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Regrind Blend and Runner Share Calculator

Cold runner molds return part of every shot as sprue and runner. This calculator shows the runner share, how much regrind you generate each hour and whether your allowed blend ratio can use it, plus the effect on resin cost.

Your numbers

Part weight per cavity

Cavities

Runner weight per shot

Total sprue and runner weight for the shot. Use zero for a hot runner.

Cycle time

Allowed regrind in the blend

20 %

Virgin resin cost

per kg

Regrind value

per kg

Include grinding, handling and any quality allowance.

Your results

Runner share of shot

16.3%

Share of every shot that returns as regrind.

Surplus regrind per hour

Regrind that must be stored, sold or reduced at source.

Shot weight

Part weight times cavities plus runner, in grams.

Shots per hour

113

Regrind generated per hour

1.575

Kilograms per hour.

Regrind that can be used per hour

1.935

Kilograms per hour at the allowed blend.

Blended resin cost per kg

This estimate ignores startup scrap and rejects. Confirm the permitted regrind percent with your customer and the resin supplier before changing the blend.

Save results:

Surplus regrind per hour versus allowed blend

Sweeps Allowed regrind in the blend from 0 to 50, holding your other inputs fixed.

Model regrind in your ERP costing

Optional: discuss how runner and regrind flow into BOM and cost roll up.

Optional. The tool and every export work without this. No spam.

How to use the calculator

Enter the part weight per cavity, the cavity count and the total sprue and runner weight for one shot. Add the cycle time and the maximum regrind percent your customer or resin supplier allows. Enter the virgin cost and the value you assign to regrind. The results show runner share, regrind produced and usable per hour, the surplus and the blended resin cost.

Weigh a complete shot to get runner weight
Use zero runner for a hot runner mold
Set regrind value below virgin cost to reflect handling and risk
Use the chart to see how blend limit changes surplus

The formulas

Shot weight equals part weight times cavities plus runner weight. Runner share is runner weight divided by shot weight. Regrind generated per hour is runner weight times shots per hour. Regrind usable per hour is shot weight times shots per hour times the blend fraction. Surplus is generated minus usable, never below zero. Blended cost is the weighted average of virgin and regrind cost.

Reading the result

If the runner share is lower than the allowed blend percent, you can reabsorb all runner regrind on that job, and surplus is zero. If runner share is higher, surplus builds up and must be sold, used on another job or reduced by redesigning the runner or moving to a hot runner. Compare the saving from regrind with the cost of the hot runner investment before deciding.

Runner share below blend limit: no surplus
Runner share above blend limit: surplus accumulates
Consider a smaller runner or hot runner for large surplus

Common mistakes

Assuming the blend limit applies to every part is a risk, since medical, food contact, aerospace and cosmetic parts often restrict or forbid regrind. Another mistake is ignoring moisture and contamination, which can make regrind perform worse than the number suggests. Remember that grinding changes particle size distribution, and screen size affects feeding and melt consistency.

Frequently Asked Questions

What regrind percent is typical?

Many general purpose applications use 10 to 30 percent, but allowed levels depend on the resin, the part requirements and customer approval. Always confirm in writing for critical parts.

Does a hot runner remove regrind?

A hot runner mold has no cold runner, so no runner regrind is produced. Startup, purge and rejected parts still create some scrap.

How do I value regrind?

Use a fraction of virgin cost that covers grinding labor, handling and quality risk. Some plants use the full virgin cost if regrind is fully reused, while others use a lower figure.

Why is there surplus even at a high blend?

If runner share exceeds the allowed blend percent, you generate more regrind than the blend can use. The excess needs another outlet or a runner redesign.

If you want regrind and scrap accounted for in your standard cost, we can talk about how to model it in your ERP's BOM and routing.

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