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What is Extrusion?
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The blown film tube can be used to make all kinds of bags, or the tube can be slit to form a flat sheet. In cast film extrusion, the molten polymer is fed to a flat die, the melt is drawn in the machine direction into a thin film, and the flat web is rapidly cooled by passing over one or more chilled rollers. As in all types of extrusion, the cooling rate and the draw-down rate affect molecular orientation, which affects physical properties of the end product. Both blown and cast films are used in many kinds of applications. A walk through the grocery store reveals a myriad of highly engineered film packages, from fresh-produce bags and wraps for meat to candy wrappers, cereal bags, stand-up pouches, and bags for frozen food. Films are also used in stretch and shrink wrap for bulk packaging. Outside of packaging, films are used in bags, agricultural covers for fields or greenhouses, and nonwoven products for healthcare or personal care like baby diapers, to name just a few.
Plastic sheet is usually defined as greater than 10 mil thick. Sheet is typically extruded with a single-screw extrusion system, although direct extrusion might use a twin-screw extruder. Like cast film, sheet is extruded through a wide, flat die, pulled through rollers, and cooled with air or water.
Sheets may be used in sheet-form for applications such as pond or landfill liners, or go to a thermoforming process to be made into a variety of containers or other parts.
Profile extrusion systems
Many types of plastic cross-sections are produced with either single or twin screw extruders in the profile extrusion process. The melt is fed to a profile die, pulled through a cooling or calibration tank, and cut to size.
Profiles made from PVC powder are usually processed on counter-rotating parallel or conical TSE. PVC is widely used in building and construction applications like windows, decking, siding, fencing, and railing. Various resins are used in many different profile applications, like edge trim or other shapes that go into automotive, recreational vehicle, industrial, consumer good and other applications.
Pipe and tubing extrusion systems
Pipes and tubing are extruded similarly to profiles through a die and a cooling tank, but dies are annular with a wide range of diameters for applications from medical tubing to water piping. As with profiles, PVC pipe is often extruded with counter-rotating TSE. Other resins are typically extruded with single-screw extruders.
Coating, laminating, and wire and cable coating extrusion systems
In extrusion systems for coatings, a thin coat of polymer is extruded onto a substrate, which could be another polymer, metal foil, paper, or other material. In extrusion lamination, a polymer is extruded between two substrates. (Giles figure 47.2 and 47.3) Many types of packages use extrusion coating or laminating.
In wire and cable coating, a polymer is extruded onto the wire or cable as it is pulled through a crosshead die. This process typically uses a single-screw extruder, which is used to melt the polymer and sometimes to add color.
In compounding extrusion, the extruder’s primary function is to disperse and distribute additives, colors, fillers, or fibers into the polymer. Typically, the melt is fed through either a strand die, where it is cooled and chopped, or a pelletizing die, where it is cut into small pellets that are then cooled. Highly dispersive and distributive mixing is needed, especially for mixing high levels of filler or for combining two dissimilar polymers. Twin-screw extruders are commonly used, although single-screw extruders with adequate mixing sections, Kneaders, and other mixing devices are also used in compounding.
Other special types of extrusion
A coextrusion process combines two or more molten polymer streams from multiple extruders to create distinct layers in the extruded product. For example, in a foam-core pipe, the foamed inner layer is surrounded by two, unfoamed outer layers. Coextrusion is often used to surround regrind or recycled resin with virgin resin. Packaging film often has many layers (up to 9 or more), including a barrier resin layer, a sealant resin layer, and adhesive layers to tie incompatible layers together. Die design is especially critical in coextrusion.
Direct extrusion usually describes a process in which resin, additives, and other ingredients such as fibers or fillers are fed into the extruder, mixed and melted, and sent directly to the forming die (sheet, profile, pipe, etc.), rather than being made into pellets first and then extruded into the end-product in a separate step. Direct extrusion is used for high-volume products like sheet or decking boards.
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