Resin Handling Guide
Drying Hygroscopic Resins: PA, PC and PET Rules That Hold Up in Production
Moisture in hygroscopic pellets turns into steam inside the barrel. The steam leaves silver streaks and bubbles on the part. In PA, PC and PET it also cuts polymer chains through hydrolysis, so the part gets weaker and more brittle. The fix is drying hygroscopic resins to known moisture limits before molding. Use a dehumidifying dryer at the right temperature and dew point. This article gives you those limits and settings, plus the shop habits that keep dried resin dry until the shot.
Dried resin is a process input with a measured number. Wet resin is a gamble that shows up as scrap only after the cycle time is already spent. Every number below is a target you can check with a moisture analyzer.
Why Moisture in the Pellet Becomes a Defect
Water hides in the pellet. Melt exposes it.
PA6 can hold 8 to 9% water when saturated. That is roughly a hundred times what slightly polar resins like HIPS can hold. Hygroscopic resins pull water deep into the pellet, not just onto the surface. PP and PE behave the opposite way. They are non-polar, and water has no chemical affinity for them, so they pick up almost nothing from humid air.
Inside the barrel the trapped water flashes to steam. The steam pushes silver streaks (splay) across the surface, traps bubbles under it, and leaves foamy, rough flow marks. The silver streaks wipe off the surface. The lost mechanical strength does not. In PA, PC and PET, water also attacks the polymer chain itself. That is hydrolysis. It breaks long chains into shorter ones, so molded parts come out weaker and more brittle than the datasheet promises.
PET is the sharpest example. Hydrolysis begins around 150°C, far below its 250°C melting point. By the time the resin is fully molten, wet PET is already losing molecular weight. Buyers blame the mold or the press. The real culprit is usually the bag the resin arrived in.
Which Resins Need Drying
Not every resin earns its drying time. Sort them first.
| Behavior | Resins | Drying rule |
|---|---|---|
| Hygroscopic | PA6, PA66, PC, PET, PBT, PMMA, TPU, PSU | Dry every lot before molding, no exceptions |
| Low sensitivity | PP, PE, PVC, PS | No drying for new pellets in normal storage; dry only if visibly damp |
Drying hygroscopic resins is not optional for PA, PC or PET. For these materials, skipping the dryer is a guaranteed defect, not a calculated risk. Resin choice sets the drying burden before steel is cut. Our injection molding part design guide covers how material selection shapes the whole project.
Watch out for one exception. Some additives are hygroscopic. A non-hygroscopic resin carrying a wet additive or filler still needs drying. If the pellet blend changed and splay appeared, check the additive’s moisture behavior before blaming the base resin.
Moisture Limits by Resin
PET at 0.1% moisture is a ruined batch. PET at 0.004% is a good bottle.
The allowable limit depends on the resin and the part. These are the numbers molders check against with a rapid moisture analyzer or a dew-point test on the dried pellets.
| Resin | Maximum moisture before molding | Source |
|---|---|---|
| PA6 / PA66 | Below 0.10%; ideally below 0.08% | Supplier drying guidelines |
| PC | Below 0.02% | Supplier drying guidelines |
| PET, injection grade | Below 0.005% (50 ppm) | Supplier drying guidelines |
| PET, bottle preform | Below 50 ppm; strict lines target about 30 ppm | PET machinery maker guidance |
| PA11 | Below 0.10% | Published drying tables |
| PMMA | Below 0.05% | Published drying tables |
Some shops run tighter in-house specs. One Chinese molder holds PA under 0.05% and PC and PET under 0.02% before every run. Buyers do not need to copy that number. They need to know their supplier has a number, measures against it, and can show the reading.
Drying Hygroscopic Resins: Temperature and Time
Temperature drives moisture out. Time lets it reach the surface. Both must fit the resin.
| Resin | Drying temperature | Drying time |
|---|---|---|
| PA6 / PA66 | 80–100°C | 4–12 hours; 12 hours for previously wetted material |
| PC | 120–130°C | 6–8 hours |
| PET | 120–165°C | 4–6 hours |
| PBT | 130°C | 5 hours |
| PMMA | 70–80°C | 4–6 hours |
The table blends two reputable sources, and the ranges reflect real practice. Hybster dries PA at 80°C for 4 to 8 hours and allows 12 hours for resin that was never dried before. A Chinese molding shop publishes PA at 90–100°C for 8 to 12 hours. Both are correct for their equipment. The resin decides the window. Your dryer decides where inside it you land.
Two warnings travel with these numbers. Never exceed the resin’s heat tolerance. Too much temperature causes caking, clumping and color change. And keep the material layer 20 to 50 mm thick in the dryer. Plastic conducts heat poorly. A layer that is too thick dries unevenly: the surface dries while the center stays wet, and the moisture reading lies.
Why a Hot-Air Dryer Fails on Nylon
Hot air moves moisture around. Dry air removes it.
An ordinary hot-air dryer raises temperature but keeps feeding humid shop air to the pellets. For PA the moisture sits deep inside the pellet. Hot air alone cannot pull it out. Hybster states this plainly: a heated ambient-air dryer is ineffective for polyamide.
The right equipment is a dehumidifying dryer. It feeds air at a dew point of −30 to −40°C. The dry air gives the water somewhere to go, and the desiccant bed keeps the air that dry. Demand a dew-point reading from your supplier. The dryer’s temperature setpoint says nothing about its real performance. A spent desiccant bed can run at 90°C all day and still deliver air at a −10°C dew point. The parts still come out with splay.
Ask two questions on a factory audit. What dew point does your drying air hold? When did you last regenerate the desiccant? A supplier who answers in numbers owns the process. A supplier who answers in adjectives does not.
What Undried Resin Does to Parts
Splay announces the problem on the surface. Hydrolysis hides it inside the polymer chain.
The visible damage is familiar. Silver streaks fan out from the gate. Bubbles sit just under the surface. Flow marks look foamy and rough. On PET preforms, poor drying shows as foggy crystallization and yellowing. The invisible damage is worse. Hydrolyzed PA and PET lose impact strength and tensile strength. Shrinkage turns uneven, so dimensions drift. One PET machinery source holds the intrinsic-viscosity drop during drying and molding to no more than 0.02 dl/g. Above that, bearing strength and impact resistance fall off, and preform transparency degrades.
These defects sit next to sink marks in the defect family tree. Our sink-mark diagnostic flowchart covers the geometry-driven side of surface quality. For the process-driven side, run your symptoms through the interactive troubleshooter below.
Moisture also changes how the part shrinks. Hydrolyzed PET crystallizes faster, which shifts final dimensions away from the values the mold steel was cut for. Our shrinkage compensation guide explains how mold dimensions are really derived from resin behavior, including why crystallinity changes matter.
The Clock Starts After Drying
Dried resin reabsorbs moisture the moment it cools in humid air. The drying job is not done when the timer ends.
Move dried pellets into production within 2 hours. If the press stops for more than an hour, seal the hopper and keep it heated. Never let the resin cool in damp air. Store unused pellets sealed, off the floor, in a dry warehouse. Re-dry any resin that sat overnight before it goes back in the machine. Recycled runner material dries separately and mixes back with new resin at no more than 20%.
The old purge test still works as a quick check. Melt a small sample and watch. If it foams with continuous bubbles, the resin is not ready, whatever the dryer display says. When the sample runs clean and smooth, the moisture content meets the target.
Sending us a PA, PC or PET part to quote? Tell us the resin grade and the conditions the part must survive. We will plan drying and material handling with the tooling, not as an afterthought.