Die Science: Ramifications of reducing blank sizes for drawing and stretching operations

With the cost of sheet steel, aluminum, and other frequently stamped metals, stampers try to use as little material as possible to make a satisfactory part. Unfortunately, trimming material may have an adverse effect on your ability to make a successful part. Making blank size changes without understanding how they will affect metal flow can result in serious failures.

In my career I have been in hundreds of stamping facilities and witnessed people make very poor decisions based on the width of the scrap cut off from a drawing operation. For instance, if they looked in the scrap bin and noticed that 2 inches of extra material had been trimmed off from a deep-drawn panel, they assumed they could cut the blank down 2 in. and save material. They forgot to consider the importance of that extra material on the final part quality.

Here’s why stampers need to be very careful when considering a reduced blank size.

The Drawing Ratio

The drawing ratio is the direct relationship between the size of the forming punch and the blank. Although drawing ratios are commonly associated with round cup draws, they also apply to any given geometry formed by the deep-drawing process.

When forming a tall cylinder-shaped drawn shell, your objective typically is to maximize the amount of material flowing into the cavity and minimize the amount of stretch. To do this, you need to minimize the amount of blank surface area outside the forming punch that is trapped between the die face and drawing pressure pad. If excessive material is trapped there, excessive stretching and splitting of the deep-drawn shell can result.

However, if you are deep drawing or forming larger contoured parts, such as inner doors and panels, sometimes the operation requires the metal to be stretched. For the metal to stretch, you have to minimize the amount of flow going into the cavity, because reducing the amount of inward flow increases the amount of stretch in the panel.

Stretching the metal over a punch promotes quality, class A part surfaces and helps to eliminate loose metal and wrinkling. Adding extra material to the blank often results in a restriction of metal flow into the die cavity. This is because more surface area is trapped between the die face and drawing pressure pad. In simple terms, the more surface area on which you have a grip, the less likely the material will feed inward. If you remove the extra material from the blank, you may cause excessive inward metal flow and excessive loose metal or wrinkling.

Possible Solutions

Keep in mind that this doesn’t mean you can’t remove any excess blank from around the part. However, if you are going to reduce the blank size, you must create a substitute for the restraining force that the extra material was providing. You can do this in a couple of ways.

1. Reduce the die entry radius in the area where the blank has been reduced. Reducing the die entry radius will create a resistance for the metal to flow into the cavity, which will be similar to the restriction created when the extra material is trapped between the die face and draw pad surface. Remember that the radius you’re changing may be a product design radius, and its size may be critical. If so, you cannot change its size without a product design change. Also keep in mind that as you reduce the radius, friction will increase, and that could result in premature die wear and potential adhesive or abrasive galling on the smaller radius.
2. Add a draw bead. If a lot of metal remains on the draw pad and most becomes scrap, you can add a draw bead in the area to restrict the metal flowing into the die cavity. A draw bead creates a restrictive force by causing the material to bend and unbend before flowing into the cavity and over the punch. The height and shape of the draw bead control the amount of metal flowing into the cavity. In addition, the radii on the draw bead cavity control the amount of material flowing into the die cavity. You will have to adjust and fine-tune the draw bead and bead cavity geometry to create the same restrictive force as the additional blank.

The people making decisions about blank size reductions should be very familiar with the blank’s effect on metal flow. They should have a comprehensive understanding of how drawing dies work, as well as a good understanding of metal flow and stretch. Reducing blank size without compensating for the effect it will have, or doing it simply because your scrap width is greater than you would like, could make you the villain rather than the money-saving hero.