Regrind percentage injection molding decisions usually start with a simple question: how much ground sprue, runner and reject material can go back into the hopper before parts stop meeting specification? There is no single answer, because the safe limit depends on the resin, the number of heat histories the regrind has already seen, and what the part has to survive in service. This guide sets out starting ratios by material, the property losses to expect, and the controls that keep a regrind programme honest.
Why regrind percentage changes part properties
Every pass through the barrel shears and heats the polymer, and chain scission gradually reduces average molecular weight. The regrind ratio property loss that follows is predictable: impact strength and elongation at break fall first, then tensile strength and stiffness, while melt flow rises because the chains are shorter. The effect is milder in commodity polyolefins and severe in condensation polymers such as PC, PET and nylon, where moisture during reprocessing drives hydrolytic degradation on top of thermal damage. Regrind also differs from post-consumer recycled material: it is same-grade, in-house material with a known history, which is why it can be fed back at far higher ratios than PCR.
Starting regrind ratios by resin
The table below lists common starting points for clean, dry, same-grade regrind in non-critical parts. Treat each same grade regrind limit as a qualification starting point rather than a specification. Regulated end uses such as medical, automotive safety and food contact frequently cap or prohibit regrind regardless of these figures, and UL-recognised components typically follow the UL 746D allowance of up to 25% in-house regrind of the same grade.
| Resin | Common starting maximum | What degrades first |
|---|---|---|
| PP | 20-30% | Melt flow rises; impact and natural colour |
| HDPE | 20-30% | Stress-crack resistance, gels, colour |
| ABS | 15-25% | Impact, then gloss and colour |
| PC | 10-20% | Impact, yellowness, clarity |
| PA6 / PA66 | 15-25% | Elongation and impact; hydrolysis if wet |
| POM | 15-25% | Thermal degradation, colour, specks |
| PET | 15-25% | Intrinsic viscosity, clarity, brittleness |
| Glass-filled grades | 15-25% | Fibre length, so strength and stiffness |
Track generations, not just percentages
A blend labelled 20% regrind behaves very differently depending on whether that fraction has been through the machine once or five times. Because regrind is fed back continuously, a steady-state programme reaches an equilibrium in which some molecules have seen several heat cycles even though virgin material dominates the blend. The practical response is to label regrind by generation, favour first-pass material for critical parts, and treat any burnt or overheated scrap as waste rather than feed.
Process and drying adjustments
Regrind needs more aggressive drying than virgin pellets, especially for PC, nylon and PBT, and its melt flow is higher and less consistent from lot to lot. Gravimetric blenders keep the ratio honest through the run; melt temperature may need a small increase while screw speed and injection speed come down slightly to limit further shear. Filtration is matched to the application: 40-60 mesh for non-visible parts, 60-100 for visible surfaces and finer stages for regulated parts. Colour programmes should expect gradual yellowing in natural grades and qualify the shift against a delta E limit before approving the ratio.
Quality control for a regrind programme
Melt flow index regrind testing on incoming lots is the cheapest early warning of degradation, because flow rises long before parts fail in service. Property verification matters most when a ratio is raised or a new regrind source is introduced: tensile, flexural and impact testing on parts moulded with the qualified blend confirms the targets still hold. Segregation is equally important, since under 1% of incompatible polymer in a granulator stream creates weak points and specks that no process adjustment can hide. Drying discipline connects directly to our resin drying guide, and dimensional problems from variable shrinkage are covered in the warpage causes guide.
How to set your own regrind percentage injection molding limit
Start at 10% and step up in 5% increments, measuring the properties that matter for the application at each level, then identify the knee where retention falls away. Keep documentation of source, lot and generation for every batch, and let the part's duty decide: non-visible industrial components tolerate generous ratios, while structural, cosmetic and regulated parts do not. Applied sensibly, regrind percentage injection molding turns scrap into margin without quietly trading away the properties the part was specified for. Browse resin options in the plastic materials category and verified suppliers in the company directory.
Related product categories
The materials and part types discussed above are grouped in our sourcing directory:
