Start with the requirement, not the polymer
Engineering plastics are often chosen by name recognition: someone has used Delrin before, so the next part is Delrin too. A better approach is to list what the part must survive and then pick the plastic that meets those needs at the lowest cost and risk. Most requirements fall into five families: temperature, chemicals, wear and friction, moisture and dimensional stability, and regulatory needs such as food contact.
This guide covers the plastics most often machined from rod and plate: POM (acetal), nylon (PA6 and PA66), PTFE, UHMW-PE, PEI (Ultem) and PEEK. Values quoted are typical figures for unfilled grades; always check the datasheet for the exact grade you order.
Requirement family 1: temperature
Continuous use temperature is the first filter because it removes most options quickly.
| Plastic | Typical continuous use limit | Notes |
|---|---|---|
| UHMW-PE | ~80 °C | Softens well before other plastics |
| POM (acetal) | ~90–100 °C | Stable in short excursions above that |
| Nylon PA66 | ~90–120 °C | Depends strongly on grade and load |
| PEI (Ultem) | ~170 °C | Amorphous, keeps stiffness to near its limit |
| PTFE | ~260 °C | Very soft, creeps under load |
| PEEK | ~250 °C | Keeps useful strength at high temperature |
If the part must handle steam sterilization, engine compartments or hot fluids, the list shrinks to PEEK, PEI and PTFE. If it stays near room temperature, cost and machinability decide.
Requirement family 2: chemical exposure
Check resistance to the specific fluids, concentrations and temperatures involved, using the resin supplier's chemical compatibility tables. Broad patterns:
- PTFE resists almost every common chemical
- PEEK resists a very wide range, including many solvents and hydrocarbons, but not concentrated sulfuric or nitric acid
- POM handles fuels, oils and solvents well but is attacked by strong acids and oxidizers
- Nylon handles oils and fuels but absorbs water and is weakened by acids
- UHMW-PE resists many acids and alkalis but swells in some hydrocarbons
- PEI handles many fluids but can stress-crack in some solvents
Requirement family 3: wear and friction
Bushings, guides, gears and sliders need low friction and good wear.
POM
Stiff, slippery and dimensionally stable. A favorite for small gears, rollers and precision sliding parts.
Nylon
Tough and wear resistant, good for gears and wear pads, especially with oil or MoS2 filled grades.
UHMW-PE
Very low friction and excellent abrasion resistance, ideal for chute liners and conveyor wear strips, but too soft for precise features.
PTFE
The lowest friction of all, but it creeps and wears quickly unless filled with glass, carbon or bronze.
PEEK
Bearing-grade PEEK (filled with carbon, graphite and PTFE) handles high speed and load at high temperature.
Requirement family 4: moisture and dimensional stability
Nylon is the cautionary case. It absorbs a noticeable amount of water from humid air, and its dimensions and stiffness change with it. A tight-tolerance nylon part measured dry may grow out of tolerance after a few weeks in a humid room. Use nylon where looser tolerances are acceptable, or choose POM, which absorbs very little water.
Plastics also expand with temperature much more than metals, typically several times the expansion of steel. A plastic part pressed into an aluminum housing will fit differently at 0 °C and 60 °C. Leave clearance and check fits across the full operating range.
Machined plastic parts can also relax after machining as internal stresses release. Annealed stock, symmetrical machining and rest time between roughing and finishing help hold tolerance on long or thin parts.
Requirement family 5: compliance and special needs
Many grades are available with food-contact compliance, such as FDA or EU food-contact compliant grades of POM, UHMW-PE, PEEK and PTFE. Some medical applications need special grades with documented biocompatibility testing. Electrical parts may need specific flame ratings, which PEI and PEEK achieve without additives. Confirm the exact grade and its documentation before ordering; a generic "natural POM" rod may not carry the certificate you need.
Material profiles
POM (acetal, Delrin type)
Machines very cleanly, holds tight tolerance and is affordable. Homopolymer is slightly stronger; copolymer resists hot water and alkalis better and is less prone to porosity in thick sections. A strong default for precise mechanical parts at moderate temperature.
Nylon PA6 / PA66
Tough and impact resistant with good wear. Cast nylon is available in large sizes. Watch moisture absorption and tolerances.
PTFE
Chemically inert, very low friction, good electrical insulator. Soft, creeps and hard to hold to tight tolerance. Filled grades improve wear and creep.
UHMW-PE
Cheap, very tough and abrasion resistant. Poor dimensional stability and low heat resistance; best for liners and wear strips.
PEI (Ultem)
Amber, strong and stiff at temperature, inherently flame resistant, and able to tolerate repeated sterilization. Notch sensitive, so avoid sharp internal corners.
PEEK
The high-performance option: strength, stiffness, chemical resistance and heat resistance together. Expensive as raw stock, so it is reserved for parts that genuinely need it. Glass and carbon filled grades add stiffness.
A quick decision path
- Over about 150 °C continuous: PEEK or PEI, or PTFE if loads are low
- Aggressive chemicals: PTFE first, then PEEK
- Precise sliding or gear parts at moderate temperature: POM
- Tough wear parts where tolerance is relaxed: nylon
- Large wear liners on a budget: UHMW-PE
- Sterilization and flame resistance: PEI or PEEK
Design notes for machined plastic parts
Plastics are softer and less stiff than metals, so the cutter pushes thin walls and they spring back. Keep walls thicker than you would in aluminum where possible, and avoid very deep narrow pockets. Generous internal radii reduce stress concentration, which matters most for notch-sensitive plastics such as PEI and some filled grades.
Tolerances should reflect the material. Tight tolerances are achievable in POM and PEEK, but nylon, PTFE and UHMW-PE move with temperature, moisture and stress relief. Applying a blanket metal tolerance to a UHMW part creates inspection disputes without improving function.
Threads in plastics are weaker than in metals. For repeated assembly, use threaded inserts rather than tapping directly into the plastic, especially in PTFE and UHMW-PE.
Molding versus machining
Machining from stock suits prototypes and low volumes, and it handles high-performance materials such as PEEK without tooling cost. At higher volumes, injection molding becomes economical, but molded parts follow different design rules, including draft and uniform wall thickness. A molded grade may also differ from the rod grade. If a part will move to molding later, design with that in mind from the start.
Common selection mistakes
- Choosing PEEK by default when POM would do the job at a fraction of the material cost
- Holding nylon to metal-style tolerances and then rejecting parts that moved with humidity
- Using unfilled PTFE as a loaded bearing and seeing it creep out of shape
- Forgetting that UHMW-PE softens at temperatures a hot vehicle interior can reach
- Specifying a food-contact application without asking for the compliant grade and its paperwork
- Ignoring sharp internal corners in notch-sensitive materials, which then crack at assembly
Each of these mistakes is common, and each is easy to prevent if the requirement list is written down before the material is chosen.
Prototyping and validation
A machined prototype is the quickest way to confirm that a plastic will work. Test it at the real temperature, with the real fluids and under realistic loads for long enough to see creep. For parts that will be molded later, remember that a machined prototype from rod may behave somewhat differently from a molded part, because the molding process introduces flow orientation, weld lines and different internal stresses. Use machined prototypes to choose the polymer family, then confirm with molded samples before final release.
If the first choice fails a test, the failure mode usually points to the next choice. Creep suggests a stiffer or filled grade, swelling suggests a less absorbent polymer, and cracking suggests a tougher material or larger radii.
Getting it right on the drawing
Name the polymer, the grade or trade name if it matters, the color, and any filler. "Natural PEEK 450G" or "black acetal copolymer" removes ambiguity. Where compliance documents are needed, list them in the notes so they are quoted and shipped with the parts.