Runners, sprues, flash, start-up waste and rejects can often be ground and fed back into production. Used well, regrind lowers material cost. Used carelessly, it causes colour variation, weak parts and complaints that are hard to trace. The difference is control.
1. Weigh it back into stock
Regrind should be a stock item like any raw material. When runners, flash or rejects are ground, weigh the regrind and book it into stock against the material and colour it came from. Without this, regrind is invisible: it disappears into the hopper and the scrap figure looks better than it is.
2. Keep it segregated
Regrind from different polymers, grades or colours must not mix. Store it in labelled bags or bins by material, grade and colour, and give each lot an identity, just like virgin material. Contaminated or mixed regrind should be sold or recycled, not fed back into the process.
3. Set the ratio in the recipe
How much regrind a product can take depends on the material, the part and the customer's requirements. Some applications allow none at all. Decide the maximum regrind percentage for each product with your quality team, write it into the recipe, and treat any change as a recipe change with approval.
4. Record it in every batch
When the blend is made, book the actual virgin resin, masterbatch and regrind weights against their lots. This gives you the actual regrind ratio per batch and makes traceability possible: if a complaint arrives, you can see whether the batch contained regrind and where that regrind came from.
5. Watch the trend
Rising regrind stock means rising waste upstream. Track regrind generated per machine and mould, and treat a sudden increase as a process problem to solve, not just a material to reuse.
Quick checklist
- Regrind weighed and booked into stock by material and colour
- Maximum regrind percentage written into each recipe
- Actual regrind weight recorded in every batch
- Regrind generated reviewed by machine and mould every week
Where regrind comes from
- Injection molding: runners and sprues from cold-runner moulds, start-up shots and rejects.
- Blow molding: flash trimmed from the neck and base, and rejected containers.
- Extrusion: start-up material, cut-offs and off-specification product.
- Film, tape and non-woven lines: edge trim and slitting trim, often recycled straight back into the line.
When not to use regrind
Some customers and applications do not allow regrind at all, or only from the same production run. Pharmaceutical and food-contact packaging, pressure pipes and safety-critical parts often have strict rules. Write the rule into each product's recipe so it cannot be ignored on the shop floor, and keep records that show it was followed.
Regrind and quality
Every time plastic is melted again its properties can change. Watch test results for products that use regrind, such as melt flow, strength and colour, and compare batches with different regrind percentages. If quality drops as the ratio rises, lower the maximum in the recipe.
Give regrind a cost
Regrind is cheaper than virgin material, but it is not free. Grinding uses energy and labour, and regrind stock ties up space. Value it at a sensible rate so that production costing shows the true cost of waste rather than hiding it.
Formezy returns runners, flash and rejects to regrind stock, records the regrind percentage in every batch and shows regrind generated by machine and mould. See injection molding ERP and recipe and formulation management.
