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.
Moisture Pores
Meaning
Internal volumetric defects formed by vaporized water during thermoplastic melt processing constitute a primary class of processing-induced voids in hygroscopic resins. Presence of moisture pores indicates that resin pellets retained excessive moisture during processing, causing steam expansion inside the melt stream. The condition governs local density reductions, mechanical property degradation, and cosmetic surface splay in moulded polymers.
Its classification stops at vapor-induced micro-voids, excluding thermal shrinkage voids, gas entrapment from poor venting, and mechanical air entrainment.
Defect Morphology
Vaporized moisture trapped within processing barrels forms smooth spherical micro-cavities distributed throughout the polymer matrix. Scans displaying moisture pores confirm that resin drying parameters failed to reduce moisture levels below recommended processing thresholds.
Process Boundary
Hygroscopic polymers like polyamide, polycarbonate, and polybutylene terephthalate undergo hydrolytic degradation when melted with excess moisture. This chemical reaction breaks polymer chains and drops molecular weight, compounding the physical weakness caused by internal steam cavities. Computed tomography scans reveal moisture pores as spherical, evenly dispersed voids, distinct from the irregular, centrally located vacuum voids created by volumetric shrinkage.
Moulders must lower hopper moisture content to prevent structural degradation, as hold pressure cannot collapse steam bubbles once hydrolytic chain scission has occurred. Datasheet tensile strength values are invalidated when high pore densities degrade effective load-bearing area.
Structural Degradation
Uncontrolled steam expansion during injection creates internal stress raisers and promotes premature ductile-to-brittle failure transitions. Identifying moisture pores through non-destructive scan audits forces strict drying protocols before processing technical resin lots.