Most defects blamed on the mould or the injection moulding machine actually start in the hopper: sink marks, silver streaks, bubbles, brittle parts or a melt flow index that changes from one batch to the next. Plastic drying is not an optional step when the material is hygroscopic. This guide summarises approximate temperatures and times by polymer, how to read the dew point and how to size the hopper of an industrial dehumidifying dryer.
Why the pellets have to be dried
Hygroscopic polymers (PA, PC, PET, PBT, PMMA, TPU…) absorb moisture from the air into the pellet. When they melt, that water evaporates or, worse, breaks the polymer chains through hydrolysis. The result is a part that may look right but has lost mechanical strength.
Non-hygroscopic polymers (PP, PE, PS) only pick up surface moisture, from condensation or from outdoor storage. A short drying cycle with hot air is enough for them, and often not even that.
Drying table by material
Typical approximate industry values for virgin material. The reliable reference is always the datasheet from the material manufacturer: each grade, filler or additive can change the temperature and the time.
| Material | Drying temperature | Approximate time | Target residual moisture |
|---|---|---|---|
| PA6 | 70–80 °C | 4–6 h | < 0.2 % |
| PA66 | 80 °C | 4–6 h | < 0.2 % |
| PC | 120 °C | 3–4 h | < 0.02 % |
| PET | 160–170 °C | 4–6 h | < 0.005 % |
| PBT | 120 °C | 3–4 h | < 0.03 % |
| ABS | 80 °C | 2–4 h | < 0.1 % |
| PMMA | 80–90 °C | 3–4 h | < 0.05 % |
| POM | 80–100 °C | 2–3 h | < 0.15 % |
| TPU | 80–100 °C | 2–4 h | < 0.05 % |
| PC/ABS | 100 °C | 3–4 h | < 0.02 % |
| PP, PE | 80 °C (if needed) | 1 h | Surface moisture only |
Two important points. The time counts from when the material reaches temperature, not from when it enters the hopper. And recycled or regrind material usually arrives wetter than virgin material, so it needs more time.
Dew point: the figure that matters
Heating is not enough. To extract water from the pellet, the air passing through it must be drier than the pellet itself. This is measured by the dew point of the process air: the temperature at which that moisture would start to condense.
Engineering plastics are normally dried with a dew point of −30 to −40 °C. If the air leaves at −10 °C, the hopper can stay at the right temperature for hours and the material will still be wet. That is why the unit should display the dew point and warn when it degrades, which is usually the first sign of saturated desiccant or a dirty filter.
Desiccant dehumidifier or hot air dryer
| Criterion | Hot air dryer | Desiccant dehumidifier |
|---|---|---|
| Principle | Heats ambient air | Dries the air with desiccant and recirculates it |
| Dew point | Ambient dew point | Typically −30 to −40 °C |
| Materials | Non-hygroscopic or surface moisture | Hygroscopic: PA, PC, PET, PBT, PMMA, TPU |
| Repeatability | Depends on the weather that day | Independent of the weather |
A hot air pellet dryer works well in winter and fails in August, because it dries with the air in the plant. If you work with polyamides, polycarbonate or PET, the safe option is a desiccant-type industrial dehumidifier .
How to size the hopper: kg/h × hours
The rule is simple: the hopper must hold all the material the machine consumes during the drying time. If it is smaller, the pellets leave before they are ready.
Capacity (kg) = machine consumption (kg/h) × drying time (h)
Example with PA66. The injection moulding machine consumes 25 kg/h and the material needs 4 h of drying:
- Material in the hopper: 25 × 4 = 100 kg.
- Bulk density of the pellets: about 0.65 kg/l (check the datasheet; it varies with pellet shape and filler).
- Useful volume: 100 / 0.65 ≈ 154 l.
- With a 20 % margin: a hopper of about 185 l, i.e. the next commercial size up.
Calculate the real consumption from the shot weight (part + runner) and the cycle: 300 g per shot with a 40 s cycle means 90 shots per hour, i.e. 27 kg/h. The dry air flow must also match the consumption; the exact ratio is given by each equipment manufacturer.
Typical mistakes
- Over-drying. Holding PA or PBT at temperature for many hours turns them yellow and degrades them. If the machine stops, lower the hopper temperature.
- Undersized hopper. When the cycle is shortened, the material spends less time in the hopper than it needs.
- Conveying with humid air. Dry material conveyed with plant air or left waiting in an open container picks up moisture again within minutes. Use a vacuum loader with a short, closed path.
- Not measuring. Without a moisture analyser you cannot tell whether the problem is drying or the process.
- Dirty filters. They reduce the air flow and worsen the dew point without any change showing in the temperature.
Drying also affects the mould temperature you need. To set it by polymer, see our mould temperature table and the guide to plastic injection moulding.
Frequently asked questions
Can plastic be over-dried?
Yes. Polyamides and PBT oxidise and turn yellow if they stay at temperature too long. Respect the maximum time on the material datasheet.
Does PP need drying?
Normally not. Only if it has picked up surface moisture from condensation or storage; in that case one hour at about 80 °C is enough.
What dew point do I need?
For hygroscopic materials, −30 to −40 °C. Above −20 °C drying loses effectiveness.
How long does it take to pick up moisture again?
It depends on the material and the environment. A dry polyamide can exceed the limit again in less than an hour in plant air.
Need to size your drying system?
Tell us the material, the consumption in kg/h and the cycle of your injection moulding machine, and we will help you choose the equipment. You can use the equipment selector or talk to a technician.