Meaning
Physical characteristic termed surface moisture refers to the water molecules that are loosely attached to the exterior of polymer pellets or flakes. This moisture is typically acquired from the atmosphere during storage or from the cooling water used during the pelletizing process. Unlike water that is absorbed into the polymer matrix, this external layer can be removed relatively easily with a hot air dryer.
While less problematic than internal moisture, it must still be eliminated to prevent processing defects and to ensure accurate material feeding.
Handling Impact
High levels of surface moisture can cause the resin pellets to stick together, leading to bridging in the hopper or inconsistent flow into the extruder. This disruption in the material feed results in fluctuations in the melt pressure and the final part weight. In some cases, the water can act as a lubricant, changing the friction between the pellets and the screw.
These variations make it difficult to maintain a stable moulding process and can lead to a high scrap rate.
Drying Requirements
Removal of this water is the first step in preparing the resin for use. A simple desiccant-free dryer can often strip away surface moisture by circulating heated air through the material bed. This is usually sufficient for non-hygroscopic resins like polyethylene or polypropylene.
For these materials, the goal is simply to prevent the water from turning into steam inside the barrel, which would cause surface bubbles or splay marks on the finished part.
Measurement Difference
Detection of this external water is usually done through a simple weight loss test at a temperature just below the resin’s melting point. Because surface moisture is not chemically bound, it evaporates quickly and produces a clear change in weight. This test is faster and less expensive than the methods used to measure internal moisture.
Ensuring the pellets are dry to the touch is a basic but essential quality check for any moulding operation.
