The tandem extruder at PPI consists of a 34 mm co-rotating
intermeshing twin screw extruder (mixing extruder) feeding a 40mm single
screw extruder (cooling extruder). Polymer pellets or powders are meter fed
into the hopper of the twin screw extruder. The polymer is plasticated in the
first section of the extruder. Physical blowing agent is pumped into the next
section of the extruder, where it is mixed with the polymer. In the last
section of the twin screw, the polymer-blowing agent charge is compressed to
high enough pressure to dissolve the blowing agent into the polymer. The
single screw extruder or the cooling extruder accepts the polymer at high
pressure and cools the melt down at constant pressure. The screw of the
cooling extruder is segmented and
hence designed to optimize the cooling, mixing and pumping characteristics.
There are three barrel sections on the cooling extruder. Heat transfer fluid
is circulated through the shell
side of the barrel to extract the heat out of the polymer. The die is mounted on the barrel of this cooling extruder.
Polystyrene, polyethylene, polypropylene and various copolymers were foamed
down to 3 pcf using carbon dioxide gas and to 1 pcf using VOC. Normally closed cell foams were produced
in this setup.
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