Voids and Bubbles in Injection Molding: The Hidden Defect

Most injection molding defects are visible on the surface. Voids are different: they are empty pockets hiding inside the part. Often invisible until a part fails in service, a clear part shows a bubble, or a cut cross-section reveals a hollow core.

Because a void represents missing material in the center of a load path, it is a structural risk. This guide covers how voids form, why they matter, and which fixes belong to part design versus process control. (See injection molding defects).

Two Defects That Look Alike

A hole inside a molded part typically originates from one of two distinct mechanisms:

• Vacuum Voids: These form in thick cross-sections as the part cools. The outer skin against the mold steel freezes first. As the molten inner core cools and shrinks, there is no new material feeding in to compensate. The shrinking core pulls itself apart, leaving a vacuum pocket. This is the exact 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: These are trapped pockets of gas. The source is usually moisture turning to steam in an improperly dried hygroscopic resin (the internal version of splay), air entrained during plasticizing, or volatile gases released from a degrading, overheated melt.

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

The Structural Risk of Voids

A void is both a loss of cross-sectional area and a severe stress concentrator. Parts containing vacuum voids can pass standard visual inspections yet fail catastrophically in the field—such as a thick boss snapping off when a screw is torqued.

For pressure-vessel, load-bearing, or safety-critical applications, voids in thick sections are a known failure mode. If your part relies on heavy wall sections for strength, you must explicitly establish an internal inspection method (like sectioning samples, X-ray/CT scanning, or precision weight monitoring) because standard cosmetic QC will miss them.

The Design Math Behind Voids

Vacuum voids and sink marks are two sides of the same coin, governed by the same geometry rules. A feature designed at full wall thickness creates a thermal mass that the process cannot adequately pack out.

Protolabs’ general guidelines recommend designing ribs and bosses at 40 to 60 percent of the thickness of the adjacent wall. Designing a boss as a massive column of plastic guarantees a void; the failure mode is a boss that splits during assembly, which is often mistakenly blamed on material strength rather than part design.

The standard engineering solution is coring. Hollowing out a heavy section and reinforcing it with thinner ribs provides equivalent structural stiffness without the concentrated mass that causes voids.

Resin selection also alters void tendency. SABIC notes that glass-fiber-reinforced compounds exhibit one-third to one-half the volumetric shrinkage of unreinforced base resins. Because they shrink less, highly filled grades are less prone to forming vacuum voids in thick sections. However, this trades a void risk for a warpage risk, as the glass fibers cause the plastic to shrink anisotropically (differently along the flow path versus across it).

Design vs. Process Responsibility

Solving a void depends entirely on its root cause:

• Design owns Vacuum Voids: Thick, unfed sections are fundamentally a geometry failure. The primary fix is coring out heavy areas and maintaining uniform wall thickness. The molder can attempt to compensate with process (increasing pack pressure, holding pressure longer, or opening the gate size so the feed path doesn’t freeze), but they cannot pack out a heavy section if the path to it freezes first.
• Process owns Gas Bubbles: Proper material drying, controlling melt temperatures to prevent degradation, and using adequate back pressure to prevent air entrainment are standard molder disciplines. If bubbles appear in a clear part, it is usually a process or material-handling failure.

Buyer Action Plan

• Core out thick sections during design: Fix the geometry before cutting steel.
• Review the gate strategy: Ask the tooling engineer if the gate size and location will allow the feed path to stay molten long enough to pack out the thickest sections.
• Establish internal inspection criteria: If a void would compromise structural integrity, agree with the supplier on how internal voids will be detected during production (e.g., destructive sectioning or part-weight limits).
• Distinguish the cause: If bubbles appear randomly throughout a part (especially in clear resins), investigate the supplier’s drying and material handling procedures.

Buyer FAQs

What causes voids inside injection molded parts?

Vacuum voids are caused by thermal shrinkage in thick sections: the outer skin freezes, and the shrinking molten core pulls itself apart because no new material can reach it. Gas bubbles are caused by trapped air, moisture in the resin flashing to steam, or material degradation.

Are voids in molded parts a structural problem?

Yes. A void is a stress concentrator and a loss of solid material. A part with an internal void can pass a visual inspection but fail under mechanical load. Thick-section voids require explicit attention for load-bearing parts.

How are voids detected if they’re invisible?

Through non-visual QC methods. Common approaches include destructively sectioning sample parts, monitoring part weight (a part with a void weighs less than a solid one), or using X-ray/CT scanning for critical applications.

How do you fix voids in thick sections?

The permanent fix is design: core out the heavy sections and follow proper rib-to-wall thickness ratios. If the design cannot change, the molder must rely on increasing pack pressure, extending hold time, and ensuring the gate is large enough to stay open while the thick section cools.

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.