PET Drying Windows and the Hydrolysis a Moulder Absorbs
Excess residual moisture causes instant ester chain hydrolysis in molten PET, dropping intrinsic viscosity and destroying mechanical part strength.
Core Moisture Gradient
Meaning
Variation in water content between the surface and the center of an individual plastic pellet creates a complex drying challenge for the processor. A steep core moisture gradient forms when the exterior of the granule dries rapidly while the interior remains saturated. This uneven distribution means that a low surface reading does not guarantee a dry melt.
Diffusion Rate
Water molecules move from the center to the surface at a pace governed by the temperature of the drying air. In a significant core moisture gradient, the slow migration of internal water becomes the limiting factor in the preparation of the resin. Increasing the airflow is less effective than raising the temperature to accelerate this internal movement.
Drying Requirement
Hoppers must provide enough residence time for the moisture at the core to reach the boundary of the pellet. If the core moisture gradient is not fully resolved, the hidden water will vaporize in the injection barrel and cause splay or structural voids. High performance engineering resins require four to six hours of heat to achieve a uniform state of dryness.
Processing Risk
Standard moisture meters that only test the surface often give a false sense of security to the operator. Only a sensor that heats the entire sample can accurately measure the remaining water trapped in the center. Reliability in moulding depends on eliminating the difference between the shell and the core.