How to Extend the Service Life of Plastic Molding Screw?

Injection mold screws operate under high temperatures, high pressures, high mechanical torque, and high friction conditions for extended periods. The first three factors are required by the process conditions, while wear caused by friction is inevitable. Generally, screws are treated with surface nitriding to enhance surface hardness, thereby improving their wear resistance. However, if the causes of wear are overlooked and no efforts are made to minimize it, the working life of the screw will inevitably be significantly reduced.

The following explains the causes of screw wear and the methods to reduce it.

1. Each type of plastic has an ideal plasticizing temperature range, and the barrel processing temperature should be controlled to approach this range. Granular plastic entering the barrel first reaches the feeding section, where dry friction is inevitable. When these plastics are not adequately heated and melt unevenly, it is easy to cause increased wear on the inner wall of the barrel and the surface of the screw. Similarly, in the compression section and homogenization section, uneven melting of the plastic can also lead to increased wear.

2. The rotational speed should be properly adjusted. As some plastics are reinforced with additives like glass fiber, minerals, or other fillers, these materials often have a much higher friction against metal than molten plastic. During injection molding these plastics, using a high rotational speed not only increases the shearing force on the plastic but also causes the reinforcement to produce more shredded fibers. These shredded fibers, with their sharp ends, significantly increase the wear. The abrasive action of inorganic minerals is also considerable when they slide at high speeds over metal surfaces. Therefore, the rotational speed should not be set too high.

3. To remove debris from the plastic. Generally, freshly purchased, original packaging plastics have little debris. However, during transportation, weighing, drying, coloring, especially when adding recycled materials, debris can be introduced. This can range from small items like metal shavings to larger ones like heating element nuts, paper clips, or even strings of warehouse keys, all of which have been found in the hopper. Such debris can cause considerable damage to the screw (the hopper will also be affected), so it is imperative to install a magnetic separator and enforce strict material handling and monitoring.

4. Moisture content in plastic can affect the wear on the screw surface. If moisture is not fully removed before injection molding, the remaining moisture forms high-pressure "steam particles" mixed with the melted plastic in the screw compression section. As the screw advances during the injection process from the homogenization section to the screw head, these "steam particles" decompress and expand during the injection process, resembling tiny, hard impurities, exerting abrasive damage on the wall surface.

Additionally, under high temperature and pressure, moisture in certain types of plastic can act as a catalyst, promoting plastic decomposition and generating harmful impurities that can erode metal surfaces. Therefore, the drying process before plastic injection is not only directly related to the quality of the finished product but also affects the lifespan of the screw.