Key decisions
- Use POM, PA 6 and PEEK as starting families, then compare specific grades under comparable tests.
- Check long-term loading and the operating environment before prioritizing room-temperature strength.
- A qualification plan is more useful than a universal claim that one plastic is best.
Start with the part’s job, not the price of a kilogram
A wear guide, an electrical spacer and a hot-process fixture can have identical external dimensions yet need different materials. The wear guide depends on contact pressure, sliding speed and the opposing surface. The spacer may be governed by insulation and contamination. The fixture may be governed by temperature, chemical exposure and dimensional recovery. A useful material shortlist starts by naming the failure that would make the part unusable.
This comparison uses three identifiable unfilled stock-shape products as reference points: Ensinger TECAFORM AH natural POM-C, TECAMID 6 natural PA 6 and TECAPEEK natural PEEK. Other formulations, cast nylon, reinforced grades and modified bearing grades can behave differently. Product data are screening information, not a guarantee for every geometry or a statement that Beigain holds these brands in stock.
Three reference materials, three different reasons to shortlist them
Ensinger describes the POM-C grade as easy to machine with low moisture absorption and dimensional stability, the PA 6 grade as tough with damping and abrasion properties, and the PEEK grade as suitable for thermal and chemical demands. Those broad distinctions help structure a shortlist. The density entries below are the manufacturer’s published grade values; the selection guidance is an engineering interpretation that requires application testing.
| Criterion | POM-C reference | PA 6 reference | PEEK reference |
|---|---|---|---|
| Example grade | TECAFORM AH natural | TECAMID 6 natural | TECAPEEK natural |
| Published density | 1.41 g/cm³ | 1.14 g/cm³ | 1.31 g/cm³ |
| Reason to investigate | Stable fits in suitable environments | Toughness, damping and wear duty | Thermal and chemical demands |
| Question that can reverse selection | Does the fluid or temperature exceed grade suitability? | Will moisture change clearance or stiffness? | Does performance justify the complete-part cost? |
| Qualification priority | Fit after actual exposure | Moisture state and loaded service | Temperature, creep and chemical combination |
A small clearance makes moisture behavior a design variable
Water absorption values depend on the test method, exposure duration and specimen condition. A 24-hour value is not the same as saturation, and mass uptake is not equal to linear dimensional growth. Avoid comparing a percentage from one test directly with a different test on another supplier’s sheet. Obtain consistent data or test the finished geometry under the exposure that matters.
For nylon, decide whether the dimension is accepted dry, after controlled conditioning or in another agreed state. A shop-floor bore measurement cannot predict an unlimited range of storage and service humidity. If a freely running fit is the requirement, document minimum functional clearance in the relevant environment and assess both mating components. A bigger nominal bore may help one condition but create excess play in another.
Service temperature is not an allowable stress
A published temperature classification cannot be used alone to size a loaded component. Tensile strength from a short laboratory test, heat-deflection temperature under a stated load and long-term service temperature answer different questions. For a spacer compressed under a bolt, time-dependent movement may matter before immediate fracture. For a rotating bushing, generated frictional heat may make the local contact hotter than the surrounding air.
PEEK becomes worth evaluating when a less costly material cannot retain the required function under the combined duty. It still requires stress, duration and environment to be specified. POM may be entirely adequate for a moderate-temperature mechanism with controlled fluids, while nylon may better suit a different impact or wear situation. Material price is relevant after the required duty has eliminated unsuitable candidates.
A bushing selection should compare assembled clearance
Create a test matrix that connects material, geometry and service rather than ordering three visually identical samples and choosing the smoothest. Measure the same functional characteristics in the same state, record load and exposure, and check whether changes recover. The following example deliberately uses acceptance targets rather than a predicted polymer growth percentage.
Worked example: a 20 mm shaft bushing
An illustrative bushing drawing has a 20.20 ±0.05 mm bore and a mating shaft of 20.00 ±0.02 mm. The initial worst-case diametral clearance is 20.15 − 20.02 = 0.13 mm. Suppose the application requires at least 0.10 mm after its defined humidity and temperature exposure. Only 0.03 mm of that initial worst-case margin remains for the combined dimensional effects under the chosen assumptions. Test candidate POM-C, PA 6 and PEEK bushings in the actual housing and duty. Do not estimate the bore change by treating a water-absorption mass percentage as a linear expansion factor.
A reinforced grade is a new material decision
Adding glass or carbon reinforcement is not simply an upgrade to the unfilled comparison. Reinforcement can change stiffness, thermal movement, wear of the mating surface, electrical behavior and machining tools. Directional properties may become more significant. A filled grade that reduces movement in one direction can still require a separate analysis at an interference fit or a thin section.
Likewise, a low-friction additive is useful only in the relevant contact system. The same nominal polymer may experience different wear against a polished shaft, a rough shaft or an abrasive process medium. Request the grade’s applicable wear data, then test the combination of pressure, speed, lubrication and counterface. A generic claim of self-lubrication should not replace a lubricant or contamination requirement in the drawing.
How to turn the shortlist into a purchasing specification
Give Beigain the functional requirements along with the preferred grade. If an equivalent may be proposed, define which properties and documents determine equivalence, and who approves it. A brand substitution should not be accepted merely because both products are called PEEK or both are the same color.
Include the planned repeat quantity and future process. Machining a stock shape is a useful route for many prototypes, but a future molded part may have a different grade, orientation and processing history. If the prototype is intended to validate material behavior, those differences belong in the qualification plan.
- Freeze the grade, filler, stock form and substitution approval rule.
- List fluids, cleaning agents, temperatures, humidity and exposure duration.
- Define critical fits in the assembled and inspection conditions.
- Request material identity and only the application documents actually required.
- Approve a test protocol with measurable pass criteria before comparing samples.
Questions about this topic
Is POM the same as Delrin?
POM is a polymer family. Delrin is a trade name commonly associated with acetal homopolymer, while the POM example here is an acetal copolymer. Confirm the specified grade instead of treating names as automatically interchangeable.
Is PEEK always the strongest choice?
No single strength label resolves a part’s duty. Compare the relevant property at the operating temperature, moisture state, loading duration and direction. Higher material cost also does not prove better wear performance in every contact pair.
Can water absorption be converted directly to size change?
No. Water absorption is generally reported as a mass change under a stated test. Dimensional change depends on the material, geometry, direction and constraints. Use appropriate dimensional data or finished-part exposure testing.
What should be tested first when changing from metal to plastic?
Begin with the limiting function: fit across temperature, load retention, wear, insulation or chemical compatibility. A metal-sized geometry may need different wall sections, bearing areas or clearances before a polymer substitution works reliably.
Technical references
- Ensinger: TECAFORM AH natural POM-C — Reference grade identity, density and qualitative dimensional and moisture characteristics.
- Ensinger: TECAMID 6 natural PA 6 — Reference grade identity, density and nylon-specific selection considerations.
- Ensinger: TECAPEEK natural PEEK — Reference grade identity, density and thermal and chemical positioning; no blanket application approval.
Continue reading
- CNC Machining for Engineering Plastics
- Defining CNC inspection requirements before production
- Request a Manufacturing Quote
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