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Prevent the injection screw from slipping

Update:13 Jul 2019
Summary:

When the injection screw is difficult to produce the ma […]

When the injection screw is difficult to produce the material at the feeding port, or if the material does not form enough adhesive force along the length of the barrel to transport the material, the screw will slip. In the screw pre-plastic stage, 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 begins 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 undercooling of the second half of the barrel, wear of the barrel or screw, too shallow groove in the screw feeding section, improper design of the hopper, lack of material or blockage of the hopper, moisture in the resin, 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 layer. 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 let's look at two simple ways to handle this problem. 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 have marble markings, black spots or light streaks, it indicates 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. Increasing the back pressure setting also 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 and will not back off, which will do more for the melt. The work significantly increases 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, then screw and barrel wear is likely to be the key issue. As with the feed section, the resin adheres to the wall of the barrel as it melts in the compression section of the screw. When the screw is rotated, 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 clearance 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 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 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 anti-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 (abruptly narrowed at the bottom) containing a mutated compression zone. 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 affect the consistency of the feed. Poor pellet shape can cause reduced screw processing performance, yield fluctuations, and screw slip. The uniformly shaped pellets can be more closely packed 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. Increasing the temperature in the rear section of the barrel allows the material to start melting faster and the melt flow to obtain greater compressibility.
When processing absorbent 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 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.

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