Scrap is unavoidable in every injection molding plant: runners, setup shots, defective parts. There is a serious difference in profit between selling them to a scrap dealer by the kilo and granulating them and returning them to production in a controlled way. The difference is hidden in the word "controlled".
What Is Regrind and Why Is It Valuable?
Regrind is production scrap reduced to granule size in a granulator. It is the same polymer as the virgin raw material; if processed correctly it can be returned to production at a ratio of 10–30%. Given the level of raw material prices, recovering the annual scrap tonnage goes straight to profit.
However, regrind does not behave exactly like virgin granulate — three differences must be managed:
1. Thermal history: The material has already been through an extrusion/injection cycle once; its molecular weight has fallen somewhat. As the regrind ratio increases, the mechanical properties decline progressively 2. Grain structure: Granulated particles are irregular — bulk density varies and there is dust 3. Contamination risk: Metal, mixing of different polymers, moisture
Granulating Strategy: Beside the Press or Central?
Beside-the-press granulator: The runner goes to the granulator at once, by robot or by hand; the regrind is fresh and of a single material, and the risk of mixing is minimal. Low-speed granulators produce little dust and are quiet — the ideal way to recover clean runners.
Central granulating: Powerful central granulators are used for large, thick parts and high-volume scrap. Flexibility is high but material separation discipline is essential: if PA scrap and PP scrap go through the same granulator, the materials cross-contaminate without a cleaning procedure.
The golden rule: separate scrap by type, label it and store it in closed containers. Floor contamination and mixed-up material are the biggest quality enemies of regrind.
Metal Risk: The Granulator Is Where the Risk Is Born
Granulator blades wear, and inserts overlooked in the scrap meet the blades — regrind is the number one source of metal contamination. A drawer separator at the granulator outlet and a hopper magnet at the machine inlet: these two points are non-negotiable.
Moisture: Regrind Takes Up Moisture Faster
The surface area of a granulated particle is many times that of a granule — in hygroscopic materials (PA, PC, ABS, PET) regrind absorbs moisture faster. In recipes that use regrind, drying cannot be skipped; regrind must go through the same drying regime as the virgin material.
The Right Ratio and Consistent Dosing
The regrind ratio is determined by how critical the part is: 20–30% for non-visible technical parts, 10–15% for aesthetically/mechanically critical parts, and zero for safety parts without customer approval. Whatever the ratio, it must be constant — regrind added at 10% today and 35% tomorrow disrupts process consistency.
This is where dosing technology comes in: because of its irregular grain structure, regrind is the material with which volume-based (volumetric) systems are weakest. Gravimetric dosing adds regrind by weighing it, so the ratio is the same in every batch — the GRAVIMIX models with a regrind inlet are designed exactly for this. A detailed comparison is in our dosing guide.
Summary Checklist
1. Separate scrap by type, store it closed and labeled 2. Metal separation at the granulator outlet + machine inlet 3. Dry hygroscopic regrind under the same regime as the virgin material 4. Set the ratio according to part criticality and hold it constant with gravimetric dosing 5. Put the first samples of a recipe with regrind through mechanical testing
For a granulator capacity, separator and dosing combination suited to your scrap volume, get in touch with us.