What regrind limit should I enter?

Enter the lower of the customer print limit and the resin datasheet limit. For UL work, UL 746D allows in-house runner and sprue regrind up to 25% of the hopper, with the rest the same virgin grade.

How much of my feed has been molded more than once?

At a steady regrind share r, a fraction r² of the feed has been molded at least twice before and r³ at least three times. At 25% regrind that is 6.25% and about 1.6%.

Can a print that allows only first-generation regrind run closed loop?

Not strictly. Runners molded from a blend already contain regrind, so grinding them again always builds some second-generation material. Keep first-pass runners separate, or send them to another job.

Which rejects are safe to regrind?

Clean rejects of the same grade and color, such as short shots, flash or sinks. Leave out burned or contaminated parts and floor sweepings: they carry the defect into every later shot.

Injection Molding calculator

Regrind Percentage Calculator

Find the regrind share a press reaches when every runner, sprue and clean reject goes back into its own hopper. You need shot weights from the scale, part and reject counts, and your regrind limit.

What this calculator does

Regrind share your press makes when runners and rejects go back in, checked against your print or datasheet limit.

Formula used

Shot weight = part weight per shot + runner and sprue weight per shot
Regrind per shot = runner and sprue weight + part weight per shot × rejects reground ÷ parts molded
Regrind share = regrind per shot ÷ shot weight × 100
Regrind to divert = MAX(0, regrind per shot − regrind limit ÷ 100 × shot weight)
Second-generation share = MIN(regrind share, regrind limit)² ÷ 100

Inputs explained

Part Weight per Shot: All parts from one shot, runner removed, weighed together.
Runner and Sprue Weight per Shot: Cold runner plus sprue per shot; 0 on a full hot runner.
Parts Molded: Good plus rejected parts over the counting period.
Rejects Reground: Rejected parts from that period that go to the granulator.
Regrind Limit: Maximum regrind on the customer print or resin datasheet.

How to use the result

Best suited to closing the loop on a cold runner tool, setting a regrind blender for a new job, answering a customer audit on regrind content.
Weight share only: property loss depends on resin, drying and generations, and needs part testing. Granulator fines and dust are not subtracted, so regrind per shot reads slightly high.

Current U.S. benchmarks

The producer price index for plastic resins and materials stands at 280.569 (BLS, Aug 2026), up 6.6% from a year earlier. Quotes priced off last quarter's material cost miss this move.
The U.S. has 9,635 plastics product manufacturing establishments employing about 677,302 workers (Census County Business Patterns, 2023).

Common questions

What regrind limit should I enter? Enter the lower of the customer print limit and the resin datasheet limit. For UL work, UL 746D allows in-house runner and sprue regrind up to 25% of the hopper, with the rest the same virgin grade.
How much of my feed has been molded more than once? At a steady regrind share r, a fraction r² of the feed has been molded at least twice before and r³ at least three times. At 25% regrind that is 6.25% and about 1.6%.
Can a print that allows only first-generation regrind run closed loop? Not strictly. Runners molded from a blend already contain regrind, so grinding them again always builds some second-generation material. Keep first-pass runners separate, or send them to another job.
Which rejects are safe to regrind? Clean rejects of the same grade and color, such as short shots, flash or sinks. Leave out burned or contaminated parts and floor sweepings: they carry the defect into every later shot.

Last reviewed 2026-10-01.