Eliminate internal voids in injection molded parts. Packing pressure, gate design, gas venting and material shrinkage analysis with 5-Why root cause and corr...
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 in an 8 mm thick PA66 boss
- Why void? → Boss not fully packed before gate froze
- Why not packed? → Packing ended (gate sealed) while boss was still shrinking
- Why sealed early? → Gate land length increased 2 mm during mold repair
- Why changed? → Mold repair restored an older gate design without a packing study
- Why not validated? → Mold repair process has no gate verification step
TRC: No gate verification step in the mold repair process
MRC: Tooling change control does not require a packing/flow validation
Fishbone (6M)
- Man: Operator delayed adjusting hold pressure after SPC alarm
- Machine: Packing pressure drift; gate wear
- Method: Hold time set on a gate-seal basis, not thick-section packing time
- Material: High-shrinkage POM/PA grade; lot-to-lot MFI variation
- Measurement: No CT/X-ray sampling; weight-check tolerance too loose
- Environment: Mold temperature difference across cavity
D3 Containment
- 100% X-ray/CT sampling on suspect lots; block parts with void > 0.5 mm
- Interim: increase hold pressure 20%, extend hold time +3 s
- Audit gate/process data on the affected tool
D5 Permanent Fix
- Restore gate land to original dimension
- Run a packing study to set optimum hold pressure / hold time per cavity
- Balance shrinkage vs. sink: validate with X-ray-sampled mold trial
D7 Prevention
- Add gate-dimension check (land length, size) to mold PM and repair sign-off
- Add packing-pressure SPC with reaction rule
- Update process FMEA: add “premature gate freeze-off” failure mode
- Require a packing study on any mold repair affecting gate design
