Nondestructive Computed Tomography Volumetric Void Acceptance Criteria for Heavy Polymer Sections
Computed tomography volumetric void acceptance requires zoning heavy polymer walls by stress field, setting pore limits below 0.1% in tensile skins.
Vacuum Shrinkage Voids
Meaning
Volumetric internal cavities created when solidifying outer polymer skins isolate a molten core undergoing thermal contraction form localized low-density pockets inside thick wall sections. Development of vacuum shrinkage voids indicates that internal volumetric contraction exceeded resin supply during the holding phase of injection moulding. The phenomenon governs core density distribution, localized structural weakness, and internal sink mark formation in thick plastic moldings.
Its definition stops at thermal contraction cavities containing near-vacuum conditions, excluding gas-filled moisture pores, air entrainment, and chemical blowing agent voids.
Thermal Contraction
Rapid cooling of outer mold surfaces locks part outer dimensions while interior polymer masses remain hot and continue contracting. Scans revealing vacuum shrinkage voids demonstrate that hold pressure was insufficient or gate freeze occurred prematurely.
Morphology Boundary
Vacuum voids exhibit distinctive, highly irregular jagged geometries located near thick cross-section centers where thermal cooling is slowest. Unlike smooth spherical gas pores, vacuum shrinkage voids follow thermal stress contours and feature rough internal surface boundaries visible under high-resolution CT imaging. Moulders suppress these internal cavities by elevating packing pressure, extending hold times, and lowering barrel melt temperatures to reduce overall volumetric contraction.
Regrind material with inconsistent melt flow indices exacerbates void formation by disrupting hold pressure transfer through thin gate sections.
Packing Optimization
Uncontrolled internal voiding reduces effective load-bearing cross-sections and induces premature fatigue cracking under mechanical load. Tracking vacuum shrinkage voids during tooling trials enables process engineers to optimize pack-and-hold profiles to deliver fully dense parts.