Short shot injection molding is a common defect in plastic injection molding that can affect the quality and performance of your products. In this article, we will explain what short shot injection molding is, what causes it, and how to prevent it.
What is Short Shot Injection Molding?
Short shot injection molding, also known as underfill, refers to the phenomenon of partial incompleteness at the end of the material flow, or partial underfill in multiple cavities of an injection mold. This means that the molten plastic does not fill the entire mold cavity, leaving gaps or thin areas in the final product. These gaps or thin areas can compromise the structural integrity, appearance, and functionality of the product.
What Causes Short Shot Injection Molding?
There are many factors that can cause short shot injection molding, such as:
- Insufficient injection pressure or speed. If the injection pressure or speed is too low, the molten plastic may not have enough force or momentum to overcome the flow resistance and reach the end of the mold cavity.
- Insufficient plastic material. If there is not enough plastic material in the barrel or hopper, the injection volume may be insufficient to fill the mold cavity.
- Inappropriate mold design. If the mold design is not optimal, it may cause uneven flow distribution, excessive flow length, large flow resistance, or poor venting. These factors can affect the filling and packing of the molten plastic in the mold cavity.
- Improper molding process. If the molding process parameters are not set correctly, such as barrel temperature, injection time, backpressure, etc., they may affect the plastic fluidity, viscosity, density, and shrinkage. These factors can influence the filling and packing of the molten plastic in the mold cavity.
- Unstable production cycle. If the production cycle is not consistent or interrupted frequently, it may cause some plastic material to stay in the barrel for too long and degrade. This can reduce the plastic fluidity and viscosity and cause short shot injection molding.
How to Prevent Short Shot Injection Molding?
- Optimize the injection pressure and speed. You need to adjust the injection pressure and speed according to the mold design, plastic material, and product requirements. You need to ensure that the injection pressure and speed are high enough to fill and pack the mold cavity completely and evenly.
- Ensure sufficient plastic material. You need to check and control the amount of plastic material in the barrel or hopper. You need to ensure that there is enough plastic material to fill and pack the mold cavity completely and avoid overfeeding or underfeeding.
- Improve the mold design. You need to design the mold with proper flow balance, flow length, flow resistance, and venting. You need to ensure that the mold design facilitates a smooth and uniform flow of molten plastic in the mold cavity.
- Adjust the molding process parameters. You need to set the molding process parameters according to the mold design, plastic material, and product requirements. You need to ensure that the barrel temperature, injection time, backpressure, etc., are suitable for maintaining a good plastic fluidity, viscosity, density, and shrinkage.
- Maintain a stable production cycle. You need to avoid frequent machine shutdowns or interruptions in production. You need to ensure that the production cycle is consistent with
the normal cycle and avoid plastic degradation in the barrel.
Short shot injection molding is a common defect that can occur during the injection molding process. It happens when the mold cavity is not completely filled with molten plastic material during the injection phase, resulting in an incomplete or “short” molded part. This defect can have several causes and can lead to significant issues if not addressed promptly and appropriately.
One of the main reasons for short shots is inadequate or inconsistent melt temperature. If the molten plastic material does not reach the correct temperature before injection, it may not flow properly into all areas of the mold cavity. This can result in incomplete filling and short shots. Maintaining proper melt temperature is crucial, and factors like material type, mold design, and processing conditions need to be carefully considered to achieve optimal results.
Another common cause of short shots is insufficient injection pressure. If the injection pressure is too low, the molten plastic material may not have enough force to completely fill the mold cavity. Injection pressure needs to be carefully adjusted based on the complexity of the part, the material being used, and the mold design to ensure adequate filling.
Inadequate venting is another factor that can contribute to short shots. If air or gas is trapped in the mold cavity, it can prevent the molten plastic from properly flowing and filling all areas. Proper venting is essential to allow trapped air to escape, ensuring smooth and complete mold filling.
Additionally, issues with the mold design can also lead to short shots. Improper gate placement, inadequate runner system, or insufficient draft angles can all impact the flow of the molten plastic material and result in incomplete filling. A well-designed mold that accounts for the material’s flow characteristics and the part’s complexity is crucial to prevent short shots.
Short shots can have detrimental effects on the final product’s quality and performance. Incomplete filling can result in weak spots, voids, or poor surface finish, which can compromise the part’s functionality and structural integrity. Short shots can also lead to increased scrap rates and production delays, negatively impacting the overall manufacturing process.
To address short shots, it is essential to carefully analyze and identify the root causes. Proper troubleshooting and adjustments to the processing parameters, mold design, and material selection may be necessary to resolve the issue. Advanced simulation tools can also be used to predict and optimize the injection molding process, helping to prevent short shots before production begins.
In conclusion, short shot injection molding is a common defect that can occur during the injection molding process. Incomplete filling of the mold cavity can result from factors such as inadequate melt temperature, insufficient injection pressure, poor venting, and issues with the mold design. Short shots can have detrimental effects on the final product’s quality and performance, leading to weak spots, voids, and poor surface finish. Addressing short shots requires careful analysis and troubleshooting to identify the root causes and make appropriate adjustments to the injection molding process. By optimizing the processing parameters, mold design, and material selection, manufacturers can prevent short shots and ensure the production of high-quality plastic parts.