PET Processing V4 - 2

8/3/2019 PET Processing V4 - 2

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South Asian Petrochem Limited

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Annexure I

The chart typically shows the thermal conditions undergone by the PET pellet before getting converted intothe final bottle.

Drying in Hopper: The pellets are heated to around 165 deg. C in the dryer, during the process of moistureremoval.

Plasticization: During this process the material undergoes the phase transformation from solid phase toliquid phase. The heat of fusion (58 J/g) is supplied to the polymer and plasticized into a homogeneous melt.

Tg

Tm


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Melt solidification: The melt is cooled in the mold to form the product. The melt is quenched at rate fasterthan the rate of crystallization at the operating temperatures, so as to get the amorphous product. Thishappens till 100 deg C, about 20 deg C above the Tg (Glass transition temperature. This temperature isknown as the stretch blow molding temperature.

AOKI Machine: These are three station machines, were the preforms at the stretch blow moldingtemperature, is directly moved to the blow station and blown into bottle, without change in heat content of thepreform.

ASB Machines: These machines operate on four-station method. Here the preforms are ejected from themold at temperature close to Tg. Then the preforms are conditioned at the conditioning station withconditioning Core and Pot, to reach the Stretch Blow Molding temperature.

ASB Machines PF Series: These machine are know as the 1 stage machines as the Injection stationoperates on a separate indexing table, and the preforms formed by every injection cycle will be blown intobottle in two blow cycle. The preforms are Re-Heated by means of IR lamps as the preforms are cooledconsiderably 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 areejected from the Injection mold at temperatures around Tg, and they are further cooled in ROBOT coolingsystem. Some machines, which do not have the ROBO cooling system, can cool the preforms till 50

oC

(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 theStretch Blow Molding temperature by means of IR heaters, which helps them to heat the entire cross-sectionof the preform with high efficiency and speed. The blow bottle is cooled in the blow mold; the blow mold istypically maintained at 18

oC in the sidewalls & 10

oC in the base.

Heat Set Bottles: In Heat Set Blow molding, the mold is maintained at higher temperature (around 120oC).

High temperature combined with the blowing residence time, when the wall of the bottle is in contact with thehot mold ensures higher crystallinity in the final bottle. The highly crystalline bottle wall ensures hightemperature stability for the bottle.

To release the bottle from the mold, a cold jet of air, cools internal surface of the bottle.


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Annexure II

IV of Preform / Bottle

The IV of the preform depends on various factors;

1. IV of the resin.2. Efficiency of the Dryer.3. Residual moisture content of the resin, on entry to the extruder.4. Plasticizing conditions.

a. Back pressureb. Screw speedc. 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 dropin plasticization due to higher shear generated and temperature, and vice versa.

Dryer performance:

Dryer with dew point less than 40oC, will have IV increase, with flow rate higher than 0.062 m

3/min for

every Kg of resin processed. Normally the IV increase is at the order of 0.01 dl/g.

Dryer with dew point more than - 25oC will have IV drop which will be determined by the actual dew point ofthe 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 indryer is maintained as per specified norms. Incase where the temperature or residual time is abnormally highthe IV drop may be considerably higher.

Moisture content of the dried resin:

The ideal moisture content for dried resin is less than 40 PPM; any higher moisture content above this willresult in IV drop due to hydrolytic degradation, during the melting of the resin.


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Plasticization of resin:

Temperature: Temperature of plasticization does a key role in the IV drop, as PET can undergoconsiderable thermal degradation above 290

oC (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 bythe 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 areavailable, which does effective plasticization at lower shear.

Residence time of the melt:

As discussed earlier the residence of melt at higher temperature for longer time will have higher IV drop.Hence is always recommended to run the machine 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 iseffectively utilized.

Under optimum conditions the machine throughput should be utilized in the range of 70 ~ 90 %. Use ofmachine throughput less than 50% will lead to higher IV drop. Again use of machine throughput more than90% 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 thecushion material held in the system for one more cycle, and thus higher IV drop in this portion 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 ofmelt 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 tomelt separation and also the melt at the barrel wall is exposed to higher temperature for prolong durationwithout 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 turnwill create higher IV drop.

Hence, the preform IV is a complex combination of various factors. Hence it is recommended to operate the

parameters at optimum, so that the IV drop is minimum.

As a thumb rule following are some of the IV drops normally encountered in various PET processingsystems, these are not hard and fast values, the result may vary between system to system.

Maximum IV drops encountered, dl/gNo. Machine Type 0.76 dl/g 0.80 dl/g 0.84 dl/g

1 Single stage systems 0.04 0.05 0.05

2 Two stage No Robot 0.03 0.03 0.043 Two Stage Lower end systems 0.04 0.05 0.05

4 Two stage Husky, Krauss Maffei, Nestal 0.01 0.02 0.02


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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 takesplace, and the highest tensile properties are achieved. The onset of strain hardening is in the transitionregion between regions B and C. According to a strict definition, the onset of strain-hardening is determinedas follows: extend lines from regions B and C until it intersect; bisect the angle adjacent to the stress/straincurve; and extend that bisecting line to intersect the curve. The point at which that line and the curveintersect is the onset of strain hardening. (Refer Fig)


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Annexure IV

Injection Molding Screws

Extruder screws work on the principle of screw pump. Forward motion of chips takes place with help flightshaving a helical profile. During rotation, plastic material from the hopper section flows vertically into therecess between flights. This material is then forced horizontally forward by friction on the barrel and screw.

The material adheres to the heated barrel walls, and slips on the cool, smooth-surfaced, rotating screw. Thisway, 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, to 1.2 at the meltstage. As the PET granules melt, air entrapped in the process is purged back through hopper. Entrapped airis 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. Screwflights are further toughened by flame treatment to a Rockwell C hardness of 50 or higher, or are protectedby application of special hard-facing alloys.

Commonly, the entire screw is polished to a finish of 100 to 200 microns. Plating with chrome or nickelprovides a smooth, hard surface that is easy to clean and is resistant to corrosion. Final dimensions of thescrew 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 polymer leakage to a low level, yet not so tight to causeoverheating.

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 the depth of the screw flights in the feed zone to the metering Zone.

For PET with single flight the L/D ratio should be 20, and the compression ration should be 2.5. A lowcompression 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 solidification of preform.

A high compression ratio can cause the very high melt shear and over heating the compression zone andmelt 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 falldirectly in screw channel. In most designs, feed section has a constant root diameter throughout its entirelength.


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Experience supports the need for a minimum of four effective flights in the feed section to avoid non-uniformfeed and/or unwanted temperature rise in the rear zone. Hence the length of feed zone will be (4 D +maximum shot length of screw). Where, D is diameter of screw, which is equal to pitch. Typically flight depthis 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 andheating of the plastic granules. This is achieved by a uniformly tapered, increasing root diameter thatreduces the available volume between flights, compressing the granules.

As granules are compressed, air is purged back through hopper. Material when compressed and movedforward, it is also heated, partly by conduction, but mainly by friction from rotary shear, it melts. Some screwdesigns achieve transition within a single 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 constant along the section length. The range for metering section 112 mmextruders is between 3.00 and 5.00 mm in depth, and five to twelve flights in length. On smaller 63 mmextruders, flight depth varies between 2.00 and 3.00 mm, and length runs from five to ten flights. Shallowscrews have a short metering length to avoid undue compression and consequent high polymertemperatures. Deep screws have a longer metering length to provide added flow stability.

Invariable low shear screws have barrier flights or a mixer added in the metering zone with a shorter lengththan normal. The mixer does the function of homogeneous melt, and the shorter metering zone reduces theeffective 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 relativevolumes per flight of one revolution in the feed and metering. In practice, compression ratio is oftenconsidered 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 asshown below. The first stage ordinarily comprises about 60 percent of the overall length, or about 17 flights ina 32:1 L/D ratio. The second stage is somewhat shorter, about 15 flights.


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Both stages are further subdivided into separate feed, transition (compression) and metering sections of

varying length. For 112 mm diameter screws, flight depths in the feed sections range between 14.0 to 19mm, 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 ensureforward polymer flow and eliminate vent flooding.

A4.3: Pump Ratio

Pump ratio is the ratio of the flight depth in the second-stage metering section compared with the flight depthof the first-stage metering section. A factor greater than 1 is necessary to avoid having polymer flow out thevent. For Pump ratios of 1.6 to 1.7 commonly provide good performance.

A4.4: Screw Cooling

Some improvement in extrusion of plastic polymers is possible by circulating cooling water through the coredsection(s) of the screw typically the feed section. The amount of cooling required is dependent on screwdesign and operating parameters.

Cooling is more critical for larger diameter screws, because the larger volume of polymer flow requires morecooling. Superior extrusion may be achieved by optimized cooling, but reduced rates or surging may resultunless proper processing temperatures are maintained. If screw cooling is used, it should be limited to thefeed (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, basically for material with flakesand 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 middleof the barrel.

3. Low shear during plasticizing, as melting and homogeneous melt is one in two stages. Thus lowerchance of shear degradation.


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Annexure V

PET Preform design

Design of preform, though looks very simple in comparison with other plastics components which havecomplicated 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.o Wall thickness of the container.o Length and Diameter of the container.o Neck finisho Weight of the container.

Processo Single stageo Two stage

Rough preform design (Considering the stretch ratio details)

o Neck finisho Length & Diameter of the Preformo Wall thickness of the preform.

Melt flow consideration of the preform.o Flow length.o Temperature of the moldo Injection pressure.

Mold making and ejection characteristics. (Basic Preform Designs)o External Taper typeo Internal taper typeo Dual taper typeo Internal step relief

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 into consideration the product presentation requirements such as the aestheticof the final container.

The basic fact that needs to be frozen at this stage is the neck finish details, wall thickness and the shape ofthe 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 flexibility 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 detailsto decide on basic preform design. From the container wall thickness and the stretch ratio, we can determinethe dimension of the preform, such as length, diameter and thickness.


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A5.4: Melt flow consideration:

Once we have arrived at the preform wall thickness, we need to apply the two limitation conditions on thedesign to get an optimum preform design. Melt flow forms the basis for the minimum wall thickness of thepreform. Specific Flow length (l/t) of a polymer is determined as the length (l) the polymer can flow throughan annular space of gap (t), at specific conditions of injection pressure and the wall temperature of theannular space.

Flow length Vs Section wall Thickness

50

75

100

125

150

175

200

225

250

275

Section Wall Thickness (mm)

Flow

length(mm)

10 degC & 700bar 50 80 145 200

40 degC & 700bar 100 125 175 240

40 degC & 1000bar 135 158 203 268

1 2 3 4

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 thepreform increase, and vise versa.

And the maximum wall thickness is determined by the limitation of material; basically the 2 % IPA grades inthe 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 wallthickness the cycle time increase drastically and process becomes less productive, as the cooling time isproportional 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 preformweight one of the following preform designs can be selected.


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A5.5.1: External Taper: In this design the core pins are straight, equal to the internal diameter of the neckfinish, and the neck ring has a diverging taper to match the diameter of the cavity. These preforms arenormally used for high wall thickness CSD preforms; the future scope for increase or decrease in weight ofthe 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 ofthe preform

We can reduce the diameter of the core pins to increase the weight of the preforms, but normally thisdesign is used for CSD bottles, which are normally at the higher end of the wall thickness, 4mm.

A5.5.2: Internal taper: In this design the neck ring surface is straight from the neck-holding diameter andmatches the cavity. The core pins have a converging taper, which converges just below the neck finish to thenominal internal diameter of the preform. These preform design are used in high wall thickness preforms withneck diameters less than 28 mm & Large size water preforms. The advantage of this preform design is thatwe 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.


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Annexure VI: PET Bottle Design

We have discussed the concept of strain hardening in Stretch Blow Molding topic, and understood thevarious 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 alwaysbeen restricted below 3 mm, since it is difficult to cool the preform to a blowing temperature uniformly acrossthe length, and further conditioning for blowing.

Single stage (Typical Values)

Lower limit Upper Limit Typical ValuesAxial 2 2.5 2.2Hoop 3 4 3.6

Blow Ratio 6 10 8

In case of three station single stage machines, it is difficult to eject the preform with surface temperature ofaround 100

oC (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)

Lower limit Upper Limit Typical ValuesAxial 2.2 2.8 2.5Hoop 4.4 5.4 4.8

Blow Ratio 9.6 15 12Gate wall thickness, as % ofPreform wall thickness

70 80

A6.2: Base Design

Base of a PET bottle has a critical function of maintaining the bottle upright, it also takes the entire load ofthe package with the product, takes care of the drop load, maintain the pack in good condition. There aredifferent 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 oneof the heaviest bases in PET containers, and it has been slowly phased of CSD, now there are fewcompanies 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 beveragesapplications, 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 inherentstrength required to withstand the higher temperature and force encountered during the filling of thecontainer with hot juice. The rugged internal ribs also prevent the base from warping outward andcreate 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 thefunctional aspects.


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Annexure VII

ASPET 22CJ

ASPET 22CJ is a special grade, with lower rate of crystallization, to suit slow cooling high wall thicknesspreforms, used in 20 lts water containers with typical wall thickness of 8 ~ 10 mm. ASPET 22CJ has higherco-polymer content to ensure higher transparency and strength. Resin IV is 0.88 dl/g +/- 0.02.

Injection Molding Machine - Setting

Parameter Significance Typical Values

Process Temperature1 Dryer settings To remove moisture from resin, to

enable plasticization with minimum IVdrop.

For Hot air dryers:170 deg C for 5hrs165 deg C for > 5 hrs (Max. 7hrs)

2 Barrel & Nozzletemperature

Temperature required forplasticization of the polymer.

Barrel: 275 +/- 10 deg CNozzle: Barrel plus 5 deg C

Plasticizing Parameters

3 Screw Speed &idle time The screw rotation does the function offeeding the pellets, melting, plasticizing.Screw Idle time is waiting time afterplasticization of melt, before injection.

Shall be operated at minimum, so thatthe idle time is kept low.

Set pre-charging delay to keep screwidle time low, ~ 10 sec.

4 Back pressure The backpressure aids in uniformplasticizing of melt.

Typically 10 ~ 15 bar.

5 ExtruderCushion

Volume of melt held in front of screw atthe end of Injection.

10 ~ 15 mm

Injection Parameters6 Injection

Pressure &Velocity

Melt pressure for injecting the meltinto cavity. Low due to large crosssectional area.

300 ~ 400 bar (Melt pressure)

Typical speed: 35% ~ 45%

7 Injection time The time required for filling the cavitywith melt. High due to higher section wall thickness,typically 15g/s ~ 18 g/s8 Hold pressure

& timeMelt Pressure & time required forpacking the melt in cavity, tocompensate shrinkage duringsolidification.

High as melt is packed for longer lengthand cross sectional area.Typical values: 500 ~ 650 barTypical values: 12 ~ 16 sec.

9 Cooling time The time required to cool thepreforms.

Typical values: 30 ~ 40 sec (for 8.5 mmwall thickness)

10 Cycle time Total time required to process on shotof preform

Typical values: 110 ~ 120 sec.


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Annexure VIII

Coloring of PET

Coloring of PET has become a important of aspect in PET packaging, with better understanding ofpackaging needs of products, like improvement in shelf life of product packed, and retaining the freshness ofproduct through out its shelf life.

Colorants have moved from just coloring the plastics to give aesthetic appearance to more product relevantneeds such as protecting the product from radiations which harm the product such as UV, fluorescent lightetc.,

Colorants such as green and blue are used in light dosages for product identification better appearance inwater and soft drinks industry.

With the fast growth of PET in drugs packaging, colorants have started taking additional roles as masking forUV and fluorescent light radiations, which decompose the organic molecules present in the drugs and reducetheir efficiency. Amber color is generally used for this purpose, as they mask the high-energy radiations. Italso 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 lowercost, easier handling and higher-quality results 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 compatiblewith PET, polyolefin, styrene and engineering resin systems.

High letdown ratios are achieved by loading up to 75 percent pigment (by weight) into the liquidvehicle. Pigment agglomerates are then mechanically broken down into individual particles, whichare wet-out in the resin vehicle to prevent agglomeration, increasing the uniformity, opacity andintensity of the original pigment by 50 to 80 percent. This high concentration of color results in highletdown ratios: for opaque colors, normally one part color to 100 parts resin; for transparent colors,as little as one part colorant to 1,000 parts resin.

Functional additives such as anti-static agents, slip aids, chemical blowing agents and UV stabilizersand absorbers may be incorporated into the colorant. Such multi-additive packages enable theplastics processor to monitor all additives by monitoring only the color.

A precision metering pump is used to inject small amounts of concentrated liquid color directly intothe 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 aflexible feed tube, forcing liquid color through the tube, and is capable of metering as little as onegram of material per cycle of viscosities up to 20,000 CPS. Output of the pump can be adjusted inmilligram increments, permitting fine-tuning of color levels with no interruption in processing.Because the colorant is metered into the processing machine mechanically, the system requires littlesupervision or labor skill to obtain consistent color.


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The colorant feed tube can be connected to injection and extrusion machines in either of twolocations: immediately below the hopper feed throat, or into a special groove machined into the baseof the barrel feed section. The liquid-to-liquid interface of colorant-to-resin in the plasticizing barrelpromotes rapid mixing and complete distribution of the concentrated color, resulting in uniformcoloration of the extrudate or molded component.

Complete color changeovers are accomplished in two to 10 minutes by simply changing to a differentcolorant feed tube and purging the barrel of the original resin color. Utilizing a liquid purgingcompound, purge waste and changeover times can be further reduced. No additional operations arerequired to change colors; natural resin in the hopper never requires emptying or drying, and theperistaltic pump head requires no cleaning. Any accidental spills are easily cleaned since liquid colorwashes 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 in

molten polymer, and color it.

Solid colorant cannot be dosed as liquid colorants in the machine throat as their rate of dispersion ina molten polymer is much lower compared to liquid colorants, as they do not have any carrier todisperse.

Solid colorants are mixed in tumbler mixers, with the resin pellets in the required dosing ratio and themix is charged into the machine. The feeding of the resin dry color mix should be by gravity hopperand it cannot be loaded using vacuum hopper loader, since the fine color powder will be separatedby 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 to color per Kg of resin, However, the overall cost ofliquid color is lower, 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 quantityof resin is stored in the hopper, which makes the color change over a cumbersome process. The finedry color powder settles on the hopper inner sides and is difficult to clean.


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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, which is solid at normaltemperatures; wax concentrates can achieve same or higher let down ratio of pigments in the finalconcentrate. Hence the dosage required is low.

These are in small spherical granular form, can be easily dosed through modified master batchdosing system at the extruder throat. Due to the presence of dispersion medium, the dispersion ofcolorant 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 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 theresin 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 master batches are lower compared to liquid and wax colorants, andhence 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 property of final product.

Advantages of Master batches:1. A mechanical system is available to meter the solid concentrates.2. Dosing system & dosing is clean.3. Color consistency is 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 at high letdown ratios.3. Long lead times are required for delivery, and small quantities are either high-priced ordifficult to obtain.

4. Needs to be blended and loaded into hopper dryer for drying. Any color change over willrequire 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 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.


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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, twin-screw extruders are employed for this process, as they are gentler on the resin and good in uniformlydispersing the color. As resin is colored to the required level, it does not require addition of colorantduring the molding operation.

This method is not employed for PET as this will involve IV drop in the resin, and other coloringmethods 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 of each color resin must be purchased and stored.2. Colors other than black and white incur significant premium charges.3. Long lead times are 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)

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PET Processing V4 - 2