A Tale of Screw Wear
7. February 2012 15:56
Recently a processor installed a new screw into their 6-in., 32:1 L/D extruder. Within a few weeks the hard facing that had been welded on the flight OD started to pop off. The flight failure was in an isolated area, so it was then assumed it was due to a poor weld bond. The screw was ultrasonically inspected and the remaining flights showed good bore, but the entire screw had to be rebuilt.
Within about 4 weeks after it was returned, the processor called and said that the flights had failed again in the same area, which was located about in the middle of the screw and about 12-in. long. The screw was returned to the manufacturer and repaired a second time, sent back to the customer and reinstalled.
In about another 4-5 weeks the customer called again to say (you guessed it) the screw had been pulled and yet again the flights had failed in the same exact location.
After much research and review of the screw design, it was considered that there was something wrong with the extruder—and not the screw—and that most likely the problem was due to thermal expansion.
It was time to make a plant visit. The first thing I noticed was that the 900-lb, 6-in. screen changer—located approximately 36 in. in front of the front barrel support—was not supported in any way, causing a cantilevered overhung load.
So to determine if the barrel was expanding properly, the die and adapter were disconnected from the extruder. Then, three dial indicators were mounted so that they contacted with the screen changer. The indicators were mounted independent to the extruder at 12 o’clock, 3 o’clock, and also on the face of the extruder, so that the movement could be measured in the X, Y and Z axis. The die and cart had been pushed up close to the screen changer so that it could be used as the independent support for the indicators.
Also, a dial indicator was mounted independently near the middle of the barrel between the front barrel support and the face of the feed throat housing, at the 12 o’clock position. This was done to observe if the barrel would bow upwards, which would be an indication that the barrel was not thermally expanding forward properly.
Calculations were made to determine the theoretical amount of thermal expansion that would be expected when the barrel zones were set at the processing temperatures of the extruder. The expected expansion was to be approximately 0.434 in. at an average barrel temperature of 410°F. Once all of the indicators had been properly “zeroed” on the “cold” extruder, the barrel zones were turned on and allowed to heat up.
Within an hour, the barrel only expanded forward approximately 0.400-in., but the screen changer had dropped 0.045-in., and the middle of the barrel had lifted 0.032-in. for a total deflection of 0.077 in. Also, it was evident and measured that the barrel had only moved forward at the front barrel support a distance of 0.253 in.
From all this it was concluded that the overhung load from the screen changer was causing a bind in the area of the front barrel support, and not allowing for smooth and uniform expansion of the barrel in the axial direction.
A support for the screen changer was fabricated and installed to eliminate the overhung load. Also, it was confirmed by a major screen changer manufacturer that they recommend a screen changer cart for all screen changers 6-in. diameter and larger, and even for smaller extruders also.
Lesson learned: Excessive overhung loads and non-uniform thermal expansion will cause premature screw and barrel wear.
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.