Drying Nylon Before Moulding: Moisture, Conditioning, Dimensions

Polyamides are hygroscopic. Pellets pick up water from the air, and moulded parts keep doing so for weeks after they leave the tool. Both effects are predictable. Many nylon faults that look like process problems start in the dryer, and most dimensional surprises start with a drawing that never said which moisture state it meant.

What water does in the barrel

At melt temperature, water and polyamide react. Too much water drives hydrolysis: chains break, melt viscosity falls, and the moulded part loses toughness and strength. DuPont's moulding guide puts the threshold plainly. Up to about 0.20% moisture, the reactions are slow enough that properties stay practically unaffected. Above 0.25%, toughness and elongation drop, the surface suffers and parts flash easily. Steam in the melt also leaves splay marks and internal bubbles. Once degraded, the material cannot be restored by reprocessing.

Very dry resin goes the other way. Polycondensation continues in the melt and molecular weight rises. For one PA66 resin at a 295 °C melt, DuPont puts the equilibrium at around 0.12% moisture. Leave pellets in a hot dryer for too long and they discolour. At melt temperatures above 315 °C, or with long cycles, the guide suggests going down to 0.1%.

Drying targets

Published producer recommendations. Always follow the TDS for the grade you are running.

Dryer practice that holds the number

• Dry at the press. DuPont advises against tray ovens with manual transfer to an open hopper. A desiccant hopper dryer with counter-current flow puts the driest air on the driest pellets at the outlet.
• Size airflow and residence time. DuPont suggests about 3.7 m³/h of air per kg/h of resin. Check that hopper volume divided by throughput still gives the full drying time at your highest output.
• Watch the dew point. The guide wants −18 °C or lower at the dryer, and −20 °C or lower at the hopper inlet throughout the cycle.
• Lower the temperature for long holds. When drying times have to be extended, DuPont's advice is to lower the temperature rather than cook the pellets.
• Protect regrind. It absorbs moisture faster than virgin pellets; keep it in sealed containers.
• Mind cold stores. BASF advises letting containers from cold rooms reach ambient temperature before opening, to avoid condensation on the granules.
• Measure, do not assume. ISO 15512 describes Karl Fischer and related methods that measure water specifically. Loss-on-weight analysers read all volatiles, so calibrate them against a water-specific method for each grade.

After the mould: conditioning and dimensional change

A freshly moulded part is close to dry. From the moment it is ejected it absorbs moisture until it reaches equilibrium with its surroundings, and its dimensions grow as it does. For unreinforced nylon, DuPont lists equilibrium at 50% RH of 2.8% for PA6 and 2.5% for PA66, and 8.5% and 7.5% respectively in water. Glass fibre lowers the figures: 1.9–2.3% for a PA6 GF30 and 1.5–1.9% for a PA66 GF30 at 23 °C and 50% RH. Thick walls take far longer to get there than thin ones.

Glass also changes the shape of the change. In a study of PA6, PA66 and PA46 published on UL Prospector, glass-filled grades changed very little along the flow between dry-as-moulded and 50% RH equilibrium, and much more across it. Unfilled grades changed about equally in all directions. The same study found unfilled PA6 absorbed more water than PA66 yet changed size less, so do not predict growth from absorption figures alone.

What that means in practice:

1. Put the moisture state on the drawing. Dry-as-moulded or conditioned, and to what standard.
2. Condition parts when first-article results must reflect service. For parts used at 23 °C and 50% RH, DuPont suggests conditioning unreinforced PA66 to about 3% water; the surface excess evaporates and roughly 2.5% stays.
3. Use ISO 1110 for test specimens. It conditions polyamide specimens at 70 °C and 62% RH until they reach at least 95% of equilibrium, close to the 23 °C / 50% RH result.
4. Expect properties to move. In BASF's PA6 GF30 data, notched Charpy rises from 10 to 18 kJ/m² on conditioning while tensile modulus falls from 9,600 to 5,700 MPa.
5. Change material if the growth is unacceptable. Semi-aromatic grades hold their stiffness when wet (see PA6T and PPA compounds), and OSATAL acetal absorbs little moisture.

Where we can help

Ask for the TDS of any OSAMIDE grade before the first lot and agree the drying conditions with us before it reaches your press. If parts are cracking or growing in service, send us mouldings and the pellet lot number. Comparing MFI and DSC results on the part with those of the virgin material can show whether degradation happened in processing; regular customers get this testing free of charge. For the choice between the two common glass-filled nylons, see PA66 GF30 vs PA6 GF30.

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