Hygroscopic Resins in Injection Molding: Drying, Moisture Control and Dimensional Drift

Two problems, not one

Moisture in plastics is usually discussed as a single issue. It is actually two, occurring at different times and requiring different countermeasures.

The first is a processing problem. If resin containing excess moisture is fed into a heated barrel, the water reacts with the polymer at melt temperature. In polyamide this is hydrolysis — the water molecule splits polymer chains, permanently reducing molecular weight. In polycarbonate the same mechanism applies. The visible symptoms appear on the molded part: silver streaks, splay marks radiating from the gate, voids, and a surface that looks cloudy or mottled. The invisible consequence is degraded mechanical performance, because shorter polymer chains mean lower impact strength and reduced elongation.

The second is a service-life problem. A part molded correctly from properly dried resin will still absorb moisture from the atmosphere after it is produced. Water molecules migrate into the polymer and act as a plasticiser: the material becomes slightly softer and more ductile, and — critically for precision components — it swells. Dimensions grow. For a tight-tolerance part, this growth can exceed the tolerance band.

Which resins absorb enough to matter

Moisture absorption varies across an order of magnitude. The table below groups the common families by practical significance.

The distinction between "must dry" and "should dry" is worth making explicit in a specification. PA and PC must be dried — processing them wet causes permanent molecular damage that cannot be corrected by adjusting machine parameters. ABS and PC-ABS should be dried where surface appearance is critical, because splay marks on a visible housing are a reject. PP and PE need no drying at all, and adding a drying step wastes time without benefit.

Drying: temperature, time and the moisture spec

Effective drying requires three things to be set correctly and one thing to be measured. The three settings are temperature, time and dew point of the drying air; the measurement is residual moisture content.

Temperature

Drying temperature is bounded from both sides. Too low and the drying rate is impractically slow — water simply does not leave the pellet quickly enough. Too high and the resin itself begins to degrade or the pellets cake and fuse in the hopper. Each grade has a window, and for glass-reinforced or flame-retardant grades the window is often narrower than for the base polymer. Temperature must be taken from the resin supplier's processing guide for the specific grade, not generalised from the family.

Time

Drying time depends on how far the moisture has to travel. Larger pellets, thicker wall sections and higher starting moisture all require longer drying. There is also a practical trap: drying time is frequently set by tradition or by what fits the production schedule, rather than by the actual drying curve. A hopper running for two hours at the right temperature may not achieve the specified moisture content if the starting moisture was high.

Dew point

This is the most commonly overlooked variable. Drying air must be drier than the target moisture content of the resin, or the process stalls — the pellets reach equilibrium with the air rather than becoming dry. Desiccant dryers are used to hold air at low dew point. If the desiccant is saturated or the dew point is not monitored, the dryer continues to run and consume energy while the resin stops getting drier.

Verifying dryness before the shot

Because residual moisture is invisible, it must be measured rather than assumed. Three approaches are in common use.

The practical routine is a moisture analyser check at the machine before a production run begins, with the reading recorded against the specification. Where a part is appearance-critical or the resin is hydrolysis-sensitive, periodic checks during the run catch gradual regressions — a partially saturated desiccant bed will slowly let moisture rise without any change to the machine settings.

What residual moisture does inside the barrel

Understanding the mechanism helps in diagnosing it. When wet pellets enter the heated barrel, the water they contain turns to steam and, at melt temperature, reacts chemically with the polymer. Two effects follow simultaneously.

The chemical effect is chain scission — the polymer backbone breaks. This reduces molecular weight, which reduces viscosity (so the melt flows more easily than expected) while also reducing impact strength and elongation. The part may look acceptable but fail a drop test that a correctly processed sample passes.

The physical effect is gas generation. Steam and reaction by-products expand and push through the melt front, leaving the characteristic streak patterns: silver streaks along the flow path, splay radiating from the gate, and sometimes voids or a dull patch near the end of fill. Position matters diagnostically — splay concentrated at the gate points to moisture, whereas splay distributed near the end of fill more often indicates trapped air or inadequate venting.

Dimensional drift in service

Post-molding moisture absorption is the slower and less obvious problem, and it is where tolerance design decisions are won or lost.

A part molded from properly dried polyamide leaves the mold essentially dry. In service it equilibrates with ambient humidity over a period of days to weeks. As it absorbs water, it swells — the dimensional change can be in the range of a few tenths of a percent for unreinforced grades under humid conditions, which on a 100 mm dimension is measurable and on a tight-tolerance fit is significant. Glass-reinforced grades swell far less in the flow direction because the fibres restrain the polymer, but they shrink more in the transverse direction, so the change becomes anisotropic rather than simply smaller.

Two consequences follow for the buying and engineering process:

• The inspection standard must state the condition. Measuring a PA part immediately after molding and again two weeks later in a humid environment will give different numbers. If the drawing does not say whether dimensions are specified dry-as-molded or at equilibrium, two parties can both be right and still disagree.
• The tolerance must reflect the service environment. A part that will spend its life in a humid climate should be toleranced around the moisture-equilibrated dimension, not the dry dimension. Designing to the dry dimension produces parts that assemble correctly at the factory and bind in the field.

For precision mechanisms where this drift is unacceptable, the engineering answer is usually to move away from polyamide. POM absorbs very little moisture and holds tolerance across humidity changes, which is a large part of why it dominates gears, bushings and precision sliding components despite a higher material cost.

Designing tolerances around equilibrium moisture

The following practices reduce the risk of moisture-related dimensional problems on production programs.

A procurement and production checklist

Conclusion

Moisture in plastics produces two separate failures on two separate timelines. In the barrel, absorbed water hydrolyses the melt — degrading molecular weight and leaving splay streaks, silver marks and reduced strength on the molded part. In service, the polymer continues to absorb moisture and swells, shifting dimensions that were correct at the moment of molding.

Both are manageable with the same underlying discipline: treat moisture as a specified and measured quantity rather than an assumption. Specify a residual moisture target and verify it before production. State on the drawing whether dimensions apply dry or at equilibrium. Where dimensional stability across humidity is critical, select a low-absorption material such as POM rather than compensating for polyamide's behaviour with tighter tolerances.

The programmes that avoid moisture problems are not the ones that dry resin longest. They are the ones that know what moisture content they started with, what they achieved, and what the part will do over the years it is in service.