A short shot is a common injection molding defect that occurs when molten plastic fails to completely fill the mold cavity. The finished part may have missing sections, incomplete edges, or unfilled details.

Short shots can affect product appearance, dimensions, strength, and functionality. To solve the problem effectively, it is important to identify the root cause rather than simply increasing injection pressure.

1. Common Causes of Short Shots

Several factors can lead to incomplete mold filling:

Insufficient Injection Pressure or Speed

If the injection pressure or speed is too low, molten plastic may not reach the end of the cavity before solidifying.


Gate and Runner Design

Small gates, narrow runners, or poorly positioned gates can restrict material flow and increase filling resistance.

Incorrect Material Processing Conditions

Low melt temperature, unsuitable material viscosity, or incorrect processing settings can reduce plastic flowability and cause short shots.

2. How to Identify the Root Cause

A systematic inspection helps engineers locate the problem.

- Check where the incomplete filling occurs.
- Review injection pressure, speed, and melt temperature.
- Inspect gates, runners, and mold vents.
- Verify the material grade and drying conditions.
- Conduct a short-shot study if necessary to understand the filling pattern.

The defect location and filling pattern can provide useful clues about the underlying cause.

3. How to Fix Short Shots

The solution depends on the actual cause.

- Adjust injection speed, pressure, or temperature when process settings are responsible.
- Clean or improve mold vents to allow trapped air to escape.
- Optimize gate and runner dimensions to improve material flow.
- Check material specifications and ensure proper drying and processing conditions.

Avoid changing multiple parameters at once. Controlled adjustments make it easier to identify which change solves the problem.

4. How to Prevent Short Shots

Prevention starts during mold design and continues through production.

Proper wall thickness, suitable gate locations, balanced runner systems, and effective venting all help ensure complete mold filling.

Moldflow analysis can also help engineers identify potential filling problems before mold manufacturing.

During production, stable process parameters and regular mold maintenance help reduce the risk of recurring defects.

Conclusion

Short shots are usually caused by problems with material flow, injection settings, mold design, or venting. Identifying the root cause is the key to achieving complete and consistent mold filling.

At CN-MOULDING, we provide custom injection mold manufacturing and plastic injection molding services, with engineering support from mold design to production.

Contact our team to discuss your injection molding project.