For hygroscopic materials (such as nylon), high resin moisture content can also cause screw slip. Improper drying will significantly reduce viscosity of material and cause phenomenon of air pockets in screw, thus reducing conveying ability of screw. In this regard, a moisture analyzer can be used to determine moisture content of dried material prior to processing.
When screw slips, material probably gathers at feeding port and cannot be conveyed to end of injection machine normally. When screw rotates and retreats in barrel to transport material and prepare for next injection, screw slip occurs in plasticizing section. At this time, rotation of screw is still inherited, but axial movement of screw will stop, that is, slippage will occur. Screw slippage often causes material degradation before injection, resulting in reduced product quality (such as material shortages) and longer molding cycles.
There are many reasons for screw slippage, which may be related to factors such as too high back pressure, too hot or too cold at the end of barrel, worn barrel or screw, too shallow thread in feeding section, blocked hopper, wet resin, overly lubricated resin, too fine material or unfair cutting of resin and recycled material.

 

Overcooling at the end of barrel is one of main causes of screw slippage. Barrel of injection machine is divided into three sections. At the end, that is, feeding section, pellets will form a layer of melt film and stick to screw during process of heating and compression. Without this film, pellets are not easily transported to front end.
Material in feed section must be heated to a critical temperature to form a melt film. However, usually residence time of material in feeding section is too short to reach required temperature. And this kind of environment is generally produced on small injection machines. A residence time that is too short will result in incomplete melting and inclusion of polymer, resulting in screw slippage or stall.
There are 2 simple and simple ways to determine if screw is slipping. One way is to add a small amount of material to the end of barrel to check melt temperature. If dwell time is too short, melt temperature will be below barrel temperature set point. Second method is to check finished product: if there are stripes or light and dark stripes on finished product, it means that material is not evenly mixed in barrel.
Once screw slip occurs, one solution is to increase temperature of barrel in feeding section until rotation and retraction of screw are unobstructed. To achieve this, barrel temperature may have to be increased beyond recommended set point.
High back pressure can also cause screw to stagnate 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 end of screw. At this time, when screw rotates at a certain position without normal retraction, it will do more work on material, thereby significantly increasing melt temperature, resulting in a decrease in quality of finished product and a prolongation of molding cycle. Back pressure of melt can be controlled by adjusting valve of injection cylinder.

If screw slip is due to equipment rather than process setup, 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 melt down and delivers it to front end. If screw and barrel are worn, it is difficult for screw to effectively convey material to front end. If you are not sure whether it has been worn, you can measure width of crack between screw and barrel. Once it is not suitable for designated public service, it should be replaced or repaired.
Screw design, especially compression ratio design, plays an important role in plasticizing. If feeding section is too short, that is, compression ratio is too small, conveying capacity will decrease and screw will slip. Resin suppliers recommend the best compression ratio for their material.

 

Reason for screw slip is probably that check valve (one-way valve) is not working properly. When screw rotates to prepare for injection, retaining ring should be at the front end (open position), and it should be in contact with convex buckle of retaining ring frame. If retaining ring is at the end (i.e. closed position), roughly halfway between tail and retaining ring frame, it will be difficult for polymer melt to pass through this crack. 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 crux of an even feed, but it is often overlooked. A square hopper with a fast compression section (i.e. abruptly tightened at the bottom) is more suitable for processing even raw meal, but is not really suitable for processing recycled material.
Since particle size distribution of recycled material is wider, it will affect uniformity of feed, which means that screw cannot deliver melt evenly at same pressure, which will eventually lead to slippage. To solve this problem, a circular hopper with a gradual compression section (that is, gradually compressing in a cone at the bottom) is used to handle materials with a wide particle size distribution.

As mentioned above, length, particle size and shape of material affects uniformity of feed - poor pellet shape can also cause screw to slip, which can lead to changes in output. Granules of average size will be tightly packed in feeding section, which is easy to melt and transport.
Different shapes of pellets will cause free volume to be too large (excessive space between pellets), making it difficult to transport, causing screw to slip. In this regard, temperature at the end of barrel can be increased, so that melting process starts earlier, and material is compressed more tightly!