Die Science: 6 questions to answer to achieve successful deep drawing

Of all the metal forming process used today, deep drawing is certainly one of the most versatile. Products such as oil filters, cigarette lighters, medical battery casings, and automotive parts are made using this process, just to name a few.

1. What Is Deep Drawing? How is it Different From Stretching?

Drawing is a process in which a metal product is shaped by controlled inward flow of the metal over a punch an into a cavity. In deep drawing, the total height of the part exceeds its minimum part width. Long, tubular parts often are made using this process.

Frequently deep drawing requires more than one operation. The secondary operation are referred to as draw reductions or redraws (see Figure 1 ).

In deep drawing, the original blank must change its perimeter dimensions. As metal flows during drawing, it may get thicker.

Note that stretching and drawing are not the same. While drawing displaces surface area through plastic flow, stretching increases the surface area via tension. Although some stretching occurs during the drawing process, the process should be engineered so that the amount of stretching is limited. Metal is at greater risk of failing during stretching than during drawing.

2. How Well Do I Know My Material?

Thousands of different types of ferrous an nonferrous metals are available in sheet form today. Although not all are suitable, many of them can be deep drawn (see Figure 2 ).

Each type of metal has its own special behavior characteristics, and to form a material successfully, you need to take the time to determine its tensile and yield strength, elongation percentage, and average n and r values. Find out if the metal is coated or pre-painted. If it is a nonferrous metal, find out if it is full hard, dead soft, or half hard. What is it drawing quality? Is it a speciality advanced high-strength steel such as dual-or trip-phase?

Knowing the material type will help you choose a suitable tool steel to cut and form it. Knowing you metal’s characteristics also will help you to determine if you need to anneal the parts between forming operations.

Annealing is a heat-treating process that returns the metal to it original properties after being cold-worked. Superalloys often require annealing between drawing operations, which explains why aircraft and medical parts typically are so expensive: They often are designed wit tight geometric tolerances and require more steps, including annealing, to manufacture. In contrast, materials such as low-carbon extra-deep-drawn-quality steel can be drawn up to 12 times before annealing becomes necessary.

If your material is high strength, you might need to add extra stations to compensate for springback. The material strength also affects your lubricant choice. Drawing high-strength steel into a given geometry usually requires a different lubricant than drawing the same geometry out of low-strength steel, because higher-strength steels require more force and energy to form. Higher forming forces result in more friction between the tool steel and the metal being formed, which generates more heat.

Most lubricants contain additives that react to heat and reduce the friction. If the heat being generated is insufficient or excessive, the additive will likely fail to reduce the friction sufficiently.

Avoid building common tools that are designed to form a variety of different metals without changing the die.

Also pay close attention to the material’s thickness. Thicker metal because there is more metal to stretch. Nothing is more difficult to stretch or draw than thin, high-strength metal because of buckling and wrinkling.

Whenever possible, avoid trying to draw materials that are extremely hard with poor stretchability. They might require additional heat to be formed.

3. Am I Using the Right Press?

Keep in mind that the press you select for deep drawing not only use have the force necessary to deform it too but it must not try to deform it too quickly. Think of your metal as being like Silly Putty®— if you pull it apart very quickly, it will snap and break, but if you stretch it slowly, you can stretch it a great deal without breaking it.

Hydraulic and servo-drive presses are suitable for deep drawing because they can be adjusted not to exceed the maximum forming speed for the metal. In addition, hydraulic presses have their full rated tonnage throughout the entire press stoke, whereas crank-drive presses often lose tonnage off the bottom of the press stroke. Eyelet presses or special cam/shaft drive transfer presses also are good for deep drawing.

Part II, which will discuss the limiting draw ratio, metal flow patterns, and draw radii.

Until next time… Best of Luck!