Proper alignment of your production machinery is a critical factor in maintaining optimum equipment performance and manufacturing the highest quality product.
The plastics extrusion process is no exception. As plastics manufacturers increase the demand on extruders for faster speeds and better quality, it is more important than ever to properly maintain the components that make up this process.
A typical single screw extruder has four major components: gearbox, feed throat, barrel and feedscrew. One of the main factors in achieving optimum efficiency is maintaining the proper alignment of the rotational center of the screw to the rotational center of the gearbox thrust shaft. If these components are not aligned, excessive contact may result between the screw and the barrel leading to:
* Premature barrel and screw wear
* Increased clearance
* Decreased through-put
* Poor mixing
* Uneven melt temperatures
* Screw flex
* Screw breakage
Extruder Alignment Methods
Misalignment can be caused by a number of factors including maintenance procedures, barrel and gearbox/motor installations, as well as machinery conversions and upgrades. Fortunately, there are alignment tools and methods available that can significantly help to decrease misalignment associated issues.
Traditionally, two primary methods have been used for measuring extruder alignment – optical alignment tools and laser based alignment systems.
This method involves the use of precision optical equipment, such as telescopic transit squares or dual axis sight levels. The optical instrument is used to align the extruder barrel to the gearbox thrust shaft. Within the eyepiece of the optical instrument there is a reticle (or cross hair) set to infinity. An optical alignment engineer uses the instrument to establish a line of sight reference. With the use of optical targets along the barrel, the engineer is able to measure any radial displacement of the centerline of the barrel in relation to the line of sight reference.
Laser Based Alignment Systems
Similar to the optical alignment method, laser alignment systems are positioned on the gearbox shaft and adjusted so that it is coincident with the centerline of the thrust shaft. As the name would suggest, laser alignment systems utilize a laser beam and light sensitive targets that are moved along the length of the barrel. As the targets detect the laser beam, measurement data is displayed on a digital readout.
Both optical alignment and laser alignment systems are highly accurate and will perform well in order to measure the misalignment of the gearbox and the barrel.
Preventative Maintenance & Troubleshooting
In addition to alignment inspections, it is highly recommended that wear testing of the barrel and screw be completed regularly in order to accurately forecast when new equipment may be necessary. Also, performing troubleshooting measures early after the appearance of the first symptoms helps to ensure that issues are resolved before they can cause excessive damage to the components, therefore lengthening the life of the equipment. Preventative and troubleshooting measures include:
=> Bore gauging – Wear profile of the barrel
Screw Diametrical Profile
=> Screw Outside Diameter (O.D.) Measuring – Wear profile of screw flights
Screw and Barrel Straightness Measurement
=> Straightness of both the screw and the barrel is quickly and accurately measured using a 3D metrology tool such as a laser tracker
Thermal Growth Studies of Barrel
=> Using a laser tracker and targets installed along the exterior of the barrel, measurements are taken in both hot and cold conditions. Target displacements are evaluated in relation to their impact on alignment.
With regular extruder alignment inspections and preventative maintenance procedures, plastics manufacturers can see substantial improvement in both process efficiency and product quality. By decreasing scrap rates, minimizing extruder downtime and avoiding high costs of component replacement – companies often see fast ROI and an increase in customer satisfaction.
In future posts on extruder barrel alignment, we will delve further into the issues caused by misalignment and how to address them – as well as, offer guidance with regards to the steps to take and questions to ask prior to an extruder alignment project.