Aluminum body Panel stamping: Draw operation

Blanks are fed to the first form, or "draw" operation

As we venture into the Draw operation (first form where-in a blank is given a majority of its part shape and depth) we will take a little side trip into metallurgy to discuss the oxidation of Aluminum. Yep, OXIDATION. At this moment some might take offense an complain, but *Die Expert (the self proclaimed is silent here) I thought that aluminum would not rust. And yes, you are right. but not rusting is not oxidizing. Aluminum is highly reactive with oxygen. The difference is that aluminum-oxide unlike ferrous oxide can form a non-permeable barrier. So the aluminum canoe that sits in a corrosive environment does not rust away like that old steel bucket because the oxidation stops after the first few micro layers of Aluminum-oxide forms.

The significance of this mention is not the "rusting" or ""oxidation" but the pervasive presence of Aluminum-Oxide on the surface of every aluminum blank you will ever process. Where else have we seen aluminum-oxide???? Asnwer: Sand paper.

Your aluminum blank? A super sized industrial form of this

Aluminum is highly abrasive in the stamping environment. Each time we try to form the aluminum blank we are dragging a sheet of giant sandpaper through the die. This is well known for most who stamp aluminum for a living, so before we ever enter the Draw operation we will have to process the blank through some sort of lubrication system to coat the blank with a friendly lubricant to ease the wear and tear that the aluminum blank will subject the dies to.

blank washer and lube application prior to loading via robot Source: Schuler group

Lubrication of the aluminum blank and proper cleaning is a consistent challenge in the stamping environment. especially for those undertaking the production of Major Body Panels. Here we should define and differentiate what might commonly be at the front end of many press-lines previously purposed for stamping steel body panels.

In some of the plants where I had worked we saw a variety of attempts at lubricating our steel and aluminum blanks, some successful and others not so much. Here are some of my completely subjective observations.

• Too little lube?  Adhesive wear of the aluminum oxide on the tool surfaces leads to galling and significant additional adhesive wear. On at least two of the products that I supported, this lead to a necessary SOP of cleaning out the die with "Scotch Brite" pads nearly every hour (they bought it by the roll). Abrasive wear of the die can result and the tool will lose its dimensional integrity. Tools for making outer skin parts were commonly chromed anyway at my plants so abrasive wear was kept very low, however chroming the die was expensive and meant there was very little chance for forming adjustments (the plant manager referred to chrome as die-maker repellent).
• Is it a blank washer or a lubrication application: The blank washer is purpose built in most cases to apply a "detergent" solution that has some lubricating properties but its purpose is to clean the blank. When issues arise that seem like they can be solved with more lube. The washer was tuned (de-tuned) to leave more fluid on the part, however since the fluid was engineered to be a detergent it meant that it carried more dirt into the die. Washers are meant to use exit rollers to squeegee the blank "dry" leaving only enough film of lube to fill the surface texture of the sheet. Effective for many steels (hot dipped or electrogalvanized) but with aluminum and Galvannealed material a bit more secret sauce was desired to reduce adhesive and abrasive wear.
• Too much lube? leaving excess wash on is not a fix to lube, and it came with the dirt issues. But other issues arise with excess lubrication (moisture) on the blank. Lubrication imprinting caused by pockets of lube trapped between the blank and the punch. For large surface area outer panels can look really bad. Lube is in-compressible so if you get a "bubble" of it trapped between the blank and the punch we got pretty bad surface defects. Aluminum is prone to this more so than steel as the incoming yield strength of aluminum is consistently lower than that of steel. Suction and poor venting lead to interesting behavior on the opening of the draw die. As a hydro-static seal forms between the punch and the blank. As the die opens the part could practically turn inside out if venting is not addressed. Excess venting to alleviate the inside-out parts did result in quality issues as the vent hole started to draw old lubricant and rust particles from the "core" of the die (the rust in this case was from the water based lube reacting with the cast-iron tools). Also, the old lube congealed over time and lead to "boogers" that would also get trapped under the blanks and lead to surface defects. Safety and handling soaking wet blanks was a major issue, during my stay at one plant two die makers suffered broken arms as they fell servicing draw dies that were dripping with lube. Also, production personnel has issues with saturated gloves after only 10-15 parts that they would have to deal with for the remainder of the production run (or until the next cycle-stop or "andon" interruption).
• Lube compatibility with paint or steel production: not all lubes behave the same and cannot be assumed that there is a one-size fits all lube. So in order to run well, there was a fully dedicated line for only aluminum panels. (This also comes up later when we discuss scrap.)

and that was just Lube (still not fully into the draw application yet).....

I guess more to come later......