PIM feedstock tips

PIM molders are discovering that, where volume warrants, modern plastics compounding technology is tough to beat for feedstocks.

Researchers at Arburg’s PIM development lab believe that advanced compounding systems—such as a shear roller compactor, a twin-screw extruder, or a continuous kneading extruder—better homogenize binders and powders by breaking down powder agglomerates. Arburg has also found such technology can reduce agglomeration caused by regrind dust. Arburg engineers suspect that agglomerates are a major cause of viscosity and filling problems in PIM; variations of less than .1 percent by weight influence material flow.

Thermat Precision Technology Inc. uses twin-screw, controlled-atmosphere reaction compounding systems to prepare its proprietary feedstocks for pelletization. Thermat sources say they need such process sophistication to ensure a thin but uniform coating of metal powders by their multicomponent binder system. All of the compounders were customized to Thermat specs and built by Readco.

Twin-screw tips

William Thiele, general manager of American Leistritz Extruder Corp. (Somerville, NJ) offers advice on improving compounding and pelletizing performance:

Feeding all binders and powders together in the main feed port of a twin-screw extruder can cause high specific energy, low output rates, quality problems, and high machine wear. Feed powders downstream. When large powder masses are added downstream to the binder system, excess friction sometimes occurs due to binder freeze-off. Preheating the powder can solve this problem, allowing for more efficient processing, and sometimes eliminating the need for a side-stuffer.Die-face hot pelletizing is generally preferred over trying to cut cooled metal-hard strands. Yet, some strand pelletizing may be done, particularly when the binder system is still soft at the point of cutting.