Two Things Every Extruder Must Do

Stephen J. Derezinski, Ph.D.

Extruder Tech, Inc.

June 18, 2006

Every extruder must provide two things:

1) a desired flow rate at

2) a desired product temperature.

There may be other requirements, such as mixing or degassing, but the flow rate and melt temperature are always basic.

However, the single screw extruder machine has only one basic control variable, and that is screw speed. Once the screw speed is set to attain the proper flow rate of a given polymer, the product temperature is also determined. Some adjustment to the product temperature is normally accomplished by barrel heating or cooling, but using a vast amount of heating or cooling to achieve the proper melt temperature leads to large thermal gradients in the melt with resulting non-uniform melt temperature distribution. Also, localized polymer degradation can occur if high heating of the melt is necessary. At a given screw speed and flow rate, there is only one melt temperature that will have minimum barrel heating/cooling. It is the job of the screw designer to size the screw channels such that the desired melt temperature occurs at the desired flow rate with conditions of minimal barrel heating/cooling.

The screw design is also complicated by the fact that the screw speed must be within the capability of the drive system. Also, the screw speed must be such that the power and torque required by the screw are within the capability of the drive at that specified screw speed. Values of speed, torque, and power on the order of 80% of maximum are prudent. If the values are too close to their maximum, then control becomes difficult. If the values are too low, then the efficiency and stability of the drive suffers.

Next: General melt temperature development in the metering section of an extruder.