POM-C

What Is POM-C Injection Molding?

Polyoxymethylene copolymer (POM-C), also known as acetal copolymer, is a semi-crystalline engineering thermoplastic built on a carbon-oxygen backbone with a controlled comonomer content that stabilizes the polymer chain. It is one of the most widely specified engineering plastics for precision mechanical parts because it combines high stiffness, low friction, excellent dimensional stability and outstanding fatigue and wear resistance at a moderate cost.

Compared with its close relative POM-H (homopolymer acetal), the copolymer version trades a small amount of mechanical strength for substantially better thermal stability and chemical resistance. POM-C degrades less readily during processing, produces less mold deposit, and resists hot water and alkalis better — which is why it is preferred for gears, pumps, valves and fluid-handling components that operate in warm, wet environments. In everyday engineering terms, POM-C is the safer all-round acetal: easier to process, more forgiving on the tool, and more predictable in long service.

POM-C is available in natural, colored, glass-reinforced and internally lubricated grades, giving designers a range of stiffness, friction and wear options without leaving the same base polymer family.

Key Properties of POM-C

Property	Typical Value	Why It Matters
Density	≈ 1.39 – 1.42 g/cm³	Dense, crisp feel; predictable part weight.
Tensile strength	≈ 60 – 70 MPa	High strength for gear and fastener loading.
Flexural modulus	≈ 2.6 – 3.1 GPa	Stiff parts that hold shape under load.
Coefficient of friction	≈ 0.2 – 0.35 on steel, dry	Self-lubricating; runs without grease in many mechanisms.
Heat deflection temperature	≈ 100 – 110 °C (1.8 MPa)	Continuous use up to roughly 90 – 100 °C.
Water absorption	Low — ≈ 0.2% after 24 h immersion	Dimensions stay stable in humid service.
Fatigue resistance	Excellent	Springs, clips and gears survive millions of cycles.

Processing Parameters We Control

POM-C is a crystalline polymer, so mold temperature and cooling rate directly control shrinkage, crystallinity and dimensional accuracy. The material is not strongly hygroscopic, but drying is still recommended to avoid splay and surface defects, and melt temperature must stay below roughly 230 °C to prevent formaldehyde evolution and tool corrosion.

Parameter	Typical Range
Pre-drying	90 – 100 °C for 2 – 4 h (recommended)
Melt temperature	190 – 210 °C (do not exceed ≈ 230 °C)
Mold temperature	60 – 90 °C
Mold shrinkage	≈ 1.8 – 2.2%
Injection speed	Medium to fast; fast fill improves surface finish

The relatively high shrinkage means tool steel must be cut with compensation, and post-mold dimensional behavior should be verified on representative samples before committing to full production tooling. Because POM-C crystallizes quickly, cycle times are short, which keeps piece costs competitive even for precision components. Venting at the parting line and at core pins prevents trapped air from burning the resin and staining the surface.

Because POM-C combines metal-like stiffness with plastic processability, it frequently replaces brass, steel and bronze in cost-reduction programs.

Common Applications

• Precision gears and gear trains: low friction and fatigue resistance make POM-C the default gear material for printers, actuators, meters and power tools.
• Bearings, bushings and slides: self-lubricating running surfaces reduce maintenance and eliminate grease fittings.
• Fasteners and clips: snap fits, screws and spring clips hold tension over years of service.
• Fluid-handling parts: pump impellers, valve bodies and fittings resist hot water and many chemicals.
• Conveyor and automation components: rollers, guides and chain parts run quietly and wear slowly.
• Electrical components: good insulation properties suit coil bobbins and switch housings.

Design Guidelines for POM-C Parts

• Uniform walls: keep walls between 1 and 4 mm; thick sections invite sink marks and voids because of high shrinkage.
• Rib geometry: use ribs at 0.5 – 0.6 × wall thickness to add stiffness without creating thick sections.
• Radii: blend corners with radii of 0.5 – 1 × wall thickness; POM-C is notch-sensitive in fatigue.
• Draft: provide 0.5 – 1.5° of draft on side walls to ease ejection.
• Shrinkage compensation: design tooling for 1.8 – 2.2% shrinkage and verify with sampling before hard-tooling.
• Snap-fit design: POM-C's flexibility and fatigue life make it ideal for snap fits — design deflections within the elastic limits of the material.

Common Defects and Prevention

• Mold deposits: white or grey deposits on the tool from formaldehyde evolution when melt temperature exceeds roughly 230 °C; lower melt temperature and improve venting.
• Splay: moisture in the resin; dry at 90 – 100 °C before molding.
• Sink marks and voids: thick sections combined with high shrinkage; redesign for uniform walls or use ribs.
• Warpage: uneven cooling in crystalline parts; balance mold temperature and cooling channels.
• Weak weld lines: knit lines reduce strength in stressed areas; reposition gates or raise melt temperature.

POM-C vs. POM-H

The two acetal families are often treated as interchangeable on drawings but differ in real engineering terms. POM-H (homopolymer) has slightly higher tensile strength, stiffness and surface hardness, plus a marginally better surface finish, making it the choice for some high-precision cosmetic parts. POM-C (copolymer) gives up a few percent of mechanical strength in exchange for better thermal stability, lower mold deposits, easier processing over a wider window, and superior resistance to hot water and alkaline environments.

In practice, POM-C is the safer general-purpose selection — especially for fluid handling and for high-cavitation tooling — while POM-H is specified when maximum mechanical properties are the priority and processing can be tightly controlled. For the design engineer, this means a wider safety margin on the machine and fewer surprises in the field. Both materials share the same design rules, so a part can often be re-specified from one to the other without geometry changes.

MOLDITQUICK Capabilities

MOLDITQUICK injection-molds POM-C for precision gears, fluid-handling components and automation parts. Because acetal parts live and die by dimensional accuracy, our process control focuses on the parameters that matter: melt temperature held inside the 190 – 210 °C window, mold temperature in the 60 – 90 °C range, and shrinkage compensation built into the tool from the first draft of the DFM.

We manufacture the molds in-house, so gate placement, venting and cooling-channel design are optimized for your specific part rather than adapted to a standard tool. Upload your drawings to our quote form, or review our injection molding service and mold making service pages to see the full scope of what we deliver.