Over-hung Loads

Recently a processor installed a new screw into their 6” x 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 that the failure was due to a poor weld bond. The screw was ultra-sonically inspected and the remaining flights showed good bore, but the entire screw was rebuilt and returned to the customer.

Within about 4 weeks the customer called and said that the flights had failed again in the same area, which was located about in the middle of the screw and within about a 12” length. The screw was returned to the manufacturer and repaired a second time, sent back to the customer and re-installed.

Once again in about another 4-5 weeks the customer calls again to say the screw had been pulled and again the flights had failed in the same exact location. This was unbelievable!!
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 most likely it was due to thermal expansion.

It was time to make a plant visit. The first thing that was noticed that the 900# 6” screen changer had no support under it and it was located approximately 36” in front of the front barrel support causing a cantilevered overhung load.

So to determine if the barrel was expanding properly, the die and adapter was disconnected from the extruder. Then, three dial indicators were mounted so that they contacted with the 6” 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 in other words 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 be an indication that the barrel was not thermally expanding forward properly.

Calculations were made to determine the theoretical amount of thermal expansion should be expected when the barrel zones were set at the processing temperatures of the extruder. The expected expansion was to be approximately .434” at an average barrel temperature of 410°F. Once all of the indicators had been properly “zero-ed” on the “cold” extruder, the barrel zones were turned on and allowed to heat up.

Within an hour, the barrel only expanded forward approximately .400” but the screen changer had dropped .045” and the middle of the barrel had lifted .032” for a total deflection of .077”. Also, it was evident and measured that the barrel had only moved forward at the front barrel support a distance of .253”.

It was then concluded that the 900# overhung load from the screen changer was causing a bind in the area of the front barrel support and not allowing the 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 confirmed by a major screen changer manufacturer that they recommend a screen changer cart be used for all screen changers 6” 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.