POLYMER EXTRUSION PROCESS

Once a polymer has been melted, mixed and pressurised in an extruder, it is pumped through an extrusion die for continuous forming (after cooling and solidification) into a final product. The most common die types are flat, annular and proŽle. Products made by extrusion include pipe, tubing, coating of wire, plastic bottles, plastic films and sheets, plastic bags, coating for paper and foil, Ž bres, Ž laments, yarns, tapes and a wide array of proŽfiles.

Polymer extrusion through dies has certain similarities to the hot extrusion of metals.However, there are also signi-ficant differences. In metal extrusion the material is pushed forward by a ram, while in polymer extrusion the material is continuously supplied by a rotating screw. In hot metal extrusion the temperatures range from 340°C for magnesium to 1325°C for steel, and the corresponding pressures range from 35 to over 700 MPa.1 5 In polymer extrusion the temperatures seldom exceed 350°C, and pressures usually do not go much above 50 MPa at the screw tip. Solid phase extrusion of polymers has been developed for the production of certain high strength products.At low temperatures, the molecular orientation imparted by the forcing of the material through the shaping die remains in the extrudate. Solid state polymer extrusion has certain similaritiesto the cold extrusion of metals.

Blown film extrusion is the most important process for the production of thin plastic Žfilms from polyethylenes.The molten polymer is extruded through an annular die (normally of spiral mandrel construction) to form a thin walled tube which is simultaneously axially drawn and radially expanded. In most cases the blown film bubble is formed vertically upwards. The maximum bubble diameteris usually 1.2 – 4 times larger than the die diameter. The hot melt is cooled by annular streams of high speed air from external air rings, and occasionally also from internal air distributors. The solidiŽ ed Žfilm passes through a frame which pinches the top of the bubble and is taken up by rollers. Coextruded Žfilms with 3 – 8 layers (sometimes up to 11) are also produced by this process, for use in food packaging.

Cast Žfilm and sheet extrusion involves extruding a poly- mer through a  at die with a wide opening (up to 10 m), onto a chilled steel roller or rollers which quench and solidify the molten material. Film is generally defined as a product thinner than 0.25 mm, while sheet is thicker than this.The cast Žfilm process is used for very tight tolerances of thinŽ film, or for low viscosity resins. Most flat dies are of T slot or coathanger designs, which contain a manifold to spread the flowing polymer across the width of the die, followed downstream by alternating narrow and open slits to create the desired flow distribution and pressure drop. Most cast film lines manufactured today are coextrusion lines, com- bining layers from as many as seven extruders into the product through multimanifold dies, or single manifold dies with the aid of feedblocks. 

In Žfilm extrusion, the shear rates at the die lips are usually ~103 s21. When the wall shear stress exceeds a certain value (usually 0.14 MPa in research papers, higher in industry with the help of additives),the extrudate surface loses its gloss owing to the sharkskin melt fracture phenomenon.Sharkskin can be described as a sequence of ridges visible to the naked eye, perpendicular to the flow direction.

Pipe and tubing extrusion involves pumping a molten polymer throughanannulardie,followingwhich the extruded product, while being pulled, passes through a vacuum sizer where it attains its final dimensions. This is followed by spray or immersion cooling and cutting to fixed lengths. Pipe of diameter up to 2 m or greater is made by this process, and tubing with diameters from 10 mm down to below 1 mm. The annular dies are normally of spider or spiral mandrel design.

In wire and cable coating processes, individual wires or wire assemblies are pulled at very high speed through a crosshead die, at right angles to the extruder axis. In high pressure extrusion, the polymer melt meets the wire or cable before the die exit, for example insulating of individual wires. In low pressure extrusion, the melt meets the cable after the die exit, for example jacketing of assemblies of insulated cables. Very high shear rates are frequently encountered in this process(up to 106 s2 1 ) and low viscosity resins are used.

ProŽfile extrusion is a manufacturing process used for products of constant cross-section. These can range from simple shapes to very complex profiles with multiple chambers and Žfingers. Examplesrange from picture frame mouldings, to automotive trim, to edging for tabletops,to window lineals. The extruded materials are classiŽ ed (roughly) as rigid or fiexible. The typical proŽle extrusion line consists of an extruder pumping a polymer through a proŽle die, followed by a sizing tank or calibrator, additional cooling troughs, a puller and a cutoff device. The design of profile dies requires considerable experience and patience.Output limitations in profile extrusion are encountered owing to either sharkskin (for thin products produced from high viscosity polymers) or the ability to cool thick walled products. Polymer pipe and proŽfile extrusion is similar to the hot extrusion of metals for the production of continuous hollow shapes of barlike objects. However, the mathematicalmodelling of these processesfor metals is based mainly on elastic – plastic flow hypotheses.

In melt spinning, the molten polymer ows through numerous capillaries in a spinneret (up to 1000). The poly- mer is delivered under pressure by a gear pump for accurate metering, after passing through a Ž lter which follows the extruder. On exiting the capillaries, the Ž laments are attenuated to the desired diameter.For the production of very thin Žfibres, the melt blowing process is used. In this process the bres are attenuated by the drag force exerted by a high velocity air jet.