Analysis of Reasons and Solutions for Short Shots and Incomplete Filling in Injection - Molded Parts

Analysis of Reasons and Solutions for Short Shots and Incomplete Filling in Injection - Molded Parts

1. Introduction

Injection molding is a widely used manufacturing process for producing plastic parts. However, issues such as short shots (parts with unfilled areas) and incomplete filling can occur, which not only affect the quality of the products but also increase production costs. This report aims to analyze the causes of these problems and propose corresponding solutions.

2. Reasons Analysis

2.1 Material - related Reasons

• Viscosity: If the plastic material has a high viscosity, it is difficult to flow smoothly in the mold cavity. For example, materials with high molecular weight or those that have been degraded due to over - heating during the drying process may have increased viscosity. High - viscosity materials require more injection pressure to fill the mold, and if the injection system cannot provide sufficient pressure, short shots will occur.

• Moisture Content: Moisture in the plastic material can cause gas generation during the injection process. When the molten plastic with moisture enters the mold cavity, the water vaporizes, creating bubbles and preventing the complete filling of the mold. This is especially common in hygroscopic materials like nylon.

2.2 Mold - related Reasons

• Ventilation Problems: Insufficient ventilation in the mold is a major cause of incomplete filling. As the molten plastic fills the mold cavity, the air inside the cavity needs to be discharged. If the vents are too small, blocked, or not properly designed, the trapped air will create resistance to the plastic flow, resulting in short shots. For instance, in complex - shaped molds, air may be trapped in narrow corners or deep recesses.

• Mold Temperature: An inappropriate mold temperature can affect the flowability of the plastic. If the mold temperature is too low, the plastic cools down quickly when it enters the mold cavity, increasing its viscosity and making it difficult to fill the entire cavity. On the other hand, if the mold temperature is too high, the plastic may over - heat and decompose, also leading to filling problems.

2.3 Injection Process - related Reasons

• Injection Pressure: Inadequate injection pressure is one of the most direct causes of short shots. If the pressure is not high enough, the molten plastic cannot overcome the flow resistance in the runner system and the mold cavity, leaving some areas unfilled. This can happen when the injection pressure setting is too low or when there are leaks in the injection system that reduce the effective pressure.

• Injection Speed: The injection speed also plays a crucial role. If the injection speed is too slow, the plastic may start to solidify before completely filling the mold. Conversely, if the injection speed is too fast, it can cause air entrapment, flash, and other problems, which may also lead to incomplete filling in some cases.

• Shot Volume: Incorrect shot volume setting means that the amount of plastic injected into the mold is not enough to fill the entire cavity. This can be due to inaccurate calculation of the part volume or improper adjustment of the injection volume control system.

3. Solutions

3.1 Material - related Solutions

• Viscosity Adjustment: If the material viscosity is too high, consider using a lower - viscosity grade of the same material. Additionally, optimizing the drying process to ensure that the material is dried to the appropriate moisture content can also help reduce viscosity. For example, adjusting the drying temperature and time according to the material's specifications.

• Moisture Control: For hygroscopic materials, proper drying is essential. Use a reliable drying equipment, such as a desiccant dryer, and ensure that the drying time and temperature are strictly controlled. Regularly check the moisture content of the material using a moisture analyzer.

3.2 Mold - related Solutions

• Improve Ventilation: Design or modify the mold vents to ensure effective air discharge. This can include increasing the size of the vents, adding more vents in critical areas, or using venting inserts. After each production cycle, clean the vents to prevent blockages.

• Optimize Mold Temperature: Set the mold temperature according to the material's recommended range. Use a mold temperature control system, such as a water - or oil - circulation system, to maintain a stable mold temperature. Conduct trials to find the optimal temperature that provides the best filling performance for the specific part and material combination.

3.3 Injection Process - related Solutions

• Adjust Injection Pressure: Gradually increase the injection pressure during the trial production until the mold is completely filled. However, be careful not to set the pressure too high, as it may cause excessive stress on the mold and the part, leading to other problems such as flash or warping.

• Optimize Injection Speed: Experiment with different injection speeds to find the optimal value. A common approach is to start with a medium - speed injection and then adjust according to the filling results. For complex - shaped parts, a variable - speed injection process may be required, with a faster speed at the beginning to quickly fill the large - volume areas and a slower speed at the end to avoid air entrapment and flash.

• Accurate Shot Volume Setting: Calculate the shot volume accurately based on the part volume, taking into account the volume of the runner system. Use the injection molding machine's volume - control function to set the correct shot volume. Regularly calibrate the injection volume control system to ensure its accuracy.

4. Conclusion

Short shots and incomplete filling in injection - molded parts are complex problems that can be caused by a combination of material, mold, and injection - process factors. By carefully analyzing these factors and implementing the corresponding solutions, manufacturers can effectively reduce the occurrence of these defects, improve product quality, and increase production efficiency. Regular monitoring and continuous improvement of the injection - molding process are also necessary to maintain stable production quality.