Frequently Asked Questions: Voids in Plastic Parts
Summary
Voids are internal empty spaces trapped inside a molded plastic part, invisible from the outside but capable of weakening the part structurally or causing it to fail a leak or pressure test. They are often confused with sink marks, which are a related but distinct surface defect, and the fix for one does not necessarily fix the other. This FAQ answers the questions that come up most often about voids: what causes them, how they differ from sink marks, how they are detected, and what process and design changes actually prevent them.
Key Takeaways
- Voids come from two main causes: short shots (insufficient material) and trapped gas pockets.
- Poor venting, moisture vaporization, and resin degradation from excessive heat are the main sources of gas-pocket voids.
- Voids are internal cavities; sink marks are surface depressions – they look similar in cross-section but are not the same defect.
- Troubleshooting voids means addressing the specific root cause: injection pressure, cooling rate, runner or gate size, or mold venting.
- Thicker sections and sections near gates are more prone to voids than thin, well-vented areas.
- Voids can be present without any visible external sign, which is why internal inspection methods matter for critical parts.
What Causes Voids in a Molded Part?
| Cause | Mechanism |
|---|---|
| Short shot | Insufficient plastic introduced during injection; as the part cools, material shrinkage creates an internal cavity |
| Poor venting | Air trapped in the mold is not fully displaced by the incoming plastic, leaving a gas pocket |
| Moisture vaporization | Moisture in the resin turns to steam during processing, forming a void as the part solidifies |
| Volatile compound decomposition | Additives or contaminants release gas when heated during molding |
| Resin degradation | Excessive processing temperature breaks down the resin, releasing gas that becomes trapped |
How Are Voids Different From Sink Marks?
Voids and sink marks both involve material loss relative to the intended solid part, which is why they are easy to confuse, but they are fundamentally different defects. A void is an internal cavity, created either by insufficient material (a short shot) or by trapped gas, and it is not visible from the part’s exterior surface. A sink mark is a surface depression, caused by uneven shrinkage in a thicker wall section as it cools more slowly than the surrounding material. A part can have a sink mark without a void, a void without a visible sink mark, or both together in the same thick section – which is part of why distinguishing them correctly matters for choosing the right fix.
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How Are Voids Detected?
Because voids sit entirely inside the part, standard visual or dimensional inspection will not catch them. Parts suspected of voiding, especially in critical or thick-walled sections, typically need to be sectioned and examined under magnification, or inspected non-destructively using X-ray or CT scanning for parts where destructive sectioning is not practical. For high-consequence applications – parts under pressure, structural load-bearing components, anything in a sealed assembly – building void inspection into the validation plan is worth the added step, since a void that never shows up externally can still cause a field failure under load or pressure.
How Are Voids Prevented or Fixed?
- Address the specific root cause, not a generic fix: a void caused by a short shot needs a different correction than one caused by trapped gas, so diagnosing the actual cause matters before adjusting the process.
- Adjust injection pressure and speed: insufficient pressure or fill speed can leave a section under-packed, directly causing a short-shot void.
- Improve mold venting: adding or enlarging vents at the last-to-fill areas of the cavity gives trapped air a path out instead of a pocket to form in.
- Dry the resin properly before processing: moisture-related voids are often resolved simply by following the resin manufacturer’s recommended drying time and temperature.
- Reduce processing temperature if degradation is the cause: excessive heat that is breaking down the resin needs to be dialed back, even if it means adjusting cycle time elsewhere to compensate.
- Review cooling rate and gate or runner size: changes to mold geometry, not just process settings, are sometimes needed to resolve a persistent void in a specific wall section.
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Where in a Part Voids Are Most Likely to Appear
| Location | Why It’s Prone to Voiding |
|---|---|
| Thick sections near ribs or bosses | Slower cooling and harder to fully pack than surrounding thin walls |
| Areas far from the gate | Material loses pressure as it travels, leaving the furthest regions under-packed |
| Last-to-fill zones | Trapped air has nowhere to escape if venting wasn’t designed for that specific flow pattern |
| Transitions between thick and thin walls | Differential cooling at the transition creates internal stress and voiding risk |
Mapping where voids actually occur on a specific part against this list is often the fastest way to narrow down the likely cause before changing process settings. A void consistently appearing in the same location shot after shot points toward a design or venting issue specific to that location, while voids appearing inconsistently across different areas point more toward a process variability issue like inconsistent material drying or fluctuating injection pressure.
Why Voids Matter More for Some Applications Than Others
Not every void is equally consequential. A small, isolated void deep inside a non-structural, non-sealed cosmetic part may have no practical effect on function. The same void in a part that holds pressure, seals against a gasket, or carries structural load is a different story entirely – it can act as a stress concentration point under load, or as a leak path in a sealed assembly. This is why acceptance criteria for voids should be tied to the part’s actual function and consequence of failure, rather than applying one blanket standard across every part a facility produces regardless of application.
How Voids Connect to Broader Design and Process Decisions
Voids rarely appear in isolation from other design and process factors. Wall thickness is one of the biggest contributors, since thicker sections are more prone to voids because they cool more slowly and are harder to fully pack; our guide to part design for manufacturing covers how wall thickness decisions affect defects like voids across a whole part. Cooling channel layout also matters directly, since uneven cooling can create the exact differential shrinkage conditions that produce both voids and sink marks together; see our guide to cooling channel design for more on that relationship.
Because voids are invisible externally, catching them before they reach a customer depends heavily on a thorough inspection process; our guide to first article inspection covers how to build internal-defect checks into a validation plan. And for tooling that has been in service for a long time, wear in the venting system can gradually introduce voids that were not present when the tool was new; our guide to mold life cycle management covers how aging tooling can contribute to defects like this.
Working With a Supplier to Prevent Voids
A molder offering low volume injection molding should be comfortable diagnosing voids specifically, including distinguishing them from sink marks and identifying whether a short shot or trapped gas is the actual cause, rather than applying a single generic fix regardless of root cause. A plastic injection molding company in Malaysia with mature process control should also be able to discuss how venting, drying procedures, and process settings are monitored on an ongoing basis to prevent voids from recurring once resolved.
Setting Realistic Acceptance Criteria for Voids
A production quality plan benefits from defining void acceptance criteria explicitly, before production starts, rather than making case-by-case judgment calls once parts are already in hand. That typically means specifying maximum acceptable void size and location by zone on the part, which dimensions or zones require zero tolerance for voiding versus which can accept some, and what inspection method and frequency will verify compliance – sectioning a sample batch periodically, X-ray on a sampling plan, or full inspection for the most critical applications. Having this defined in advance also makes it far easier to react consistently when a void does show up in production, rather than debating acceptability under schedule pressure.
Conclusion
Voids are a defect that hides well – invisible from the outside, but capable of weakening a part or causing it to fail under conditions a solid part would handle fine. Distinguishing them correctly from sink marks, diagnosing whether the cause is a short shot or trapped gas, and addressing that specific root cause through process adjustment or mold design changes is what actually resolves voids, rather than guessing at a generic fix and hoping it happens to address the real problem.
Frequently Asked Questions (FAQs)
What exactly is a void in a molded plastic part?
An internal empty space inside the part, not visible from the surface, caused either by a short shot (insufficient material) or by trapped gas from poor venting, moisture, or resin degradation.
Are voids and sink marks the same defect?
No. Voids are internal cavities; sink marks are surface depressions caused by uneven shrinkage in thicker sections. A part can have either, both, or neither, and they need different fixes.
How can voids be detected if they aren't visible externally?
Through sectioning and magnified examination, or non-destructive methods like X-ray or CT scanning for parts where cutting into a sample isn’t practical.
What's the most common cause of voids?
It varies by part, but short shots (insufficient injected material) and poor mold venting trapping air are two of the most frequent causes.
Can drying the resin properly prevent voids?
Yes, when the cause is moisture-related. Following the resin manufacturer’s recommended drying time and temperature resolves voids caused by moisture vaporizing into steam during processing.
Do voids always mean a part should be scrapped?
Not automatically – it depends on the part’s application and where the void is located. For structural, pressure-bearing, or sealed parts, voids are a more serious concern and usually warrant rejection or further investigation.
Should void acceptance criteria be defined before production starts?
Yes. Defining maximum acceptable void size and location by zone, along with the inspection method and frequency, before production begins avoids inconsistent case-by-case decisions once parts are already being produced.
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