Improving Plant Performance
by Roger A.P. Fielding, BENCHMARKS
Improving the performance of an extrusion press installation must start with measurement and understanding of the issues. The Recovery Billet—Figure 1—illustrates the factors affecting the overall recovery of aluminum between the supply of log or billet to the extrusion press installation and the delivery of good extrusions to the aging oven.
The size of the extrusion butt—which is sheared off at the end of each extrusion cycle—ensures that no “back-end defect” enters the extrusion. The size of the butt is controlled by the quality of the billet, the extrusion alloy, the diameter of the billet in relation to the bore of the liner, the control of the billet “upset,” and the removal of air from the container during and after upset.(1)
The quality of billet is measured by its conformance to the specified composition limits (and particularly the level of hydrogen gas), the thickness of the shell zone, the level of inverse segregation, and the surface finish of the billet. In general terms, the higher the level of alloying elements (or the “harder” the alloy), the larger the butt. To minimize air entrapment—and at the same time maximize billet weight—the diameter of the billet should be close to the diameter of the bore of the liner. Upset must be controlled to minimize air entrapment; with proper control, a “burp cycle” is usually unnecessary.
All of the items listed in Figure 1 contribute to “Extrusion Scrap.” They can be minimized by using best practices, and by the configuration of the extrusion press, its handling system, the stretcher, and the finish-cut saw.
Best practices recognize that the first and last billet in each production lot should be different sizes from the second and subsequent billets, since this allows for filling the die at the start of production and minimizes scrap at the end of each lot. Similarly, the use of best practices can minimize the back end and the transverse and longitudinal weld scrap.
However, the “shape scrap”—recorded as bow, twist, and wave—is usually a function of the extrusion die and specifically the control of the mechanical and thermal alignment of the extrusion press and container, the billet and the die.
The proper configuration and operation of the press run-out table, the puller(s), the quenching systems, the handling system which moves extrusions into and out of the stretcher, and the finish cut saw have a major impact on minimizing scrap.
Improving Press Performance
While the performance of a given extrusion press installation can usually be quickly improved by improving the recovery of good extrusions from billet, the ultimate performance of the press can only be achieved by controlling every second of available time—where available time is taken to be 7 x 24 hours in each and every week.
Rigorous measurement is (again) the only way to understand where time is lost in the production system.
The “Dead Cycle” (see figure 2) is established by the mechanical design of the extrusion press. Any time lost over and above the original manufacturer’s timing is referred to as “Waste Time,” as is other time lost—waiting for the billet, lubricating the die, or, more commonly, operating the press manually instead of using the automatic-repeat-cycle control.
In an extrusion press equipped with double die-slide, much of the “Die Change” time occurs within the dead cycle, and the only lost time is that taken to lead out the front end of the extrusion.
The “Extrusion Cycle Time” is self-explanatory, and for any extrusion alloy it can be minimized by the use of best die design, optimizing the composition and structure of the extrusion alloy billet and using best extrusion practice to achieve the highest extrusion speed while achieving the desired mechanical properties.
“Maintenance Downtime” can be reduced by the care taken in the original design of the extrusion press, its hydraulic systems, and the electrical and electronic control systems. Proper maintenance of the press and its attendant systems and specifically the employment of autonomous maintenance by the press operators will minimize maintenance downtime.
Full tables are the most common cause of “Other Downtime” and are generally caused by an imbalance between the productive capacity of the extrusion press and the finish-cut saw. The extrusion press—being the prime production unit—should be the bottleneck to production. All downstream devices should be capable of working faster than the extrusion press.
1) Jowett, C. W., “Upset,” Eighth International Extrusion Technology Seminar ET ’04, Orlando, Florida, May 18-21, 2004, Volume 1, pp. 23-37
Abridged from “Managing the Performance of an Extrusion Plant,” Proceedings of the Eighth International Extrusion Technology Seminar ET ’04 Orlando, Florida, May 18-21, 2004, pp. 565-570