16. May 2012 10:41
Many processors don’t realize that the screw flight diameter clearance has a significant impact on the performance of their screws. You need to periodically pull your screw, measure the flight diameter, and record the information. Engaging in this regular maintenance routine will help production personnel predict when they may need their next shutdown so that they have can their screw repaired to buy an altogether new one.
Typically, it is not recommended to rebuild any size screw more than three 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. Then the root is polished to remove possible root wear or scarring if 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 the pumping or melting capacity of the screw can be impacted . 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 in. and smaller) than larger ones.
Also, if a screw is rebuilt too many times not only is the metallurgy of the base material degraded but the narrowing of the flights can cause an even more increase 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, record the data along with the production rate as a means to predict a preventative maintenance shut-down.
Tim Womer is a recognized authority in plastics processing and machinery with a career spanning more than 35 years. He has designed thousands of screws for all types of single-screw plasticating. He now runs his own consulting company, TWWomer & Associates LLC. Contact: (724) 355-3311; firstname.lastname@example.org; twwomer.com.