For hygroscopic materials (such as nylon), the high water content of the resin can also cause the screw to slip. Improper drying will significantly reduce the viscosity of the material and generate water vapor in the drum, resulting in a reduction in the conveying capacity of the screw. In fact, there are many reasons for slippage, which may be related to high back pressure, overheating or overcooling of the end of the cylinder, wear of the cylinder or screw, conical screw barrel too shallow threads in the feed section, unfair hopper planning, and hopper blockage, resin wetness, excessive resin lubrication, Resin materials are too fine and unfair recycling of raw materials is related to other factors.
The overcooling at the end of the cylinder is one of the main causes of screw slippage. The cylinder of the injection molding machine is divided into 3 parts. At the end, the feeding part, the granular material will form a molten film that sticks to the screw during heating and compression. Without this film, the particles are not easily transported to the front end. The material in the feed section needs to be heated to the boundary temperature to form a molten film at this stage. However, usually the material stays in the feed section for a short time and cannot reach the required temperature. This environment is usually generated on small injection molding machines. If the residence time is too short, the process of melting and inclusion of the polymer will be incomplete, causing the screw to slip or stall.