The Part Passed Inspection—Then It Changed
Engineering plastic moisture absorption can make a machined part change size after it has already passed inspection.
A nylon guide was machined, measured, and packed on a dry day. Several days later, the customer reported that its sliding fit had become tight. The machining dimensions had not changed—but the material had absorbed moisture from the surrounding air.
Unlike metals, many engineering plastics exchange moisture with their environment. Water molecules enter the material over time, changing its mass and sometimes its dimensions, stiffness, and fit with mating parts.
Why Moisture Changes the Size
Some polymer structures attract and retain water more readily than others. When moisture diffuses into the material, it can separate polymer chains slightly. The result is often dimensional growth.
The speed of this change depends on relative humidity, temperature, exposure time, and the part’s thickness. Thin sections may reach moisture equilibrium faster than thick sections. A part stored in a humid environment can therefore measure differently from the same part measured immediately after machining.
The movement may not be uniform in every direction. Material grade, fiber reinforcement, molding history, and internal stresses can all influence the final result.
Which Materials Need Attention?
Nylon grades, including PA6 and PA66, are well known for moisture sensitivity. They can gain moisture after machining and show noticeable changes in dimensions and mechanical behavior.
Other materials behave differently:
- POM/acetal generally has low moisture absorption and good dimensional stability.
- PEEK and PPS are often selected where stability is important, but the exact grade still matters.
- Polyimide may absorb moisture and requires careful environmental control for precision applications.
- Glass-filled grades can reduce movement in some directions, but may also create directional behavior.
Never make a material decision from the base polymer name alone. Confirm the supplier data for the specific grade, filler content, and service environment.
Do Not Inspect Only in the “As-Machined” Condition
A dimension checked immediately after machining describes the part at that moment. It may not describe the part after storage, shipping, or assembly.
For moisture-sensitive components, define the inspection condition. The drawing, purchase specification, or quality plan should state whether dimensions apply:
- After a defined conditioning period
- At a specified temperature and relative humidity
- In the dry-as-machined condition
- After equilibration in the expected service environment
Without this information, a supplier and customer may both measure correctly but obtain different results.
Machine for the Intended Condition
If the final application is humid, machining every feature to its dry-state nominal size can create an assembly problem later. The required machining target may need an allowance based on validated conditioning tests.
This should be established through material trials, not a general rule. Record the starting moisture condition, conditioning environment, part thickness, elapsed time, and measured dimensional change. Use production-representative material and machining conditions.
Control the Material Before and After Machining
Keep incoming material sealed and identified. Avoid mixing dry stock with stock that has been exposed to unknown humidity. After machining, package parts in a controlled way if their inspection condition must be maintained.
For close fits, include conditioning and inspection requirements in the control plan. The goal is not to prevent every moisture change. It is to ensure the part reaches assembly in the dimensional state the design requires.
Conclusion
Moisture is part of the operating environment for many plastics. When a precision plastic part changes size after machining, the root cause may be material conditioning—not a machining error.
Define the intended moisture condition early, then machine and inspect to that condition.
