Fareed Mohmed · Engineering
Plastics & Non-Metals Selector
Which plastic, rubber or other non-metal goes in which part of a machine, and why.
Machines are not only steel and aluminium. Gears, bushings, wear strips, seals, guards, insulators and pads are often plastic, rubber or ceramic, and picking the wrong one is a common cause of early failures. Choose the part, enter the conditions it will see, and the tool lists the materials that fit, with the usual first choice on top and what to watch out for.
Values are typical figures for each material family, from general engineering practice. Grade, filler, supplier and moisture all change them. Confirm every choice against the maker’s datasheet for the exact grade, and test a sample under real conditions before you commit. For safety guards, follow the machinery guard standard and the supplier’s tested ratings. See also Friction Coefficient Finder and Material Properties.
Start Here: The Selection Process
Six steps from the part to the material. Read this once, then use sections 01 and 02. Steps with a button link to where you enter or read that step.
Say what the part must do
Carry a load, slide, roll, seal, insulate, absorb knocks or let you see through? The job decides the shortlist. A gear and a guard need very different things.
Pick the part in section 01 →Find the worst temperature
Take the hottest and the coldest the part will see, including nearby motors, hot product, sunlight, wash water and cold stores. Use the peak that lasts minutes, not just the average.
List what touches it
Oil, grease, water, steam, cleaners, solvents, sunlight and food. Most plastic failures come from a fluid nobody thought about, such as a cutting fluid or a cleaning spray.
Read the ranked list
The top row is the usual first choice. A yellow “with care” tag means it works but needs a particular grade or a check. Open “eliminated” to see which conditions ruled out the others.
See the result in section 02 →Compare the numbers
Use the tables to compare strength, temperature, wear, friction and cost. Expansion matters more than most beginners expect, so try the growth calculator.
Open the material tables →Get the datasheet and test
Order a sample of the exact grade, read its datasheet for the values at your temperature, and test it in the machine before buying the full quantity.
Common mistakes
Your Part and Its Conditions
Materials That Fit
Material Comparison Tables
All materials in the tool, with typical values. Filter by family or search by use. The dots are relative ratings from 1 to 5 within this list: more dots means better wear, lower friction or stiffer.
| Material | Family | Typical use | Max °C | Min °C | Strength MPa | Density g/cm³ | Expansion µm/m·°C | Water absorbed | Wear life | Low friction | Stiffness | Cost | Watch out |
|---|
For comparison: steel expands about 12 and aluminium about 23 µm per metre per °C. Strength is a tensile or bending figure at room temperature and falls as the temperature rises. Cost is relative: $ is cheapest, $$$$$ is most expensive.
Rubbers: Which One for Which Fluid
The rubber letters on a seal or pad tell you what it survives. Match the rubber to the fluid first, then to temperature.
| Rubber | Range, °C | Good with | Bad with | Typical use |
|---|---|---|---|---|
| NBR (nitrile) | -30 to 100 | Mineral oil, grease, fuel, air | Ozone, sunlight, strong acids, ketones, brake fluid | O-rings, pneumatic and oil seals, vacuum cups |
| HNBR | -30 to 150 | Hot oil, ozone, wear | Strong acids, brake fluid | Hot-oil seals, toothed belts |
| EPDM | -45 to 130 | Water, steam, brake fluid, ozone, weather | Mineral oil, fuel, petroleum grease | Outdoor and water seals, door seals |
| FKM (Viton type) | -20 to 200 | Hot oil, fuel, many chemicals | Hot water and steam, amines, ketones, very cold | Engine and hot-oil seals |
| FFKM | -15 to 260 | Almost everything | Cost | Semiconductor and chemical seals |
| Silicone (VMQ) | -55 to 200 | Extreme heat and cold, food, ozone | Oils, fuel, abrasion, tearing | Food and medical seals, gaskets, soft cups |
| Neoprene (CR) | -35 to 100 | Weather, ozone, moderate oil | Strong oxidisers, ketones | Pads, mounts, hoses, belts |
| Natural rubber (NR) | -50 to 70 | Abrasion, rebound, vibration | Oil, ozone, sunlight | Anti-vibration mounts, abrasion liners |
| Cast PU | -30 to 80 | Wear, cuts, mineral oil, ozone | Hot water (polyester type), strong acids | Rollers, bumpers, wipers, rod seals |
Hardness is in Shore A: 40 is very soft, 70 is a typical seal, 90 is hard. Shore D is used for harder plastics: 50 Shore D is about 90 Shore A. These ranges are typical for standard grades, and special compounds do better.
How Much Will It Grow?
Plastics expand 5 to 15 times as much as steel as the temperature changes. Enter a length and a temperature rise to see how much extra room the part needs.
Ideas Worth Knowing
Types of plastic
Thermoplastics (nylon, acetal, PE, PC) soften when heated and can be re-melted. Thermosets (phenolic, epoxy laminates) harden for good and do not melt. Elastomers (rubber, PU) stretch and spring back. Engineering plastics are the stronger, more stable group used for machine parts; commodity plastics (PE, PP, PVC, ABS) are cheaper and used for covers and light duty.
Stiffness and creep
Plastics have a modulus of about 1 to 4 GPa, against 200 GPa for steel. Under a steady load they also creep: they keep slowly deforming. Bolts through plastic loosen as it creeps. Use metal washers, sleeves or inserts so the bolt load is carried by metal.
Moisture
Nylon, phenolic and some polyimides soak up water and change size and strength. Acetal, PET, PE, PP and PTFE barely absorb any. Where size matters in damp air, choose the low-absorption plastics.
Heat and cold
Plastics soften over a range, not at a point. Maximum continuous temperature is where properties hold for a long time, and strength is already much lower than at room temperature. Some plastics (PP, PVC) turn brittle near freezing.
Wear and friction
Plastics slide on smooth, hard steel with low friction. The PV limit (pressure × velocity) sets how fast and hard they can slide before heat builds up. Two different plastics usually wear less than two of the same. Look up friction values in the Friction Coefficient Finder.
Chemicals and stress cracking
Liquids that do not dissolve a plastic can still make a stressed part crack. This is worst for polycarbonate, acrylic and ABS with oils, coolants, adhesives and cleaners. Hot water breaks down some plastics over time (hydrolysis): PET, PC, polyester PU and nylon in particular.
Fixing and joining
PE, PP, PTFE and acetal are hard to glue. Use screws, rivets or press fits for them. Drill and tap with care: cut threads in plastic are weak, so use thread inserts for repeated assembly.
Machining and tolerance
Plastic stock holds stress from the factory and moves after machining. Rough cut, let it rest, then finish. Expect looser tolerances than metal, and measure at the temperature and humidity it will work in. See Process vs. Tolerance.
Related tools
Plastic belts and pulleys: Timing Belt Drive Selector. Bearings and bushes: Bearing Type Selector. Pneumatic tubing and seals: Pneumatic Component Selector.