News

The Screw and Barrel alignment methods
 
 
By: Richard Henry 
 
              Alignment Specialist The ADVANCED Team
 
 
Extruder barrel alignment is one of the most important aspects of a properly and efficiently operating plastics extrusion process. Whether it’s a blown film line, injection molding or extruded sheet process, the relationship between the rotational center of the thrust shaft in the gearbox and the rotational center of the screw are vital.
 
 
Misalignment is known to cause excessive screw wear and barrel wear. If the misalignment is severe enough, the screw can fail prematurely as a result of the fatigue caused by the cyclic bending of the screw as it rotates in the barrel. In some cases, the clearance that is opened up between the flights of the screw and the barrel will allow significant losses of system efficiency and through-put of the system.
 
 
Alignment Methods 
 
 
Historically, there have been three methods utilized to align the extruder barrel to the gearbox thrust shaft. 
 
 
A Level 
 
The first is to measure everything with a level. The idea being that if the machine is level, everything should be aligned. This is a serious misconception and will almost certainly result in a misaligned process and certain failure. This method of extruder barrel alignment does not take into consideration the horizontal position of the barrel. Misalignment in the horizontal plane is just as big of a problem as misalignment in the vertical plane. Other problems arise when the level used is not a precision measurement machinist level. The resolution and accuracy of a carpenters level or torpedo level is far inferior and will not give the desired results. 
 
 
A Borescope 
 
Another alignment method is to use an optical Borescope. A precision bore mounted telescope is installed in the gearbox thrust shaft and “bucked-in” to the mechanical center of the through hole. The eyepiece of the bore scope has a precision cross hair that is focused to infinity. The operator can then site down the barrel and measure the displacement of the optical target at multiple points along the barrel ID, provided the section is accessible and clean. The accuracy of the optical bore scope is sometimes less than desirable and can cause an apparent “accurate” alignment to fall well outside the prescribed alignment tolerances.
 
 
A Laser-based Extruder Barrel Alignment System 
 
There are several laser manufacturing companies offering off the shelf extruder alignment systems. A laser system improves accuracy, speed of measurement and reporting on the alignment of the barrel to the thrust shaft rotational center. The typical accuracy of the laser system is approximately 0.001”. While the laser system is going to improve the accuracy of the alignment, the money spent on the laser alignment system and the time involved in learning how to properly utilize the technology can sometimes be daunting for a maintenance department. Simply purchasing a laser system does not qualify someone to perform precision alignment; any more than buying a stethoscope makes someone a medical doctor. 
 
 
Let’s discuss the operation of a laser based extruder barrel alignment system. Rather than ambient light traveling up the barrel to the operator’s eye looking through the Borescope eye piece; a laser system utilizes a columnated beam of laser light directed from a transmitter that is mounted on the thrust shaft. The laser transmitter can be quickly qualified to project the true rotational center of the thrust shaft at any point along the barrel (or even beyond the end of the barrel if mold position measurements are needed). The qualified laser beam strikes the surface of a Position Sensitive Detector (PSD) mounted in the barrel. Rather than a cross hair and scale to measure displacement, the PSD very accurately measures the center of the laser beam to less than half a micron (0.0005 mm) and displays the measurement value on a display unit.
 
 
 
When the laser transmitter is properly qualified to project the rotational center of the thrust shaft to the point of measurement, the results are geometrically infallible. Depending on the operator and machine configuration, laser based barrel alignment measurements can be completed about 30 minutes with alignment data documented at each barrel support and at the feed throat, provided they are accessible. Corrections are made using live values that are updated in real time in both the horizontal and vertical plane, cutting the correction time significantly.

  [Return Home] [Print] [Go Back]   Next

Contact Us