Almost all processes require use of regrind. The amount of regrind consumed by a process depends largely on the process itself. In injection molding, for example, since a relatively minimal amount of scrap is generated in the first place, there isn’t much regrind used. The story is quite different in sheet extrusion-thermorforming operations, where the entire skeleton has to be fed back into the process; or in blow molding, which almost 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. The percentage of regrind also has an impact on the overall bulk density of the feedstock. Both of these parameters will affect the overall plasticating rate of the screw, whether it be extrusion or molding.
Best practices for reprocessing regrind hold that the overall bulk density be close to the bulk density of the virgin pellet, and that the size of the regrind be 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-in. or 10-mm 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 be “dust free.” This requires that the granulator knives are kept sharp, and that a good dust collection system be 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. Melt blocks occur because the fines will melt before the virgin pellets and regrind particles, potentially of causing severe after effects.
Tim Womer is a recognized authority in plastics processing and machinery with a career spanning more than 35 years. He has designed thousands of screws for all types of single-screw plasticating. He now runs his own consulting company, TWWomer & Associates LLC. Contact: (724) 355-3311; firstname.lastname@example.org; twwomer.com