Summary
Voids are internal cavities or bubbles within an injection-moulded part, typically forming in thick sections where the outer skin solidifies before the core has fully packed. As the core cools and contracts, insufficient material is available to compensate, and the internal material pulls away from itself, forming a void. Unlike sink marks, which are visible on the surface, voids are internal and may only be detected by sectioning, X-ray or other inspection methods. Voids may affect structural performance, dimensional stability and, in transparent parts, appearance.
What It Looks Like
Voids are not visible on the part surface unless the part is transparent or translucent. In transparent parts, voids appear as internal bubbles or cloudy areas. In sectioned parts, voids appear as hollow cavities within the material.
Appearance may vary with:
- In transparent or translucent materials, voids may be visible as internal bubbles or distortions.
- In opaque materials, voids are typically only detectable by sectioning or non-destructive inspection.
- Void size and shape vary with the degree of underpacking and the geometry of the thick section.
- Multiple small voids may form in a thick section rather than a single large void.
Where It Commonly Appears
- In thick wall sections, particularly at rib bases, boss roots and other heavy sections.
- At the centre of thick areas where the core is last to cool.
- In areas remote from the gate where packing pressure is lowest.
- In parts with large variations in wall thickness.
Possible Causes
The following are possible contributing factors, not confirmed diagnoses. Multiple causes may be present simultaneously.
Part design
- Excessively thick wall sections that cannot be adequately packed.
- Rib or boss geometry that creates a heavy section at the base.
- Large variations in wall thickness that make uniform packing difficult.
Mould and tooling
- Gate size that is too small, freezing before adequate packing is achieved.
- Gate location remote from thick sections, reducing packing pressure.
- Insufficient cooling in thick sections, extending the time before the core solidifies.
- Runner system that freezes before adequate packing pressure is transmitted.
Material handling and selection
- Semi-crystalline materials with high volumetric shrinkage are more susceptible to voids.
- Materials with high melt viscosity that resist packing flow into thick sections.
Moulding process
- Insufficient holding pressure or holding time.
- Melt temperature that is too high, increasing shrinkage.
- Mould temperature that is too high, slowing solidification.
- Insufficient shot size or cushion.
Machine or equipment
- Worn or leaking non-return valve reducing effective packing pressure.
- Insufficient shot capacity for the part.
Practical Checks
Begin with observation and verification before recommending changes.
- 1Confirm whether voids are present by sectioning a sample part or using appropriate inspection methods.
- 2Identify the location of voids relative to thick sections, ribs and bosses.
- 3Check whether voids occur consistently or intermittently.
- 4Review the part geometry for thick sections that may be difficult to pack.
- 5Review process records: holding pressure, holding time, melt temperature, mould temperature and cushion.
- 6Inspect the gate for signs of early freeze-off.
- 7Check the non-return valve for wear or leakage.
- 8Assess whether the void affects structural requirements, dimensional accuracy or appearance.
Possible Corrective Directions
These are possible directions for investigation, not guaranteed solutions. Process changes must be evaluated against other part-quality and validation requirements. Do not change several variables simultaneously without first identifying the likely cause.
Design considerations
- Core out thick sections to reduce wall mass and make packing more effective.
- Reduce rib and boss thickness at the base.
- Introduce more uniform wall thickness to improve packing uniformity.
Tooling considerations
- Increase gate size to allow packing flow for longer.
- Reposition the gate closer to thick sections.
- Improve cooling in thick sections to reduce the time before the core solidifies.
Material considerations
- Consider a material with lower shrinkage if design changes are not feasible, noting that material changes require validation.
Process considerations
- Increase holding pressure within the limits of the part and mould design.
- Increase holding time.
- Reduce melt temperature to lower shrinkage.
- Reduce mould temperature to accelerate solidification.
- Process changes must be evaluated against other part-quality and validation requirements.
Risks and Trade-Offs
- Increasing holding pressure may increase residual stress.
- Coring out thick sections changes part geometry and may require re-evaluation of structural requirements.
- Reducing rib thickness to eliminate voids may reduce structural performance.
- Increasing holding time increases cycle time.
- Voids and sink marks are related — conditions that reduce voids may increase surface sink marks, and vice versa.
When to Involve Your Moulding Partner
Consider requesting an engineering review when:
- Voids are in a structurally loaded area and may affect mechanical performance.
- The part is subject to pressure, load or safety requirements.
- The defect is recurring despite process adjustments.
- Tooling changes are being considered.
- The material grade or its shrinkage behaviour is uncertain.
- Regulated or validation-controlled parts are involved.
- Non-destructive inspection is needed to characterise the void distribution.
Sources and Further Reading
- Autodesk Moldflow. Troubleshooting Sink Marks and Voids. https://help.autodesk.com/cloudhelp/2021/ENU/MoldflowInsight-CLC-Troubleshoot/files/Troubleshooting-molding-problems/MoldflowInsight_CLC_Troubleshoot_Troubleshooting_molding_problems_Troubleshooting_sink_marks_and_html.html(accessed )
- BASF. Injection-Molding Problems in Engineering Thermoplastics: Causes and Solutions. https://download.basf.com/p1/8a8082587fd4b608017fd6631d5a24b1/en/Injection-Molding_Problems_in_Engineering_Thermoplastics_-_Causes_and_Solutions(accessed )
- Autodesk Moldflow. Molding Problems and Troubleshooting. https://help.autodesk.com/cloudhelp/2017/ENU/MoldflowInsight/files/GUID-59A81F21-DC33-489D-B0A9-9436E5A9A32C.htm(accessed )