Here’s a number that should bother any plant manager: requisition 1,000 pounds of resin to ship 1,000 pounds of product, and that’s the goal — but at the end of a real run, production has often drawn 1,200 pounds to ship those 1,000. The extra 200 pounds didn’t evaporate. It went somewhere, and on a tight job that somewhere is the difference between making money and quietly losing it.
Material is usually the single largest cost in a molded part — frequently the majority of the piece price. That makes a 15–20% material leak one of the most expensive problems a shop can have, and one of the least investigated, because it never shows up as a single event. It bleeds out a few pounds at a time, everywhere, all run long.
This guide doesn’t reproduce historical WJT Associates material. It uses the same applied logic — account for every pound before you blame the job — to find where material actually goes and how to stop guessing about it.
The missing 200 pounds didn’t vanish
Material leaves the building in product, and it leaves in a dozen other ways that nobody requisitioned for. When you actually list them, the gap stops being a mystery:
| Where material goes | Why it adds up | Recoverable? |
|---|---|---|
| Sprues and runners | Every shot makes runner you didn’t ship; on small parts it can rival part weight | Often, as controlled regrind |
| Purge and color changes | Flushing the barrel between jobs/colors burns pounds with nothing to show | Rarely — usually scrap |
| Startup and setup scrap | The first shots while the process stabilizes are material, not product | No |
| Process rejects | Every scrapped part is material that was bought, melted, and binned | Partly, via regrind |
| Flash and overpack | Material escaping the cavity or packed beyond net weight | No |
| Fines and dust from grinding | Granulator fines that don’t feed well and get swept up | No |
| Spillage and handling | Hopper spills, dropped boxes, transfer-line losses | No |
| Residual in barrel and hopper | What’s left at shutdown and changeover | Sometimes |
| Miscount and unrecorded scrap | Pounds that left with no paperwork | No — but findable |
The point isn’t that any one of these is huge. It’s that together they’re the 200 pounds, and you can’t manage what you’ve never itemized. A shop that can’t say where its material goes is a shop guessing at its own cost.
The “net part weight” trap
A lot of the leak starts before production, in how the part was weighed when the job was quoted. “Net part weight” sounds like one number. It’s actually several, and quoting on the wrong one bakes a loss into every shot:
| Weight definition | What it is | The trap |
|---|---|---|
| Actual part weight | Just put a finished part on a scale | Lazy and incomplete — ignores everything you consume beyond the part |
| Net (design) part weight | The part by itself, per design | Real, but it’s not what you actually buy per part |
| Gross / shot weight | Part(s) plus sprue and runner per shot | Closer — this is what the press consumes each cycle |
| Consumed per good part | Gross weight adjusted for yield, purge, and loss | The honest number — what you actually pay for per shippable part |
Quoting on actual or net part weight and then running with full runners, a real scrap rate, and color-change purge is how a job that “should” be profitable loses money on material every shift. The estimate has to be built on what you consume per good part, not on what a single clean part happens to weigh.
How to actually account for it
Material accountability isn’t exotic — it’s reconciliation. The method is simple; the discipline is the hard part:
- Weigh real shots, including runner. Establish the true gross shot weight, not the catalog part weight.
- Track requisitioned versus shipped, by job. The gap is your total loss to explain — the 200 pounds.
- Apportion the gap to the list above. How much is runner you do reclaim, runner you don’t, purge, startup, rejects? Put numbers on each bucket.
- Apply a real yield factor. If 5% scraps, you consume material for ~105 parts to ship 100 — cost the job that way, not at a perfect yield.
- Reconcile regrind. Runner reclaimed and actually fed back is recovered material; runner ground and never used is just delayed scrap. Track which it is.
Once the gap is itemized, the big leaks are usually obvious and fixable: a runner that’s heavier than the part, a color-change procedure that wastes pounds, a scrap rate nobody costed in, regrind generated faster than it’s consumed.
You can’t beat the loss out of people
A note on what doesn’t work, because plants reach for it first. The “beatings will continue until morale and profits improve” approach to material loss produces nothing but resentment, and forcing a lean program to the point of a labor camp shows a brief blip and then drifts right back. Material loss isn’t a discipline problem you can shout away — it’s a measurement problem. You find it by accounting for every pound, naming the biggest leaks, and fixing the process that creates them. The pounds come back when the leaks are closed, not when the floor is yelled at.
FAQs
Why does my plant requisition more material than it ships?
Because product is only one of the places material goes. The rest leaves as sprues and runners, purge and color-change flushing, startup and setup scrap, process rejects, flash, granulator fines, spillage, and material left in the barrel and hopper at shutdown. None of it is individually dramatic, but together it’s commonly 15–20% of what you draw. The gap between requisitioned and shipped weight is real material — the work is itemizing where it went so you can close the biggest leaks.
What’s the right “part weight” to quote on?
The material you consume per good part — not the weight of a single clean finished part. That means starting from the gross shot weight (part plus sprue and runner), then adjusting for yield, purge, and process loss. Quoting on actual or net part weight understates material cost, and since material is usually the largest part of the piece price, that understatement turns into a loss on every shot once you’re running with real runners and a real scrap rate.
Is runner material a loss if we regrind it?
Only partly. Runner you grind and actually feed back into the same job at a controlled ratio is recovered material — a legitimate offset. Runner you grind and never consume is just scrap with an extra step: it piles up, can degrade, and may cost more to dispose of than it’s worth. The accountability question isn’t “do we regrind?” — it’s “is the regrind we generate actually being consumed?” Track generation against consumption, not just the fact that a granulator is running.
How do I find where the material is leaking?
Reconcile. Weigh true gross shots, track requisitioned versus shipped weight per job to size the total gap, then apportion that gap across the known buckets — reclaimed runner, lost runner, purge, startup, rejects, fines, spillage. Apply a real yield factor so scrap is costed in. Once each bucket has a number, the largest leaks are usually obvious: an oversized runner, a wasteful color-change procedure, an uncosted scrap rate, or regrind generated faster than it’s used.