Unfilled acetal (POM) grades

Unfilled acetal (POM) grades: precision, low friction and stable operation of moving parts

Unfilled acetal (POM) grades are engineering thermoplastics for parts where stable geometry, a low coefficient of friction, good wear resistance, stiffness and dimensional repeatability in series production matter. Acetal is widely used in gears, bushings, guides, rollers, retainers, latches, precision mechanical elements and parts that operate in motion or in contact with other materials.

The technical value of unfilled POM comes from its high crystallinity, low water absorption, stable modulus, good fatigue endurance and its ability to work without additional reinforcement in many mechanical assemblies. Unlike materials whose properties change substantially after moisture conditioning, acetal better preserves the dimensions, fits, clearances and functionality of precision parts in service.

The engineering role of unfilled POM in a mechanical assembly

POM works where a part must not simply withstand a load, but repeat a mechanical function many times: slide, rotate, latch, snap, transmit force or maintain a precise clearance. For such products, what matters is not only tensile or flexural strength, but also friction stability, wear in the mating pair, creep, fatigue behavior and accuracy after prolonged operation.

Unfilled POM grades are often chosen precisely because they combine stiffness with sufficient processing ductility, deliver a smooth as-molded surface, machine well and suit parts where glass fiber or mineral filling may be undesirable due to abrasiveness, noise, increased friction or the risk of counterface wear.

POM-H and POM-C: why the acetal type matters

In technical selection, it is important to distinguish between homopolymer acetal POM-H and copolymer acetal POM-C. POM-H typically has higher crystallinity, stiffness, strength and fatigue endurance, so it can be appropriate for precision mechanical parts with high requirements for modulus and service life.

POM-C typically offers better chemical stability across a wider range of environments, a lower risk of degradation during processing and more forgiving molding behavior. In many series-production parts, POM-C is the practical choice thanks to its combination of stable processing, good mechanical properties and lower sensitivity to certain chemical factors. The choice between POM-H and POM-C should be based on the function of the part, not merely on the generic name “acetal”.

Low friction and wear resistance without additional reinforcement

Unfilled POM is valuable in friction assemblies thanks to its low coefficient of friction, smooth surface and stable behavior in sliding or rotation. It can run against metal, plastic or other materials, provided the contact pressure, motion speed, temperature and lubrication conditions match the capabilities of the specific grade.

For gears, bushings, rollers, guides and sliding elements, it is important to evaluate not only the overall wear resistance, but the actual mating pair. A material that runs well against steel may behave differently against another polymer; dry friction, lubricant, dust, moisture, temperature and load cycling significantly change the service life of the part.

Dimensional stability, low water absorption and precise fits

Acetal has low water absorption compared with polyamides, so it better preserves dimensions in parts with precise fits, clearances, gear profiles, snap-fits, guides and mechanical joints. For products that must operate without jamming, play or geometry changes in service, this property has direct manufacturing significance.

In a production part, the stability of POM depends not only on the material, but also on shrinkage, mold temperature, holding pressure, part geometry, wall thickness and proper cooling. For precision parts with tight tolerances, it is important to control not only the TDS values, but the actual dimensional repeatability in the specific mold.

Fatigue endurance, elasticity and snap-fit performance

Unfilled POM is well suited for parts that deform repeatedly within the allowable elastic range: snap-fits, spring retainers, clamps, button mechanisms, locks, hinge elements and small mechanical components. For such parts, the key factors are fatigue endurance, modulus stability, low creep and the material's ability to return to its working geometry after loading.

At the same time, the design must be engineered for a polymer, not copied from a metal part. Excessively sharp corners, stress concentrators, insufficient thickness at the base of a snap-fit or too much assembly travel can lead to failure even in a correctly selected POM. For functional retainers, the critical factors become radii, the length of the flexible element, the melt flow direction and weld line quality.

Chemical resistance and limitations of acetal

POM has good resistance to many fuels, oils, salt solutions, weak alkalis and technical fluids, which makes it suitable for mechanical and technical assemblies. At the same time, acetal has limitations regarding strong acids, oxidizers, some chlorinated media and conditions that can trigger chemical degradation of the material.

For a part in contact with chemicals, compatibility should be verified not against a general material description, but against the specific medium, temperature, contact duration, mechanical stress and POM type. POM-H and POM-C can behave differently in particular environments, so critical parts require a case-by-case assessment of the service conditions.

Processing of unfilled POM grades

Acetal processes well by injection molding, but demands temperature discipline. POM is sensitive to overheating and excessive residence time in the barrel, so stable processing requires control of the melt temperature, cycle time, mold venting, equipment cleanliness and the absence of incompatible polymer residues in the machine.

It is especially important to avoid mixing POM with incompatible materials, notably PVC or certain acidic components, as this can cause hazardous degradation. Production must also account for shrinkage, crystallinity, mold temperature, holding pressure and cooling stability, since these parameters directly affect the dimensions, warpage and mechanical behavior of the finished part.

Typical applications of unfilled POM

Unfilled acetal grades are used in parts that require precision, low friction, dimensional stability and repeated mechanical operation:

• gears, toothed wheels, worm drives and mechanism elements;
• bushings, rollers, guides, sliders and sliding elements;
• snap-fits, retainers, spring elements, buttons and lock mechanisms;
• precision molded parts with tight tolerances and stable fits;
• parts for household appliances, industrial equipment, automation and mechanical systems;
• components of fuel, hydraulic or technical systems after chemical compatibility verification;
• moving parts requiring a combination of stiffness, low friction and wear resistance;
• parts where glass filling is undesirable due to counterface wear, noise or increased abrasiveness.

Critical parameters for selecting unfilled POM

To select a standard acetal grade correctly, evaluate not just hardness or strength, but the full function of the part:

• the acetal type: POM-H or POM-C;
• the friction level, wear and the material of the mating pair;
• the accuracy of fit dimensions, clearances and geometry after molding;
• creep, fatigue endurance and the duration of mechanical loading;
• the operating temperature and permissible deformation over time;
• contact with fuel, oils, water, solutions or chemical media;
• the presence of snap-fits, spring elements, thin sections or weld lines;
• the flowability of the grade, wall thickness and the ability to fill the mold consistently;
• shrinkage, warpage and dimensional repeatability in series production;
• processing safety and compatibility with equipment.

Selection of unfilled POM grades from Material Wizard

Material Wizard selects standard unfilled acetal grades based on the real function of the part: friction, wear, precision, load, operating temperature, contact with the environment, mechanism type, service-life requirements and the stability of series molding. We help determine whether you need POM-H, POM-C, a high-flow grade, a material with increased fatigue endurance, or a switch to a special modification.

This approach lets you choose not simply “acetal”, but a material that matches a specific mechanical function: a gear, bushing, guide, snap-fit, retainer or precision molded part. For a manufacturer, this means stable processing, fewer dimensional problems, controlled wear and predictable part performance in series production.