Runners, sprues, and rejected parts contain the same material that went into good parts. Throwing them away throws away profit. Yet regrind management is surprisingly neglected in many operations, treated as an afterthought rather than a process requiring the same rigor as virgin material handling. The difference between profitable regrind programs and problematic ones comes down to understanding what reprocessing does to material properties and implementing systems that maintain quality while capturing value.
What Regrind Is
Regrind consists of runners, sprues, start-up scrap, and rejected parts that have been granulated for reuse. This material has already experienced at least one complete molding cycle: melting, shearing through the injection system, and cooling. When granulated and blended with virgin material, it enters the process again.
The critical distinction separates regrind from post-consumer recycled (PCR) material. Regrind has known history: you know exactly what polymer it is, what additives it contains, and how many times it has been processed. PCR comes from external sources with unknown history, potential contamination, and unpredictable properties. The closed-loop concept applies to regrind: material cycles within your facility under your control, enabling quality management that external recycled streams cannot match.
Regrind represents immediate cost recovery. A mold with a cold runner system might generate 15 to 25 percent of shot weight as runner scrap. Without regrind use, that material becomes waste. With proper regrind management, most of that material returns to production as saleable parts.
Why Ratios Matter
Each heat history degrades the polymer. High temperatures and mechanical shear during plastication break polymer chains into shorter fragments. Shorter chains mean lower molecular weight, which affects viscosity, impact strength, and other properties. This degradation accumulates with each processing cycle.
The degradation rate varies by material. Some polymers tolerate multiple heat histories with minimal property loss; others degrade significantly with each cycle.
| Material | Degradation Sensitivity | Typical Maximum Regrind |
|---|---|---|
| ABS | Low | 25-30% |
| Polycarbonate | Medium | 15-25% |
| Polypropylene | Low | 20-30% |
| HDPE | Low | 20-25% |
| Nylon (dry) | Medium-High | 10-20% |
| Acetal (POM) | High | 10-15% |
| PET | High | 10-15% |
Higher regrind percentages increase the average number of heat histories in the blend. At 25 percent regrind addition, material recirculates: some fraction of today’s regrind contains material that was regrind yesterday, and last week. Over time, the blend reaches equilibrium where some molecules have experienced many cycles while fresh virgin material enters continuously.
This equilibrium effect means steady-state regrind programs actually contain more degraded material than the nominal percentage suggests. A 20 percent regrind ratio eventually produces blends where some portion has seen five or more heat cycles, even though fresh virgin material dominates.
Establishing the Right Ratio
Determining the appropriate regrind ratio requires balancing material recovery against property retention. The right ratio depends on your specific material, application requirements, and quality tolerance.
Testing approach: Run controlled trials at increasing regrind percentages. Start at 10 percent and increase in 5 percent increments. At each level, measure the properties that matter for your application: typically impact strength, tensile strength, melt flow index, and color consistency. Plot property retention against regrind percentage. Identify the knee point where properties begin declining unacceptably.
Property retention requirements vary by application. Structural parts requiring consistent impact strength tolerate less regrind than non-critical components. Cosmetic parts sensitive to color consistency may limit regrind more than technical parts where appearance is secondary.
Conservative starting points: When test data isn’t available, industry conventions provide guidance. Most commodity materials tolerate 15 to 25 percent regrind without significant property concerns. Engineering resins typically require 10 to 20 percent limits. Materials known for degradation sensitivity (POM, PET, moisture-sensitive nylons) should start at 10 percent maximum until testing confirms higher ratios are acceptable.
Some operations run higher regrind ratios successfully by using first-pass regrind only (material that has seen exactly one previous heat history) rather than recirculating regrind indefinitely. This approach limits degradation but requires more sophisticated material handling.
Quality Control Systems
Regrind quality depends on systems that prevent contamination and ensure blend consistency.
Material segregation prevents the most damaging contamination: mixing different polymers. Even small amounts of incompatible material (under 1 percent) cause defects, weak points, and inconsistent properties. Dedicated granulators for each material family, clear labeling on regrind containers, and physical separation in storage areas prevent cross-contamination. Color-coded containers by material type add visual confirmation.
Granulator maintenance affects regrind quality directly. Dull blades tear rather than cut, creating dust (fines) and irregular particle sizes. Fines melt faster than larger particles, potentially degrading before bulk material reaches temperature. Regular blade sharpening (typically every 40 to 80 hours of operation) maintains clean cuts. Screen condition determines particle size consistency; damaged screens produce oversized particles that feed inconsistently.
First-in-first-out material handling prevents aged regrind from accumulating. Regrind sitting in containers absorbs moisture (for hygroscopic materials), collects dust, and may degrade. Using oldest regrind first maintains fresher average material age.
Blend consistency requires accurate metering. Volumetric feeders estimate blend ratios based on bulk density, which varies between virgin pellets and irregularly shaped regrind. Gravimetric (weight-based) feeders provide accurate ratios regardless of density differences. For critical applications, gravimetric blending justifies the equipment premium.
Calculating Regrind Generation
Understanding how much regrind your process generates helps size equipment and predict material flow.
Runner-to-part ratio determines base regrind generation. Calculate: (shot weight minus part weight) divided by part weight. A 100-gram shot producing a 75-gram part generates 25 grams of runner, or 33 percent of part weight.
Runner design impact: Larger runners, longer flow paths, and cold sprue bushings increase regrind generation. Optimizing runner sizing reduces waste. Three-plate molds with automatic degating often have larger runner systems than two-plate molds, generating more regrind despite the automation benefit.
Hot runner systems dramatically reduce regrind generation by maintaining runners in molten state. The material in hot runner manifolds stays there, eliminating runner scrap entirely. Hot runners cost more initially but eliminate ongoing regrind handling for high-volume applications.
Startup and purge scrap adds to regrind volume, especially for operations with frequent changeovers. Tracking this separately helps identify improvement opportunities in setup procedures.
Rejected parts contribute to regrind if defects don’t affect material integrity. Parts rejected for dimensional issues or surface defects can be reground. Parts rejected for degradation (burning, brown streaks) should not enter regrind streams because the damage would propagate.
When Regrind Doesn’t Work
Certain applications and materials don’t tolerate regrind, regardless of ratio.
Medical applications often require virgin material documentation for regulatory compliance. Traceability requirements may preclude regrind use even when properties would be acceptable. Material certifications typically cover virgin material only.
Optical applications requiring clarity suffer from regrind’s typical property degradation. Small amounts of degraded material create haze or specks visible in transparent parts.
Materials that degrade quickly may not tolerate any regrind. Acetal (POM) releases formaldehyde during degradation, creating both part quality and safety concerns. Some nylon grades, especially when not perfectly dry, degrade rapidly. PET crystallizes during reprocessing, changing properties significantly.
High-cosmetic parts with strict color matching requirements may show regrind effects as color shift or inconsistency. Heat history affects colorant dispersion and can cause yellowing in some systems.
Contamination-sensitive applications where even small foreign particles cause functional problems should avoid regrind. The granulation process can introduce metal particles from blade wear, dust from the environment, or trace contamination from previous materials processed in shared equipment.
When regrind cannot be used in primary production, alternatives exist: selling to recyclers, using in less-demanding internal applications, or building inventory for future product lines with lower requirements.
Regrind management is basic manufacturing efficiency. Doing it well reduces material costs while maintaining quality. Doing it poorly creates quality problems that cost more than the material saved. The investment in proper handling systems, quality controls, and process discipline pays back through lower material costs and consistent part quality.
Sources
- Rosato, Donald V. “Plastics Processing Data Handbook.” Springer.
- Society of Plastics Engineers. “Regrind Management Guidelines.”
- Plastics Technology. “Best Practices for Regrind Use.” https://www.ptonline.com/
- Material Supplier Technical Bulletins on Reprocessing Limits.