Sources and scope for this page: Industry convention — no single governing standard (conventional usage, definitions vary between sources), covering the definition of water absorption (plastics). No independent source check is recorded for this page: the content rests on the sources listed and on the working shown on the page. How sources, methods and updates are recorded.
Definition
Absorbed water acts as a plasticiser — it pushes the polymer chains apart, so the part swells and softens. Nylon 6/6 takes up 1.2 to 1.5% in 24 hours and about 2.5% at equilibrium, which grows a machined part by 0.5 to 1.5% linearly.
That movement happens after machining, so no amount of precision in the cut controls it. PTFE and UHMW-PE absorb almost nothing, which is why their tolerances look tighter relative to how soft they are.
Common misuse
Machining nylon dry to a tight tolerance and expecting it to hold. Condition the stock to the service humidity before the final cut, or design around the swell.