Did you know that metal has a memory? Imperfections that are a natural result of the steel making process can impart a level of stress on a piece of metal—levels that are often difficult for the metal to fully “forget”.
Take stainless steel sheet or aluminum sheet, for example. In their rawest state, these products are not perfectly flat. They contain a range of imperfections in the form of such things as cross bowing and wavy edges. And it can be a meticulous battle in trying to stop metal from wanting to return to this natural form.
And naturally those “memories” tend to come back at the most inopportune times. For instance, when you are laser cutting a piece of material. Consider yourself lucky if the material simply bows a bit; with the worst-case scenario being that the sheet snaps up and damages the head of your laser. In that case, you are not only losing a piece of material, but possibly facing major equipment repair or replacement as well.
There are many advantages to reducing the level of stress on material. Doing so can improve the surface finish, produce closer gauge, flatness and shape tolerances, and even reduce additional costs and steps in the manufacturing process.
But that means wiping that “memory” of the metal clean. There are two main schools of thought for best practice: temper pass and stretcher level.
The temper pass process, which occurs at a temper mill, involves the application of pressure to the metal. Picture two giant rolling pins that apply an extreme amount of pressure as they compress the plate or sheet, thus squeezing out the stress. This not only eliminates inconsistencies, but also delivers more uniform thickness across the entire width of the material.
Quality assurance is essential throughout this process. Personnel working within the temper mill must pay attention to the material, interpreting the level of inconsistencies to make proper adjustments to the machine.
The stretcher level process removes bowing and warpage by applying uniform tension between the ends so that the piece is elongated to a definite amount of permanent set. Picture grippers attached at each of the four corners of piece that stretch the material beyond its yield point. No bending is used as part of this leveling process.
Both processes typically operate in conjunction with a cut-to-length line. That means after the piece is stretched or pressed, it is subsequently cut-to-length to a specific size (see below)
So, whether it is through temper passing, stretching or leveling—the importance of achieving consistent, stress-free metal is something you should keep in your memory.