Regrind Particle Size

Almost all processes, extrusion, injection and blow molding require that regrind be used in the process. Depending on the process greatly determines the amount of regrind that is generated. Some processes produce very minimal amounts of regrind, such as an injection molding process using a hot runner system versus a flat sheet extrusion process producing thermoformed sheet and the entire skeleton has to be fed back into the process or a blow molding process which always generates large amounts of regrind.

The particle size of the regrind has a direct effect on the bulk density of the material being blended with the virgin pellets. Also the percentage of regrind also has an effect on the overall bulk density of the feed stock. Both of these parameters will affect the overall plasticating rate of the screw, whether it be extrusion or injection molding.

The best overall practice for reprocessing regrind is that the overall bulk density if as close to the bulk density of the virgin pellet and the size of the regrind is also approximately the same size as a pellet. Typically, if the screens in the granulator that is being used to produce the regrind are 3/8” (.375”) or 10mm in diameter the particle size of the regrind will be close to the size and bulk density of the virgin pellet.

Finally, the other important issue in using regrind in the plasticating process is that the regrind is “dust free”. This requires that the granulator’s knives are sharp and a good dust collection system is part of the process. “Fines” in the regrind will cause problems such as black specks in the finish part, loss in rate, and melt blocks because the “fines” will melt before the virgin pellets and regrind particles and in turn have a potential of causing severe after affects.