Why does the screw of the injection molding machine slip?

Update:08 Jan 2021

People use high-speed injection molding machines to shorten the injection cycle and improve production efficiency. However, in the production process of injection molding products, the slippage of the injection molding machine screw is a big problem. Especially for hygroscopic materials (such as nylon), the high water content of the resin will cause the screw of the injection molding machine to slip. Improper drying will significantly reduce the viscosity of the material and generate water vapor in the barrel, which will reduce the conveying power of the screw. In this regard, a humidity analyzer can be used before processing to determine the moisture content of the dried material, so that the drying level of the material reaches the supplier's recommended value.

The slippage of the screw of the injection molding machine will cause material degradation, which will affect the quality of the product. What are the reasons for the screw slippage? How to "catch" the slippery screw? When the screw of the barrel is slipping, the materials probably gather at the feeding port and cannot be conveyed to the end of the injection machine normally. When the screw rotates and retreats in the barrel to transport the material and prepare for the next injection, screw slippage will occur in the plasticizing section.

At this time, the rotation of the screw is still inherited, but the axial movement of the screw will stop, that is, slipping occurs. The slippage of the screw often causes the degradation of the material before injection, the quality of the product will be reduced (such as lack of material), and the molding cycle will be prolonged. There are many reasons for screw slippage, which may be related to too high back pressure, overheating or too cold at the end of the barrel, wear of the barrel or screw, too shallow threads in the feeding section, unfair hopper planning, and blocked hopper, resin wetting, and excessive resin Lubrication, too fine materials, or unfair cutting of resin and recycled materials are related to factors.

Too cold at the end of the barrel is one of the main causes of screw slippage. The barrel of the injection machine is divided into 3 sections. At the end, the feeding section, the pellets will form a layer of melt film and stick to the screw during the process of heating and compression. Without this film, the pellets are not easily transported to the front end.

The material in the feeding section of the injection molding machine must be heated to the critical temperature to form that critical melt film. However, usually the residence time of the material in the feeding section is very short and cannot reach the required temperature. This kind of environment is generally generated on a small injection machine. If the residence time is too short, the melting and inclusion process of the polymer will be incomplete, which will cause the screw to slip or stall.More about:screw barrel for Injection molding machine factory