Metal replacement with modified engineering plastics — glass-fiber-reinforced PA66 and PP, plus flame-retardant PC/ABS — cuts part weight by up to 40% and lowers finished-part cost against aluminum, zinc, and steel die-castings, without giving up the stiffness and heat resistance load-bearing parts need. This guide maps which polymer replaces which metal part, shows the real property numbers, and explains how to spec a grade straight from your drawing.
Why Are Engineers Replacing Metal with Engineering Plastics?
For decades, aluminum and zinc die-casting was the default for load-bearing structural parts — pump housings, intake manifolds, power tool bodies, electronic enclosures. Cast metal is stiff and heat-tolerant, but it is heavy, needs secondary machining, and corrodes without a protective coating.
High-performance modified engineering plastics now replace aluminum, zinc, and steel alloys across these parts for three reasons:
- Weight. Glass-filled PA66 and PP weigh a fraction of the metals they replace: PA66 with 30% glass runs about 1.36–1.38 g/cm³ against 2.70 g/cm³ for aluminum, roughly 6.6 g/cm³ for zinc, and 7.85 g/cm³ for steel.
- Cost. Injection molding produces net-shape parts in a single shot — snap-fits, bosses, ribs, and threaded inserts molded in — eliminating machining, plating, and assembly steps.
- Corrosion and design freedom. Polymers don’t rust, and they open up geometries that die-casting cannot produce.
The net result is roughly 40% weight reduction and meaningful total-cost savings on many programs, with stiffness and heat resistance tuned by glass loading and formulation.
How Much Weight and Cost Can Metal Replacement Cut?
The headline number is weight. A glass-filled PA66 part weighs about half its aluminum equivalent, and roughly a fifth of a zinc or steel part. On real assemblies the saving compounds: one injection-molded part often replaces several stamped and fastened metal pieces, so you drop the brackets, screws, and bosses that metal design forces on you.
Cost savings come from three directions:
- One-shot molding removes secondary machining, deburring, and anti-corrosion coating.
- Part consolidation cuts part count and assembly labor.
- Lighter parts lower shipping and handling costs.
The table below compares the workhorse grades against the metals they replace.
| Property | Aluminum die-cast | Zinc (Zamak) | PA66 + 30% GF | PP + 30% GF | FR PC/ABS |
|---|---|---|---|---|---|
| Density (g/cm³) | 2.70 | ~6.6 | 1.36–1.38 | ~1.12 | 1.17–1.21 |
| Tensile strength (MPa) | 240–320 | ~280 | 180–200 | ~90 | ~50 |
| Flexural modulus (GPa) | ~70 | ~85 | 8.5–9.5 | ~3.5 | ~2.1 |
| HDT @ 1.8 MPa (°C) | — | — | 240–250 | ~150 | 88–110 |
| UL94 rating | — | — | HB | HB | V-0 @ 0.75–1.5 mm |
| Corrosion resistance | Needs coating | Good | Excellent | Excellent | Excellent |
Which Polymer Replaces Which Metal Part?
PA66 + Glass Fiber (15–50%) — the aluminum substitute
PA66 compounds reinforced with 15–50% glass fiber deliver rigidity comparable to aluminum at a fraction of the weight. At 30% glass, tensile strength reaches 180–200 MPa and flexural modulus 8.5–9.5 GPa, while heat deflection temperature holds above 240°C at 1.8 MPa — enough for under-hood and engine-bay heat, with continuous service comfortable to about 150°C.
Typical metal-replacement targets: engine covers, intake manifolds, power tool housings, pump housings, and structural brackets.
PP + Glass Fiber / Mineral — the cost play
For large, cost-sensitive parts that don’t need PA66’s heat resistance, PP filled with glass fiber or mineral is the cheapest route to stiffness. It is the workhorse of the reinforced plastics range and the standard choice for automotive modules, washing machine tubs, and HVAC fan blades — parts where material cost per kilogram and cycle time matter as much as the mechanical numbers.
FR PC/ABS — enclosures that must not burn
When the part carries current or sits near a heat source, PC/ABS compounds with halogen-free phosphorus/nitrogen flame retardants reach UL94 V-0 at 0.75–1.5 mm while keeping RoHS and REACH compliance. That combination is why FR PC/ABS is the default for EV charging stations, battery modules, and smart meters — impact toughness and dimensional stability plus a documented flame rating.
What Else Changes When You Drop Metal?
Beyond weight, metal replacement changes how the part is made and how it performs over its life:
- Part consolidation. Snap-fits, living hinges, ribs, and mounting bosses are molded in one step, so a single plastic part replaces multi-piece metal assemblies.
- Corrosion resistance. Polymers don’t rust, eliminating plating and paint steps and cutting warranty claims in wet or chemical environments.
- System-level weight. Lighter vehicles and products improve fuel economy and cut freight and handling costs through the supply chain.
How Do You Spec a Metal-Replacement Grade?
Specifying a grade is not a catalog pick — it is a formulation exercise. We compound from your drawing and requirements: target stiffness and flexural modulus, surface appearance and color, flame rating (UL94 V-0, halogen-free for RoHS/REACH), and the processing window for your tool and press. Sending the drawing together with the load, temperature, and compliance targets is enough to start.
FAQ
How much weight can I save replacing aluminum with PA66 + glass fiber?
At 30% glass, PA66 runs about 1.36–1.38 g/cm³ versus 2.70 g/cm³ for aluminum — roughly half the weight per unit volume. Combined with part consolidation, most programs see around 40% total weight reduction.
What glass loading should I choose — 15%, 30%, or 50%?
15% glass gives moderate stiffness and better flow for thin walls; 30% is the balance point, lifting tensile strength from about 80 MPa (unfilled PA66) to 180–200 MPa; 50% delivers maximum rigidity and the highest HDT, at the cost of elongation and surface finish.
What flame rating do EV charging and battery parts need?
UL94 V-0. FR PC/ABS achieves V-0 at 0.75–1.5 mm, and halogen-free phosphorus/nitrogen systems keep the part RoHS and REACH compliant.
Can one plastic part really replace a metal assembly?
Yes. Snap-fits, ribs, bosses, and living hinges are molded in a single shot, so a one-piece molded part often replaces several stamped, welded, or fastened metal pieces — cutting part count and assembly labor.
Need a Metal-Replacement Compound?
Tell us which metal part you want to replace and send your drawing. We’ll recommend a PA66, PP, or PC/ABS grade matched to your stiffness, heat, and flame targets, and supply samples for tool trials. Contact FitPoly to get a recommendation and a quote.
