POM

Polyoxymethylene, often called Acetal, is one of the key engineering plastics. Among plastic materials, it is one with the most crystalline structure. Acetal is a good choice if low friction and good creep/fatigue resistance are required.

There are two types of POM: homopolymer (POM-H) and copolymer (POM-C). The difference between the two is not considerable. It is explained in this article by Hardie Polymers. The Plasticprop sample is a POM-C copolymer type.

Why choose POM?

  • Excellent bearing properties
  • POM has good fatigue and creep-resistance. POM is usually selected as the material for plastic springs, although any prolonged tension of the spring can not be recommended.
  • Retains its properties well in temperatures significantly below zero.
  • Excellent chemical resistance

Why not choose?

  • High shrinkage easily leads to warpage and dimension inaccuracy.
  • Due to small friction co-efficient, POM can be slippery to touch which can be a problem if you intend to use it for a handle.
  • Wear resistance is limited, although it is commonly mentioned as a strength of the material.
  • The surface appearance of POM components is typically oily.
  • POM is flammable; fire-retardant grades are difficult to find.

Typical applications:

  • Petrol tank lid in a car
  • Backpack snap harnesses
  • Plastic zippers
  • Pulley wheels
  • Gears

By examining the Plasticprop sample made of POM, you can draw for example the following conclusions:

  • Grip the sample tightly – you will notice how slippery it is compared with any other sample in the kit. POM makes for good bearing material.
  • When comparing the POM sample with glass-reinforced samples in particular, you can see that it is smaller. The shrinkage of POM is substantial.
  • Considerable corner effect and the slight distortion characteristic of semi-crystalline plastics can be noticed. The sink marks caused by ribs are also clearly distinguishable.
  • The filling window is almost completely filled; POM is more fluid than most other plastic materials. The fluidity also depends on the product grade used.
  • If you stress the sample’s creep indicator for several hours, you will see that POM also suffers from creep, even though it is known for its excellent creep resistance.
  • The sample surface is relatively clean, but ‘splashes’ typical for POM can be noticed around the injection point.
  • The surface of POM feels greasy. Printing on POM is difficult – test this by carefully scraping the stamp on the upper edge.
  • Scratches, especially on the rougher spark-eroded surface side, accompany smaller draft angles. This is somewhat surprising because you would expect POM to shrink apart from the scratching mould surface. Fair draft is recommended.
  • The integral hinge fills up and even functions for a short time before breaking off. For example, as an element facilitating assembly, the hinge could still function.
  • If you freeze the sample overnight, its feeling hardly changes. The properties of POM are maintained well at low temperatures.

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