POM (Polyoxymethylene/Polyacetal)

Definition and Classification as an Engineering Plastic

POM (polyoxymethylene), also called polyacetal, is a thermoplastic engineering plastic. Thermoplastic means that the material can be deformed by heating and solidifies again on cooling. POM is semi-crystalline — that is, it has ordered (crystalline) and disordered (amorphous) regions in its structure.

This semi-crystalline structure explains the typical combination of properties: high stiffness and rigidity, low friction, good wear resistance, very good dimensional stability, and low water absorption. In sealing technology, precisely this combination matters, because components are often tightly toleranced and slide over many cycles.

Relevant Material Properties for Sealing and Guide Elements

In hydraulics and pneumatics, the question usually is what POM is used for within the sealing system. POM is frequently chosen when a component has to guide, support, or slide and at the same time has to reliably maintain its geometry. The low water absorption helps, because dimensions remain comparatively stable in moist environments or under condensation.

Creep tendency is also important. Creep is a time-dependent deformation under sustained load. In back-up and guide elements, creep can change the gap size and thereby worsen friction, guide clearance, or the support effect. POM frequently shows a comparatively low creep tendency here, which stabilizes the function over time.

For the classification in sealing technology, the following applies: POM is typically a hard structural material. In many cases it does not take over the actual sealing against the medium but mechanically supports the sealing system.

Friction and Wear (Tribology)

Tribology describes friction, wear, and lubrication between moving contact partners. POM is frequently used in dynamic applications because it can offer low friction and good wear resistance. As a result, friction forces often drop and uniform motion is supported — for example with piston or rod guides.

In design, however, what the friction and wear behavior depends on should always be considered: mating material (e.g., steel), surface roughness, lubrication, speed, and load strongly determine the result. In many systems, POM combines well with metallic sliding partners; nevertheless, the tribological assessment remains a system question.

Dimensional Stability, Water Absorption, and Creep

Dimensional stability is directly noticeable in sealing systems, because even small geometry changes affect sealing gaps and contact conditions. Compared to many other polymers, POM frequently absorbs little water. As a result, dimensional scatter caused by moisture or condensation is reduced.

Creep becomes critical particularly when POM works as a support element under continuous loading. When deformation remains small, the support effect is preserved as well. Therefore, POM is attractive for many guide and back-up applications, as long as load, temperature, and time stay within the permissible range.

Typical Applications in Hydraulics and Pneumatics

In practice, the question is mostly where POM sits in the sealing assembly. It frequently complements a soft sealing element (e.g., an elastomer) and takes over a mechanical secondary function. A typical configuration is: the soft material seals, while POM supports or guides and positively influences friction and wear.

Common components are guide rings, slide rings, and back-up rings. These parts help to absorb side loads, prevent metal contact, and stabilize motion. Particularly in dynamic cylinders, this can extend the service life of the overall system, because sealing edges and mating surfaces are less stressed.

Back-Up Ring (Anti-Extrusion Ring) Against Gap Extrusion

A core application is the back-up ring (also called anti-extrusion ring). It addresses the problem of gap extrusion: under pressure, a soft sealing material can be pushed into an existing gap. This gap exists, for example, between piston and cylinder tube or between rod and guide. When the seal is pressed into the gap, it can shear off or be permanently damaged.

A POM back-up ring acts here as a pressure-resistant, gap-bridging partner. It reduces the tendency to extrude, because it mechanically “covers” the gap and thereby relieves the soft seal. This is particularly relevant when high pressure, larger gap sizes, or dynamic load changes occur.

Variant Selection, Media Resistance, and Limits (Including Safety)

For material selection, what is decisive is which POM variant is used and which media and temperatures actually apply. POM is roughly divided into POM-H (homopolymer) and POM-C (copolymer). As a rule of thumb, POM-H is often described as somewhat stiffer and stronger, while POM-C is frequently considered more robust against thermal and chemical influences. In sealing technology, this classification should only serve as a starting point, however, because the actual characteristic values depend strongly on grade and manufacturer.

On media resistance, the following applies: POM is in many applications frequently compatible with oils, fuels, and certain hydraulic media, but suitability depends noticeably on fluid class, additives, and temperature. A blanket approval is therefore risky. Temperature usage also depends on grade; in sealing practice, a rough range of approx. $-40/-50\hspace{0.17em}°C$ to $+100/+120\hspace{0.17em}°C$ is frequently cited. Above this, mechanical parameters and long-term stability typically decrease.

For quick orientation, looking at a few data sheet points often helps:

POM-H vs. POM-C: Selection Principle

In practice, selection often begins with the question of whether higher stiffness or higher robustness against temperature and media is more important. POM-H is frequently chosen when high stiffness is in the foreground. POM-C is frequently chosen when thermal and chemical operating conditions are more critical.

In the end, however, the data sheet comparison for the specific load case decides. Relevant parameters include E modulus, tensile strength, heat distortion resistance, water absorption, and the permissible continuous-use temperature. For dynamic applications, it is also worth looking at tribological test values, when they are available for the matching mating material.

Media and Temperature Limits and Safety

The most common misconception is to treat media compatibility as a fixed property. In reality, temperature, additives, and exposure time act together, which is why tests or approvals are often tied to media and grade. As temperature rises, strength and wear reserve usually drop, and safety against creep becomes smaller.

Thermal loading is also safety-relevant: under overheating or thermal decomposition, POM can release formaldehyde. This concerns in particular incidents in operation or processing. Therefore, thermal limits should be respected and suitable measures in design and occupational safety should be considered.

A brief note in closing: for critical media, high pressures, or tight gap sizes, data-sheet- and application-based specialist consultation is sensible, because small changes in operating conditions can noticeably affect component function.