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Zhoushan Hengguan Machinery Manufacturing Co., Ltd

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04-07, 2021

A brief introduction to the method of preventing injection screw from slipping

When the injection screw is difficult to press at the feeding port, or the enough adhesion force can not be formed along the length of the barrel to transport the material, the screw will slip. In the pre molding stage, the screw rotates in the barrel to transport materials along the direction of the screw and back to accumulate the material to prepare for the next injection molding, the screw will also slip. If the screw starts to slide in the pre plastic stage, the axial movement of the screw will stop when the screw continues to rotate. Generally, screw slip will lead to degradation of materials before injection molding, and there will be short shot, prolonging the cycle cycle and other product quality problems.
The reasons for the screw slipping include too high back pressure, overheating or supercooling in the second half of the barrel, wear of barrel or screw, too shallow screw groove in screw feeding section, improper hopper design, hopper missing or blocking, resin humidity, too high lubricant content in resin, too fine material size, poor cutting shape of resin or recycled material.
Influence of process parameters
The low temperature in the back section of the barrel is usually the main reason for the slipping of injection screw. The injection molding machine barrel is divided into three sections. At the back of the feeding section, a thin layer of molten polymer is formed during the heating and compression process. The melting film is attached to the cylinder. Without this thin layer, it is difficult for the granular material to be transported forward.
The material in the feeding section must be heated to the critical temperature to produce a critical melting film. However, it is often that the material is too short to reach the temperature. This situation may be caused by the small equipment size and the corresponding configuration of smaller barrel and screw. The short residence time will lead to the polymer melting or insufficient mixing, which will lead to the screw slipping or stopping.
Now, two simple methods to deal with this problem are introduced. Add a small amount of material to clean from the end of the barrel and check the melting temperature. If the dwell time is short, the melting temperature will be lower than the set value of the barrel temperature. The second method is to observe the molding products. If it is found that there are marble spots, black spots or light stripes, it indicates that the material has not been mixed well in the barrel.
One of the solutions to the problem is to gradually raise the temperature of the feeding section until the screw rotates and retreats to a consistent level. Sometimes the temperature of the barrel has to be raised above the recommended setting value to reach this range.
Setting too high back pressure can also cause the screw to stop rotating or slipping. The increase of the back pressure setting also improves the energy entering the material. If the back pressure is set too high, the screw may not produce enough forward melt pressure to overcome the back pressure, and the screw will rotate in a certain position and will not retreat, which will do more work on the melt and significantly improve the melt temperature, which will have adverse effects on the product quality and cycle cycle. The back pressure applied to the melt can be adjusted by the control valve on the syringe cylinder.
Impact of equipment
If the reason of screw slip is caused by processing equipment rather than process parameters, the wear of screw and barrel is likely to be the key problem. As in the feeding section, the resin is adhered to the cylinder wall when the resin melts in the screw compression section. When the screw is rotated, the material is sheared and then it leaves the cylinder wall and is transported to the front. If there is wear area on the screw and barrel, the screw can not effectively forward the material. If there is any doubt that the equipment is worn, check the screw and barrel and check the fit clearance between the two. If the fit clearance between screw and barrel exceeds the standard value, it is necessary to start to replace or repair.
The design parameters of screw, especially the compression ratio (depth of feed section to homogenization section), play an important role in the plasticization uniformity. Too shallow feeding section (getting a smaller compression ratio) will reduce the output and cause screw slip due to insufficient feeding. The suppliers of various resins generally have the best compression ratio of injection materials recommended.
Failure of the check ring (single-way valve) can also lead to screw slipping. When the screw is rotating and plasticizing
When materials are used, the check ring should be in the front (open) position, contacting the fixed ring seat. If the check ring is in a backward (closed) state or between the forward and the backward, the molten polymer will have resistance when passing through the gap between the check ring and the ring seat. If you suspect a problem with the check ring, replace it immediately.
Resin feed hopper will also be the cause of sliding of various injection screw. The correct hopper design is the key to ensure the stable delivery of materials, but this is often ignored. Generally speaking, new particles of uniform size operate well in square hopper with sudden narrowing of bottom containing sudden compression zone. However, this is not the case when recycled materials are added. The shape and size of the crushed particles are very different after re grinding, which will affect the uniformity of the feed. Incoherence of feed means that screw can not maintain uniform conveying pressure on melt, and then slip occurs. To solve this problem and solve the difference between the size of recycled material and new aggregate, we can try to use a circular hopper with a gentle compression zone (with a gentle gradient at the bottom).
Material uniformity
As mentioned above, the shape and size of the material particles will affect the consistency of the feed. The poor shape of granular material will cause the decline of screw processing performance, fluctuation of output and screw slipping. The granular materials with uniform shape can be piled together more closely in the screw feeding section. The more granular materials are packed together, the more time the material is melted and transported forward in the screw. The particles with poor shape will have larger free volume (lower volume density between particles or more vacuum area), and the feeding is difficult, which causes the screw to slip. The higher the temperature of the back section of the barrel can make the material melt faster and the compression of the melt flow is greater.
When processing hygroscopic materials such as nylon, humidity can also lead to screw slipping. The improper drying of materials will obviously reduce the material viscosity and produce water vapor in the barrel, which makes it difficult for screw to transport materials forward. The moisture value of the particles shall be measured by a hygrometer at the bottom of the drying hopper and compared with the humidity content recommended by the material supplier.

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