Die geometry for embossing and stretching

STAMPING JOURNAL® JULY/AUGUST 2003

August 28, 2003

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Stretching or embossing, not to be confused with drawing, is the process in which the part's geometry is obtained by stretching the metal into a forming cavity.

Embossing usually refers to the stretching of metal into a shallow depression. Unlike drawing, embossing entails little or no inward metal flow. Some automobile hoods and parts are made using a stretching operation, while others use a combination of drawing and stretching (see Figure 1).

Figure 1
Some automobile hoods and parts are made using a stretching operation, while others use a combination of drawing and stretching.

The ability to make the finished part geometry successfully in a stretching operation is limited by three main factors:

  • The metal's mechanical properties
  • The die geometry
  • The frictional value

The latter two are discussed here.

Punch Radius

Figure 2
A large punch radius pulls material from the product area and allows more surface area of the metal to be stretched. This is one of the main reasons that increasing a punch radius helps to reduce fractures. For stretch forming operations, larger punch radii are preferred.

The punch radius is very influential in obtaining stretch.

A small punch radius prevents the metal from stretching from the product area and forces the metal to be pulled from the blank holder area. If the draw ratio (the relationship between the punch edge and the blank edge) is excessive, the metal will not flow, and fracture will likely result.

A large punch radius pulls material from the product area and allows more surface area of the metal to be stretched. This is one of the main reasons that increasing a punch radius helps to reduce fractures. For stretch forming operations, larger punch radii typically are preferred (see Figure 2).

Die Entry Radius

Figure 3
Wherever a large radius is used, the metal is allowed to stretch over a larger surface area and pulls or feeds areas needing metal.

The size and surface quality of the die entry radius also are important to consider when stretching metal.

A small radius prevents metal from stretching from the blank holder area, and it decreases the length of line to be stretched. A large radius allows metal to be stretched from the blank holder area and increases the length of line to be stretched. Basically, a large radius allows the metal to stretch over a larger surface area and pulls or feeds areas needing metal (see Figure 3).

Unlike the radius sizes needed for drawing, stretch radii generally are much larger. The reason primarily is that when metal is drawn into the cavity, it is flowing over the radius in an effort to feed the punch metal. In a drawing operation, the blank decreases in size; in a stretch operation, the blank remains the same size.

Figure 4
Open wall angles increase the length of line and surface area of the metal being stretched, decrease the draft angle, and allow the metal to be pulled in a more horizontal fashion with respect to the punch and the blank holder.

For part geometries created by drawing and stretching, the radius size is a judgment based on your knowledge of draw ratios, metal properties, and stretching principles.

Wall Angle

While a true drawing operation works best with vertical walls, stretching operations work best when wall angles are opened up. Open wall angles increase the length of line and surface area of the metal being stretched, decrease the draft angle, and allow the metal to be pulled in a more horizontal fashion with respect to the punch and the blank holder (see Figure 4).

Figure 5
Avoid opening up wall angles in the profile radius area, especially when using thin metal, because the metal is in compression and may have a tendency to wrinkle before the bottom of the press stroke. Source: ESI Group

Avoid opening up wall angles in the profile radius area, especially when using thin metal, because the metal is in compression and may have a tendency to wrinkle before the bottom of the press stroke (see Figure 5). With heavy metal or metal with excellent stretching ability, you can apply more tension through the use of a draw bead or increased pad pressure to reduce the wrinkling.

Frictional Values

The surface finish of the radii is critical to the success of the stretching operation. Be sure to finish all working radii to a mirrorlike surface finish to promote metal sliding action. The process of stoning followed by polishing with diamond dust works very well.

Make sure that all radii are free from imperfections. Use a good high-pressure barrier lubrication to allow the metal to slide over the punch and die entry radii. This barrier lubricant actually keeps the sheet from touching the die by forcing it to ride on a thin layer of lubricant. Best of luck!



Dieology LLC

Art Hedrick

Contributing Writer
Dieology LLC
8579 River Oak Circle
Greenville, MI 48838
Phone: 616-225-2170
Author of the "Die Science" column in STAMPING Journal®, Art also has written technical articles on stamping die design and build for a number of trade publications. A recipient of many training awards, he is active in metal stamping training and consulting worldwide.

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