WEB DEFECTS
PEER REVIEWED
The m e chanics of w rinkling rinkli ng
w
DAVID R. ROISUM
RINKLING ISA MOST COM-
mon and costly web defect encountered in the manufacturing and converting of lightweight grades of paper, film, foils, nonwovens, and textiles. Wrinkling is so common that it has been given synonyms and appellations no less diverse than the wide variety of materials, machines, and applications it has appeared on. In this paper, however, we use the word wrinkling to denote any deviation from absolute flatness of the web. Its severity can range from an almost imperceptible troughing of the web in an open span to a trough that is so pronounced that it folds over and creases when crossing a roller or upon entering a nip. We also include the baggy web or the layflat problem as subsets of this encom passing definition of wrinkling. Despite the ubiquitous nature of wrinkling, however, there are only a few references that offer a brief treatment (1, 2). In this paper, we try to remove the mystery by explaining in simple but relevant terms how wrinkles are formed and how they can be reduced. We begin this by s howing that all wrinkling results from a single behavior. Later, we describe some common mechanisms of wrinkle causation and how to reduce wrinkles after the fact. COMPRESSIVE BUCKLING OF WEBS
A web wrinkles because it has buckled in compression (3). We all know from experience that you cannot push a rope, because it folds under the smallest of compressions. A web also shares this behavior, because it, too, buckles under just a few psi (lb/in.2) of compression.
Another analogy is a yardstick, which bows outward if too much axial load is applied. Unlike the yardstick, which is only loaded in one direction, however, a web can be loaded in three directions. One direction is the MD (machine direction line tension), and the other two directions are what can cause wrinkling. Because all wrinkles are a compressive buckling of the web, it is important to determine how webs can get loaded in compression. We use Mohr's circle to show what types of forces may load a web in compression. Mohr's circle, as seen in Figs. 1 — 3, 3, is a plot of shear stress (y axis) vs. tensile or compressive stress (x axis). All possible states of stress for a member in planar loading, such as webs, can be represented as a circle on this plot. This is a useful tool for showing both how to combine different kinds of stresses and how the web sees stresses in different directions. We first look at Mohr's circle for the simpler case of an unwrinkled web in pure MD (machine-direction) tension, as shown in Fig. 1. The right side of the circle is the MD face, which is loaded by web line tension. The left side of the circle, or CD (crossdirection) face, has no stresses and thus sits at the origin. Just to the left of the origin and under slight compression is the wrinkling boundary. As seen here, a web that is loaded in pure MD tension has a Mohr's circle that does not cross into the buckling boundary and thus has no tendency to wrinkle. While the figure shows a large distance between the circle and the wrinkling boundary for illustration purposes, in real life, the separation is very small for thin materials. The
Wrinkling is a common proble, thin webs of almost any material. This paper shows how all wrinkling is a compressive buckling of the web induced by either CD compression or in-plane shear stresses. Common causes of wrinkling are then discussed; these include roll(er) misalignment, deflection and diametral variations, and constrained expansion as well as residual stresses of manufacturing. Wrinkle removal via web flattening and web spreading spreading are briefly reviewe d. Application: The primary application of this paper is to diagnose the mode of wrinkling, which then leads to options for wrinkle prevention or removal.
amount of compression a web can withstand is primarily determined by the cube of the web caliper. Web modulus, span width, span length, and other factors are not as strong. This is why wrinkles are common with materials that are less than 10 mils in thickness, regardless of their composition. This is also why wrinkles are rare for materials thicker than 100 mils. If we add a slight amount of CD compression to the MD web tension, we push Mohr's circle past the wrinkling boundary, as seen in Fig. 2. Constrained expansion is one way to cause this to happen. Constrained expansion is where a web wants to get wider going through a process, such as due to a tension drop, moisture increase, or temperature increase. Since the web cannot effectively push its edges outward, it may instead buckle out of plane. Also, a web may gather because a slender roller deflects too much, or at a spreader that is not operating properly. The distinctive feature of CD
VOL. 79: NO. 10 TAPPI JOURNAL 2 I 7
WEB DEFECTS
///«/
I
(oMD=IO,T = 0)
\
/o"X(Co np.) a (Tensile) | / / / s
^X
\
1
T
//A //// Element stress
Web
Element stress
Causes
Web Causes Constrained expansion Roller crown Roller deflection Bad 'spreading'
11
No wrinkles
/. Pure MD tension
compression is that the wrinkles run exactly in the machine direction (4). If we add a slight amount of shear stress to the MD web tension, we again push Mohr's circle into the wrinkling boundary, as seen in Fig. 3The causes for shear stress share a common theme: either a web and/or a roller is crooked. The distinctive feature of shear wrinkles is their angled orientation (5). As we have seen, Mohr's circle is a powerful tool for diagnosing wrinkles. First, you can cut troubleshooting time in half by merely noting whether the wrinkle is oriented exactly in the machine direction or at a slight angle. Second, you can gauge shear stress severity by the angle of the wrinkle. Third, you can theoretically pull out shear wrinkles with enough tension (left as an exercise for the reader). However, wrinkles return when the product is cut or sheeted if its source was the material itself. Finally, Mohr's circle shows
218 TAPPI JOURNAL OCTOBER 1996
—
1
2. AID tension + CD compression
just how close to trouble we run, even under the best of conditions. CONSTRAINED EXPANSION
Many web machines contain an expansive process that tends to increase the width of the web. For example, plastic films may be heated in air-float ovens or over heated rollers. Here, the web tries to grow wider due to hygrothermal expansion. Similarly, many paper webs see a moisture increase in coating and printing operations, which tries to increase the width due to hygroscopic expansion (6). Finally, two other processes that often expand the web are calendering and embossing. However, expansion can also occur in web handling merely due to a drop in tension somewhere in the line. Here, the web tries to expand because of Poisson's ratio. While Poisson's ratio is typically approximately 0.3 for most materials, nonwovens can exceed a ratio of
3. This means that the changes in width are three times the changes in length. The fractional expansion of width for linear elastic processes can be calculated as: V
• Ten
4r--H ----- +a Ar + p AX 1 E.c
2 (1)
where w
= width u = Poisson's ratio Ten = lineal web line tension E, = MD modulus c = caliper a, T 62
= CD coefficient of thermal expansion = temperature = CD coefficient of solvent expansion
2
4. MD trough wrinkle
bt.iL
Web travel Misalignment Tension profile Draw profile Material skew
Deflection Net Tension Weight
3. MD tension + shear
5. Excessive deflection wrinkling
These expansive forces do not always result in the edges of the web moving outward (to a greater width), especially on lighter grades. Rather, the web may take that extra width as the arc length of a buckled sinusoidal shape. There are many reasons that this buckling behavior is more common than edges moving outward, but the "you cannot push a rope" analogy suffices as a visual tool. As seen in Fig. 4, the constrained expansion of a free web results in a curtain-like appearance, where the troughs are oriented precisely in the machine direction. Furthermore, the spacing or wavelength between the troughs is 0.25 calculated as (7): 1+ 1.95 JL -Ten ( %-
m
The amplitude can be estimated from: Aw w
_ / TL4_\ " 2b " V % ) £
where £_,
= buckled lateral strain
ID
A
= amplitude A.
= wavelength.
(2) where X = wavelength L = web span length Ten = lineal web line tension E = web modulus oCj = longitudinal stress = Ten /caliper.
A web begins to show a noticeable troughing at CD buckling strains of as little as 0.1%. By the time the strains reach 1%, the web looks like curtains and is at great risk for foldovers and creases. A 0.1% thermal expansion of polymers may require a temperature rise on the order of 100-200 CE In one case, a converter built an expensive vacuum treater with a heated roller that increased the film temperature to nearly 400°F in a single step. Unfortunately, the web buckled off the hot roller surface due to expansion so that a uniform treatment was not possible. A more common situation is passing a film through an air flotation oven. To address the MD troughing, the VOL79: NO. 10TAPPIJOURNAL2I9
WEB DEFECTS
Web
Web
Region I Planar
Increase L/W
O
Region III Wrinkled
Increase gauge
8. Misalignm ent wrinkle relationsh ips
Shell
Groove
Wide grooving: Web may slide inward into grooves
Shell
Thread
Raised thread: Web may slide inward to lands and gather due to high diameter
LU
R egion 6. Wrinklin g at wide grooves/threads
Partial drive II Rivers & lakes Slack
edge
b¥/l
MISALIGNMENT ANGLE
-c
>
A
-c
-Ci
Zh
T-
7. R o l l er m i s a l i g n m e n t w r i n k l e
Material skew
first roller after the oven should be a spreader to avoid creasing the web over a conventional roller. Unfortunately, this first position after the dryer is also coveted by guide roll and/or vacuum pull roll functions. Paper is generally thicker than many films and thus is slightly more buckling resistant. However, problems should be anticipated on paper when the hygroscopic expansion reaches 1%; this is approximately equivalent to a 5% moisture increase. This amount of water addition is common in most latex coaters and some treaters and printing operations. Again, we would consider a spreader in the first position after the coater station.
Trim removal
Zh -c
Corrugation OTHER MD TROUGH WRINKLE CAUSES
Other causes of MD trough wrinkles include excessive roll or roller deflection. We illustrate this tendency with the schematic given in Fig. 5. As seen here, the roller deflects downward due to roll weight and moves to the right due to a normally smaller web tension. The net shape of the deflected roller is similar to a bowed roller, except that it is pointed backward or in the contracting direction. The web does not know whether we call an
Misalignment
3-
~C D-
-C
fcZh
Zh -c 9. Other shear wrinkle causes
element a mere idler roller, a spreader, or a contractor. It reacts to a given geometry in the same manner. Thus, if the "roller" looks like a bowed spreader pointed backward, the web reacts with the 220 TAPPI JOURNAL OCTOBER
tendency to trough wrinkle. Other orientations, such as reversing the path of the web in the figure, result in neutral or even spreading effects. For a variety of reasons, however, the web may tend to wrinkle more easily than it tends to spread. Thus, the net effect of excessive deflection of a system of rollers is the tendency for MD trough wrinkling of the web. To avoid this type of wrinkling, rollers should not deflect more than about 0.015% of roller width. This
Symptoms: Wrinkle oriented precisely in the machine direction.Troughs are evenly spaced with the exception of some diametral variations. Fundamental behavior: CD compressive buckling strain Modes: Constrained expansion Moisture increase Solvent increase Temperature increase Tension drop Roll(er) geometry Roller deflection excessive Grooving width excessive Raised threads or features High-diameter position (tracking) Low-diameter position (sliding) /. MD trough wrinkle summary
value is about one-tenth of the 0.15% bow value often specified for the bowed-roller spreading of unslit stiff webs. Thus, for example, it would take a typical bowed-roller spreader to clean up the troughing caused by ten rollers whose deflection just met standard but whose deflection was pointed in an inopportune direction. As a rule of thumb, rollers that are adequately sized for deflection have a slenderness ratio (length/diameter) of 10-15 if nipped and 15-20 if unnipped (8). Another cause of wrinkling is excessive grooving width. It is ironic that many people believe that grooving, especially if spiraled outward, has a spreading effect. Unfortunately, the tendency of all grooving is just the opposite. As seen in Fig. 6, the web tends to pull into the overly wide groove. It may do this elasti-cally, in which case there is no damage. However, the web could slip slightly sideways, especially at the edges, which can induce trough wrinkling. To avoid MD troughing caused by grooving, the groove width should be no wider than 10-20 times the minimum web
Roll diameter increase Roll wrap angle decrease Roll/web speed difference (sliding or floating vs. tracking) Roll or web roughness change (depends on application) Speed increase Tension decrease Web permeability decrease Width increase //. Factors to increase web flattening
caliper (9). Another reason to avoid wide grooves on film is to avoid marking of the web. Many film products bear the tractor-tire tread marks of idler rollers they have touched. A final source for MD trough wrinkling is diametral variations. If the variation is large and intentional, such as with raised threads, the web may be pulled inward, just as in the case of wide grooves. However, any largediameter area of a roller also tends to gather material there into a trough for another reason. The principle for this is the mechanism for the centering of belts on a crowned pulley (which also creates CD compression) and is opposite to that of a concave spreader. Because the diametral variation need be as little as 0.1% to start gathering thin-gauge materials, rollers must be cut accurately and caliper controlled closely for wound rolls. If the web is slipping instead of tracking, it gathers at small-diameter areas. As we have seen, there are many mechanisms that can cause the MD trough wrinkle. They can be broadly categorized as due to constrained expansion or roller geometry. Table I summarizes the various modes of the MD trough wrinkle. However, they all share the common features of a trough that is oriented exactly in the machine direction and result from a compressive CD buckling strain.
Simplex Concave roller Bowed spreader roller D-bar or bent pipe Duplex Dual bowed roller Pos-Z Expander rollers Slatted Banded Covered Edge stretchers Edge pull stretcher rollers Tenter chains ///. Spreading devices
DIAGONAL SHEAR WRINKLES
From the Mohr's circle of Fig. 3, the superposition of a shear stress on a web line tension produced a circle that could cross beyond the buckling boundary. However, the direction of maximum compression for a web in shear is at a slight angle from the cross direction. Thus, the troughs are oriented at a right angle to the maximum compression, which is, in other words, at a slight diagonal to the machine direction. The angled wrinkle uniquely identifies the presence of a shear component; the steeper the angle, the larger the shear stress. Aside from the commonality of an angled wrinkle, the numerous different sources share another feature. Shear wrinkles always result from something being crooked, such as a profile variation problem. In many cases, the crooked element is a machine component, such as a roller, instead of the web. If the web is crooked, it often displays a discernible rope or corrugation, or it is baggy in spots. Since there are so many sources of shear, we cover only the more common cases here (5). ROLLER MISALIGNMENT
Undoubtedly, the classic shear wrinkle case is caused by a misaligned VOL. 79: NO. IDTAPPI JOURNAL 22 I
WEB DEFECTS
. n i / b l , N O I S N E T
x- -------- Yield \ A' (yc /
)CJ / s / s
/ / / /
k /
|
Unwinding: Outside loosens
Unwinding: Inside tightens
Top view of a crepe wrinkle
Winding: inside loosens
MD
c
ZD MD
Side view of a crepe wrinkle (enlarged)
'\f\f\f\f\f\f\. / /. Interlayer slippage and the crepe wrinkle
roller pair. It has the fingerprint of diagonal wrinkle that walks sideways on the downstream roller of the misaligned roller pair. As seen in Fig. 7, the specific misalignment that causes this wrinkle is the component of misalignment that is in the plane of the roller pair. One can use several rules to determine the direction of misalignment. First, the trough points to the short side of the downstream roller. Second, the wrinkle walks uphill or toward the long side of the downstream roller. Misalignment in this sense is the lack of parallelism of one roller with respect to the other, rather than with respect to an alignment datum. Thus, both rollers can be misaligned in the machine sense of level and square but not cause wrinkles in that span because they are mutually parallel. Alignment tolerances needed to avoid this mode of wrinkling can either be calculated or determined by experience. A model has been pioneered for misalign222 TAPPI JOURNAL OCTOBER
s
/ /
s
s
' ' /
/"
tl°
•
' ' / / /
\/s
10. CD wrinkles and air entrainm ent
/^TpX Winding: / \ Outside ti htens ( ♦ 6 ) z
* /
l B'
/ •
A"
/ •
B"
STRAIN, in./in. or %
12. Baggy w eb stress-strain diagram
ment wrinkling in a joint research project by 3M and the Web Handling Research Center (10). The results are summarized in Fig. 8. There are three web states on this plot of web tension vs. misalignment angle. Region I is for a flat and unwrinkled web, which occurs when misalignment is sufficiently small. Region II is for a web with a slack edge but no wrinkling, which occurs when tension is sufficiently small. Region III is where the diagonal wrinkle crosses the downstream roller. There are several courses of possible treatment given a web in the wrinkled state denoted by the heavy dot. The most obvious and preferable would be to reduce the misalignment angle; this would move the point to the left and into the flat web region. Another possibility would be to increase tension, which may move the point above the wrinkling boundary. By Mohr's circle, this reduces the intrusion into the buckling zone. Ironically, however, one could also reduce tension to move the point below the wrinkled region. By reducing tension, web-to-roll traction is also decreased; this can allow the web to partly break instead of conforming to the violent bend required to make normal entry to the downstream roller (11). In other words, low tension provides a safety valve for misalignment. However, using tension to reduce wrinkling may not be desirable, because there are other webhandling limits that may be exceeded. If tension is too low, the web becomes floppy and ill behaved. Conversely, if tension is too high, the web may be damaged or even break. An appropriate web tension for most applications is between 10 and 25% of the web's tensile strength (12).
Bow method Tension is not uniform
o UJ
Front
1. Lay a long strip of web on floor and flatten as best possible.
Back
Short (tight)
2. Measure bow (B) & length (L).
Some deckle positions are 'longer' than others
-H *k- w bagginess as:
Bag(%)=I
A
Back
Front
OTHER DIAGONAL WRINKLING
There are other ways that something can be crooked even if the axes of all rollers are aligned. For example, the
OO%x4B 2
W/L
Fold method
A"!
13. Baggy web tension and length
If all of above remedies are exhausted, it is still possible to move the wrinkling boundary itself. The boundary between Regions I and III has been modeled and can be moved to the right by increasing the aspect ratio of the span (length/width) or by increasing web caliper. The boundary between Regions II and III is empirically determined and can be moved up by reducing any traction factor and by increasing speed. Despite the important relationships revealed by this analytical approach, it may not be sufficiently conservative for good web machine practices (13-15). Obviously, the wrinkled Region III is unacceptable for any operation. However, the slack web Region II should also be undesirable, because if one side is slack, the other side must be at least two times the line tension; this is probably an excessive and unhealthy variation. Also, slack webs do not go through nips well. Even the flat Region I may not always be adequate, because a tension profile variation exists with any misalignment, even if the web does not wrinkle in obvious protest. Thus, a flat web is necessary but not sufficient for applications where tight tension control is desirable. In lieu of modeling, we can use the experience of machine builders to establish alignment tolerances. The tolerances used by large paper machinery builders on the dry end is 2-3 mils per 100 in. of face. A few larger film machinery builders also use the 3 mils/100 in. tolerance. Those few converting machine builders who have established standards often use a looser 1 mil per foot of face. In any case, standards of this order require alignment by optical transit rather than by hand tools (16).
3. Calculate
2B->
1. Lay a strip of web on floor and fold lengthwise down
L
middle. 2. Measure bow (B) & length (L). 3. Calculate bagginess as: Bag(%)=IOO%x4BW/L2
±. VJ -H N-w/2 14. Bagginess measured as camber
diameter of the roller may vary along its face. Then, the highdiameter areas have a faster surface speed than the low-diameter areas and thus force an in-plane shear stress. While small diametral variations may be tolerated on some rollers, nipped rollers are very unforgiving for two reasons. First, the nip pressure itself is nonuniform. Second, the web cannot slip in a nip to relieve itself as it could on a wrapped roller. Several common cases of diagonal wrinkling are shown in Fig. 9- The corrugation or rope is usually found after surface winding of a web that has an abrupt caliper variation. Rivers and lakes are formed when the crowns are not appropriate for a nipped roller pair (17). Wrinkles can also be generated at nips where the nip pressure is not uniform on a deformable roll(er) (18). Finally, any portion of a baggy web that is not sufficiently tightened to bring it into tension everywhere will likely wrinkle when entering a nip. This wrinkling happens because the baggy lane gets behind the rest of the web until it is dragged through in a gulp by shear stresses. The partial drive wrinkles form on the boundary of a driven/braked element that does not span the entire web width. Material skew occurs when the principal (stiffest) axis of an anisotropic material is not aligned with the machine direction. The principal axis of paper from a paper machine and film from a tenter is oriented straight VOL79:NO.10TAPPIJOURNAL 223
WEB DEFECTS
\ ^
^
1
AL
D-L
^
C«I*iiAii UA.;.i
~\
Perpendicular Chalk lines
i 6. Traction resi sts web flattening
J L
ti
.Z
c
Another mechanism for creating CD-oriented wrinkles is by interlayer slippage (20). As seen in Fig. 11, the
MD Perpendicular CD
1. Carefully scribe two perpendiculars to slit edge 50-100 ft apart.
LJ
4. W i t h
2. Mark two narrow deckle positions within a tight (t) and loose (I) band.
s c i s s o
3. Strike chalk lines on each side of the tight and loose bands. rs, cut out the two strips. 5. Drag strips from one end to flatten. 6. Align perpendiculars. 7. Calculate bagginess as: Bag(%)=IOOxl/2xAUL
15. Bagginess measurement by strip method
in the middle but outward at the machine edges. Finally, whenever a portion of a web is not tensioned, such as at a trim slitter, a diagonal trough forms. CD WRINKLES AND WINDING
Winding and unwinding processes can generate wrinkles that are primarily oriented in the cross direction either due to air entrainment or interlayer slippage. Some air is always entrained into a winding roll. The amount of air brought in is dependent on the type of nip roller (if any), web permeability, web caliper, web width, speed, and other factors (19). The problem that can occur is that the air may get trapped behind a nip as a bubble, as seen in Fig. 10. If this bubble grows, it eventually goes through the nip in a gulp, causing a wrinkle or crease that is primarily oriented in the cross direction. Finally, air trapped in a wound roll can outgas over the course of several days; this causes the roll to collapse on itself, forming the buckles defect. The solution to these defects is to exclude sufficient air by using a heavily loaded nip roller (>5 pli) that is smooth or narrow-grooved (to avoid tunneling the air into the roll) and which has a soft cover (to conform to wound roll diametral variations).
wound roll layers may slip upon themselves in the presence of an internal or external nip and during winding or unwinding. If the slippage is nonuniform and in the uncinching (loosening) direction, the roll may crepe wrinkle. Nip-induced defects are common with light slip pery grades of paper (lightweight coated or newsprint) and with bulky grades such as tissue. THE BAGGY WEB
Most of the wrinkling cases discussed earlier are due more to a shortcoming in web handling or machine design rather than any prior web defect. That is, while web properties and nonuniformities can affect the propensity to those wrinkles, the basic mechanism operates even on perfect webs. However, there are at least as many web defects that are the result of a prior manufacturing defect, such as web bagginess. Here, the web may be ill behaved even going through a perfect machine. 224 TAPPIJOURNALOCTOBER1996
The baggy web has two distinct visual differences from the MD troughs discussed earlier (21). First, the tight and loose bands are irregularly spaced across the width. Second, the product does not lie flat and straight. However, it is important to determine if the web is naturally crooked or is elastically (temporarily) forced that way through the machine. If the web lies flat on a table but not through the machine, then the web is not being handled properly through the machine, perhaps due to causes discussed earlier. If the web does not lie straight and flat on a table, then you have a case of residual stresses. A web that does not lie flat on a table has uneven stresses built into it during manufacturing and, in some cases, rough handling after manufacturing. It is helpful to understand what residual stresses are and how they cause the web to be crooked. Figure 12 shows a stress-strain curve for a material lying on a table under no external stress. On this sample are two points labeled A and B. Note how one is compressive and the other is tensile. Residual stresses indicate that the web has nonzero stresses inside it, even though no external stresses are imposed on it.
The effect of the residual stresses is made clear by two examples. If A and B were at the same point (x, y) but one was on the top and the other on the bottom, the web has a curl in the machine direction or the cross direction. If the points had the same MD position but were on different edges of the web, the web would be baggy or cambered. Of course, there are many other combinations of MD, CD, and shear stresses that can cause crookedness or lie-flat problems in a web. Thus, one way to define a baggy web is as a variation of the web tension across the width of a parallel roller pair, as seen in Fig. 13. However, the difficulty in using this definition is that measuring the tension variation is extremely difficult. The instrumentation that is capable of tension profiling may have poor measurement quality, be extremely expensive, or be very complex. Alternatively, a baggy web can be defined as one where the "natural length" of the material is different across its width. Surprisingly, a web where lengths vary by only 0.1% results in severe bagginess on almost any material or product. Lighter webs may tolerate only about 0.01% before the web appears visibly baggy or has a noticeable lie-flat problem. The baggy lane is a position that is just slightly longer than the rest. It buckles out of the plane of the web to accommodate the extra length, because it cannot push and stretch the adjacent tight bands. There are several ways to measure the bagginess of a web. Figure 14 shows a bow and a fold method, which are most suitable for narrow webs whose residual tension or natural length varies linearly from front to back. Camber is the term used for this idealism of real web bagginess. These methods would not, for example, be able to detect bagginess that is symmetrical about the centerline. The advantage of these methods is
simplicity of measurement, so they are occasionally used for quality control testing. A more general method of quantifying bagginess is shown in Fig. 15Here, we use a strain- or length-based measure of bagginess in any CD position(s) of interest. Indeed, one could cut the entire width into numerous strips to completely profile bagginess with respect to the average length. Unfortunately, this method is tedious and time consuming, so it is not suitable for routine use. The specific causes for baggy lanes are as diverse as the names given to the defect. However, there are some universal troubleshooting techniques. First, the source of the defect must be where the material has sufficient mobility, such as the point of manufacture, or later in processing, where there are high stresses, moistures, and temperatures. Second, the deckle position and width correspond to its source. The source is in general a profile precision problem. The problem should be identified and corrected at its source, because there is very little that can be effectively done after the fact. The one rare cure of the baggy web is to yield it (22). This is illustrated in Fig. 12 by pulling the web to A'B' and releasing it to A"B". However, it may be more difficult to yield the web uniformly than it would be to make it uniform in the first place. Another advertised treatment is to use spreaders. However, the effect is almost always temporary and local to the spreader area. Also, the effects are usually disappointingly small, because spreaders act in the cross direction while the baggy web problem is in the machine direction. Thus, it is only the weak Poisson coupling between a CD pull of the spreader that increases the effective MD tension
and thus may temporarily pull a baggy portion of a web into tension. FLATTENING AND SPREADING
There would be little need for spreaders in a perfect world where the web and machine are true. The realities are, however, that wrinkles may remain despite our best efforts to correct them at their source. Then we can, as a last resort, attempt to remove the wrinkles via flattening and spreading. Web flattening is a passive process that allows the web to be as wide as it would prefer to be. Web spreading is an aggressive process that forces the web to be wider than it would prefer to be. Flattening works on the principle that the web would avoid compound curvature because it is at a highenergy state. The web, which must first follow the arc of the roller or bar, would prefer not to hold a second arc as a trough or wrinkle crossing the roller or bar. As seen in Fig. 16, the wrinkle crossing the roller induces compressive CD forces that would tend to push the edges outward. These forces are very weak and thus can seldom overcome the friction or traction between the web and the roller. If the friction can be reduced sufficiently, however, the trough pushes itself outward until the web is flat (23) ■ The factors for reducing web/roller traction are given in Table H (24). All that is usually needed to flatten a web is to lightly wrap a largediameter bar (or roller if sliding cannot be tolerated). Caution must be exercised, however, because loss of traction also means a certain loss of web-handling control. Spreading is a more aggressive process than flattening and thus can deal with webs that are in need of more serious treatment. The various types of spreading devices, listed in approximate order of strength, are shown in Table in (25-27). The
VOL. 79: NO. 10 TAPPI JOURNAL 225
WEB DEFECTS
KEYWORDS corrugation, crease recovery, creasing, design, flattening, rolls, tolerance, web spreaders, webs, wrinkles.
most common pitfall to avoid with the common concave and bowed-roller spreaders is loss of traction. Slipping causes a loss of the spreading function and may even cause wrinkling. Slipping is avoided by reducing bows, wrapping the rollers adequately, and making sure the surface has an aggressive traction with the web. However, spreading is only able to effectively treat MD trough wrinkles or diagonal shear wrinkles. Bag-giness and most other web defects are not appreciably affected by spreading. For example, if a web has
bubble-shaped patches of bagginess, the spreader could not pull them out, because the flat areas around the bubble would carry all of the spreading forces. Also, spreading is greatly limited on unbonded multiply materials. Here, spreading may be limited to the ply in contact if web/roll friction is greater than web/web friction. Also, spreading is limited to the point where the first ply becomes taut in the cross direction. Finally, the effect of spreading is almost always temporary and local to the spreader area, lasting perhaps only a span or so downstream.
sive buckling of the web. The first troubleshooting technique is to observe the angle of the wrinkle. If it is directly in the machine direction, the cause is one of a few easily distinguishable cases of constrained expansion or related behavior. If the web is at a slight angle, determine if the crookedness is web or machine related. Finally, baggy webs are irregular in character and most difficult to treat. In most cases, bagginess is the result of manufacturing profile problems and must be corrected at the source. TJ Roisum is president of FinishingTechnologies, Inc., 1305 Orchard Ct, Neenah.WI 54956.
SUMMARY
Wrinkles may appear to be as varied as the webs and applications on which they are found. As we have seen, however, wrinkles and lie-flat problems share a common feature in that they all result from a compres-
Received for review Dec. 11, 1995. Accepted Feb. 9, 1996. Presented at theTAPPI 1996 Polymers, Laminations and Coatings Conference and the TAPPI 1996 Finishing and Converting Conference.
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