Age Ovens

by Roger A.P. Fielding, BENCHMARKS

Reviewing the hundreds of extrusion plants—located all over the world—I’ve visited in the last 27 years, I’m forced to conclude that the age oven is one of, if not the last thing, on the minds of the planners of new facilities.

Age ovens have often been selected without regard for the configuration of the extruded product and how it is to be stacked in the oven. They have been installed without proper regard for the flow of material and the movement of handling devices. And, very often, they are poorly maintained and operated. But, artificial aging is essential to maximizing the mechanical properties of most common aluminum alloys. Artificial aging is the longest part of the process of converting aluminum billet to an extruded lineal, it is the single major cause of in-process inventory, and, in most cases, the major disruption to the smooth flow of aluminum extrusions between the billet inventory and the packed and shipped extrusions.

Most extrusion plants schedule and sequence dies to the extrusion press in the order that the extrusions are to be shipped. The extrusions move from the extrusion press down a run-out table, where they are transferred to the cooling table, the stretcher, and the saw table. After cutting to a pre-determined, often standard, length, the extrusions are stacked—manually or automatically—in aging baskets, skips, or oven racks. The oven racks are then transferred to the age oven area, where numerous racks are collected to build an oven load. Most extruders load the oven racks “first in—last out” (FILO). Most extruders place the first oven racks to be loaded at the front of the oven, where they are then “buried” under the next two or three oven racks to be packed—further compounding the problem. And most extruders wait for a complete oven load before starting the aging process—which can take 8 or more hours. After aging, the oven racks are removed from the oven to be cooled and then packed.

Planners, please take note—there is a better way! Depending on the finished length of the extruded products, their shape, their configuration in the loaded oven racks, and the availability of space, age ovens are usually configured to process a fixed number of oven racks with the air flow in the age oven arranged as single end flow, double end flow, or side flow. Although each configuration—if properly maintained and operated—will ensure that the designed load will be uniformly heated to the correct temperature for the correct time, the extrusions leave the oven as much as 16 hours after leaving the extrusion press. Then, each load of extrusions must be “sorted” at the packing station to restore the shipping sequence.

Continuous aging is the solution: automatic indexing of oven racks through a properly configured oven ensures continuous production flow—with the oven racks flowing one-by-one through the oven. When matched to a stacker at the saw that is equipped to load the extrusions to be handled “first in—first out” (FIFO), the lead time—from delivery of billet to the press, to the arrival of extrusions at shipping—is minimized. The in-process scrap that is generated during multiple handling of oven racks and extrusions is also minimized.

The continuous oven, handling oven racks one-by-one, is not new. Other attempts have been made at transporting racked extrusions through an age oven—as in an anodizing or paint line. And the concept of aging full-length extrusions, demonstrated by Boal and reported at ET ’96, has merit because it reduces in-process inventory and each extruded length is aged before the finish cut saw, further reducing in-process scrap.