Engineering plastics
Chosen for mass, insulation, chemical resistance or running without lubrication. They are machined on the same machines as metal, but behave entirely differently about dimension - and that is where most parts are lost.
Usual grades
How it behaves at the cutting edge
They machine fast, but dimensions behave differently from metal: thermal expansion is several times greater - for POM, close to ten times, and PA6 takes up moisture from the air and keeps changing size for days after machining. POM is the most stable of the three and the right choice for close tolerances. PEEK withstands heat and chemicals but is expensive and needs sharp tools, since it melts readily at the edge. For all of them, tolerances and measuring conditions belong on the drawing rather than being assumed.
What to state on the drawing
Specify the working temperature and whether the part sits in a humid environment. PA6 absorbs moisture and changes size after machining - a part measured good in the workshop can fall out of tolerance after a week in storage. POM is dimensionally stable and is the usual choice where the dimension must stay. PEEK withstands temperature and chemicals, but the material cost changes the calculation: there the allowance is kept tight, not generous.
What it means for the routing
Heat does not leave with the chip as it does in metal - it stays in the part. Passes are therefore taken with sharp tools and constant feed, and stopping the tool in the material leaves a mark. Tight tolerances are stated at the temperature at which they are measured, not at which they are cut: warm plastic is larger, and the difference is measurable on a dimension of a few tenths.
Where it is used
- Guides and dry-running bearings
- Electrical insulators
- Parts for aggressive chemical environments
- Light mechanism components
- Food-contact parts
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