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What is Extrusion...[continued]
Plastic sheet is usually defined as greater than 10 mil thick. Sheet is typically extruded with a single-screw extruder, 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 extrusionMany 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
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.
In extrusion coating, 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.
Compounding extrusionIn 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.
