When the injection molding screw has difficulty pressing the material at the feeding port or fails to form sufficient adhesion along the length of the barrel to convey the material, the screw will slip. During the pre-plasticizing stage of the screw, when the screw rotates in the barrel, conveying materials along the screw direction and retreating to accumulate materials for the next injection molding, the screw may also slip. If the screw starts to slip during the pre-plasticizing stage, the axial movement of the screw will stop as it continues to rotate. Usually, screw slippage can cause material degradation before injection molding, and also lead to product quality problems such as short shots and extended processing cycle periods.
The reasons for the slippage of the screw include too high back pressure, overheating or overcooling in the latter half of the barrel, wear of the barrel or screw, too shallow screw grooves in the feeding section of the screw, improper design of the hopper, shortage or blockage of the hopper, moisture in the resin, excessive lubricant content in the resin, too fine particle size of the material, poor cutting shape of the resin used or recycled material.
The influence of process parameters
Low temperature at the rear end of the barrel is usually the main cause of the slippage of the injection molding screw. The injection molding machine barrel is divided into three sections. At the rear of the feeding section, the material forms a thin layer of molten polymer during the heating and compression process. The molten film layer is adhered to the barrel. Without this thin layer, it would be very difficult for the granular materials to be conveyed forward.
The materials in the feeding section must be heated to the critical temperature in order to form the crucial molten film layer. However, often the material stays in the barrel for too short a time to allow the polymer to reach that temperature. This situation might occur due to the fact that the equipment is too small in scale and the corresponding barrel and screw configuration are relatively small. Too short a residence time can easily cause the polymer to melt or mix insufficiently, which may lead to the screw slipping or stopping.
Now, two simple solutions to this problem will be introduced. Start by adding 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 the barrel temperature. The second method is to observe the formed product. If you find marble-like patterns, black spots or light stripes, it indicates that the material has not been well mixed in the barrel.
One of the solutions to the problem of screw slippage is to gradually increase the temperature of the feeding section until the screw rotates and retreats in harmony. Sometimes, the barrel temperature has to be raised above the recommended set value to reach this range.
Setting an excessively high back pressure can also cause the screw to stop rotating or slip. Increasing the back pressure setting value also increases the energy entering the material. If the back pressure is set too high, the screw may not generate sufficient forward pressure to convey the melt to overcome the reverse pressure of the back pressure. The screw will rotate at a certain position without moving backward, which will do more work on the melt and significantly increase the melt temperature, thereby having an adverse effect on the quality of the product and the cycle period. The back pressure applied to the melt can be adjusted through the control valve on the injection barrel.
The influence of equipment
If the cause of the screw slippage is due to the processing equipment rather than the process parameters, then the wear of the screw and barrel is very likely to be the key problem. Just like in the feeding section, when the resin melts in the screw compression section, it adheres to the barrel wall. When the screw rotates, the material is subjected to shearing force and leaves the barrel wall, then is conveyed forward. If there are wear areas on the screw and barrel, the screw cannot effectively convey materials forward. If there is suspicion of wear on the equipment, the screw and barrel should be inspected, and the fit clearance between them should be checked. If the fit clearance between the screw and the barrel exceeds the standard value, replacement or maintenance work should be initiated.
The design parameters of the screw, especially the compression ratio (the depth of the feeding section compared to the depth of the homogenization section), play a crucial role in the uniformity of plasticization. If the feeding section is too shallow (resulting in a smaller compression ratio), the output will be reduced and the screw will slip due to insufficient feeding. Suppliers of various resins generally recommend the best compression ratio for injection molding materials.
A malfunction of the check ring (one-way valve) can also cause the screw to slip.
When the screw is rotating and plasticizing the material, the anti-reverse ring should be in the front (open) position and in contact with the fixed ring seat. If the anti-reverse ring is in a backward (closed) state or in a state between forward and backward, there will be resistance when the molten polymer passes through the gap between the anti-reverse ring and the ring seat. If there is suspicion that the anti-reverse loop is faulty, it should be replaced immediately.
The resin feeding hopper can also be the cause of slippage in various injection molding screws. The correct design of the hopper is the key to ensuring stable material transportation, but this point is often overlooked. Generally speaking, new granular materials of uniform size operate well in a square hopper with a sudden compression zone (where the bottom suddenly Narrows). However, this is not the case when recycled materials are added. After being crushed again, the shape and size of the granular materials are very inconsistent, which will affect the uniformity of the feed. Incoherent feeding means that the screw cannot maintain a uniform conveying pressure on the melt, which in turn causes slippage. To solve this problem and address the difference in size between recycled materials and new granular materials, a circular hopper with a gentle compression zone (with a gentle gradient at the bottom) can be attempted.
Material uniformity
As mentioned above, the shape and size of the material particles will affect the continuity of the feed. Poor shape of granular materials can lead to a decline in the processing performance of the screw, fluctuations in output, and screw slippage. Uniform-shaped granular materials can be more closely packed together in the screw feeding section. The more closely the granular materials are packed together in the screw, the more time the materials have to melt in the screw and be conveyed forward. Poorly shaped granular materials will have a larger free volume (lower volumetric density between granular materials or more vacuum zones), and it will be difficult to feed, causing the screw to slip. Increasing the temperature at the rear end of the barrel can make the material start to melt more quickly, and the compressibility of the melt flow will be greater.
When processing hygroscopic materials such as nylon, moisture can also cause the screw to slip. Incorrect material drying can significantly reduce the viscosity of the material in the barrel and generate water vapor, making it difficult for the screw to convey the material forward. The moisture content of the granular material should be measured at the bottom of the drying hopper using a hygrometer and compared with the readings recommended by the material supplier.