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
Total sprue and runner weight for the shot. Use zero for a hot runner.
Include grinding, handling and any quality allowance.
Your results
This estimate ignores startup scrap and rejects. Confirm the permitted regrind percent with your customer and the resin supplier before changing the blend.
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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