South Asian Petrochem Limited Annexure – I
Tm
Tg
The chart typically shows the thermal conditions undergone unde rgone by the PET pellet before getting converted into the final bottle. Drying in Hopper: The pellets are heated to around 165 deg. C in the dryer, during the process of moisture removal. Plasticization: During this process the material undergoes the phase transformation from solid phase to liquid phase. The heat of fusion (58 J/g) is supplied to the polymer and plasticized into a homogeneous hom ogeneous melt.
South Asian Petrochem Limited Melt solidification: The melt is cooled in the mold to form the product. The melt is i s quenched at rate faster than the rate of crystallization at the operating temperatures, so as to get the amorphous product. This happens till 100 deg C, C , about 20 deg C above the Tg (Glass transition temperature. This temperature is known as the stretch blow molding temperature. tem perature. AOKI Machine: These are three station machines, were the preforms at the stretch blow molding temperature, is directly moved to the blow station and blown into bottle, without change in i n heat content of the preform. ASB Machines: These machines operate on four-station method. Here the preforms are ejected from the mold at temperature close to Tg. Then the preforms are conditioned at the conditioning station stat ion with conditioning Core and Pot, to reach the Stretch Blow Molding temperature. ASB Machines – PF Series: These machine are know as the 1 ½ stage machines as the Injection station operates on a separate indexing table, and the preforms formed by every injection cycle will be blown into bottle in two blow cycle. c ycle. The preforms are Re-Heated by means of IR lamps as the preforms are cooled considerably lower than the Tg. ASB Machines – PB Series: These machines are high capacity versions of the PF series machines. They normally have two or more rows of Injection Cores & two Blow stations match the performing rate. Injection Molding Machines: These machines make only the intermediate stage the preforms, they are ejected from the Injection mold at temperatures around Tg, and they are further cooled in ROBOT cooling o system. Some machines, which do not have the ROBO cooling system, can cool the preforms till 50 C (normally) at the injection mold itself. The preforms are packed at room temperature. Stretch Blow molding Machines: These machines Re-Heat the preforms from the room temperature to the Stretch Blow Molding temperature by means of IR heaters, which helps them to heat the entire cross -section of the preform with high efficiency and speed. The blow bottle is cooled in the blow mold; the blow mold is o o typically maintained at 18 C in the sidewalls & 10 C in the base. o
Heat Set Bottles: In Heat Set Blow molding, the mold is maintained at higher hig her temperature (around 120 C). High temperature combined with the blowing residence time, when the wall of the bottle is in contact with the hot mold ensures higher crystallinity in the final bottle. The highly crystalline bottle wall ensures high temperature stability for the bottle. To release the bottle from the mold, m old, a cold jet of air, cools internal surface of the bottle.
South Asian Petrochem Limited Annexure – II
IV of Preform / Bottle The IV of the preform depends on various factors; 1. 2. 3. 4.
IV of the resin. Efficiency of the Dryer. Residual moisture moisture content of the resin, on entry to to the extruder. Plasticizing conditions. a. Back pressure b. Screw speed c. Barrel temperature. d. Screw design 5. Melt residence conditions. a. Machine throughput utilized. b. Machine shot weight utilized. c. Barrel / Shooting pot cushion. d. Cycle time for the preform. e. Screw idle time. Thus the IV of the preform in simple sense will be
IV of Preform = IV Resin – IV Dryer – IV Moisture - IV Plasticization – IV Residence Where, IV Resin = IV of the resin. IV Dryer = IV drop or increase in dryer. IV Moisture = IV drop due to moisture in resin plasticized. IV Plasticization = IV drop due to plasticization conditions. IV Residence = IV drop due to residence time of melt in barrel . IV of the resin: IV of the resin forms the basis for the preform IV. Subtracting the losses at various stages, from resin-IV, derives the preform-IV. IV of the resin has effect in plasticization, i.e., higher IV resin will have higher IV drop in plasticization due to higher shear generated and temperature, and vice versa. Dryer performance: o
3
Dryer with dew point less than – 40 C, will have IV increase, with flow rate higher than 0.062 m /min for every Kg of resin processed. Normally the IV increase is at the order of 0.01 dl/g. o
Dryer with dew point more than - 25 C will have IV drop which will be determined by the actual dew point of the dryer, which again will be at the order of 0.01 dl/g. All the above discussions are based on the fact that the temperature of the dryer and the residual time in dryer is maintained as per specified norms. n orms. Incase where the temperature or residual time is abnormally high the IV drop may be considerably considerabl y higher. Moisture content of the dried resin: The ideal moisture content for dried resin is less than 40 PPM; any an y higher moisture content above this will result in IV drop due to hydrolytic degradation, during the melting of the resin.
South Asian Petrochem Limited Plasticization of resin: Temperature: Temperature of plasticization does a key role in the IV drop, as PET can undergo o considerable thermal degradation above 290 C (Standard bottle grades, there are special grades available with better thermal stability for different molding applications) Backpressure: The backpressure in the screw is directly proportional to the shear generated on the melt by the screw. Hence higher backpressure will lead to higher IV drop. Screw speed: Higher the screw speed higher will be the generated shear on the melt. Screw design: Screw designs are a vital factor in the IV drop, nowadays more gentle barrier screws are available, which does effective plasticization at lower shear. Residence time of the melt: As discussed earlier the residence of melt m elt at higher temperature for longer time will ha ve higher IV drop. Hence is always recommended to run the machine m achine at the lowest possible cycle time, at optimum throughput. Machine throughput: Nowadays PET is not run just in any machines, but they designed a line. The combination of dryer, extruder mold & other equipments are synchronized, so the output of each stage is effectively utilized. Under optimum conditions the machine throughput should be utilized in the range of 70 ~ 90 %. Use of machine throughput less than 50% will lead to higher IV drop. Again use of machine throughput more than 90% will create un-melt, which will require higher temperature to process and finally result in higher IV drop. Shot Capacity: Ideally the shot capacity should be used in the range of 60 ~ 85%. Extruder / Shooting pot cushion: The cushion should be as low as possible, higher cushion will lead to the cushion material held in the system for one more cycle, and thus higher h igher IV drop in this portion p ortion of the melt, and so the preform. Recommended cushion for Extruder is 10 ~ 15 mm. Recommended cushion for shooting pot is 5 ~ 8 mm. Cycle time: Cycle time should be as low as possible, higher cycle time will lead to higher residence time of melt at higher temperature and thus higher IV drop. Screw Idle time: Screw idle time should not be any case more than 2 sec, higher screw idle time leads to melt separation and also the melt at the barrel wall is exposed to higher temperature for prolong duration without motion, this also will create higher IV drop. Also by having higher screw idle time the screw is made to run at speed more than normal, which also in turn will create higher IV drop. Hence, the preform IV is a complex combination com bination of various factors. Hence it is recommended to operate the parameters at optimum, so that the IV drop is minimum. minim um. As a thumb rule following are some of the IV drops normally encountered in various PET processing systems, these are not hard and fast values, the result may vary between system to system.
No. 1 2 3 4
Machine Type Single stage systems Two stage – No Robot Two Stage – Lower end systems Two stage – Husky, Krauss Maffei, Nestal
Maximum IV drops encountered, dl/g 0.76 dl/g 0.80 dl/g 0.84 dl/g 0.04 0.05 0.05 0.03 0.03 0.04 0.04 0.05 0.05 0.01 0.02 0.02
South Asian Petrochem Limited Annexure – III
Definition of Natural Stretch Ratio (NSR) In the case of biaxial orientation (as in a stretch-blown PET bottle), the NSR is defined as A2/A1, where: A2 = Area of stretched surface at the onset of strain hardening. A1 = Area of original (unstretched) surface.
(The curve is typical for PET preforms at 100 degC) The curve turns upward into region C, the strain-hardening region, where strain-induced crystallization takes place, and the highest tensile properties are achieved. The onset of strain hardening is in the transition region between regions B and C. According to a strict definition, the onset of strain-hardening is determined as follows: extend lines from regions B and C until it intersect; bisect the angle adjacent to the stress/strain curve; and extend that bisecting line to intersect the curve. The point at which that line and the curve intersect is the onset of strain hardening. (Refer Fig)
South Asian Petrochem Limited Annexure – IV
Injection Molding Screws Extruder screws work on the principle of screw pump. Forward motion of chips takes place with help flights having a helical profile. During rotation, plastic material from the hopper section flows vertically into the recess between flights. This material is then fo rced horizontally forward by friction on the barrel and screw. The material adheres to the heated barrel walls, and slips on the cool, coo l, smooth-surfaced, rotating screw. This way, a continuous flow is assured and the necessary shear for frictional heating and subsequent melting. There is a change in polymer bulk density from approximately 0.8 at the granule stage, s tage, to 1.2 at the melt stage. As the PET granules melt, air entrapped in the process is purged back through hopper. Entrapped air is either removed at the vent on a vented extruder or entrapped in the melt. Screws are made of rugged alloy steel such as SAE 4140, with Rockwell C hardness of 35 to 40. Screw flights are further toughened by flame treatment to a Rockwell C hardness of 50 or higher, or are protected by application of special hard-facing alloys. Commonly, the entire screw is polished to a finish of 100 to 200 microns. Plating with chrome or nickel provides a smooth, hard surface that is easy to clean and is resistant to corrosion. Final dimensions of the screw are controlled so clearance between the barrel and outside flight diameter is 0.05 to 0.10 mm per side. To minimizes screw removal difficulties and keeps p olymer leakage to a low level, l evel, yet not so tight to cause overheating. The two critical parameters of the screw are 1. L/D ratio, i.e. ratio between the length and the diameter of the screw. 2. Compression ratio, the ratio of of the depth of the screw flights in the feed zone to the metering Zone. Zone. For PET with single flight the L/D ratio should be 20, and the compression ration should be 2.5. A low compression ratio can cause bubbles and also cause hazy preforms, due to crystallites not broken properly during the plasticization phase, which forms the nucleus for the crystal growth during solidific ation of preform. A high compression ratio can cause the very high melt shear and over o ver heating the compression zone and melt degradation. The pitch of the screw is always equal to major diameter of the screw. A4.1: Single-Stage Screw: The single-stage screw has three separate sections, as shown below: Feed Transition (compression) Metering A4.1.1: The Feed Section Feed section is located in rear barrel zone. Channel depth is maximum in this zone. The granules to fall directly in screw channel. In most designs, feed section has a constant root diameter throughout its entire length.
South Asian Petrochem Limited
Experience supports the need for a minimum of four effective flights in the feed section to avoid non-uniform feed and/or unwanted temperature rise in the rear zone. Hence He nce the length of feed zone will wil l be (4 D + maximum shot length of screw). Where, D is diameter of screw, which is equal to pitch. T ypically flight depth is 12.50 to 18.75 mm for 112 mm extruder. A4.1.2: Transition (Compression) Section The transition (compression) section of full-flighted screws is designed to promote both the compression and heating of the plastic granules. This is achieved by a uniformly uniforml y tapered, increasing root diameter that reduces the available volume between flights, compressing the granules. As granules are compressed, air is purged back through hopper. Material when compressed and moved forward, it is also heated, partly by b y conduction, but mainly by friction from rotary shear, it melts. Some screw designs achieve transition within a single sin gle flight. A transition section that is too short can promote overheating. Normally, the transition section is from one-fourth to one-third the entire screw length for PET. A4.1.3: Metering Section The metering section provides polymer melt stability and helps ensure a uniform delivery rate. Flight depth is at a minimum and is normally n ormally constant along the section length. The range for metering section 112 mm extruders is between 3.00 and 5.00 mm in depth, and five to twelve flights in length. On smaller 63 mm extruders, flight depth varies between 2.00 and 3.00 mm, and length runs from five to ten flights. Shallow screws have a short metering length to avoid undue compression and consequent high polymer temperatures. Deep screws have a longer metering length to provide added flow stability. Invariable low shear screws have barrier flights flig hts or a mixer added in the metering zone with a shorter length than normal. The mixer does the function of homogeneous melt, and the shorter metering zone reduces the effective shear on the melt. A4.1.4: Single-Stage Screw, Compression Ratio Screws are frequently referred to by their compression ratio. This factor is calculated from the relative volumes per flight of one revolution in the feed and metering. In practice, compression ratio is often considered to be the ratio between the relative flight depth in the feed and metering sections. A4.2: Two-Stage Screw The two-stage screws used in vented extruders can be considered as two separate screws in tandem as shown below. The first stage ordinarily ordinaril y comprises about 60 percent of the overall length, or about 17 flights in a 32:1 L/D ratio. The second stage is somewhat shorter, about 15 flights.
South Asian Petrochem Limited
Both stages are further subdivided into separate feed, transition (compression) a nd metering sections of varying length. For 112 mm diameter screws, flight depths in the feed sections range between 14.0 to 19 mm, with a greater depth in the second stage to provide a decompression zone at the vent. Metering section depths range between 3.80 to 6.40mm, with a greater depth in the second stage to ensure forward polymer flow and eliminate vent flooding. A4.3: Pump Ratio Pump ratio is the ratio of the flight depth in the second-stage second -stage metering section compared with the flight depth of the first-stage metering section. A factor greater than 1 is necessary to avoid having polymer flow out the vent. For Pump ratios of 1.6 to 1.7 commonly comm only provide good performance. A4.4: Screw Cooling Some improvement in extrusion of plastic polymers is possible by circulating cooling water through the cored section(s) of the screw – – typically the feed section. The amount of cooling required is dependent on screw design and operating parameters. Cooling is more critical for larger l arger diameter screws, because the larger volume of polymer flow requires more cooling. Superior extrusion may be achieved by b y optimized cooling, but reduced rates or surging may m ay result unless proper processing temperatures are maintained. If screw cooling is used, it should be limited to the feed (hopper) section. A4.5: Key advantage of two stage / Vented barrel screws: 1. Ability to use material with lower bulk density. Less than 0.8 g/cc, g/cc, basically for material with flakes and regrinds dosage greater than 10%, and still offer consistent melt quality and charging. 2. Ability to process with higher moisture, as the excess can be released through the vent at the middle of the barrel. 3. Low shear during plasticizing, as melting and homogeneous melt is one one in two stages. Thus lower chance of shear degradation.
South Asian Petrochem Limited Annexure – V
PET Preform design Design of preform, though looks very simple in comparison with other plastics components which have complicated shapes, but the basics right so that we have a perfect container at the end of Preform making & blowing process. While designing a preform we need to consider the following aspects;
Final container design. Wall thickness of the container. o Length and Diameter of the container. o Neck finish o Weight of the container. o Process Single stage o Two stage o Rough preform design (Considering the stretch ratio details) Neck finish o Length & Diameter of the Preform o Wall thickness of the preform. o Melt flow consideration of the preform. Flow length. o Temperature of the mold o Injection pressure. o Mold making and ejection characteristics. (Basic Preform Designs) External Taper type o Internal taper type o Dual taper type o Internal step relief o
A5.1: Final container Design: The final container design is based on the product packed, its shelf life requirements, and its characteristics. The final package also takes tak es into consideration the product presentation requirements such as the aesthetic of the final container. The basic fact that needs to be frozen fro zen at this stage is the neck finish details, wall thickness and the shape of the container, based on the requirements discussed above. From the dimensional details we can also calculate the weight of the container. A5.2: Process: The PET container can be made with two processes, single stage and the two-stage process. Each of the process have a specific characteristics, i.e. the stretch ratio of the Single stage is lower than the two stage, similarly, single stage has more flexibilit y in terms of oblong shaped bottles compared to two stage process. A5.3: Basic Preform design: Once the container design is frozen and the process of manufacture determined, we have adequate details to decide on basic preform design. From the container wall thickness and the stretch ratio, we can determine the dimension of the preform, such as length, diameter and thickness.
South Asian Petrochem Limited A5.4: Melt flow consideration: Once we have arrived at the preform wall thickness, we need to apply appl y the two limitation conditions on the design to get an optimum preform design. Melt flow forms the basis for the minimum wall thickness thick ness of the preform. Specific Flow length (l/t) of a polymer pol ymer is determined as the length (l) the polymer can flow through an annular space of gap (t), at specific conditions of injection pressure and the wall temperature of the annular space.
Flow length Vs Section wall Thickness 275 250 ) m m ( h t g n e l w o l F
225 200 175 150 125 100 75 50
1
2
3
4
10 degC & 700bar
50
80
145
200
40 degC & 700bar
100
125
175
240
40 degC & 1000bar
135
158
203
268
Section Wall Thickness (mm) Typical values of flow length at different conditions of melt ( 0.82 IV)
From the graph we understand that the wall thickness of the preform needs to be more as the length of the preform increase, and vise versa. And the maximum wall thickness is determined by the limitation of material; basically the 2 % IPA grades in the market can retain good clarity, till a wall thickness of 4 mm, beyond which the cooling the preform is slow, which results in higher crystallinity in the preform, and thus a hazy appearance. Also with higher wall thickness the cycle time increase drastically and process becomes less productive, as the cooling time is proportional to the square of the wall thickness. A5.5: Mold and Ejection requirements: Based on the mold design requirements and the scope for future increase and decrease of the preform weight one of the following preform designs can be selected.
South Asian Petrochem Limited A5.5.1: External Taper: In this design the core pins p ins are straight, equal to the internal diameter of the neck finish, and the neck ring has a diverging taper to match the diameter of t he cavity. These preforms are normally used for high wall thickness CSD preforms; the future scope for increase or decrease in weight of the preform is limited, with the change of core pins.
As the core pins are straight, we cannot increase the diameter of core pins to decrease the weight of the preform We can reduce the diameter of the core pins to increase the weight of the preforms, but normally this design is used for CSD bottles, which are normally at the higher end en d of the wall thickness, 4mm.
A5.5.2: Internal taper: In this design the neck ring surface is straight from the neck-holding diameter and matches the cavity. The core pins have a converging taper, which converges just below the neck finish to the nominal internal diameter of the preform. These preform design are used in high wall thickness preforms with neck diameters less than 28 mm & Large size water preforms. The advantage of this preform design is that we can have weight reduction of preforms with change of core pins
A5.5.3: Dual taper: There two type of dual taper preforms; 1) Converging core and diverging Neck finish. 2) Converging core and converging neck finish.
South Asian Petrochem Limited A5.5.4: Converging core & Diverging neck finish: finish : This design is used for high wall thickness CSD preforms, it normally have very good scope for weight reduction with the change of core pins.
A5.5.5: Converging core & Converging Neck finish: This design is normally used for low wall thickness water preforms, basically of 28mm & 30/25 mm neck finish, where the neck finish diameters are considerably higher. These preform designs provide possibilities for weight reduction & increase.
A5.5.6: Internal step relief design: Internal step relief are provided in high wall thickness and very long preforms, with length more than 150 mm, the relief in the internal core diameter helps in easy e asy ejection of preforms with less force. As longer preforms will require higher ejection force due to higher sliding force required during the ejection of the preform.
South Asian Petrochem Limited Annexure – VI: PET Bottle Design We have discussed the concept of strain hardening in “Stretch Blow Molding” topic, and understood the various factors, which affect the Stress – Strain Curve for PET.
A6.1: Stretch ratio Single stage stretch ratios have always been lower because the wall thickness of the preforms has always been restricted below 3 mm, since it is difficult to cool the preform to a blowing temperature uniformly across the length, and further conditioning for blowing. Single stage (Typical Values) Axial Hoop Blow Ratio
Lower limit 2 3 6
Upper Limit 2.5 4 10
Typical Values 2.2 3.6 8
In case of three station single stage machines, m achines, it is difficult to eject the preform with surface temperature of o around 100 C (blowing temperature), and hence the ejection temperature is always lower. Thus, if extreme Blow ratios of 12 ~ 14 is used in these machines, it will lead to pearlescence or bursting. Two stage (Typical Values) Axial Hoop Blow Ratio Gate wall thickness, as % of Preform wall thickness
Lower limit 2.2 4.4 9.6 70
Upper Limit 2.8 5.4 15 80
Typical Values 2.5 4.8 12
A6.2: Base Design Base of a PET bottle has a critical function of maintaining the bottle upright, it also takes the entire load of the package with the product, takes care of the drop load, maintain the pack in good condition. There are different types of bottle bases depending on the end use applications; following are some of the bases used, with their field of application;
Flat Base: For mineral water, oil and other non-pressure applications.
Champagne Base: originally used for Carbonated Beverages (High Pressure) application, it is one of the heaviest bases in PET containers, and it has been slowly phased of CSD, now there are few companies using for their beer application. The base is expensive and difficult to control the process.
Petaloid Base: This base has widely accepted in the high-pressure carbonated beverages applications, due to its stability and lower weight. There other variants with six legs and eight legs, but these do not have the wide acceptance.
Hot fill base: The hot fill base is also one of the heavy bases in PET container, due to inherent strength required to withstand the higher temperature and force encountered during the fillin g of the container with hot juice. The rugged internal ribs also prevent the base from warping outward and create rocking during the ejection of bottle at higher temperatures from hot fill blowing machine.
There are many base designs used in PET packages but they widely fall under these categories, and the functional aspects.
South Asian Petrochem Limited Annexure – VII
ASPET 22CJ ASPET 22CJ is a special grade, with lower rate of crystallization, to suit slow cooling high wall thickness preforms, used in 20 lts water containers with typical wall thickness of 8 ~ 10 mm. ASPET 22CJ has higher co-polymer content to ensure higher transparency and strength. Resin IV is 0.88 dl/g +/- 0.02. Injection Molding Machine - Setting Parameter 1
Dryer settings
2
Barrel & Nozzle temperature
3
Screw Speed & idle time
4
Back pressure
5
Extruder Cushion
6
Injection Pressure & Velocity Injection time
7 8
Hold pressure & time
9
Cooling time
10
Cycle time
Significance Typical Values Process Temperature For Hot air dryers: To remove moisture from resin, to enable plasticization with minimum IV 170 deg C for 5hrs drop. 165 deg C for > 5 hrs (Max. 7hrs) Temperature required for Barrel: 275 +/- 10 deg C plasticization of the polymer. Nozzle: Barrel plus 5 deg C Plasticizing Parameters The screw rotation does the function of Shall be operated at minimum, so that feeding the pellets, melting, plasticizing. the idle time is kept low. Screw Idle time is waiting time after plasticization of melt, before injection. Set pre-charging delay to keep screw idle time low, ~ 10 sec. The backpressure aids in uniform Typically 10 ~ 15 bar. plasticizing of melt. Volume of melt held in front of screw at 10 ~ 15 mm the end of Injection. Injection Parameters Melt pressure for injecting the melt 300 ~ 400 bar (Melt pressure) into cavity. Low due to large cross sectional area. Typical speed: 35% ~ 45% The time required for filling the cavity High due to higher section wall thickness, with melt. typically 15g/s ~ 18 g/s Melt Pressure & time required for High as melt is packed for longer length packing the melt in cavity, to and cross sectional area. compensate shrinkage during Typical values: 500 ~ 650 bar solidification. Typical values: 12 ~ 16 sec. The time required to cool the Typical values: 30 ~ 40 sec (for 8.5 mm preforms. wall thickness) Total time required to process on shot Typical values: 110 ~ 120 sec. of preform
South Asian Petrochem Limited Annexure – VIII
Coloring of PET Coloring of PET has become b ecome a important of aspect in PET P ET packaging, with better understanding of packaging needs of products, like improvement in shelf shel f life of product packed, and retaining the freshness of product through out its shelf life. Colorants have moved from just coloring the pl astics to give aesthetic appearance to more product relevant needs such as protecting the product from radiations which harm the product such as UV, fluorescent light etc., Colorants such as green and blue are used in light dosages for product identification better appearance in water and soft drinks industry. With the fast growth of PET in drugs packaging, colorants have started taking additional roles as masking for UV and fluorescent light radiations, which decompose the organic molecules present in the drugs and reduce their efficiency. Amber color is generally used for this purpose, as they mask the high-energy high -energy radiations. It also protects the product colors from becoming pale, as in case of fruit juices and colored drugs. Coloring of plastics can be done by five methods; 1. Liquid color dosing (liquid) 2. Dry color dosing (Powder) 3. Wax concentrate dosing (Granular) 4. Master batches dosing (Pellet) 5. Pre-colored resin Pellets (Pellets) Liquid color dosing (liquid): Liquid color for thermoplastics has become the coloring system of choice because it offers lower cost, easier handling and higher-quality results r esults than any other coloring system available today. What is liquid color? Liquid color is a dispersion of pigments or dyes in specialized liquid vehicles, which are compatible with PET, polyolefin, styrene and engineering resin r esin systems. High letdown ratios are achieved by loading up to 75 percent pigment (by weight) into the liquid vehicle. Pigment agglomerates are are then mechanically broken broken down into individual particles, which are wet-out in the resin vehicle to prevent agglomeration, increasing the uniformity, opacity and intensity of the original pigment by 50 to 80 percent. This high concentration of color color results in high letdown ratios: for opaque colors, normally one part color to 100 parts resin; for transparent colors, as little as one part colorant col orant to 1,000 parts resin. Functional additives such as anti-static agents, slip aids, chemical blowing agents and UV stabilizers and absorbers may be incorporated incorporated into the colorant. Such multi-additive packages enable the plastics processor to monitor all additives by b y monitoring only the color. A precision metering pump is used to inject small amounts of concentrated liquid color directly directl y into the plasticizing screw of injection molding machine during screw recovery cycle. The pump is equipped with a three- or six-roller peristaltic head which compresses the outside of a flexible feed tube, forcing liquid color through the tube, and is capable of metering as little as one gram of material per cycle of viscosities up to 20,000 CPS. Output of the pump can be adjusted in milligram increments, permitting fine-tuning of color levels with no interruption in processing. Because the colorant is metered into the processing p rocessing machine mechanically, the system requires little supervision or labor skill to obtain consistent color.
South Asian Petrochem Limited The colorant feed tube can be connected to injection and extrusion machines in either of two locations: immediately below the hopper feed throat, thr oat, or into a special groove machined m achined into the base of the barrel feed section. section. The liquid-to-liquid interface of colorant-to-resin colorant-to-resin in the plasticizing barrel promotes rapid mixing and complete distribution of the concentrated color, resulting in uniform coloration of the extrudate or molded component. Complete color changeovers are accomplished in two to 10 minutes by simply changing to a different colorant feed tube and purging purging the barrel of the original original resin color. Utilizing a liquid purging compound, purge waste and changeover changeover times can be further reduced. No additional operations are required to change colors; natural resin in the hopper never requires emptying or drying, and the peristaltic pump head requires no cleaning. Any accidental spills are easily cleaned since liquid color washes up with water. Dry color dosing (Solid): Dry colorants are in fine powder form. What is a Dry color? Dry colors are essentially finely ground pigments from various sources, which can easily disperse disp erse in molten polymer, and color it. Solid colorant cannot be dosed as liquid colorants in the machine throat as their rate of dispersion in a molten polymer is much lower lo wer compared to liquid colorants, as they do not have any carrier to disperse.
Solid colorants are mixed in tumbler mixers, with the resin pellets in the required dosing ratio and the mix is charged into the machine. The feeding of the resin – dry color mix should be by b y gravity hopper and it cannot be loaded using vacuum hopper loader, since the fine color powder will be separated by the vacuum loader in filters. Hence requires considerable manual handling of the resin. Advantages of dry color: 1. Available in large or small quantities. 2. Economical to buy. 3. Less storage space. Disadvantages of dry color: 1. Dust is a health, housekeeping and cross-contamination problem. 2. Poor reproducibility, especially for critical colors. 3. No adequate metering system is available. 4. Resin to be mixed in batches to distribute the pigment. 5. Color changeover and/or clean up is very difficult. By comparison, dry colorant cost is the least l east to color per Kg of resin, However, the overall cost of liquid color is lower, lo wer, as economies are achieved in the form of lower housekeeping costs, less downtime, reduced waste, reduced labor and reduced rejects. Dry color is not used in PET as it requires drying of pellets for 5 ~ 8 hrs, hence considerable quantity of resin is stored in the hopper, which makes the color change over a cumbersome process. The fine dry color powder settles on the hopper inner sides and is difficult to clean.
South Asian Petrochem Limited Wax concentrates dosing (Granules): Wax colorants in small granular form easy to dose in the extruder feed throat, by dosing equipments. What is a Wax concentrate? Wax concentrates are pigments dispersed in wax-based medium, m edium, which is solid at normal temperatures; wax concentrates can achieve same or higher let down ratio of pigments in the final concentrate. Hence the dosage required is low. These are in small spherical granular form, can be easily dosed through modified master batch dosing system at the extruder throat. Due to the presence of dispersion medium, the dispersion of colorant in the final component is good. Advantages of wax concentrate: 1. High loading of pigment is possible. 2. Carrier is universal. 3. May use modified concentrate feeders. 4. Can be handled similar to concentrate. Disadvantages of wax concentrate: 1. They may cause slippage of screw. 2. They may melt in in feeding feeding unit. (Pre-dried resin) 3. Consistent color is difficult to maintain in light color shades, due to variation in particle size. Master batches dosing (Pellets): Master batches are colorant granules in pellet form, normally having the same polymer base of the resin colored. What is a Master batch? Master batches are pigments dispersed in same polymer or polymers, which are freely miscible with the resin being colored. The let down ratios employed in i n master batches are lower compared to liquid and wax colorants, and hence the dosing percentages are higher. PET master batches are made with low IV resin as carrier, for freely mixing with the base polymer, without any changes in the physical ph ysical property of final product. Advantages of Master batches: 1. A mechanical system is available to meter the the solid concentrates. 2. Dosing system & dosing is clean. 3. Color consistency is generally generally superior to dry color. Disadvantages of Master batches: 1. More expensive than dry color or liquid color. 2. Difficult to achieve uniform color distribution distribution at high letdown ratios. 3. Long lead times are required required for delivery, delivery, and small quantities are are either high-priced or difficult to obtain. 4. Needs to be blended and loaded into hopper dryer for drying. Any color change over will require draining the entire material from hopper. 5. Concentrates normally require four times the storage space as dry or liquid colors. 6. High moisture content in master batch can create defects defects in product. 7. Variations in dimension of pellet affect accuracy of colorant metering. Compared to master batches, liquid color costs from 10 to 40 percent less.
South Asian Petrochem Limited Pre-colored resin (Pellets): The raw material itself is colored by plasticizing and pelletizing. What is a Pre-colored resin? Pre-colored resins are resins, which is plasticized with dry colors, and then palletized. Normally, twinscrew extruders are employed for this process, as they are gentler on the resin and good in uniformly dispersing the color. As resin is colored to the required level, it does not require addition of colorant during the molding operation. This method is not employed for PET as this will involve IV drop in the resin, and other coloring methods are easy to follow for PET economically. Advantages of pre-colored resin: 1. They are simple to use. 2. No additional colorant dosing is required. 3. They provide consistency of color. Disadvantages of pre-colored resin: 1. Large quantities quantities of each color resin must be purchased purchased and stored. 2. Colors other other than black and white incur significant premium charges. 3. Long lead times are required required for reorders and new colors. 4. Changing colors may require long periods of downtime to empty hoppers and dry resin. Factors to be considered during selection of coloring methods:
Availability (lead time) Availability (small quantities) Cost (per Kg of resin colored) Storage space required Color consistency Metering system Cleanliness Clean-up time Wasted colorant/resin during clean-up Simplicity for operator Letdown ratios (higher = better) Color control Color changeover times (faster = better)
