Regrind and Recycled Content in Injection Molding: How Much Is Too Much

Regrind is not automatically a defect

Regrind, granulated sprues, runners, and rejected parts fed back into the process, is standard practice in injection molding, not a corner-cutting shortcut. Virgin resin producers themselves expect a portion of scrap to be reprocessed; it's built into how the material is priced and specified. The question isn't whether to use regrind, it's how much a given part can tolerate before its physical properties start to slip.

Why regrind ratio has a ceiling

Every heat cycle through a barrel degrades polymer chain length slightly, thermal and mechanical stress during melting and injection cause chain scission, which shows up as reduced impact strength, increased brittleness, and inconsistent melt flow. For commodity resins like HDPE and PP, 20 to 25 percent regrind blended with virgin material is a common, safe ceiling for most non-structural packaging parts. Push past 30 to 40 percent, especially with resin that has already been reground once, and mechanical properties degrade in ways that show up as field failures, not just lab numbers.

Where regrind ratio actually matters

Cosmetic closures and thin-wall parts under continuous flex stress, snap-fit hinges, trigger sprayers, are the least tolerant of high regrind content because they depend on consistent elongation and impact resistance. Thicker structural parts, like bottle bases or jar walls, tolerate higher regrind ratios because they have more material cross-section to absorb property variation. Color-critical parts are a separate problem: regrind carries pigment history from whatever it was molded from before, which is why color-matched programs typically cap regrind at a lower percentage or require dedicated, single-color regrind streams.

What to ask a supplier about their regrind practice

Ask what percentage regrind a quote assumes, whether it is virgin-plus-regrind blended in-house or a purchased regrind stream from an unknown source, and whether that ratio is documented per job or informally eyeballed on the floor. A supplier who can't answer specifically is not controlling for it, which means your part's mechanical properties are drifting between production runs even if nothing else about the process changed.

What we run

Our regrind ratios are set per part, per resin, and documented in the process sheet, not left to floor discretion. Structural parts run higher regrind ratios where the mechanical margin supports it; closures, hinges, and color-critical cosmetic parts run tighter limits or virgin-only. For how resin selection affects part performance more broadly, see PP vs. HDPE vs. PET: Choosing the Right Resin.

Learn more about our sustainability and PCR resin program.