Regrind and recycled-material management: how much should be blended in?

Using regrind reduces cost, but every pass fatigues the polymer a little. Managed incorrectly, it brings inconsistent shrinkage, brittleness and color problems.

What happens with every pass?

With every thermal and mechanical processing step (grinding, drying, melting, injection), plastic loses some molecular weight (chains shorten). How much it loses varies a lot by material:

• PP, PE: Generally tolerate a few cycles, but viscosity and impact strength drift slowly.
• PC, PET, PA, PBT: Sensitive to moisture and heat; damp regrind undergoes hydrolysis and degrades quickly. Drying and ratio control are critical.
• Fiber-filled grades: Fiber length shortens during grinding, reducing strength.
• POM, PVC: Degrade at excessive heat; temperature and residence time matter.

Control points

1. Cleanliness: Regrind must be free of dust, fines and foreign matter. Use screens and magnetic/metal separators.
2. Moisture: Dry regrind under the same conditions as virgin material; fine dust absorbs moisture faster.
3. Blend ratio: Keep the ratio consistent and limited. For certified-quality or critical parts, follow the customer specification; many applications discuss an upper limit (e.g. 10–25%), but this depends on the customer and quality system.
4. Homogeneity of the blend: Do the ratio by dosing (gravimetric) and in a blending unit; scooping it into a bucket by hand makes the ratio fluctuate.
5. Particle size: Grains that are too coarse or too fine disrupt dosing accuracy and feed stability.
6. Traceability: Record the regrind ratio, source and number of passes for every batch.

Where it's not suitable

• For parts requiring safety and full product specifications (e.g. food contact, medical, automotive safety), regrind use is generally limited or forbidden; check customer and regulatory requirements.
• On optical, transparent and very thin-wall parts, even small degradation becomes visible.

A hidden cost: part fluctuation

Fluctuation in the regrind ratio changes viscosity, and therefore filling and holding behavior. If your process is set for a certain regrind level and scrap increases when the ratio changes, this is the reason.

Note: Ratios and limits are determined by your product, specification and the material manufacturer's recommendation. This article is a general framework.

See the defect guide for details on silver streaks, brittleness and color problems that come up when using regrind.