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There are many reasons for the screw slip, such as the wear of the screw and the barrel is likely to cause the screw to slip. The compression ratio of the screw of the injection molding machine is designed. If the feeding section is too short, that is, the compression ratio is too small, the conveying amount is reduced and the screw is slipped. When the check valve (check valve) is not working properly, it may cause the screw to slip. In addition, the screw slip may be caused by the resin feed hopper.
Uniform feeding of the hopper is often overlooked in injection molding. A square hopper equipped with a fast compression section (ie, abruptly tightened at the bottom) is suitable for processing uniform raw materials, but it is not suitable for processing recycled materials with a wide particle size distribution.
This will affect the uniformity of the feed, which means that the screw cannot deliver the melt evenly under the same pressure and will also cause slippage!
Effect of process parameters
The low temperature in the rear section of the barrel is usually the cause of the slip of the injection screw. The barrel of the injection molding machine is divided into three sections. At the back of the feeding section, the material forms a layer of melting during the heating and compression process.
A thin layer of polymer. The molten film layer is attached to the barrel. Without this thin layer, the pellets are difficult to transport forward.
The material in the feed section must be heated to a critical temperature to produce a critical molten film.
The material in the feed section must be heated to a critical temperature to produce a critical molten film. However, it is often the material that stays in the barrel for too short a time.
The polymer is brought to this temperature.
This can happen due to the small size of the equipment and the correspondingly small barrel and screw.
If the residence time is too short, the polymer may melt or mix insufficiently, which may cause the screw to slip or stop.
1. Add a small amount of material from the end of the barrel for cleaning and check the melting temperature. If the residence time is short, the melting temperature will be lower than the barrel temperature setting.
2. Observe the molded product. If it is found to have marble markings, black spots or light streaks, it means that the material is not well mixed in the barrel.
3. Gradually increase the temperature of the feed section until the screw rotates and retracts to the same level. Sometimes it is necessary to raise the barrel temperature above the recommended setting to reach this range.
Setting too high a back pressure can also cause the screw to stop or slip.
Increasing the back pressure setting increases the amount of energy entering the material. If the back pressure setting is too high, the screw may not produce enough forward melt pressure to overcome the back pressure back pressure,
The screw will rotate at a certain position without retreating, which will do more work on the melt and significantly increase the melt temperature.
This has an adverse effect on the quality of the product and the cycle time. The back pressure applied to the melt can be adjusted by a control valve on the injection barrel
If the reason for the screw slip of the injection molding machine is due to the processing equipment and not the process parameters, it is the possibility of wear of the screw and the barrel. Like the feeding section, the resin adheres to the wall of the barrel when the screw is compressed.
When the screw rotates, the material is subjected to shear force and then leaves the wall of the barrel and is transported forward. If there is a wear zone on the screw and barrel, the screw will not be able to effectively transport the material forward.
If you suspect that the equipment is worn, check the screw and barrel and check the fit clearance between the two. If the fit clearance between the screw and the barrel exceeds the standard value, it should be replaced or repaired.
The screw design parameters, especially the compression ratio (feed section depth is greater than the homogenization section depth), play a crucial role in plasticizing uniformity. Too shallow a feed section (getting a smaller compression ratio) will reduce the yield and cause the screw to slip due to insufficient feed.
Suppliers of various resins generally recommend the optimum compression ratio for injection molding materials.
Failure of the check ring (check valve) can also cause the screw to slip.
When the screw is rotating and plasticizing the material, the check ring should be in the front (open) position to contact the retaining ring seat. If the check ring is in a backward (closed) state, or between forward and backward,
The molten polymer has resistance as it passes through the gap between the check ring and the ring seat.
If you suspect that there is a problem with the anti-reverse ring, you should replace it immediately.