Before resin reaches the barrel, its moisture content already decides whether parts come out strong and clear or streaked, brittle, and weak. Drying is the step that’s easy to underrate.
Here is a defect that a perfectly set-up machine cannot prevent: the one caused by wet plastic. Before resin ever reaches the barrel, its moisture content is already deciding whether your parts come out strong and clear or streaked, brittle, and weak. Drying is the least glamorous step in molding and one of the most consequential, and it is routinely underestimated. This is a plain guide to why drying matters, which materials demand it, and what happens when you skip it.
For hygroscopic resins, moisture is not just surface water you can shrug off. It bonds into the polymer and, at molding temperatures, drives a chemical reaction (hydrolysis) that breaks the long polymer chains into shorter fragments, permanently reducing the strength and quality of the part.[1] You cannot fix that at the press. It has to be prevented by drying.
Two kinds of plastic: hygroscopic and not
Plastics fall into two camps for drying. Non-hygroscopic resins like polyethylene and polypropylene only pick up surface moisture, which is easy to remove. Hygroscopic resins absorb water into their molecular structure, and that bonded moisture cannot be removed by surface heating alone, it requires drying in low-dew-point air.[2] The common hygroscopic engineering resins are nylon (PA), polycarbonate (PC), PET, PBT, and ABS.[2][3] These are the materials where drying is not optional.
How much moisture, and how dry
The numbers show why this is serious. Nylon can absorb up to roughly 2.5% moisture by weight, the most of the common engineering plastics, and if exposed directly to water some grades absorb far more.[2] Yet the target moisture for molding is a tiny fraction of that. PET, for example, should generally be dried to about 0.005% (50 ppm) or less before processing.[3] The gap between how much moisture a resin will grab from the air and how little it can tolerate in the barrel is the whole reason drying equipment exists.
| Resin | Absorbs (approx.) | Drying note |
|---|---|---|
| Nylon (PA) | up to ~2.5% by weight | Most hygroscopic common engineering plastic; bonds water tightly[2] |
| PET | ~0.3% | Dry to ~0.005% (50 ppm); IV degrades above ~0.02%[2][3] |
| PC (polycarbonate) | ~0.15–0.35% | Susceptible to hydrolysis at processing temperature[2] |
| PBT | ~0.08–0.1% | Dry before every processing run[2] |
| PP / PE | surface only | Non-hygroscopic; minimal drying needs |
What wet plastic does to your parts
Skipping or shortcutting drying shows up fast, and in more than one way.
- Cosmetic defects: moisture flashing to steam in the barrel causes splay, silver streaks, bubbles, and poor gloss on the surface.[4]
- Lost strength: hydrolysis shortens polymer chains, so hygroscopic resins molded wet come out mechanically weaker, an invisible defect that only shows up when the part fails.[1]
- Process instability: moisture causes drooling, inconsistent viscosity, and pressure variation, making a stable process window hard to hold.[2]
The strength loss is the dangerous one, because a wet-molded part can look fine and still be compromised. That is why drying is treated as a required processing step for hygroscopic resins, not a nice-to-have.[5]
The moisture gap: what nylon holds vs. what molding allows
Hygroscopic resins absorb far more moisture from the air than molding tolerates. Values approximate, from published resin-drying references.
Approximate moisture absorption vs. molding target, from published resin-drying data.[2][3] Follow your resin supplier’s datasheet for exact figures.
Drying done right
Two rules cover most of it. First, match the method to the material: surface moisture on non-hygroscopic resin can go with hot-air drying, but hygroscopic engineering resins usually need a desiccant dryer with controlled low dew point, because hot air alone cannot pull bonded moisture from inside the pellet.[5] Second, follow the supplier’s datasheet for temperature and time, and do not overdo it, over-drying can cause its own problems like discoloration, brittleness, and degradation.[5] Storage matters too: open bags on a humid shop floor re-absorb moisture, so material handling is part of drying, not separate from it.
For hygroscopic resins, drying is not preparation, it is processing. Wet plastic causes cosmetic defects you can see and strength loss you cannot, and neither can be fixed at the press. Match the dryer to the material, hit the supplier’s target moisture, and a whole category of defects simply never appears.
Frequently asked questions
Which plastics actually need drying?
Hygroscopic resins, nylon (PA), PET, PC, PBT, and ABS, absorb moisture into their structure and require drying, usually with a desiccant dryer. Non-hygroscopic resins like PP and PE only pick up surface moisture and need far less. Note that adding fillers to a non-hygroscopic resin can make the blend hygroscopic.
What happens if I mold wet nylon or PET?
Two things. Cosmetically, moisture flashes to steam and causes splay, silver streaks, and bubbles. Structurally, hydrolysis shortens the polymer chains and permanently weakens the part. The cosmetic problem you’ll see; the strength loss you often won’t, until the part fails.
Can I over-dry resin?
Yes. Excessive drying temperature or residence time can discolor, embrittle, or degrade some resins. The goal isn’t “as dry as possible,” it’s the supplier’s target moisture at the recommended temperature and time. Follow the datasheet rather than guessing high.
Chasing splay or brittle parts?
It may be moisture, not the machine. LOG’s application engineers help with difficult-to-process resins and optimizing your process around your material.
