Twin Screw Compounding Extruders - Torque and Shear Simplified

Every extruder has a maximum torque rating per shaft. When the machine is running, torque is usually displayed as percentage of maximum allowable at that revolutions per minute (RPM). This indicated percent Torque is defined as:
Indicated % Torque α Power Input ÷ Screw RPM

This means that rest being the same, as a percentage of maximum, higher power means more % torque on the screw shafts whereas higher RPM means less % torque on the screw shafts. At high RPM, the extruder is capable of accepting high power input and hence higher feed rate. Conversely, at lower RPM, the extruder cannot use the full installed motor power (there may be some exceptions).

Shear Rate: Another important consideration is Shear Rate which is defined by the following equation:
Shear Rate = (3.142 X Screw dia X Screw RPM) / (Actual Flight Depth X 60)

The “Actual Flight Depth” is NOT the mechanical channel depth of the screw flights but the actual depth of the material that fills up this mechanical depth. A completely filled screw will have an actual flight depth almost equal to the mechanical flight depth while a partially filled screw will have an actual depth significantly lower than the mechanical flight depth. So, at a given RPM, actual flight depth depends upon the feed rate.

Since Pi, Screw diameter and 60 are constants for a given extruder, from the equation above we have the following conclusions:
- Shear Rate is directly proportional to Screw RPM
- Shear Rate is inversely proportional to Actual Flight Depth and Feed Rate

This reminds me of my early days in compounding when I thought that to increase shear, one must increase motor amperes by increasing feed rate. This “logical” thinking was probably due to the feel-good factor of putting in “more power” into the extruder which was assumed to improve mixing. Now, after understanding the relationship between screw RPM, feed rate and shear rate, I know that it was a wrong assumption.

IN CONCLUSION, perhaps torque is the MOST important and the most neglected control parameter. Used with expertise, Torque is the one parameter that can give the most complete indication of process health so much so that if the same RPM and feed rate results into a similar % torque reading then one can rest assured that the product is good and no quality checks may be needed. I believe that running a compounding process without proper torque display and control is like driving a car blindfolded.

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Dr. Pradeep Bakshi
Consultant – Plastics Technology
Trainer - Polymerupdate Academy