Injection molding machine screw barrel process settings

One of the main reasons for screw slippage is the overcooling of the barrel end. 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 process of heating and compression. Without this film, the pellets cannot be easily 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, usually the residence time of the material in the feeding section is too short to reach the required temperature. And this usually happens on small injection machines. A residence time that is too short will result in incomplete melting and mixing of the polymer, resulting in screw slippage or stall.

There are 2 easy ways to determine if the screw is slipping. One method is to add a small amount of material to the end of the barrel to monitor the melt temperature. If the dwell time is too short, the melt temperature will be below the barrel temperature set point. The second method is to check the product: if there are patterns or light and dark stripes on the product, it means that the material is not mixed evenly in the barrel.

Once screw slip occurs, one solution is to increase the temperature of the feeding section barrel until the screw rotates and retracts without hindrance. To achieve this, the barrel temperature may need to be raised above the recommended set point.

High back pressure can also cause the screw to stagnate or slip. Increasing the back pressure setting will increase the energy applied to the material. But if the back pressure is set too high, the screw will not have enough pressure to overcome the back pressure and will not be able to deliver the material to the front. At this time, when the screw rotates at a certain position without normal retraction, it will do more work on the material, thereby significantly increasing the melt temperature, resulting in a decline in product quality and a prolonged molding cycle. The back pressure of the melt can be controlled by adjusting the valve of the injection cylinder.