Regrind comes from recycled runners, sprues, and rejected parts. It reduces material cost and waste, making it a common part of many molding operations.
That reuse changes how the material behaves. Each time the polymer is processed, its structure shifts slightly.
During regrind, the material is exposed to heat, shear, and air multiple times.
Polymer chains break down, reducing molecular weight. Oxidation can occur during grinding or storage. Additives may degrade or evaporate. Contamination can also be introduced if handling isn’t controlled.
These effects accumulate, changing both flow behavior and final properties.
As molecular chains shorten, the material becomes less robust.
Tensile strength and impact resistance drop as the polymer loses its ability to absorb stress. The material becomes more brittle and less durable under load.
Dimensional stability can also shift as viscosity changes, affecting how the material fills and shrinks.
You end up with a process that may require adjustment just to maintain the same part performance.
Regrind typically increases melt flow.
Lower molecular weight reduces viscosity, allowing the material to flow more easily. This can alter how the cavity fills and how pressure builds during packing.
That shift changes the process window. Settings that worked with virgin material may no longer produce the same result.
Regrind can also affect how parts look.
Color variation can occur from mixed batches or pigment breakdown. Black specks may appear from degraded material or contamination. Surface finish may lose gloss or show flow lines due to inconsistent melt behavior.
For cosmetic parts, these differences are often unacceptable without strict control.
Regrind is more exposed to the environment than virgin resin.
Hygroscopic materials can absorb moisture during grinding and storage. If not dried properly, this leads to additional degradation during processing.
This compounds the effects already introduced by thermal history.
Consistency depends on how regrind is managed.
Limiting the percentage used keeps material properties within a predictable range. Blending must be uniform to avoid shot-to-shot variation. Material sources should be tracked to prevent contamination or mixing of incompatible batches.
Without this control, variation increases quickly.
Regrind use should be tested, not assumed.
DOE studies can compare different regrind ratios against part performance, including strength, dimensions, and process stability. Monitoring viscosity or melt flow helps detect changes between batches.
This turns regrind from a variable into a controlled input.
Regrind is treated as a defined material condition.
Acceptable ratios are established through testing, documented, and tracked through production. Each batch is linked to source material and process data, ensuring traceability.
That structure allows cost savings without introducing uncontrolled variation into the process.