Process Window Compensation versus Tool Steel Alterations for Heavy Wall Sections
Altering tool steel to eliminate heavy wall sections cuts cycle time and unit cost far more reliably than forcing process windows past physical thermal limits.
Hydrostatic Tension
Meaning
Mechanical stress state where a material is subjected to equal pulling forces in all directions can lead to rapid void initiation and structural failure in solid polymers. Within the core of thick injection-molded parts, hydrostatic tension arises during the solidification phase as the outer skin freezes and the inner melt contracts due to thermal shrinkage. This localized stress state is the primary cause of internal voids and micro-cracking in heavy-walled plastic components.
Void Mechanism
Volumetric shrinkage of the polymer melt during cooling creates a high localized negative pressure when the material cannot draw more melt from the gating system. This hydrostatic tension can literally pull the molten polymer apart, creating vacuum voids that are invisible from the surface of the part. If the hold pressure is terminated before the gate freezes, this phenomenon is exacerbated, leading to a high scrap rate.
This problem is particularly troublesome in precision gears and thick-walled structural blocks where internal integrity is critical.
Material Susceptibility
Semi-crystalline polymers with high volumetric shrinkage are particularly prone to this form of internal stress during molding. Under hydrostatic tension, the polymer chains in these materials are pulled outward toward the rigid, frozen mold walls, disrupting the crystallization process. This disruption can result in a brittle region at the center of the molded component, reducing its load-bearing capacity.
Mitigation Strategy
Molders adjust the packing profile to ensure that material continues to flow into the cavity until the core has solidified. Preventing hydrostatic tension requires optimizing the gate location and maintaining holding pressure until the gate freezes.