Separate the Types of Loss
Lumping all waste into scrap hides the fix. Startup and purge are process losses tied to changeovers. Runner and sprue material in cold runner tools is planned loss that can usually be reground.
Rejected parts have causes such as short shots, flash, sink, splay, black specks or dimensional failures. Quality holds are inventory, not scrap, until dispositioned. Each category has a different owner and a different remedy, so the ERP needs distinct transaction types or reason codes for each.
- Startup and purge by job and changeover
- Runner and sprue weight per shot, by tool
- Reject reason codes aligned with the plant's defect list
- Quality hold versus final scrap disposition
Reconcile Resin Issued Versus Output
Material reconciliation is the control that catches hidden loss. For each job compare resin issued (or backflushed) to good parts times part weight, plus runner weight, plus scrap weight, plus regrind returned. A gap beyond tolerance means unrecorded scrap, wrong part weight in the item master, overpacking, or drying and handling loss.
Many plants backflush resin from the BOM at standard weight and never find out that actual weight is 3 percent higher. Weigh a sample of shots and update the standard when the process changes.
Regrind Rules and Costing
Regrind use is limited by customer specification, resin type and part criticality. Many commodity applications allow 10 to 25 percent regrind, while medical, optical and some automotive programs prohibit it or limit it tightly. Track regrind by resin lot family, with blend percentage recorded per job and storage kept clean and dry.
For costing, choose a policy: value regrind at a percentage of virgin cost, or credit it back at cost with the quote assuming recovery. Whatever you choose, document it, because the choice changes margin on every cold runner job.
- Blend limit per part number from customer specification
- Regrind lot identification and dryness control
- Credit regrind consistently in job cost
- Never use regrind on parts where the quality agreement prohibits it
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Capture at the Press, Not at End of Shift
Scrap reported by the operator at the end of the shift is partial and late. Capture rejects at the press with a touchscreen or scan, by reason code, and let weight-based scales on regrind and purge bins feed totals. A real-time view lets supervisors react during the run, for example when short shots spike after a resin lot change.
In DELMIAworks, Plex, Epicor or SyteLine, check whether production reporting supports reason codes by job and machine, or whether you need an MES layer.
Where AI Helps
A private assistant can answer questions like which three reject reasons cost the most last month on glass-filled nylon jobs, which jobs show a material reconciliation gap over 4 percent, and how scrap changed after a resin supplier switch. It reads ERP and MES data without write access by default. It cannot fix data capture, so a plant where operators skip reason codes will get confident but wrong answers.
When Detailed Tracking Is Excessive
On a low scrap, high margin program with a stable process, per-shot reconciliation adds cost without change in decisions. Focus detailed tracking on high volume and high resin spend jobs, where one point of scrap equals real money, and on regulated programs where traceability is mandatory.
Frequently asked questions
What is an acceptable scrap rate in injection molding?
It depends on part complexity and program. Simple high volume parts may target 1 to 2 percent, while complex, cosmetic or medical parts commonly run higher. Track by job and reason so you can compare like with like.
How do you cost regrind in injection molding?
Either value regrind at a fixed percentage of virgin resin cost or credit it at the cost of the resin it replaces. The key is a consistent documented policy, matched to how you quote runner and scrap allowances.
How much regrind can be used in injection molding?
Commonly up to 10 to 25 percent for non-critical parts in many resins, but it is limited by the material supplier guidance, customer specifications and part function. Some programs forbid regrind entirely.
If scrap shows up as one monthly percentage and nobody can say why, we can help structure reason-code capture and resin reconciliation from your existing ERP and press data.