PlasticsPlastic or metal? When to replace bronze, aluminum or steel with an engineering plastic
More and more parts that used to be made of bronze, aluminum or steel are now made of nylon, acetal, polyethylene or PEEK. The reason isn't just price: an engineering plastic weighs four to eight times less, doesn't rust, slides without lubrication and reduces noise. But it also has clear limits in temperature and stiffness. In this guide we explain when it makes sense to replace a metal with a plastic and when it doesn't.
What a part gains when switching from metal to plastic
- Weight: nylon (1.15 g/cm³) weighs one seventh as much as steel (7.85 g/cm³) and less than half as much as aluminum (2.73 g/cm³); UHMW polyethylene (0.93 g/cm³) is up to 8 times lighter than steel. Less weight means less inertia in moving parts, less load on structures and easier assemblies.
- Sliding without lubrication: self-lubricating Nylon AL has a coefficient of friction of approximately 0.08 and Teflon 0.06 to 0.09, versus metal pairs that require grease or oil. Less maintenance and less downtime.
- No corrosion: plastics do not rust; polypropylene and polyethylene resist chemicals and moisture, and Teflon is practically inert.
- Less noise and better damping: a plastic gear or roller absorbs vibration and shock that metal transmits.
- Easy machining: it can be turned, milled and drilled with conventional tools; acetal, PET and nylon also give an excellent finish.
- Electrical insulation: plastics and thermosets insulate against current; metals conduct it.
Density: plastics and metals, side by side
Reference values from our data sheets for plastics and from public density tables for metals:
| Material | Density (g/cm³) |
|---|---|
| Polypropylene (PP) | 0.91 |
| UHMW Polyethylene PE 1000 | 0.93 |
| HD Polyethylene PE 300 | 0.955 |
| Nylon PA 6 | 1.14 a 1.15 |
| Polycarbonate (PC) | 1.2 |
| PETG | 1.27 |
| PEEK | 1.31 |
| Bakelite / Celoron | 1.34 a 1.35 |
| PET | 1.38 |
| Acetal (POM) | 1.41 |
| G10 / G11 | 1.9 a 2.0 |
| Teflon (PTFE) | 2.13 a 2.21 |
| Aluminum | 2.73 |
| Carbon steel | 7.85 |
| 304 Stainless Steel | 7.93 |
| Brass | 8.5 |
| Bronze | 8.9 |
| Copper | 8.94 |
What you give up, and how to compensate
- Stiffness: the modulus of elasticity of plastics (from 750 MPa for UHMW to 7,500 MPa for Bakelite) is far lower than that of metals, so a plastic part deflects more under the same load. This is offset with thicker sections, ribbing, or a stiffer material (PEEK, PET, acetal, G10).
- Temperature: most engineering plastics work up to 100 to 120 °C; thermosets up to 180 °C, and only PEEK and Teflon reach 250 and 260 °C. Above that, metal remains the option.
- Thermal expansion: plastics expand several times more than steel (for example 110 × 10⁻⁶/K for acetal and 50 × 10⁻⁶/K for PEEK). For parts that undergo temperature changes, allow larger clearances.
- Moisture absorption: nylon absorbs about 2.5% water and changes size; for tight tolerances in humid environments, acetal (0.2%), PET (0.25%) or polyethylene (0.01%) are better choices.
- Creep: under constant load a plastic can slowly deform over time; PET, for example, reports low creep on its data sheet. This figure should be checked for each material when the load is permanent.
Which plastic replaces which metal?
| Typical part | Usual metal | Plastic that replaces it | Why? |
|---|---|---|---|
| Bushings and bearings | Bronze | Nylon SL, Nylon AL, acetal, PET | Less weight, sliding without lubrication, no corrosion |
| Gears and pinions | Steel, bronze | Acetal, PET, nylon, Celoron | Quiet, lubrication-free, easy to machine |
| Rollers, pulleys and wheels | Steel, aluminum | Nylon PA 6, UHMW polyethylene | Lightweight, does not damage the mating part, absorbs shock |
| Guides and wear plates | Steel, bronze | UHMW Polyethylene PE 1000, Ceramic UHMW, Teflon | Maximum abrasion resistance and low coefficient of friction |
| Hopper and chute liners | Steel plate | UHMW and HMW polyethylene | Low coefficient of friction, abrasion resistance and less weight |
| Tanks and ducts for chemicals | Stainless steel | Polypropylene, HD polyethylene | Chemical resistance, weldable, no corrosion |
| Guards and windows | Glass, sheet metal | Polycarbonate, PETG | Clear and virtually unbreakable |
| Electrical insulators and spacers | Not applicable (the metal conducts) | Bakelite, Celoron, G10/FR4, G11, Teflon | Dielectric strength and heat stability |
| Precision parts at high temperature | Stainless steel, aluminum | PEEK | Up to 250 °C, self-extinguishing, stable and lightweight |
When does it make sense to stay with metal?
- When continuous working temperature exceeds 250 °C.
- When the part is structural and the stiffness or fatigue resistance of steel is essential.
- When heat or electricity must be conducted.
- When the part works under very high impact loads and the cross-section cannot be increased.
In summary
- An engineering plastic weighs 4 to 8 times less than steel and does not rust
- Slides without lubrication (self-lubricating nylon ≈ 0.08, Teflon 0.06 to 0.09) and reduces noise and maintenance
- Gives up some stiffness, temperature resistance, and expansion; offset with section, design, and the right material
- Bushings, gears, rollers, guides, liners, tanks, guards and insulators are the most common replacements
- Above 250 °C or in highly demanding structural parts, metal is still the choice
Why buy your engineering plastics at NAT MAC?
We have more than 54 years of experience in the industry and ISO 9001 certified quality. We carry all ten families of engineering plastics, each with its own data sheet, and we help you choose the right material for your part, your load and your working environment.
With us you get
- Material cut to the size you need
- Data sheets with verified properties for each grade
- Advice on choosing the right plastic for your load, temperature and environment
- Stock in bars, plates and bushings with delivery throughout Mexico
- Backed by ISO 9001 certified quality
Tell us what part you'll make, the conditions it operates in and the sizes you need, and we'll recommend the right material. Request your free quote.
Sources: NAT MAC Engineering Plastics data sheets and catalog; TecnoQuim (material data sheets); Projectmaterials, Metal density chart.
Note: the information, values and recommendations in this article are provided for general reference only and may vary depending on the standard, manufacturer, heat or lot, treatment and application conditions. They do not constitute guaranteed specifications; for your project, consult the current data sheet for the material and confirm the data with our technical team.
