Short Shot in Injection Molding: Causes, Detection & Solutions

Short shot in injection molding is one of the most frequent and critical defects encountered in the plastic manufacturing industry. It occurs when molten plastic fails to fill the mold cavity completely, leading to incomplete, underfilled, or malformed parts. This issue can result in elevated scrap rates, increased production costs, and compromised product reliability, especially in sectors requiring high precision, such as automotive, electronics, and medical device manufacturing.

Further reading: Common‌ Injection Molding Defects: Causes, Types, and Solutions

A short shot happens when the injected plastic injection molding material fails to fill the mold cavity before it cools and solidifies. Common manifestations include missing edges, thin or incomplete sections, and voids, particularly in thin walls or areas far from the gate. The defect affects the appearance of molded parts and undermines their mechanical strength and performance.

Several variables contribute to short shots, including resin viscosity, mold design, process settings, and external temperature conditions. A precise understanding of these root causes is essential for consistent, defect-free production.

• Thin-wall sections or ribs
• Far-end cavities in multi-cavity molds
• Corners and thick-to-thin transitions
• Undervented areas and obstructed flow paths

Short shots often result from material, machine, mold, or injection molding process issues. Here’s a breakdown of the most common causes and corresponding industrial solutions:

Resins with high viscosity struggle to flow through long or narrow mold paths, especially in intricate geometries.

Solution:
Increase melt temperature, switch to lower-viscosity or high-MFI resins, and ensure uniform melt through proper screw design and back pressure control.

Trapped air creates back pressure, preventing the molten plastic from wholly occupying the cavity.

Solution:
Improve or clean vent channels near parting lines, ejector pins, or corners. Add micro-vents in persistent short shot zones.

Suboptimal injection molding machine parameters, such as low injection speed, short hold time, or inadequate shot volume, can reduce fill pressure.

Solution:
Fine-tune speed, shot size, and hold pressure using scientific molding techniques. Apply Design of Experiments (DOE) for parameter optimization.

If either the melt or the mold is too cold, the plastic may prematurely solidify before filling the cavity.

Solution:
Raise barrel and mold temperatures. Use temperature controllers to stabilize mold heat and prevent cold spots.

Low pressure may be inadequate to push the material to distant or narrow mold areas.

Solution:
Increase injection pressure gradually. Inspect the hydraulic system, screw, and non-return valve to ensure reliable pressure transmission.

Long flow paths, sudden cross-section changes, or unbalanced runner layouts can lead to inconsistent fill.

Solution:
Use Moldflow simulations to validate the design. Optimize runner balance, gate size and placement, and wall thickness uniformity.

Wet pellets or foreign particles can vaporize under heat, introducing bubbles or inconsistent flow.

Solution:
Pre-dry material using dehumidifying dryers to below 0.003% moisture content. Avoid excessive regrind and ensure material cleanliness.

Inconsistent dosing due to inadequate back pressure or screw retraction settings can lead to shot volume fluctuation.

Solution:
Ensure stable screw retraction, apply moderate back pressure, and verify consistent melt preparation.

Identifying short shots quickly is critical for timely corrective actions and minimizing scrap:

• Visual Inspection: Basic yet effective; check molded parts for incomplete sections or surface voids.
• Dimensional Analysis: Use calipers or vision systems to measure and compare part geometry.
• Process Monitoring: Track screw position, injection pressure curves, and fill time. Sudden pressure drops may indicate incomplete filling.
• Sensor-Based Systems: In-mold pressure or temperature sensors can provide real-time data on cavity fill behavior.
• AI-Powered Vision Systems: Advanced Indian factories are adopting machine learning vision tools for automatic defect classification and rejection.

To effectively resolve short shots, improvements should be made across mold design, processing parameters, raw material handling, and machine condition：

• Revise runner and gate dimensions: Increase size to reduce pressure drop and enhance flow efficiency.
• Enhance mold venting: Add venting grooves or utilize ejector mechanisms for better air release.
• Increase mold temperature: Use mold temperature controllers to maintain ideal conditions and improve flow stability.

• Increase injection pressure and speed: Adjust based on product geometry and flow distance to overcome resistance.
• Extend packing and fill times: Allow enough time for full filling and cooling to reduce shrinkage and underfilling.
• Verify proper temperature settings: Adjust barrel, nozzle, and mold temperatures according to resin properties to ensure optimal flow.

• Use low-viscosity, high-MFI resins: Select materials with better flow characteristics for complex or fine-detail parts.
• Ensure thorough drying: Use a dehumidifying dryer and follow appropriate temperature/time guidelines to reduce moisture content.
• Maintain consistent material sourcing and formulation: Avoid batch inconsistencies or excessive regrind that may alter melt behavior.

• Regularly check screw, non-return valve, and heaters to ensure proper function.
• Match machine capacity to product and mold size, especially injection volume and clamping force.
• Utilize monitoring systems (e.g., pressure sensors, screw position feedback) for real-time diagnostics and anomaly detection.

Preventive actions reduce the likelihood of recurrence and improve long-term operational stability:

1. Scientific Molding Protocols: Use structured DOE to identify optimal process windows and ensure repeatable results.
2. Material Handling Best Practices: Keep resin dry, especially in humid Indian climates. Install hopper dryers and use sealed material conveyance systems.
3. Scheduled Mold Maintenance: Clean vents, runners, and gates regularly. Use thermal imaging to detect mold temperature imbalance.
4. Operator Training: Train shop-floor staff to recognize early symptoms of short shots and implement stepwise troubleshooting.
5. Smart Factory Monitoring: Adopt advanced monitoring systems like HFM (Huarong Factory Management) to visualize shot curves, detect anomalies, and enable traceability.
6. Design for Manufacturability (DFM): Collaborate with designers to avoid abrupt thickness changes, place gates logically, and simplify the flow path for better moldability.

Effectively addressing short shot defects requires thorough technical analysis and cross-functional collaboration among mold designers, process engineers, and material suppliers. Only by optimizing designs from the outset and integrating precise machine settings with controlled material management can manufacturers achieve complete cavity filling, stable quality, and improved injection molding productivity.

• Group Name: Huarong Group
• Brand: Huarong, Yuhdak, Nanrong
• Service Offerings: Injection Molding Machine, Vertical Injection Molding Machine, Injection Molding Automation
• Tel: +886-6-7956777
• Address: No.21-6, Zhongzhou, Chin An Vil., Xigang Dist.., Tainan City 72351, Taiwan