Regrinding Plastics

With today’s upward spiral of resin prices, it has become ever more important to optimize resin usage. A significant part of optimizing resin usage is proper use of regrind. Regrind is material that has undergone at least one processing method such as molding or extrusion and the subsequent sprue, runners, flash, rejected parts etc are ground or chopped. Both the heat history of processing and the grinding may lead to degraded physical, chemical and flow properties for the thermoplastic resin and subsequent parts made from this regrind. Normally it is considered that the heat history is most detrimental as stabilizers and antioxidants get consumed.

For perspective, note that even virgin resin may have one or more heat histories as it must be compounded or extruded into pellets when made. Virgin resins can have additional heat history as additives such as color, fillers (fibers, talc, mica, etc) or a host of additives may require a second compounding step. This may be done by the resin supplier or at a separate compounding facility. This is not considered regrind but it is interesting to note that virgin resin right out of the bag, box or rail-car can already have a “Heat History”.

To develop a strategy and optimize the value of regrind it is important to know the issues involved. Appropriate shop floor procedures and discipline must be in place to avoid potentially catastrophic results. The list of issues is significant and there are subtleties under each.

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