Extruder Screw Rebuilding
Many people don’t realize the importance of making sure that the screw flight diameter clearance has a significant effect on the performance of their screws. There are some companies that do pull their screws, measure the flight diameter, and record the information. By doing this regular maintenance routine will help production to predict when they may need their next shut down to have a screw repaired or the purchase of a new screw.
Typically, it is not recommended to rebuild any size screw more then 3 times. This is mainly because of the dilution of the base metal that occurs between the hard surface weld material and the 4140 H.T. or carbon steel base metal. Also, geometric dimensions start to deteriorate, such as flight widths, channel depths and channel radius. The geometry of the screw changes because every time that the screw is rebuilt it is either milled or flight ground to clean the over weld on the sides of the flights and the root is polished is remove possible root wear or scarring due to the fact that a foreign object had entered the screw.
If the root of the screw in the metering section is polished deeper then by deepening the screw channel can affect the pumping or melting capacity of the screw. Also, if the screw is rebuilt and the concentricity between the flight outside diameter and the root is not maintained, then the variation of the channel depth could cause the stability of the screw. This is more of an issue with small screws (2.5” and smaller) than larger screws.
Also if a screw is rebuilt too many times not only is the metallurgy of the base material degraded but as mentioned earlier, the narrowing of the flights can cause an even more increase in wear due to less flight land width to support the screw.
The best tool when it comes to screw maintenance is to do regular measuring of the flight OD and record the data along with the production rate as a means to predict a preventative maintenance shut-down.