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Why does the injection molding machine screw slip?

Zhoushan Dinghai Jinsheng Bimetallic Plastic Machinery Factory | Updated: Oct 22, 2018

In the case of hygroscopic materials (such as nylon), high water content of the resin can also cause the screw to slip. Inappropriate drying can significantly reduce the viscosity of the material and cause water vapor in the barrel, thus causing a reduction in the conveying power of the screw. In this regard, a moisture analyzer can be used prior to processing to determine the moisture content of the dried material so that the dryness of the material reaches the supplier's recommended value.

The screw slip of the injection molding machine causes material degradation, which affects the quality of the product. What are the reasons for the screw slip? How to "catch" the slippery screw?

When the barrel screw slips, the material collects at the feed port and cannot be normally delivered to the end of the injection machine. When the screw rotates and retracts within the barrel to deliver the material and prepares for the next injection, the screw slip occurs in the plasticizing section. At this point, the rotation of the screw is still inherited, but the axial movement of the screw will stop, ie, slippage will occur. Screw slippage often causes the material before injection to degrade, the quality of the product is reduced (such as lack of material), and the molding cycle is prolonged.

The reason for the screw slip is various. The back pressure is too high, the end of the barrel is too hot or too cold, the barrel or screw is worn, the feeding section thread is too shallow, the hopper plan is unfair and the hopper is infested, the resin is wet, and the resin is excessive. Lubrication, the material is too thin, probably related to the unfair cutting of the resin and recycled materials.

Subcooling at the end of the barrel is one of the main causes of screw slippage. The barrel of the injection machine is divided into three sections. At the end, that is, the feeding section, during the heating and compression process, a layer of melt film is formed on the screw. Without this film, the pellets are not easily transported to the front end.

The material of the feed section must be heated to the critical temperature of the light to form the melt film of that layer. However, usually the residence time of the material in the feed section is very short and the required temperature cannot be reached. This environment is usually produced on small injection machines. Too short a residence time can result in incomplete melting and inclusion of the polymer, causing the screw to slip or stall.


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