POM plastic properties, applications and processing considerations

What POM plastic is and why it is specified

POM plastic is an engineering thermoplastic used where a part needs stiffness, low friction, wear resistance and stable dimensions without the weight or corrosion concerns of metal. It is also called acetal, polyacetal or polyoxymethylene. In production, engineers often specify POM for gears, bushings, rollers, clips, valve parts, conveyor components and precision mechanical parts that slide or rotate against another surface. It is not a universal substitute for nylon, ABS, PC or metal, but it is often the right choice when moisture uptake, friction and repeatable tolerances matter. For more processing topics, visit the Plastic Processing section.

For most projects, the decision is not simply whether to use POM. It is which POM family, grade and processing route fit the part. Product and processing literature from Delrin, Celanese, Mitsubishi Chemical Group and Ensinger consistently describes acetal as a semi-crystalline material with a narrow but manageable processing window. That combination gives POM its useful performance, but it also explains why mold temperature, residence time, shrinkage and thermal degradation require close attention.

Key properties that define POM performance

POM is valued because it combines several properties that are difficult to obtain together in commodity plastics. It is stiff without glass fiber reinforcement, has good fatigue behavior, absorbs little moisture compared with nylon and maintains a smooth bearing surface in many sliding applications. These benefits are grade-dependent, so the figures below should be treated as typical guide values rather than design limits.

Property or behavior	Typical range or description	Why it matters in processing and design
Density	About 1.41-1.42 g/cm3 for many unfilled grades	Heavier than many commodity plastics, but much lighter than metal alternatives.
Tensile modulus	Often around 2.7-3.2 GPa for unfilled general-purpose grades	Supports rigid gears, clips and housings without an immediate need for reinforcement.
Moisture absorption	Low compared with nylon; equilibrium values around 0.2-0.3% appear in common datasheets	Helps hold tolerances in humid environments and after machining.
Friction and wear	Generally low friction with good sliding behavior	Useful for bushings, rollers and gears, especially where lubrication is limited.
Mold shrinkage	Often around 1.8-2.2% for many unfilled grades, depending on flow direction and part design	Requires realistic mold compensation and careful control of crystallinity.
Processing temperatures	Common melt windows are roughly 180-230°C, depending on grade and supplier guidance	Overheating or long residence time can cause degradation and formaldehyde release.

Low moisture uptake is one of the clearest reasons to choose POM instead of nylon for precision parts. Nylon can be excellent for toughness and wear, but it absorbs significantly more moisture, which can change dimensions and stiffness. POM usually provides more predictable dimensions in applications such as gears, sliding blocks, metering components and machined shapes.

The trade-off is that POM is not suitable for every environment. Strong acids, strong oxidizers and some high-temperature wet conditions can be problematic. Unmodified grades also have limitations in ultraviolet exposure and flame performance. Any safety-critical, food-contact, medical, fuel-contact or electrical application should be checked against the exact resin datasheet and the relevant certification, not a generic POM description.

POM homopolymer and copolymer are not identical

Most commercial discussions divide acetal into homopolymer POM and copolymer POM. Homopolymer POM is often associated with the Delrin family. Copolymer POM includes many grades sold under other trade names. Both are acetal plastics, but their structure and performance balance differ enough that processors and designers should not treat them as automatic equivalents.

Selection point	POM homopolymer	POM copolymer
Mechanical strength and stiffness	Often chosen where higher stiffness and strength are priorities.	Usually slightly lower in some mechanical values, though grade choices vary.
Dimensional stability in wet or chemically exposed uses	Good, but the grade and environment must be checked carefully.	Often preferred for hot water, chemical resistance or fluid-handling environments.
Centerline porosity in thick stock shapes	Can be a consideration, especially in thicker extruded shapes.	Often selected to reduce risk in thick machined components.
Processing behavior	Requires strict temperature and residence-time control.	Also requires control, but some copolymer grades are selected for stability in processing.

A useful starting point is the application environment. If the part is a dry precision gear, spring element or mechanical component where stiffness is the leading requirement, a homopolymer grade may be attractive. If the part works around hot water, cleaning chemicals, pumps, valves or wet sliding conditions, a copolymer grade is often evaluated first. This is not a final specification, because additives, viscosity, lubrication packages and regulatory approvals can change the answer.

Common applications for POM plastic

POM is most successful in parts where dimensional stability and mechanical movement meet. Typical applications include small gears, cams, conveyor chain components, bearing cages, guide rollers, zippers, fasteners, pump housings, valve seats, electrical connectors and appliance mechanisms. Many of these parts are not highly visible to consumers, but they depend on repeatable movement over thousands or millions of cycles.

In gears, POM offers stiffness, fatigue resistance and a low-friction surface. It does not remove the need for tooth design, load calculations or noise testing, but it gives designers a proven engineering plastic platform. In bushings and rollers, its low friction can reduce lubrication demands, although abrasive particles, high loads and poor alignment can still shorten service life. In clips and snap-fits, POM can provide spring-like recovery, but sharp corners and over-stressed latch designs should be avoided.

POM also competes with machined metals in some low-load precision components. It is easier to machine than many abrasive reinforced plastics and can reduce corrosion concerns. However, it has lower heat resistance, lower stiffness and higher thermal expansion than metals. POM can replace metal only when the operating loads, temperature, tolerance stack-up and long-term creep behavior allow it.

Processing considerations for injection molding

POM can be injection molded on conventional equipment, but it should not be treated as a forgiving commodity resin. The material is semi-crystalline, so mold temperature, packing, cooling and part thickness strongly affect shrinkage and final dimensions. A short molding trial that only checks appearance may miss post-mold shrinkage or warpage that appears later.

Drying and material handling

Many standard POM grades do not require aggressive drying when supplied and stored correctly, because acetal is not as moisture-sensitive as polyamide or polycarbonate. However, drying may be recommended if the material has been exposed to humid air, if the supplier specifies a moisture limit, or if the part has strict cosmetic and mechanical requirements. Delrin processing literature, for example, commonly refers to drying when moisture is above a specified limit or when material handling conditions are uncertain.

Pellets should be stored dry, protected from contamination and kept away from incompatible materials. Cross-contamination is especially important for POM because acidic decomposition products from other polymers, including PVC, can accelerate degradation. Processors should follow supplier purge recommendations rather than relying on a generic color-change or shutdown routine.

Temperature, residence time and ventilation

The main processing risk with POM is excessive heat history. If the melt temperature is too high, if material sits too long in the barrel, or if dead spots trap resin, acetal can degrade and release formaldehyde gas. Celanese and Delrin processing guides both warn that excessive temperature or residence time can cause discoloration and formaldehyde release. This is both a quality issue and a workplace safety issue.

Good practice includes using the supplier temperature range, verifying actual melt temperature, minimizing unnecessary residence time, avoiding long interruptions with material at processing temperature and ensuring suitable ventilation. If degradation odor, discoloration, splay, bubbling or unstable shot behavior appears, the process should be stopped and investigated rather than corrected only by adding heat or pressure. See also: Buying Guides.

Shrinkage, crystallinity and tooling

POM shrinkage is higher than that of many amorphous plastics because it crystallizes during cooling. Mold temperature influences crystallization, cycle time and post-mold dimensional change. Lower mold temperatures may shorten cycle time, but they can increase the risk of later dimensional movement. Higher mold temperatures can improve crystallinity and dimensional stability, especially for precision parts, but may lengthen cycle time.

Tooling should account for balanced filling, effective venting and consistent cooling. Thick sections should be avoided where possible because they increase sink, void risk and cycle time. Uniform wall thickness, generous radii and well-placed gates help reduce internal stress and warpage. Because POM parts are often used in moving mechanisms, dimensional inspection should include functional features such as bore roundness, gear tooth geometry and flatness, not just overall length and width.

Machining and secondary operations

POM is also widely used as rod, sheet and plate for machined components. Its low moisture uptake and good dimensional stability make it attractive for jigs, fixtures, rollers and precision guides. Machining is usually clean compared with reinforced plastics, but sharp tools, controlled feed rates and heat management still matter. Excessive machining heat can distort parts or affect tolerance.

Secondary operations need realistic expectations. POM is difficult to bond with standard adhesives because of its low surface energy and chemical resistance. Mechanical fastening, press fits, ultrasonic welding, special surface treatment or design changes may be more reliable than ordinary glue. Painting and printing also require surface preparation and testing. If decoration or adhesive bonding is central to the product, another plastic may be easier to process.

How to decide whether POM is the right material

A practical selection process starts with the function of the part. If the part slides, rotates, snaps, indexes or holds a close tolerance in a humid environment, POM deserves consideration. If the part must be transparent, continuously exposed to high heat, bonded with standard adhesives or used in strong acid environments, POM may be a poor fit.

• Define the motion: sliding, rolling, gear meshing, snap-fit recovery or static support.
• Check the environment: humidity, hot water, fuels, cleaners, acids, UV exposure and operating temperature.
• Choose the grade family: compare homopolymer and copolymer options before selecting a trade name.
• Review exact datasheets: use grade-specific values for modulus, impact, shrinkage, melt flow and certifications.
• Validate processing: run trials that measure dimensions after conditioning, not only immediately after molding.
• Test the real assembly: friction, noise, wear and creep depend on mating materials, load, lubrication and alignment.

The value of POM plastic is its balance. It can be molded or machined into precise mechanical parts, and it gives designers a low-friction alternative to metal and moisture-sensitive plastics. The limitation is that this balance depends on grade choice and process discipline. When those are controlled, POM remains one of the most useful engineering plastics for moving, load-bearing and dimensionally sensitive components.

Frequently asked questions

Is POM plastic the same as acetal?

Yes. POM, acetal, polyacetal and polyoxymethylene are commonly used to describe the same engineering plastic family. The important distinction is whether the grade is homopolymer POM or copolymer POM.

Is POM better than nylon?

It depends on the application. POM usually absorbs less moisture and holds dimensions better in humid conditions. Nylon may offer advantages in toughness, abrasion behavior or high-load bearing applications depending on grade and reinforcement. The choice should be based on the environment, load and tolerance requirements.

Does POM need to be dried before molding?

Many standard grades do not need intensive drying if stored properly, but drying may be required after moisture exposure or when the supplier specifies a moisture limit. Processors should follow the resin supplier guide for the exact grade.

What is the main processing risk with POM?

The main risk is overheating or excessive residence time, which can degrade the polymer and release formaldehyde gas. Temperature control, purge discipline and ventilation are therefore important parts of safe and stable POM processing.

Can POM plastic be glued?

POM is difficult to bond with ordinary adhesives. Mechanical fastening, welding, surface treatment or a design change is often more reliable. Any adhesive approach should be tested under the real service conditions of the part.