Every molder has seen it: a glossy ABS housing comes off the press streaked with silver, or a polycarbonate lens that should survive a 2-joule impact cracks in the drop test. Before anyone blames the mold or the machine, check what was in the hopper. Moisture in resin is the most common and most underestimated defect source in injection molding, and drying it properly is one of the cheapest quality controls in the whole process. Here is what it actually involves.
Why Moisture Ruins Molded Parts
Wet resin fails in two different ways, and it helps to separate them:
- Physical: splay, voids, bubbles. Water flashes to steam in the barrel. The steam disrupts the melt front and leaves silvery streaks on the surface (usually called splay), internal voids, or foamed purge from the nozzle.
- Chemical: hydrolysis. In condensation polymers like PC, PET, TPU, and PA, water at melt temperature actually cuts the polymer chains. Molecular weight drops. There is no visual warning at all: the part comes out looking fine and then fails brittleness testing. For polycarbonate this is the classic silent killer of lens and enclosure programs.
The first failure shows up on the surface and gets fixed eventually. The second one ships. That asymmetry is why drying discipline matters even when parts look acceptable.
Which Materials Actually Need Drying
Resins split into hygroscopic (they pull moisture inside the pellet) and non-hygroscopic (moisture sits on the surface only). Non-hygroscopic materials like PP, PE, and most PVC usually need nothing beyond ambient handling. Everything else needs a real drying step. Typical starting points, always confirm against the data sheet:
- PC: 120 degrees C for 3 to 4 hours, target moisture below 0.02 percent
- ABS: 80 degrees C for 2 to 4 hours, target below 0.1 percent
- PA66 (dry as processed): 80 degrees C for 4 to 6 hours, aim for roughly 0.1 to 0.2 percent; do not chase zero
- PMMA: 80 degrees C for 3 to 4 hours
- POM: 80 to 90 degrees C for 2 to 3 hours, mainly to strip surface moisture
- TPU: 80 to 90 degrees C for 2 to 4 hours
Two cautions. Drying temperature is a ceiling, not a suggestion: push ABS toward 100 degrees C and pellets cake into a solid brick. And for polyamide, extremely low moisture can make parts brittle in a different way; nylon is normally processed with a trace of moisture on purpose.
Hot Air Versus Desiccant Dryers
A hot air dryer simply blows heated ambient air through the pellets. That works for surface moisture and non-hygroscopic materials, but it cannot dry hygroscopic pellets below the humidity of the shop air. In a coastal summer plant, hot air alone can leave ABS and PC effectively wet no matter how long they sit.
Hygroscopic resins need a desiccant (dehumidifying) dryer with a dew point around minus 20 to minus 40 degrees C. If the dryer has a dew point indicator, glance at it weekly; saturated desiccant is a common hidden failure. Older units need desiccant regeneration or wheel checks, and a dryer that runs hot but not dry is worse than none, because everyone trusts it.
Residence Time Is Where Most Plants Slip
Drying happens in the hopper, and only if the pellets stay in it long enough. The hopper must hold at least (machine throughput multiplied by required drying time). If a 100 kg hopper feeds a press consuming 50 kg per hour and the material needs 4 hours, the math is already broken: the freshest pellets ride down and get injected after 2 hours or less.
Related habits worth enforcing:
- Keep the hopper lid closed; airborne moisture re-enters faster than it leaves.
- Never leave pellets in an open bag or unsealed gaylord overnight. Rebag and re-dry what has been exposed.
- Regrind absorbs moisture faster than virgin pellets because of its higher surface area, so blend in regrind after drying or dry it longer.
- After any stoppage longer than a few hours, assume the hopper contents have re-absorbed something and allow the timer to reset.
Diagnosing Splay: Moisture or Something Else?
Silver streaks have three usual parents, and they leave different fingerprints:
- Moisture splay appears randomly across the part, worsens with higher melt temperature and higher injection speed, and improves immediately when drying is fixed. With polyamide, the fastest test is a purge: wet nylon foams and spits from the nozzle.
- Trapped gas and venting problems concentrate near the last areas to fill and often show a dull or scorched edge. That is a mold issue, not a dryer issue, and it needs different fixes such as deeper vents or added vent pins.
- Thermal degradation produces brown or black streaks, grows with residence time in the barrel, and follows a machine that has been idling hot.
A cheap moisture analyzer that weighs a small sample before and after heating gives a number in about 15 minutes. For PC and PET work it is worth having; arguing about streaks without data burns more money than the instrument costs.
Prototype and Low-Volume Traps
Short runs have their own failure pattern. Material arrives pre-dried in sealed bags, gets opened for a small job, and the remainder sits in a partially open liner until the next run weeks later. The mold is then blamed for streaks that the second half of that bag caused. For any material-sensitive program, treat opened bags as wet by default: re-dry before use.
Vacuum drying is a useful accelerator for small batches: pulling vacuum lets moisture leave at lower temperature, so polyamide can be dried in roughly half the time without oxidizing the pellets. And when a prototype program runs bridging tools or quick-turn soft production tooling, the temptation to skip drying entirely is highest exactly when the schedule has no slack for a re-shot trial.
Finally, remember that resin choice interacts with process everywhere. If you are still selecting a grade, our plastic injection molding service team can flag the moisture-sensitive ones before they reach the shop floor.
A Five-Point Drying Checklist
- Confirm the material is hygroscopic and look up its drying temperature, time, and moisture target from the data sheet.
- Verify desiccant dryer dew point is in the minus 20 to minus 40 degrees C range.
- Check hopper capacity against throughput times drying time; measure, don't estimate.
- Keep opened bags sealed with desiccant; re-dry any exposed material.
- Inspect the first shots under angled light for splay before running the full batch.
Drying resin is unglamorous, consumes trivial energy compared to the molding machine, and prevents the two most expensive failure modes in plastics: visible scrap and invisible brittleness. Make it a gate, not an afterthought.
