Injection Molding Machine Screw Noise and Slippage: What to Do?

When the injection molding screw encounters difficulty in feeding material at the feed port, or when it cannot generate sufficient adhesion along the length of the barrel to convey the material, the screw experiences slippage. During the screw plastication stage, while the screw rotates inside the barrel to convey material along the screw direction and retracts to accumulate material in preparation for the next injection, slippage can also occur.

If the screw begins to slip during the pre-plasticizing stage, the axial movement of the screw will stop while the screw continues to rotate. Screw slippage typically leads to material degradation before injection, as well as product quality issues such as short shots and prolonged processing cycles.

The causes of screw slippage include excessively high back pressure, overheating or overcooling in the rear half of the barrel, wear of the barrel or screw, too shallow screw threads in the feed section, improper hopper design, lack of material or blockage in the hopper, damp resin, excessive lubricant content in the resin, too fine material granularity, suboptimal resin cutting shape, or the use of recycled material.

The impact of process parameters

A low temperature in the rear section of the barrel is usually the main cause of injection molding screw slippage. The injection molding machine barrel is divided into three sections. In the rear part of the feeding section, a thin layer of molten polymer is formed during the heating and compression of the material. This molten film adheres to the barrel wall. Without this thin layer, it would be very difficult for the pellets to be conveyed forward.

In the feeding section, the material must be heated to a critical temperature to generate the essential molten film layer. However, often the material's residence time in the barrel is too short to allow the polymer to reach this temperature. This situation may occur due to the small scale of the equipment, with correspondingly smaller barrels and screws. Insufficient residence time can easily lead to inadequate melting or mixing of the polymer, which may cause the screw to slip or stop turning.

Here are two simple methods to address this issue. Start by adding a small amount of material from the end of the barrel for cleaning and check the melt temperature. If the residence time is short, the melt temperature will be lower than the set temperature of the barrel. The second method is to observe the molded products; if you find marble-like streaks, black spots, or light stripes, it indicates that the material has not been well blended in the barrel.

One of the attempted solutions for screw slippage is to gradually increase the temperature of the feed zone until the screw’s rotation and retraction become properly coordinated. Sometimes, it is necessary to raise the barrel temperature above the recommended set value to achieve this range.

Setting excessively high back pressure can also lead to screw stoppage or slipping. Increasing the back pressure setting increases the energy entering the material. If the back pressure is set too high, the screw may not generate enough forward melt conveying pressure to overcome the counter-pressure of the back pressure. Consequently, the screw will rotate without moving backward at a certain position, which will do more work on the melt and significantly increase the melt temperature, adversely affecting product quality and cycle time. The back pressure applied to the melt can be adjusted through the control valve on the injection barrel.

Impact of Equipment

If the screw slippage is caused by the processing equipment rather than the process parameters, then wear on the screw and barrel is likely the key issue. Similar to the feeding section, when the resin melts in the screw’s compression section, it adheres to the barrel wall. As the screw rotates, the material is sheared off the barrel wall and conveyed forward. If there are worn areas on the screw and barrel, the screw cannot effectively convey the material forward. If equipment wear is suspected, the screw and barrel should be inspected, and the clearance between them should be checked. If the clearance between the screw and barrel exceeds the standard value, replacement or repair should be initiated.

Screw design parameters, especially the compression ratio (the depth ratio of the feed section to the metering section), play a crucial role in the uniformity of plasticizing. If the feed section is too shallow (resulting in a smaller compression ratio), it will reduce output and may cause screw slippage due to insufficient feeding. Suppliers of various resins generally recommend the optimal compression ratio for injection molding materials.

A malfunction of the non-return ring (check valve) can also cause the screw to slip. When the screw is rotating and plasticizing material, the non-return ring should be in the forward (open) position, in contact with the fixed ring seat. If the non-return ring is in the backward (closed) position or somewhere between forward and backward, there will be resistance as the molten polymer passes through the gap between the non-return ring and the ring seat. If a problem with the non-return ring is suspected, it should be replaced immediately.

The resin feed hopper can also be the cause of various injection screw slipping issues. Proper hopper design is crucial to ensure stable material conveyance, but it is often overlooked. Generally, uniform-sized new pellets work well in a square hopper with a sudden compression zone (where the bottom suddenly narrows). However, this is not the case when recycled materials are added.

The shape and size of the re-crushed pellets are highly inconsistent, which can affect the uniformity of feeding. Inconsistent feeding means that the screw cannot maintain a consistent conveying pressure on the melt, leading to slippage. To solve this problem and address the size differences between recycled and virgin pellets, it is advisable to use a circular hopper with a gentle compression zone (with a bottom that has a gradual gradient).

Material uniformity

As mentioned above, the shape and size of material particles can affect the consistency of feeding. Poor particle shape can lead to decreased screw processing performance, output fluctuations, and screw slippage. Uniformly shaped pellets can be packed more tightly together in the screw feeding section. The more tightly the pellets are packed within the screw, the more time the material has to melt and be conveyed forward. Irregularly shaped pellets have greater free volume (lower bulk density or more void spaces between particles), making feeding more difficult and causing screw slippage. Increasing the temperature in the rear section of the barrel allows the material to begin melting more quickly, giving the melt greater compressibility.

When processing hygroscopic materials such as nylon, moisture can also cause screw slippage. Inadequate drying of the material will significantly reduce the viscosity of the material in the barrel and generate steam, making it difficult for the screw to convey the material forward. A hygrometer should be used at the bottom of the drying hopper to measure the moisture content of the pellets and compare the readings with the moisture levels recommended by the material supplier.