The deceleration principle of the lab extruder:
In most extruders, the change of screw speed is achieved by adjusting the speed of the motor. The motor usually rotates at full speed of about 1750rpm, but this is too fast for an extruder screw. If it turns at such a fast speed, it produces too much friction heat and the retention time of the plastic is too short to produce a homogeneous, well stirred melt. The typical rate of deceleration is between 10:1 and 20:1.
The first stage can be either geared or pulley set, but the second stage is geared and the screw is positioned at the center of the last big gear. In some slow running machines (such as the twin-screw for UPVC), there may be 3 deceleration stages and the maximum speed may be as low as 30rpm or lower (the ratio is 60:1).
Another extreme is that a few long twin screws used for mixing can run at 600rpm or faster, so a very low deceleration rate and a lot of deep cooling are needed. Sometimes the deceleration rate is wrong with the task matching – there will be too much energy that can’t be used – and it is possible to add a pulley group between the motor and the first deceleration stage that changes the maximum speed.
