Injection molding defects
Voids are hollow pockets inside a molded part, often hidden until you section, drill, or X-ray the piece. This guide explains shrinkage-driven vacuum voids versus trapped-gas voids, how to tell them apart on the floor, and a fix order that starts with geometry and pack before you chase mystery material lots.
Defect troubleshooting
A void is empty space inside solid plastic, distinct from an unfilled cavity or a surface sink.
Most voids show up in the center of heavy cross-sections or where melt flows around thick bosses and ribs. As the part cools, the polymer contracts. If the outer skin stiffens before enough melt can be pushed in during pack and hold, the interior pulls apart and leaves a vacuum pocket. When that same shrinkage can still collapse the surface, you see sink instead of a void.
Not every internal bubble is a shrinkage void. Trapped air or volatiles can leave gas pockets, especially where vents are blocked or melt races around a thick island and traps air in the core. Industry references such as BASF engineering thermoplastics troubleshooting guides and PlasticsToday part-design notes on internal voids stress that the fix path depends on which mechanism you have, because pack pressure fixes shrink voids but only shrinks trapped-air voids.
For new programs, coring heavy sections in CAD and reviewing gate location before steel is cut prevents the most expensive void loops. NetProto returns DFM feedback with instant quotes so teams can adjust geometry while a hardened steel tool is still in planning, depending on part size and complexity.
Diagnosis
Use different levers for shrink-driven voids and gas-driven voids. Mixing them wastes cycles on hold pressure that cannot displace air already sealed inside the part.
Root causes
Match the symptom pattern before you change steel, material, or every temperature zone at once.
| Likely driver | What you see | First lever to try |
|---|---|---|
| Inadequate pack or hold for shrink compensation | Void repeats in the same thick boss; sink on thinner nearby skin | Raise hold pressure and time until gate freeze; verify melt cushion is stable shot to shot |
| Premature skin freeze on thick sections | Void grows when cycle time is shortened or mold coolant is aggressive | Balance mold heat and cooling so the heavy section can feed longer; core geometry in CAD is the durable fix |
| Pressure loss in nozzle, runner, or gate | Void appears after gate wear or when production drops pack to reduce flash elsewhere | Restore pack transfer; open flow restrictions only after confirming hold actually reaches the thick section |
| Nonuniform fill or reverse flow | Void near flow weld or where melt meets from two directions around a thick island | Relocate gate, improve venting, or slow fill so air escapes before the flow front closes |
| High-shrinkage material or fiber orientation | Void worse in flow direction on reinforced grades; lot change shifts severity | Compare to known-good lot; consider lower-shrinkage grade or filler strategy with supplier guidance |
| Heavy section in part design | Void and sink alternate as pack is tuned; cosmetic surface hides internal hollow | Core out thick areas, taper ribs toward thinner sections, and gate into the heaviest region per mold-design practice |
Troubleshooting
Confirm void type, then prove pack reaches the thick section before you redesign steel.
Section or NDT
Cut a sacrificial part or use appropriate NDT on opaque housings. Open unfilled regions point to incomplete fill. Surface dimples without an internal pocket point to sink. Document location relative to gate and thickest section.
Mechanism first
If voids track thick sections and worsen when hold time drops, treat as shrink compensation. If voids follow vent-starved last-fill points or backflow around a boss, treat as gas entrapment and fix vents or gate before maxing hold pressure.
Process window
Graph pressure at the transfer point and verify cushion remains positive through hold. Inspect the non-return valve and nozzle for leakage that steals pack volume. Adjust hold in small steps and re-section after each change.
When process hits a ceiling
If voids persist at acceptable pack without flash, core heavy sections in CAD, rebalance fill with mold-flow simulation, and review gate placement into the thick region. Production timing after tooling changes varies by program and is confirmed through the instant quote workflow.
Design context
Voids and sink share the same shrink budget. Design guides on thick-wall parts explain when coring removes both defects at once.
Prevention
Most recurring voids trace to thick geometry, pack window drift, or venting that was marginal on day one.
Core heavy cross-sections in CAD so nominal sections stay more uniform along the flow path.
Gate into the thickest functional region when flow length allows, so pack pressure transfers before the gate seals.
Run mold-flow or similar simulation on high-shrink grades to predict void-prone regions before tooling release.
Size vents for last-fill and rib tips so air escapes before melt encircles thick features.
Document hold pressure, time, and cushion on the setup sheet; voids often return when operators shorten hold to gain cycle time.
Upload CAD early for DFM feedback on qualified programs so thick bosses are flagged before a hardened steel tool is built, depending on part size and complexity.
FAQ
Shop floor language overlaps, but voids usually mean shrink-driven hollow space in thick sections, while bubbles often imply gas or moisture vapor. Section the part and review venting and material drying before you treat every pocket as the same defect.
When the outer skin is stiff enough to resist collapse, shrink in the core forms an internal pocket instead of a visible dimple. Opaque parts can look fine cosmetically while still failing structural or leak tests.
Higher pack helps shrink voids if melt can still flow through the gate. It does not remove air that was trapped and sealed before hold. Trapped-air voids need venting, fill profile, or geometry changes.
Yes. Colder conditions raise viscosity and can freeze the skin early, which limits pack feed to the center. Raise temperatures within the resin supplier window and re-check voids after the skin has time to feed.
Upload CAD for an instant quote and DFM feedback. Engineers can flag heavy sections, gate strategy, and fill risk while the hardened steel tool is still in planning, depending on part size and complexity. There is no minimum order quantity once production starts.
Ready to quote
Bring section photos and CAD into an instant quote so DFM review and tooling plans align with your pack and cosmetic targets. One hardened steel tool is built for the life of the part and can typically achieve 100,000+ parts when the process window is proven.
