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Polymer Extrusion

POLYMER EXTRUSION
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Description: The commercial process of polymer extrusion is the conversion of a raw material, usually in the form of a powder or pellet, into a finished product or part by forcing it through an opening. The process consists of pumping a molten state polymer, under pressure, through a die, producing a continuos cross section or profile.

For continuos extrusion the pumping action is typically performed by a screw inside a barrel or a combination of screws. The polymers used are typically thermoplastics and are melted by heating the barrel. The opening in the die is the guide after which the extrudate takes its final form.

For batch extrusion a reciprocating member transports the feed, pushing it thorough the die. This process is repeated for each product or part.

Extrusion is the most used, and perhaps the most important method of plastic fabrication today.

This is a picture of an extruder at the University of Connecticut in Professor M.T. Shaw's laboratory.

The following pages contain information on:
* Feed Materials
* The Process
* Equipment
* Other Designs and Uses
* Products Formed
* Other Links
* References
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Feed Materials

Most of the polymers used in extrusion are of high molecular weight and by nature are highly viscous when in the melted, molten state. In order to process such materials the pump of the extruder must work under high pressures and temperatures. Thermoplastics are the predominant feed for extrusion processes. Due to the shearing action inherent in the screw feed mechanism, the process lends itself to dividing, heating up and melting the extradite. This does not exclude thermosetting polymers though. Thermosetting polymers and elastomers can be fed, mixed with additives and crosslinking initiated by the heat in the barrel, usually completing the crosslinking after passing through the die. Examples of this are rubbers with vulcanizing agents and HDPE crosslinked by radiation.
Abbreviation Base Polymer
AMORPHOUS POLYMERS
ABS acrylonitrile-butadiene-styrene
PMMA poly methylmethacrylate
CAB cellulose acetate butyrate
PC polycarbonate
PS polystyrene
PVAC polyvinyl acetate
PVAL polyvinyl alcohol
SAN styrene-acylonitrile
UPVC unplasticised (rigid) polyvinyl chloride
PPVC plasticised (flexible) polyvinyl chloride
HIPS high impact polystyrene (rubber toughened)
SEMI-CRYSTALLINE
POM polyoxymethylene; polyformaldehyde (polyacetal)
EVAC ethylene vinyl acetate copolymer
PA polyamide (nylon)
PETP polyethylene terephthalate (polyester)
PBTP polybutylene terephthalate <./td>
LDPE low density polyethylene
LLDPE linear low density polyethylene
HDPE high density polyethylene
PP polypropylene (homo- and co-polymers)
TPX poly 4 methyl pentene 1
PTFE polytetrafluoroethylene (ram extrusion only)
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Process

The solid polymer feed material is melted under the action of the screw and barrel with heat, friction and pressure. The polymer melt is then forced through the die, cooled and collected. The cooling system is blown air or commonly a water bath. For a continuos process of thin or very flexible material after cooling it is collected on rolls. For rigid material it is cut to lengths of up to 20 meters depending upon the transportation facilities available and collected in stacks.

Due to the combination of viscous and elastic nature of polymers there is some recovery by the polymer after the die. That is the elastic nature of the polymer is to remember its shape, resist change and recover the shape it had prior to the distortion imposed by the die. The viscous nature of the polymer is without memory and quickly accepts the shape forced upon it by the die. The combination, viscoelastic behavior results in a swell after passing through the die. Since there is a considerable necking down from the barrel size to the passage in the die, the polymer swells to partially return to its former shape. So the final product from the extrusion is not the size of the die passage but larger. This must be planed so that the final product is of the desired size. This is no small task. The factors that affect the swell are; polymer type, molecular weight distribution, temperature, shear rate, fillers present and sometimes recent shear history (especially LD polyethylene).
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Equipment

1) Hopper Feed
The hopper contains a large amount of feed material in the form of powder or pellets. This is gravity fed onto the upper surface of the exposed screw which continuously pulls material into the barrel between the flights.
2) Barrel

The barrel is normally heated to melt the polymer or initiate crosslinking. The barrel is of constant inner diameter and has heavy walls to withstand high pressures. A heating element is usually wrapped around the outside of the barrel. The barrel runs the entire length of the screw from the hopper, where its upper side is fitted to the hopper, to the die where it narrows, with the only opening being through the die. The range of barrel sizes is 3/4 to 24 inches of inside diameter.
3) Conventional Flighted Extruder Screw
The screw is the vehicle upon which the feed travels. The shape of the screw as described in figure #1 below, determines along with the rpms at which the screw turns, the speed at which the feed moves and the pressure attained in the barrel. The screw is so names because its general shape is that of the spiral of a screw. The continuos center rod of the screw is called the core. The diameter of the core is a major factor in determining the pressure in the barrel. The ratio L/D is the characteristic used to describe the size of the screw. The L is the total length of the screw, while D is the inside diameter of the barrel. The shortest extruders have a ratio of 12, the longest 42. The conventional plasticating or single stage screw has three basic regions; the feed, transition and metering sections . The feed or solids conveying zone which transports the feed away from the hopper into the enclosed barrel. The feed is still in a solid powder or pelletized state and the screw has deep flights in this section. The transition zone is where compression occurs as the root diameter increases and the melting process takes place from the friction and heat from the barrel. The depth of the flights decreases along this section because the root changes size. The metering zone is the last section before the die so the polymer is molten and the depth of the flights are shallow and rather constant.
4) The Extruder Die
The extruder die has an opening in it the shape of the cross section of the product. The die has to be able to withstand the high temperatures and pressures exerted on it from the polymer being forced through it. The polymer adopts the shape of the flow channel of the die. The pressure built up called diehead pressure depends upon the properties of the polymer, temperature of the polymer, the shape of the die and the flow rate through the die. Most polymers experience some form of swelling upon exiting the die. This is discussed further under process.
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Other Designs

Vented Extruder
Has one or more openings in the barrel from which volatiles driven out from the polymer can escape. The root of the screw drops in diameter just before the opening in the barrel so that pressure is not above atmospheric at the opening.

There are several types of twin screw extruders. They may be intermeshing, or nonintermeshing, corotating, counterrotating or coaxial. They have more design variables than the single screw, but some can be used in very different applications. There is a planetary roller extruder with a set of planetary gears located between a conventional screw and the die. The convention screw is the plasticating section and the gears act as a pump to build pressure. The disk and drum extruders based on a spinning disk or drum. The ram extruder is used mostly for solids extrusion where a piston tightly fitted to a barrel forces the feed through the die.
OTHER USES
The single-screw extruder is also used as a continuos stirred reactor, as a feed for blow molding machines, as a feed for injection molding and an effective conveyor of particulate solids.
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Products

Products may be formed with solid cross sections or hollow cross sections. Those of solid cross sections such as angles, rods or strips is predominantly done under a very uniform, controlled process. The barrel is maintained at an unvarying temperature to provide a constant melt, the screw speed is also unwavering so these result in a uniform flow rate. With conditions well controlled the die-swell will also be constant so that a product with dimensions that are very consistent can be produced. For very exacting work or situations where normal controls are not adequate, sizing after the die is sometimes used.

Products with hollow cross sections such as tubes, pipes, and channels are also extruded. These require a die-core or mandrel which is the form around which the feed forms its shape. These are supported in the rear near the end of the screw by a spider" of arms or by a restricting the feed to entering on only a fraction of the circumference with the rest being filled by the support. Sizing is frequently used for hollow sections, fixing the internal or external dimensions. The dimension that is not sized is determined by the polymer output rate, the haul-off rate and the size of the forming die used.

Forming flat sheet brings up a special set of problems. A small deflection in the die lips can cause large error in the thickness of the final sheet. Uniform heating of the die is difficult. Extrusion of a film is very similar to a sheet but the thickness variation due to deflection in the lips has even greater importance. Since the thinner films are more flexible the unsupported gap between the die lips and the cooling must be reduced.

To form a tubular film by the blown method is very different than the sheet method. The melted polymer is forced around an annular or crosshead die to form the film. Through the die air is pumped inflating the tube and providing some cooling as does a outer ring pumping cooling air over the surface of the expanded tubular bubble. A set of boards then guides the tube to a set of rollers called nip rollers, the film is then wound ----------------------------------
References
* Progelhof, R.C. and J.L. Throne, "Polymer Engineering Principles: Properties, Processes, and Tests for Design", Hanser Publishers, New York, 1993.
* C. Rauwendaal, "Polymer Extrusion", Hanser Publishers, New York, NY (1986)
* M.J. Stevens, "Extruder Principals and Operation", Ellsevier Applied Science Publishers, New York, NY.(1985)
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This homepage was written by Ralph Bridge as the Honor's project for Dr. R.A. Weiss's Chemical Engineering 256 (Polymeric Materials) class at the University of Connecticut.
Last Updated on 5/15/97

Ralph Bridge