Why does the screw of the injection molding machine slip?

Update:27-02-2021
Summary:

For hygroscopic materials (such as nylon), the high wat […]

For hygroscopic materials (such as nylon), the high water content of the resin can also cause the screw to slip. Improper drying will be significantly lower

The viscosity of the material and the generation of water vapor in the barrel will reduce the conveying power of the screw. In fact, there are many reasons for 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 hoppers, 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 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. And this kind of environment is generally produced on a small injection machine. If the residence time is too short, the melting and inclusion process of the polymer will not be complete, which will cause the screw to slip or stall.

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