1. Excessive back pressure: If the back pressure is set too high, the screw may not be able to generate sufficient forward pressure to transport the melt to overcome the reverse pressure of the back pressure. The screw will rotate in a certain direction without retreating, which will do more work on the melt and significantly increase the melt temperature, thereby having an adverse effect on the quality of the finished product and the cycle period.
2. Overheating or overcooling in the rear part of the barrel: At the rear of the feeding section, a thin layer of molten polymer forms during the heating and compression process of the material. The molten film layer is adhered to the barrel. Without this thin layer, it would be very difficult for the granular materials to be transported 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 are relatively small. Too short a dwell time can easily lead to polymer melting or insufficient mixing, which may cause the screw to slip or stop rotating.
3. Wear of the barrel or screw: Similar to 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 detach from the barrel wall, then transported forward. If there are wear areas on the screw and barrel, the screw cannot effectively transport materials forward.
4. Improper hopper design: Correct hopper design is the key to ensuring the stable transportation of materials. Generally speaking, new granular materials of uniform size operate well in a square hopper containing a sudden compression zone. However, this is not the case when recycled materials are involved. By further disrupting the shape and size of the granules, which are very inconsistent, it will affect the uniformity of the feed. Incoherent feeding means that the screw cannot maintain a uniform transport pressure on the melt, and thus slippage occurs.
5. When processing hygroscopic materials: Incorrect material drying will significantly reduce the viscosity of the material in the barrel and generate water vapor, making it difficult for the screw to move the material forward.
6. Failure of the anti-reverse ring: When the screw is rotating and plasticizing the material, the anti-reverse ring should be in the front position and in contact with the fixed ring seat. If the anti-reverse ring is in a backward 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 ring is faulty, it should be replaced immediately.