Guide
Nylon machining: how do you set tolerances when nylon swells with moisture?
Nylon machining is the cutting of polyamide rod, tube or plate, most often PA6 or PA66, into bushings, wear pads, rollers, gears and insulators. The cutting itself is easy; the hard part is that nylon absorbs water from the air and grows, so a dimension that measured in tolerance at the machine can drift out weeks later. The fix is to tolerance the part for the humidity it will live in and to state the condition at which it is measured.
By Saola Manufacturing engineering team · Published · Updated
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Why does machined nylon change size after machining?
Nylon absorbs moisture from the air until it reaches equilibrium with the surrounding humidity, and the absorbed water makes it swell. The process is slow and reversible, so a part’s size depends on its moisture history as well as on the machining.
DuPont’s design information for Zytel nylon resins states that 66 nylon equilibrates at about 2.8% water at 50% relative humidity and about 8.5% at 100% RH, and that moisture absorption increases the dimensions of 66 nylons by about 0.6% at 50% RH and about 2.6% at 100% RH. The same guide notes that strength and stiffness rise and dimensions shrink again as the part dries out.
The speed is what catches buyers out. The DuPont guide gives the example that a dry specimen 1.5 mm thick takes about 125 days to reach equilibrium at 50% RH. A thick machined part takes longer still, so it can keep changing for months, and the outer skin and the core may be at different moisture contents when it is measured.
How much will a nylon part grow?
For a dry PA66 part moving to normal indoor air, the DuPont figure implies growth of roughly six thousandths of each dimension, and several times that if the part ends up wet. That is far larger than the tolerances most machined metal parts carry.
For a typical 50 mm dimension, at the 0.6% growth that the DuPont guide gives for 66 nylon from dry to 50% RH equilibrium, it grows by about 0.3 mm; at the 2.6% figure for 100% RH, by about 1.3 mm. Real stock is rarely bone dry when it is machined, so the actual change from machine to service is usually smaller, but its direction and order of magnitude are what the tolerance has to absorb.
Nylon grade changes the answer. The DuPont guide says 612 nylons absorb less water, about 1.3% at 50% RH and 3.0% at 100% RH, and therefore have better dimensional stability, while it describes PA6-based grades as more sensitive to moisture than PA66. Glass or mineral reinforcement also improves dimensional stability.
| Item | PA66 | PA612 | PA6 |
|---|---|---|---|
| Equilibrium water at 50% RH | about 2.8% | about 1.3% | Higher sensitivity than PA66 |
| Equilibrium water at 100% RH | about 8.5% | about 3.0% | Higher sensitivity than PA66 |
| Dimensional growth, dry to 50% RH | about 0.6% | Smaller than PA66 | Larger than PA66 |
| Dimensional growth, dry to 100% RH | about 2.6% | Smaller than PA66 | Larger than PA66 |
| 50 mm dimension, dry to 50% RH | about 0.3 mm | Smaller | Larger |
| Datasheet water absorption test | ASTM D570, 24-hour or equilibrium immersion, reported as percent weight gain | ||
How should a nylon part be toleranced?
Give nylon dimensions wider tolerances than the same metal part, open them up in proportion to the size, and state the moisture condition the tolerance applies to. Tight tolerances belong only on features where the function needs them, and those features may need a different material.
Start from where the part lives. A bushing in a dry indoor machine sees small changes; a roller in a wash-down line or an outdoor part sees large ones. Then size the tolerance so the part works at both ends of its moisture range, not just at the moment it is inspected. For fits, design the clearance for the swollen state: a nylon bushing pressed into a metal housing closes up on its shaft as it absorbs water, so the bore needs extra running clearance. For a press fit, check that the interference at the dry end still holds and that the wet end does not crack the housing or crush the bushing.
Avoid stacking nylon dimensions against tight metal dimensions in one chain. Where a nylon part locates a metal part, let a metal feature carry the precise location and use the nylon part only for wear or insulation.
At what condition should a nylon part be measured?
Write the measuring condition on the drawing: either “dimensions apply dry as machined, within a stated time” or “dimensions apply after conditioning to equilibrium at a stated temperature and humidity.” Without it, supplier and customer can measure the same part days apart and both be right.
The practical choice for most machined parts is dry as machined, measured soon after machining and shipped in sealed bags so the moisture state does not change in transit. It is quick and repeatable, but the customer must accept that the part will grow in service. The alternative is conditioned dimensions, which match service better but require a conditioning step. The DuPont guide describes moisture conditioning, by boiling in water or by immersion in a potassium acetate solution, as a way to speed up moisture uptake and to produce test parts that show whether the expected changes affect performance.
Water absorption on a material datasheet is usually reported by the immersion test in ASTM D570. The test laboratory Intertek describes it as drying specimens, then immersing them, often at 23 °C for 24 hours or until equilibrium, and reporting the weight gain as a percentage. It is a material comparison, not a dimension prediction, so do not use a 24-hour value to set a tolerance.
What else affects dimensions when machining nylon?
Heat, stress and clamping. Nylon expands far more with temperature than metal, can move when internal stresses are released by cutting, and deflects under clamping force, so finishing passes, sharp tools and light clamping matter.
Extruded rod and plate carry internal stress that cutting releases, which can bow long or thin parts after they come off the machine. A common shop practice is to rough machine, let the part rest or anneal it, then finish machine. The DuPont guide notes that rods, tubes and other extruded shapes can be machined into small parts and that prototypes and short runs are commonly machined from rod or slab stock; its section on annealing covers the stress relief side.
Temperature is the second variable. A part measured warm from machining reads larger than it will at room temperature. For any dimension with a tight tolerance, measure at a stated temperature after the part has cooled, and say so on the inspection plan.
When should you pick a different plastic instead of nylon?
When the tolerance is tight and the part lives in changing humidity or water, choose a low-absorption plastic such as acetal (POM), or a reinforced or lower-absorption nylon. Nylon earns its place where toughness, wear resistance and impact strength matter more than size stability.
Acetal is the usual swap for small precision parts such as gears, cams and valve components; our Delrin vs acetal guide covers how to call it out. A 612 nylon or a glass-reinforced grade keeps more of nylon’s character with less movement. For bearings and wear pads, an oil-filled or lubricated nylon reduces friction, but it still absorbs water. If the drawing shows a nylon part with metal-like tolerances everywhere, the material choice is worth a second look before release.
How does Saola handle nylon parts?
Plastics are not on the material lists of the Saola capability pages, so whether a nylon part can be made, and from which grade and stock form, is confirmed at drawing review. The measuring condition and the packaging are part of that review.
The CNC turning and CNC milling pages describe the metal parts the lead plant runs. If your assembly mixes metal parts with nylon components, send the full drawing set with the nylon grade, the moisture condition for inspection and the service environment through the quote form, and the review will say which parts can be quoted.
Frequently asked
Does nylon shrink or grow after machining?
It usually grows as it absorbs moisture from the air, and shrinks again if it dries out. Parts can also move once internal stress from the stock is released.
Should I dry nylon before machining?
Drying matters most for molding. For machining, what matters more is stating the moisture condition at inspection and keeping it stable through shipping.
Is cast nylon more stable than extruded nylon?
Cast and extruded stock differ in internal stress and properties, so check the stock maker’s data. Both are still nylon and still absorb water, so treat moisture growth the same way.
Can I hold tight tolerances on nylon?
Only at a defined moisture condition and temperature, and only for as long as that condition lasts. For fits that must hold in service, consider acetal or a reinforced grade.
Does a nylon gear need special tolerancing?
Yes. Tooth thickness and center distance both change with moisture, so backlash must be designed for the wet state. Many designers choose acetal for small precision gears for this reason.
Sources
- Minlon and Zytel Design Information, Module II, DuPont, 2001
- ASTM D570-22, Standard Test Method for Water Absorption of Plastics, ASTM International, 2022
- Water Absorption 24 Hour/Equilibrium ASTM D570, Intertek, accessed November 2026