What is metal-to-plastic conversion?

Redesigning a machined, cast or fabricated metal component as an injection-moulded plastic part, typically in glass-filled engineering resins. Drivers are weight reduction (often 40-80%), part consolidation, eliminated finishing steps, and lower unit cost at volume.

Which plastics can replace metal parts?

Glass-filled PA66 (GF30-50) covers most brackets, housings and mechanisms; PPA and PPS for hotter environments; PEEK for extreme temperature, wear or chemical duty; POM for gears and low-friction parts. Die-cast aluminium is the most commonly replaced material.

How much lighter is a plastic part versus metal?

PA66-GF30 has a density of about 1.36 g/cm³ versus 2.7 for aluminium and 7.85 for steel — roughly half the weight of an aluminium part and 80% less than steel at equal volume, before accounting for geometry changes like ribbed structures.

What design changes does metal-to-plastic conversion need?

Uniform walls instead of machined profiles, ribs for stiffness instead of thickness, draft on pull faces, generous radii at stress points, and metal inserts for threads and bearing seats. A direct geometry copy fails — the part must be redesigned for moulding, not merely re-materialised.

When does metal-to-plastic conversion not make sense?

Very low volumes (tooling never amortises), sustained loads at high temperature (creep), tight long-term dimensional stability requirements, high bearing loads without inserts, and where stiffness-to-volume truly requires metal's modulus. A DFM review will say so honestly.

What is the cost case for converting a metal part to plastic?

A machined aluminium bracket at €15-40/unit becomes a moulded GF-nylon bracket at €1-5/unit plus a €5,000-15,000 tool — breakeven typically in the low thousands of units, with consolidation (one moulded part replacing an assembly) often delivering the largest saving.

Metal-to-Plastic Conversion: When Injection Moulding Replaces Machined Metal

Metal-to-plastic conversion is the quietest cost-reduction programme in manufacturing: take a machined or cast metal part, redesign it for injection moulding, and collect savings on weight, assembly steps and unit price for the rest of the product's life. The engineering is real but bounded — this guide covers when it works, what changes, and when to leave metal alone.

What are the benefits in numbers?

Driver	Typical magnitude	Mechanism
Weight	40-80 % reduction	GF-nylon at ~1.4 g/cm³ vs aluminium 2.7, steel 7.85
Unit cost	50-90 % reduction at volume	Net-shape moulding vs machining time and scrap
Part consolidation	3-10 parts → 1	Brackets, fasteners and features moulded in
Finishing elimination	Paint/coating removed	Colour moulded in; no corrosion protection needed
Corrosion & insulation	Qualitative	No rust; electrical and thermal insulation built in

Consolidation is often the biggest winner: a stamped-and-welded five-piece assembly becomes one moulded housing with snap-fits and bosses included — deleting welds, fasteners, stock and assembly labour at once.

Which resins replace which metals?

Resin	Replaces	Strengths	Watch out for
PA66-GF30/50	Aluminium brackets, housings	Stiff, strong, cheap, proven	Moisture uptake, creep under sustained load
PPA-GF	Aluminium at higher temperature	Better heat and moisture behaviour than PA66	Cost above PA66
PPS-GF40	Aluminium in hot, chemical duty	Dimensional stability, inherently flame-retardant	Brittle-ish impact, tool wear
PEEK-GF/CF	Metal in extreme duty	Temperature, wear, chemicals, strength	€80-120/kg resin cost
POM	Small steel gears, cams	Low friction, fatigue, precision	Not for high heat or strong acids

How does the design change?

A machined part's habits become liabilities: thick sections (sink, voids, cycle time), sharp internal corners (stress concentration), uniform draft-free walls (ejection), and tapped holes everywhere (plastic threads are weak). The redesign applies the standard moulding toolkit — uniform 2-4 mm walls with ribs per the rib and boss rules, draft, radii, and metal inserts for threads and bearing seats per the threads and inserts guide.

Stiffness needs honest attention: even GF50 nylon's flexural modulus (~10-15 GPa) is far below aluminium's 70 GPa, so a plastic beam of identical geometry deflects more. The answer is geometry — ribbed sections raise stiffness per gram dramatically — but load cases should be checked, not assumed.

What are the real limitations?

Creep: plastic under constant load deforms slowly over months — clamped joints and permanent preloads need design allowances or metal inserts. Temperature: sustained service above the resin's limit quietly eats strength. Fatigue and impact behave differently from metal and need grade-level data. And the business case has a floor: below roughly a few thousand lifetime units, tooling amortisation kills the saving — the arithmetic sits in the cost-per-part guide.

How do you run a conversion project?

Four steps: screen the candidate assembly (volume × current cost × consolidation potential), redesign for moulding with load cases checked, prototype in the target resin (bridge tooling gives production-representative parts), then validate — including creep-relevant tests at temperature — before releasing production tooling. Nordmould's free DFM review includes a conversion feasibility read: resin shortlist, geometry changes and the breakeven volume for your specific part.