How to Prevent the Dual-alloy Screw from Slipping?

When the bi-alloy extruder screw is difficult to press at the feeding port, or the adhesive force along the length of the extruder barrel cannot be formed to convey the material, the screw of the bi-alloy extruder will slip.


In the pre-plastication stage of the extruder screw design, the bi-alloy extruder screw rotates in the barrel, conveys the material along the direction of the bi-alloy extruder screw, and retreats the piled material. When preparing for the next injection, the screw of the bi-alloy extruder will also slip.


Assuming that the bi-alloy extruder screw initially slips during the pre-plastication stage, the axial movement of the screw of the bi-alloy extruder will stop when the screw of the bi-alloy extruder continues to rotate.


Generally, the bi-alloy extruder screw slips, which will cause the material to degrade before injection, and there will also be product quality problems such as short shots and prolonged processing cycles.


There are some reasons for the slippage of the bi-alloy extruder screw which includes too high back pressure, overheating or overcooling in the second half of the extruder barrel, wear of the barrel or double alloy screw, too shallow groove in the feeding section of the double alloy screw, improper hopper design, lack of material or blockage in the hopper, and resin moisture, too high lubricant content in the resin, too fine material particle size, poor cut shape of resin used or recycled material.


Ⅰ. The low temperature in the back section of the extruder barrel is the main reason for the slippage of the dual-alloy screw.


The barrel of the injection molding machine is divided into three sections. At the rear of the feeding section, the material forms a thin layer of molten polymer during the heating and shrinking process. The molten film layer is attached to the extruder barrel. Assuming that there is no such thin layer, the pellets are difficult to transport forward.


The material in the feeding section must be heated to a critical temperature to produce a critical molten film layer. But often the residence time of the material in the extruder barrel is too short to allow the polymer to reach this temperature. This situation may occur because the equipment plan is too small, correspondingly equipped with smaller barrels and bi-alloy screws. If the residence time is too short, it is easy to cause the polymer to melt or mix insufficiently, which will cause the dual-alloy screw to slip or stop.


Now introduce two simple solutions to this problem. From the end of the extruder barrel, initially add a small amount of material for cleaning, and check the melting temperature. Assuming that the residence time is short, the melting temperature will be lower than the set value of the barrel temperature. The second method is to check the molded product. If it is found that there are marble patterns, black spots or light streaks, it means that the material is not well mixed in the barrel.


One of the solutions to the dual-alloy extruder screw slip test is to gradually increase the temperature of the feeding section until the dual-alloy screw rotates and retreats in harmony. Sometimes the barrel temperature has to be raised above the recommended setting value to reach this range.


Ⅱ. The high back pressure will also cause the screw of the dual-alloy extruder to stop or slip


Increasing the back pressure setting also increases the energy entering the material. Assuming that the back pressure is set too high, the bi-alloy screw may not be able to deliver enough melt pressure forward to resist the back pressure of the back pressure. The screw of the bi-alloy extruder will rotate in a certain position and will not retreat, which will do more work to the melt and significantly increase the melt temperature, which will adversely affect the quality of the product and the cycle period. The back pressure exerted on the melt can be regulated by the control valve on the injection barrel.

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