POM / Acetal Injection Moulding: Properties, Grades & Design Guide
POM (polyoxymethylene), also called acetal or polyacetal, and commercially known as Delrin (DuPont homopolymer) or Hostaform / Celcon (copolymers), is the go-to engineering thermoplastic for lubrication-free mechanical components. It combines the highest stiffness of any standard thermoplastic with exceptional dimensional stability, low friction, and outstanding fatigue resistance. Nordmould uses POM routinely for gears, sliders, snap mechanisms, and precision mechanical housings across automotive, medical, and industrial applications.
What are the mechanical and thermal properties of POM?
POM is a highly crystalline polymer with the highest degree of crystallinity of any common injection-moulding plastic. This crystallinity drives its outstanding stiffness, creep resistance, and wear performance — and also its high shrinkage and need for careful processing.
| Property | POM Homopolymer | POM Copolymer | Test Standard |
|---|---|---|---|
| Tensile Strength | 65–70 MPa | 60–65 MPa | ISO 527 |
| Flexural Modulus | 2,700–3,200 MPa | 2,500–2,900 MPa | ISO 178 |
| Izod Impact (notched, 23°C) | 70–120 J/m | 60–110 J/m | ISO 180 |
| Heat Deflection Temp (1.8 MPa) | 100–110°C | 95–110°C | ISO 75 |
| Vicat Softening Point | 150–165°C | 155–175°C | ISO 306 |
| Density | 1.41–1.43 g/cm³ | 1.40–1.42 g/cm³ | ISO 1183 |
| Mould Shrinkage | 1.8–2.5% | 1.8–2.2% | ISO 294-4 |
| Friction Coefficient (vs steel) | 0.10–0.35 | 0.10–0.35 | ASTM D1894 |
| Water Absorption (24 h) | 0.2–0.3% | 0.2–0.3% | ISO 62 |
POM's tensile strength (65–70 MPa) is notably higher than ABS (40–50 MPa) or PP (30–40 MPa), and its stiffness is the highest of the five standard materials at Nordmould. Water absorption is very low, meaning in-service dimensions are stable even in humid or immersed conditions.
Where is POM injection moulding used?
POM's tribological properties — low friction, resistance to wear, and fatigue endurance — define its application space. It is the standard material wherever plastic must slide against plastic or metal without lubrication.
Precision gears and cams: POM gears operate reliably against each other and against steel, aluminium, or nylon mates. Appliance timer gears, automotive HVAC flap actuators, and pump impellers are common applications.
Sliding and rolling mechanisms: Door lock components, seatbelt buckle slides, conveyor chain guides, roller wheels, and cable tensioners. POM's resistance to abrasive wear extends component life substantially compared to ABS or PP.
Fasteners and snap-fit assemblies: POM's high stiffness and fatigue endurance make it the preferred material for repeated snap engagement. Belt clips, buckles, spring-loaded latches, and retainer rings all exploit this.
Plumbing and fluid handling: Valve bodies, ball-cock floats, pump housings, and spray nozzles. POM's dimensional stability in water is a key advantage over nylon in these applications.
Medical device components: Insulin pump gear trains, drug delivery device mechanisms, catheter valves, and dental unit components. FDA- and ISO 10993-compliant copolymer grades are available.
Consumer goods and sporting equipment: Zip slider bodies, ski binding mechanisms, bicycle derailleur parts, and pen internals.
What are the moulding characteristics of POM?
POM requires disciplined processing. It has a narrow processing window, is sensitive to overheating, and produces formaldehyde gas on decomposition — requiring good ventilation and screw/barrel management.
Melt temperature: 190–230°C (homopolymer), 190–240°C (copolymer). POM decomposes rapidly above 240°C (homopolymer) or 250°C (copolymer). Material must never stagnate in the barrel — purging before and after runs is essential. Do not mix POM with acidic or basic materials (e.g. PVC or nylon) in the same barrel.
Mould temperature: 60–100°C. Higher mould temperature improves surface quality and reduces internal stress; lower mould temperature speeds cycle time but increases the risk of surface sink at thick sections.
Injection speed: Moderate to fast. POM crystallises quickly and can freeze off in thin sections. Relatively fast fill rates are used, but excessive speed causes jetting and surface defects.
Drying: POM has low water absorption and normally requires no pre-drying if stored correctly. If moisture is suspected, 1–2 hours at 80°C in a dehumidifying dryer is sufficient.
Shrinkage: 1.8–2.5%. High and semi-anisotropic, similar in magnitude to PP. Wall thickness uniformity is critical; thick sections shrink more than thin sections and cause warp and sink. Tool compensation must be grade-specific.
Draft angles: Minimum 1–1.5° on all walls. POM parts eject well due to the material's crystalline surface, but inadequate draft in deep ribs produces pull-out or drag scoring.
Sink and warp: Sink marks occur at ribs and bosses exceeding 50% of the nominal wall. Warp is a risk in flat or asymmetric parts. Symmetrical gating and uniform cooling are essential for flat precision parts.
Contamination: Critical. Even trace amounts of acid, alkali, or incompatible polymer can cause violent decomposition. Dedicated barrels or extremely thorough purging between POM and other materials is required.
Which POM grades and variants should you consider?
| Grade / Variant | Key Modification | Typical Use |
|---|---|---|
| POM homopolymer (Delrin-type) | High crystallinity | High-performance gears, precision parts |
| POM copolymer | Better thermal stability, chemical resistance | General engineering, medical |
| POM + PTFE (10–20%) | Reduced friction, dry-running bearings | Bearing surfaces, slide guides |
| POM + GF (10–30%) | Higher stiffness, HDT improvement | Structural brackets, pump bodies |
| POM + UV stabiliser | Improved UV stability | Outdoor mechanisms |
| Food-contact POM | NSF/FDA compliant | Water fittings, food processing |
| Antistatic POM | ESD-safe | Electronics assembly fixtures |
PTFE-filled POM has a friction coefficient against steel as low as 0.05–0.10, enabling dry-running bearing applications that would otherwise require external lubrication. Glass-filled POM at 25% GF achieves a flexural modulus of approximately 6,000–7,000 MPa.
What are POM's advantages and limitations?
Advantages:
- Highest stiffness of standard injection-moulding materials without glass reinforcement
- Exceptional fatigue endurance and creep resistance under sustained loading
- Very low friction coefficient and outstanding wear resistance without lubrication
- Low, stable water absorption: dimensions do not change with humidity or immersion
- Good chemical resistance to fuels, oils, and many organic solvents
- Excellent surface finish directly from the mould
Limitations:
- High shrinkage (1.8–2.5%) demands tight tooling compensation; warp risk in asymmetric parts
- Narrow processing window; decomposes to formaldehyde above 240–250°C — requires ventilation and discipline
- Poor resistance to strong acids and bases; not suitable for acid or alkali chemical equipment
- Cannot be bonded with most adhesives without surface preparation; welding options limited
- Not UV-stable in standard grades; standard grades yellow and embrittle outdoors
- Lower impact strength than PC or ABS at equivalent stiffness
When should you choose POM over alternative materials?
POM vs nylon (PA6/PA66): Choose POM when dimensional stability in wet environments is essential — POM absorbs far less moisture than nylon. Choose nylon for higher-temperature continuous mechanical loading (above 110°C) or where impact rather than wear is the dominant failure mode.
POM vs PP: Choose POM for precision sliding contact, high-stiffness snap-fits, and dimensional accuracy. PP is preferred for chemical resistance, living hinges, food packaging, and lower cost.
POM vs ABS: Choose POM for any sliding, wearing, or precision mechanical application. ABS is preferred for housings, aesthetic parts, and surface-finish-sensitive components.
POM vs PEEK or PPS: If service temperature exceeds 110°C continuously, or if the application involves aggressive chemical exposure that POM cannot tolerate, engineering super-polymers such as PEEK or PPS may be considered. These carry a substantial cost premium and require separate tooling considerations.
Is POM recyclable?
POM carries resin code 7 and is not widely collected in consumer recycling streams. Industrial regrind of sprues, runners, and reject parts is standard practice in professional moulding operations and can be reincorporated at low percentages (typically up to 15–20% of virgin material) without significant mechanical property loss. POM copolymer regrind is slightly easier to manage than homopolymer due to its broader thermal stability window.
End-of-life POM parts present challenges for municipal recycling. Design for disassembly — avoiding metal inserts in POM parts where possible — improves recycling separation in circular-economy supply chains.
Frequently asked questions
What is POM / acetal used for in injection moulding?
POM is the preferred material for precision mechanical parts: gears, cams, bearings, sliders, and snap-fit assemblies. Its low friction, excellent wear resistance, and tight dimensional tolerance make it the standard choice for lubrication-free sliding contact in consumer, automotive, and medical applications.
What is the difference between POM homopolymer and copolymer?
POM homopolymer (e.g. Delrin) has higher crystallinity, giving marginally better stiffness, strength, and fatigue resistance. POM copolymer is more thermally stable, has better resistance to alkaline chemicals, and produces less formaldehyde during processing. Copolymer is the default for most applications; homopolymer suits the most demanding mechanical parts.
What shrinkage should I expect with POM injection moulding?
POM shrinks 1.8–2.5%, one of the higher rates among common engineering plastics. Like PP, POM's semi-crystalline structure causes anisotropic shrinkage. Nordmould compensates for POM shrinkage in the tool design and specifies grade-matched shrinkage factors during DFM review.
Can POM be used in food-contact and drinking-water applications?
Yes — specific POM grades carry NSF 61, FDA, and EU (EC) No 10/2011 food-contact approvals. POM is widely used in water meter components, beverage dispenser parts, and food processing equipment. Nordmould can confirm grade compliance for your application during DFM review.
Why does POM sometimes produce gas or formaldehyde during moulding?
POM can decompose to release formaldehyde when overheated, especially above 220°C (homopolymer) or 240°C (copolymer). Proper melt temperature control, adequate screw design, and avoidance of stagnant material in the barrel are essential. POM must never be contaminated with acidic or basic materials in the barrel.
How does POM compare to nylon (PA6 / PA66) for mechanical parts?
POM has lower water absorption than nylon, so its dimensions and mechanical properties are more stable in humid or wet environments. Nylon offers higher temperature resistance, better impact strength, and broader chemical resistance at elevated temperatures. For sliding wear parts in wet or humid conditions, POM is usually preferred.
Is POM suitable for injection-moulded gears?
POM is the standard thermoplastic for low-to-mid load gear applications. Its fatigue resistance allows long gear life under cyclic loading; its low friction coefficient (0.10–0.35 against steel) reduces power loss and heat generation. Nordmould has produced gears, cams, and rack segments from POM for automotive, appliance, and medical applications.
Submit your STEP file to Nordmould for a free DFM review — our team will confirm wall thickness, draft, and shrinkage compensation for your POM part before the tool is cut.