Short Shots: Why Molds Don't Fill and How to Prevent It?

Short shots are a frequent defect in injection molding, occurring when the molten material fails to completely fill the mold cavity. This results in incomplete parts, missing sections, or thin areas, which can compromise mechanical strength, assembly precision, and visual quality. The issue is particularly critical in high-precision industries such as automotive, electronics, and medical devices, where even minor dimensional inaccuracies can lead to functional failures or product rejection. Understanding the causes and contributing factors of short shots is essential for improving production yield and maintaining product quality.

Causes of Short Shots

Short shots are typically caused by material flow issues, mold design limitations, or improper process parameters. Key factors include:

Low Injection Pressure or Slow Injection Speed

If the injection pressure is insufficient or the injection speed too low, molten plastic may solidify before filling the entire cavity. This is particularly problematic for thin-walled sections, long flow paths, or complex geometries.

Poor Gate Design

The location, size, and number of gates directly affect how the material flows into the mold cavity. Incorrect gate positioning or undersized gates create high flow resistance, preventing material from reaching all areas of the mold, leading to incomplete filling.

High Material Viscosity

Materials with high viscosity resist flow, especially when used in thick or long parts. Viscous polymers or composites may struggle to fill intricate mold features, resulting in short shots.

Low Mold Temperature or Improper Temperature Distribution

A cold mold or uneven mold temperature can cause the plastic to solidify prematurely, preventing complete cavity filling. Hot spots or cold spots can exacerbate this issue, particularly in large or complex molds.

Contaminated or Degraded Material

Foreign particles, moisture, or thermally degraded resin can increase flow resistance, reducing the ability of the material to fill the mold completely.

Solutions to Short Shots

Eliminating short shots requires optimizing both the injection molding process and mold design:

1. Increase Injection Pressure and Speed: Raising injection pressure ensures that molten plastic reaches all areas of the mold cavity before solidifying. Adjusting injection speed helps maintain a consistent flow front, reducing the risk of premature freezing in thin or long sections.
2. Optimize Gate Design: Gate size, shape, and location should be carefully engineered to minimize flow resistance and balance filling. Multiple gates or side-entry gates can improve flow into complex cavities and reduce the likelihood of short shots.
3. Adjust Mold Temperature and Heating Zones: Maintaining a uniform and appropriate mold temperature helps prevent premature solidification of the plastic. Zoned heating or conformal cooling can ensure consistent filling, particularly in molds with varying thicknesses.
4. Use Materials with Better Flow Properties: Selecting low-viscosity grades or materials specially formulated for high-flow applications improves cavity filling. For high-melt-temperature polymers, using appropriate additives can reduce viscosity without compromising mechanical properties.
5. Ensure Material Quality and Preparation: Dry hygroscopic plastics to remove moisture, filter out contaminants, and avoid degraded material. Proper material handling ensures consistent flow characteristics and reduces defects.

Prevention Strategies

Proactive measures can help prevent short shots before production begins:

• Conduct CAE flow simulations to predict filling patterns, identify potential flow restrictions, and optimize gate placement.
• Regularly inspect and maintain the screw, barrel, and nozzle to ensure efficient plasticizing and consistent flow.
• Design parts with uniform wall thickness and gradual transitions, reducing flow resistance and uneven cooling.
• Implement process control monitoring to track injection pressure, speed, and melt temperature during production.
• Use pre-trials and pilot runs to validate mold filling before full-scale manufacturing.

Impact on Part Quality

Short shots can have significant consequences on the mechanical, functional, and visual aspects of injection-molded parts:

• Dimensional Inaccuracy: Missing sections or thin areas compromise part assembly and functionality.
• Reduced Strength: Incomplete filling creates weak points that reduce load-bearing capacity or durability.
• Surface Defects: Short shots may leave rough or incomplete surfaces, affecting aesthetics and post-processing requirements.
• Increased Costs: Defective parts require rework, additional material, or scrap, increasing production costs and lead times.

Conclusion

Strategies such as optimizing gate placement, adjusting injection pressure and speed, controlling mold temperature, and conducting CAE simulations can ensure uniform cavity filling. Additionally, maintaining clean, properly dried materials and regularly servicing equipment helps reduce the likelihood of incomplete parts. By implementing these measures, injection molding manufacturers can minimize defects, improve efficiency, and consistently produce high-quality molded components suitable for demanding applications across automotive, electronics, consumer, and industrial products.