How to Prevent and Address Short Shots and Flash (Overflow) Issues During the Injection Molding Process?

Injection molding is a complex yet versatile manufacturing process that often encounters challenges such as short shots and flash (also known as overflow or burrs). These defects can negatively impact product quality, production efficiency, and increase manufacturing costs. Understanding how to prevent and resolve them is essential for achieving optimal results in injection molding operations.

Short Shots: A short shot occurs when the molten plastic fails to completely fill the mold cavity, resulting in incomplete parts. To mitigate this issue:

1. Material and Process Parameters: Ensure that the injection volume is sufficient to fill the entire mold cavity. Adjust the melt temperature, injection speed, and pressure to ensure adequate flowability without causing excessive shear or cooling too quickly. A slower injection speed may allow for better packing and complete filling.

2. Mold Design: Optimize the gate location and size to facilitate uniform filling. Include proper runner and gate designs, such as hot runners or balanced cold runners, to minimize pressure drop and promote material flow.

3. Ventilation: Adequate venting in the mold can relieve trapped air or gases, preventing voids that might cause short shots.

4. Machine Settings: Verify that the injection molding machine is set up correctly and has the appropriate tonnage capacity to handle the required clamping force.

Flash (Overflow): Flash refers to the excess material that leaks out between the mating surfaces of the mold. Prevention and resolution tactics include:

1. Clamping Force: Increase or adjust the clamping force if the mold halves are not held tightly enough together. This prevents the molten plastic from seeping through gaps.

2. Mold Alignment and Tightness: Ensure that the mold is properly aligned and tightened to avoid misalignments that can cause flash.

3. Ejection System: Well-designed ejection pins and stripper plates prevent flash by evenly pushing the part off the core.

4. Process Parameters: Reduce the injection pressure or holding pressure if they exceed what is necessary to fill the mold. Overpressure can cause the mold to slightly open, leading to flash.

5. Mold Design: Modify the parting lines and draft angles to reduce the likelihood of flash formation. Additionally, radii at corners can assist with equal distribution of pressure.

In both cases, continuous monitoring and adjustment of process parameters, combined with periodic mold maintenance and inspection, are critical. Advanced simulation software can also aid in predicting and resolving these issues before production commences. Continuous operator training and adherence to established procedures are equally important to maintain a defect-free injection molding process.

By addressing short shots and flash effectively, manufacturers can enhance product consistency, reduce scrap rates, and improve overall profitability and competitiveness in the market.


Abstract: This article focuses on addressing two common defects encountered in the injection molding process—short shots and flash (overflow). It discusses preventive measures and solutions for each issue. For short shots, adjustments in material and process parameters, mold design improvements, and effective ventilation are highlighted. In dealing with flash, emphasis is placed on correct clamping force, mold alignment, ejection system optimization, and process parameter control. The importance of continuous monitoring, mold maintenance, and the use of simulation software is stressed to ensure a high-quality, efficient injection molding operation.

Keywords: Injection Molding, Short Shots, Overflow, Flash, Process Parameters, Mold Design, Clamping Force, Ventilation, Ejection System, Optimization, Defect Prevention, Simulation Software.