Kaivanya Extrusion – Strategies for Solving Process Problems in Twin-Screw Compounding

When investigating compound quality defects or determining the root cause(s) of processing problems, look for spatial and/or temporal patterns to provide clues. The strategy to identify an assignable cause and cure is to first determine whether the problem is chronic or transient.

You can draw a parallel between the diagnosis of twin-screw compounding extruders and of the human anatomy. When you go to a physician to diagnose your ailment, the first question is designed to lead the investigation in one of two directions: “How long have you had this problem?” If you respond that the problem started when you were a child, this will lead the doctors down the “disease” pathway, eliminating the possibility that this is a temporary situation. If you reply that the problem started yesterday, this will limit the investigation to your activities within the past 48 hr only.

Chronic compounding problems are like diseases—they are always present, and the cure requires extensive treatment, while transient problems can be thought of as “illnesses.” An illness is temporary, and the treatment is relatively simple (e.g., take some medication and rest for 24 hr).

We identify problems in compounding as “chronic” if the results are the same every time a particular formulation is run—every lot produces the same results: poor physical properties, black specks, strands dropping, die holes freezing, vent ports fouling, etc. In these cases, the problem cannot be the result of a particular lot of raw materials, worn extruder components, or any other temporary condition. The symptoms of chronic problems suggest that the process itself is the problem: improper screw Manufacturer/die design and/or operating conditions. For these problems, the treatment requires redesigning the screw configuration or modifying the compounding process. This is the only way to cure such problems.

We identify problems in compounding as “chronic” if the results are the same every time a particular formulation is run.

On the other hand, if a particular compound has been produced successfully in the past, and today it runs differently—this is the symptom of a temporary illness. Now we must determine why the current condition exists and whether it is causing compound quality problems or processing problems. Illnesses are most commonly associated with specific lots of raw materials, mechanical failures (burned-out heater, leaky solenoid valve, etc.), environmental conditions, operator error, or machine wear—all of which appear over different time periods.

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