Voids, sometimes misidentified as bubbles, are internal pockets that form when a molded part fails to compensate for volumetric shrinkage during cooling. Unlike moisture-driven gas bubbles—which appear as small, irregular clusters—true voids form as large, smooth cavities that sit deep in the material. Many quality teams confuse these two defects at first glance. Are you sure the defect inside your part is a void and not moisture-related bubbling? If the cavity looks clean-walled and sits in the thickest region, you’re likely dealing with a true internal void.
For high-performance programs—automotive housings, structural brackets, optical components—voids are unacceptable. A void becomes a stress riser and directly reduces load-bearing capability. When voids appear consistently in the same region, it is often a signal that the internal pressure decay profile is not matching the part’s volumetric shrinkage curve.
Among injection molding defects, internal voids are unique because they form deep within the material rather than on the surface, making them easy to overlook until the part is cut open or begins to fail under load.
What Are Voids in Injection Molding?
A void is an internal cavity formed when the outer shell of the polymer solidifies early, while the molten core continues shrinking without sufficient packing compensation. During cooling, the core’s specific volume drops, but if the gate freezes before the holding pressure can feed additional material, an empty pocket forms.
This is why voids are strongly linked to gate freeze time, packing efficiency, and local wall thickness. Thick sections (ribs/boss bases, load-bearing pads, battery enclosures, lens anchors) are always more vulnerable. If you cut a part open and see a single round, smooth pocket in the last-to-freeze zone, that’s a classic internal void.
Many engineers search for “voids vs bubbles” because the corrective actions differ. Bubbles = moisture. Voids = volumetric shrinkage + inadequate compensation.
Primary Causes of Void Formation
Insufficient Holding Pressure or Time
This is the dominant cause in most production environments.
Too little pressure:
Without enough holding pressure, the molten core cannot be packed firmly. The center shrinks freely while the shell remains rigid, leaving a cavity.
Holding time too short:
If the gate freezes early, the packing pressure is cut off even if the press is still applying it. Many automotive tools show voids appearing exactly at the moment the holding time drops below the actual gate freeze time.
Have you ever done a part-weight vs holding-time curve? That curve usually shows the exact moment voids start to form.
Excessive Wall Thickness or Wall Thickness Variation
Voids almost always appear in the last-to-freeze region.
If a part has thin-to-thick transitions, the thin region freezes first, sealing off the flow path before the thick section completes its shrinkage. The thick area becomes completely isolated from holding pressure.
A typical example:
A thick boss placed on top of a thin panel. The panel freezes, the boss core continues shrinking, and a void appears right at the center. Many engineers search for “voids in thick sections” because this exact scenario is so common.
Melt and Mold Temperature Imbalance
Higher melt temperature = higher initial specific volume = more shrinkage later.
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High melt temperature: increases shrinkage → demands stronger packing pressure.
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High mold temperature: slows cooling → extends core shrinkage → increases risk of gate freeze mismatch.
If you see voids on one side of the cavity but not the other, mold-temperature imbalance is usually the first thing to check.
Restrictive Gating or Runner Design
Any restriction reduces the effective transmission of packing pressure.
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Undersized gates freeze early
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Long, narrow runners lose pressure
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Poor cold-slug removal increases shear variation
When the pressure is lost before the core finishes shrinking, voids appear.
This is why “internal voids injection molding” often leads engineers to inspect gate geometry first.
Practical Solutions to Eliminate Voids
Optimize Holding Pressure and Time
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Increase holding pressure gradually and monitor part weight.
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Increase holding time until the weight curve becomes flat—this indicates the gate freeze time has been reached.
A simple weigh-study often reveals that the existing holding time is too short by 0.3–0.8 seconds. On thick automotive housings, the difference between a void and a perfect core sometimes comes down to just 1 second of additional holding time.
Have you checked whether your holding time truly matches your gate freeze time?
Adjust Melt and Mold Temperatures
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Lower melt temperature slightly to reduce total volumetric shrinkage.
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Reduce mold temperature if the tool is running too warm and extending the shrinkage window.
A lower melt temperature also reduces material residence-time degradation—something that helps with PC, POM, and PA66 applications where internal voids often blend with discoloration issues.
Increase Initial Injection Speed
A faster fill minimizes early skin solidification, improving pressure transmission during packing. This is especially effective for PA66-GF, PC/ABS housings, and any part with deep ribs or thick bosses.
Faster filling = longer open packing channel = fewer voids.
Tooling or Design Modifications
When process adjustments are not enough:
Enlarge the gate
Delays freeze-off and allows longer packing pressure transmission.
Add secondary gates
Useful for battery housings, automotive brackets, large connectors, and lens carriers.
Improve cooling layout
Uniform cooling reduces localized freeze-off and stabilizes the volumetric-shrinkage profile.
Increase transition radii
This reduces thin-to-thick choke points that lead to pressure isolation.
Engineers searching “voids troubleshooting injection molding” almost always end up modifying cooling or gating after process tuning hits its limit.
Conclusion
Voids are not gas defects—they are volumetric defects caused by inadequate compensation during solidification.
The core shrinks. The gate freezes early. Pressure can’t reach the center. A cavity forms.
The fastest way to solve voids is to focus on:
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holding pressure
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holding time
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gate freeze timing
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local cooling balance
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and, when necessary, gating redesign
Which one gives you faster results in your press—pressure or time? Many shops find pressure helps first, but time eliminates the defect completely.
Ready to Eliminate Internal Voids?
If you’re dealing with voids, sink marks, packing imbalance, or thick-section molding challenges, our engineering team can analyze your tool and process.