Environmental Engineering Reference
In-Depth Information
4.3.2 Extrusion
Extruders are designed to heat, melt, and homogenize the plastic pellets
or powder and to transport the melt forward to eventually force it through
an open die to obtain a long, continuous shape such as plastic pipe. The
equipment consists of a long heated barrel (similar to that in injection
molding machines), but the nozzle at its terminus forces the melt through
a die. A single screw (or a reciprocating pair of screws) in the barrel moves
the partially melted plastic forward toward the die. However, unlike with
injection molding, the screw does not act like a piston. The process is more
akin to a paste being pressed out of a tube to obtain a cylindrical extrudate.
Extruded shape that is produced in a continuous profile is rapidly cooled to
ensure that it is not deformed and is cutinto suitable lengths. Plastic tubing,
plastic sheets, window/door frames, molding used in building industry, and
all thermoplastic pipes are extruded products. Figure 4.7 shows the basic
featuresofasingle-screwextruderandasheetextrusiondieincrosssection.
However, twin-screw extruders are also used. Naturally, the throughput of
extrudate from the latter will be higher compared to single-screw extruder.
 
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