Summary
A short shot is an incomplete injection-moulded part where the plastic melt did not fill the entire cavity before solidifying. The result is a part with missing sections, typically at the ends of flow paths or in thin areas. Short shots always render the part non-conforming and are typically detected at first inspection. They may result from insufficient material, restricted flow, inadequate venting or process conditions that cause premature solidification.
What It Looks Like
The part is visibly incomplete, with one or more areas where material is absent. The unfilled edge is typically ragged or rounded, reflecting where the melt front stopped.
Appearance may vary with:
- The unfilled area may be at the far end of the flow path, in thin sections, or in areas with restricted venting.
- In multi-cavity moulds, short shots may affect only some cavities if filling is unbalanced.
- The boundary between filled and unfilled areas may be sharp or gradual depending on how quickly the melt front froze.
- Very thin features such as fins or snap-fit arms are often the first areas to show short shots.
Where It Commonly Appears
- Ends of flow paths, furthest from the gate.
- Thin wall sections where the melt front cools and freezes quickly.
- Areas with restricted or blocked venting.
- Fine features such as thin ribs, fins or snap-fit arms.
- Areas in multi-cavity moulds that receive less material due to runner imbalance.
Possible Causes
The following are possible contributing factors, not confirmed diagnoses. Multiple causes may be present simultaneously.
Part design
- Wall sections that are too thin for the material and flow length, causing premature freeze-off.
- Long flow paths relative to wall thickness (high L/t ratio) that exceed the material's flow capability.
- Features such as thin ribs or fins that are difficult to fill.
Mould and tooling
- Insufficient or blocked venting, trapping air that resists the advancing melt front.
- Gate size that is too small, restricting flow rate.
- Runner system that is undersized or poorly balanced, reducing pressure at the cavity.
- Cold slug wells that are absent or undersized, allowing cold material to block flow.
- Sprue or runner freeze-off before the cavity is filled.
Material handling and selection
- Material with insufficient flow for the part geometry and wall thickness.
- Degraded or contaminated material with reduced flow properties.
- Moisture in hygroscopic materials causing splay or flow disruption.
- Material that has been over-dried or processed outside its recommended window.
Moulding process
- Insufficient injection pressure or speed to fill the cavity before freeze-off.
- Melt temperature that is too low, increasing viscosity and reducing flow.
- Mould temperature that is too low, accelerating freeze-off at the flow front.
- Shot size that is too small, providing insufficient material to fill the cavity.
- Transfer point set too early, reducing the volume injected.
Machine or equipment
- Worn or leaking non-return valve reducing effective injection pressure.
- Insufficient injection capacity for the part size.
- Barrel temperature inconsistency causing cold spots in the melt.
Practical Checks
Begin with observation and verification before recommending changes.
- 1Confirm whether the short shot occurs on every part or intermittently.
- 2Identify which area of the part is consistently unfilled.
- 3In a multi-cavity mould, check whether all cavities are affected or only specific ones.
- 4Check whether the defect started after a material, mould, machine or settings change.
- 5Review the shot size and cushion to confirm sufficient material is being injected.
- 6Inspect vents for blockage, contamination or insufficient depth.
- 7Check the gate and runner for restriction, cold slugs or damage.
- 8Review process records: injection pressure, injection speed, melt temperature, mould temperature and transfer position.
- 9Confirm the material has been dried to specification and is within its recommended processing window.
- 10Check the non-return valve for wear or leakage.
- 11Assess whether the wall thickness at the unfilled area is within the material's recommended range for the flow length.
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
- Increase wall thickness in areas that are consistently unfilling, if the design permits.
- Reduce flow length by repositioning the gate closer to difficult-to-fill areas.
- Simplify or thicken thin features that are consistently short-shooting.
Tooling considerations
- Clean, deepen or add vents in areas where air entrapment may be restricting fill.
- Increase gate size to improve flow rate into the cavity.
- Review and rebalance the runner system to ensure even pressure distribution to all cavities.
- Add or enlarge cold slug wells.
Material considerations
- Consider a higher-flow grade of the same material if the part geometry requires it, noting that material changes require validation.
- Ensure material is dried to specification.
- Verify the material is within its recommended processing window.
Process considerations
- Increase injection pressure or speed to improve fill.
- Increase melt temperature within the material's processing window to reduce viscosity.
- Increase mould temperature to slow freeze-off at the flow front.
- Increase shot size if the cushion is insufficient.
- Adjust the transfer point to ensure the full shot is injected.
- Process changes must be evaluated against other part-quality and validation requirements.
Risks and Trade-Offs
- Increasing injection speed may introduce jetting, flow marks or burning in some geometries.
- Increasing melt temperature improves flow but may increase shrinkage, degradation risk and cycle time.
- Increasing mould temperature slows freeze-off but increases cycle time.
- Deepening vents too aggressively may introduce flash.
- Increasing shot size beyond what is needed may cause flash or overpacking.
- Switching to a higher-flow grade may reduce mechanical properties.
When to Involve Your Moulding Partner
Consider requesting an engineering review when:
- Short shots are recurring despite process adjustments.
- The part geometry includes very thin sections or long flow paths that may be at the limit of the material's capability.
- Tooling changes to venting, gate or runner are being considered.
- The material grade or its flow properties are uncertain.
- The part is subject to customer approval or validation requirements.
- Multiple variables may be contributing.
Sources and Further Reading
- Autodesk Moldflow. Troubleshooting Short Shot Problems. https://help.autodesk.com/cloudhelp/2021/ENU/MoldflowInsight-CLC-Troubleshoot/files/Troubleshooting-molding-problems/MoldflowInsight_CLC_Troubleshoot_Troubleshooting_molding_problems_Troubleshooting_short_shot_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 )