If you mold nylon parts, you have almost certainly hit this "mystery": right out of the mold, the dimensions pass inspection. Yet a few months later, the customer reports the parts no longer fit. The mold hasn't changed, the process hasn't changed, and no one touched the dimensions — so what went wrong?
The answer is one sentence: nylon keeps "drinking" water.
Here is a nylon part (PA6) we mold in our workshop. It once came out of the mold slightly undersized, but after moisture absorption it settled right back inside the drawing tolerance — exactly the rule we just described.
Why Does Nylon Absorb Water?
Nylon (polyamide, PA) is a classic hygroscopic engineering plastic. Its molecular chains are lined with hydrophilic amide groups (—CONH—), which act like countless tiny "hands" that grab water molecules from the air and pull them between the chains through hydrogen bonds.
So as long as nylon is exposed to air, it keeps absorbing moisture until it reaches moisture equilibrium (saturation) — a process that takes months, not days.
How Do Dimensions Change After Moisture Absorption?
Water molecules slip between the molecular chains, acting like "spacers" that push the chains apart. On a macro scale, this means:
External dimensions grow — and so do internal holes.
This is precisely why nylon parts are "hard to dimension" — it is not unstable processing, but the material itself expanding over time.
So how much does it grow? We have measured it in our workshop, and the pattern is very consistent:
| Material | Dimension at moisture equilibrium |
|---|---|
| PA6 / PA66 (unfilled) | 1.005× original |
| PA6+30%GF / PA66+30%GF | 1.003× original |
Put simply: a 100 mm unfilled nylon dimension becomes 100.5 mm when fully saturated; with 30% glass fiber, it becomes 100.3 mm.
Why does glass-fiber reinforcement grow less? Because glass fiber itself does not absorb water — it occupies part of the part's volume and effectively "dilutes" the nylon matrix's moisture expansion. That is also why glass-filled nylon has better dimensional stability.
Moisture Does More Than Change Dimensions
Once inside the nylon, water also acts as a plasticizer — it breaks the hydrogen bonds between molecular chains, letting them slide more easily. The result:
- Toughness and impact strength rise (dry nylon is brittle; conditioned nylon is tough)
- Strength, modulus, and hardness drop
- Electrical insulation degrades
That is why nylon gears, snap-fits, load-bearing parts, and electrical parts are often moisture-conditioned before shipping — to bring the part close to its in-service equilibrium moisture content ahead of time, locking in both dimensions and performance.
How to Put This Rule to Work
The real value of this 1.005 / 1.003 rule is working backward to set production dimensions:
Since the part will already be at equilibrium when it is delivered and assembled, we can reverse-calculate — what dimensions should the part reach 24 hours after molding, so that it lands within tolerance once it reaches moisture equilibrium?
Paired with accelerated moisture conditioning (per ISO 1110, the table below shows conditioning times for nylon parts of different wall thicknesses), the part can reach equilibrium before it leaves the factory — so the customer receives a dimensionally stable part from day one.
Final Word
Moisture absorption is not a defect in nylon — it is the nature of the material. Once you understand the "absorb water → expand" rule, you can control dimensions instead of being controlled by them.
Next issue: I will share solutions for warpage and distortion in glass-fiber-reinforced parts — another classic headache in injection molding. Stay tuned.