The last function of the extruder is to control the rate at which the parison flows. Extrusion rate is determined by the speed at which the screw rotates inside the barrel. The faster the screw turns, the faster the parison flows from the tooling.
Barrel Screw
The screw is turned by a motor which is capable of variable speeds. The speed is established by manually setting the motor speed control. The tachometer indicates the speed that screw is turning. There is a limit to the speed at which the screw can be turned, and it is not necessarily a limitation imposed by the motor or the drive system.
Screw rotation must be restricted to a speed that will allow the plastic to make a smooth, gradual transformation from pellets to parison. For this reason, different size extruders are designed to process different volumes of plastic.
Extruder Volume
Extruder volume is expressed in pounds of plastic output per hour. Extruder size is normally expressed in terms of the size of the screw it uses. This extruder is an 80/24:1 extruder. That means that the screw is 80 millimetres or about 3 inches in diameter. The 24 to 1 part means that the length of the screw is 24 times its diameter, or about six feet long.
Expression of Extruder Size
On this extruder, the screw is turned by a sixty horsepower motor and a belt driven gear reducer. It has an output of up to 300 pounds per hour. Thus, the last function of the extruder is to control the rate of plastic output. A manual setting establishes the drive motor’s speed. Through a gear reducer, the motor causes the screw to rotate inside the barrel. The speed of this rotation determines the rate at which the parison extrudes from the tooling.
Let’s go over what’s been covered so far. An extruder transforms plastic pellets using external and internal heat sources. It maintains temperature profiles by automatically adding heat or by taking it away. It uses the extruder head, head tooling, and parison programming to form parisons. And finally, it provides for control of the rate at which the parisons can be extruded – up to a maximum design output.
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