Die Science: Understanding and calculating draw reductions

Splitting, wrinkling, and surface defects are common problems in drawing, a process in which metal flows into a die cavity. Often these problems result from incorrect performance of the draw reduction process.

A draw reduction is a multistep process in which the surface area of the original blank is displaced through a series of drawing operations. A draw reduction could involve two to 16 or more drawing operations. Having the right number of stations and the right diameters and heights is critical to the success of a multistage drawing process.

Why Are Draw Reductions Necessary?

To understand why draw reductions are necessary, you need to understand the draw ratio theory.

The draw ratio is among the most important elements to be considered when attempting to deep-draw a round cup. The draw ratio is the relationship between the size of the draw punch and the diameter of the blank. The draw ratio must fall within acceptable limits to allow metal to flow.

During forming, a blank is forced into circumferential compression, which creates a resistance to flow. If the blank diameter is too large with respect to the punch diameter, the resistance for the metal to flow will be excessive and the metal will overstretch, resulting in excessive thinning and most likely splitting. And obviously, if the punch is too small with respect to the blank diameter, the same failures will likely occur.

If the punch is the appropriate diameter with respect to the blank diameter, and other variables such as die entry radius size and blank holder force are correct, the metal will flow freely, progressively thickening as it enters the die cavity. The objective in choosing the draw ratio is to maximize material flow while minimizing thinning.

To form a small-diameter blank into a very tall, deep-drawn vessel, you have to take a systematic approach, making sure the metal can flow freely in each drawing station. Using multiple drawing stations or reductions decreases the resistance to flow by minimizing the amount of material in compression in each drawing station.

For example, if you were attempting to draw metal using a ½-inch-diameter punch and a 5-in.-dia. blank, flow would be minimal or nonexistent, so the punch would not be able to enter very far into the material before the metal would overstretch and split. However, if the drawing punch were 3 in. dia. and the blank were 5 in. dia., the metal would flow freely, and the part draw would be successful.

Performing a Draw Reduction

The most important factor to remember when performing draw reductions is that all of the surface area necessary to make the final part shape must be present in the first draw. After the first drawing station, the surface area remains the same. In other words, the same surface area that is in your final part must be present not only in the blank, but in every drawing station before your finish station.

A draw reduction chart can help you determine the relationship between the blank and all of the preceding draws before the final form. The chart in Figure 1 is designed for three drawing operations. If additional operations are needed, use the same reduction percentage as the third drawing operation.

Before you can determine the punch diameter and height of the first draw, you need to calculate the total surface area of the finished part. If the part is to be trimmed, allow additional material during this calculation. Then convert the calculated surface area into a flat blank diameter. For engineering purposes, keep the surface area of each drawing station constant.

To find the diameter of the first drawing punch, multiply this blank diameter by the percentage given in Figure 1 and subtract the result from the original blank diameter. Remember that all dimensions are taken through the centerline of the material.

Now you can determine the second drawing punch diameter by multiplying the diameter of the first drawing punch and the second drawing reduction percentage. Repeat this process until you reach the minimum diameter of the feature you want to make.

Fit the Draw Reduction to the Application

If you're attempting to make a very deep-drawn part, you likely will have to perform a draw reduction to achieve the desired part geometry. Although draw reduction charts can be helpful, there are no hard rules for the calculations. Many companies have developed their own draw reduction percentages based on their past experience with different materials, as these percentages change with respect to metal type and thickness.