Voids and Bubbles in Injection Molding: The Hidden Defect

Most molding defects show on the surface. Voids do not. They are empty pockets inside the part, often invisible until a part fails in service, a clear part shows a bubble, or a cut section reveals a hollow core.

A void is missing material on a load path, so it is a structural risk. Know how voids form, why they matter, and which fixes belong to design versus process. See injection molding defects.

Two Defects That Look Alike

A hole inside a molded part usually comes from one of two mechanisms:

• Vacuum Voids: Form in thick cross-sections during cooling. The outer skin against the mold steel freezes first. As the molten inner core cools and shrinks, no new material feeds in. The shrinking core pulls itself apart and leaves a vacuum pocket. Same physical mechanism as sink marks. If the outer skin is rigid enough to resist being pulled inward, the core tears open internally instead.
• Gas Bubbles: Trapped gas. Usually moisture flashing to steam in an improperly dried hygroscopic resin (the internal version of splay), air entrained during plasticizing, or volatiles from a degrading, overheated melt.

Diagnostic check: Vacuum voids sit almost exclusively in the thickest sections (heavy bosses, rib intersections). Gas bubbles can appear anywhere the flow front carries them.

The Structural Risk of Voids

A void removes cross-sectional area and acts as a stress concentrator. Parts with vacuum voids can pass visual inspection and still fail in the field, such as a thick boss snapping when a screw is torqued.

For pressure-vessel, load-bearing, or safety-critical work, voids in thick sections are a known failure mode. If heavy walls carry strength, set an internal inspection method (sectioning, X-ray/CT, or precision weight monitoring). Cosmetic QC will miss them.

The Design Math Behind Voids

Vacuum voids and sink marks share the same geometry rules. A feature at full wall thickness creates thermal mass the process cannot pack out well.

Protolabs’ general guidelines recommend ribs and bosses at 40 to 60 percent of adjacent wall thickness. Designing a boss as a solid column of plastic invites a void. The failure mode is often a boss that splits in assembly, blamed on material strength instead of design.

The usual engineering fix is coring: hollow the heavy section and reinforce with thinner ribs for stiffness without the concentrated mass.

Resin selection changes void tendency. SABIC notes glass-fiber-reinforced compounds show one-third to one-half the volumetric shrinkage of unreinforced base resins. Less shrink means less vacuum-void tendency in thick sections. That trade often raises warpage risk, because glass fibers shrink anisotropically (along flow versus across it).

Design vs. Process Responsibility

• Design owns vacuum voids: Thick, unfed sections are a geometry failure. Primary fix: core heavy areas and keep uniform wall thickness. The molder can try more pack pressure, longer hold, or a larger gate so the feed path does not freeze early, but they cannot pack a heavy section if the path to it freezes first.
• Process owns gas bubbles: Proper drying, melt temperatures that avoid degradation, and enough back pressure to limit air entrainment are molder disciplines. Bubbles in a clear part are usually process or material-handling failures.

Buyer Action Plan

• Core out thick sections during design before cutting steel.
• Review the gate strategy: Ask whether gate size and location keep the feed path molten long enough to pack the thickest sections.
• Establish internal inspection criteria: If a void would compromise structure, agree how voids will be detected in production (destructive sectioning or part-weight limits).
• Distinguish the cause: Random bubbles throughout a part (especially clear resins) point to drying and material handling.

Buyer FAQs

What causes voids inside injection molded parts?

Vacuum voids: thermal shrinkage in thick sections; outer skin freezes and the shrinking core pulls itself apart when no new material can reach it. Gas bubbles: trapped air, moisture flashing to steam, or degradation. Ask the supplier which type they diagnosed before you authorize a design or process change.

Are voids in molded parts a structural problem?

Yes. A void concentrates stress and removes solid material. A part can pass visual inspection and still fail under load. For load-bearing thick sections, require an internal inspection method in the quality plan before production release.

How are voids detected if they’re invisible?

Non-visual QC: destructive sectioning, part-weight monitoring (a voided part weighs less), or X-ray/CT for critical applications. Write the method into the inspection plan; do not rely on cosmetic checks alone.

How do you fix voids in thick sections?

Permanent fix is design: core heavy sections and follow rib-to-wall ratios (typically 40–60%). If design cannot change, the molder must raise pack pressure, extend hold, and keep the gate open while the thick section cools. Do not approve production on pack-only fixes without proving a process window.

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.