Desktop
Resin Data
POM

Polioximetileno (Acetal)

POM·Engineering·Semi-crystalline

POM (Polyacetal or Polyoxymethylene) is the engineering plastic with the best combination of rigidity, dimensional stability and wear resistance on the market — earning it the nickname "the metal of plastics" for its ability to replace zinc, brass and aluminum in precision gears, snap-fits, electrical connectors, pumps, valves and thousands of close-tolerance mechanical parts. You also know it by its brand names: Delrin (DuPont, homopolymer), Celcon and Hostaform (Celanese, copolymer).

But it carries a unique risk: above 230°C it decomposes releasing formaldehyde, a toxic, irritant and carcinogenic gas. And the process window is narrow — a bad purge or a 20°C overheat can clear the shop floor. Here we have compiled the reference ranges from the PDS, plus the questions that come up over and over on the shop floor: when homopolymer (Delrin) makes sense vs copolymer, why you need good ventilation, how to avoid centerline porosity in thick parts, snap-fits and hinges, and when to switch to PA6.

Share your experience in the comments — ranges vary by manufacturer and grade, and collective discussion is what gets us out of trouble on the floor.

The ranges shown in these data tables were compiled by the MVPS team from various parameter sheets and literature, integrating the lower and upper limits for each material type.

This information must be carefully reviewed when developing injection molding processes. Final ranges and processing tolerances are the responsibility of the engineer in charge.

These ranges are not recommended for developing specific process tolerances. MVPS always recommends requesting and consulting the supplier's data sheet.

General Properties

Chemical StructureSemi-crystalline
Specific Gravity (Density)1.42:1
L/D Ratio18 – 22:1
Compression Ratio2.5 – 3:1
Tonnage Factor4.63 – 6.18kN/cm²
Thermal Diffusivity0.1489mm²/s
Max Shear Rate40,0001/s
Shrinkage1.5 – 2.5%
Regrind⚠ Caution
Heat Deflection (HDT) @ 1.82 MPa110°C
Glass Transition (Tg) @ 10°C/min-85°C
Vicat Softening @ 50N144°C

Drying

Drying Temperature91 – 110°C
Drying Time2 – 4h
Recommended Moisture0.2%
Recommended Dryer TypeAir
Dew Point-40°C

Temperatures

Melt193 – 216°C
Nozzle185 – 221°C
Front185 – 216°C
Middle185 – 199°C
Rear185 – 199°C
Demolding66 – 116°C
Mold (Cooling)49 – 104°C
Feed Throat10 – 49°C

Processing

Back Pressure2.1 – 4.8bar
Screw Speed30 – 50RPM
Injection SpeedMedium – High
Barrel Occupancy25 – 75%
Injection Pressure700 – 1,200Pbar
Holding Pressure175 – 960Pbar
Cushion6.4 – 12.7mm

Mold

Runner Diameter3.05 – 6.1mm
Gate Diameter0.76 – 1.52mm
Gate Area0.46 – 1.82mm²
Wall Thickness0.76 – 3.81mm

Venting

Depth (Vent Depth)0.0203 – 0.0406mm
Land (Vent Land)0.508 – 1.02mm
Width (Vent / Clearance)3.05 – 10.2mm
Relief (Relief Channel)0.127 – 0.254mm

Frequently asked questions

POM (polyoxymethylene, also called polyacetal) is a semi-crystalline engineering thermoplastic made of repeating (–CH₂O–) units — literally chains of polymerized formaldehyde. That simple, highly crystalline structure (75–80%) gives it its unique signature: extreme stiffness, exceptional dimensional stability, natural low friction and outstanding wear resistance. Density ~1.41–1.42 g/cm³ (denser than most plastics).
Homopolymer POM-H (Delrin): pure (–CH₂O–) chain. Higher stiffness (~10% more), better fatigue resistance (gears that cycle millions of times), greater mechanical strength. Cons: narrower process window, prone to centerline porosity in thick sections (>25 mm) from outside-in crystallization, weaker chemical resistance to alkalis. Copolymer POM-C (with dioxolane): better thermal stability, wider process window, no centerline porosity (key for parts machined from stock), better chemical and hot-hydrolysis resistance. Rule of thumb: high-demand gears and snap-fits → Delrin. Parts machined from thick stock, hot water or alkali contact → POM-C.
Yes — formaldehyde is toxic, irritant and classified as carcinogenic by IARC (group 1). At normal processing temperatures (190–220°C) emission is low but detectable; above 230°C depolymerization accelerates and release can become dangerous. Exposure symptoms: eye/nose/throat irritation, headache, occupational asthma. Mandatory measures: local exhaust over the machine, good general ventilation, TWA limit 0.75 ppm (OSHA) / 0.5 ppm (EU). If the machine smells of 'formaldehyde' when you're near it — shut it off and check temperatures NOW.
POM absorbs very little (0.2–0.25% at equilibrium vs 3% for PA6), but surface moisture on pellets plus ambient humidity can cause bubbles, silver streaks and localized degradation via catalytic hydrolysis. The PDS marks desiccant at 80°C for 2–4 h, dew point ≤ –30°C. In dry climates you can skip with material that comes in sealed desiccant bags and is opened just for use, but in humid climates (tropical, coastal) always dry. Moisture also accelerates decomposition to formaldehyde.
It is a defect unique to POM-H (Delrin) in thick sections. POM-H solidifies from the outside in during cooling; on crystallization it shrinks ~2.5%, and the already-rigid surface material cannot compensate for core shrinkage → microvoids or hollow channels form along the central axis. Invisible from outside but catastrophic if the part is later machined and the core is exposed (broken seal, fluid infiltration). Solutions: (1) use POM-C which doesn't have this problem, (2) if you insist on POM-H, keep sections <25 mm, high holding pressure and long hold time, (3) wide gates to keep flow open during packing.
Very high shrinkage: 1.8–2.5% (typical semi-crystalline), and it shrinks more in the flow direction than perpendicular → warping tendency in flat parts. Solutions: (1) hot mold (80–110°C) for complete symmetric crystallization — fundamental, (2) generous holding time to compensate shrinkage, (3) uniform walls <15% variation, (4) multiple balanced gates on large parts. For submillimeter precision → always POM-C with mold at 90–110°C. Good news: once molded and stabilized, POM holds tolerances better than almost any other plastic (vs PA6 which keeps growing with humidity).
POM wins on: dimensional stability (absorbs <0.25% vs 3% for PA6), precision, dry-running (self-lubrication at low speed), broader chemical resistance, no ambient humidity effects. PA6 wins on: impact and toughness, wear resistance under high sustained load (PA6-GF even better), higher continuous temperature tolerance, cost (~30% cheaper). Rule: watch gears, precision, electronics → POM. Industrial gears under load and impacts → PA6-GF. Gears in variable humidity environments → POM (PA6 changes dimensions).
The PDS marks 60–105°C. Hotter (90–110°C) = higher final crystallinity, better dimensional stability, better surface, less stress, but longer cycle. Cooler (60–80°C) = shorter cycle but parts with incomplete crystallinity that will keep shrinking days/weeks in storage. For gears and precision parts: minimum 90°C, ideal 100–110°C. Never below 60°C — cold POM generates frozen-in crystallization and the part changes dimensions later.
The PDS marks 20–25% as the maximum regrind. Each cycle slightly degrades molecular weight and increases sensitivity to formaldehyde formation. Critical purging when changing material: never mix POM with PVC, halogens or acids — contamination can cause massive spontaneous formaldehyde release (danger). To purge POM when changing: (1) lower temperature to 180°C before purging, (2) use stabilized purge (polystyrene or commercial compound), (3) never purge POM with standard LDPE/HDPE — use specific purge. If someone ran PVC before and you start POM without purging = potential plant evacuation.
Yes — specific POM-C food-grade resins are approved by FDA, EFSA, NSF and BfR for food contact. Thanks to its low moisture absorption and excellent wear resistance, it's ideal for food machinery components (conveyors, valves, bushings in bottling lines, blender parts, water dispensers). Important: not all POM is food-grade — ask the supplier for a specific lot certificate, especially for colored or lubricated grades (PTFE, MoS₂) that may contain non-certified additives.

Sources

Discussion (0)