Sliding screw of vertical injection molding machine will cause material degradation and then affect product quality. In this regard, most users want to know more about the cause of screw slippage? How to "catch" a slippery screw? The following is explained by the editor of Baizan ...
When the screw starts to slip, the material may collect at the feeding port and cannot be normally transported to the end of the injection machine. When the screw rotates and retracts inside the barrel to transport the material and prepare for the next injection, screw slip will occur in the plasticizing section. At this time, the rotation of the screw is still continuing, but the axial movement of the screw will be stopped, that is, slipping will occur. Screw slippage often results in degradation of the material before injection, product quality degradation (such as lack of material), and the molding cycle will extend.
There are many reasons for the screw slipping, which may be related to too high back pressure, overheating or cooling of the end of the barrel, wear of the barrel or screw, too shallow thread in the feeding section, unreasonable hopper planning, blocked hopper, wet resin, and excessive resin. Smooth, material is too thin, maybe the resin and recycled materials are cut unreasonably.
One of the main causes of screw slippage is the overcooling of the barrel at the end of the process. The barrel of the injection machine is divided into 3 sections. At the end, that is, the feeding section, the pellets will form a layer of melt film and stick to the screw during the heating and shrinking process. Without this film, the pellets are not simply transported to the front end.
The material in the feed section must be heated to a critical temperature to form that critical melt film. However, the residence time of the material in the feeding section is usually short and cannot reach the required temperature. This condition usually occurs on small injection machines. Too short a residence time can lead to incomplete melting and mixing of the polymer and then cause the screw to slip or stall.