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The extrusion of polymers differs from the extrusion of metals. The “billet” material is not in the form of
bar stock, but rather in a pellet or powder form. This substance is added to a machine that utilizes a screw-type
action to assist in mixing and pushes the plastic material through the die opening. The polymer is fed onto the
rotating screw, heated, and mixed.
The screw is made up of three sections:
·- Feed Section - the stock is moved from the hopper and preheated.
- Compression Section - the polymer is transformed into a liquid consistency, air entrapped among
the pellets is extracted from the melt, and the material is compressed.
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- Metering Section - the melt is homogenized and sufficient pressure is developed to pump it
through the die opening.
As the melt reaches the die, it must pass through a screen pack, a series of wire meshes supported by
a stiff plate (called a breaker plate) containing small axial holes.
The screen pack functions to:
- Filter the contaminants and hard lumps from the melt,
- build pressure in the metering section, and
- Straighten the flow of the polymer melt and remove its memory of the circular motion imposed by
the screw.
This melted polymer is finally forced through a tapered die/nozzle and the extrudate is formed.
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polymer extrusion
