How a Plastic Extruder Works
By Susan Kristoff, eHow Contributor
Plastic Extruder Parts
A plastic extrusion system converts a granulated plastic resin into a final product with a constant cross section. A plastic extruder consists of four parts: a gravity-fed hopper, a drive screw, a die system and a cooling bath.
The plastic begins the extrusion process as granulated pellets. The pellets are added to the gravity-fed hopper at the front of the extrusion machine. More than one type of plastic can be added to the hopper, or other products such as pigments or plasticizers can be added to the base plastic to affect the final extruded part.
The plastic resin pellets exit the base of the hopper and enter the drive screw. The screw slowly moves the plastic pellets toward the die, heat the pellets to their melting point and mix the plastic and any additives to create a homogeneous mixture. The drive screw often contains several temperature sensors to help maintain a consistent temperature along the length of the screw. The pitch of the drive screw also changes as it approaches the die to increase the pressure of the molten plastic.
The die assembly consists of a screen to filter out any solids from the liquid plastic, a breaker plate to remove rotational effects from the plastic mixture and provide back pressure and the die itself. The pressure created by the drive screw forces the plastic through the die at a specified rate. For hollow cross sections, the die includes a pin to create the center portion of the shape, which is then pressurized with air to maintain the cross section during cooling. The die often contains a gradual shape change with a conical draw down to eliminate stresses in the extruded part.
After the plastic has been extruded it is immersed in a cooling bath. Plastic is an excellent insulator and retains heat well, so it takes time to remove the heat from the plastic. For products such as plastic sheeting, rollers are used to cool the extrusion. When the extrusion reaches a workable temperature, it is either cut to length or wound onto a core.
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