Meaning
Hydraulic pressure and heat requirements for polymer transformation define the critical limits of raw material water content. A processing moisture threshold governs the chemical stability of hygroscopic resins during the injection phase by setting the maximum permissible percentage of water allowed before hydrolysis occurs. Exceeding this limit causes polymer chains to fracture under heat, which reduces the molecular weight of the final part.
This degradation produces brittleness and surface defects known as silver streaking.
Moulding Limit
The industry determines these values based on the specific resin chemistry and the geometry of the part under production. Resin suppliers publish maximum moisture percentages on datasheets that remain constant for the virgin material regardless of the specific machine brand. A moulder manages this threshold by installing desiccant dryers that pull atmospheric humidity away from the pellets before plasticisation.
If the drying time fails to reach the required duration, the machine pushes material into the cavity that carries enough internal water to generate steam during the melt phase. This steam leaves voids or bubbles inside the finished component that act as stress concentrators.
Regrind Economics
The addition of regrind to the virgin supply chain alters the baseline saturation point of the material mix. Every cycle of heat history increases the surface area or changes the porosity of the plastic, which creates a higher affinity for moisture uptake during storage. Practitioners must adjust the processing moisture threshold downward when high percentages of ground scrap exist in the hopper to compensate for the faster rate of absorption.
This reduction in acceptable moisture lowers the throughput capacity of the drying equipment and requires more precise control over the ambient humidity in the facility.
Tooling Feedback
Small gas traps in the mould often mask the presence of moisture by creating burn marks that look like short shots at the extremities of the flow path. Engineers distinguish these defects by comparing the location of the gas mark to the venting strategy of the steel tool. When the defect appears in a non-vented area, the material moisture likely exceeds the threshold.
High-performance grades require a stricter moisture discipline because the long molecular chains in engineering resins collapse under minor hydrolytic interference. Consistent control of this variable remains the primary technical requirement for achieving expected physical properties in precision moulding.
