What if the screw of injection molding machine slips?

When it is difficult for the injection screw to press down the material at the inlet, or the length of the barrel cannot form enough adhesion to transport the material, the screw slips. In the pre-forming stage of the screw, when the screw rotates in the barrel to convey materials along the screw direction and retreat to accumulate materials for the next injection molding, the screw will also slip. If the screw starts to slide in the preforming stage, the axial movement of the screw will stop when the screw continues to rotate. Usually, screw slippage will lead to material degradation before injection molding, resulting in product quality problems such as short shot and prolonged processing cycle. Therefore, the screw technicians in Hua Hong summarized the causes of screw slippage, including: too high back pressure, overheating or supercooling of the lower half of the barrel, wear of the barrel or screw, too shallow spiral groove in the feed section of the screw, improper design of the hopper, lack or blockage of the hopper, wet resin, too high lubricant content in the resin, too fine particle size of the material, and bad cutting shape of the used resin or recycled material. First, the influence of process parameters The low temperature at the back of the barrel is usually the main reason for the slip of the injection screw. The barrel of injection molding machine is divided into three parts. Behind the feeding section, the material forms a thin layer of molten polymer during heating and compression. The molten film layer is attached to the barrel. Without this thin layer, it is difficult for particulate matter to move forward. The material in the feed section must be heated to a critical temperature to produce a critical molten film layer. However, the residence time of materials in the barrel is often too short to make the polymer reach this temperature. This situation may be caused by the small scale of the equipment and the corresponding small barrel and screw configuration. If the residence time is too short, it will easily lead to polymer melting or insufficient mixing, which will lead to screw slippage or stall. Hua Hong screw technicians put forward two simple solutions to this problem: one is to add a small amount of material from the end of the barrel for cleaning, and check the melting temperature. If the residence time is short, the melting temperature will be lower than the set value of barrel temperature. Secondly, observing the molded product, if there are marbles, black spots or light stripes, it means that the materials are not mixed well in the barrel. In addition, according to the customer's usage, our company analyzed another scheme to solve the screw slip, and gradually increased the temperature of the feeding section until the screw rotates and retreats in harmony. Sometimes it is necessary to raise the barrel temperature above the recommended set value to reach this range. Setting too high back pressure will also cause the screw to stop running or slip. Increasing the back pressure setting will also increase the energy entering the material. If the back pressure is set too high, the screw may not generate enough pressure to convey the melt forward to overcome the back pressure of the back pressure, and the screw will rotate at a certain position without retreating, which will do more work for the melt and significantly increase the melt temperature, thus adversely affecting the product quality and cycle time. The back pressure exerted on the melt can be adjusted by the control valve on the injection cylinder. Second, the influence of equipment If the screw slip is caused by processing equipment rather than process parameters, then the wear of the screw and barrel is likely to be the key to the problem. Like the feeding section, when the resin melts in the compression section of the screw, it will stick to the barrel wall. When the screw rotates, the material leaves the cylinder wall and is conveyed forward after being subjected to shear force. If there are wear areas on the screw and barrel, then the screw can't effectively transport materials forward. If it is suspected that the equipment is worn, check the screw and barrel, and check the fit clearance between them. If the fit clearance between the screw and the barrel exceeds the standard value, it should be replaced or repaired. Hua Hong screw technicians found that the screw design parameters, especially the compression ratio (the depth of the feeding section is greater than that of the homogenizing section) play a vital role in plasticizing uniformity. Too shallow feeding section (reduced compression ratio) will reduce the output, resulting in insufficient feeding and screw slippage. Suppliers of various resins generally recommend the best compression ratio of injection molding materials. Failure of retaining ring (check valve) will also lead to screw slippage. When the screw is rotating and plasticizing materials, the retaining ring should be in the front (open) position and contact with the fixed ring seat. If the stop ring is in a backward (closed) state, or in a state between forward and backward, the molten polymer will be resisted when passing through the gap between the stop ring and the ring seat. If there is a problem with the retaining ring, it should be replaced immediately. Note: Due to different plastic materials, the compression ratio is also different. See the details in the figure below. The resin feed hopper is also the reason for the slippage of various injection screws. The correct hopper design is the key to ensure the stable transportation of materials, but it is often ignored. Generally speaking, new particles with uniform size work well in a square hopper with a sudden compression zone (sudden narrowing of the bottom). However, this is not the case when recycled materials are added. The shape and size of particles after re-crushing are very different, which will affect the uniformity of feed. Inconsistent feeding means that the screw can't keep the uniform conveying pressure on the melt, which leads to slipping. To solve this problem, as well as the particle size difference between recycled materials and new granular materials, we can try to use a circular hopper with a gentle compression zone (with a gentle gradient at the bottom). Three. Uniformity of materials As mentioned above, the shape and size of material particles will affect the consistency of feeding. Poor particle shape will lead to the decline of screw processing performance, output fluctuation and screw slip. Granular materials with uniform shapes can be packed together more closely in the feeding section of the screw. The denser the granular material in the screw is, the longer it takes for the material to melt in the screw and be transported forward. Particles with bad shape will have larger free volume (low packing density between particles or more vacuum areas), which makes it difficult to feed, leading to screw slippage. Increasing the temperature at the rear end of the barrel can make the material melt faster, and the melt flow can obtain greater compressibility. When processing hygroscopic materials such as nylon, moisture will also cause the screw to slip. Incorrect drying of the material will obviously reduce the viscosity of the material in the barrel and produce water vapor, which makes it difficult for the screw to convey the material forward. The moisture content of the particles should be measured at the bottom of the drying hopper with a hygrometer and compared with the moisture content recommended by the material supplier.