Vacuum Void in Injection Molding — Root Cause & 8D C...

Symptoms Internal cavity visible on X-ray or cross-section — round, smooth-walled void at thickest section Void located at thickest section of part (boss, rib junction, gate-remote thick wall) Part weight variation correlating with void size (void reduces weight 0.2–2%) Cosmetic sink mark on surface opposite an internal void Affected Components Injection molded housings, gears, brackets with thick-wall sections (> 4 mm) Bosses and rib junctions where material accumulates POM, PA, PBT, PP grades with high crystallinity and high volumetric shrinkage Parts with long flow length from gate to the thick section Detection Methods X-ray / CT: round internal cavity with smooth walls (vs. gas traps: irregular, near-surface) Cross-section / microtome: locate void and measure depth below surface Part weight: compare against baseline; void reduces weight 0.2–2% Warpage / differential shrinkage check: indicator of uneven cooling Possible Causes Packing pressure too low or hold time too short for the thick section Gate freeze-off before the thick section is fully packed Gate land too long / gate too small → premature freezing High volumetric shrinkage from high-crystallinity material Uneven cooling causing differential shrinkage and internal cavitation Nozzle / sprue / runner freeze before cavity completion 5-Why Example Problem: 3 mm internal void...