The reason why the screw of the injection molding machine slips


The reason why the screw of the injection molding machi […]

The reason why the screw of the injection molding machine slips: the feeding port is difficult to press and the length of the barrel cannot form enough adhesive force, so the screw slips.

The following is a detailed analysis of the slip:

When the screw of the injection molding machine is difficult to produce the feeding material at the feeding port, or the adhesive force cannot be formed along the length of the barrel to convey the material, the screw will slip. In the pre-plasticizing stage of the screw, the screw rotates in the barrel to transport the material along the direction of the screw and retreats the accumulated material to prepare for the next injection, and the screw also slips. If the screw starts to slip during the pre-plastic phase, the axial movement of the screw will stop as the screw continues to rotate. Usually, the screw slip will cause the material to degrade before injection molding, and there will be problems such as short shots and prolonged processing cycle.

The reasons for the slippage of the screw include too high back pressure, overheating or overcooling of the second half of the barrel, wear of the barrel or screw, too shallow groove of the screw feeding section, improper design of the hopper, lack of material or blockage of the hopper, resin wetting, and resin. The lubricant content is too high, the material particle size is too fine, and the resin used is not well cut or recycled.

Effect of process parameters

The low temperature in the rear section of the barrel is usually the main cause of the slippage of the injection 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 compression process. 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. 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 can happen due to the small size of the equipment and the correspondingly small barrel and screw. Too short a residence time may cause the polymer to melt or mix insufficiently, which may cause the screw to slip or stop.

Now introduce two simple treatments for this topic. 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. The second method is to observe the shaped article. If it is found to be like marble markings, spots or light streaks, it means that the material is not well mixed in the barrel.

One of the solutions to the screw slip attempt is to gradually increase the temperature of the feed section until the screw is rotated and retracted. 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. The improved back pressure setpoint also improves the 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 and will not back off, which will do more for the melt. The work is significantly improved by the melt temperature, which adversely affects product quality and cycle time. The back pressure exerted on the melt can be adjusted by a control valve on the injection barrel.

Equipment impact

If the cause of the screw slip is due to processing equipment rather than process parameters, screw and barrel wear are likely to be the key issues. As with the feed section, the resin adheres to the barrel wall as it melts in the compression section of the screw. 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 the barrel, the screw cannot effectively transport the material forward. If you suspect that the equipment is worn, check the screw and barrel and check the fit between the two. If the fit clearance between the screw and the barrel exceeds the standard value, replacement or repair work should be initiated.

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 have the best compression ratio for recommended injection molding materials.

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

For the material, the check ring should be in the front (open) position and contact the retaining ring seat. If the check ring is in a rearward (closed) state, or between forward and backward, the molten polymer will have 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 reverse ring, you should replace it immediately.

The resin feed hopper is also responsible for the slippage of various injection screws. The correct hopper design is the key to ensuring stable material handling, but this is often overlooked. In general, new pellets of uniform size work well in a square hopper with a critical compression zone (the bottom is suddenly narrowed). However, this is not the case when adding recycled materials. The shape and size of the pulverized pellets are very inconsistent, which affects the uniformity of the feed. Inconsistent feed means that the screw does not maintain a uniform delivery pressure on the melt, which in turn causes slippage. To solve this problem and solve the problem of the difference between the size of the recycled material and the new pellet, try a circular hopper involving a gentle compression zone (the bottom is gently graded).

Material uniformity

As mentioned above, the shape and size of the material particles can affect the consistency of the feed. Poor pellet shape can cause reduced screw processing performance, yield fluctuations, and screw slip. Uniform pellets can be packed closer together in the screw feed section. The more the pellets in the screw are packed together, the more time the material melts in the screw and is transported forward. Poorly shaped pellets have a greater free volume (lower bulk density between pellets or more vacuum zones) and are difficult to feed, causing the screw to slip. Improving the temperature in the rear section of the barrel allows the material to start melting faster, and the melt flow obtains greater compressibility.

When processing absorbent materials such as nylon, wetting can also cause the screw to slip. Incorrect material drying can significantly reduce the viscosity of the material in the barrel and produce water vapor, making it difficult for the screw to transport the material forward. The humidity value of the pellets should be measured at the bottom of the drying hopper using a hygrometer and compared to the moisture content recommended by the material supplier.

There are two solutions for screwing the injection molding machine:

The first method: adding a small amount of material to the end of the barrel to start cleaning, while checking the melting temperature, short residence time will cause the melting temperature to be lower than the barrel temperature setting.

The second method: pay attention to the observation of molded products, if there are black spots or light streaks, marble markings, it means that the material is not well mixed in the barrel.