Material Preparation
Regrind is material obtained by grinding runners, flash and defective parts in a granulator and reused by mixing it into new raw material at a certain ratio. It turns scrap into value; but with each thermal cycle the polymer chain shortens a little, the bulk density falls and the particle size changes. That is why the ratio must be controlled, dosing must be gravimetric and metal separation is essential.
The bulk density of regrind is lower and more irregular than that of granulate; that is why volume-based (volumetric) dosing cannot hold the ratio in a regrind mix. Weight-based gravimetric dosing keeps the ratio constant even if the density changes.
During granulating, metal particles can get in from blade and screen wear; on a regrind line a magnetic separator is the insurance for the screw and the mold. The ratio is usually kept in the 10–30% band; it may be lower for technical parts and higher for packaging.
It depends on the material and the requirements of the part. For technical parts where strength is critical 10–15% is common, and for general-purpose and packaging parts 25–30%. The ratio should be verified by testing the part and held constant with a gravimetric doser.
Yes, and more carefully: granulated material has a large surface area and takes up moisture quickly. With hygroscopic material, regrind goes through the same drying regime as the granulate.