Last updated: 2026-09-02 · 5 min read

How Do You Pick a Resin for Metal Replacement Parts?

Pick a metal-replacement resin by working the load case backwards: identify which property of the metal the part actually uses — stiffness, strength, temperature, creep, or mass — then choose the polymer family and glass loading that meets it with margin, and redesign the geometry to help the polymer rather than forcing the polymer to mimic metal. The materials rarely fail; the designs that treat plastic as die-cast metal do.

Parting line — The line where the two mold halves (A-side and B-side) meet. Parting line location affects: gate placement, ejector pin marks, witness lines, and cosmetic appearance. A well-designed parting line minimizes visible witness marks. See also related guide.

Key Facts

  • Metal-stiffness is out of ; metal-stiffness per gram is not: glass-filled engineering resins carry a fraction of steel’s or aluminum’s modulus, but the part uses far less material, so the achievable stiffness-to-weight can still win. The redesign budget — ribs, corrugation, boss geometry — is where the swap is really engineered.
  • Creep is the silent disqualifier: metals barely creep at room temperature; plastics always do under sustained load. Any metal-replacement part carrying continuous load — brackets, clamps, spring elements — must be checked for creep rupture and stress relaxation at the part’s real service temperature, not at 23 degrees on the datasheet.
  • Temperature moves every other number: modulus, strength and impact all degrade as the part heats. The datasheet’s dry-as-molded, room-temperature values flatter every resin; the decision lives in the property curves at service temperature and humidity.

    • Parting line — The line where the two mold halves (A-side and B-side) meet. Parting line location affects: gate placement, ejector pin marks, witness lines, and cosmetic appearance. A well-designed parting line minimizes visible witness marks. The related guide covers adjacent topics.

    Candidate Families and Where They Win

    Family Strengths for Metal Replacement Watch Out For
    Glass-filled PA6 / PA66 High strength and stiffness, tough, proven in structural brackets Moisture uptake moves properties; dry values flatter; warpage engineering needed
    PPA (aromatic nylon, glass-filled) Better heat and moisture stability than PA66; holds stiffness near fluids and heat Higher processing demands; cost above commodity nylons
    PBT glass-filled Low moisture sensitivity, good electrical properties, stable dimensions Notched impact lower; bonding and painting need surface prep
    Long-fiber thermoplastics Highest stiffness and impact per molding step; metal-like load paths Fiber length survives only with gentle processing; tool wear class rises
    High-strength PC blends Impact with stiffness, good appearance, wide color range Creep under sustained load limits pure structural use

    The Selection Walk-Through

    1

    Write the real load case down

    What carries what, for how long, at what temperature, with what impact events, in what environment — fluids, UV, humidity. One paragraph of service truth beats forty datasheets. Continuous loads and impact events drive the shortlist faster than any stiffness table.

    2

    Filter at service conditions

    Screen candidates on their property values at the part’s working temperature and moisture state — conditioning included for nylons. The resin that survives this filter honestly is usually a class above the one the room-temperature table suggested.

    3

    Redesign the geometry for the polymer

    Ribs across the load path, corrugated sections instead of flat plates, generous fillets at every intersection, and bosses designed for insert or heat-stake assembly. The polymer pays back this engineering with weight, corrosion immunity, and consolidated functions metal never offered.

    4

    Verify the swap with tests that match service

    Load to failure at temperature, creep under sustained load, impact at cold end of service, and assembly endurance. Prototype tools or printed fixtures can carry this stage before the production steel is cut — the failures found here cost weeks, not molds.

    Where Metal Replacement Quietly Fails

  • Copy-cat geometry: the same walls and bosses as the die-cast original, now carrying load through plastic sections that were never ribbed for it.
  • Continuous load ignored: a bracket that holds for a month and sags by summer — creep relaxation that no short test would have caught.
  • Dry datasheet values: nylon parts specified on dry-as-molded stiffness, then living their lives at conditioned moisture where stiffness has dropped.
  • Threaded steel inserts pulled out because boss walls were copied, not designed for the insert’s pull-out loads.
  • Assembly by impact: dropping or hammering parts that the metal original shrugged off, at a cold winter loading dock.

    • Frequently Asked Questions

    Glass-filled or long-fiber for a structural bracket?

    Short glass fills most bracket duty at moderate cost; long-fiber earns its place where load paths are abrupt, impacts matter, or wall sections are thick enough to starve short fibers of orientation.
    If the bracket already passed with short glass at margin, long-fiber adds cost without benefit — run the load case first, not the material fashion.

    How do I compare resin stiffness to the aluminum part it replaces?

    Never wall-for-wall: run the deflection target instead. Compute the deflection the assembly can tolerate, then shape the plastic section — ribs, curvature, section depth — until the composite stiffness meets it. The plastic part will be thicker and shaped differently; that is the redesign working, not failing.

    Does the mold cost change much for these resins?

    Glass-filled and long-fiber grades raise the wear story: harder steels on flow fronts, robust gate lands, planned maintenance intervals. The tooling quote should name the wear provisions — a shop that quotes the same steel for glass-filled as for unfilled is under-engineering one of them.

    Key Takeaways

    1. Work backwards from service: real loads, real temperature, real moisture — then pick the family and glass loading that meets them with margin.
    2. Creep and conditioned properties are the honest filters; room-temperature dry datasheet values are marketing’s contribution.
    3. The redesign is the swap: ribs, corrugation and fillets make metal replacement work — copied metal geometry makes it fail.

    Bring the metal part, its loads, and its service environment to the DFM, and aCatMould will run the replacement analysis item by item — shortlist with conditioned values, geometry redesign notes, and the wear provisions priced into the tool — so the swap is proven before the first kilo of glass-filled resin ever sees a hopper.

    Sources
    DuPont — metal replacement and engineering polymer guidance
    BASF — structural plastics and creep data
    Plastics Technology — reinforced materials processing

    Sources: Plastics Technology · Plastics Technology Online · Society of Plastics Engineers (SPE) · ThomasNet