Analysis of Screw Slipping
Category: Industry news
Date: 2018-06-04
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When the screw of the injection molding machine is difficult to produce feeding material at the feeding port, or the sufficient adhesive force cannot be formed along the length of the barrel to convey...

When the screw of the injection molding machine is difficult to produce feeding material at the feeding port, or the sufficient adhesive force cannot be formed along the length of the barrel to convey the material, then the screw of the injection molding machine is slippery. In the screw plastic molding stage of the injection molding machine, the screw of the injection molding machine rotates in the barrel to convey the material along the direction of the screw and retreat accumulates the material for the next injection molding. The screw of the injection molding machine also slips. If the screw of the injection molding machine starts to slip during the pre-molding phase, the axial movement of the screw will stop when the screw continues to rotate. In general, screw slippage will lead to degradation of the material before injection molding, as well as product quality problems such as short shots and prolonged processing cycles.

The reasons for slippage of the injection molding machine screw include high back pressure, overheating or undercooling in the second half of the barrel, wear of the barrel or screw, shallow screw groove in the screw feed section, improper hopper design, missing or blocked hopper, damp resin, The content of lubricant in the resin is too high, the particle size of the material is too fine, and the used resin cuts poorly in shape or is a reclaimed material.

1, the influence of process parameters

The low temperature in the rear section of the barrel is usually the main cause of screw slippage in the injection molding machine. 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 heating and compression. The molten film is attached to the barrel. Without this thin layer, granules are difficult to transport forward.

The material in the feed section must be heated to a critical temperature to produce a critical melt film. However, it is often the case that the residence time of the material in the barrel is too short to allow the polymer to reach this temperature. This may be due to the small size of the equipment and the correspondingly smaller barrels and screws. Too short a residence time may cause the polymer to melt or mix insufficiently, which may cause the screw to slip or stall.

Now introduce two simple ways to deal with this problem. Add a small amount of material from the end of the barrel to clean it, and check the melting temperature. If the residence time is short, the melting temperature will be lower than the barrel temperature setting. The second method is to observe molded articles. If there are marble markings, black spots or light stripes, it means that the materials are not well mixed in the barrel.

One of the solutions to the occurrence of screw slipping attempts is to gradually increase the temperature of the feed section until the screw rotates and retreats in a coordinated manner. Sometimes the barrel temperature must be raised above the recommended setting to reach this range.

Setting excessive back pressure can also cause the screw to stop or slip. Increasing the back pressure setpoint also increases the energy that goes into the material. If the back pressure is set too high, then the screw may not be able to produce enough forward delivery melt pressure to overcome the back pressure back pressure, the screw will rotate in a certain position will not back, this will do more to the melt The work also significantly increases the melt temperature, which adversely affects the product quality and cycle time. The back pressure exerted on the melt can be adjusted by the control valve on the injection cylinder.

2, the impact of the equipment

If the cause of screw slip in injection molding machines is due to processing equipment rather than process parameters, then screw and barrel wear are likely to be critical. As with the feed section, the resin adheres to the barrel wall as the resin melts in the screw compression section. When the screw is rotated, the material is sheared off and leaves the barrel wall and is conveyed forward. If there is a wear zone on the screw and barrel, the screw cannot effectively feed the material forward. If you suspect that there is wear on the equipment, check the screw and barrel and check the fit clearance between the two. If the clearance between the screw and the barrel exceeds the standard value, replacement or repair work should be started.

Injection molding machine screw design parameters, especially the compression ratio (feeding section depth than the homogenization section depth) play a crucial role in plasticization uniformity. The shallow addition of the feed section (which results in a smaller compression ratio) will reduce the yield and cause the screw to slip due to insufficient feed. Suppliers of various resins generally have the best compression ratio recommended for injection molding materials.

3. Failure of the check ring (check valve) can also cause the screw to slip. When the screw is spinning and plasticizing

In the case of material, the check ring should be in the front (open) position and contact the retaining ring seat. If the check ring is in a backward (closed) state, or in a state between forward and backward, the molten polymer will have resistance when passing through the gap between the check ring and the ring seat. If you suspect there is a problem with the loop, you should replace it immediately.

The resin hopper is also responsible for the slippage of many injection screws. The correct hopper design is the key to ensure the stable delivery of materials, but this is often overlooked. In general, new pellets of uniform size work well in a square hopper containing a sudden compression zone (suddenly narrowing at the bottom). However, this is not the case when adding recycled material. The shape and size of the crushed pellets are inconsistent again, which affects the uniformity of the feed. Incoherent feeding means that the screw rod of the injection molding machine cannot maintain the uniform delivery pressure on the melt, resulting in slippage. To solve this problem and solve the problem of the difference between the size of the regrind and the new pellets, try using a circular hopper that involves a gentle compression zone with a gentle gradient at the bottom.

4, material uniformity

As mentioned above, the shape and size of the material particles will affect the consistency of the feed. Poor shape of the pellets can cause screw processing performance, output fluctuations, and screw slippage in the injection molding machine. The uniformly shaped pellets can be packed closer together in the screw feed section. The more closely the pellets in the screw are packed together, the more time the material will melt in the screw and transport it forward. Poorly shaped granules will have a greater free volume (lower bulk density between granules or more vacuum area), and feeding is difficult, causing the screw to slip. Increasing the temperature at the back of the barrel allows the material to start melting faster, and the melt flow is more compressible.

When processing hygroscopic materials such as nylon, moisture can also cause the screw to slip. Incorrect material drying can significantly reduce the viscosity of the material in the barrel and generate water vapor, making it difficult for the screw of the injection molding machine to feed the material forward. Hygrometers should be used at the bottom of the drying hopper to measure the moisture content of the pellets and compare them with the moisture content recommended by the material supplier.


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