Taking advantage of simulation technology

By Art Hedrick

October 24, 2002

One of the most valuable high-tech tools introduced in the last decade has been finite element analysis (FEA) simulation software that stamping tool makers can use to test forming conditions and design dies in the virtual world. This reduces tooling and product design time and saves costs of prototyping and experimentation to find the right design. Training the tool designer or process engineer how to use simulation software can provide a quick ROI and improve the bottom line.

Figure 1
Object

Tremendous advancements have been made in sheet metal stamping during the last decade. Even during my short 22 years of tooling experience, I have seen dies designed with a pencil and a drafting board; with a computer; with solid modeling; and with real-world 3-D methods.

I've seen steel cut with saws, oxyacetylene, and plasma torches, to CNC machining centers, waterjets, lasers, electrical discharge machines (EDMs), and CNC plasma arc cutting machines. The speed at which this equipment can perform also has increased, from cutting steel a slow 1 inch per minute (IPM) to 600 IPM.

Such high-tech tools allow today's dies to be processed, designed, and built in a fraction of the time than previously required.

Virtual Solutions

Perhaps one of the most valuable high-tech tools introduced in the last decade or so is forming simulation. Forming simulation is a special type of finite element analysis (FEA) that simulates metal being deformed by almost any deforming method.

One of the biggest uses for this technology is the stretch and deep drawing industry. This specific type of FEA typically is referred to as drawing simulation software. It also is called a one-step solver, inverse solver, or virtual press.

What Can Forming Simulation Do?

Figure 2
Object

Forming simulation software offers numerous advantages for toolmaking.

1. It helps you design stamped parts so they can be formed in the most cost-efficient way using the most economical process and material. Simulation also can help you develop a part shape that can be formed or drawn easily and that will fit and function properly.

Major areas of concern can be identified and changed easily with simulation software, such as excessive thinning, fractures, or wrinkling. Simply by inputting different sheet steel properties, you can introduce a variety of materials into the process to determine which works best. You then can design the product using the optimal material type and thickness.

This software also allows you to predetermine a blank size and shape, which helps determine the final part cost.Figure 1shows simulation data derived from a one-step simulation process.

2. Special crash simulation software can determine the final strength of the part by virtual destructive or crash testing.
3. Special incremental software can show the final stamping die geometry. Figure 2shows incremental simulation process data.

Other benefits of forming simulation include:

• Reduces product design cost and unnecessary redesign.
• Reduces costly die tryout time.
• Reduces frustration during tryout.
• Reduces or eliminates the need for prototyping.
• Reduces the forming severity of the stamped part, resulting in a more robust stamping operation and less scrap.
• Removes the guesswork from the tryout process.

In addition, simulation software is designed to show details of the design, such as:

• Areas where the metal tore or fractured.
• Areas where the metal wrinkled or buckled.
• Finished part thickness anywhere on the part.
• Strain distribution of the finished part.
• A forming limit diagram representing the forming severity of the part.
• Major springback tendencies.
• Necessary blank holder pressure.
• Tonnage needed to form or draw the part at any point during the forming process.
• A virtual simulation movie file of the part being incrementally formed.

The software also helps to identify possible tooling changes that would result from:

• Draw bead additions.
• Metal gainer and draw bar additions.
• Blank size or shape changes.
• Radii changes.
• Binder or blank holder changes.
• Addendum changes.
• Blank holder pressure changes.
• Frictional lubricant changes.
• Process changes.
• Addition or subtraction of hits.

Remember that FEA software does not design the die for you, but rather shows you the results of your die design based on the data you input into the computer. A lot of people invest in this technology thinking that it will design the die for them. Not true.

Limitations of Simulation Software

The initial cost of FEA software can be substantial, so you need to compare that cost to your anticipated cost and efficiency savings over the long term. It would be surprising if your calculations do not reflect a good return on investment (ROI). I know toolmakers who have saved millions of dollars using this software.

Also, significant training is required to run the software properly. An average training period might be about two months if training is done every day. However, the amount of training needed depends on the person learning the software. Using the software and training appropriate personnel to run it effectively can provide a quick ROI.

The person operating the software will be creating and inputting the die geometry and is simply using the computer as a tool. Therefore, my recommendation is that you train a person with a great deal of formability expertise, such as a product designer, tool designer, process engineer, or diemaker. It's much easier to train a person who knows about forming and drawing how to run a computer than it is to train a computer operator how to design and process deep-drawing and forming dies.

What the Computer Needs to Do a Simulation

Certain kinds of information must be input into the computer for the software to be efficient, including:

Mechanical properties of the material being formed. The computer doesn't recognize "good" steel so it needs more finite information, such as:

Thickness

n value

r value

Yield stress

Young's modulus

Poisson ratio

Density

Lankford coefficient

Frictional value—Coulomb coefficient of friction.

Die and forming geometry created by the operator or die designer—completely meshed.

Making Virtual Success a Reality

Forming simulation can help you design and build both your parts and your tooling. The real glory of forming simulation is that it does so in the virtual world, before the tool is actually built. Die geometry changes can be made in minutes instead of hours—no need for welding, grinding, or proof prototype tooling.

Forming simulation is very accurate. However, to achieve the same results after the tooling is actually built, the same variables must be achieved that were used in the simulation.

Today's simulation software doesn't tell you how to design your dies—it just gives you the result of your design. Tomorrow, who knows? It might just design the die for you. As for me, I wish they'd come up with a program that could help me put my thoughts on paper a little easier. Best of luck!