Plastics manufacturers — injection molders especially — tend to have a BOM structure most ERPs handle badly: one production run consumes resin and produces a primary molded part plus, frequently, a regrind by-product or a co-product from a multi-cavity mold. Acumatica's BOM engine supports co-products and by-products natively, but the default configuration treats them as an afterthought unless you set the cost allocation method deliberately.
Co-products and by-products are different BOM output types, and the distinction matters
On a BOM output line, Acumatica lets you flag additional outputs beyond the primary finished item, and distinguishes co-products (planned, valuable outputs — think a two-cavity mold producing two different parts in one shot) from by-products (secondary material, usually low-value, like regrind plastic scrap that gets reprocessed). The distinction is not cosmetic: co-products participate in cost allocation across the production order's total cost (by relative value or by a fixed percentage you configure), while by-products are typically valued at a nominal recovery cost and credited back against the run's material cost, reducing the primary product's burden.
Primary output: Molded Housing (Item A) Qty: 1000 Cost alloc: 85%
Co-product: Molded Cap (Item B) Qty: 1000 Cost alloc: 15%
By-product: Regrind Plastic (Item C) Qty: 40 lb Value: nominal
(credited to material cost)Get the allocation percentages wrong and your finished-goods costing is wrong in a way that does not show up until someone asks why the housing's margin looks worse than the quote implied — the regrind credit and cap allocation were never configured, so 100% of resin cost is landing on the housing alone.
Resin shrinkage and yield variance
Injection molding has real material shrinkage between raw resin weight and finished part weight — some of it expected (process shrinkage), some of it scrap (short shots, flash trim). Acumatica's standard BOM quantity is a fixed expected consumption; actual consumption varies run to run based on cycle time and mold condition. I handle this the same way as the metal-fab scrap problem: a reason-coded scrap/yield capture on the production order rather than letting shrinkage silently absorb into a generic material variance account, because plastics clients specifically want to know whether yield is trending down (a sign of mold wear) — that trend is invisible if scrap is not captured as its own data point.
If regrind gets blended back into virgin resin at a fixed ratio for the next run, that blending is itself a small production order or assembly transaction (regrind + virgin in, blended resin out) — it is not something the co-product/by-product output configuration does automatically. Model it as its own simple BOM if your client actually reprocesses regrind rather than selling or discarding it.
Wrapping up
Plastics manufacturing maps well onto Acumatica's BOM co-product/by-product model, but only if cost allocation percentages and by-product recovery values are configured deliberately rather than left at BOM defaults, and if shrinkage/scrap is captured as its own tracked event. Both are configuration decisions you make once, correctly, at implementation — not something to patch after the first quarter's margin numbers look wrong.
Independent software engineer in Nairobi specialising in Acumatica customisations, Laravel backends, and tax fiscalisation integrations across East and Southern Africa.