The short answer
For most metal brackets and parts, start with 30–50% glass-filled nylon 66 or nylon 6. Step up to glass-filled PPA for hot or wet service, glass-filled PPS for heat and chemicals, and PEI or carbon-fiber PEEK for the most demanding parts. Use long-glass PP for large, cost-driven parts at moderate temperature. Whatever you pick, plan on redesigning the part with ribs, because no plastic matches metal stiffness.
This page compares the candidate resins by the numbers that decide a metal conversion. For the step-by-step checks, common failures and what each resin costs you, see replacing a metal part with plastic.
When metal-to-plastic works
- Weight: glass-filled nylon is about one-sixth the density of steel and half that of aluminum.
- Part consolidation: one molding can replace a bracket, its fasteners and a second machined part. This is usually where the cost savings come from, more than the material itself.
- Corrosion: no rust and glass-filled grades cause no galvanic corrosion (carbon-fiber grades can, against aluminum).
- Noise: plastics damp vibration and run quieter in gears and mounts.
The stiffness gap, in numbers
Steel has a modulus of about 200 GPa and aluminum about 70 GPa. A 30% glass-filled nylon 66 is about 8.5–10 GPa dry, roughly one-eighth of aluminum and one-twentieth of steel. Even 40% glass-filled PPS (13–15 GPa) and carbon-fiber PEEK (about 20–24 GPa) are far below aluminum.
Geometry closes the gap. Bending stiffness of a plate rises with the cube of its thickness, so a glass-filled nylon plate about twice as thick as an aluminum plate has similar stiffness at similar weight. Against steel, the plastic plate needs to be nearly three times as thick but weighs about half as much. In practice you get there with ribs and deeper sections, not solid thick walls, which sink, void and add cycle time.
Candidate resins compared
| Material | Flexural modulus, GPa | Density, g/cm³ | Max continuous temp. (approx.) | Relative cost tier | Typical replacements |
|---|---|---|---|---|---|
| PA6, 30% glass | 8.0–9.5 dry | 1.35–1.37 | ~100–130°C (212–266°F) | Moderate | Brackets, housings, clips; zinc and aluminum die-castings |
| PA66, 30% glass | 8.5–10.0 dry | 1.37–1.39 | ~120–150°C (248–302°F) | Moderate | Brackets, engine covers, pump housings |
| PA66, 50% glass | ~14–16 dry | ~1.56–1.58 | ~120–150°C (248–302°F) | Moderate | Structural brackets, pedal and seat parts |
| PPA, 30–50% glass | ~10–17 | ~1.43–1.62 | ~140–170°C (284–338°F) | Higher | Hot or wet brackets, coolant and fuel parts |
| PBT, 30% glass | 8.0–9.5 | 1.52–1.54 | ~120–140°C (248–284°F) | Moderate | Housings and brackets needing steady size |
| PPS, 40% glass | 13–15 | 1.65–1.68 | ~200–220°C (392–428°F) | High | Hot, chemical-exposed brackets; pump parts |
| PEI, unfilled / 30% glass | 3.3–3.5 / ~9 | 1.27 / ~1.51 | ~170–180°C (338–356°F) | High | Hot structural parts, flame-resistant brackets |
| PEEK, 30% carbon fiber | ~20–24 | ~1.40–1.44 | ~240–260°C (464–500°F) | Highest | Aerospace and oil-field brackets, machined metal parts |
| Long-glass PP, 30–40% | ~5–8 | ~1.12–1.22 | ~100–110°C (212–230°F) | Low | Large carriers, trays, front-end and battery structures |
Typical values for injection molding grades at room temperature; nylon values are dry as molded and drop once the part absorbs moisture. Continuous use limits depend on load, heat stabilization and environment. Grades vary, so design from the data sheet for your grade. For comparison: steel ~200 GPa at 7.85 g/cm³, aluminum ~70 GPa at 2.7 g/cm³.
What limits a conversion
- Creep at temperature: under a constant load, plastics keep deforming. Size long-term loads from creep (isochronous) curves at the service temperature, not from the short-term modulus.
- Fatigue: vibrating brackets fail at stresses well below the tensile strength, and weld lines and sharp corners lower the limit further. Ask for fatigue data at temperature.
- Moisture in nylon: conditioned glass-filled nylon loses roughly a quarter to a third of its dry stiffness. PPA, PBT and PPS absorb far less. See glass-filled nylon and PPA.
- Thermal expansion: unfilled plastics expand roughly five to ten times as much as steel; glass-filled grades come closer along the flow but not across it. Over a temperature swing, bolted joints lose clamp load and long parts bind against metal frames.
- Fiber orientation: glass-filled parts are stiffer along the flow than across it, so gate location matters. See glass-filled plastics.
Fasteners and inserts
- Use metal compression limiters at bolted joints so the sleeve, not the plastic, carries the clamp load.
- Use threaded brass inserts (heat-staked, ultrasonic or molded-in) where screws are removed and refitted; use thread-forming screws designed for plastics elsewhere.
- Slot holes on long parts fixed to metal so the plastic can move with temperature.
Glass, carbon or long fiber
Glass fiber is the economical default. Carbon fiber gives more stiffness at lower weight and adds conductivity, at a much higher price; see carbon vs. glass-filled nylon and carbon-filled resins. Long-fiber grades keep longer fibers in the part, which improves impact and creep resistance over short-fiber grades of the same resin.
What to send us for a quote
- The metal part, what it is made of and a drawing or photo
- Loads, service temperature range and any chemicals
- The candidate resin if you have one, annual volume and ship-to location
Data for the two most common choices: PA66 GF30 properties and PPS GF40 properties.
Common questions
Can plastic replace a metal bracket?
Often, when the bracket's load, temperature and stiffness needs fit a glass-filled engineering resin and the part is redesigned with ribs. Glass-filled nylon 66 is the usual first choice; PPA, PPS and PEEK cover hotter or harsher service.
How stiff is glass-filled nylon compared to aluminum?
Much less stiff. Aluminum is about 70 GPa and steel about 200 GPa, while 30% glass-filled nylon 66 is about 8.5–10 GPa dry and lower once it absorbs moisture. Ribs and deeper sections make up the difference.
What is the strongest plastic for replacing metal?
For stiffness and strength, carbon-fiber-filled PEEK and highly glass-filled PPA, PPS and nylon lead, with flexural modulus roughly 13–24 GPa. They are also the most costly, so most conversions use 30–50% glass-filled nylon or PPA.
Is long-glass polypropylene good for metal replacement?
Yes, for large, moderate-temperature parts where cost and weight matter, such as carriers, trays and housings. Long glass fibers give much better impact and creep resistance than short-glass PP, though stiffness and heat stay below glass-filled nylon.
Why does a plastic replacement part loosen at its bolts?
Creep and thermal expansion. The plastic relaxes under constant clamp load, more so when hot, and it expands several times more than steel. Metal compression limiters or inserts at bolted joints carry the clamp load.
Does moisture affect glass-filled nylon brackets?
Yes. Nylon absorbs water from the air, which lowers stiffness by roughly a quarter to a third and grows the part slightly. Design to conditioned properties, or use PPA, PBT or PPS where moisture matters.
Tell us about the metal part
A polymer chemist reviews every request, then matches your grade or finds a qualified equivalent and prices it.
What to send for an accurate quote
- The grade name, from any supplier, or a data sheet
- Color (natural, black or a custom match)
- Fill type and percentage, if any
- Melt flow, if you know it
- Quantity for this order and what you use in a year
- Ship-to city and state or province