Last updated: 2026-09-05 · 5 min read

Hygroscopic resins — polycarbonate, the polyamides, ABS, PMMA, PET, PBT and TPU among them — absorb moisture from the air, and molding them wet shows up immediately as splay, voids and weak weld lines, and, for polycarbonate in particular, as a chemical loss of toughness that no process setting can undo. Dry to the grade datasheet’s moisture target, hold that target with a desiccant dryer and verified dewpoint, and check the resin at the hopper — that is the whole discipline, and skipping any part of it shows up on the part. This article sets out what moisture does inside the machine, which resins actually need drying, how dry “dry enough” is, and what a credible drying record looks like.

Key facts

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  • Hygroscopic resins absorb moisture into the pellet itself; polyolefins like PP and PE only collect surface water, which is why they usually skip the dryer.
  • Wet melt flashes to steam in the barrel — splay, silver streaks, voids and weak weld lines are the visible signatures, and they mimic other defects if you have no moisture record.
  • For polycarbonate, moisture does not only blemish: hydrolysis cuts molecular weight during molding, and the impact strength is lost chemically — the part looks fine and fails the drop test.
  • The datasheet carries the moisture target; many polycarbonate guides set it at around 0.02 percent, while polyamides are typically specified at levels several times higher — the grade governs, not the habit.
  • Verification happens at the hopper, not only at the bag: moisture measurement on the resin actually entering the screw is the number that predicts the parts.

    • Cycle time — The total time for one injection molding cycle: injection + packing + cooling + mold open/close + ejection. Typical range: 15–60 seconds. Cycle time determines production rate and cost per part — reducing cycle by 5 seconds saves 10–15% annual production cost. The related guide covers adjacent topics.

    What does moisture actually do inside the barrel?

    Two different mechanisms run at once, and separating them matters because they produce different evidence. The first is physical: water trapped in the pellet flashes to steam at melt temperature, and the expanding vapor tears the melt — the result is splay and silver streaking radiating from the gate, voids inside thick sections, and weld lines that knit weakly because steam films keep the two fronts from bonding cleanly. The second mechanism is chemical, and it is the more dangerous one for condensation polymers: at melt temperature, water attacks the polymer chains themselves. In polycarbonate this hydrolysis cuts the molecular chain, the melt’s viscosity drops, and the finished part quietly loses the impact toughness the resin was chosen for. A splayed part announces its own problem; a hydrolyzed part can look glossy and presentable and still fail the drop test it was designed to pass. That asymmetry is why the moisture record matters more than any cosmetic argument — it is the only way to prove which failure mode you were protecting against.

    Which resins need drying — and which skip the dryer?

    The dividing line is whether the resin absorbs water into the pellet or only onto it. Polyolefins — PP and PE — are hydrophobic: water sits on the surface from condensation or a wet silo, and ordinary handling clears it, so a dryer is rarely justified. Everything with polar groups in the chain — carbonates, amides, esters, urethanes — drinks water from the air at rates that depend on humidity, pellet size and residence time. The table below is the classification as the molding floor uses it.

    Family Representative resins Drying discipline
    Non-hygroscopic PP, PE Surface water only; dry only if condensation or a wet silo is suspected.
    Hygroscopic, cosmetic-critical ABS, PMMA, PC/ABS Must dry; splay is the penalty, and clear or textured surfaces show it first.
    Hygroscopic, hydrolysis-critical PC, PET, PBT, TPU Must dry to a tight target; toughness loss is chemical and irreversible.
    Hygroscopic, high baseline PA6, PA66 and their glass-filled grades Must dry; polyamides tolerate more moisture but condition-dependent properties make the target grade-specific.

    Blends inherit the stricter parent: PC/ABS dries to the polycarbonate requirement, not the ABS one. Glass-filled grades of the polyamides are no exemption either — the filler changes shrinkage and stiffness, not the amide groups that absorb the water, and the dimensional stakes on a glass-filled nylon part are higher because reinforcement amplifies any property direction the moisture pushes.

    How dry is dry enough?

    Moisture target
    The maximum moisture content the grade datasheet allows at the machine — expressed as a percentage of resin weight. It is a property of the grade, not of the resin family.

    The numbers are set by the supplier’s processing guide, and they differ by an order of magnitude across families: many polycarbonate grades call for roughly 0.02 percent or below, while polyamide grades are commonly specified at several times that level — not because polyamides forgive water, but because their chemistry and conditioning behave differently, and the datasheet has already priced that in. Treat the following as typical starting points for orientation, and let the datasheet of the exact grade govern: amorphous resins such as ABS and PMMA often dry in the region of two to four hours at temperatures near 80 °C, polycarbonate dries hotter — commonly near 120 °C for two to four hours — and polyamides typically sit between, with times that stretch as humidity rises. Two rules survive every grade change. First, more heat is not a substitute for more time: overheating degrades additives and can oxidize the resin while the pellets at the core are still wet. Second, the target applies at the machine — resin that left the dryer at target can reabsorb moisture in a humid plant within hours if it sits in an open container, a paper bag or an uncovered hopper.

    What does correct dryer setup look like?

    1Match the dryer type to the resin. Desiccant dryers are the default for hygroscopic resins; hot-air dryers only move surface water, which is why they suit PP and PE and not the families above.
    2Set temperature and time from the datasheet. Enter both on the process sheet with the grade name — “dry as usual” is not a parameter.
    3Check the dewpoint. A desiccant dryer is judged by the dryness of its air; a typical target is around −40 °C. A saturated desiccant bed delivers warm, humid air and the pellets never the datasheet target no matter how long they sit.
    4Verify airflow and hopper fill. Air takes the path of least resistance — an overfilled hopper or a collapsed insulation jacket short-circuits the drying air past the pellets.
    5Sample at the hopper. Pull resin from the hopper outlet, not from the dryer’s original bag, and measure moisture on that sample.
    6Log the result with the shot. Time, temperature, dewpoint and measured moisture go on the process sheet next to the shot data — the record that travels with moisture-sensitive programs like polycarbonate builds.

    Why does drying time matter as much as temperature?

    Drying is diffusion: water migrates from the pellet core to the surface, and diffusion runs on time at temperature, not on temperature alone. Cut the time in half and doubling the temperature does not recover the lost dryness — it just drives the surface hotter while the core stays wet, which is exactly the state that produces a part with splay on the cosmetic face and hydrolysis damage hidden in the load path. This is also where regrind earns its reputation: ground pellets have more surface area and have usually sat open longer, so a blend with regrind content holds more water and needs longer at temperature than virgin resin. If the program runs a regrind percentage, the drying record should say so explicitly — a moisture target met on virgin resin says nothing about the blend actually in the hopper. And the exposure clock keeps running downstream: every transfer, every open container and every hour in an unsealed hopper in a humid workshop recharges the resin, which is why hopper covers and short residence windows are part of the same discipline, not housekeeping extras.

    How do you verify moisture at the press?

  • Measure, do not assume — a moisture analyzer reading on resin pulled from the hopper outlet is the only number that predicts the parts; the dryer’s settings panel is a claim, not a measurement.
  • Sample before first shot — verify after startup and after every resin change or dryer interruption, not once per week.
  • Watch the defects that mimic wet resin — splay also comes from overheating and decompression; if moisture reads on target, escalate to melt temperature and venting before chasing the dryer.
  • Keep the sealed bag sealed — unopened, moisture-barrier bags from the supplier are protected for the supplier’s stated shelf life; once opened, the exposure clock starts and the resin goes back through the dryer before reuse.
  • Record the reading per shot window — the moisture value belongs beside temperature and holding pressure on the process sheet, so a cosmetic verdict later can be traced to evidence.
  • Clear or textured surfaces get the strict eye — on clear polycarbonate and PMMA parts, splay shows at levels plain housings hide, so the verification frequency steps up.

    • What belongs in the drying record?

    A drying record is six lines, not a system: grade name, dryer used, temperature, time, dewpoint at the hopper inlet, and the measured moisture at the machine, stamped with the time of the shift. That single block answers the two questions every cosmetic and impact dispute eventually reaches — was the resin dry to the datasheet when it entered the screw, and can you prove it. Programs that keep the block ship fewer arguments: when a splay complaint arrives, the record separates a moisture problem from a melt-temperature problem in one phone call, instead of a week of re-runs conducted by opinion. On tools we build, the drying record is part of the process sheet from T1 onward, and the T1 evidence package refers to it — because the cosmetic verdicts at first shots are only defensible when the moisture state behind them was measured, not presumed.

    “Wet resin is the one defect that shows up on the part and in the tensile bar long before it shows up in a complaint. The moisture log is the least expensive line on the process sheet and the one that wins the arguments.”
    — Arthur, Tooling Engineer at aCatMould

    Frequently asked questions

    How long can dried resin sit before it reabsorbs moisture?

    Hours, not days, in a humid plant. In a covered hopper fed by the dryer it stays at target; in an open container it starts reabsorbing immediately. Keep resin in covered hoppers or sealed containers, and return anything that sat out overnight to the dryer before molding.

    Can I dry regrind together with virgin resin?

    Yes, and you usually should dry the blend — but the blend needs longer than virgin resin, because regrind carries more surface area and more absorbed water. Verify moisture on the blend, not on a handful of virgin pellets, and note the regrind percentage in the drying record.

    Will drying fix every splay problem?

    No — splay has several causes, and moisture is only the most common. If the moisture reading is on target, escalate to melt temperature, decompression and venting before touching the dryer. The value of the moisture record is precisely that it eliminates the lowest-cost suspect first.

    What dewpoint should a desiccant dryer hold?

    Around −40 °C is the typical target for a healthy desiccant bed. If the dewpoint climbs toward room levels, the desiccant is saturated or the seals are leaking, and the pellets will not the datasheet target regardless of time on the hopper. Check the dewpoint reading as part of every startup.

    Do sealed bags need drying before molding?

    Unopened, moisture-barrier bags are protected for the supplier’s stated shelf life, and many programs mold straight from the bag inside that window. Once the bag is opened, the clock starts — follow the supplier’s guidance for exposure time, and when in doubt, re-dry. The cost of an unnecessary drying cycle is trivial next to a splayed T1.

    Does moisture matter as much for glass-filled polyamide as for unfilled?

    Yes. The glass changes stiffness and shrinkage, but the amide groups in the polymer still absorb water, and wet PA66-GF30 splays, voids and loses weld-line strength exactly like the unfilled grade — with higher dimensional stakes. Dry to the grade datasheet and verify at the hopper.

    Key takeaways

    1. Moisture in hygroscopic resins is a chemical problem as well as a cosmetic one — hydrolysis removes toughness that no re-run or process tweak restores.
    2. The grade datasheet carries the moisture target, the temperature and the time; habits from the last grade do not transfer.
    3. Desiccant drying is judged by dewpoint and verified at the hopper — settings panels are claims, measurements are evidence.
    4. Regrind and open storage recharge the resin with water; dry the blend you actually mold and log the blend percentage.
    5. Six lines of drying record — grade, dryer, temperature, time, dewpoint, measured moisture — settle most cosmetic disputes in one phone call.

    If your program runs hygroscopic resins — polycarbonate, polyamide, ABS, PMMA or their blends — send us the 3D model, the resin grade and the service environment before the deposit. You will get the drying and moisture-verification plan in writing, the process sheet format with the drying record built in, and the T1 evidence list that ties cosmetic verdicts to measured data — from a named tooling engineer, before any steel is ordered.

    Sources
    MatWeb — material property database for hygroscopic resin grades and datasheets
    Plastics Industry Association — industry resources on plastics processing practice
    ISO 20457:2018 — Plastics moulded parts: Tolerances and acceptance conditions, International Organization for Standardization
    ASTM D638 — Standard Test Method for Tensile Properties of Plastics, ASTM International

    Sources: Plastics Technology · Plastics Technology Online · Prospector — Resin Datasheets · MatWeb — Material Property Database