What the floor team often tries first
- Raising pack pressure without knowing gate seal time or cooling balance.
Defect troubleshooting
Voids in Injection Molding: Causes, Fixes and Prevention
Voids are internal bubbles or surface sinks from insufficient pack, uneven cooling, or thick sections that shrink internally.
Defect troubleshooting
What are voids in injection molded parts?
Voids are empty pockets inside the part or visible pits when they break the surface. They often appear in thick sections, ribs, or bosses where the core cools slower than the skin and shrink pulls material inward.
Surface voids can look like sinks but may not show a large external depression. Confirm with sectioning or CT on critical parts when the spec requires it.
Pack pressure and gate seal timing matter more than arbitrary hold increases. Map void location to last-to-pack regions and cooling layout before changing steel.
Void signals vs levers
| Signal on the part | Common process lever | Tooling / DFM lever |
|---|---|---|
| Internal void in thick boss | Extend pack while gate open; tune cooling to boss | Core pin cooling; reduce boss thickness |
| Surface pit at rib base | Increase local pack; check venting at rib | Rib-to-wall ratio per DFM guides |
| Void after wall thickness increase | Revisit fill and pack profile for new volume | Redesign wall stock before enlarging steel |
More pack vs better cooling
What usually stabilizes the defect
- Instrument pack and cooling, then adjust geometry or gate size with data.
Void reduction workflow
- 1
Classify
Internal only vs broken surface; mark thick sections involved.
- 2
Pack window
Sweep pack/hold with gate freeze monitoring.
- 3
Cooling
Verify turbulent cooling lines and baffle flow at thick cores.
- 4
Design
Escalate wall or rib changes through DFM if void persists.
Pack and cooling checks
Use this order before opening steel or rewriting fill profiles without data.
Gate seal time recorded for the cavity.
Mold temperature map stable across the thick section.
Material shrink behavior noted for the grade in use.
Quality plan defines acceptable void size if non-cosmetic.
Thick-section review in DFM
Voids are often designed in when bosses are too thick for the selected resin. Upload CAD for thickness and pack-risk feedback with your quote.
Related defect guides
Gas assist molding
Coring out thick sections from the inside.
Learn moreFAQ
Are voids the same as bubbles?
Bubbles often trace to moisture volatiles; voids often trace to shrink. Use location and material history to split them.
Will higher pack always remove voids?
Only while the gate is open and cooling allows. Otherwise flash or stress may appear elsewhere.
Ready to quote
Close pack and thickness risks early
Share CAD for DFM on ribs, bosses, and wall transitions before cutting steel.