Regrind particle size significantly influences bulk density in processes like extrusion, injection, and blow molding, with different processes generating varying amounts of regrind. The optimal practice is to match the bulk density and particle size of regrind to that of virgin pellets to enhance plasticating rates. Additionally, ensuring regrind is dust-free is crucial to avoid quality issues in the final product.

1. 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.