Nylon (PA6 / PA66) Injection Molding: Moisture, Conditioning and Dimensions

The material that changes size after it leaves you

Polyamide — nylon 6 and nylon 66 — is the workhorse engineering plastic for anything that needs toughness, wear resistance, temperature resistance and resistance to oils and fuels. Gears, bushes, cable ties, connectors, under-bonnet parts, power-tool housings and conveyor components are all nylon.

It also has a property that causes more quality disputes than any other in injection moulding: it absorbs water and grows. A dry PA66 part left in a normal room reaches equilibrium at 1.5 – 2.5 % moisture and grows by 0.3 – 0.6 % in every dimension. That is 0.3 mm on a 50 mm part. If the drawing does not say whether the tolerance applies to the dry part or the conditioned part, the drawing is incomplete.

Grades

Table 1 — Polyamide grades. PA12 and PPA exist mainly to solve the moisture problem.

Moisture: drying, then re-absorbing

There are two completely separate moisture issues with nylon, and confusing them is the root of most problems.

Table 2 — The two moisture problems. They have opposite targets.

Table 3 — Drying schedules. Over-drying above 110 °C oxidises nylon and yellows it.

Note the second row of Table 2: dry nylon is brittle. A freshly moulded, bone-dry PA66 part will snap where the same part conditioned for two weeks will bend. This is why nylon parts are often conditioned deliberately — either by leaving them in a humid chamber, or by boiling them in water for 30 – 60 minutes, which accelerates uptake to near-equilibrium in an hour.

Table 4 — Conditioning routes. Hot water is the practical answer for production.

Processing window

Table 5 — Processing windows. PA66 needs roughly 35 °C more melt temperature than PA6 for the same viscosity.

Mould temperature is the lever that controls crystallinity, and therefore strength, wear resistance and dimensional stability. A PA66 part moulded at 40 °C has a thin amorphous skin, low crystallinity in the core, and poor wear resistance; the same part at 90 °C is fully crystalline and performs as the datasheet says. For gears and wear parts, a hot mould is not optional.

Design rules

Table 6 — Nylon design rules. The tolerance row is the one that causes the most arguments.

Gears and wear parts

Nylon is the default material for moulded gears and bushes, and the reasons are specific: it has a low coefficient of friction, it tolerates shock loads, it runs without external lubrication in many applications, and it damps noise.

Table 7 — Gear material selection. Never run nylon against nylon in a loaded pair — it welds and tears.

Gear tolerances need a note: a moulded nylon gear is normally specified to AGMA 8 – 9 or DIN 9 – 10, not to the precision of a machined gear. If the application needs better, the gear is machined from cast nylon or from POM.

Defects and causes

Table 8 — Nylon defects. "Brittle" and "wet" can both be true at once — they are different faults.

The moisture gradient: why thick parts keep moving

Moisture enters nylon from the surface. On a 1 mm wall, equilibrium is reached in a few weeks and the whole section moves together. On a 5 mm wall, the outside reaches equilibrium in weeks while the core is still dry, and the part continues to change shape for months.

Table 9 — Moisture diffusion times for unfilled PA66. Thick sections are never in equilibrium in service.

The engineering consequence is that a thick nylon part should be designed as a dry part with generous tolerances, or made from a glass-filled grade, or made from PA12. Specifying ±0.05 mm on a 5 mm thick unfilled PA66 part is not achievable in any state, because the part is a different size in every month of its first year.

Glass fibre: what you gain and what you lose

Table 10 — The cost of glass. Stiffness and stability go up; elongation, weld strength and tool life go down.

The weld-line row is the one that fails parts in the field. In a glass-filled material, the fibres do not cross the weld — they lie parallel to the weld line, so there is a plane of unreinforced resin across the part. A 30 % GF nylon part with a weld line in a loaded location behaves like an unfilled part there, and often worse, because the fibres create a notch. If a weld line cannot be avoided, move it out of the load path or use a mineral-filled grade where the filler is more isotropic.

Under-bonnet: the real requirements

Table 11 — Under-bonnet requirements. The heat-stabiliser package is what separates an automotive grade from a general-purpose one.

Heat stabilisation is a real additive package, not marketing. Unstabilised PA66 loses about 70 % of its tensile strength after 1,000 hours at 150 °C; a properly stabilised grade loses 20 – 30 %. The difference is a copper or organic stabiliser system that interrupts the thermal oxidation chain. It costs 10 – 20 % more per kilogram and it is not optional for anything that lives near an engine.

Frequently asked questions