For hygroscopic materials (such as nylon), high water content of resin will also cause screw to slip. Improper drying will significantly reduce viscosity of material and cause air bagging in screw, thus reducing conveying capacity of screw. In this regard, a humidity analyzer can be used to determine moisture content of dried material before processing.

 

When screw slips, material may gather at feeding port and cannot be transported to the end of injection machine normally. When screw rotates and retreats in barrel to convey material and prepare for next injection, screw slippage will occur in plasticizing section. At this time, rotation of screw is still continuing, but axial movement of screw will stop, that is, slippage will occur. Screw slippage often leads to material degradation before injection, product quality will be reduced (such as lack of material), and molding cycle will be prolonged.
There are many reasons for screw slippage, probably related to too high back pressure, overheating or overcooling at the end of barrel, wear of barrel or screw, too shallow thread in feeding section, blocked hopper, wet resin, excessive lubrication of resin, too fine material, or unreasonable cutting of resin and recycled materials.

Overcooling at the end of barrel is one of main causes of screw slippage. Barrel of injection machine is divided into 3 sections: at the end, that is, feeding section, pellets will form a melt film and stick to screw during heating and compression process. Without this layer of film, pellets are not easily conveyed to front end.
Material in feeding section must be heated to a critical temperature to form a melt film. However, usually residence time of material in feeding section is very short, and required temperature cannot be reached. And this kind of environment is generally produced on a small injection machine. Too short a residence time will result in incomplete melting and inclusion of polymer, resulting in screw slippage or stall.
There are two simple and simple ways to determine whether screw is slipping. One way is to add a small amount of material to the end of barrel to detect melt temperature. If residence time is too short, melt temperature will drop below barrel temperature set point. Second method is to check finished product: if there are spots or light and dark stripes on finished product, it means that material is not evenly mixed in barrel.
Once screw slippage occurs, a solution is to increase temperature of barrel in feeding section until there is no obstacle to rotation and retraction of screw. To achieve this, barrel temperature may need to be raised above recommended set point.
High back pressure can also cause screw to stall or slip. Increasing back pressure setting will increase energy applied to material. However, if back pressure is set too high, screw will not have enough pressure to overcome back pressure, so that material cannot be conveyed to front of screw. At this time, when screw rotates at a certain position without retracting normally, it will do more work on material, thereby significantly increasing temperature of melt, resulting in a decrease in quality of finished product and an extension of molding cycle. Back pressure of melt can be controlled by adjusting valve of injection cylinder.

If screw slippage is due to equipment rather than process settings, screw and barrel wear is likely to be culprit. Resin melts in transition section and sticks to barrel wall, just like feed section. As screw rotates, it shears off melt and delivers it to front end. If screw and barrel are worn, it becomes difficult for screw to efficiently move material to front end. If you are not sure whether it has been worn, you can measure gap width between screw and barrel. Once it does not meet specified tolerance, it should be replaced or repaired.
Screw design, especially compression ratio design, plays an important role in plasticization. If feeding section is too short, that is, compression ratio is too small, it will result in reduced delivery and screw slippage. Resin suppliers will recommend best compression ratio for their material.
Cause of screw slippage is probably that check valve (one-way valve) is not working properly. When screw is rotating for injection, retaining ring should be at the front end (open position) and connected with convex buckle of retaining ring holder. If retaining ring is at the end (i.e. in closed position), roughly halfway between tail and retaining ring holder, it will be difficult for polymer melt to pass through this gap. If there is a problem with retaining ring, it should be replaced in time.
In addition, resin feed hopper may also be one of factors that cause screw to slip. Precise planning of hopper is the key to even feeding, but this is often overlooked by people. A square hopper with a fast compression section (i.e. a sharp tightening at the bottom) is more suitable for processing even raw materials, but is not suitable for processing recycled materials. Due to wide particle size distribution of regrind, it will affect uniformity of feed, which means that screw cannot convey melt evenly under same pressure, which will eventually lead to slippage. To solve this problem, a circular hopper with a gradual compression section (that is, a gradual conical compression at the bottom) is used to handle materials with a wide particle size distribution.

As mentioned above, length, particle size and shape of material will affect uniformity of feed - poor particle shape can also cause screw slippage, resulting in production changes. Pellets with uniform size will be closely accumulated in feeding section, which is easy to melt and transport.
Pellets of different shapes can cause excessive free volume (excessive space between pellets) to be conveyed, resulting in screw slippage. In this regard, temperature at the end of barrel can be increased, so that melting process starts earlier and material is compressed more tightly!