Steps 5 - 8
June 12, 2007
Determining the best die geometry to produce multicontoured formed parts can be difficult. A full understanding of the drawing and stretching process is necessary, as well as a good understanding of all tooling factors to make complex geometries.
Editor's Note: This is third installment of a three-part series that discusses die development for producing nonuniform, contoured parts by breaking the process down into eight steps. Part I covers the part material, its form, and its function. Part II discusses length-of-line analysis and tip angle. Part III discusses unfolding the part, blank holder shape, addendum features, and virtual die tryout.
Part I discussed designing dies for contoured parts from critical data, provided by finite element analysis. It also walked you through the initial steps you need to take to develop such a die design — Step 1: Determine Metal Type and Grade, and Step 2: Study the Part. Part II walked you through Step 3: Conduct a Length-of-line Analysis and Step 4: Determine the Tip Angle. Now we move on to Steps 5, 6, 7, and 8.
In many cases, it's not possible to create the entire part geometry in a single forming operation. It's often necessary to 'unfold the part,' which means incorporating bends and flanges into the forming die.
To see a classic example of the unfolding process, look at an outer automotive door panel. An automotive door panel must contain a flange around its parameter so it can be attached and hemmed to the inner door panel. The process of unfolding the part also allows for a larger portion of the part to be direct-trimmed (see Figure 1). Direct trimming is a less expensive process than cam trimming (see Figure 2). Three guidelines for unfolding part geometry are:
The shape of the blank holder, or binder, is critical when developing forming die geometry. A poorly developed blank holder shape can result in forming defects such as splits, wrinkling, and surface defects. The shape the sheet metal takes when it conforms to the blank holder often is referred to as the binder or blank holder wrap.
Wrap geometry is important if you're attempting to form an exposed body panel. For example, if the blank takes a shape that puts a severe wrinkle or crease in the metal before punch contact, chances are there will be evidence of this crease deformation at the end of the forming process. Using a poorly developed blank holder shape also can result in wasted material, as well as poor punch contact conditions.
When determining a blank holder shape, maximize the area of the forming punch that contacts the blank simultaneously. Design the blank holder so the depth of form is as uniform as possible. This is usually done by shaping the blank holder to a profile similar to the top of the forming punch.
Figure 3 shows two possible scenarios for a blank holder shape. With a flat blank holder, the depth of draw is excessive in the part's center and very shallow in other areas. This creates a poor forming condition, increasing the probability of splitting and wrinkling.
Using flat blank holder geometry will result in wasted material. Curving the blank holder with respect to the top profile of the part maximizes punch contact, unifies and reduces the depth of form, and helps save material. Avoid blank holder shapes that cause the metal to severely buckle or crease the blank. Pay close attention to how much material will be wasted with respect to your blank holder's shape.
Addendum features are defined as added features placed outside the part areas to aid in stretching and forming. In most cases, these features are trimmed away. Items such as draw beads, draw bars, draw walls, and shelves are common addendum features.
The most common practice for determining the need for addendum features is to perform a length-of-line analysis (see Part II). A length-of-line analysis measures the amount of metal consumption through each area of your part. This may be performed using a piece of string, a map reader, or tape.
Areas needing less metal may require a draw bead or draw bar. Very deep part areas most likely will not need a draw bead or bar. The exact geometry of the addendum features most likely will be finalized during the forming simulation or die tryout (see Figure 4).
If you don't use forming simulation software, the next step is to design and build the die. Any die geometry changes would have to be determined by putting the die in the press and making adjustments through grinding, welding, and shimming.
This process can take as little as one hour or as long as two months, depending on a die's complexity. Using this old-fashioned process usually results in a more expensive tool and it takes longer to finalize. Forming simulation software may not account for all of the variables that take place in the actual tool, but it can predict and correct failures such as splits and wrinkles (see Figure 5).
Forming simulation often is referred to as virtual die tryout. Unlike actual physical die tryout procedures, changing addendum features in a computer may take an hour as opposed to a week. Using forming simulation software also takes a great deal of risk out of processing and designing a die.
Although I have attempted with this series of articles to lay out a step-by-step process for developing die geometry, true success depends on a great deal of personal forming die development experience. Attempting to develop complex die geometry without experience is very risky, especially if you don't invest in forming simulation software.
Until next time ... Best of luck!