Yield, scrap and the real cost of a moulded part
A 20 litre container that should weigh 820 g and runs at 845 g costs 3% more in material on every single piece. Here is how to see it while it is happening.
The standard cost of a moulded part is easy to write down. Weight per piece, multiplied by the resin rate, plus masterbatch, plus a share of machine hours and power. The actual cost is harder, because it depends on what happened at the machine on that shift.
Where the difference hides
- Weight drift: parts running heavier than the standard weight
- Rejects: pieces that fail inspection and go to regrind
- Start up loss: material purged at every mould change and restart
- Regrind ratio: how much ground material goes back in, and at what value
- Changeovers: hours a machine is not producing while a mould is changed
None of these are visible in a monthly consumption figure. Together they can move the real material cost of a part by 4 to 8 percent.
Measure at the job card
The only place all five can be captured is the job card. For each batch or shift, record material issued, good pieces, rejected pieces and kilograms sent to regrind. From those four numbers, yield and actual weight per good piece calculate themselves.
For example: 984 kg issued, 1,096 good pieces, 38 rejected, 21 kg to regrind. The actual material per good piece is (984 - 21) / 1,096, or about 879 g, against a standard of 820 g. That gap is worth looking into before the next batch, not at the year end stock count.
Treat regrind as stock
Regrind is often thrown back into the hopper without being counted. That makes yield look better than it is, and it hides how much virgin resin you are really buying per piece. Recording regrind as its own item, returned to stock at a set value, keeps both the yield and the stock honest.
Sequence to save changeovers
Two orders on the same mould, run back to back, save a changeover. Two orders in the same colour, run light to dark, save a purge. These choices are made by the planner every day, usually from memory. When orders, moulds and colours are in one system, the sequence can be suggested, and the hours saved can be measured.
What to try this month
Pick your highest volume part. For one week, weigh ten pieces at the start of every shift and record the average next to the standard weight. If the average is more than 2 percent above standard, you have found material cost that no purchase negotiation will recover.