What should I do if the screw of the injection molding machine slips?

Update:12 Mar 2021

When the injection screw is difficult to press at the feeding port, or the length of the barrel cannot form enough adhesive force to convey the material, the screw will slip. In the screw pre-molding stage, the screw rotates in the barrel to convey materials along the direction of the screw and retreat to accumulate materials for the next injection. The screw will also slip. If the screw starts to slip during the pre-plastic stage, the axial movement of the screw will stop when the screw continues to rotate. Usually, the screw slippage will cause the material to degrade before injection, and there will also be product quality problems such as short shots and prolonged processing cycles.

Therefore, the screw technicians summarized the reasons for the screw slippage, including: too high back pressure, overheating or overcooling in the back half of the barrel, wear of the barrel or screw, too shallow screw groove in the screw feeding section, improper hopper design, and lack of hopper. Material is clogged, the resin is wet, the lubricant content in the resin is too high, the particle size of the material is too fine, the resin used has a poor cutting shape or is recycled material.

1. The influence of process parameters

Low temperature in the back section of the barrel is usually the main reason for the slip 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 feeding section must be heated to a critical temperature to produce a critical molten film layer. However, it is often that the residence time of the material in the barrel is too short to allow the polymer to reach this temperature. This situation may be caused by the small scale of the equipment and the correspondingly configured smaller barrel and screw. Too short residence time can easily cause the polymer to melt or mix insufficiently, which can cause the screw to slip or stop.

Screw technicians proposed two simple solutions to this problem: First, add a small amount of material from the end of the barrel to clean, and check the melting temperature. If the residence time is short, the melting temperature will be lower than the set value of the barrel temperature. Second, observe the molded product. If you find marble patterns, black spots or light streaks, it means that the material is not well mixed in the barrel.

In addition, our company analyzes another solution for screw slippage according to the customer's usage, and gradually increases the temperature of the feeding section until the screw rotation and retreat are in harmony. Sometimes the barrel temperature has to be raised above the recommended setting to reach this range. Setting a too high back pressure can also cause the screw to stall or slip. Increasing the back pressure setting also increases 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 of the back pressure, and the screw will rotate at a certain position and will not retreat, which will do more to the melt. The work and significantly increase the melt temperature, thereby adversely affecting product quality and cycle time. The back pressure exerted on the melt can be adjusted by the control valve on the injection barrel.

Second, the impact of equipment

If the screw slip is caused by processing equipment rather than process parameters, then screw and barrel wear is likely to be the key to the problem.

Like the feeding section, when the resin melts in the screw compression section, it adheres to the barrel wall. When the screw rotates, the material leaves the barrel wall and is conveyed forward after being subjected to shearing force. If there are wear areas on the screw and barrel, the screw cannot effectively transport the material forward. If it is suspected that the equipment is worn, the screw and barrel should be inspected and the clearance between the two should be checked. If the fit clearance between the screw and barrel exceeds the standard value, replacement or repair work should be started.

Screw technicians found that the screw design parameters, especially the compression ratio (the depth of the feeding section than the depth of the homogenization section) play a crucial role in the uniformity of plasticization. The feeding section is too shallow (to get a smaller compression ratio) will reduce the output and will cause the screw to slip due to insufficient feeding. Suppliers of various resins generally recommend the best 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 materials, the non-return ring should be in the front (open) position, touching the fixed ring seat. If the non-return ring is in a backward (closed) state, or a state between forward and backward, the molten polymer will have resistance when passing through the gap between the non-return ring and the ring seat. If you suspect that there is a problem with the non-return ring, replace it immediately.

The resin feed hopper is also the cause of the slippage of many injection molding screws. The correct hopper design is the key to ensure stable conveying of materials, but this is often overlooked. Generally speaking, new pellets of uniform size work well in a square hopper (with a sudden narrowing of the bottom) with a sudden compression zone. However, this is not the case when adding recycled materials. After being crushed again, the shape and size of the pellets are very inconsistent, which will affect the uniformity of the feed. Incoherent feed means that the screw cannot maintain a uniform delivery pressure on the melt, which results in slippage. To solve this problem and solve the problem of the size difference between the recycled material and the new pellets, try to use a circular hopper that involves a gentle compression zone (with a gentle gradient at the bottom).

3. Material uniformity

As mentioned above, the shape and size of the material particles will affect the consistency of the feed. Poor shape of pellets will cause reduction in screw processing performance, output fluctuations and screw slippage. The uniformly shaped pellets can be packed more closely together in the screw feeding section. The more closely the pellets in the screw are packed together, the more time the material will melt in the screw and be transported forward. Poorly shaped pellets will have more free volume (lower bulk density or more vacuum zone between pellets), and difficult to feed, causing the screw to slip. Increasing the temperature in the back section of the barrel can make the material start to melt faster and the melt flow can obtain greater compressibility.

When processing hygroscopic materials such as nylon, moisture can also cause the screw to slip. Incorrect drying of the material will 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. A hygrometer should be used at the bottom of the drying hopper to measure the moisture value of the pellets and compare the reading with the moisture content recommended by the material supplier.More about:screw barrel for blowing film extruder machine