Drying temperatures and times by resin
This is the table to keep next to the machine. Values are for a dehumidifying dryer delivering air at −40°C dew point or lower. If your dryer cannot achieve that, see the dew point section below — it changes the picture significantly.
| Material | Drying temp | Time | How critical | Symptom if wrong |
|---|---|---|---|---|
| PA6 / Nylon 6 | 80°C | 4–6 h | Critical | Bubbles, splay, dimension change, brittle parts |
| PA66 / Nylon 66 | 80°C | 4–6 h | Critical | Bubbles, splay, dimension change, brittle parts |
| PC / Polycarbonate | 120°C | 4 h | Critical | Silver streaks, hydrolysis on long runs, black specks |
| PET / PETG | 120–150°C | 4–6 h | Critical | Hydrolysis, loss of viscosity, black specks |
| PBT | 120–130°C | 4 h | Critical | Bubbles, splay, electrical property loss |
| PMMA / Acrylic | 70–80°C | 2–4 h | High | Silver streaks, stress crazing, dimension change |
| TPU / TPE | 80–100°C | 2–4 h | High | Bubbles, splay, inconsistent properties |
| ABS | 80°C | 2–4 h | Moderate | Silver streaks, loss of impact strength |
| PC/ABS | 90–110°C | 3–4 h | High | Silver streaks, haze, dimension change |
| PP / Polypropylene | 80°C | 2–4 h | Moderate | Silver streaks on critical parts |
| PE / Polyethylene | 70°C | 2–4 h | Moderate | Silver streaks, bubble formation |
| POM / Acetal | 80–120°C | 2–4 h | Critical — see note | Fumes, dimension change, degradation |
Dew point — the part most people get wrong
Here is the thing that undermines even a correctly-configured dryer: drying capability depends on how dry the air is, not only how hot it is.
Dew point is the temperature at which air becomes saturated with water. It is the honest measure of how much moisture the air can still absorb. A dehumidifying dryer should deliver process air at −40°C dew point or lower for most engineering materials. Below that, the air is genuinely dry.
In a cool, dry climate that may be acceptable. In a humid climate — or in summer, or in a plant with air conditioning exhaust nearby — the same dryer cannot dry anything properly, because the air it delivers is already close to saturated.
This is the single most common reason a correctly-set dryer still produces damp material. The temperature is right, the timer is right, and the material is still wet.
How to check your dryer is actually working
- Ask the supplier for the dew point specification, not just the temperature rating. A dryer sold without a dew point figure is a hot-air box.
- Check whether it is a dehumidifying (desiccant) type or a simple hot-air type. The desiccant type is the one with a drying chamber and a regeneration cycle.
- Measure the actual melt temperature before and after drying, if your machine allows it — wet material requires noticeably more energy to melt.
- Look at the material when you open the hopper. If it looks or feels damp, it is damp. This is the most reliable check available.
What under-drying actually causes
Under-drying produces defects that look exactly like machine faults, which is why it costs so much diagnostic time.
| Symptom | Moisture mechanism | Usually mistaken for |
|---|---|---|
| Silver streaks | Water vapourises at melt temperature and forms micro-bubbles that reflect light | Melt temperature too high; screw problem |
| Bubbles inside the part | Vapour trapped in the melt, released as the material cools | Air entrapment; mould venting |
| Splay marks | Vapour bubbles breaking at the surface, leaving fine streaks | Contamination in the machine |
| Dimensional variation | Moisture reduces melt viscosity, changing shrinkage | Mould temperature instability |
| Reduced impact strength | Absorbed water plasticises the polymer chain, softening it | Wrong resin grade; worn screw |
| Brittleness in PA parts | Hydrolysis — moisture chemically breaks the polymer chain at high melt temperature | Over-drying; degraded material |
| Black specks | Hydrolysis degradation in the barrel, producing black particles | Dead-corner contamination (which moisture then makes worse) |
Drying in practice
The sequence that works
- Dry the material to specification — temperature, time and dew point from the table above or the supplier datasheet.
- Keep it dry in transit — this is where most plants lose the drying. The material is dried correctly and then re-absorbs moisture in the hopper, the conveyor, or while sitting on a pallet.
- Use a sealed hopper with a lid. An open hopper exposes dried material to humid air continuously.
- Insulate the hopper connection between the dryer and the machine where possible.
Re-drying after a process interruption
If the machine stops for a period, or if material has been left in the barrel or the hopper overnight, the material must be dried again from scratch. Material that has absorbed moisture is not fixed by an extra hour of drying — it needs the full cycle, because the moisture is throughout the pellet, not just on the surface.
Using regrind
Regrind absorbs moisture faster than virgin material, and the more regrind in the mix, the worse the situation. As a general rule, any material containing regrind needs the full drying cycle at the correct temperature, even if the same material at 100% virgin would not.
Choosing a dryer
Three criteria, in this order of importance:
1. Dew point specification — check this first
−40°C dew point or lower for engineering materials. This is not a luxury specification; without it, drying may not be achievable at all in humid conditions. Many dryers are described as "3-in-1" units combining dryer, hopper and heater — the quality varies enormously between them, and the dryer's actual capability is where the cost and reliability differences sit.
2. Hopper size — size for a reasonable changeover
Hold enough material for 1 to 2 hours of running. Note that dry time scales with fill level: a full hopper takes considerably longer than a half-full one, because material at the bottom has been exposed longer than material on top. Over-filling to get more capacity per dryer is a false economy — you will extend the dry cycle to compensate.
3. Throughput — match your consumption
The dryer should deliver dried material at least as fast as the machine consumes it, or you will get clumping and inconsistent supply. Match the kg/h rating to your actual consumption.
Process air dew point: −40°C or lower
Hopper capacity: 1–2 hours of machine consumption
Throughput: at least your consumption rate
Sealed hopper with lid
Insulated connection to machine
Stainless steel hopper if running POM
Our range and what we will tell you
We supply hopper dryers and 3-in-1 dehumidifying units, and the full range of auxiliary equipment around them. When you send us your resin and consumption rate, we will size the dryer and state the dew point specification — because we would rather you bought the right size than the biggest one. If your current dryer is a hot-air type, we will say so, and tell you whether upgrading would actually fix your problem. Full specification range on our automation and auxiliary equipment page.
Common questions
Which materials actually need drying?
Strictly, all hygroscopic materials; in practice most material delivered in open bags. The critical ones are PA6 and PA66, PC, PET and PBT — these produce bubbles and dimension variation without proper drying. PP, PE and ABS absorb less, but pick up enough surface moisture to cause silver streaks on a critical part. POM and PMMA need drying too, and POM needs a stainless hopper for safety reasons.
What is dew point and does it matter?
Dew point is the temperature at which air becomes saturated with water, and it is the honest measure of how much moisture the air can still absorb. It matters because drying capability depends on how dry the air is, not just how hot. A dehumidifying dryer should deliver at −40°C dew point or lower. A simple hot-air dryer delivers air at ambient dew point — acceptable in a dry climate, useless in a humid one. This is the most common reason a correctly-set dryer still produces damp material.
How long does material need drying?
Typically PP and PE 2–4 hours, ABS 2–4 hours, PA6 and PA66 4–6 hours, PC 4 hours, PET 4–6 hours, PMMA 2–4 hours, TPU 2–4 hours. These assume −40°C dew point air. Longer times are needed for a full hopper or a cold start, and re-moistened material needs the full cycle again from scratch.
What size hopper dryer do I need?
Enough for 1 to 2 hours of running. Dry time scales with fill level, so over-filling to get capacity is a false economy — you will extend the cycle to compensate. Throughput should match your actual consumption. If you run POM, the hopper must be stainless steel, not acrylic.
Can I dry material overnight instead of before each shift?
Better to dry before each shift than to leave material in the hopper overnight, because it re-absorbs moisture continuously from any air ingress. If you must dry overnight, the hopper must be properly sealed with a lid, and the connection to the machine insulated — otherwise the material at the top will be damp again by morning.
Does regrind need different drying?
Yes. Regrind absorbs moisture faster than virgin material, and the more regrind in the mix, the worse the situation. Any material containing regrind should get the full drying cycle at the correct temperature, even if the same material at 100% virgin would not need it.