Drying plastic resin before molding is not optional for hygroscopic materials, and it cannot be judged by eye. Moisture absorbed into the pellets turns to steam inside the barrel, and the same moisture breaks the polymer chains. Splay at the gate is the visible symptom; lost impact strength is the expensive one. If your part arrives with silver streaks, the resin was wet.
Key takeaways
- Two resin families, two answers. Hygroscopic resins pull water into the pellet itself and must be dried. Non-hygroscopic resins only hold surface moisture, and often need nothing more than a short warm-up.
- Moisture does two different kinds of damage. One is cosmetic and reversible on the next shot. The other is chemical and permanent, because it cuts the polymer chain length.
- Temperature, time and dew point are three separate settings and a dryer only delivers on all three at once. A hot-air oven at the right temperature but the wrong dew point is a wet oven.
- Over-drying is a real failure mode, not a safety margin. Held too long or too hot, many resins oxidize or yellow before they ever reach the screw.
- Drying is a process topic that shows up as a tool problem. Trapped gas has to leave the cavity somewhere, so venting is part of the same conversation.
Why some resins absorb water and some do not
Whether a resin needs drying is a property of its chemistry, not of how it was stored.
Hygroscopic resins contain polar groups that attract water and hold it inside the pellet. Polyamide (PA6, PA66), polycarbonate (PC), PBT, PET, PMMA, ABS and TPU are all in this group, and the amount varies from a few hundred parts per million to more than a percent by weight. The moisture is not on the surface, so wiping or blowing it off achieves nothing.
Non-hygroscopic resins such as PP, PE, PS and rigid PVC do not hold water internally. They can still pick up condensation on the pellets during a cold night in an unheated warehouse, but that moisture sits on the outside and a short residence in a warm hopper usually removes it.
The practical consequence: a molder who dries everything the same way is wasting energy on the resins that do not need it and probably under-drying the ones that do. Our resin selection guide covers which family a given application belongs to.
What moisture actually does inside the barrel
When wettish pellets meet a 280 °C barrel, two things happen at once, and only one of them is visible.
The mechanical effect is steam. Water flashes to vapour, and the vapour pushes its way through the melt. Where it reaches the cavity surface it leaves a silver streak or a splay mark radiating from the gate. Where it does not reach the surface it leaves a void, and a void is a crack waiting for a load.
The chemical effect is hydrolysis. In polyesters and polyamides, water reacts with the polymer backbone at melt temperature and splits it. Molecular weight falls, and so does every property that depends on it: impact strength, elongation at break, and fatigue life. A hydrolysed part looks acceptable and fails early in service.
That is why the second failure mode is the expensive one. Splay marks get a part rejected at the press. Hydrolysis ships it to the customer. The injection molding defects guide covers the full set of moisture-related defects and how to tell them apart.
Drying temperature and time, by resin
These are typical starting points for a desiccant dryer. Treat them as an orientation, not a specification — the resin supplier’s datasheet always wins, because a 30% glass-filled grade and an unfilled grade of the same polymer do not behave identically.
| Resin | Drying temp. | Typical time | Target moisture |
|---|---|---|---|
| PA6 / PA66 (unfilled) | 80 °C | 4 h | < 0.20% |
| PA66 glass-filled | 80–90 °C | 4–6 h | < 0.15% |
| PC | 120 °C | 3–4 h | < 0.02% |
| PBT / PET | 120–140 °C | 4–6 h | < 0.05% |
| POM | 80–100 °C | 2–3 h | < 0.20% |
| ABS | 80 °C | 2–3 h | < 0.10% |
| PMMA | 80 °C | 2–4 h | < 0.10% |
| TPU | 80–100 °C | 2–3 h | < 0.05% |
| PP / PE | 70–80 °C | 1–2 h | surface moisture only |
Two of these rows are the ones that cause most production arguments. PC at 120 °C needs a genuine desiccant dryer, not a hot box, and its target is an order of magnitude tighter than the polyamides. PET and PBT sit in the same region. Where a buyer sees unexplained brittleness in a PC part, drying capability is the first place to look.
Our PA66 vs POM comparison walks through how the two most common engineering grades differ on this and on several other points that affect tool design.
Temperature, time and dew point are three settings, not one
A dryer can be correct on temperature and time and still deliver wet resin. The third variable is the dew point of the air being pushed through the hopper.
The dryer’s job is not to heat the pellet, it is to make the air drier than the pellet so that moisture migrates out. Air at a dew point of −20 °C to −40 °C will do that. Air at +10 °C, which is what a tired desiccant bed or a saturated cartridge produces, will not, no matter how long the cycle runs. This is the failure that hides in plain sight, because the controller still reads 80 °C.
Ask for the dew point to be logged, not just set. On a polyamide running at 80 °C for four hours, a dew point that has drifted above the target is the single most common reason a process that worked last week produces splay this week.
Desiccant dryers, hot-air ovens and vacuum dryers
A hot-air oven is a convection oven. It heats the pellets and the air around them to the same temperature and holds them there, which means the air can never be drier than the resin is trying to become. It works for non-hygroscopic resins and for surface moisture. It does not work for PC or PET.
A desiccant dryer passes the air over a regenerating bed of desiccant before it enters the hopper, so it arrives well below the moisture level of the pellets and keeps pulling. This is the default for hygroscopic engineering resins.
A vacuum dryer pulls moisture out under reduced pressure, which lowers the boiling point and shortens the cycle considerably. It costs more, and it earns its place where drying time is the constraint on throughput.
How to prove the resin is actually dry
You cannot tell by looking at pellets, and you cannot tell by holding a handful. There are two credible methods.
Loss-on-drying weighs a sample, dries it in a lab oven, and weighs it again. It is simple and it is slow, which makes it a verification method rather than a control method.
A moisture analyser, either a Karl Fischer titration or a bench instrument, gives a number in minutes and can be run at the press. If a molder is claiming a moisture target, this is the instrument that proves it.
What you should receive, on a project where moisture matters, is the resin lot certificate together with the dryer settings and the dew point log. Every claim on a part should be backed by a document issued against your mold number, and drying records belong in that set.
Over-drying is a real failure mode
Drying is not a case where more is safer. Every resin has a temperature above which it starts to degrade in the hopper, and the degradation is cumulative with residence time.
PC held too long at the top of its range yellows and loses impact strength. POM is worse: held too hot it can generate formaldehyde and begin to depolymerize, and the effect does not reverse when the resin reaches the screw. Polyamide left in a hot hopper over a shutdown weekend oxidizes and darkens.
The practical rule is to dry to the datasheet and to run the hopper down rather than leaving it charged through a long stop. Where a plant is fighting both splay and brittleness in the same part, over-drying is worth ruling out before anything else is changed.
Where drying stops being a process problem and becomes a tool problem
Even correctly dried resin produces some gas, and that gas has to leave the cavity. If it cannot, it shows up as a burn mark or a short shot at the last point to fill — and then the conversation moves from the dryer to the tool.
Venting is a mould feature, and it is designed at the same time as the runner and the cooling layout, not added later. Vents at the parting line, at the last point to fill and around any deep rib give the gas somewhere to go. On a part with a large wall section, or a resin with a narrow processing window, a vent that is too shallow or too far from the flow front will produce burns even on perfectly dry material.
This is one of the checks a DFM review is for. At RCH Plastic your STEP or IGES file is modelled in UG, run through Moldflow, and returned as a written DFM report within 3 working days, at no cost. Our DFM rules reference lists the design-level checks worth doing before steel is cut, and venting is on it.
Vent depth also interacts with tolerance. Our machining tolerance is ±0.005 mm on critical mold dimensions, and the molded part tolerance that tool can hold is ±0.01 mm on critical dimensions, subject to the resin and the part geometry. A vent that has to be cut to a hundredth of a millimetre is a machining question before it is a process question. The tolerances guide covers how to write that into a specification.
What to put in the purchase specification
Drying is easy to leave vague in a purchase order, and vagueness is what turns a process disagreement into a rejected shipment. Four lines close the gap:
- The resin lot certificate, showing grade and manufacturer, filed against your mold number.
- Dryer settings on the process parameter sheet: temperature, time and dew point, not just temperature.
- The moisture measurement method and the target, agreed in advance so that “dry enough” has a number.
- The venting plan from the DFM report, so that gas is designed out rather than tuned out.
On a project where the material is doing real work — a glass-filled bracket, a medical housing, a part that sees impact — those four lines are worth more than a tighter tolerance on the drawing, because they protect the properties the part was specified for in the first place. Our injection molding design guide covers the rest of the specification set.
Related guides in this series
- How to choose the right resin — which family suits which application.
- PA66 vs POM — the two engineering grades buyers most often compare.
- Injection molding defects guide — splay, burns and voids, and how to tell them apart.
- Cooling system design — the other half of the cycle-time equation.
- How tight injection molding tolerances can be — why shrinkage is a range, not a number.
Send us the drawing and the resin
Tell us the polymer, the wall thickness and the properties that matter, and we will flag the material risks in the DFM report before the tool is cut — including whether the drying window is wide enough to run reliably at your volume. Request a DFM review and quote.