Replacing metal parts with engineering plastics can cut cost and weight by a third or more, but only for the right parts. How to screen candidates, design for plastic rather than copying metal, and validate the result.
An equipment manufacturer used a machined and welded aluminium bracket assembly: five parts, four fasteners, two secondary operations and a painted finish. Converting it to a single glass-filled nylon moulding removed every fastener, all the machining and the painting, cut weight by more than half and reduced cost by over a third. The design looked nothing like the metal original, and that was the point.
Metal-to-plastic conversion is one of the biggest cost-reduction opportunities in product design, and one of the easiest to get wrong.
Why convert
- Part consolidation. Several metal parts and their fasteners can become one moulding, as our metal-to-plastic bracket case study showed.
- Fewer operations. Machining, welding, deburring, painting and plating disappear.
- Weight reduction. Engineering plastics are typically one-fifth to one-seventh the density of steel and around half that of aluminium.
- Corrosion resistance without coatings.
- Design freedom: integrated clips, hinges, ribs and cable routing.
- Lower cost at volume, once tooling is amortised.
When it does not make sense
- Very low volumes, where tooling cost cannot be recovered.
- Very high, sustained loads at high temperature, where creep will eventually win.
- Parts that need metal's electrical or thermal conductivity, unless special grades or inserts are used.
- Extremely tight tolerances over large dimensions in high-shrink materials.
- Applications where the regulatory or certification cost of change outweighs the saving.
Screening candidates
Good candidates usually share several features: medium to high volumes, multiple metal parts and fasteners in an assembly, secondary operations, moderate loads and temperatures, and weight or corrosion problems. Score each candidate on these, then estimate cost and feasibility for the top few.
Design for plastic, not like metal
The most common failure is copying a metal design in plastic. Plastics have a lower modulus, creep under sustained load, are sensitive to temperature, moisture and notches, and in filled grades are anisotropic. Good conversions:
- Use ribs and geometry for stiffness, not thick sections, following the rib design rules.
- Keep walls uniform and add generous radii at load-bearing corners.
- Account for fibre orientation in glass-filled materials, which can halve strength across the flow direction.
- Design around creep for sustained loads, using long-term data at the operating temperature.
- Use metal inserts or overmoulded metal only where needed, for threads, bearing surfaces or high local loads.
Material choice
Common choices include glass-filled nylons (PA6, PA66), PBT, PPS and PEEK for higher temperatures and chemical resistance, and long-fibre reinforced grades for structural parts. The choice depends on load, temperature, chemicals, moisture, dimensional stability and cost. Moisture absorption in nylon, for example, changes both dimensions and stiffness.
Validate properly
FEA with appropriate material models, including fibre orientation from mould flow where possible, is essential. Then test real moulded parts, not machined ones, under realistic loads, temperatures and durations, including creep and fatigue where relevant. The extra validation is small compared with the savings, and with the cost of a field failure.
