Resin Drying for Injection Moulding: Grade Requirements and Moisture Control

Updated: 2026-10-22

Resin preparation should start with the exact grade's current processing instructions and the condition of the delivered material. A polymer-family drying table is not a production specification. Temperature, time, airflow, air dryness and initial moisture interact; the endpoint that matters is suitable material at the machine, not simply an elapsed timer on a dryer.

Which materials need drying?

Moisture absorption varies by polymer and formulation. Many polyamides, polycarbonates, polyesters and polyurethane grades require careful moisture control. PP and PE generally absorb little moisture into the polymer, but surface condensation or contamination can still matter. TPE is a broad group: do not assign every elastomer the same drying requirement. Use the actual supplier instructions for blends, fillers, flame-retardant systems and recycled compounds.

Sealed material may already meet the supplier's processing condition, while damaged packaging or an opened container may require a different preparation step. Allow cold containers to reach an appropriate environment before opening to reduce condensation risk. Check storage and transport history rather than assuming that every bag of the same family starts equally wet.

Which settings and measurements should be agreed?

Record the grade, supplier document revision, permitted residual moisture and applicable measurement method. Establish the dryer type, actual material temperature, airflow, air dew point where relevant and residence-time requirements. The grade may also limit total exposure to heat. A universal minus 40 °C dew-point rule or one temperature/time pair cannot replace that specification.

Dew point describes moisture in the drying air; it does not measure water remaining in the pellets. A satisfactory air reading cannot compensate for inadequate airflow, bypassing material, a short residence time or moisture picked up downstream. Agree where and when pellet samples are taken and how they are protected before testing. The measurement must be suitable for the grade and the low moisture level being assessed.

How can throughput undermine an apparently correct setup?

Fictional planning example: a hopper contains 30 kg of usable material and the machine consumes 10 kg/h. Dividing inventory by consumption gives three hours of nominal average residence. If consumption rises to 15 kg/h, that estimate falls to two hours. This calculation is not a validated drying time: hopper flow, filling level, uneven heating, initial moisture and equipment performance can change the actual treatment of individual pellets.

Review capacity when changing cavitation, cycle time or part mass. Record start-up loading and when newly added material can reach the feed outlet. An adequate overnight preparation does not prove that later additions receive the same treatment. Conversely, a production stop can increase heat exposure, so shutdown and restart instructions should come from the grade and equipment requirements.

Does a good-looking part prove that resin was dry?

No. Moisture can contribute to streaks, bubbles and other surface problems, but similar symptoms can have other causes. Some moisture-sensitive polymers can suffer molecular degradation during melt processing without an obvious cosmetic warning. A visual improvement after changing drying conditions is useful evidence to investigate, not a substitute for material and process checks.

Use controlled comparisons: identify resin and lot, record preparation and machine conditions, retain samples and check the relevant dimensions and performance. Separate moisture diagnosis from contamination, thermal degradation, venting and other possible causes. Do not rescue a suspect batch solely because its final surface appears acceptable.

What about conveying, regrind and repeated drying?

Protect material between dryer and machine. Investigate open containers, long conveying paths, leaks and periods of exposure to ambient air. The rate of moisture uptake depends on the grade and conditions; there is no single safe number of minutes for every material. Document any handling step used to preserve the agreed condition.

Regrind requires its own traceability and assessment of contamination, previous thermal history, moisture and the allowed blend. Drying does not restore damage from previous overheating or make contaminated material acceptable. Follow the compound supplier's limits for reuse and cumulative heat exposure. Do not treat repeated drying as unlimited or assume that every weekend shutdown can leave material hot in the hopper.

What should a buyer request before production approval?

• Exact grade and colour, approved substitutions and current supplier preparation instructions.
• Packaging/storage checks, material-lot identity and any permitted regrind proportion.
• Dryer and conveying arrangement, start-up, throughput changes and shutdown procedure.
• Defined moisture sampling, acceptance criteria, instrument/method and retained records.
• Treatment of deviations and any verification of part function after a suspected preparation problem.

Prepare the RFQ brief, connect material requirements to the inspection plan, and discuss the required scope. Confirm records and tests for the proposed production route before placing the order.

Primary processing references

BASF's Ultramid processing guidance and the B3K6 grade processing sheet illustrate product-specific requirements. Novatec's drying questions discusses equipment and material conditions. Their example settings are not instructions for a different grade.