Thermal expansion: 4 to 10 times higher than steel
Steel expands by about 12 µm per metre and degree Celsius. POM-C is around 110 µm. PE-1000 is around 200 µm. This means that an 80 mm diameter POM bushing changes by 0.7 mm between winter (5 °C) and summer operation (90 °C). That tolerance is not even visible on the drawing if you designed in steel-world dimensions.
Practical rule: always calculate assembly clearance at the lowest operating temperature, not at 20 °C. For gears and bearings with varying loads: add 0.5 to 1 % of the diameter as extra clearance.
Moisture absorption in polyamide
PA6 and PA6 G absorb moisture from the air: up to 3 % by weight at 100 % RH. The dimensional change is roughly the same percentage. A 100 mm PA6 part grows 2 to 3 mm from dry delivery to moisture saturation. This is not theoretical; it happens in a workshop between January and July.
For PA6 fits: condition the part in a humid environment (60 % RH for 7 days) before finish machining. Or choose POM-C, which absorbs less than 0.4 % moisture.
Creep under load
Plastic does not spring back like steel. A POM screw joint tightened to a torque will lose preload over time: 15 to 25 % in the first week, then slowly towards a plateau. If the joint is critical, design for retightening or use a spring washer to take up the settlement.
Three areas to keep track of, then. Thermal expansion during design, moisture absorption before final dimensions, creep after assembly. Classic workshop handbooks treat plastic as a variant of aluminium. It is not.