How much lighter is a moulded plastic part than a die-cast one?

Typically 50-80% lighter for the same envelope. Aluminium die casting runs 2.7 g/cm³ and magnesium 1.74 g/cm³, against 0.9-1.6 g/cm³ for engineering plastics — and plastic designs often remove wall because moulding forms ribs cheaply. A 400 g aluminium housing commonly becomes a 120-180 g moulded one.

Which process wins on per-part cost at volume?

Injection moulding, in most comparisons. Cycle times are similar (20-90 seconds), but resin at €2-6/kg beats aluminium at €2-3/kg per kilo of a much lighter part, and moulded parts emerge net-shape with threads, bosses and finishes — die castings usually add €1-5 of machining, deburring and coating per part.

Where does die casting genuinely beat engineering plastic?

Four places: continuous temperatures above ~200°C; structural loads where metal's stiffness and fatigue margin are decisive; electrical and thermal conductivity, including EMI shielding and heat-sinking; and thin-wall strength below 1 mm, where die casting flows and plastic cannot. If any of these is a hard requirement, stay metal.

How do I get EMI shielding in a moulded plastic housing?

Three standard routes: conductive coatings (sprayed copper/nickel or zinc arc-spray, €1-4 per part), conductive fillers in the resin, or moulded-in metal shields and foil. Coatings reach 40-70 dB attenuation — enough for most electronics enclosures, though not the 80+ dB of a solid metal can.

When does metal-to-plastic conversion pay back?

When volume is above ~1,000 parts/yr and no hard metal requirement applies. Savings stack from three directions: the lighter part cuts material cost, net-shape moulding deletes machining and finishing, and part consolidation — one moulded housing replacing a casting plus brackets — removes assembly labour. Payback on the mould is typically under 12 months.

Injection Moulding vs Die Casting: Weight, Cost & the Metal-to-Plastic Decision

The die-cast-or-moulded question is really a metal-to-plastic question: keep aluminium or magnesium where temperature, stiffness or conductivity is a hard requirement, and take the plastic option — 50-80% lighter, net-shape, consolidation-friendly — everywhere else. At volumes above roughly 1,000 parts a year, engineering plastic wins the cost comparison in most housings, brackets and enclosures; this guide frames where each process genuinely earns its place.

How do the two processes compare?

Factor	Aluminium / magnesium die casting	Injection moulding (engineering plastic)
Density / part weight	Al 2.7 g/cm³, Mg 1.74 g/cm³	0.9-1.6 g/cm³ — typically 50-80% lighter
Tooling cost	€15,000-100,000+, 8-16 weeks	€10,000-60,000, 6-12 weeks
Per-part cost at volume	€3-25 plus machining, deburring, coating	€1-8, net-shape with features in-cycle
Continuous service temperature	200-400°C+	80-150°C typical; 200°C+ for PEEK/PEI at a price
Stiffness / strength	E ≈ 70 GPa (Al), 45 GPa (Mg)	E ≈ 2-14 GPa (unfilled to GF grades)
Conductivity	Electrical and thermal; inherent EMI shield	Insulator; shielding needs coatings or fillers
Tolerances	±0.1-0.3 mm as-cast; tighter with machining	±0.05-0.2 mm, repeatable, no post-machining
Features	Bosses and threads usually machined afterwards	Ribs, bosses, threads, snaps formed in-cycle
Corrosion / finish	Needs coating in most environments	Inherent; colour and texture from the mould

What do the per-part economics look like at volume?

Cycles are similar — 20-90 seconds depending on size — so the comparison turns on material and secondary operations. Resin at €2-6/kg feeding a part one-third the metal weight gives moulding a structural material-cost advantage. The bigger gap is downstream: a die casting typically needs €1-5 per part of trimming, deburring, machining of threads and datums, and powder coating or plating; a moulded part ejects with threads, bosses, colour and texture already on it.

Part consolidation multiplies the saving. Because the mould forms features in-cycle, one moulded housing routinely replaces a casting plus two or three brackets and the fasteners and labour that joined them. Programmes converting to plastic regularly report 20-40% total part-cost reduction once assembly is counted — the cost per part guide covers how to model it before committing.

How do tooling cost and lead time compare?

Die-cast dies and injection moulds live in the same order of magnitude — tens of thousands of euros — because both are hardened steel cavities holding back hundreds of bar. Moulds are usually 20-40% cheaper for a given part and a few weeks faster, and they last longer: a die-cast die managing molten aluminium at 660°C may need major refurbishment after 100,000-300,000 shots, while a well-maintained mould runs a million cycles or more on engineering resins.

That durability difference matters to whole-life cost. Two die refurbishments over a programme's life can equal the original tooling delta — include them in any honest comparison, alongside the coating line and machining cell the cast part will visit.

Where does metal genuinely win?

Four requirements settle the decision immediately. Temperature: continuous service above ~200°C is metal territory — PEEK and PEI stretch plastic's ceiling but at €60-120/kg and with stiffness still far below metal. Structural load with fatigue margin: aluminium's 70 GPa modulus and well-characterised fatigue behaviour beat any thermoplastic for suspension, drivetrain or high-cycle load paths. Conductivity: where the part must sink heat, carry current or shield at 80 dB+, metal is the material. Ultra-thin walls: die casting flows below 1 mm walls that plastic cannot fill reliably.

Where none of these binds — housings, covers, brackets, handles, enclosures — glass-filled PA, PBT or PC/ABS at 2-14 GPa with ribbed geometry is mechanically adequate for most industrial and vehicle applications, at half the weight. The metal-to-plastic conversion guide walks through the full qualification checklist.

What about EMI shielding and other metal behaviours?

A solid aluminium housing shields 80 dB+ for free; a plastic housing shields nothing by default. The standard fixes are conductive coatings — sprayed copper/nickel or zinc arc-spray at €1-4 per part reaching 40-70 dB — conductive-compound resins, or moulded-in metal shields. For most consumer and industrial electronics enclosures, a coated plastic housing passes the same EMC tests as the cast one it replaced, at lower weight and cost.

Nordmould's free DFM review handles conversion candidates directly: send the die-cast part's STEP file or drawing, and the review returns a plastic redesign proposal with rib layout and wall sections, a transparent quotation covering tool and part price, the weight saving, and a T1 sampling plan to validate the conversion in the real material before you retire the die.

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Send Nordmould your die-cast part drawing — the free DFM review returns a conversion proposal, transparent quotation, weight saving and a T1 sampling plan.