
n
I
Appendices A E~emplarypropertis of thennoforming materials 205 B E ~ e m p wpraperties of film materials 209 C Trade nmes and materials manufacturers D Conversion factors 225
I
201
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.I:,
Foreword
kith air "overlay" of t b% of detail$, stnwE mgatbxl the principles hd&l. Hence, 'his book is an ideal t&t for all plstics engineers and &xMeians and for pdymer &emis& Ye$it is at l&ast equally valuable to &emoformas and those who utilize thermoformed products. The generic terminology employed is espwially appreciated in that students are not lost in a maze of unfamiliar, nondescriptive jargon, Dr. Gruenwald has, however, provided an invaluable delineated description of thermoforming prowses replete with practical 'bow-to" problem-solving guides for even the most sophisticated professional in the field. And these experts will be every bit as comfortable with his terminology as novices. Themofurmhg is a highly interdisciplinary technology encompass4ng chemistry, physics, materials, mechanical engineering, and thqmodynamics competence. Its breadth, shady grandiose, may f a n e even to metals; several alloys are now known to have remarkable plasticity, and probably many more will be di~overed.Indeed, it is quite likely that glasses will be comrnody thmoformed. But the great growth in therinofodng will undoubtedly continue injlastics materials. Dr. @menwald has indimted the c b h M e properties of polymerics for differihg applications; thus, Ms tmt'ls eqecidly useful for palmer chemists who must 'WIoPph&cnMieri& 'foT specific groups of applications. Engimrn i~ extndiq'md ehnetehg A h and sheet will benefit from the processing parameters img and the products thereof. guide that will enable them effort so often experienced fw tihe %VG&@QB of melds for (eqmially) complex parts. This sok is sspecMy uscfulhforr n e c h ~ engineers d in that it delineates of the fact-, bkt$ing ~ r m o d p a q i c s important , in the design and constru&m iBPt3lmaformingsystems. Operators will find "everything you ever wanted to know" about the technology of thermoforming, enabling them to improve productivity, minimize down-time, and greatly keduce map.Quite likely, Dr. Gruenwald's suggestionswill lead to considerable benefits to those who read and practice by this rematlrablk exposition of
$
I
ROBERT K.JORDAN Director-Metalliding Institute Director-Engineering Research Institute Scientist in Residence Gannon University University Square Erie, PA 16541
operational sequences of standard equipment, the properties of normally used raw materials, and the peculiarities and advantages of major and minor forming processes. Also examined were cognate fo&g processes that compete with thermoforming. This guide is intended to be comprehensive, a goal whose attainment is limited by the breadth of the field and the rapid evolution of new products. Several areas of intense development have been emphasized in the original book and have been further expanded and brought up to date for the second edition. These are as follows: (1) The vast expansion in regard to the availability of polymeric materials. This relates not only to the widening in copolymer and alloy compositions but also-as seen especially in polyolefins-in the increased multitude of polymerization processes. (2) The remarkable expansion of the materials' properties has further been amplified by the application of orientaticm and crystallization processes. (3) Where desirable properties have not become obtainable by these two means, one has found ways to obtain them by laminations or coextrusio~sof different materials or by surface treatmats.Thesetypes of materials can be expected to gain the fastest g6iW.b rate in bani= packaging materials. After studying rbe main chapters, the reader should glance at Chapter and Comp3ag Forming Processes." At that stage, the information d6nived from a few experiments should to envisionUiemwt efidient processes and operational that the ma&r can join the successful businesses that produ&on methods to manufacture thermoformed parts at dlPmdb tw$ts.Many zrf these methods were considered impossible b y m p 25 y m s ago. The limit&and disadvantages of thermoforming pracesses are explicitly dhxmsd in this book, not to discourage, but to shaq.~nthe t&mician's mjnd in finding ways to minimize them. Same tables and an extended index are found on the concluding pages to blp the casual reader find numerical d&a and additional Momtion on subjects of interest. The reader should be cautioned u,in mmY
Introduction
?$
I! represent only part of all the possibilities for plastics items, but the total market is so large T that if only a fraction of it to thermoforming, sizeable business
HERMOFORMING WILL ALWAYS O N shaping goes opportunities exist. In 1997 the growth rate for the next 5 years is -ted to maintain 4 to 6%.TheBvsiness Communications Co., Inc.
'
(Norwalk, CT) writes that "packaging of all types is a $100 billion business in the U.S. and that the market value of plastics used to package medical, pharmaceutical and specialty health-care products was $801 million in 1995." The Rauch Guide (Impact Marketing Consultants, Manchester Center, VT) to the U.S. Plastics Industry states that "the U.S. sales of plastic resins and materials reached $150.5 billion in 1996. Among the several (five) technological advances that will contribute to future growth of plastics, the further development and use of barrier plastics for packaging applications occupies the first rank,the expansion of coextrusion film in flexible end uses were mentioned as second.'Both are pointing to a respectable growth in thin film themoforming. The third reason mentioned, "the further development of plastic blends, alloys and copolymer technology," will attain benefits also for the heavy-gauge thermoformer. The various thermoforming processes are based on the recognition that rigid thermoplasticsbecome pliable and stretchablewhen heated but will to their original rigidity and strength when cooled. For most plastics molding processes the temperature of the material is r&d until it turns into a liquid-like but highly viscous material. This m a b it
The formed sheet must be cooled to become rigid again, and, finally excess material around the circumference must be trimmed off. cooling and heating processes represent the most time-consuming ste and must, therefore, be painstakingly arranged.
sf conveying heat to the plastic
4
Yeating of the Plastic
Steam heat represents an ideal convection heat source both in regard to heat output and uniform temperature distribution. Its convection heat transfer coefficient is 1000 times greater than that of air. However, the application of steam as a heat source is limited to cases where the generated condensed water poses no problem and where this very limited temperature range (212°F) can be tolerated. Presently, steam heat dorninates the production of polystyrene foam parts and foam boards. Steam heat had its day at the beginning of thermoforming, when toys etc. were fabricated from celluloid using the twin-sheet steam injection process
for thin films is approximately 0.02 to 0.05 W/sq in. x OF
e temperature of practical radiant heaters ranges between
Physics of radiation heating
1200°F Surface
y-
sheet, reflection losses, which normally account to no more than 4%, can be ignored. Energy losses due to transparency in certain wavelength regions, however, can be significant for v e v h i n films. From transparency in the visible region one cannot conclude whether the sheet will be opaque or transparent at the frequency of maximum heat radiation. As seen in the graphs of Figure 2.2 (pages 8 and 9), transparency increases markedly as the thickness of the film decreases. Sheets of 118" thickness will absorb practically all infrared radiation. On the other hand, thin films of less than 5 mils will be inefficiently heated when irradiated in certain infrared regions, e.g., fluorocarbon polymer films are transparent up to 7.5 microns. This is similarly true for thin polyolefin (polyethylene, polypropylene) films. In these cases radiation passes through the film and heats the surrounding chamber, which then indirectly heats the film at a higher wavelength. But the rise in heater temperatureresults in higher energy losses. On the other hand, a radiation in a less suitable frequency be applied to advantage by letting the energy penetrate deeper into the sheet material, thus avoiding scorching the sheet surface and making it possible to irradiate at a higher wattage. The relationship between heater surface temperature and heat energy transfer can be estimated using the Stefan-Boltzmann relationship:
E = Radiation energy per unit area in tefan - Boltzmann constant 0.17 13
vi@ of heater and plastic surfaces (assumed to be 0.9)
in OR (Rankine) equal to O
WAVELENGTH, MICRONS
Figure 2.1. Blackbody radiation (courtesy of I ~ C OInc., ~ , Niles, IL 60648)@
higher wavelength range, 4 to 7 microns. This is important when herrnoforming thin sheets or films because each' @tStic materid &?sorbs infrared radiation in distinct regions. Only absorbed radiation is utilized for heating the plastic directly. The remainder will just b a t Up the even enclosure and thus i d i m t v heat the sheet or will be lost to the outside. The *onion of radiation that is reflected by the sheet is insignificant and nearly independent of the wavelength and namrally independent of sheet thickness. Unless one wishes to themofom a mplasti~ L
1-
7
F
+ 459.6
$1& emitted energy E when heater surface ternor 15WF (1960°R)and the plastic surface
; I
sq ft - hr
sq in.
If emissivity would be lowered to only 0.5 at a constant w Wlsq in., the heater surface temperature would rise to 1225OF. the radiation in each case is spread out over a wide band of (see Figure 2.1, page 6), the bulk of energy would be radiat 1000°F at approximately 3.5 micron 1225°F at approximately 3.0 micron 1500°F at approximately 2.5 micron Energy emitted from a heater surface rises drastically with inoreas emperature. The primary emitter surfaces of near-infrared F , / !lrelatively small (tungsten wires), but the bulk of heat will be radiated the indirectlyheated quartz tube. Still heaters are spaced at wide interv and must, therefore, be increasingly distanced from the pl relative motion or oscillation of either heater or plastic is advanta for streak-free heating. Because radiation from the wires travels directions, reflectors behind these heater elements are essential. Heaters with totally enclosed resistance wires or bands occupy a lar area but still require the reflection of the radiation emitted toward opposite direction. This type of heater can be seen in the photographs of large, automatic thermoformers (Figures 2.3 and 7.15, page 91). Finally, surface ceramic or metallic heaters are spread out over the entire area of the heater assembly. These units are backed by a thermal insulation to reduce energy losses (Figure 2.4). The importance of high sugace emissivities for the efficiencies of resistance heaters is frequently overemphasized. One hundred percent of 94 ,the electric power will be converted to heat in all cases. Most of the heat -*absorbedby the plastic will be due to radiation, and most of the heat lost o the environment will be due to air convection. Reduced emissivity will raise the temperature of the heater, thus shifting.fie radiation toward the hear-infrared range. Only a few thin, clear films, such as polyolefins and :J fluorocarbon polymers, tend to absorb this kind of radiation less efficiently. For such pigmented or heavier films and sheets this shift is actually an advantage, because the shorter wavelength radiation will penetrate and generate heat deeper in the plastic. The concentrations of pigments in colored ~fieetsiue generally low
P
da 8 .'J"
C w d c Warm radiators- Asray of heaters mounted on top of oven U a t e d Internationale, Ltd-, B d ~ d h o bRep. , Ireland).
12
Physics of &on : * ? t I
enough so that no significant variations will occur. Carbon black may be one exception and may increase absorption in thin films. White pigmented sheets may show somewhat higher reflection losses, but only in the near-infrared region. Although conventional hot air convection ovens might have to be modified to obtain better temperature uniformity over the entire sheet, the problems of obtaining consistently homogeneous heat distribution when using radiant heat are enormous. This is especially true if the sheets suffer from excessive gauge variations. There is a great temperature differential between the heaters, the frames, and the various clamping devices. In some cases the latter two must be water cooled; in others they are heated or at least preheated. Nevertheless, every square inch of the plastic should ideally have exactly the same temperature at the end of the heating cycle unless special heating schemes are desired. The difficulty of obtaining a low temperature gradient also throughout the thickness of the plastic sheet will later be covered. The sketches in Figure 2.5 illustrate how to compensate for heat losses occurring at the outside edges. It can readily be seen that less radiation strikes the plastic on the outskirts of the sheet especially when tl: distance between the heater and the plastic is great [arrow points hitting each square of the sheet in Figure 2.5(a)]. Therefore, either additional heaters must be mounted at the outer edges and especially outer comers [Figure 2.5(b)] or the inside heaters must be spaced farther apart [Figure 2.5(c)] or energized at lower wattage. To appraise these inevitable energy losses at the corners and edges, one must consider the view factor of an installation. In this case the surface area of the heater is identical to that of the plastic sheet. The parameters, which can be obtained by dividing the sides by the distance between the heater and the sheet M = heater widthldistance and N = ---. -.( ,-heater lengwdistance), just must be entered into the graph in Figure 2.6 to read the resulting view factor on the y-axis of the graph. This value is identical to the fraction of the total avekae " radiant ieat that strikes the plastic. Example: Assuming that the dimensionsfrom Figure 2.7 for the square sides are 25" and the distance between the heater and sheet 5", the values for M and N become 2515 = 5. The resulting view factor is, therefore, F = 0.7, meaning that 30%of the energy is directed outside of the plastic sheet. I* In practice the results are much worse because this reduced amount of energy is not distributed evenly over the whole surface. Figure 2.7 illustrates this uneven distribution. in which case only the most central ~
~
heating
C
~~
- -
----
--------
-
-
.!-
mw
.;,;j
RADI'ANT HEATER
\ 0
0
0
0
1
I
- , -
~
I
Figure 2.5. Heater arrangements for obtaining good heat distribution. \
at (he heater edges will lessen these edge losses. Sandwich (simulflttDWs exposure of the plastic to radiation from both $ m y 221~0 mitigate both radiation and convection losses. When h heam are u d , the lower heater banks should ha
r~~~
I
-.
ve the sarfmeto prevent the W t from contacting the 1
Thermal properties of plastics
I
1
Roll-fed automatic thermoformer. Length of solid arrows inaicates ume of film are exposed to cooling prior to forming.
P""-=--
~ a ~ a d ii.e., n using ~ , wire mesh screens of various densities and h-ported on metal grates under or above the heaters), one can &t&lish the most effective temperature profile. For long pro,runs it becomes more practical to obtain such zoning by usWad of screens-individually controlled heater banks, which bpms-%me should keep in mind that the leading edge of the R which leaves the oven first, loses more heat than the trailing
The heater edge with baffles to reduce heat losses (Figure 2.8). properties of plastics
s are poor heat conductors (Table 2.1). Therefore, thick sheets
healing time caq be rduced by &eheating the material and :hatan intermediatetemperature. This is, however, seldom done kdds under 114" thickness. One vendor (Fiwre 2.9) sumlies for
,
by the manufacturer of thermoforming equipment. Again, polypropylene can serve as an important example. During heating the sheet is expanding to such an extent-partly due to the melting of the crystalline portion-that it would contact the lower heater banks if the parallel clamping rails would not be led at a diverging path at those regions. These difficulties might become exasperated by the shrinkage that might occur in the longitudinal direction if the sheet has been pulled excessively during extrusion. The necessity for observing the upper boundary of the thermal stability of the plastic material will be emphasized later. The surface appearance of plastics, such as gloss or mat finish and especially imprinted surface
the oven for irregular time intervals until the forming equipment or manpower is ready for continued processing. At higher and steadier production rates, the oven temperature will probably be set higher, but the sheets must be withdrawn of the oven according to an established time schedule to prevent tearing or thermal degradation.
the sheet is recommended. Heating equipment for plastic sheets
Convection ovens were originally the most common devices used heat plastic sheets for thermoforming. They are still the preferred w
200 feet per minute are crucial to obtain temperature uniformity and adequate heat transfer. Good thermal insulation of the oven walls and the strategical position and size of entrance and exit doors increase energy efficiency. A typical forced-circulation air oven is shown in Figure 2.10 and a practical means for hanging sheets on trolley tracks in Figure 2.11. When sheets are heated horizontally to better utilize the heater space, they are supported in trays. These should lined to protect the surface of the plastic. A polytetrafluorocarbon-coatedglass fiber cloth lining is ideal. SECTlm THROUGH OVEN
&idadm
ahx: .-
-1L
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0fR
o b and Haas
co,,
.
2 . u
The lowest cost gas-jired infrared heaters are surface combustio burners in which the gas-air mixture is burned at the surface of either
combustion occurs in a metallic tube with no openings inside the chamber. This also minimizes drafts. Cross section of catalytic heater (coufiesy of Vulcan Catalytic Systems,
SINGLE SIDED
43
Fig@@? 2.4). The flat plate heaters have an integral insulator at their back side and should be arranged to occupy the whole surface of the heats assembly. They are the ones that are most interchangeable radiant panel heaters. The concave heaters radiate also to the ersed in front of a surface. They require a greater distance to the plastic sheet. B e c m the heating wires are tightly imbedded in a highly emitting white ceramic body, they are protected from oxidation and can deliverup to 40 W/sq in. at surface temperatures up to 1400°F.Their heat-up time is approximately 5 minutes. Intricate heating patterns can be established either by means of individual heater embedded mputer program utilizing a scanning infrawith a coating providing a color indication per heating operation are also available. heaters (usually called calrod heaters) ater construction due to their ruggedness, life. They consist of nichrome wires or into flat strips. Magnesium oxide, which is r, serves as an electrical insulator to prevent 1sheath. Heat-up time is very long, and heat output is only moderate, not easily controllable, and deteriorating with time. Their low efficiency is mainly based on the required extended distances between heaters and the plastic.
minutes is required.
M i s c o n c e ~ t i dm a ~ persist regarding an alleged drop in energy efficiency with radiant heat elements due to the fact that often heater times =st be increasml with passage of time. The emitted energy of quam kting denlen@ iWMins fairly constant. If less than the original heat the plastfo ahat, it is usually due to the reflectors having accu*.ted as ~rodlt~tion progresses. On the other hand, metal-sheathed and plate hea@t'$ anit less heat as the nichrome wire gradually Bxidizes- The -, now with a smaller diameter, will conduct less 'wtricit~, a d , -m, will uwand deliver lower wattage. A variable compensate for this
Heating of the Plastic
life span, and the larger panel or plate heaters have the longest. The life expectancy will drop somewhat with increasing temperature, but frequent and deep temperature cycling may also affect heater life, mainly on account of movements caused by differences in the thermal expansion of resistance elements, ceramic insulation, and metal sheathing. There-
4
6
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Judging the con&
ternpemture of the hated sheel
., t
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F'4 i J ' ) li
27
areas where they escape can sometimes be spotted from the rising plume. ~tis advisabb to install at those points a very low-powered exhaust hood s u f f c i F f@ s k b off the fumes without causing drafts.
bmperature of the heated sheet
~ombnded sheet temperatures and the maximum
mabli~hingthe most praotical spot within the b tQ start out with heating a sheet to its maxi-
full-time utilization o ers are provided with for optimal heat utili hanged-the obtained part other detailed areas that the
Sophisticated heaters employ schemes to cut down heating times by moving heater and sheet in unison over the mold OF by starting the heating cycle with a surge of power. For single-unit heaters this is possible only with quartz heating elements with tungsten filament heaters that will instantly emit full power and, therefore, need only be energized while the material is heated. Ceramic heating elements have a greater heat lag. Therefore, more time must be allotted for changes in temperature settings until the heater output corresponds to the new irdjustnent. At the other extreme,-convection ovens may need hours of warm-up time until they reach temperature stability.
30
Heating of the Plastic
arranged in a row or the installation must be equipped with a sensor having a 90" rotating mirror. A low-cost method for establishing surface temperatures can be util-
Enav
for radiation pymxneay (adapF ,Slwkie*IL 6(XKI),
heating process. Some control devices utilize the sagging phenomenon and terminate heating when signaled by a photoelectric cell. This criterion, however, cannot be indiscriminatelyapplied to all plastics, because some materials may already be overheated when they start to sag. A once established optimum heater output adjustment set by a timer may not repeatedly result in the same sheet temperature. Minor line voltage fluctuations, ambient temperature changes, or sheet gauge fluctuations may significantly alter the actual sheet temperature. Multiple heat-sensing devices inside the heater tunnel are used ex@nsively for control purposes. Even though their indicated temperature does not coincide with the sheet's temperature, the relative temperature changes detected by these devices might still be indicative of the aberrations
occur due to the
,
Heater controls '
-
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A
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t e
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,'
s
8
,
To control the temperature of the plastic sheet or film, two avenues can be taken. In simple cases the heaters are powered at a constant energy level, and the desirable sheet temperature is obtained by controlling the heating time of the sheet or the advancement rate in case of automatic roll-fed thermoformers. If acceptable production cycles can be maintained?there should be nothing wrong with such a simple arrangement. In those less sophisticated thermoformers the heaters are directly powered by the electric line voltage. The temperature of the heater will rise until oven losses will qua1 current input. Thermally well-insulated ovens overheat unless plastic sheers are continuously being fed into it. Therefore thermal overload relays must be present in heaters. Until equilibrium is reached, the owator must individually decide on heating time. Furthermore, small line voltage variations must be by heater time adjustments. Similarly, heater output-but "Ot energy efficiency-is lowered in time when belectrical resistance of the heater wires or bands increases doe to oxidation of the or when heater and reflector surfaces become contaminated, Identical replacement heater elements will have a higher wattage than adjacent older tieaterr
,
-
{eating of the Plastic
In general one can state that, in resistance heating, all electrical energy is converted into heat. The efficiency with which this energy is conveyed to the plastic to be heated is mitigated by the amount of heat conducted or radiated to the machinery and mainly by convection of heated air. Unfortunately, these losses can range from 50 to 90%. The claims of some heater manufacturers showing that their elements provide 96% energy efficiency do not make allowance for such cited losses. Losses occur mainly in the overall equipment and not in the heater elements. The abounding information published in regard to energy e energy cost, applicability, and cost of heaters for thermoforming considered biased or pertain to special circumstances only. The many thermoformer producers offer a choice of heaters confi present, still no clear winner has emerged. When thermoforming takes place on automatic machine production scheduling requires a certain output, cycle times constant. In such cases heater output, which can compensate for ch in line voltage, sheet gauge variations or fluctuating heat losses, m established. Two basic electrical controls have found applications. In a less sophisticated open loop control mode the heaters can be adjusted by an operator via percentage tim maintain a constant output. For a closed loop control necessary that the power-switching device either obtains the heat-sensing thermocouple in the heater so that its tem constant or, preferably, that an infrared sensor scans the make sure it remains constant at the desired level wit of an operator. In both cases simple mechanical contactors, merc switches, solid-state relays, or silicone-controlled rectifiers are be used as switching devices. Applicable controllers would be temperat controllers and programmable controllers. With the increasing spread of computer technology the possibil for using infrared sensors for measuring film and she have been expanded. The operator can not only display tures on a computer screen and compare those valu can also, by means of an operator interface, program tern or set up the heating process. Apparatus manufacturers are respondi to these needs by supplying heater banks with individual heat control a monitoring indicators for each element, thus making it also easy to sp burned-out heating elements. For example, one thermoformer is keeping erocess d by applying computer-integratedmanufacturing (CIM) methods. A grammable controller (PLC 5 from Allen-Bradley Co.) is logging data points each cycle. Most of them relate to temperatures, both for heaters and the molds. Other data keep track of press moveme in kes and constancy of vacuum, always comparing act 1
!I ,I,?
2.15. Articulating clamp frame (courtesy of Brown Machine, Beaverton, M
sheet ai' fihheating, as well as for the forming and stripping s, the plastic-stockmust be restrained firmly between two frames. frames usually consist of profile irons to which nonslip gripping s are mount&d (coarse abrasive cloth, rough rubber pads, weld
Thermoforming Molds
IVERSEAND UNIQUE kinds of molds are used for thermoforming. As
Da
matter of fact, the low cost of molds and the short lead time required for tooling up have led to this forming method being favored over others in many applications. . Generally, only one side of a mold is required, which-depending on +&& s ape of the formed part, thk desired appearance, and the process wsed-may be a male, or positive mold for drape forming or a female, . or negative mold for cavity forming. The determination of which one to choose becomes more critical the deeper the part to be formed is. When forming shallow, low-profile parts, the reduction in wall thickness is minimal; therefore, the selection will depend more on appearance. If fine . mold details must be duplicated, then the side of the plastic sheet which touches the mold surface should ultimately be the one that becomes visible. Sometimes, a more rounded or smoother appearance is desirable, Pr the sheet material may already have a pleasant textured surface that Wuld be affected when it touches the mold. In these instances the side .that does not touch the mold should eventually be the one that becomes visible in the formed part. It must be realized that a closer dimensional i control will be obtainable at the mold surface side.
h
Figure 2.17. Gripping clips. Mounted on film transport chain of thermo tesy of Kramer & Grebe GmbH, Biedenkopf-Wallau, Germany).
spatter, etc.). These frames are pressed together by C-clamps, tog clamps, air cylinder operated jaws (Figure 2.15), and cam or spring-h devices. Sufficient clamping force must be provided to prevent liberated internal strain, frozen in during the extrusion of the sheet, fr pulling the sheet out of the frame during reheating. Roll-fed automatic thermoformers grip the sheet securely at the sid by means of spring-loaded pin chains running in chain rails (Figure 2.1 or gripping discs mounted on the transport ch@n(Figure 2.17). On equipment where only a few pieces a day are formed become necessary to heat the clamping frame so that the sheet material : at the flanges will also heat up. The heating of these frames may also be necessary to obtain a better grip of the sheet. In other cases, such as the chain rail of an automatic thermoformer (Figure 2.16),water cooling is r;+d to keep sheet-gripping pins at toleraw temperatures.
,
Ir
i
Reduction in wall thickness: male and female molds
Under all thermoforming conditions in which pieces are shaped from a flat sheet or film, the surface area must become larger and, therefore, the gauge thickness thinner. One of the decisive factors for this thinning is the draw ratio. Many times a draw ratio of 3:l just means that the thickness of an area of a part is just one-third of the original sheet thickness. Unfortunately, three different ways to define that ratio with a comparable numerical value exist, and in each of these cases the values depend also very much on the specific shapes of the part formed. (1) For irregularly shaped objects the draw ratio is difficult to establish
35
ssure-forming procof the unformed sheet.
projecbd line passing through the deepest depression of the them
mainly biaxially stretched.
me foamed or biaxially oriented sheet products and the cast
The difference between unidirectional and biaxial stretching can
always nonuniform and increasingly progressive with the advancement of the forming process steps. Furthermore, each shape yields another number. Three simple examples should be cited to illustrate how these numbers relate. The values in the table refer to a cubical container formed from a square sheet, a hemispherical bowl, and a cone formed from a circular sheet. Formed Shape
Common Draw Ratio
the three-dimensionalillustrations will show
Draw Ratio
Draw Ratio
Cube Hemisphere Cone (60") Final material
-
Available sheet area
AxB-CXD A x B - C X D+E(2C+20)
A x B - C x D, resulting in a flange of heavy
Available material area &a to to fom& aut d-ik Figure 3.1. Drape forming over male or positive mold.
culated wall thickne cut into small pieces-or s thickness determined with banslucent or transparent dark line on one side w
*so show the extent of draw in any one direction on obl
I
43
Reduction in wall thickness: male and female rnoldr
~nthe c s e of a female mold the opposite occurs. The plastic sheet will apart until it contacts all four vertical mold surfaces (Figure be 3.71, resut~ngin extremely thin-walled comers. Again, the rounding of edges will h e b to retain sheet thickness at tolerable values. - in Figufe: 33, several other means for eliminating webbing are illus-
1
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1 (
1
,
.;
(a) positisn square or rectangular parts diagonally on vacuum box. When a snapback box is applied, the same effect can be obtained by equipping it with four 45" gussets in the comers, instead of the 4% - -rotation of the mold. (b) Use web catchers or web pullers. These are smaller parts of variable shape that utilize a portion of the excess corner material. (c) Mount a ring assist (approximately 2 times the diameter of the mold) and one-eighth its height to the vacuum box. s e a combination of male and female molds. In effect this is equivalent to lowering the mold height by the amount the base is dropped, ' -Techniques of preventing webbing are not limited to changes in mold design. At lower sheet-forming temperatures the sheet will retain more tenacity and mb'beriness, enabling it to more easily retract during the latter stages of forming, which should also be slowed down somewhat so that the makerial has more time to contract. For large parts, increasing b e temperature ,in localized areas could be beneficial. Using extruded sheets with a higher drawdown ratio (higher orientation) can also reduce the likelihood of webbing. Sheet materials that contain a small amount of cross-links will also reduce webbing. Multiple male m ~ l d smust be spaced sufficiently apart to prevent webbing. A distance of 1.75 times the mold height should be adequate. In many instances, to save material, male molds are located closer ese cases drod or strip fastened to the upper clamping frame must be used to sepaa'tely form each part. One such illustration is shown in Figure 10.6.Automatic,roll-fed machines?which have no upper frame, use the identically performing grid-assist male forming technique. hterchangeable mold cavities are used for many plastic processing methods, but none of them can parallel the benefits achievable with &rmofoming. Because thennoforming molds are neither subject to mely high pressures nor require tight tolerances to prevent penetraof a liquid resin, which could bond the various parts together, ng mold depth or adding dividers for a certain overall shape has e standard procedure. The photo in Figure 3.9 pichues five differte shapes all formed in the same basic late mold- w ----h n a ~rnnfiml6iwdwas &hanged by easily replaceable i&s. Figure 3.10 contains -r
1. -
-.-8
.
--.-a
..
.,
w
Figure 3.9. Plates fonned in molds with qui Gabler Maschinenbau GmbH, Luebeck D-23
Figure 3.10. Dividable formsel
-.-----.,,".'b ",""- -. --- ----'-- --- rexcessive flashing. In thermoforming, however, circumstances are more complex. The finite-element method, when applied to injection molded parts, allows a modest grid density to be selected and still receive satisfactory information. The finite elements usually consist of triangular shapes to allow better representation of curved edges. In deep thermoforming the most desirable information must be searched in a highly stretched area that. occupies only a small speck on the original sheet. Therefore, the analysis must be conducted in several steps so that the system is not overloaded with an unmanageable number of elements nor misses out on important detailed information. Condensing the model by neglecting the sheet thickness (the third dimension) and by assuming membrane formulations is seen as an acceptable simplification, ?cause bending forces are only encountered at the sheet edges. The viscoelastic behavior of all the thermoplastics used for thermoforming presents the greatest stumbling bl&k to obtaining an allencompassing solution. This process is carried out at elevated temperatures (250 to 750°F) where the modulus of elasticity is approximate 10to 150psi, reflecting also the applied forming pressure and at rates around 1 second. The forming predominately takes pl biaxial (not necessarily equibiaxial) extension mode. Unfortunate1 physical data that could be inserted in such mathematical equa have not been gathered on those polymers and are difficult to o experimentally.
I rn
I
m
i
W
mntebediate model, incorporating experimental observationis with aim-
E' ' At present there are t h m
97.2) derived from C-PITA and Accuform @fin, T-dm2.2. MI three programs allow the uer ter screen, apply pmsslue observe in &ps how it
49
The designer must put some ledges, protrusions, or dimples into th mold, not only for providing reference points for taking measurement but also to keep critical regions better constrainedduring cooling. These markings are ?specially helpful when cutouts or holes are incoyorated into the p a i .Apositive support is also required if accurate trim dimensions must be upheld. Because maintainable tolerances can vary so widely with the kind of equipment andmaterial, no specific numerical data are given here. Past shmM he the best criterion. As a guideline, mold shrinkage should be assumed to be approximately 0.005 in./in. for male molds and nearly double @at for female molds. A lower mold shrinkage is found in' the cellulosics and rigid polyvinyl chloride (0.0035 in./in.); high mold shrinkage materials are the acrylics, polycarbonate, thermoplastic polyesters, rind oriented polystyrene (0.008 in./in.). Materials with excessively high mold shrinkages include all the crystalline materials, such as high-densfty polyethylme, polypropylene, and the fluorocarbon poly-(~.025 in./ia). However, these numbers must be used cautiously because the following conditions may alter them significantly: (1) Mold temperature: A 15°F temperature difference can change shrinkage by up to 0.001 in./in. (2) Size and shap$.?&@rs: These refer to the degree to which the part is restrained in tlie mold and to 'the effect a greater wall thickness has on the temperature profile. (3) End-use temperature: Due to normal expansion or ccintraction proportional to the coeff~cfentof thermal expansion, part dimensions continually vary with changes in ambient conditions. (4) Extreme use conditians: Shrinkage will reach peak values after first exposure b the highest use temperature. This is especially important to considei-when heat s,terilizationis applied, (5) Sheet orientation of'mtfutied sheets: Drawdown during extrusion might cause a shrinkage differential of 0.002 in./in., with the shrinkage in the machine dinsfion being higher thzin in the transverse
8 will have to be a
I
symmel imbalan irregular ~4 transverce of mold can occur i-i
draft sides i ing lugs i~
Where part tiimrnsion b h w m lm than Q.Win./itl., it is advieable t~ W m e w h a t ovmized m1& in the c m of male molds mol&, This is ,so that subsequent mqt~lid to the mold t&e need d s d b . jammed or digx
use tempemre iand d . e r to gome water inside) f o r t W t
danced molds a~ more likely to cause warpage. Any f the following conditions should become suspect: ded sheet wall thickness due to irregularity in gauge, uneven heater output, unequal cooling air draft. Slight shrinkage and thus movement hours after forming. Perfectly formed parts ocde warped if either packaged too soon or that strain becomes introduced in the parts. In cases ay lead to blocking of stacked parts along the e advantageous to incorporate several denestto prevent crushing of stacked parts during f Figure 3.15 and magnified in the foreground ng shell are shown. This sample At the right side the uniform shown. In the background the merchandise is shown. The positions g formed parts from becoming aed during transDort is illustrated in
Molds u ais g e ~ s l hy id omrt as possible.
Radii at ed They not Ohr improve the ?j there can be mr' formation on of the part. R ~ $ I and are mere 13
t b pa% from t b mold but also mf chill m d a 325).Shaq c m w cao lead to web so astry the sf brittle failure
€t a
9 A .
I J
Surface appeamcaa-
f
. ;,
I F l p f16. Undi.drmt forming can prevent s t a c k . problems (courtesy of Marbach mi L ~ ~ ~ - I Q G . , M 4516). ~ ~
I?-
appearance of large
3.16 %'bw -FtB t%@ Lblld do& not odymaintab the distance lids b~&*-s g d f i t and tightness despite minor l!Wwpm container and lid
cloth is also effective in
t
GROOVE
With little pmpmthxt Ulese shells cao be made to serve productiol as molds for! W m temperature curing epoxy-casting compound. These vided with the air passages and cooling castings ca$ me useable as mold blanks for alasting multicavity tubes SO thdd perties of epoxies are well suited for long producform. The tE@ ,cycles. If copper cooling coils are embedded in them, tion runs at $3 ill improve only somewhat, but it still will their heat I ction runs. g over several square feet, plastic molds are For large m or spray-up process using glass fibers as prepared by a uns-ed polyester resins as binder. At reinforcemen1 called gelcoat) is applied to the polyvinyl fist, a smood ed by a thin layer of a formable fine glass ts, chopped glass mat or up the surface layer. A an be bonded with a resin paste to theshell to con mold, If msamratixl polyesters have beg should not exceed 150°F;e p ~ ' 4 The manufacturitlg &ds was drastically reduced when metal spray prwr eposition on suitable patterns were introduced. LI b stat with the deposition of a thin stainless steel backing; another G with a thicker la3 preferably with other
~~
Figure 3.17. Mold details for raised characters. Small circles show air vent holes.
1
not become visible. Mold materials
Aluminum moll either prepared bj t % ~ aluminum ~k shal m e s it necessary to ~ W I I W ~ E # J ha water through the F-doI'ing, e t heat. For rapid forming -Ids. This should t
~ulus,ano ml; wS
datb+*uw+
B&kn
.~~~
paitem. initla j
are needed Tdfthy
ng. ?'his can fi2Ik,,.=, olds can not^ agce*afn@ , L-.
should be considered.
L-%
f'v
Air M
C
holes
porous meM molds have the advmtage that no d d h g of vent holes is required. Their disadvantagesam:prolonged cooling times,increased fragility,and the possibility of reducd porosity over h e .
COOLlfdCl DUCT
Mold plugs
OUCH MECHANICAL FORCES are of increasing interest, air pres-
re is still widely used to accomplish the forming task. It is agentle
of this pressure is attained. If higher forces are required or if it becomes
and pressure forces
1 can wit alti ma, is e incj fac~
case, atmospheric pressure, wherever it oint. The pressure difference or vacuum
, I.in.Hg = 0.4912 psig or
8 7 1 <; t
nical F -
Vacuum, Air I
'
I
Therefore, working parameters shodd not be selected too close to the vacuum capabilities of the equipment, because these conditions may not be obtainable under adverse low-pressure weather conditions, making it necessary to readjust cycle time. For any vacuum-formed part one must consider the volume of air that must be removed to retain the actual force sufficient to form the last dimple in the part. Both requirements are counteracting each other. Voluminous parts will require the evacuation of a large amount of air. This may drain the leftover power of the vacuum, resulting in improper figuration of the last detail. On the other hand, shallow parts will be formed quickly, leaving adequatejforce in reserve. In the first case some power can be saved by filling large voids in male molds with closed cell foam.
Many kin& of vacuum pumps can be used. Reciprocating piston, d i a p m , sliding vam~otary~ eccentric rotor pumps, etc., all establish s g d v w u m but & 1- capable of e v ~ a large ~ volume g of air qwkkly. FW this remm &my t q connected to an air ~.eservoir(surge tank) @atservesas a vacuum atxxufiwlzltor. tbe otha hand, air blowers can ~ o v large b m ~ i x of air, although somewhat limited in the due to their c c m f i d vacuum. Com& e @ m are ideal in both respects a d are very b1,kw5 Timy m,ilot fottpdl h cammereidly built equipment. They ~ i o pb ~ 6 w 1 m e hr .soair and should be equipped with silamm mia mund-attenuating enclosure.
Vacuum agcwdabrs or surge tanks With the exception of some of the rmly used vacuum sources, the vacuum for forming the par@will be suppki f s ~ mvmu s-9 af 8 bald. Far that can rapidly evacuate the s p w small parts in a roll-fed automatic tbemoZ-t -o~,fl&+@m for finding space f a r ,stank having volume that is sixfdd the: volume of the spme7to be evacuated. This Id correspond ta a 15-gal surge ta& wJtm4bdngtwenty 16-ozcups under 4 net forming pressure of 12.5 @g (approximately 25 in. Hg) vacuum. These parameters are, &!&fore, generally cited in the literature. However, to apply the same conditions for forming aspa, the surge tank would need to have acapacity
%
64
Vacuum, Air Pressure, and Mechanical Forces
If the forming process includes a pressure-induced billow or prestretch forming step, the surge tank of larger capacity should be selected. Otherwise, the pressurized air must first be vented before the application of vacuum to prevent excessive loss of vacuum from the surge tank. It is evident that the positioning of the heated sheet in reference to the mold must ensure an airtight seal. This may sometimes be difficult to attain on very tall male, drape fonnings.
Application of vacuum forces
ssures or may deflect excessively. For pressure muons applicable to pressure vessels must be obeyed. mold might require a clamping force of several tons,
,
In applying a vacuum, many variables are encountered. In general, vacuum pumps operate constantly to maintain vacuum in the surge tank. The gauge readings fluctuate with each cycle. On the other hand, blowers and air ejectors are turned on only for the time vacuum is required. They quickly establish a constant pressure differential. In either case the vacuum on the formed part must be maintained until it is sufficiently cooled to resist the material's inner forces, which tend to revert the part to the original shape and may cause warpage. The faster the vacuum can be applied, the better the appearance of the part. On occasion, a slower forming rate may be called for when large pieces made up of heavy sections are produced. The slower cooling rate in these cases will allow a longer forming time, enabling the material to flow in a viscous mode. When using tall male molds and where webbing becomes a problem, slowing down the forming step and reducing forming temperature may give the plastic more time to contract in the transverse direction, thus eliminating web formation (see Figure 3.6 on page 41). In many cases the vacuum supply line leading to the forming table can also be used to conduct compressed air to the mold box. After the main vacuum valve between the surge tank and vacuum box is closed, a brief blast of compressed air can detach the part from the mold by the reversed flow of air through the multitude of air passages in the mold. However, better results are obtained-specially with tall male molds-when air
process.
in which the vacuum force is replaced by an air pressure pposite side, one must consider that it is more complia satisfactory seal for pressurized air. The forming force
.
pressure gauge should be installed. It is important mtmce to the mold, so that the cold air will never
he heated sheet. Sometimes, the air is guided over rn blowing large billows that must stay hot
formed onto the mold.
Vacuum, Air P
Mechanical farming
mica1 B P ~ S
Compressed Air
Continuous CLamP
Plastic Sheet Mold
"
I
Air Vent
-
Ywuun Ducts
43. EWafgedemstw&i@ v k of~a reprochetiondetail abtahedby pressure Ctq?) and vwum fonmisg @tm8113). ' L
h
m f o h~ g has become popular mainly for s m d parts. Its tolerances are improved and that formderably. It is capable of duplicating fine e. previously mentioned dielectric heaters
?
Mechanical forming
4
As indicated previously, thermoforming is not limited to pn techniques; various mechanical forces may be applied instead. The simplest application of mechanical forces is utilized in mensional forming. In this case the heated sheet is laid over a c u d mold that usually has a soft thermally insulating surface. Gravity normally suffice to bend the sheet and duplicate the shape of the mo d~ and no stretching, which would change wall thickness, occurs.
from distol male mold
candsctive r n d molds c
n n o d i . M y controlled The qplir:&on of mold heat
or irietalpdi-filled plastic m l d s sr otlbrer waterhawing low thermal condtw:tivity am"& Qn sheet-fed
~fsuch a hi ie migh nother possltal losen, to alten ~olant.The wa le importa one realize ' ' :ne shou~a Eibility c engineeril exceeding the pt at higher p -one of the111c;i :speed of heat lply defined a lse experience ~s I second coo11 e above example. ning production ti tion in running speed served cooling wmtctr to the expected s, derived from ti :t between metal :dly reduced. Limitec r crystalline n ?s that impede coolini rime instances it may pay tu L rming temperatures. s~lcl Icooling time will all more, on rollming cycle only one cooling cycle We the help of m ~ l t Ilc;* , ~ ~ ~ n described in Chanter'
_
'-•
Ids--due to heir heat buildup-are
.,.,
&est ( 4 2 wide*numb
a mld temp-
of
frorn $le mld €itJ160°F.
"
wrature, the more shrinkage can be expected.
'.
IV.
I
f.BLA if; L
1
. 4.
I
Tdmmlng of Thermoformed Pads
II
ssion-molded parts and degatmetimes pose problems, the r thermoformed parts is an
round ones. If round mount of trim can be y 10%(see Figure 7.17 page 95). Only if an in-line extruder is will continuous regrinding and recycling of excess material tly. Still the reworking of materials involves iated specifically with it. no single, immutable time for trimming. Often, a smoother the material is still warm. Electrically heated in special cases, but low-heat conductivity of the plastics heat-up time. High-production parts are frequently old. When stacking is not immediately pere machine, multicavity formed parts should be left connected skeleton with a few narrow links to prevent that one part
are formed, which must match in size with metal parts, stacking are mostly performed in another piece of equipto room temperathe formed part was any different ways, originating paration can occur plied. The force to be exerted is scribed in the stress-strain diarittle fracture, requiring generally much less force, takes place abrasion and saw cutting, including routering and nibbling
.
Trimming of Tlwmodormed Parts
75
hot gas jet, and laser beam cutting and, maybe, w
Tb minimize dust problems,
The thermal separation pmxssear entail odor pdliem8. Some disad-
a d f e d rafes m s t be selected tie avoid heat
Tools for Mmming
The simplest tools represent sometimes specially shaped knives and scissors. Steel rule dies are convenient to manufacture and work v W well with thin parts if all cutting is performed in one plane. They c0nsis-t of hardened, sharpened strips of steel 1/16 in. wide and 1 to 2 in. tall-
arance should be provided. As can be seen in the next chapter,'a heavier piece of m i n g than for forming (Figure 7.10).One uses a trimming die to cut the hot sheet to a thickness down to 10 mils, regardless of still suffkient to keep the sheet in place trimming is completed
76
Trimming of T
h
a
Tookfor trimming
w Parts
77
'on of star cracks. The published shear strength values for
E &!m
not indicate the required forces accurately, because too much sheet thickness, accuracy and sharpness of the tooling, and d cpdity of. the cut. heated wire can be used to advantage for separating c ham parts, resulting in a very smooth edge. xes, for which dies would be too expensive, must be manually r ith programmed automatic tools. Nibblers represent an
4 @!
se theythe do not generate many dust particles. However, popularity that routers and saws have gained.
E O ~ $&ed
i W t become necessary to debur the cut edges. This can
Hshd 1&er with a sharp edged scraping bar, a file, fine S.
in secondary processing, the three to impacting the sheet at a speed of 3000 feethec severs the plastic much faster and more at the opposke side of the sheet to collect the small amount of w&r and ral or glass fiber-filled materials a fine But ahis also causes some wear in the
of a laser beam results in a C02laser may cut through
b t B M for'heat damage and environ-
B nsocssary, parts should be annealed
b f \ ~ e ~ ~ l o s i c sother m d s h e e ~ v e very n thick .
&*
--
F to prevent
N THE PRECEDING chapters the various components involved in the overall thermofonning process were described. Due to the great variety of processes, thermoforming equipment may assume many dissimilar shapes. At one extreme, hardly any equipment is required. Aside from an oven, makeshift molds alone may satisfy all requirements for a operation in which only a limited number of simple curvature parts have to be produced. At the other extreme, every 2 seconds a fully automated thermoformer. as shown later in Figures 7.23 and 7.24, can start with pellets and shape multiple parts occipying an area UP to 21" by 13". The material-if supplied in sheet form-may be fed manually or lifted by suction arms to the forming machine where it is clamped in a metal frame. Larger and heavier sheets may first be heated in any of the described heating ovens, either while suspended or lying flat. Mediumsized sheets are conveyed into the oven clamped in a frame. Ovens with infrared heaters can temporarily be slid over the frame. On the other hand, the frame with the sheet is usually pushed in to the oven when sandwich heaters are employed. If the material is taken from rolls, it is essential that a dancer unwinder is utilized to convert a gentle unwinding rotation of the heavy roll into the sudden movement required for the advancement of a new section of the sheet to be formed. The edges of the sheet are grabbed by pins or wedge-shaped teeth attached to chains, on both sides of the sheet. The chains, which glide in rails, transport the sheet through the heated tunnel ovens, the form press area, and sometimes also the trim section if it is part of the machine. Depending on material and machine layout temperature control (heating or cooling) must be provided to keep the plastic in grip. The endless chain returns for picking up new material. Electric servomotors, which provide a Smoother and better controllable indexing motion, have replaced the type mechanical rack-and-pinion arrangements. To obtain high productivity, the heater tunnel is divided into two to four sections, each identical in length to the indexing stroke. Temperature zones are usually laid out in receding order. Heat p;ofiling does 'not only 79
I ,
-s-on
Thermoforming Equipment
Rotary thermoforming equipment
wand
heating oven for forming heavy-walled parts that require an heating cycle. Subsequently, the fourth station was utilized for t o r robotic trimming. Again, the sheet heating cycle will dictate prr,d Qn speed. Because all other steps are performed during the same be as the heating cycle, good production efficiencies are achievable. machine is shown in several of the following illustrations. a drawing and a plan view of a four-station rotary thermotop and bottom heaters can be subdivided, and the bottom nL !wma&ommodated for a downward motion to follow the sag of the %i J
g :
--
Figure 7.3. Cut sheet modular themofonnet ( c o r n y of Brown Machine,Beaverton,
MD 48612). Shuttle thermoformer
Shuttle-forming equipment virtually doubles output and conserves energy consumed by the heaters. This necessitates a second mold and forming station and also two clamping frames, which can be shuttled ia succession from the oven to mold station A and from the oven to mold station B, respectively. A double-ended therrnoformer as shown in Figure 7.4 can be operated with two crews producing two different parts. An infrared sandwich heater occupies the encased space between the two workstations. Rotary thermoforming equipment
Greater productivity can be generated if three or four workstations are arranged around a central point. Only one mold and one forming station are needed. Three or four clamping frames mounfid on a horizontal wheel and a means for rotating them from one station to the next are required, compared with the single-stage formers. An operator must be present only at the first station, where the frame is opened, the formed part removed, and a new sheet inserted. Station two provides heat to soften the plastic, whereas station three is the site of both thermofoming and cooling. The fourth station was originally added to accommodate a
Therrnoforming Equipment
Continuous in-line thennofonners
87
cools in mold B. ennoformers have an intermittent film-feed me&to the next position after completion of the forming of drive systems are employed to obtain smooth Figure7.8. Horizontal shuttle mold machine.Sheettravels vertically downward. Molds shuttle left to right. Forming station ;emaim s W i 0 n (w-Y ~ of F%-Js&~ & Equipment, h. 1978). ,
ntinuaus thermoformers are depicted in the followFor thin films, efficient cooling can take place while the plastic remains on the mold. In other cases, after adequate rigidity is attained to allow removal of the part from the mold, final cooling is canied out down line by forced air convection. The output of formed parts can be nearly doubled when two cooling cycles in a row are utilized. This may iy especially necessary when working with materials of low thermal diffisivity, such as polY~ro~~lene. m e previously mentioned shuttle-mold principle has, therefore, been applied alm to continuous thennoformers by utilizing horizontally or vertically reciprocating mold pairs. As shown in Figures 7.8 and 7.9, the heated sheet travels v e ~ c a l l ydownward (Zdirection), whereas the two
icts the Model 44 Thermofomer, protunnel. Figure 7.14 is a close-up of the be considered the smoothest running
Figure7.14. Mechanical drive lieage of thennoformer shown in Figure 7.13 (C b of Irwin Research & Development, Inc., Yakima, WA 98902).
Continuous in-line themfonners
Therrnoforming Equipment
Automatic chain rail adjust (optional) MP I1 precisely controls the chain rail positioning facilitating immediate changes in sheet width. A gear box and motor operate each of the adjust points. The rails can also be automatically moved into a V shape, either narrower or wider at the former, to compensate for severe sag or orientation in the sheet.
Heating elements Calrod Standard. Ceramic and quartz optional. PID Heat control (optional) PID Heat individually controls up to 48 thermocouples and 84 heat zones. Heaters can be assigned to different thermocouples to match the heat zone layout to the product. Tunnel length and width can also be adjusted by turning off heaters as part of the product recipe. All parameters are part of the product recipe, stored on hard disk for fast access. MP 11's optional video control shows configuration of the tunnel, all temperature setpoints, and all actual tem-
Model $4: Maximum 2Sa" above or k l ~ w sheet line Model 44Mini-MagMaximum 5.50'' above or below sbeet line
: 3.78" per platen (at 6" closed height)
Mini-Mag: 8.50" per platen (at 2 0 closed height)
ntrol functions are accomplished via the Micro-Phaser
standard. Other colors availble upon request.
1clamps. Tools can be simply modified to accept the t h d e d inserts to each tool.
shows the camelback arrangement of the Model would be positioned to the right of the thermo7.13. For a l l control functions it utilizes the
'
converts the trim
Themdts&w Equipment
EP$ine 7 a h-Wfhemwfod* system for solid a@ foatlbsra thee%parts (Kiefel Extarmat (cmttbq dPJctetics~ ~c& &- r Julyy1*q.
A comparable setup is shown in Figm 7.26. This Wefel Extrumat in-line system is designed fm converting both solid and foamed sheet to formed articles. Linear thermoformers
There are, however, other types of automatic feed thermoforming machines in which the sheet keeps moving at a steady speed instead of the usual start-stop mode. They can be fed either from rolls with the film first traveling through a heater tunnel or from a film emerging directly from an extruder. In one case the set of molds, the clamps, the plugs, and all vacuum connections travel in unison with the sheet on a moving conveyor and return in a loop when the forming is completed. The set of molds could also be arranged firmly at the circumference of a drum. In both cases the need of multiple molds restricts such formers to large volume productions. Figure 7.25 shows the schematic of a simplified continuous linear thermoformer by the Linear Form Pty. Ltd. A simple conveyor with 18 molds proceeds at the linear speed of 69 ftlmin, qctated by a film or sheet extruder or a film-heating tunnel. A second conveyor, driven by the same motor, transports the heated film and preforms the p"t by means of a cam-actuated protruding plug. Final forming takes p h e ' b ~ means of a blast of air. The need for le_ak-prone rotating'manifolds for mold-cooling water is circumvented by the use d W W air for part and mold cooling. A new rotarv thermoforming: m a c ~
Thermoforming Equipment
be adjusted for the right length of travel. Another food packaging, the possibility of contamination by include the facts that most tvues are not
rmoformers are as versatile as, but more complicated and than, pneumatic thermoformers. One reason is that n always need a return line. Hydraulic movements are
low, noisy, and dirty. This is no longer the case. Better Figure 7.26. An in-line continuous rotary thermoformingdrum 1s a~recdyfed by a sheet extruder to provide 71,000 container lids per hour (courtesy of Irwin International, Yakinta, WA 98902).
mounted on a continuously revolving drum rotating at approximately 10 rpm. The film or directly above the necessity for providing heating tunnels and saves energy; however, extruder output must be tightly controlled to the roll former. Because molds for deep drawn parts would squander much material at the edges, the depth of formed parts is limited to 1 in. Pneumatic thermoformers A number of different motions must be performed b thermoforming equipment. Besides the transportation of the sheet we$ the movements of the molds, the plug assists, the in-mold trim die, and the stripper plate must all be well coordinated. These motions can be accomplished by pneumatic, hydra of them. Pneumatic thermoformers were popular because they are least corn.. because m plicated and very versatile. They are fast and moving parts are lightweight tly f .aW '"
'
1
tenkce hours a& required for hydra&ic equipment, airing leaks, especially where stringent demands items. More care must be exercised to set up in vrouer seauence. However. because these
e platen stroke depth must be fixed action design is used. The shortest &&a connection with-independentlypowered precise indexing-necessary &mmplished with DC servocontrols are used with A4bugh this machinery L md for a certain
104
Themofarming Equipment
105
thermofomers have the lowest frequency of maintenance reguirements. Power consumption for the me&mkal part of this equipment is negligible, whereas energy requirements for heating and cooling the plastic ase dependent on bulk of output. Skin packaging equipment
operation completes the process as in all other Thermoforming processes are not only employed to manufacture structural parts or packaging containers but have been specifically modified to incorporate packaging and sealing. For instance, with skin packaging, the items to be enclosed are placed on a piece of printed cardboard, which is air permeable and rests on the plenum chamber. After the plastic film is sufficiently heated, the mounting frame is lowered to obtain an air seal with the cardboard, and the vacuum is quickly applied. The softened sheet will stick to the specially coated cardboard and tightly adhere to the packaged goods. No molds are required for this process.
the utiGz&on of the machine's entire work area.
SP~P Prsokagina:
Heal~ng
Stallon
\
Web feed sysiem
Package separallon
Load~ngslatton
Figure 7.28. Small thermoform packaging machine (courtesy of Paul Kiefel GmbH Thermoformmaschinen, D-83395 Freilassing, Germany).
Tirornat VA automatic form, fill and seal machine (courtesy of Kraemer
GmbH & Co.KG, Maschinenfabrik, Biedenkopf-Wallau, Germany).
introduction of the inert atmosphere and the motions performed by the package container, the vacuum box, and the sealing film are pointed out in Figure 7.32. Figure 7.33 presents a close-up view of the equipment shown in Figure 7.30. The sealing chamber in which evacuation and flushing also take place is seen in the center. The photo in Figure 7.34 demonstrates the variety of jobs performed by this machine and the environment in which it is used.
VACUUM,
h' *re
7-31.schematic of vacuum packaging process.
packaging machine in processed meat plant (courtesy of Kraemer KG,Maschinenfabrik, Biedenkopf-Wallau, Gemany).
Figure 7.32. Layout for vacuum and gas flushing pmess.
Themoforrning Equipment
~icro~rocc@ors and the-now at low cost available-personal computers have the advantage that they can be programmed to automatically and continually readjust certain processing conditions to correct for inevitable aberrations, such as gauge thickness, heater output, and other variations, detectable by sensors attached to the machine.
.
I
F;
materials should be suitable for B thewfbnniog processes. Such materials, when heated, will exhibit a reductib in their modulus of elasticity, their stiffness, and their ASIC-,
.
ALL THERMOPLASTIC
load-bearing ~ q w i t y To . understand these relationships it becomes h o w how temperature changes affect the physical propernecessary to . plastia. We are too much accustomedto assume that our everyday materials, suph tiis wood, concrete, glass, metals, and textiles, remain unchanged b&ween 0 and 200°F. i.
I I
as mercury is at room temperature. High materials (most thermoplastics) or materi-
the material remains a
ill never become a fluid liquid between each link of the long easily sliding past each W P thermal decompo-
112
Thermoforming-Related Material Properties
Sof-tning range and bt strength
113
will exhibit still another transition at their crystalline melting point, which is usually much higher than the glass transition point. Heat deflection temperature
The heat deflection temperature, a change in mechanical properties, represents a more practical temperature limit for the materials used in thermoforming processes. The literature usually lists two values. The first, determined under 264 psi loading, is the value far determining the temperature up to which rigidity for light mechanical load applications relevant to large parts is retained. The second value, determined at a quarter of that l d , 66 psi, represents the upper temperature limit for applications to small, stubby parts. This temperature, or in some cases a still higher temperature limit-sometimes called the no-load deflection temperature-is extremely significantfor thermoforming, since the material temperature of the formed part must be below this temperature to be safely talcen from the mold, Otherwise, gravitational force would collapse or distort the formed part. In view of this fact, the mold temperature setting should be at least 20°F below this temperature. For many plastics the differences in loading have a minimal effect on the deflection temperature; however, for others they are significant. Tbe lower loading k a m e introduced when tests on nylon resulted in an unacceptably law value of 160°F for the heat deflection temperature versus 450"~;which was obtained when loading became reduced to just one-quarter of it (66 psi), Softening range and hot strength
At still higher temperatures the material's behavior will again stabilize
heated sheet when the relatively low force of gravity k c ~ m e ss
n areas or, in more severe cases, to punctures in the
Specific hear
Thermoforming-Related Material Propetties
polyvinyl chloride (PVC), polystyrene (PSI, plymethyl m e t h ~ l a t e (PMMA), and polycarbonate (PC), is indicative for the much wider processing window for rhese materials. In comparison, the semicrystalline polypropylene (PP) has only a very narrow range. The height of the plateau becomes indicative for the force that has to be exerted during themofonning;in this case PVC and PC are higher than the other plastics and polypropylene is the softest at forming temperatures. To W r understand the stmctural differences in these materials, one should visualize the two extremes: (1) a low molecular weight thermoplastic, having low intermolecular forces on one side and (2) a cured, highly cross-linked thermosetting resin, such as a cured phenol formaldehyde plastic, on the other. The first will rapidly turn into adwreasingly viscous liquid, having passed quickly through the elastomeric range. Thus, it exhibits a very narrow softening range and a low hot strength. The W r will remain quite rigid up to a temperature where thermal decomposition takes place. It has hardly any softening range and such a high hot strength, that thermal forming becomes highly restricted. Both kinds of plastics are unsuitable for thermoformhg. Therefore, plastics suitable for thennoforming have been specifically selected and developed over the years. Generally, much higher molecular weight thermoplastics are used for extrusion and calendering purposes than for other processes. Thus, the film and sheet materials n o m d y fabricated by these processes contain a higher proportion of the higher molecular weight polymer. The opposite is true for injection molding and rotomolding processes. In these, lower molecular weight thermoplastics are used because the heated resin must have a sufficiently low viscosity to flow through the gates of the injection mold, or the p u l a must be able to coalesce (melt together) easily when rotomolded. In both cases the plastic will cool and solidiQ while its shape remains constrained in the mold. In the extmsion process, the material will leave tern-=, and the sheet will cool and solidify in the need for constraint. However, in most cases the ~heetpasses thereafter through roll stands, not only to cool the sheet but improve gauge uniformity and swface evenness (gloss) Or to surface texture. These are some of the teasons can be expected to produe parts with better met other Process. parts made of the same generic material by Aside fmm high molecular weight, some tendencies at level will favorably affect the softeningrange and the arrangement of tbe elements in a ring structure will make mom volumirmus and less likelyYo slip past each other. po]ycarbonates, anal the aromatic (ben among others, fall into this m ~ pm. .
There ; of differe
116
Thermofoming-klated Material Properties
ture by lo.Water serves again as the standard, receiving the number 1, to which all other materials are compared. For instance, polystyrene has a specific heat of 0.32, which means that 0.32 Btu is required to heat 1 lb of it by 1°F. In Table 2.1 @age 16) the values of a few plastics are listed in comparison with other common materials. With the exception of water, most materials have a lower specific heat than plastics. A more detailed listing can be found in Appendix A, which presents properties of a wider range of thermoformable plastics. To use these values, one must realize that they represent a weight relationship. Because substitutionsare mostly done without changing the gauge thickness, the variations in specific gravity must be taken into account. The specific heat, which can be accurately determined by subjecting the material to a differential scanning calorimeter test, increases slightly with rising temperature and jumps markedly when crossing the glass transition temperature. Therefore, listed values must cautiously be applied for calculations spreading over a wide temperature range. Published data can show great variations whether measurements were taken at room temperature or at a temperature range related more to thermoforming.
Heat offusion
117
for thermal expansion and contraction are identical. Prob-
with a high mff~cientof thermal expansion in the range of 7 to 10 x in./in:"P are the cellulosics, polyolefins, and plasticized polyvinyl
Thermal conductivity
There are also differences among plastics in regard to heat conductivity. These values are listed as Btueftlsq ft.hr."F-the num which are conducted through each square foot in 1hour if the difference is 1°F and the thickness of the piece is 1 foot. Th values between plastics and metals is apparent. Espec sheets are being formed, the low~onductivityof plastics heat energy transfer. With excessive heat inputs the plastic su blister or start to scorch,even though the center region has not Y its softening stage. That is why sandwich heaters are recommen why a brief delay between the heat and forming cycle may for letting the heat soak into the center of the sheet. ~ccordingto observations heating times can be reduced for heavy-gauge sheets w shorter wave radiation (visible light), which peneQtes deeper into sheet, is being employed. Thermal expansion
The linear coefficient of thermal expansion is eXpre Because the plastic sheet expands more in @ Mllbr a sheet is observed during tb@h t flof@
es: the almost clear low-density
slowly and accelerates until the crystalline
118
Water absorption
Thermoforming-Related Material Properties
shrinkage to final dimensions will take days, because the rate ofrecvstallization decreases as the temperature drops. Rigid plastics will stop crystallizing once cooled below a certain temperature. Proper design of the mold-cooling system can ensure uniform cooling, thereby forestalling warpage in formed parts.
Thermal Wfbdvity The use of the material constant, thennd diffusivity, would be ideal for establishing cooling times for thermofonned p m because the time required for cooling the heated and formed plastic sheet is propaioaal to the second power of the material thickness and inversely proportional to its thermal diffusivity. Furthermore, tbermal diffusivity is clearly defined by its relationship to other establishable constants: Thermal conductivity
x Specific heat However, a problem arises when one considers that all three materid constants are not constant over the whole temperature range encountered in thermoforming. In addition, the latent heat of fusion becomes absorbed into the thermal diffusivity too. Therefore, published values for thermal diffusivity vary widely, depending on the temperature limits selected fm their determination.
material of lower density, etc.
Thermal stability
forcibly, due to 1Ss load [ ~ R O P . 271. a m v e n t i d
-
119
mocouples or thermistors) are not suitable for checking
water absorption
~h~rmoformable plastics vary greatly in their capacity to absorb water. some plastics, such as the polyolefins, absorb almost no water, whereas and nylons may absorb it assiduously. Although no problems based on water absorption may appear under normal conditions-on occasion unpredictably-it can intermpt production. Therefore, it is important to understand this phenomenon. Freshly extruded film and sheet tend to be completely dry. Even if a sheet were submerged in water period of time, the material would still be practically bone absorbed just at the surface will rapidly dry off during heating e-absorbed water will only slowly permeate sheet. Although listed water absorpafter 24 hours of submersion at room ay occur only after weeks or months. Depending on the relative humidity of the ambient air, the water content of a sheet-if stored detached freely-will vary significantly. Fortunately, several months of high humidity are required for moisture @penetratetightly stacked sheets or rolled film. Problems may arise only a weekend or other lengthy producwhen warm moist air enters the cool The slow rate of water permeation represents also the cause of diffid t i e s during thennoforming. Although the moisture in thin films or on @B surface of sheets will rapidly escape during heating, the absorbed s stays trapped and vaporizes inside ue haze or bubbles of various sizes. The result ill appear to be hazy or foamy and display a ing with the thickness of the sheet, a drying time of one to several in an air convection oven at a temperature below the heat distortion eets must be dried, individually supported, ly, for many processors this is not worth st discarded. Probably it would be more pped in paper only) for a period twice high humidity (months or years) in a d dry, heated room. If only a few pieces and no ovens are available, drying can be accomplished in ofoming equipment by the use of several repeated very brief ycles. It is always advisable to keep rolls and sheet stacks of
120
Orientation and crys&lltzation
I henofomling-Related Material Properties
moisture-absorbing plastics tightly wrappa+--usually in two layers of polyethylene film-whenever not used. Chemical effects, which means breakdown of the polymer molecule due to moisture content, are not to be expected during thermofoming. But because webs and edge trim are mostly utilized during reextrusion of the material, thorough drying is necessary for all the plastics subject to hydrolysis (polycarbonates,polyesters, urethanes, ek.1 before reprocessing. Water absorption data for plastics are listed also in Appendix A. But its magnitude is not proportional to the trouble it can provoke. In materials listed with values greater than I%, the water is acting as a plasticizer and can be seen essential for warding off brittleness (nylons and cellulosics). A polycarbonate sheet having just 0.1%moisture content will blister during thermoforming, whereas a 5 times higher moisture content in cellulosics or nylons is readily tolerable. Materials with values less than 0.03% should not be expected to cause trouble. The detrimental effect of moisture on materials depends very much on the rate of absorption and the rate of hydrolysis at the processing temperature.
121
o n m@her hand, hardly any force must be exerted to form or stretch
these differences in orientation, two examples each are polystyrene and polymethyl methacrylate. extruded as a sheet will have an orientation due to its flow and the gentle pull of the takeoff rolls. This can be proven
I
Orientation and crystallization
Orientation and crystallization of plymetie m8,tetials bring about peeuliar arrangements of polymeric chain segments in the o t h e r w i ~ amorphous base material. Theirgreat e f f aon materialVQ~efies it essential to know and control them. As is often the case, gemralid
ot-
supporting a square of the sheet with known dimensions between thin plates coatd with polytetrafluoroethylene or covered with talc and exposing it for several minutes to a temperature approximately 50°F below the W t temperature. The amount of shrinkage will be proporthd mount of orientation, approximately 10% in the extrusion direction atlddess than half of that in the cross direction. Excessive orientation mfiy y e problems in thermoforming if the sheet is not clamped tighkl~r,or it may cause excessive mold shrinkage in the orientation directiq, resulting in distorted parts. If an extruW polystyrene sheet is given a biaxial orientation by stretching it fsrcibly at distinctly lower temperatures than its glass transition t e m m ~ r (220°F), e the resulting biaxially oriented polystyrene will have drastically altered properties. The sheet has
Orientation and crystallization
Thermoforming-Related Material Properties
stresses. When improperly balanced, a PMMA part that comes into contact with certain chemicals can become crazed and cracked. Fo reason, part surfaces and edges are usually protected. Generally, the thermoforming of polymethyl methacrylate parts should always be done at sufficiently high temperatures to prevent the possibility of crack formation during later use. This is also recommended for many other materials, e.g., polyethylene terephthalate. The biaxially oriented film materials that have reached high-volume applications for packaging are polyethylene terephthalate, polypropylene, polystyrene, and polyamide. Similar drastic changes in properties are experienced in several forming processes that compete with thermoforming, such as solid-phase forming and cold forming (see Chapter Twelve). On the other hand, with some polymers, crystallization will take place spontaneously-without applying external forces-but never instantaneously. For crystallization to occur, the material's temperature must be lower than the melting point but not too low, since the chain segments must remain mobile enough to arrange themselves in a three-dimensional orderly fashion. Due to restraining forces of the angular chain link interconnections, polymer materials cannot crystallize completely. Because the chain links within the crystalline region are more closely spaced than in the surrounding amorphous regions, semicrystallinepolymers always consist of a mixture of two different identities. They are (with some exceptions) opaque and become transparent only when heated close to or above their melting point. The higher the crystalline content in a polymer, the higher will be the specific gravity and the modulus of elasticity (rigidity), but brittleness can increase too. Crystallization can sometimes be prevented or reduced by rapid cooling (quenching). If chain segment alignment has been accomplished by orientation, alignment to ofderly crystallites is usually excluded. Caution: crystal polystyrene is not at all crystalline. It was so termed because parts molded from it have a "crystal" clear appearance. ~emicrystalline polystyrene, having a (syndiotactic) stereospecific arrangement of the styrene monomer units, is just now, under the trade name Questra by Dow Plastics, being developed for special engineering applications Its heat deflection temperature of 210°F (versus only 170 to 200°F for amorphous polystyrene) is comparable with that of engineering *ermoplastics. Light can be shed on these complexities by considering the behavior of several currently available thenyoplastic polyesteks. First, a low molecular weight poly(ethy1ene glycol terephthalic acid) ester was pm duced. This material was unsuitable for molding useful Parts be weight material became transformed within a this ]ow days into a highly crystalline Substance *at
#d above its crystalline melting point, it converts into a viscous @wever, it matured into very important materials when its
tRT textile fibers are gained by highly orienting unidirec-
known pc tionally tl sionally s that are b vroducts r
,
aexhibited by chemically identical polyester segments. AS @ 8.2, the injection-molded preform (at left) represents a clear partk t c~nsistsof randomly arranged polymer segments. I ~ S mechanical mrties are isotropic because no orientation has yet taken +phc~.It is GI? !W because crystallites have not been able to form due to rapid cooling is stretched t; When such 2 held at belod transition pol' somewhat mu moving the n polymer segn
3
exposures 911 t ing mass. By exchant Iments ;
rial will crystallize into a completely white appearethylene glycol with butylene glycol the chain ~ ~ rof this i composition ~ s are semicrystalline
Thermoforrning-Related Material Properties
Orientation and crystallization
and mainly used filled and glass fiber-reinforced as injection-molding compounds. A highly mineral-filled version, Enduran from GE Plastics, with its ceramic or marble look and feel is thermoformable and used for solid-surfacing applications in kitchen and bath, rivaling DuPont's Corim, the composition of which is not disclosed. The processing of these sheet materials at 112 in. thickness represents the extreme in production time requirements. At 350°F the heat-up time can stretch up to 1 hour and the following cooling time half an hour. Later on, many more thermoplastic polyester formulations that have found wide applications both for injection molding and thennoforming with great emphasis on the packaging sector were introduced. Various schemes have been found to obtain the wanted properties of the final product by regulating or completely eliminating the crystallization process. This has been accomplished by disturbing the regularity in the polymer chain with the introduction of differently shaped monomers or by compounding in micronized filler particles. The ethylene glycol component (E in PET) may be substituted by cyclohexanedimethano1or cyclohexylene glycol (C in PCT and also G in the copolymer PETG) and the terephthalic acid by either isophthalic acid or 2,6-naphthalene dicarboxylic acid (N in PEN for polyethylene naphthalate ester). It is important to remember that, with the increasing molecular fraction of ring segments or ring size in the polymer structure, the use temperature becomes elevated. Thus, the melting points of the various polyesters increase from polybutylene terephthalate (PBT) 437"F, over PET 482"E PEN 523"F, to finally PCT with 545°F. Other desirable properties are often also improved. The gas barrier properties of PEN are approximately 5 times better than those of PET. Due to the great variability of possible combinations caused also b the multiplicity of copolymer formulations, the chemical identification of commercial products bkcomes confusing. The crystallizable polyethylene glycol terephthalate (CPET) contains a nucleating agent *at speeds up crystallization. However, due to a remarkable increase in molecular weight the crystallinity content can be restricted to approximately 30%. This results in a product combining some optimal properties:
NUCLEATED PET
(1) Higher rigidity and better temperature resistahce (2) Good low-temperature toughness (3) Good barrier properties
These attributes have made CPET an ideal material for opaque' ovenable food-packaging trays. They are suitable for cooking of food in either a microwave or a conventional Oven- When consulting Table 2'2
A: ,
~ m r #A e fallizationP a sheet. On the
125
Thermoforming-Related Material Properties
Manufacture of starting materials
There are two reasons for the thermoformer to become familiar with the various processes employed by manufacturers of film and sheet materials. First, different processes may require variations in formulation, such as stabilizers and lubricants, and they usually demand polymers of different molecular weight and melt viscosity. Second, the suitability for thermoforming of materials from different manufacturers may vary. Changes in thermoforming processing must invariably be made when using film or sheet of different origin, even though the basic plastic material remains the same. These differences are primarily caused by variations in melt viscosity and frozen-in stresses. Stresses released during the thermoforming process may also have an effect on shrinkage or warpage of formed parts. Most available sheet materials are produced by the screw extrusion process, which employs medium to high molecular weight polymers that should be subject to minimal heat stress. Because pull-down is required to maintain a constant thickness, the produced sheets contain a certain degree of stretch (approximately 10%). This will become noticeable when the sheet is being heated prior to thermoforming. The sheet will tend to shrink in the machine direction (approximately 5 to 15%) but possibly expand somewhat (0 to 5%) and thus sag in the cross direction. The degree of stretch can easily be determined by placing a square piece of it on a silicone or polytetrafluoroethylene-coatedmetal sheet for 15 minutes in an oven at a temperature approaching the forming temperature. Thinner films are usually produced by the chill roll casting process in which the polymer is heated to a higher temperature, but less strain is incorporated in the film due to the short distance between die and chill roll. Polyethylene film and'some olefin copolymers are produced in large volume by the blown film extrusion process. The melt is extruded through an annular die and blown up into a very large-diameter bubble. The film is then collapsed, folded or slit, and wound up in rolls. Because the polymer is stretched in the longitudinal and transverse direction, the obtained film has good mechanical properties. These products, however, differ considerably among various suppliers. Vinyl sheets, a few polyethylene, high acrylonit~le,and acryl0nitrilebutadiene-styrene polymer sheets have also been produced by a dendering process. This process requires a considerably higher ca~ifal investment and is, therefore, restricted to very high-volume usages' Usually, even higher molecular weight resins are used. The compounded material is first homogenized in high-intensity mixers, planetary ge? extruders, or on tv~o-rollmills and then the sheet formed in the llp
Coextruswns and laminates
129
rolls of a four-roll stack. Further smoothing occurs while sheets are free of
materials must be used if high gloss becomes a
rusions and laminates
t r u W and laminated sheets have gained favor for thennoforming s no other practical molding process es so easily obtained by thermoforming. is meant the formation of a sheet product by re extruders. It is not necessary to ne die. The layers may also be combined ally as long as the materials are hot enough be employed when either two or viOusl~ W I I Jor~calendered ~~ sheets are combined or bonded, an a l w extruded ~ sheet or material layer is coated with an ginally, laminates were obtained e-plated steel sheets. Now, they act pressure between rotating
Therrnoforming-Related Material Properties
(2) Parts are subject to ultraviolet radiation on the outside, but a lower cost material suffices to provide mechanical strength. Three outstanding examples in this area are: (a) the acrylic multipolymer film (see Korad in Appendix C) laminated to acrylonitrile-butadiene-styrene (ABS) or other sheets; (b) coextruded sheets consisting of acrylonitrile-styrene-acrylateon the outside over (high impact) ABS; and (c) polyvinyl fluoride film (see Tedlar in Appendix C) bonded to ABS or other thermoplastic sheets.
determined by the quality of the outside layers of a sheet. Inferiorquality center layers may have negligible detrimental effects, as long as good bonds between the layers can be ascertained. (4) No material that could provide all the properties required for the particular application is available. In food packag~ng,where lowoxygen permeability, low-moisturepermeability, and heat sealability must be provided, coextruded films have gained wide usage. These barrier materials.
above). In the case of an in-line thermofoming p m s s
\
Mechanical properties
Material economics
veral y-, long-term performance data should not be overlooked. For materials, graphs can be found which relate to and sometimes lohg-term performance. This kind of behavior cannot be expressed in simple numerical values, as the other above-mentioned properties. consequently, even for the expert, it is difficult to apply valid data for comparisons. The three primary factors should be briefly cited: (1) Creep, the slow deformation occurring when plastic parts are subjected to a constant force at a certain temperature. This can change the shape of the part in time. (2) Fatigue, sudden failure in a part subjected to long periods of cyclic loading or vibrations. ) Aging, the slow degradation of properties due to the influence of environmental factors (temperature, chemicals, radiation) acting on the part material. Exposure to strange environments combined with external loading may cause sudden failure, which is more related to ntal stress cracking.
132
Regrind utilization
Thermoformins-Related Material Properties.
133
This formula is based on the fact that when materials are replaced, dimensional variations are, at first, not a factor. However, because the replacement material may result in differing properties, additional aspects can affect the actual part cost. Two of these should be elaborated: (1) Rigidity: Most parts used for any structural application (including packaging) are designed for certain minimum rigidity values at the highest temperature of use andlor a minimum resistance to impact. Therefore, if the replacement material is superior in these respects, reductions in wall thickness should be considered concurrently. (2) Processing: Shear resistance and heat stability (among other properties) will greatly influence extruder output and the reprocessability of trim regrind. The latter is especially important for many thermoforming processes. Again, if the replacement materiJ ensures either higher outputs or higher yields, savings in processing could be realized.
yield in square feet per pound the following equation
Some important formulas should be specified, because the designer must always keep in mind the relationship between sheet size and weight. The specific gravity of the material, which is contained in any materials brochure or handbook (seeAppendix A), must be ascertained to determine the sheet weight or the weight per square foot.
film of 7 mil thickness with outer diameter of 4 11.4''
Sheet weight:
lb = Length, in. sheet
0.1923
roll form, the following formula is helpful to length of film in feet: (Outer dia, in.)2 - (Inner dia, in.)2 Film thickness, mils
x Width, in. x Thickness, in.
P*
x Thickness, in. x 5.20
Example: A pigmented acrylonitrile-butadiene-styrenecopolymer s 0.125"thick is listed as having a specific gravity of 1.07 @cm3.A s 2 ft. by 3 ft. will weigh:
C
One square foot of this sheet weighs:
are made on a roll-fed, thin-film machine or heavyEhmmd~smer.This necessitates, unless just,a few pieces are
134
Therrnofotming-RW Material Properties
to maintain a set virgin-regrind ratio. If this ratio stays high in favor of regrind, contamination and degraded polymer may accumulate over time and may make it difficult to maintain highquality specifications. Under those conditions it might become advantageous to seek applications where mere regrind material will suffice. After cascading down in properties a few steps, eventually some of the resulting material will ehd up being discarded or fed into other recycling schemes; see the end of the next chapter. As stated elsewhere, the thennoforming conditions are seldom so severe that molecular breakdown might occur. This usually takes place during extrusion where the material is exposed to high pressure, shear and high temperatures, and the danger of introducing moisture with an improperly dried regrind material. Polyolefin resins generally degrade due to thermal oxidation and result in a material impossible to process due to excessive sagging of the sheet. Polyvinyl chloride and other chlorine-containing plastics suffer from hydrogen chloride formation. The reduction in molecular weight and with it the deterioration in properties becomes f b t n o t i d l e in the drop of impact strength or in yellowing of the material. Only severe degradation will also show up in loss of modulus of elasticity and yield strength.
~hermoforming Materials' Chemical Descriptions
:FORE SOME KEY properties of the most important plastic materials r e desdmd, a few remarks that may apply to any of them must be 3.
)pendig k lists most plastics used for thermoforming processes ped by &emical name to avoid favoring any one brand. Because in noforming applications commodity resins are often used, this tabun appears logical even though in a few cases only one commercial 's available. On the other hand, some important y.not known by their chemical composition, and :are still b&ng sold without any indication of their composition. To the reader $ these cases, another list has been added to the Appen;.This supple~nentdlist, Appendix C, identifies proprietary plastic al order using proprietary trade narnes-some of names-along with the manufacturer's name, , and application (thermoforming, packaging, or ial machinery parts). Ir the sake &consistency and avoidance of misunderstandings, all k a l name#,ma spelled out. Occasionally, the better known abbreare added ia brackets. rical values are taken, where possible, from manufacturers' -1-5s charts and f b mhandbooks. Unavoidably, not all these values ative crf a specific material within a group. Those
Thermoforming Materials' Chemical Descriptions
ms in trimming. They are especially appropriate for The cellulosics contribute strength and stiffness to ich can be transparent and also easily colored or ved stabilization has made them suitable for outdoor they are available in food grades. rming, cellulose-acetate-butyrate (CAB) has gained cellulose-propionate has better clarity, toughness, and is
importance of the various materials used for thermoforming processes, these materials will be treated in related groups. Descriptions of them are brief and cover only general properties and behaviors. The final selection of a material from the vast number of commercially available materials is, of course, an individual decision that will include other criteria, as well as personal preferences.
I
I:;
Acrylics
Acrylics, which are more descriptively designated as polymethyl methacrylate, are well suited for thermoforming due to their high hot strength and wide processing temperature range of 290 to 350°F. No other material behaves similarly during thermoforming. Their excellent optical properties and clarity are augmented by outstanding outdoor stability and rigidity. Cell cast acrylic sheets have the best optical properties and lowest shrinkage but are highest in cost. Continuous cast sheets are produced by casting a viscous, partly polymerized monomer onto a metal belt. They are nearly as good in physical properties but may show slight optical distortions. Extruded acrylic sheets are lower in cost but are not optically perfect and can have up to 15% orientation in the length direction. Modifications of acrylics-many of proprietary composition-have mitigated their brittleness, reduced their cost, and improved many of their other properties. Cellulosics
cs
At one time the cellulosics were the preferred material far f
d
regain their position. All cellulosics represent chemical reaction products of cellul which can be obtained from either wood or cotton linters.
Polyolefins comprise a great number of plastics, which have in com-
man that they consist only of carbon and hydrogen atoms and do not n-.
contain any ay~licgroups. They can be recognized by the feel of their surface, whit& resembles paraffin wax, their low molecular weight They we all resistant to water and aqueous solutions and may swell, but do ndi dissolve, in organic solvents unless heated. Low-density polyethylene (LDPE), polymerized according to the high-pressureprocess, represents the earliest commercial product. This process resulkdin highly branched polymers that are soft and translucent due to their l w crystallinity. Variations and improvements in polymerization catalp@ have yielded polymers with significant variations in densities, su* u the high-density polyethylenes (HDPE), polymerized in a low-pres~$%jsolution polymerization process. The structure of these products C O & ~ of mainly linear chains that facilitate crystallization and are, therefow, white and opaque. Further developments have folnal property alterations possible, such as the linear lenes (LLDPE) and ultralow-density polyethylenes wt metallocene catalysts, which are based on singlegemdry catalyst technology, allow the production of ith &&ly tailored properties due to th.ose catalysts' capatrolling molecular weight and copolymer structures within . But lei^ higher prices must be balanced by their imce, and slight variations in their processing must be preferred m l t index values, which are reflective of the fsr thennoforming polyethylene plastics only ght, OPylene and its copolymers have a low specific gravity of 0.90 s), thus resulting in very ed on their high deg arge parts even if they fore, polypropylene becomes engineering thermoplastic resins. Ma
Therrnoforming M&tia$'
Chemical Descriptions
p y l m copolymers at9 on the d e t for pmb that must resist impact f o m . S;irnilar to polyethykm~~ hpvemenrs in polperiza.tim techiqum have lad to low-mduk~~po1ypropylerxgrades, which exhibit a wider ternm g e than presently a d flexible plyvinyl chloride
Stymrre polymers
139 olefin monomers have that can generate poly-
forming, yielding a high degree of orientation and g of the &wi.This sagging can
e been described extensively. Their
Thermoforrning hbt&&'
141
Chemical Descriptions
of p o l y s in~ their products lene oxide alloy platks.
lities. B ~ l wwell q
propylme copolymer, as well the ethylo o p ~ ~ ecan r , be ~ m o f o r m e dand pro(3) @@@U-raistant I) stysne copoljnnm and (high) impact styrene ,(BlR$)copdyme~shave gained great acceptance for thermofonned pam to their ease of processing, reasanable cost, and good werall properties. (4) Byrene~methylmethacrylate copolymers have improved outdoor and wether resistance. 8&
pletely by the extended use of .coextmda;d swp, acrylonibrile-styrene-acryl* coplphers (AS&). ,' ,?
\ '
and are well suited for thermofoming.
tance is required. Plastics in this last group few industrial and aviation applications due
e monomers are commercially used to produce
ies zlati I acc
rhez
copolymers are more accurately deution of the various monomers within
Ling 3po
eve C01
me it bi %C
mt L ~ S I
@Y I
On
to manomem that combine under
Transparent materials
Thermoforming Materials' Chemical Descriptions
& Azdel, hc., DuPont Automotive Products and Quadrm
available coT. and
acrylonitrile-butadiene-styreneterpolymer, which can be made by graft copolymerization as well as by blending two copolymers, Blends of different polymers have also been called alloys, to signify the synergistic effect observed in certain properties of these compounds, In most cases, however, the properties of alloys lie somewhere in between the properties of their component polymers. This may also be true for their prices. The properties desired in an alloy could be any one of the following: clarity, ultraviolet resistance, heat resistance, low-temperature impact resistance, strength, processability, and last but not least, decreased cost. Many copolymers, blends, and alloys have already been mentioned in this chapter, and a large number are listed in Appendix C under their proprietary names. Alloys have gained importance, given the broadening use of plastics for highly specific applications in large quantities. I
Fiber-reinforcedthermoplastics
recent years a number of very long fiber-reinforced thermoplastic c at least 50°F beyond the melting temperature or the softening ran the resin before it is formed and solidified by cooling in a mold. inexpensive forming process is often referred to as thempkastic
provided for forming. matched-mold aompresrsicm rnoldi
b
$listed in Appendix C.
m e optical clarity of materials is critical where thermoformed parts are intended either for gluing-type or for packaging applications. Naturally, any transparent material can be made opaque by the addition of pigments. A small mount of pigmentation renders a material translucent, which m m s that although much of the light will pass through it, objects behind it are concealed or cannot be seen clearly. Thin films of translucent m~Grialsmay appear transparent as long as the object is laced in contact with it. Surface roughness will also affect the clarity and brilliance of transparent materials. The table below lists some of the naturally transparent and translucent a l s : Transparent P W c s
Translucent Plastics
ui Acrylic Cellulose acetatet . Cellulose propionate Cellulose acetate ktyrate Ethylene-vinyl aleb@kcopolymer Polymethylpentem Ionomer Polystyrene Styrene-butadiene copolymer Styrene-acryloniuile copolymer Polyvinyl chloride Nnylidene chloride &polymer Polycarbonate 1 Polyethylene tgr=phthalag Polysulfone , Pol~ar~lsulfone
Polyethylene Polypropylene Polyallomer High-impact polystyrene Acrylonitrile-butadiene-styrene terpolymer Polyvinylidene fluoride Polyamide Fluorinated ethylene propylene copolymer Polyethylene terephthalate Pdybutylene terephthalate Polyarylketone
A number of commonly opaque polymers have been made available also as transparent or nearly transparent materials. Because opacity is On the presence of crystallinephases, transparency can be obtained by Preventing the formation of crystals, e.g., the addition of another cQmunomeror the rapid cooling and stabilizing of the W o r n polymer chain segments by cross-linking or orientation. If the size of the crystal&& can be restricted to a size below the visible wavelength, the plastic
.
Fm
Thermoforming Materials' Chemical Descriptions
Barrier materials
147
will also appear clear. This can be achieved through the addition of microcrystalline nucleating agents. Barrier materials
For the food-packaging industry the barrier properties of the plastic material are of great importance. Because glass and metals are totally impervious to any other substance, they have provided idad packaging materials for centuries. However, their high cost and weight have limited their use, When lower cost plastics became available, they were tried as
also give either rise to permeability or diminish it. The
unappealing. The permeation of oxygen into the can turn fatty foods rancid, but in other cases the
(P), which according to the equation P=DxS
weight) of the gas.
Barrier materials
149
t by the film material to become useful as a packaging Is0 I J. One the oldest and still low-cost film material-the surfaceand b$ materr f it were stronger (especially in tear resistance) and ,e fc y a mE ~tthat ! .merits. .e plasr proper' 3ce: )rd, CA pplied 1 e ca I. In 0th dpreac ting lperatul 3 introauce ent high-A. m nd thur ' the CQI thoroug lume f o ~ r e larninatc als, a tougl e good ma1 adhes~ >rial layers ation of thl ce of m
occur,which means swelling and buck-
L-vl'm~i~t~lc ue goods the .cal Co. ca ng film. The same is utiliz The specia f scaven mi lr vacul sed cor a product of u Amoco Chemicals (Chicago, L)unaer .- ..ater-containing content requires a steal., mLb,,,
a
150
Therm~formin~ Materials' Chemical Descriptions
it might become necessary to select a somewhat higher water permeable outer layer, e.g., by replacing the polypropylene film by a polycarbonak film layer. Its water permeability at sterilization temperature is about equal, but at ambient temperatures it is 10 times larger than that of polypropylene. Such a structure facilitates after retorting the drying out of the intermediate polyvinyl alcohol layer. The best barrier materials cannot be extruded into a film material unless they are copolymerized with other monomers. The neat polyacrylonihile, polyvinylidene chloride, and polyvinyl alcohol films can be obtained only by a costly solutioncasting process, whereas the corresponding copolymers can *beinexpensively converted into a film material by an extrusion process that becomes even more attractivew h e ~ several layers are produced simultaneously in a coextrusion process. Some common three-layer coextruded films are:
(1) High-density polyethylene inside low-density polyethylene to increase stiffness and reduce loss of moisture (2) Acrylonitrile copolymers inside polypropylene to produce gas b rier properties (3) Ethylene-vinyl alcohol copolymers inside nylon or polypropyle to obtain gas and aroma barrier properties (4) Vinylidene chloride copolymers inside high-impact polystyrene polyolefins to obtain gas and moisture barrier properties (5) Nylon within lowdensity polyethylene to obtain gas and moistu barrier properties. As some of the distinct packaging structuresjust two examples sho be cited: (1) For blister packaging of pharmaceuticals 2fluomethylene, 2-mil low-density polyethylene, vinyl chloride laminate, each bonded with a thin adhesivelay recommended. (2) An old competitive material, the regenerated cellulose film phane, coated on both sides with nitrocellulose lacquer (the synthetic multilayered packaging film) should be mentioned as first moisture-resistant, gas-, and aroma-barri(;r film. The structuring of barrier films has been enlarged by increasi number of layers to five and seven. The development for packaging applications is still progressing, an market domination-partly initiated due to customer already taken place during the last 10 names (old and new) are listed alvhabeti
Barrier materials
Barer Byne.
Lama
-Camm Modic
MXDt Mylar
Polyvinylidene-fluoride-based (PVDF) tie-layer resin for nylon 12 to PVDF Acrylonitrile-methylacrylate copolymer, rubber modified Coextrudable ethylene copolymer tie-layer for multilayer barrier structures Coextrudable tie-layer material for Barex Ethylene-vinyl alcohol (60 to 75 molQ) copolymer Co. Biaxially oriented, high barrier polyvinyl alcohol film onal Urethane prepolymer adhesive Ethylene-vinyl alcohol copolymer, laminated within nylon or.polyolefin films Coextrudable grafted polyolefin tie-layer material Aromatic polyamide highbarrier resin Polyvinylidene chloride coated (both sides) on polyethylene terephthalate film Coextrudable ethylene copolymer tie-layer for multilayer barrier structures Vinylidene chloride-vinyl chloride copolymer film solvent-sofubleainylidene chloride copolymer for high-barrier coatings Amorphous nylon resin (hexamethylene diamine and iso/&rephthalicacid polymer) sladitive for ethylene-vinyl dcohol copolymer for
152
153
Thermoforming ~aterfals'Chemical Descriptions
Selar RB
DuPont Films
SIS
Shell Chemical
Soarnol D Morton International Aristech Chemical Unite
Laminar blend of nylon or ethylene-vinyl alcohol with polyolefin for packaging Coextrudable styrene-isoprenestyrene block copolymer tie-layer material Ethylene- (29 mol%) vinyl alcohol copolymer Polyolefin with anhydride functionality
It is evident that for applications where mass production and automation
are so important, the development of suitable machinery and an efficient process is of equal importance. For some of these the reader is referred to Chapter Twelve, "Related and Competing Forming Processes." The usage of plastics in packaging has made great inroads during the past decade and is expected to expand further in coming years. Besides performance attributes the packaging industry is much affected by appearance, physical size and shape, and customer preferences. Therefon, many changes for the future should be anticipated, including efforts cw developing workable recycling processes. Some of the possibilities afe to improve the compatibility of the various multilayer components or &e mechanical separation of incompatible components. Electrical properties
Plastics surfaces will easily become electrically charged
its effectiveness environment and
108-10'2 103-lo8
Thermoforrning M&#V
Chemical Descriptions \
b gmund, Wfi4 an ~ ~ m a d g a $ e h a v e b owt of t md appjicationof v&ws them under expenditureof c d processes into a useful pxv&&, No problems &~ulQexist with IQ tmse ~f f w o y generated was such as sheet and film edge trims p m ' f h t d in-rbe extrusion paces hag a the mgrttld is 01~831 and well mixed in a constant propo W-the M n material. U the dormer has to c o b t the con-
ch
I
made to reutilize the materid extensively low mlded plydhylene am bn m t suitable plasti
+as t&
biomass dais, intcr~tht2mtfiment
textile fibem if tbe rnoleca
the repmxdng of the drnany of t h w p m VioIet radiation,heat, Pmabili
he chap
Thermodoming
mgad t~ tbe behavior of pl&m&
Chemical Descriptions
157
To+~#pkrsrics
case of fire.
1 cases legislative ruling$ will dictate which of the various
to viobnt o M u n e o n s in
ExtraMy f l m a b 1 pla&.cs, ~ m h as ni€rocellulm, have Iwg been m h d . Makwn plastics am about gn a pas with the above-mentioned other circum~tances, q d c 9natural materials. Hawever, dm to &, afpbtics &becoming a involved m fires seems to be higher. tias am mw available cornwith flame rewant i u e i m k l3eawe t b m contribute r s a n e proprties, and make processing more , etiffkwlt, they @re4 only when necemary. n m k d Wmtw qrts for the detmnkatjesn of flammability ' hm multipiied tial ally during the l a dec&. Wl, they cannot dnpkmte di the possible fim d m .This becomes cbar to all those who want to start a f hwithot&reso&@ to gasdim. For the beginning, the fuel must be available in a s u f f i c thin ~ form (pslper, film, or small sticks) to raise its tempemre beyad i@bmuddwampdtion point.
still be cognizant of potential witn tne materiak supplier to ensure that the most suitable % for a particular application is secured.
mdecular weight polymers constitute the major component materials. They are basically nonreactive under conditions. Most of them can be used for food, and some of them even find uses as body
I
ties we made of highly chemically reactive, low -campounds commonly called monomers, which as does not pose. a.problem to the plastics user. These
bular e hi
_ _ . A *
..
omen were de-
th r h&& -tat of hydrogen atoms exhibit hig ikmM with a low content. T k w two extremes side and t h W y arom
W; awy were eliminated from all plastics intended for @kms.Other materials, including catalysts, emulsi'awa,ldlredto perform the polymerization, and ddi-
rs,'lubricants, pigments, fillers,etc., are eompunciing.Some of those that could
lynxss on the other. 'lashcs,
m now available, should be employed ."-# - - widely contact can be anticipated.
&so highly mineral
More worrisome
the themud
have dwys k e n traced to e a h n moao~[i&, position of the Etlel source mu.
nave illso D ng conditions.
nn a cormive acid. '&%the padaging field by improper
'!_',.,
-
'..
I
,.
.
,*
ihermc
fl THERMOFORMING PROCESSES have evolved from the first w n g method where live steam was used both for heating and 5 flat thermoplastic sheets (camphor plasticized nitrocellulose) b t o u r e d mold. The various ways of preparing a plastic for the g and most of the machinery for accomplishing the formdiscussed in detail. Here, only the proper selection and forming of a sheet or film material will be described. e basic forming processes employing pneumatic pressimple forming tools-a clamping frame, a concave or and a convex or male mold-we shall examine increasingly installations. All such variations were developed to offset less proficient, basic methods. to follow, the illustrations commence with an area t (not showing the heaters) and concentrate an th nts taking place during forming.
I;C&e
ilthrming
~ m eshapes d can be obtained without @euse of a mold. the uniformly heated sheet is sealed to a plenum vacuum atd box or table). If either vacuum chamber, the pressure difference will outward. To repeatedly obtain the same ssure must be controlled for each piece Figure 10.1 depicts Ive that regulates the of thce perimeter can 102ciepicts one part I wi* a W-dimen-.
Dptical .
:--7
ihi
Cavityforming
0.20 THINNEST
TOGGLE CLAMP CLAMPING RING
, PHOTO TUBE UNIT
LIGHT SOURCE FOR PHOTO TUBE
7
VACUUM BOX
7
Material thinning at apex of spherically blown shapes: (a) height to 0.2, apex thickness 80%. (b) Height-to-diameterratio 0.5, apex thickness
SOLENOID VALVE CONTROLLED BY PHOTO TUBE
B, including transparency with
Figure 10.1. Free forming with vacuum (courtesy of Rohm and Haas Co., Philadelphia, PA 19105).
minimal optical distortion, be-
B soft sheet does not touch any mold surface. Unfortunately, sfthe sheet proceeds at a very low rate.
mmonly assumed that the wall thickness of a blown shape will &I thickness as is observed when inflating a rubber balloon. b,this is only true for cross-linked polymers. Thermoplastic $% thin out nonunifonnly: the area in the center becomes the W theBtreanear the clamping frame the thickest. Because small bdi%@mes affect the modulus of elasticity, unevenly heated lot be formed by this method. In fact, most thermoplastics shaped forms that are easily punctured. Only three transBkk, due to their tenacity and thermoelastic behavior at
excessive reduction in thickness at the -apex, a
ratio of 75% appears to be the practical limit. Cavi
formed shapes obtained with a flat and a three-dimensional (courtesy of Rohm and Haas Co., Philadelphia, PA 19105).
Figure 10.2.
&ape forming: (a) male mold with heated sheet, (b) and (c) vacuum zlfter frame was lowered, (d) drape formed part still on mold.
Figure 10.4. Cavity forming: (a) female mold with heated sheet, (b) and (c) vacuum applied to mold, (d) cavity formed part.
cavity, Otherwise, the process is ideal for fabricating parts wlUl a sturay frame, because the wall thickness at the frame or edges will remain close _ _ ._ pans i:, p a to the original sheet thickness. The thinning of ccDntoureu ,lLticularly noticeable on sharp inside comers, which, therefore, snoula uc
-
.
.
_^_
iapp"Gu[W
C 1
l l e third basic themoforming method, shown in Figure 10.5, utilizes a male, positive, convex, or drape mold. The sheet is again cl-----' '-" frame. When the material is sufficiently heated, the frame until the sheet seals to the vacuum box. Because the mold projects w w aa 5 or UIG the sheet's plane, the sheet will first contact the eleivatea ----. m _ -r-c'hamnld "hPPt) and start to solidify there at nearly the original trucwss 01 UIG DII--The remaining area of the sheet is stretched during iwownward movefhs ment, and further thinning occurs when the vacuum quickly sheet to contact all mold surfaces. Here, too, the areas formed I the weakest areas of the part. improv~~ In some cases the material's distriburim . .can. be marginally ,laham cphP
.
.
.
A -
I I ~ - - ~
mities Y are to be uspd for cavity forming, no special provisions
aL drape formiag, however, the upper frame member should
!A a
, and grid to pull down the sheet to the vacuum box nvidual mold. Otherwise, the sheet might form a web molds (see Figure 10.6). Since conveyor-fed thermo.Y w upper frame, a grid assist must be used, which is & - top movable platen. ".be produced either by the cavity or drape forming &ling one to pinpoint where to have the greatest m a k i d mers of a female mold cornspond to outside corners F.nowever, many other aspects, such as surface finish and Jf also dictate which type of mold to use (See Chapter
i
165
Bilkw drapefoming
Themoforrning Processes
kshould neither mark nor cool the heated sheet during contact.
elv vet), will not need temperature control. and the following processes suR~cientmachine opening platens is required. The opening must be at least 3 times the
I
I
If ory& wants to use a male mold for producing the same part, a alanced distribution of the material can be obtained by utilizing anb r modification supplementary to that used in plug-assist forming. rf these processes is called billow drape forming or reverse draw ng ( s e e - ~ i ~ u r10.8) e and the other process i; termed vacuum snap-b@ fmming. f i e b t e d sheet is first sealed to the plenum chamber and a bubble is
-
I
I
Figure 10.7. Plug-assist forming with cavity mold: (a) heated sheet inserted between mold and plug, (b) details of mold showing efficient air removal through slots and grooves, (c) mold raised and plug inserted, (d) vacuum applied to mold and part formed.
Plug-assist forming
Because in general a balanced distribution of the material over the whole part is desirable, prestretching the sheet prior to contacting the cooled mold surface is advantageous. There are several techniques for doing this. Naturally, this additional step prolongs production time and machinery investment expense. In plug-assist forming, illustrated in the drawingeof Figure 10.7, the shortcomings of the cavity and drape forming processes cancel each other out. When using a female mold, the bottom part would become thinnest; therefore, a male plug should first contact the*heated sheet without cooling it and drape it until the plug is close to the bottom of the cavity mold. Only then is the air between the female mold and the sheef evacuated to accomplish the final cavity forming step.
---EUef valve, (,-) mold @ insdrted, vacuum may also be applied- to mold.
166
Themadwming Processes
Reverse draw withplug-assistforming
extended outside the box by air pressure. This will stretch the sheet, resulting in a spherical shape thinnest at the apex. When the male mold is lowered, it will first contact the thinnest part of the sheet at the apex of the bubble. On further closing, some of the air must be vented, as the bubble decreases in size and eventually reverses itself completely. The final forming of the article is again done by a vacuum drawn through the male mold. Snap-back f o m
This forming method,sketched in Figure 10.9, is closely related to the preceding one. The o d y difference is that the bubble is first by vaouum md, therefore, is concave instead of convex. This process ctrill m b or mark-offs bemuse no plug is used. The sheet daes not have to be drawn the full height of the male mold if the draw rsltio is high. after inserting the male mold, the vacuum in the plenum chamber is released, and the material is allowed to snap back. A vacuum must be applied to the d e mold to complete the forming step, and/or the pressure in the box must be raised
ntinues to stretch uniformly until the male mold
Figure 10.9. Snap-btxAc f d w :(a) heam$ hillnw formed with vacu~n.W
throughout or strengthdicibusly varying the size lister, the penetration of the plug, and the temperamre ofthe plug. e reverse draw with plug-assist forming method (Figure 10.10), a is first blown upward. Instead of a male mold. the h u h h l ~initiallxr
Twin-SRastfoming
169
just by'simultaneously e v a c d n g both mold halves. The formed
Tine 1fl.U. Tnppedsheetforming. Leftsheet hdd by vacuum to porous heater plate; 0-& % d i s k & to mold. Righe heated sheet famed by vacuum and/or pressure and fmd part when press is closed further. h i v e s will
air cushion fisl front of this plug. The billow will be reversed in a folding or rolling &mr while the plug is l o w d pgure 10.1Ofb)], After the plug reaches its frnal position, located at a set distance from the female mold, a vacuum is applied to the female mold, and the forming and cooling of the article are completed. Trapped sheet pressure forming
Another thermoforming process that has captured a very wide application is the trapped sheet pressure forming process. It is the method of choice for thin, biaxially oriented materials, such as polystyrene or the polyolefins. If highly oriented sheets are clamped only in frames and heated, the liberated retractile forces may easily lead to excessive thinning or rupture of the sheet in certain areas. In the trapped sheet pressure forming process (Figure 10.1I), the sheet is primarily held by vacuum to a porous heater plate heating the thin sheet rapidly and uniformly. The edges of the sheet are restrained to the female mold. The sufficiently heated sheet is rapidly depressed into the cold mold when the vacuum is switched to air pressure. By using an elastic seal and equipping the molds with steel knives, trimming can be accomplished through a brief additional exertion of pressure. This process represents the fastest thennoforming method. n
Twin-sheet forming
For twin-sheet forming two sheets of plastic must be heated. They are then inserted together between an upper and lower female mold half, illustrated in Figure 10.12. The pressure of the closing molds will weld at the circumference the two sheets together An opening or hollow ;tllows air to enter. Forming can be done either with compresscd
'1
we bonded together. WzMn $hem e t h e apm the part mus be cooled,
Figure 1U.1 uressure fon
mmbay detail of a double oven, four-station twin-sheet W C Machinery Corporation, Itasca, IL 601%3.
sheet forming the mac
detail of such a ring
parallel a@me,nt of boa molds is thin fil small'r 'I
m a must be wide enough t~ 0xmt aa inward f o d m d a pad at the inside. Raking the heat by there will impmve the bond and will becorn essenfi
POssibi low we the rnt
Qateri;
k con;
-sheet forming is widely practiced in the que for the efficient packaging of mostly
with injection molders. Many parts can now be produced by this thermoforming process which, in regard to surface appearance, cannot
components. The processes described up to this point utilize a vacuum or an air pressure limited to practically 14 psi. Although these pressures suffice to shape a heat-softened plastic sheet or film into rough contours, they cannot force the plastic into distinct engravings and low- or high-relief designs of a mold. However, many of these outlined forming methods can accomplish that if the relatively feeble vacuum force becomes
.
1
this additional pressure at the former's disposal or by matched-mold techniques. It must be stressed that this change makes it necessary to secure molds that have considerably higher strength, better surface quality and details, as well as vent holes or slots that are smaller and more prudently located. Molds and pressure boxes need a solid foundation on the mold tables. The seals must be well designed and the clamps strong enough to prevent
Mechanical thermoforming
A number of the previously described forming methods incorporate mechanical forming steps applied concurrently with the action of an air pressure differential. Those were the bar assists and the hekious applicausually heate; to a slightly lower temperature, because higher t'orces Carl be safely implemented. If forming takes place at taperatwe6 below fhe glass transition temperature of the plastic, is not w m i u thermoforming (see Chapter Tivdve).
'OC-
that
the
thehe: & 'ns duction in sheet 4m-9 l=mblg, md cooling, 111~into the farmed part. .&ni$iizd ~ ~s m s cracking, ~
them indistinguishable from injection-molded parts. (This is illustrated
to pressure vessels have to be obeyed. Although all these changes increase expenses, mold costs should still amount only to one-quarter of that for injection molds.
aofa
FORMING OR RIDGE FORMING
' 1
I
,-
8
,
174
materials that retain appreciable itable for this process, ,
;
8
.'
HED-~~IOLD FORMING
as developed into an important
s it possible to maintain local mismatch between rocess for making egg tics are formed, it is mmodate the thickest yield laterally within SUP FORMING
ER PAD AND FLUlD PRESSURE OR DIAPHRAGM FORMING
,+&.,
are several a l W m in rubber pad and fluid pressure-formi* . What-they have in common is that only one mold half is , that higher forming pressures are applicable, and-probably atest advantage-that all forming takes place under compression, reduces the likelihood of tearing. The high mobility of low
thermoforming processes
Adjusting ~RWBA' parametens
ubmk d i q & p p fbmhg: (a) heated sheet inserted in opened press, and pun* s a k k m&Eainidg hydraulic pressure bnstant. (c)
t
plastic Wore f o h n g &d leid high
pn a new w h i n e , an
'a a&e'Ule rckguired
LEXAN
-
-
a?z=G?ZL n*ssr*..
% $87 m
1 ----me
, , , sheet and film THERMOFORMING TROUBLESHOOTER
1 I
I
well WOEthe satisfactory material runs out. Under no circumstances s h h d the old material be ueuscd to the last sheet. It is only possible troubles on vwiations in material if samples from material can be submitted to a laboratory. One t of always retaining at least a dozen sheets of the se can either be used--in case of trouble with the new hine settings are all right or otherwise tests conducted side by side with both old and new rxWxial will show differences in:
(1) M a t e r k ~ e n s i o n s sheet , gauge uniformity (2) Surface i ~ r f e c t i o n s (3) Degree of;a$entation (affecting shrinkage and distortions)
After ascertain* that the above conditions are praperly m& it is best to follow the m a t w ~upplier'smcommendations. Invariably, they will stress in their brq#a&i which conditions are of importance and which must be followed.'%n praxss contiitionsare altered, it is wise to keep record of it. In a shoae+@, one am compile one's own troubleshooting guide, which will l e d @?avery high percentage af c o m t assessments
that may arise are reviewed in this equipment,makrhk, and processing.
see pages 179-180), the Thermo8h& a d Film is.reproduced in
4U d W apparently similar problems.
~bablythe iagthfd wall thicE ng, and pn tions, all t :ss must bt :t may unl d may turn I The desig ?art's applicati the overall de5 of environmer ~ermoformed1 rials were rank to everyday envir Such a listing \ noplastics, (b) pl er), (c) thermoset define the envin :e, since those conaltlons kart's use, e.g., durinv tr itions, rough handling lanical stresses form "" Or pressures, flexing, impacting, ana vibratio cs are quite sensitive toward changes in tempb,,,,,, 8s aggressive chemicals. Combinations of these influc their application or at least require the selection of a higl nore costly grade. Low-temperature exposure endangers brittlei excessive dimensional shrinkage and high temperatures, pos183 I
184
D-
sible softening and buckling. to swelling, softening, c&cd tensile loading,
Considerations
Assembly d bonding
185
n, Occupational Safety and Health Administration, and Un-
LLi
,',.
'
,
I
,
,-.
,
'
.--
'9 . .
, ,
.'
..
The orby t and Ei Iimita ing mi
washers ? sheet can Probed ta I within tolg additic
Design Considemtions
i : .
; ,it2,'4 fixtures must be provided for rolling out the air bubbles and maintaining the desired shape while the resin cures under exothermic conditions. Such fixtures may be constructed from wood or reinforced polyester resin without the need for labodans smoothing of the outer surfaces. A thermoplastic sheet that is compatible with the unsaturated polyester resin must be selected. The contact surface might have to be cleaned, roughened, or coated with an interlayer to ensure a lasting bond and @ extended use of such parts. Figure 11.? b @&process. rermoformed container which has provisions For properly stacking and of Portage Industries Corp., Portage. WI 53901).
I b ,
nlieve mwtrining stresses may be advantageous (exposure to 10to 20°F below thehait deflection temperature for 1 hour). Attentim~must also be paid to the packaging and storing of thermoTo reduce required space, one may be tempted to stack formed p a s tightly. This not only may cause deformations of the formed dso lead to cracking during storage or shipping. Figure on the left-hand side how incorporation of support steps in the d e b af the farmed part can produce parts that can be placed securely of each othec, whether for nesting e m ~ t ycontainers or shcking h a
E: k
1 . rL.
?'
-
194
195
VAJ. b this s a a ~ h s s
(Newport
of my of these stretching methods, thia type of orientation has found m y urn. TIM properties most hpmved are modulus of e l d e i t y and rigiditytregismwe to fracture, and sometimes to environmental stress c&w. Chemical mistmix L enhanced, but gas or vapor barrier pmpith only in sonre caseis.
warm forging k geryerally uaed when the dwve parts with m s ~ i vm e sections. The heating times 1 b u r for 1-in.-thick blanks, which is not a poblem rn available. b some caws microwave heating
ab lowe tempem-
12.1. Polycarbonate distribution transformer cover formed on metalworking sat ambient temperature(courtesy of Madern Plastics, September 1960, p. 137).
3
.
197
Related and C o m e r i g Fonning Processes
e by ib elaqpd lip. A plug stretches the preform
corn ;rts a ~urprise.In mttdurgy
to^,
alloy composition and aystal
um of successive cold drawing and annealing
over conventional thermoforming are the total equipment, savings in energy, and complete s by National Can Co. (Jacksonville, FL)is also a It starts with the thermoforming of a barrier lamisr the structural plastic is injection molded and the ite is subsequently inflated to the size of the pack-
materials mu& be free of surface irregularities, and lubrication is essential. The elastic spring-back is considerably larger t h that experienced with metals. 'Iherefore, in many cases the extent of forming with plastics must be exaggerated. Sometimes, excessive frozen-in stresses will have, to be relieved by solvent vapor treatment or by annealing.
chapters. These advantages
Packaging contahar
two main disadvantages connected with the commo
hitations fer tbrmoforming
LDlNG AND ROTOMULNMG
ng pmcwss sin aixommodate two she&
of dif-
Al deve
~ m w t d m b e ~ w ~ ~ ~ c i a
Prm
pild
and conti
chapter.
.
halv~ by P
P W
,m with fhe help of a spin-welding
gy
properties of thermoforming materials -..
materials most frequently used in thennoforming two tables (Appendix A and Appendix B). As formed parts are differentiated throughout the on of materials has also been chosen for the two different table in Appendix A refers to the material properties interest for heavy gauge structural parts. Therein are anical properties and the thermal boundaries specific gravity and the prices reflect 1997 ose; natural color extrusion compounds in listed because material costs occupy the overe cost of durable structural parts. The relative potential savings when substituting higher speals with such of lower specific gravity. In some cases, be obtained &om primarily sheet producing compaor.lower prices than extrusion molding comwunds.
200
0.55 O M
354CI
W
I80
O,M,O OIIC1,S 1QP-180 2-3
14543B 1-
0.4 1-7 AW.8
1151QO
ll!s-@s
445 285
081P
10-15
2-7
D,tP
11-17 ll-IT
1-2
0.12
116(~p$)P10-240 189 pi) -280 182-140 3 3 4 5 1 s 4454%
l7waE ?7%2LX %@-&XI
-
pwm,mu
~ptjefkwimide,PES pd@hemIm,Pm
.
M H - & PEEK ~ B ,
'
I .OB
4.3
1.3 1.2
1 1-54 IS4
7.3 6.7
6.7 1.7 1.5 1.2
12
1522
~0.01 <0.01 0.01
0.92-0.93 0.50 0.950.96 0.45 0.904)91 0.35
9-11
0.014.1
0.92-0.94 0.70
2.5
4-3
1.05 1.06 1.08 1.02-1.08
0.44 0.46 0.86 0.W
1.6 1.7 3.3 3.4
1.50 1.3 2.15
0.35
1.9
-
-
10.05
76
1.27 1.F1.4
0.98 0.60
4.5 2.5
-
1
0.34 ,4
0.05-0.15
5-10
0.01-0.03 0.05
0.11-0.19
3-7
0.24.4
5-10 4-14
0.01 0.1M.7
6.5
IWgW8~On hl* @m3tramrni&oll
1-3
1.15
0.1-0.2
-
1.50 5.8 1.80 7.0 4.40 1$,8
.
136
properties of film materials MATWIALS LISTED in Appendix B are preferably chosen for kaghg applications. Since they have just a short-lived use, they films yielding either flexible containers, or if be accomplished through surface s are burst and tear strength. Especially ospace packaging, gas and moisture transmisdered. Because of great variations in film kness, 'eost comparisons per weight are not very useful; therefore, r prigti%are no longer listed in this edition.
/.
t.
R
ASTM Test Method
100 in2.24 hr.atrn
-Msterlal
cedlulm ffltns 1. Wlulose m e fCA) 2. Cslluloseacete~~t~(CAB)5-9 3. R e g e n e r ~ ~ , o o a t & 8-18
50-100 15-40
4Q-70
2040
4, ,lw density (LDPE) 15-3 100-700 1040 5. FU&@Q%W, high density(HDPE) 2.5-5 2 5 4 0 NA 6- -men0 (pMR 2.54 10 7. Ethyle~svlnylacetate capolymef
-
w4
8. IOnOmer Halogen atam oantainingRlms S. w
l chl019de.rigld Ipn=]
10.Poryvinyichloride, plestidzed 1 1. Pdy&i fluaide 12. Fluethybnepropyene
(m
19.5 5004M l&?5 H3CO-400-
7-10 24
8-11
80-100 110610
tE-575 N.A
5-10 2-15 1O!X3OQ XI200
-
BQMX)
180
2000-2500
1
210
300400
S100
5W1000
15-20 20-750 3.2
10-20 1040 ' 0.25
30-60 75-3000
2-5
11.1
3-1 5 957
140-160 135-150 140
150 300350
3060
500
0.5
215
50
900-1400
-
50-200 25-100
I
26-50 W-40 ll(r-490 10-700 loo400 20 11o-280 60-1000 s l 1 9 17-48. 33a600 17-46
0 3 300 11 13. ~ t ~ i f l w x ~ e n e 5-10 56150 23-33 33o-wl 2.5-40 14. EtIqiemio h M f l u m aap.IEmE) 1 150-250 35-40 3!5o-mu ~ 1 3 0 0 BlPudrrffyor21#rfedmms 15. W V W COPP) 7-24. 30308 1600-1500 5-10 ' 16. 6-12 3-30 13-35 27Mjoo 5-26 17.Pdyme(hylmethaaylete,(PMMA) 8-12 5-20 340388 18. F!oly(&ylet?a tmphthalate)
m
-
m(m)
-
PPrn
19. PdyuiMchlaride (OP\1C] 20. Mnylidene chloride wpolpw
(PJdC) 21. F'dymlfde (nylon 6) Other Important films 22. Qpolye~W(PETG) 23. l'dymbxprts (PC)
'
8-16
70350
-
7-20
2Q-40
3o-iXl
8.6
-
400 9 &lo6 35-43 8-14 25Q44 1313
1 1 f j Q 1 m
1GfN-llm
(2min)
Man-
s
Pmw
p€%mmkm-anlalw
Carp.
O p q a supam&Sembpkdline
--
AmoooChemicafs
Nwa Ch€m&k Inc
wcmltcal &&,lfl
-
Funchrtal grmps containingpo&d%fin mywn Chygm&a+erPorfitms asArarroedmmm
,
&&I.Inc.
auyWhermapl,laminate
~~
248°F
wcm==mm-~@
HlgWieatpdywtmate Co-CXt~resSn
~~r&n
PoIy&&dme
-mY=w, ~
I
C
3
u
p Q E y m -
r
&
~
~
p=miWi
N
~
~
~
~
r aw a l3w r ksam
.
B & % f i b ~ r r r z l t h l ~ .~ ~ 31PF M Glass fiber mat rdnf, ameKph.enginwing ptastic shad 2WwF
5 P Chemicals lnc. (Food) packaging
Dow Chemical
Bayer Corp. Shdl Chemical DuPont Co. DuPont CoriEvl Products Shell Chemical BP Chemicals GE Plastics GE Plastics
Styrene-maleic anhydride terpolymer Polycarbonate resin Polyarnide 6 film (PVdC-coated) Biaxidly oriented pdyamide 6 film (WdC-coated) Polyamide 6 resin and polyamide-ethylene copolymer Aliphatic polyketone resin Long strand glass fiber reinf. thermoplastic ASA and aaylonitrile-ethylene-styrene Nucleatedcrystalline polyethylene terephthalate resin Biaxially oriented (irradiated) polyethylene shrink film Polytrimethyleneterephthalate resin Coextrudable tie-layer for polyacrylonitrile Acrylonitrile-butadiene-styreneterpolyrner resin Polycarbonate/acryionitrilebutadiene-styenblend
Coextnrsion adhesive tie-iayer 205-250°F 270°F Food packaging film Food packaging film 147°F Engineering plastic Weatherable polymers 400°F Packaging Thermoplastic sheet Barrier packaging 20G215"F 220°F (continued)
TW
E
name
Polymer Cop. Solutia Alucobond Technologies Schulman Soivay Polymers Solvay Polymers Xcona Corp. Garland Mfg. GE Plastics Geon Co.
Polytdrafluoroethylenebased composition Extruded polystyrenefoam board (Foamed) p o l y c a r u lonomer resins - --- '
f
Mihg
.
fn;gneered palypropyiemmiin
r-,
-
High barrier mterial, 150°F l S l 4 5 5 , weatherable
nylether copolymer
Use up to 450°F
4 .
*m USA
230Y Chem. resistance
-
**. M.k W r C e s
Ph1Ci Chemiml
eb
i
-.
rr ~a RL I
.
.
l2lPotlt Films DuPont Films Segm&Co.
~ m o c ol'dcmmm rod. Amoco PerformanceProd. Mitsubighi Gas Tredegar Film Products M t d l North America EastmanChemical DuPont Fjlw DuPont F&IS Durn-
-
~otyetttylene'terephthalk&film resin I High density polyethylenermhn, m e n e resin Polyethyleneterephthalate film Myethylene tefephthalate film msh Porws, machinable Mocks (4-powderfined epaxy) P ~ y s u t f o n e / a c r y i o n i t r iday t~~~ Pdysulfone/polyeth~eneterephthalate alloy PdydeAn, anhydride grafted Cornpms&n-raw monodir. oriented HDPE film Pdypropgtew Extra-kw densitv oolvethvliene resin Pdyethyleneterephthaktefilm P~~ twphthalate film \finyiidene chloride copolymer coated PET film Polyethyleneterephthalate film m-XvlYlene diamine-adoic acid nvlon resin Moldable potyeathylenefoam Modified polyphengened d e PdyphmyteneoAe/nylon alloy AqiofWte-bu-e-Wene terpdyrner P c W m w resin ~th;jen&metha~ic add cqxdyrnea resin Ccexhuded polyolefin-poiyamide film
.
129°F (Food) packagingfilm Ifaod) p a ~ Q n film g Mdd material ~[M"F
ckftmwmdm MaicJatraMam Rlm regin (Food) W n Q Weat shrink film High M a r film OvanaMe lidding High barrier (co)extrusionresin
.Pulse
..
%f
ri
C
'irWF
PYrope1 QLF Quadrax Quarite Questra Radel A Radel R Regaltech Repete Resmite Retain Rexene Reynolon Rilsan Rovel RoyaEex Rynite Rytm Saranex Selar PA Selar PT, Selar RB:
-- -
Manufacturer
Chemical identikation
Mobil Chemical Ferro Corp. Toyo Seikan AlltedSignal Elf Atochem North America Dow Plastics Dow Plastics Klockner Pentaplast Klockner Pentaplast Klockner Pentaplast Klockner Pentaplast ICI Acrylics Shell Chemical Millennium Petrochemicals Millennium Petrochemicals AtoHaas Goo4/ear Polycast Technology Schulman Plastic Suppliers Schulman Schulman Dow Chemical Dow Chemical Montell North America Dow Chemical
Oriented polypropylene film laminate Clay/calcium carbonate filled polyolefin alloy Oxygen-absorbingplastic film layer Nylon 6 laminated ethylene-vinyl alcohol copolymer hlm Polyether-polyamideblock polymer Expanded polystyrene Polyurethane elastomers Food grade film and sheets Thermoform film and sheets Medical grade film and sheets Pharmaceuticalfilm and sheets Acrylic polymer resin and sheet Crystallizable polyethylene terephthalate Polyethylene, high and low density and polyprcpytene Modifiedpolyolefin Polymethyl methacrylate sheet Polyvinyichloride resin Cast acrylic sheet Acrylonitrile-butadiene-styreneterpolyrner compounds Oriented and biaxially oriented polystyrene film Amonitrile-butadiene-styrene terpolymer alloys Polyvinyl chloride compounds Pdyurethane1AB.S blend Extrusion compound Polypropylene resins Polycarbonate/aaylonitrile-butadrene-styrene alloy
Heat deflection temperature (264 psi) or application Packaging 190-240°F (66 psi) Oxygen barrier High barrier 125-21 0°F (66 psi)
Low-density cellular sheet Coextrusion tie-layer resin 205°F
185°F Heat seal or tie-layer
Albany International
Dow C h e d d
Am~mPerformanceProd.. &num PerformanceProd. 0'8uiliian Shd Chemical Bordm Packaging DOWChemical Rexene Products Ids Metals f Atochem
.
Uniroyal Technoloav -. CWD. ~lnir& Technology Corp. Uniroyal Technology Corp. DuPont Engg. Polymers Phillips Chemical Advanced Elastomer Dow Plastics Dow Plastics DuPont Padcaging DuPont Packaging DuPont Packaging
Carbon, glass or aramid fiber reinforced prepregs , Acrylic sheet with a aranite mmarance ~emiaystallinesyndiotactic bb~~styrene Polyethersulfoneresin Polyphenylsulfoneresin Medical grade WC and PP sheet and film Polyethyleneterephpalate resin Flexiblefood packagingpolyvinyl chloridefilm PoLycarbonateIa~onitrile-butadiene-styrene alloy Polypropyleneand copolymer resins Polyvinyl chloride shrink film Polyarnide 11 and 12 resins Styrene-acrylonitrile-definicelastomer alloy 5-layer closed-cell ABS or WCIPBS sheet Aaylonitrile-butadiene-styrene sheet ABS/WC or HDPE static control sheet Polyethyleneterephthalate resin and elastomer alloy Pdyphenylenesulfide resin Polypropyleneelastomer alloy Polyvlnylidenechloride film resin Coextruded polyolefin-polyvinylidenechloride film Amorphous nylon resin as adclitive to EVAL resin Modified amorphous ethylene terephthalate copolyester Laminar Mend of polydefin and nylon or EVOH resins
Food packaging up to 275°F (66 psi) (Food) packaging 275°F (66 psi) 210°F Canoes 225°F 174°F or 156°F (66 psi) 444°F W F High barrier food packaging Food packaging Improved coext., high barrier layer Clear mondayer, high temp. barrier High barrier concentrate for packag. (continued)
-
-- - --
-
-
-
-,-
High barrier coextnrsim film
d:.e:e
DSM Engineering Plastics DSM Engineering Plastics DSM EngineeringPlastics BFGoodrich Specialty Dow m t i c s
GER&K:S:: FlmO-Gl& ' DuPont Padaging Emerson & Cuming
hrPont Fnn@-'- '
'-
-
e-
Nylon-416 resin PdycarbonatefABSor polyester alloys Bastform modified meme mabicanhydrideresin Static dissipativepolyester Extruded foam Clear s ~ b u t a c l i ~ block n e copolymer d-@h-irllpact] p d ~ m e Polyvinyl &Mde/.ABS allay sheet I J o l r p h ~ sulfide e oompounds fanomerfilm ~ y l l e n e - m a t add h ~ salt copotymer,i mresin Glass sphere syntactic faam Polyvinyl fluoride film, noworiented ~etrafluoroethylene '.. Tetrafluwoethylene-hemfluoropropylene copolymer PerRuoroalkoxy-fluorocivboncopolymer resin Ethylene-tetraftuoroethylene c o p o m r resin C
-.
3f32OF, (food) packaging film Pharm-cal packagingfilm sod) barrier packaging High clarity, 15PF
-sing (1820F),203°F
Flexible film and skin pd@ng Mdd plug material, up to 450°F Surfacing, weathering film High temperature material
High performancefilm High performancefilm
,
---
-
(Food) packaging (Food3 pac@wg
535°F tooling -7 -*Food packaging 21z24_3*?
L+-
Packaging .
L
d
-=sn-
t
345°F 375-408"F 160°F
.-. (continued)
-
Advanced Elastomer Systems Albany lnternat~onaiCorp. AlliedSignal Inc AlucobondTechnolqes, 1°C.. ',I;$ American Mirrex Corp .-,
Bamberger Polymers, Inc. BASF Pla$rcs Ma*Ws Bayer Corp. BFGoodrich Specialty Chemicals BOC CoatingTectmatcgy Borden Padt3ing Div. Borden Inc. BP Chemicals lnc., Earex Div. Charlotte C h e w Inc. Chisso America, Inc. Colorite Plastics Co. CYRO Industries Dow CherniczjlCompany DSM Engineering Plastics DuPont Automotive GuPmt Wan Products @WMEngineeringPolymers
(800)352-7866
W. ~ a ~ m~erttan: 7 . m 42025 . 3 W&hcd Hwse Rd., New Castie, DE 19720 , W E . RaneldphDr., Chicago, IL 60601-7125 46lX W i n n i s Ferry Rd., Apharetta, GA 30202-3914 3801 West Chester Pike,Newtown Square, PA 19073-2387 7350.EmpireDr.,Florence, KY 41042 Independence Mall West, Philadelphia, PA 19105 44 Whippany Rd., Morristown, NJ 07962 925Washburn Switch Road, Shelby, NC 28150 1983 Marcus Ave., Lake Success. NY 11042 3000 CMtinential Dr.North, Mwnt Olive, NJ 07828-9909 100 Bayer Rd., Pittsburgh, PA 15205-9741 9911 Brecksville Rd., Cleveland, OH 44141-3247 4020 Pike Lane, Concord, CA 94520-1297 One Clark St., North Andover, MA 01845 4440 WarrensvilleCtr. Rd., Cleveland, OH 44128 250 Wilaest, Suite 300, Houston, TX 77042 1 185 Ave. of the Americas, New York, NY 10036 101 Railroad Ave., Ridgefield, NJ 07667 PO. Box 5055, Rodcaway, NJ 07866 2040 Dow Center, Midland, MI 48674 2267 West Mill RCf., Evansville, IN 47732-3333 950Stephenson Highway, Troy, MI 48007-7013 Barley Mill Plaza, Wilrrdngton, DE 19880 Chestnut Run, Wilrnington. DE 19805
(506)339-7300 (800)821-9292 (800)626-3366 (800)488-7608 (800)621-4590 (800)621-4557 (610)359-5642 (800)354-9858 (215)785-8290 (800)221-0553 (810)351-8000
-
w')
(800) a73746 (800)622-6004 (800)331-1 144 (510)680-0B1 (508)686-9591 (800)272-4367 (713)954-4855 (212)3024500 (201)941-2900 (800)631 -5384 (800) 441-4369 (800)438-7225 (810)583-8000 (302)992-2072 (800)441-0575 (continued)
PO. Box302!3,Edison, NJ 08818-3a29
E
1
400 Frankfort Rd., k mMy Sb;Monaca, PA 15081-,2298
I W Wdw M., WIndwst~, VA 22801
RO.43a 58966, Houston, TX 77258-8966
Wctrex USA Inc. Westlake Plastia Co.
2887 Johnstawn Rd., Columbus, OH 43219 70 Cadiste Place, Stfanford, CT 06902 PO. Bdx 30010, Wlnona, MN 55987-1010 212)F&?TIOII~Am, R m , PA 19612 297 Feny St., Newark, NJ 07105 300 High'point Ave., FWtsrnouth, RI 02871 5005 LBJ Frmay, Dallas, TX 75244 6601 W. Broad St., Rimond, VA 23230 3550 W. Market St., Akron, OH 43893 11 Kent St.. Milford, CT O&E€l 119 Salisbury Rd., Sheffidd, MA 01257 PO. BOX2463, Hous~OI, TX 77252-2463 800 N. Lindbergh Blvd., St. Louis. MO 63167 PO. Box 27328, Houston, TX 77227-7328 90 Monis Awe., Summit, NJ 07901 50 B&er Ave., North Kingstown, RI 02852-7500 444 N. Michigan Ave., Chicago, IL 60611 1100-TBoulders Pkwy., Richmond, VA 23225-4035 1840Trans-Canada Hwy., Dorval, Quebec, Canada H9P 1H7 312 N. Hill St., Mishawaka, IN 465484668 PO. Box 860, Valley Forge,PA 19482 601 Willowbrook Lane, West Chester, PA 19382 PO. Box 127, W. Lenni Rd., Lenni, PA 19052
krsion factors L
3FOLLOWINGPAGES present conversion factors for property vdues
Allen-Bladley Co., 32 AlliedSignal Inc., 141, C210, C211, C214, (216,221
Alucobond Technologies, Inc., C213, C218,221 Aluminum, T16 Americas M i m Corp., C2 18,221 Amoco Chemicals, 1'49.C219 221 Amom Perfonnanm Prodwts, IC210, C214, (215, (217, (219, C220,221 Amadel, (210
.
ps, 31,128 im, l a , 1 4 , m,$93, T202,C211, C!ZlC317. (219
126. T151,217 ethylemterephthdmc o p o ~T2,9.124, Amosorb, 149, C210 Anmdhg, 77,173,187.1'90
Apec. 'a 10 Appeel, ' a 10 A@, (210 ARCXI ChemicalCo.. (3212,221
233 B m r g w PO~~BES,
Inc.. C211.221
JhBx, Tt51,621l
Badermaterids 1,@, 1% @ 1% 139, I#6-152.194,19%,C%1i(J-21%. BAWPIRS~~QM a i & C218, C219, C Z D t 221
Carboxyl groups, chemical component of polyesters, 142 Cargill Inc., 155 Carilon. C211 Cascading, see recycling Casting process. 129 Catalytic gas heaters, 23 Cavity forming, see &o female mold. 35,161 Cell cast acrylic sheets, 136 Cellophane, 149. T206
sable) poly&qba~
151,f311. €2,152(221%
B h M orj:enmtion,46.115, 123, 1B9#1193,
Birweially oriented dm,T28, 140; T148,168, BWally oriented shaet.37 B h b l l y sttetekd, 36
Billow hmbg, 37,60,6;4,159,165 Bid-
phticg, 155,157, C211
Cellulose acetate, T8, T28.136, 145, T148, T202, T206,C219 Cellulose acetate-butyrate,T28,136, 145, 196, T202, T206 Cellulose (acetate-)propionate,T28,54,136, 145, T2Il.2, C219 Cellulosics,T28,21,37,49,76, 114, 117, 119,120,136,137,161 CelsiStrip., 30 Celstran. C211 Centrex, C211 Charlotte Chemical Inc., C214,221 Chemical vapor deposition, C217 Chill roll casting, 128 Chill-marks, 53,70, TI81
. 112,138,194 'I292
DuPQot~~T151,IC210,C211,C213. W 5 , C217, =I%, 222 DnPmt Todlar,C218.222 DWMBB,a 1 2 Dycz12
W l ~ y(212 , l3wBpak a 1 2
C b d & Cb.. 81-2S,C212, a15 C218,4319.222
I3&mkdo=#
Chisso America, Inc., C212.221 Clamping frame,33
Bya&Tt6l. OX1
Cold forming, 122, 195 Cold stamping, 195 Colorite Plastics Co.. C220.221 Competing forming processes, 193 Compression molding, 53 Computer aided engineering, 45 Computer-integrated man@cturing. 32 Conduction of heat, 3.15.20.69 Continental Can Co., 194 Continuous in-line thermoformer, 85
ffusivity.see permeability Dlmnsional t o l m , 30,35,48,66, 117 C214-217.221 Draft in the mold, 52, TI 80, 185
E
235
Index Pnamed prodactg
p a l m ,a 1 3
,103,105,124,130,146
Wogen lamps, 24 Rampshire Chemical Corp., (212,222 ~ w dT16,54,60,70 , mPE,see highdensity polyethylene Head, see ako thermal ~eddeflectiontemperature, T28,ll2, T202
Heat penetration, 10,24,116 Heat pNing, 25 Head spaling, 104-108,130,149,198, C210,
Heat sSnk compound, 70 Heat W e r liquid, 4 H e a t - s W fib, 127. C215 arrangomf 18,26
34.128 Rmhmd C213
Gablar Mwhhenbau. GmbH, 75, % Earlpad Me.Ca.(213.222 OIIs ionrate, TI&& TaMT W-B& i n f h d bestem, 22 GE R&IW. 1'24. Tim, ( 2 1 1-215, Highdensity polyethylenes, 137, T202 High-impact polystpm, T28,148,145, T202 High-temperatuyephities containing ring structures, 141
Ranas mudant wmpounds, 140.156, C213 Flmm&ilW d p h t i a , 158 ~~, CZ13 R m W h *Inc., CZ18.222 flm-forming pmxss, 176 PEuid ~IWSUI&!b-g, 175
Impax, (214 Implex, C214
Inert gas atmosphem for packaging, 104. 106 Inftandabsorption, 3, T8, T9
Inhmd heaters, l o , % , 79 Infrand senSors, 27.30.32 Iqjedhn molding, 50,53,66,114,118, 172,198 Inlie themoformer, 95.98 Insehr, 186 Insufficient draw, TI80 Ionomer resin, 139,145, T206,a l l , C213 Irwin Research & Development, Inc, 89-91,M lnvin International. 100, 102 Izod impact strength, T202 K-resin, C214 Kadel, C214 Kaladar, C214 KamaCorp.. C214.222 Kapton. C214 Keldax, C214 Kiefel, GmbH, 15.7475,lOO. 105.106 Kleerdex, Co.. (214.222 Klockner Pentaplast, C216.222 Korad, 130, C214 K r a e m & Grebe GmbH,107,108.109 Kraton, C214 Krystaltite, C214 Kydex, C214 Kyner, C214 Lamal, T151, C214 Lamicon. T151, C214 Laminar blend barrier plastic, T151, C217 Laminations, 129, 149 hmisan, C214 Landfills, 155 Laser beam cutting. 74.77
R u h Co., (31%222
~~~Q
chpk4I Tfr6
Lexan, C214
&a-
Linear bending. 115 Linear Form Pty. Lfd. 100.101 Linear low-dens* palyeltyleag, 137 Linear thermofomtm, 100 Lip rolling, 76.93 Liquid carbon dioxide for cooling, n Liquid-crystal polymer. ~ 2 2 0 LLDPE, see Linear lowdensity polyethylene Lowdensi@ p~lyethyle~e, 137, T202 Ludte.C214
Qtiddet
RIW, 65,312
faElnin& 43
Ckh&d, C213 cMlbmd. C213
€me&
-
-
I -1
a14 L I K ~a~1, 4 l.a&mx,
&4e&dmm=qing
layer i paehghg films,
149
M M 3dliw 56
Idman,a 1 4
Mold w 36-59 Mdx3t@l&M&%
Mold plug inat&&, C218 Mdd lp?ags,59
,3S.W,%2,4&,@+~%,&69, got*
~213,
&Ad tempmb~, RB, 48,7fF, 71 M W m &vmssm,53 M&-,49,71 . M 4 3 t w f m 1% ~ - a 1 5 Mmauto, Crr., €213, CZ20,223
Moct08 & 1 kc., Tk51, T152, (210, Ca14, m18, C220,223 M-, T151, a 1 5
.
s *' I.
,
-*
,.' ' : .
.
-1
,
I
I
N y l q T9,EB, 1 12.115, 142,145, T l l , C 3 l
-. .
a58 CtreaPfeal Co,
am, ai% 22%
-
4
149,Tl51,
mtli ~ c a l s CZ1Q , (21, Mitad M%kc., C214,222
Mo&ii(2I-amkd aJCe,, a 1 1 , a16,m.
yacrylonttnle copolymer, T29, 128, 150, T151, T202, C211, C2I8 Polyallomers. 139, 145 Polyarmde, see nylon
Pol~~benylene ether (oxide), T29, 140,142, m,C215 Pol~phenylenesulfidede. 115,143,144, ~ 2 0 2 , C213, C217,
--
Polyphthalamide, C210 Polypropylene, 7, T9,15, 18.2 1, T28.49.60, 71,86.114,122,137, 139,144,145, T148,150,194.196,T202, T206, C210, (212, (213. C215-220 Polystyrene, T9,T16, T28,98, 114, 115, 116, 121,122,139,140,145, T148.168, ~202.~~06, (3210, ~ 2 1 4a, 1 8 Polysulfone, 21, T29.143, 145, T202, C215, C217, C219 PolyeetraRuomarbon-coating,18 Bolyt~uoroethylene,T16,60, C211, C213, C218 Polytrlmethyleneterephthalate ester, C211 Polyunthane compounds, C216, C219 Polyvinyl alcohol, T148.150, T151, TI52 Polyvinyl chloride, 4, T8, T29,49,67,76, 114,115, 117. 127. 134,138,140,145, Tl48,155,156,199, T202.TU)6, C213, C216, C217, a 2 0 Polyvinyl chloride alloy, (212-214, C218 Polyvinyl chloride film and sheet, 128, C214, (217-220 Polyvinyl fluoride. 127.130, C218 Polyvinylacetateemulsion, C212 Polyvinylidenechloride, 127,141, 145. T148, 150, ~151,194, ~ 2 0 6~, 2 1 2~, 2 1 5 , C217 Polyvinylidew fluoride, 141,145, T151, C210, C213. C214, a 1 8 Porous melal molds. 56, C215 Portage Industries Corp., 191 Post-consumer generated waste, 154 PP,see polypropyleae F%&e&m, 138 Preprinted films, 190 Press polishing, 129 Presswe forming, 50,60,65,85,171,198 Restretching, 37,59.64.164 Prevail, (3216 Primacor, C216 Printing. 190 Pro-Fax. C216 Propagation tear strength of films, TU)6 PS, see polystyrene Pulse, C216 Punch-anddie trimming,75 PVC, see polyvinyl chloride Pyropel. C217 QLF, 149. C217
Quarite, C2 17 Quartz, T 16 Quartz-like film, 149,C217 QuesTech Packaging Inc., 195 Questra, 122, C217 Quick change locks, 45
& Co., Inc., 45, C215, B%#
I
polymetg, 1 1 1 , 1 1 4 , 1 1 7 : ~ ~ ~ Radel, C217 Radiation heating, 3,5, 12,24,25 Radiation pyrometry, 27 Radio hquency interference, 153 Recycling, regrinding of plastics, 73,97, 130, 132,133,134,152-154 Reflection losses. 7 Regaltech, C217 Regenerated cellulose, see cellophane Repete- C217 Resinite. C217 Retain, C217 Reverse draw forming, 165 Revem draw with plug-assist forming, 167 Rexene Products Co., (217,223 Reynolds Metals Co., C217,223 Reynolon, C217 Ridge forming, 173 Rigidity. 59, 112, 113, 115, 122,130,132, 137,139,194 Rigidizing, 187 Riian, C217 Riveting, 186 Roll-kd themformer, 26, Rotomoldmg. 114, 199 Rovel. C217 Royalex, (217 Royalite, C217 Royalstat, C217 Rubber diaphragm or pad forming. 175 Rubbery state of plastics, 46.11 1, 112 Rynite. C217 Ryton. C217
Plastics, h.(213, , C214, CW2,
.
Sm-1
llldwvw c ~ @ y ~ e r140 ,
Skymfeam, C218 Stp011~4, €218 Styroo, a 1 8
machim, 80,169, T7Q
Otllluac, C218
a18 Sw-Piex, (218 Surfkc appemnce, tilitem, d d , textsRe. 18.27.35, a, 57.66, 119,120, 129, T180, Tl11.190.197 SwWe mi&&, see ~ M o ap lm p t b Surge taak, 58,62,64,109
S&p, C218 SynWactic plawics, 122,138,139, (317 Syn@, C218 Synauic foam, T16,60,165, (218
T-PoraEad 5.0,46 TgrIkl2.2,47 ' h r slnmgfb of films, initial and papgation,
b
Sagging of heated sheet, 15,27,30,83,112, 117.138 SAN, see styrene-acrylonitrile1-
7
-
Testing at highesr use temper
pdymecs, 140,141,a l l , a 1 3 ,
41,43.48.53,64,163, TI80
FWics Co., (214.223 m w r a p ,a 2 0 W e s , TI81
X-TC, C220 Xemoy,
plyethylem 16, IS, lg.& 4%ll6,117, ULRPE, m~uftsrdow-densiq.
e&Wmt of-
XT Polymer, C220 Xydar,
U l ~C219 , UbW,a19 55,125 T b n m s W d y amtmIId &iikcsl 70 T B c m C219 W&nwvarietions, 3,12,30,39,41,4,51, T ~ ~ x mat&&, o W
Ul-w-deesity plyethylem, 137 hlla4m19
w
Uln=m,C2U) Ulmbne, &U)
Ultravide4 wdfetion prowion, 130,140, 144, (214 fombg,2.85
. lrcsaatap., a i s , a*, 223 T~~.I~SIBS%T~ C2.k S0I,,C211, (2216 ~ ~ ~ 6 5 . a , M 3
Tmy Piaetics (Ammiwl,Inc., a 9 , m
m,a 1 9 Mon, C219
