Plastic Product Material and Process Selection Handbook Part 8 pptx

Howcvcr, TPs cxtrusion depends almost entirely on the rotating screw as a melt delivery device.143,476 TPs arc characterized by low thermal conductivity, high specific heat, and high mel

Cellulose acetate, extrusion 1.28 80.2 21.6 0.781 380 0.40 2.50

Cellulose acetate, injection 1.26 79.0 2t.9 0.794 0.36 2.40 0.20

Cellulose proprionate, extrusion 1 77 76.1 22.7 0.821 380 0.40 1.70

Cellulose proprionate, injection 1.22 75.5 22.9 0.828 0.40 2.00 0.25

'~Specific information on all machine settings and plastic properties is initially acquired by using the resin supplier's data

sheet on the oarticular compound or resin to be used

2 3 0 Plastic Product Material and Process Selection Handbook

discs or rotors arc used to generate shear Howcvcr, TPs cxtrusion depends almost entirely on the rotating screw as a melt delivery device.143,476

TPs arc characterized by low thermal conductivity, high specific heat, and high melt viscosity Preparation of a uniform homogeneous melt and its delivery at adequate pressure and a constant rate could pose considerable problems if not properly processed (Chapter 3) The principal extruder variants arc the single-screw and the twin-screw types O f these, the single-scrcw cxtruder is by far the most versatile and popular in use

The single-screw extruder consists essentially of a screw that rotates in

an axially fixed position within the close-fitting bore of a barrel Extruder sizes are identified by the inside diameter of their barrel Size range from 1/4 to 24 in diameter with the usual from 1 to 6 in (Europe and Asia sizes rangc from 20 to 600 mm with the usual from 25 to 159 mm.) The screw is electrically motor driven through different devices such as a gear reduction train or belt to meet different performance and cost requirements These gear reducers arc rated in mechanical horse- power and thermal horsepower as defined by the American Gear Manufacturers (AGMA) The AGMA rating system is based on the understanding that not all gear reducers are used the same way There are also gearlcss drive systems such as those using Siemens high-torque motor with an unusual low-inertia hollow shaft 476

The output rate of the extruder is a function of screw speed, screw geometry, and melt viscosity The pressure dcvclopcd in the extruder system is largely a function of die resistance and dependent on die geometry and melt viscosity Extrusion pressures are lower than those encountered in injection molding They are typically 500 to 5000 psi (3.5 to 35 MPa) In extreme cases, extrusion pressures may rise as high

as 10,000 psi (69 MPa) Variants on the single screw include the barrier

or melt extraction screw and the vented screw (Chapter 3)

The twin-screw extruder may have parallel or conical screws, and these screws may rotate in the same direction (co-rotating) or in opposite directions (contra-rotating) Extruders with more than two screws are known as the multiple-screw extruder These extruders are normally used when mixing and homogenization of the melt is very important,

in particular where additives, fillers, a n d / o r reinforcements arc to be included in the plastic

They are extensively used for plastic compounding that includes heat- sensitive materials such as PVC, proccssing of difficult-to-feed materials (such as certain powders), reactive processing, 197 and for plastic devola-

5 Extrusion 231

tilization Twin-screw extruders particularly offer a wide processing variability They can be starve-fed so that residence time, amount of shear, and control of melt temperature can be controlled by means of their segmental modular designs

Component

There are different components that make up the extruder each with their specific important function M1 components have to operate efficiently otherwise the extruder's operation is inefficient A very important and essential parameter in the extruder is the plasticator's pumping process It is the interaction between the rotating flights of the screw and the stationary barrel wall For the plastic material to be conveyed, its friction must be low at the screw surface but high at the barrel wall If this basic criterion is not met, the plastic will usually rotate with the screw and not move in the axial output direction

In the plasticators output zone, both screw and barrel surfaces are usually covered with the melt, and external forces between the melt and the screw channel walls have no influence except when processing extremely high viscosity plastics such as rigid PVC and U H M W P E The flow of the melt in the output section is affected by the coefficient of internal friction (viscosity) particularly when the die offers a high resistance to the flow of the melt (Chapter 3) Figure 5.2 shows the extruder's components where the following identifications are listed:

1 Drive motor from 20 to 2000 hp infinitely variable speed drives directly coupled to reducer for maximum efficiency deigned to save floor space

Gears and gearless to provide high efficiency capability to process plastics 476

Efficient performance heat treated helical or herringbones (gears equipped with shaft-driven oil pumps and oil cooler)

Thrust bearing with long life expectancy (of well in excess of 30 years' continuous operation)

5 Large rectangular standard feed opening (round with lining, optional, for use with crammer feeders)

6 Long lasting barrel heater/cooler elements that heat quicldy

Cooling tubes run parallel with heating elements The cast-in stainless steel tubes closed-loop system provide non-ferrous distilled

232 Plastic Product Material and Process Selection Handbook

water that is automatically adjusted via microprocessor-based temperature controllers providing uniform, efficient cooling

8 High-performance screw with bimetallic lined cylinder designed for processing a specific plastic; can be cored for cooling

9 Prepiped and prcwired power installation

10 Safety heat conservation and heat protection guards that are one- piece, hinged, no loose parts insulated

11 Heavy single unit steel base machine foundation prcassembled so all parts are in place ready to be used

12 When required, patented two-stage vented plasticator is used (that can be plugged in minutes)

13 Screen changer for continuous operation without shut down using standard hinged swing-bolt gate

14 Gear pump to ensure absolute volumetric output stability

15 Static mixer to provide thermal and viscosity homogeneity

16 Die designed to produce single or multi-layer sheet without modifi- cation; strand dies, etc

Figure 5~ Schematic identifies the different components in an extruder (courtesy of Welex Inc.)

Purpose of the screens is primarily twofold: (1) to change the melt's spiraling motion, caused by the screw rotation; and (2) to filter contaminants out of the melt Most plastics contain contaminants and these particles can be conveniently removed by means of a screen placed after the extruder barrel and before the melt flow reaches the extrusion die The simplest means for filtering plastic melts are woven wire mesh disks of about the same diameter as that of the extruder barrels Several

5 Extrusion 233

layers of different screens are usually made up into one screen pack The innermost layer is the finest mesh screen that determines the particle size that will be caught by the screen pack

Against the forces exerted by the melt flow, the screen packs are backed

by a thick, densely perforated steel disk called a breaker plate The outer rims of the breaker plate and of the screen pack fit into a round recess

in the end of the extruder barrel and are clamped in place by the adapter flange of the adjoining piece of equipment, usually that of the extrusion die To change a clogged screen pack, the die adapter flange has to be removed, the old pack taken out and replaced with a new one, and the equipment reassembled

Screen changers arc mechanical devices that permit changing screens in

a faster and more convenient way Screen changers fall into three main categories: (1) manual, ( 2 ) i n t e r m i t t e n t (reciprocating), and (3) continuous screen changers Other types of reciprocating screen changers employ valves by means of which the melt flow may bc diverted from one screen pack to the other, and back again The ever- changing pressure conditions that are inherent in all intermittently operating machines can bc eliminated by the use of continuous screen changers

If it is at all possible to do without screen packs they should not be used Various reasons exist Complete and continuing displacement of melt from all points in the screen pack is rather difficult Hundreds of small dead spots are filled with melt as soon as the pack is put into service, and the material in these spots is moved only very slowly, if at all, by the drag of neighboring melts This action can cause contami- nating and degrading of the extrudate

The gear pump is a component that has been standard equipment since the 1930s in textile fiber production During the 1980s they established themselves in all ldnds of extrusion lines Gear pump is used

to generate even melt pressure Two counter-rotating gears transport a melt from the pump inlet (extruder output) to the pump discharge outlet Gear rotation creates a suction that draws the melt into a gap between one tooth This continuation action from tooth to tooth develops a surface drag that resists flow, so some inlet pressure is required to fill the cavity 492

Static mixer, also called a motionless mixer, provides a homogeneous mix by flowing one or more plastic streams through geometric patterns formed by mechanical elements in a tubular tube or barrel These elements cause the plastic compound to subdivide and recombine in order to increase the homogeneity and temperature uniformity of the

2 3 4 Plastic Product Material and Process Selection Handbook

melt There are no moving parts and only a small increase in the energy

is needed to overcome the resistance of the mechanical baffles These mixers are located at the end of the screw or before the screen changer

or between the screw and gear pump

The temperature profile required along a barrel, adapter, and die depends largely on the specific extrusion process line with its screw design, plastic used, and available process control (Chapter 3) The thermal condition of the plastic is essentially determined for a given material by screw geometry with its rotational speed and the total restriction or pressure existing in the die The electrical heaters are normally placed along the barrel grouped in separate and adjoining zones; each zone is controlled independently Small machines usually have two to four zones Larger machines have five to ten zones Table 5.2 provides information on the different types of heater bands

TabJe 5.2 Selection guide for barrel heater bands (courtesy of Spirex)

6 5 0 F

6 5 0 F ,,

1 2 0 0 F

M A X

A D V A N T A G E S

W S l LOW cost,

Versatility,

E n e r g y efficiency

E n e r g y efficiency

Cost, Low t e m p e r a t u r e

Information on dies and process control is in Chapter 3 Different control systems are used to process the different extruded products Simplified examples of different controls are provided in Figures 5.3 and 5.4

In the past with the development of single-screw extrusion techniques for newer TP materials, it was found that some plastics with or without additives required higher pressures (torque) and needed higher tempera-

236 Plastic Product Material and Process Selection Handbook

Figure 5~4 Sheet line control

turcs Thcrc was also the tendency for thc plastic to rotate with the screw The result was degraded plastics The peculiar consistency of some plastics interfered with the plasticators feeding and pumping process The problem magnified with bull~ materials, also certain typcs of emulsion PVC and HDPE, as well as loosely chopped PE film or sticl~ pastes such

as PVC plastisols

In the past twin and other multi-screw extruders were developcd to correct the problems that existed with the single-screw cxtrudcr Later the single-screw designs with material dcvclopmcnts practically elimi- nated all their original problems

The conveyance and flow processes of multi-screw extruders are very different from those in the single-screw extruder The main charac- teristic of multi-screw extruders include:

1 their high conveying capacity at low spccd;

2 positive and controlled pumping ratc over a wide range of

temperatures and coefficients of frictions;

3 low frictional (if any) heat gcncration which permits low heat operation;

low contact time in the extruder;

relatively low motor-power requirements self-cleaning action with high degree of mixing;

6 very important, positivc pumping ability which is independent of the friction of the plastic against the screw and barrel which is not reduced by back flow

5 Extrusion 237

Even though the back flow theoretically does not exist, their flow phenomena are more complicated and therefore far more difficult to treat theoretically than single-screw flow Result has been that the machine designer has to rely mainly on experience

Although there are very few twin-screw (TS) extruders in comparison

to the many more single-screw extruders, they are used also to produce products such as window and custom profile systems Their major use is

in compounding applications The popular common twin-screw extruders (in the family of multi-screw extruders) include tapered screws or parallel cylindrical screws with at least one feed port through a hopper,

a discharge port to which a die is attached, and process controls such as temperature, pressure, screw rotation (rpm), melt output rate, etc ~43

Twin-screws with intermeshing counter-rotating screws are principally used for compounding Different types have been designed that include co-rotating and counter-rotating intermeshing twin screws The non- intermeshing twin screws are offered only with counter-rotation There are fully intermeshing and partially intermeshing systems and open- and closed-chamber types In the past major differences existed between co- rotating or counter-rotating; today they work equally well in about 70%

of compounding applications, leaving about 30% where one machine may perform dramatically better than the other

Similar to the single-screw cxtrudcr, the twin-screw extruder, including multi-screw, has advantages and disadvantages The type of design to be used will depend on performance requirements for a specific material to produce a specific product With the multi-screws, very exact metered feeding is necessary for certain materials otherwise output performance will vary With overfeeding, there is a possibility of overloading the drive or bearings of the machine, particularly with counter-rotating screw designs For mixing and homogenizing plastics, the absence of pressure flow is usually a disadvantage Disadvantages also include their increased initial cost due to their more complicated construction as well

as their higher maintenance cost and potential difficulty in heating

The market for counter-rotating twin-screw (TS) extruders is basically dominated by two designs One has cylindrical screws called parallel TS extruder and the other TS extruder is fitted with conical screws Performancewise, the superiority of the conical principle to parallel does not only appear in the theoretical comparison, but in practice as confirmed by users Flexibility of conical turns out an extrudate of consistent quality at both low and high output rates which are not sensitive to raw material fluctuations It appears that the parallel have reached their efficiency limit unless a means of drastically increasing the

238 Plastic Product Material and Process Selection Handbook

screw torsional strength is developed Conical continue to offer what appears to be endless improved benefits through further development

An example of a conical extruder is Milacron's CM92 that is the world's largest of this design It produces the highest output extruder for processing wood flour filled plastics Depending on the flour-plastics ratio, output rate ranges from 1,000 to 1,800 l b / h It uses a feed crammer to properly handle the low bulk density and fluffy wood flour The tapered screw design that allows for a larger feed zone and applies a natural compression on the material during processing, results in the wood flour being more effectively "wetted out" by the plastic melt The large diameter screws [184 tapering to 92mm (7.24 to 3.62in.)] with a 27:1 L/D ratio optimize feed zone surface area for faster, more uniform heat transmission from screws to material Small exit diameter reduces rotational shear and screw thrust, while increasing pumping efficiency into the die High torque at low speed of 34 rpm enables gentile plasticizing and a wide processing window

Critical to this extrusion process is maintaining consistent, controllable heating and cooling It has five-barrel zones with a total heating capacity of 86 kW Four of the barrel zones arc provided with cooling, using a heat-transfer fluid designed to dissipate heat Six die zones (including entry adapter) are provided with maximum heating capacity

of 4:5 kW This extruder was designed with high output capacity in order to provide economic advantages in volume markets such as composite lumber, fencing, decking, windows, and doors

Operation

Startup

Machine operation can take place in three stages that go from startup

to shutdown The first stage requires operating the extruder for warm-

up with operational settings of up-stream and down-stream equipment The next stage involves setting the required processing conditions to meet product requirements at the lowest cost The final stage is devoted to fine-tuning and problem solving the complete line A successful operation requires close attention to many details, such as the melt quality, temperature profile adequate to melt but which does not degrade the plastic, production of a minimum of scrap, and procedures for startup and shutdown that will not degrade (or minimize) the plastic Processors must also become familiar with troubleshooting guides 143

5 9 E x t r u s i o n 2 3 9

Extrusion operation differs based on the type of product to be produced and plastic to be processed However on startups there are some aspects which all processes have in common The process differs somewhat if one has a clean, empty machine, or one which contains plastic and is reheated A main source of difficulty in starting an extrusion run is impatience of people It is necessary to wait until the barrel and die is at the correct operating temperature before starting otherwise problems develop such as having hot or cool melt spots, overstressed melt sections, overloading the screw with plastics, plastic bridging at the hopper, degrading plastic, etc Starve feeding of plastic

on startup at a low screw speed and until melt is pumped from the die helps prevent bridging of the screw

Consider purging the extruder plasticator when it contains plastic that can be detrimental to startup a n d / o r producing unacceptable products (Chapter 3) If a plastic was left in the barrel for a while, with heat off, the processor must determine if the material is subject to shrink It could have caused moisture entrapment from the surrounding area, producing contamination that would require cleanup (this situation could also be a source of corrosion i n / o n the barrel/screw) Even with the same plastic in the machine from a previous run, the entire machine should be cleaned a n d / o r purged, including the hopper, barrel, breaker plate, die, and downstream equipment

When starting up a new extrusion setup, start the screw rotation at about 5 rpm Gradually look into the air gap between the feed throat and throat housing and makc sure the screw is turning Screws have been installed without having their key in place, or the key has fallen out during installation Also make sure that antiseize material is applied

to the drive hub, to help installation and removal Mso if the key is left out and the drive quill is turning and the screw is not, the screw will not gall to the drive quill

Prior to startup one must check certain machine conditions and process control that should be listed on some ldnd of worksheet from the machine manufacturer, plastic supplier, a n d / o r the more important plant setup person with experience (Chapter 3) Checkup includes the careful handling of:

(a) heater bands and electrical connections,

(b) thermocouples, pressure transducers, and their connections,

(c) inspect all machine heating, cooling, and ventilation systems to

ensure adequate flow,

(d) be sure flow path through the extruder is not blocked,

2 4 0 Plastic Product Material and Process Selection Handbook

(e) have a bucket or drum, half filled with water, to catch extrudate wherever purging or initial processing of plastics were contaminated gaseous by-products exist,

(f) review operating manual of the machine for other startup checks and requirements that have to be met such as motor load

(amperage) readings

Within various types of the family of plastics (PE, PVC, PP, etc.) each type usually have different heat profiles and other settings (Table 5.1) Experience shows how to set the profile a n d / o r obtain preliminary information from the material supplier Degrading or oxidizing certain plastics is a potential hazard that occurs particularly when the extruder

is subject to frequent shutdowns In this respect, the shutdown period

is even more critical than the startup period

In setting up the barrel temperature profile start with the front to rear zones (die end to feed section) The heat controllers are set slightly above the plastic melting point prior to turning on the heaters Heat-up should be gradual from the ends to the center of the barrel to prevent pressure buildup from possible melt degradation With this startup follow through with:

1 gradually increase heaters, checking for deviations that might indicate burned-out or run-away heaters by slightly raising and lowering the controller set point to check if power goes on and off,

2 following with all heaters slightly above the melt point, adjust to the desired operating heats; time required to reach temperature equilibrium may be 1/2 to 2 h, depending on the size of the extruder,

3 if overshooting occurs it is usually observed with the o n / o f f controllers,

4 after set heats have been reached, one puts the plastic in the hopper and starts the screw at a low speed such as 2 to 5 rpm; some plastics, such as nylon, may require 10 to 20 rpm

5 processor should observe the amperage required to turn the screw, stop the screw if the amperage is too high, and wait a few minutes before restart,

6 observe and remain at the required melt pressure, the extruder barrel pressure should not exceed 1,000 psi (7 MPa) during the startup period,

7 machine should run a few minutes and purge the initial run until a good quality extrudate is obtained visually; experience shows what it should look like such as a certain size and amount of bubbles or

5-Extrusion 241

fumes may be optimum for a particular melt, based on one's experience (or the trainer's experience) after setting up all controls, turn up the screw to the required rpm if not already properly set, checking to see that maximum pressure and amperage are not exceeded,

when time permits, after running for a while, the processor should consider stopping the machine, let it start cooling, and remove the screw to evaluate how the plastic performed from the start of feeding to the end of metering Thus one can see if the melt is progressive and can relate it to screw and product performances

10 adjust the die with the controls it contains, if required, at the desired running speed

Once the extruder is running at maximum performance, set up controls for takeoff/downstream equipment, which may require more precision settings a n d / o r changes in the extruder to meet downstream equip- ment requirements

Extrudate can start its tract from the die by threading (or the term also used is stringing up) through the cooling and take-off downstream equipment to its haul-off initially at a slower speed than production operation When possiblem rather than taldng the extrudate from the die and being directed through the equipment, the hot melt is made to weld to the thread-up end already in the equipment that is usually a left over from the previous run In turn the thread-up is pulled through the line carefully and safely If a welding action does not occur, a metal hook may be pushed into the melt Cooling of this joint is required to give it strength Care is needed to avoid malting a lump too large to go through the line

This operation requires the personal sldll of the startup person That person is required to integrate/interrelate extruder and down-stream equipment Extruder screw speeds and haul-off rates may then be increased Downstream equipment is adjusted to mcet their maximum operating performance, such as having the vacuum tank water operate with its proper level and vacuum applied The extruder can be fine- tuned to obtain the final rcquired setting for meeting the desired output rate and product size

Startup operations arc made at rates people can handle The process is very slow compared to standard operating speeds The puller starts its movement at just about the same speed the person has been pulling or therc may be a pile-up or tear-off of melt at the die That will usually mean threading up again

242 Plastic Product Material and Process Selection Handbook

Skill on the part of the person will involve pulling continuously at a steady rate The person acts like a machine or robot for a few minutes Skill on the part of a good operator is very evident at startup

Cooling the extrudate during hand pulling is important It gives strength and form stability to the extrudate Without cooling, the melt will string out and pull apart The steadiness of pulling and the evenness

of cooling determine what the hand-pulled product will look like and how easily it can be threaded and fed into the take-off

After this operation follow up on the product's dimensions or what would determine that the product is meeting requirements Minor changes in speed may be needed Adjustments to centering of the die or die opening may be necessary if there are thick or thin spots Product measurement and die adjustment is continued until a satisfactory product is made Frequently this process may take an hour or more During this time, scrap is produced and when practical should be used

as a regrind and reused To reduce this time schedule significantly program controllers provide a quick means to balance out all the control settings to produce the desired product

Shutdown

It is c o m m o n to run the extruder to an empty condition when one is shutting down This action ensures that there is no startup with cold plastic, a condition that could overload the extruder if improper startup occurred Some extruders, such as those processing PE film, are shut- down with the screw full of plastic This prevents air from entering and oxidizing the plastic Because PVC decomposes with heat, to ensure that this material is completely removed at shutdown, purging material such as low melt PE is processed that can remain in the barrel (Chapter 3) On startup, it is preferable to raise barrel heat slightly above its normal operating temperatures The higher temperature ensures that unmelted plastic will not produce excessive torque in the screw In regard to the downstream equipment, such as with film or sheet lines, consider leaving some "threading" for an easy startup as reviewed in startup

The shutdown is usually very simple Procedures for shutdown without clcanout starts by stop feeding plastic into the plasticator and reduce all heat settings to the melt heat Reduce the screw speed to 2 to 5 rpm, purging the plastic if requires into a water bucket or drum prior to reducing the melt heat The screw rotation continues until no more plastic exits the die Rotations of the screw stops resulting in the so- called pumping the screw dry of plastics

5 Extrusion 243

With the screw stopped, shut off the heaters and disconnect the crosshcad (or die) heaters Reduce other heaters to about 170 to 330C (400 to 625F) depending on the plastic's temperature at the melt point

If a screen pack with breaker plate is used, disconnect the crosshead (or die) from the extruder and remove the breaker plate and screen If necessary, appropriate action is taken to clean them (Chapter 17)

For clean out of the extruder at shutdown, disassemble the crosshead and clean it while still hot Remove the die, and gear pump if used, and remove as much plastic as possible by scraping with a copper spatula or brushing with a copper wire brush Remove all heaters, thermocouples, pressure transducers, and so on Consider using an exhaust duct system above the disassembly and cleaning area, even if the plastic is not a contaminating type This procedure keeps the area clean and safe

Follow by pushing the screw out gradually while cleaning with a copper wire brush and copper w o o l 93 Care should be exercised if a torch is used to burn and remove plastic; tempered steel may be altered and the screw distorted or weakened as well as subjected to excessive wear, corrosion, or even failure (broken)

After screw removal, continue the cleaning, if necessary Follow by turning off the main electric power switch Final cleaning of products, particularly disassembled parts, is best done manually, or much better,

in ventilated burnout ovens, if available, operating at about 1,000F (540C) for about 90 rain For certain parts with certain plastics, the useful life could be shortened by corrosion; check with the part manufacture After burnout, remove any grit that is present with a soft, clean cloth If water is used, air-blast to dry With precision machined parts, water cleaning could bc damaging because of the potential of corrosion when certain metals are used

Film and sheet

Films and sheets arc produced in several ways, including extrusion, calendering, and casting M e t h o d used involves the properties required

of the basic plastics and finished products as well as cost usually based

on quantity The following classification can be helpful as a guide to film and sheet thicknesses: (1) film is generally less than 0.010 in (0.003 ram) and (2) sheet at 0.010 in or more In turn sheet can be classified as:

244 Plastic Product Material and Process Selection Handbook

(a) intermediate sheet in the range of 0.04:0 to 0.250 in (0.01 to 0.06 ram);

(b) thin gauge sheet up to 0.060 in (0.015 mm);

(c) heavy gauge sheet at 0.080 to 0.500 in (0.02 to 0.13 mm)

Most commercial plastic films are produced with a thickness of less than 0.005 in and most packaging films are less than 0.003 in Different groups within the different industries (plastic, packaging, aluminum, clothing, etc.) may have their own thickness definitions; they call it what their buyer/customer use Some of them use 0.004 in (0.10 mm)

as the dividing line between film and sheet ~99

Film

Film can be produced either by extrusion tubular blowing or flat process Each has its advantages and disadvantages These processes result in film with a molecular orientation predominantly in the machine direction (MD) As reviewed later, orienting the film can be in two orthogonal directions that develop superior optical, mechanical, and physical properties The process is known as biaxial orientation and

it can bc applied to both tubular and flat film

Regardless of process, film production lines include common down- stream equipment such as haul-off, tensioning, and reeling stations Other common features include static control units and corona discharge treaters to prepare the film surface for subsequent printing processes A high purity melt, free of inclusions, is essential for film production This is achieved by filtering the melt through a screen pack upstream of the die

Blown Film

Figure 5.5 provides an example of a complete operating line that produces film Table 5.3 provides an introduction to production output yields

The blown film process involves extruding a relatively thick tube that is then expanded or blown by the usual internal air pressure or the water quench process to produce a relatively thin film (Figure 5.6) The tube can be collapsed to form double-layer layflat film or can be slit to make one or two single-layer film webs The water quench process is the generally preferred method of producing blown PP type film

5 9 E x t r u s i o n 2 4 5

[::igt~re 5o5 Assembled blown film line (courtesy of Battenfelt Gloucester)

Blown film is usually extruded vertically upward through a circular die This forms a tube that is then blown into a bubble that thins or draws down to the required final gauge Orientation takes place in two directions horizontally (transverse direction/TV) as the bubble is formed, and in the machine direction (MD) as controlled by adjustable- speed haul-off nip rolls

Air ring either with single lips, or two or more lips direct air to cool the bubble at the dic exit Internal bubble cooling is used to cool the inner surface of the extruded bubble to gain high production rates (typically 50% higher than with external air only) The bubble enters a collapsing frame and, after passing through upper nip rolls, becomes a tube that can then be processed into bags, flat film by slitting open the tube, etc Because convection cooling is relatively slow, blown film tends to bc hazier than flat cast film