Voids and Bubbles in Injection Molding: The Hidden Defect

Most molding defects announce themselves on the surface. Voids don’t—they’re holes hiding inside the part, often invisible until a part breaks where it shouldn’t, a clear part shows a bubble, or a cut cross-section reveals a cavity in a thick boss. That’s what makes them worth a buyer’s attention: a void is missing material in the middle of a load path. This guide covers voids and bubbles as part of the injection molding defects section.

Two Different Defects That Look Alike

A hole inside a part has two distinct origins, and the fix depends on which one you have:

• Vacuum voids form in thick sections as the part cools. The outside skin freezes first; as the molten core then cools and contracts, there’s no new material to feed the shrinkage, so the core literally pulls itself apart, leaving an empty pocket. This is the same mechanism as sink marks—if the skin is stiff enough to resist being pulled inward, the shrinkage opens a void inside instead of a dimple outside. Sink and voids are siblings.
• Gas bubbles are trapped gas: moisture turning to steam in an under-dried hygroscopic resin (the internal cousin of splay), air entrained during plasticizing or injection, or volatiles from an overheated, degrading melt.

A practical tell from shop-floor practice: vacuum voids live where the part is thickest—bosses, rib junctions, chunky sections—while gas bubbles can appear anywhere the flow carried them.

Why They Happen

Cause	Type	Mechanism
Thick sections / non-uniform walls	Vacuum void	Core shrinks after skin freezes; nothing feeds it
Insufficient pack/hold pressure or time	Vacuum void	Shrinkage in thick areas isn’t compensated with material
Gate freezes too early / gate too small	Vacuum void	Packing path closes before thick sections finish shrinking
Under-dried resin	Gas bubble	Moisture flashes to steam inside the melt
Entrained air	Gas bubble	Air pulled in during plasticizing/injection (back pressure too low)
Overheated melt	Gas bubble	Degradation generates gas volatiles

Why Buyers Should Care

A void is a stress concentrator and a missing cross-section in one. Parts can pass visual inspection and fail in service because a boss with a void inside snapped under screw torque, or a structural section carried less material than the drawing assumed. In clear parts, bubbles are also a cosmetic reject. For pressure-bearing, load-bearing, or safety-relevant parts, voids in thick sections are a known failure mode worth asking about explicitly—especially since the defect is internal and won’t show up in a standard cosmetic inspection.

What’s Design, What’s Process

The split follows the two mechanisms:

• Design owns vacuum voids. Thick, unfed sections are a geometry problem: core out heavy areas, keep walls uniform, follow rib and boss thickness ratios. Process (more pack pressure and time, a bigger or relocated gate that stays open longer) can compensate at the margins, but a section the packing can’t reach will void.
• Process and handling own gas bubbles. Proper drying, sane melt temperatures, and adequate back pressure are molder discipline—the same discipline that prevents splay, which is why bubbles in parts and splay on parts often travel together.

What a Buyer Should Do

• Design out thick sections before tooling—the void conversation is cheapest on the drawing.
• Ask how thick features will be packed, and whether gate size/location keeps the feed path open long enough.
• For critical parts, ask about internal inspection—sectioning samples, or weight checks that catch missing material—since voids hide from visual inspection.
• If bubbles appear in clear parts, treat it as a drying/process-discipline question first.

This is an independent buyer resource and not a substitute for engineering analysis. Whether a void problem is geometry, packing, or material handling depends on the specific part—work the diagnosis with your supplier.

Buyer FAQs

What causes voids inside injection molded parts?

Two different mechanisms. Vacuum voids form in thick sections: the skin freezes first, and as the molten core cools and shrinks with no material feeding it, it pulls apart internally—the same mechanism as sink marks, driven inward. Gas bubbles are trapped gas: moisture in under-dried resin, air entrained during processing, or volatiles from an overheated melt. Thick-section location points to vacuum voids; bubbles anywhere point to gas.

Are voids in molded parts a structural problem?

Yes, potentially. A void is missing material and a stress concentrator inside the part—a boss with a void can snap under screw torque, and a structural section carries less cross-section than designed. Because voids are internal, parts can pass visual inspection and still fail in service, which is why thick-section voids deserve explicit attention on load-bearing or safety-relevant parts.

How are voids detected if they’re invisible from outside?

Common approaches include sectioning sample parts at thick features, part-weight monitoring (a voided part weighs less than a sound one), and for critical applications non-destructive methods like X-ray/CT. In transparent parts they’re visible directly. If voids matter for your part, agree with the supplier on a detection method at sampling rather than assuming visual inspection covers it.

How do you fix voids in thick sections?

Primarily by design: core out heavy sections, keep walls uniform, and follow rib/boss thickness guidelines so shrinkage has nothing thick to hollow out. Process helps at the margins—more pack pressure and hold time, and a gate sized and placed so the feed path stays open while thick areas shrink. If the geometry can’t change, expect the supplier to lean on packing, with limits to what it can reach.

Evidence Box

The guidance on this page—covering voids and bubbles—summarizes molding failure mechanisms, causes, and corrective directions that are consistently documented across industry troubleshooting references and supplier technical literature, combined with buyer-side review logic. No defect can be diagnosed from a description alone: cause and fix on a real part depend on the specific tool, resin, and process, and should be confirmed with your supplier against physical evidence.

Disclaimer

PlasticsTechnologyAlliance.com is an independent buyer resource. It does not manufacture parts, diagnose production problems remotely, or certify suppliers. Confirm defect causes and corrective actions with your supplier against your specific part, tool, and process.