How does moisture content affect injection molding materials?

The short answer

Hygroscopic versus non-hygroscopic resins

The dividing line is whether water dissolves into the polymer or only sits on the surface of the granule. Polypropylene and polyethylene are effectively non-hygroscopic: they pick up surface moisture, which is easy to drive off, and in normal conditions they can be moulded straight from a sealed bag. Polystyrene, PVC and PMMA sit in between. Nylon, polycarbonate, PBT, PET, PPS, PEEK and TPU are genuinely hygroscopic, and water absorbed into the pellet does not come off in a hopper at production temperature - it needs a defined drying cycle in the right kind of dryer. The trap is that a wet pellet looks and feels exactly like a dry one. There is no visible warning until the parts come out streaked.

What the water actually does: hydrolysis

At 200 to 300 C, absorbed water is no longer a passive filler. It attacks the polymer backbone in a reaction called hydrolysis, cutting long chains into shorter ones. The molecular weight drop is what does the damage, because impact strength and elongation depend far more on chain length than on any additive. The visible result is often only cosmetic, which is why the mechanical loss is so easy to miss: a part can look acceptable after a quick wipe and still fail a drop test it would previously have passed. Polyester and polyamide families are the most sensitive because the same ester and amide bonds that give them their properties are exactly the bonds water attacks, which is also why over-drying is not a safe answer - heat and time degrade them too.

The defects you will see first

• Splay and silver streaks. Glossy, silvery lines radiating from the gate, where steam has flashed and stretched the melt. This is the classic wet-material signature and it appears on the first cosmetic part.
• Bubbles and internal voids. Trapped steam expands inside thicker sections and leaves holes that may not break the surface. On a pressure-tight part this is a functional failure, not a cosmetic one.
• Dull, cloudy or mottled surfaces. A wet material rarely fills and packs the same way, so gloss drops, weld lines become visible, and transparent parts come out hazy.
• Loss of impact and tensile strength. The reduction is real and permanent because it is molecular. It is typically 10 to 30 percent on a badly hydrolysed engineering resin, and it shows up in service rather than on the production line.
• Dimensional drift and warpage. Reduced melt viscosity and unstable packing change shrinkage, so a part that was in tolerance last week drifts out of it.
• Nozzle drool and screw slip. In severe cases the gas pocket at the front of the screw disrupts the shot and the machine cannot hold a stable cushion. That is usually when the cause is finally investigated.

Recommended drying setpoints

Drying is defined by three numbers: temperature, time and a target moisture level. All three must be right, because drying at the correct temperature for too short a time leaves water in the core of the pellet, and drying at too high a temperature damages the resin while the moisture reading looks correct.

Dryer selection and the dew point rule

For low-hygroscopic resins a conventional hot-air dryer is adequate. For PA, PC, PBT, PET, PPS and TPU, hot air cannot get below ambient humidity, and in a humid climate it will not reach the target moisture at all: those materials need a desiccant dryer or a vacuum dryer holding a dew point of -30 C or lower, with -40 C as a better working target. Practical settings matter as much as the machine: air speed around 2 to 3 m/s so the hot air actually penetrates the bed, a hopper temperature held within about 5 C, and a bed depth matched to the dryer so the top and bottom of the charge dry equally. In the rainy season extend the cycle by 20 to 30 percent. Nylon that has been exposed to open air for more than about 15 minutes should be re-dried before use, and re-dried material always needs a fresh drying cycle because regrind absorbs moisture faster than virgin pellets.

Over-drying is a real failure mode

• Yellowing and discolouration. Transparent polycarbonate held above about 130 C turns yellow, and no amount of process tuning brings it back.
• Additive loss. Plasticisers, lubricants and flame retardants can migrate or degrade, so a correctly dried resin can still lose the property the additive was there to provide.
• Viscosity and flow changes. Thermal degradation lowers melt viscosity, which changes fill pressure, flash and dimensional behaviour, and POM in particular can release irritant formaldehyde vapour if overheated.
• Sticking and caking. Softening at the top of the window makes pellets clump in the hopper and feeds unevenly.
• The safe rule. Keep the drying temperature at least 20 to 30 C below the melting point and 5 to 10 C below the onset of thermal decomposition, and treat the supplier's temperature and time as a window rather than a suggestion.

Verify dryness, then keep it dry

Drying without verification is optimism. The reliable shop-floor check is a moisture analyser working by loss-on-drying, or a dew point meter on the dryer outlet. A quick process check is a short shot: inject a partial part and look for splay, bubbles or steam at the nozzle, then re-check after any break in production. Once the material is dry it has to stay dry, which means sealed hoppers, dry-air conveying rather than an open bucket, loadings matched to consumption so nothing sits in the hopper overnight, and first-in first-out stock rotation. Many apparent machine faults - short shots, streaks, weak parts - are actually a drying audit, and the cheapest fix in a moulding shop is usually a closed loop between the dryer and the press. Send us the resin and the cosmetic requirement and we will confirm the drying route, the target moisture and the risk to the finished part. See plastic injection molding for the process, surface finishing for what splay and dulling do to a cosmetic surface, and aluminium die casting where a metal part avoids the drying question entirely.