Preventing screw slippage on injection molding machines

For hygroscopic materials (such as nylon), high resin moisture content can also cause screw slippage. Improper drying can significantly reduce the viscosity of the material and cause the phenomenon of air-packed material in the screw, thus reducing the conveying capacity of the screw. In this regard, a moisture meter can be used to determine the moisture content of the dried material before processing, so that the material is dried to the supplier's recommended value.

When the screw slips, the material may collect at the feed port and not be conveyed properly to the end of the injector. Screw slip occurs in the plasticizing section when the screw is rotating and retreating in the barrel to deliver material and prepare for the next injection. At this point, the screw rotation continues, but the axial movement of the screw stops, resulting in slippage. Screw slip often results in degradation of the material before injection, reduced product quality (e.g., missing material), and longer molding cycles.

The causes of screw slippage are many, probably related to too high back pressure, overheating or overcooling of the barrel end, barrel or screw wear, shallow threads in the charging section, blocked hopper, wet resin, over-lubrication of resin, too fine material or unfair cutting of resin and recycled material.

Process settings

Over-cooling of the barrel end is one of the main causes of screw slippage. The barrel of the injection machine is divided into 3 sections. In the end, i.e. the filling section, the granule is heated and compressed to form a molten film that sticks to the screw. Without this film, the pellets are not easily transported to the front end.

The material in the charging section must be heated to a critical temperature to form a molten film. However, usually, the residence time of the material in the charging section is too short to reach the required temperature. And this environment is usually created in small injection machines. Too short a residence time can result in incomplete melting of the polymer and the intercalation process, which can lead to screw slippage or stall.

There are two simple ways to determine if the screw is slipping. One method is to add a small amount of material to the end of the barrel to check the melt temperature. If the dwell time is too short, the melt temperature will be below the barrel temperature setpoint. The second method is to check the finished product: if there are mottling or bright and dark streaks on the finished product, it implies that the material is not evenly entrained in the barrel.

Once screw slippage has occurred, one solution is to advance the barrel temperature in the charging section until the screw rotates and retracts without hindrance. To achieve this, the barrel temperature will probably need to be increased beyond the recommended set point.

High backpressure can also cause screw stalling or slippage. Advancing the back pressure setting will increase the amount of energy applied to the material. However, if the backpressure setting is too high, the screw will not have sufficient pressure to subdue the back pressure and thus will not be able to convey the material to the front of the screw. At this point, when the screw is rotating in a certain position without normal retraction, it will do more work on the material, thus significantly advancing the melt temperature, resulting in lower finished product quality and longer molding cycles. The backpressure of the melt can be controlled by regulating the valve of the injection cylinder.
 

Hardware problems

If screw slippage is caused by the equipment rather than the process setup, screw and barrel wear is probably the culprit. The resin melts in the transition section and sticks to the barrel wall, just as it does in the charging section.

As the screw rotates, it shears the melt-off and transports it to the front end. If the screw and barrel are worn, the screw will have difficulty conveying the material to the front end effectively. If you are not sure if you are suffering from wear, you can measure the width of the crack between the screw and the barrel, and once it is not suitable for the defined public service, it should be replaced or patched.

The screw plan, especially the compression ratio plan, plays an important role in plasticization. Too short a charge section, i.e., too small a compression ratio, will result in low delivery and screw slippage. Resin suppliers will recommend the best compression ratio for their materials.

The cause of screw slippage may also be the check valve (check valve) is not working properly. When the screw is rotating in preparation for injection, the check ring should be at the front end (open position) and connected to the tabs of the check ring holder. If the retaining ring is at the end (i.e., the closed position), presumably in the middle of the tail and retaining ring holder, it will be difficult for the polymer melt to pass through this crack. If the retaining ring is found to be defective, it should be replaced in real-time.

In addition, the resin feed hopper is probably one of the factors causing screw slippage. Accurate planning of the hopper is the key to even dosing, but this is often overlooked. A square hopper with a fast compression section (i.e., steep tightening at the bottom) is more suitable for processing evenly spaced raw material, but not really for processing recycled material.

The wide particle size dispersion of the recycled material affects the evenness of the feed, which means that the screw cannot deliver the melt evenly at the same pressure, which can eventually lead to slippage. To deal with this problem, use a round hopper with a tapering compression section (i.e., tapered at the bottom) to dispose of materials with a wide particle size dispersion.
 

Material uniformity

As mentioned above, the length, particle size, and shape of the material affect the uniformity of the feed - poor particle shape can also cause screw slippage, which can result in yield variations. Uniformly sized pellets accumulate tightly in the charging section, making them easy to melt and transport.

Different shaped pellets can cause excessive free volume (too much open space between pellets), making it difficult to convey and thus causing screw slip. In this regard, you can improve the barrel end temperature, so that the melting process starts earlier, but also to make the material compressed more closely to start!