Aluminum stamping: Major Body Panels

Following up on the request for information on Aluminum Stamping ins and outs from an SME member in South Africa I made a quick brain dump which I will now try to elaborate upon. To frame this discussion should start with a brief overview of the "stamping process".

From coil to part- a stamping process

To produce major body panels the stamping process commonly will take the coil of flat stock (for this discussion aluminum sheet most likely in the grade 6000 series for outer or 5000 series for inner panels). The coil will be cut into regular sized and shaped pieces we will call "BLANKS". Those blanks will be collected, stacked and brought to the stamping production line. An example of a stamping lines are shown below.

source: Spartanburg Steel Flickr site

source: Schuler press USA

The stack of blanks will then be "DESTACKED" and fed into the first forming or "DRAW" operation. It is at this point we can first start talking about the challenges in forming Aluminum sheet when you are already tooled up to run Steel.

Blanking and Destacking challenges:

Source: the fabricator.com

Aluminum is non-magnetic. This elementary fact raises issues when it comes to destacking. Commonly in the stamping plant they will use suction cups to lift the blanks from the stacks and load them into the press to prep for first forming. The stacks of blanks will be commonly 200-300 parts deep. To ensure that only one steel blank is picked up we commonly use what are referred to as "fanner magnets". These are electromagnets positioned at the edge of the stack of blanks. As the suction cups lift the top blanks, the "fanner magnets" assist in separating the individual sheets. This prevents the accidental loading of multiple blanks into the first forming operation. Without the benefit of fanner magnets alternative methods to break the surface tension between the blanks is needed.

source: magnetics.com

Air knive, Source: atlas pressroom automation 

Air Knives as replacement for fanner magnets spread "DIRT" onto blanks. A workable solution to the "fanner magnets" is the use of powerful "air knives" which blow on the edge of the blank in an attempt to separate the blans. This proved problematic as any slivers, dirt, or debris that is on the edge of the blank would be blown across the blank. The blanks which are commonly shipped with an oily film on the surface of the blanks will invariably attract the dust, dirt, debris to its surface and carry the foreign material into the forming die. During the forming process any debris on the blank could lead to surface imperfections as the minute sliver of material is embedded into the sheet surface (top or bottom) and cause significant surface defects.

Aluminum sheet trims less cleanly than steel, so slivers are a consistent problem. It was near unavoidable that some slivers would be on the edge of every blank. Air knives and other part handling issues spread the slivers in a consistently random way onto the parts. It was very hard to predict where the potential surface problems could arise. As a result we had very high rate of inspection for these parts to segregate the parts that might require surface finishing from those that would not (very few).

So that is the problems that happen prior to forming. In following posts we can further explore the potential problems with Forming, Trimming, Piercing, Flanging, and Hemming.