Preventing Hydrolytic Degradation: The Critical Role of Proper Drying

Moisture as a Hidden Variable

A molding process can be dialed in perfectly and still produce poor results if the material isn’t properly prepared. Moisture is one of the most damaging variables because it isn’t visible during handling but becomes highly reactive during processing.

When hygroscopic polymers absorb water, that moisture follows the material into the barrel. Once heated, it begins to break down the polymer at a molecular level.

What Happens During Hydrolytic Degradation

As wet resin is heated, water reacts with the polymer chains and splits them into shorter segments.

That change reduces molecular weight, which directly lowers strength and impact resistance. At the same time, vaporized moisture forms gas in the melt, leading to visual defects like splay or bubbles.

The material also becomes less viscous, making it harder to fill and pack consistently. In many cases, color shifts or haze appear as the material degrades further.

Once this damage occurs, it cannot be reversed.

Materials Most at Risk

Hygroscopic materials are the most sensitive because they absorb moisture into their structure rather than just on the surface.

Common examples include polycarbonate, nylon, PET, PBT, and high-performance resins like PEI and PSU. These materials often require moisture levels below 0.02–0.04% before processing.

If that threshold is exceeded, degradation can begin as soon as the material reaches melt temperature.

How Moisture Affects the Process

Moisture-related issues build through each stage of processing.

If drying is incomplete, water remains inside the pellets. During handling, material can reabsorb moisture from the air. Once in the barrel, that moisture turns to vapor and begins breaking polymer chains.

By the time the material reaches the cavity, both physical defects and mechanical weakening are already in place.

Signs of Moisture Damage

Certain defects consistently point back to moisture.

Silver streaks or splay usually come from steam escaping during injection. Brittle parts often indicate chain breakdown. Poor surface finish, bubbles, or inconsistent shot size can all trace back to trapped moisture in the melt.

Color changes, especially yellowing or haze, suggest deeper chemical degradation.

These issues often persist even when machine settings appear correct.

Drying as Part of Process Control

Drying isn’t just preparation. It’s a controlled step that directly affects material performance.

Effective drying removes both surface and absorbed moisture, and it must be verified with measurement rather than relying on time alone. Protecting material after drying is just as important, since exposure to ambient air can quickly undo the process.

In practice, this means sealed handling systems and controlled transfer into the machine.

The Aprios Approach

Drying is treated as a validated part of the process, not a preliminary step.

Each material has defined drying conditions, including temperature, time, and dew point, all documented and controlled. Systems are monitored to ensure moisture levels remain within specification before molding begins.

That control carries through the entire process, helping maintain consistent flow, surface quality, and mechanical performance in every part produced.