Deep drawn stampings: design rules and failure modes

What deep drawing is

Deep drawing is a sheet forming process that turns a flat blank into a cup, a shell or a box. A punch pushes the blank into a die cavity while a blank holder presses on the flange. The material flows inward from the flange and is drawn over the die radius to form the wall. The wall does not simply stretch: much of it comes from material that started in the flange.

The process is used for housings, cans, caps, sleeves, shields and shells. It is different from a shallow form or an emboss, where the material is stretched locally and the wall stays close to its original thickness. In a deep draw, the depth is a significant fraction of the diameter, and the material has to move a long way without tearing.

Draw ratio and blank size

The draw ratio is the blank diameter divided by the cup diameter. A ratio of 1.8 means the blank is 1.8 times the diameter of the finished cup. Each material has a limiting draw ratio for the first pass. Mild steel can often take a first draw around 1.8 to 2.0. Stainless and high-strength steels take less. Aluminium depends on the grade and the temper.

If the part needs a ratio above the limit, it is drawn in stages: a first draw to an intermediate diameter, then a redraw, then another redraw. Each stage adds a tool, a press stroke and a handling step. The blank size for a simple cylindrical cup is calculated from the surface area of the finished part, with an allowance for the flange and the corner radius. The calculation is a starting point; the toolmaker will adjust the blank after the first trial.

The blank diameter for a cup with a flat bottom is often estimated from the area of the base plus the area of the wall, with a correction for the corner radius. That estimate assumes the wall thickness stays close to the blank thickness. In a real draw the wall thins at the punch radius and thickens at the rim, so the blank has to be adjusted. A draw simulation is useful for a deep part because it shows the thinning and the strain before the tool is cut.

The process sequence

A deep drawn part usually moves through a sequence rather than a single stroke. The sequence is designed around the draw ratio and the features that have to be added after the wall is formed.

1. 01

   Blank

   A flat blank is cut to the calculated diameter. The blank quality and the edge condition affect the draw.

2. 02

   First draw

   The punch draws the blank into the die while the blank holder controls the flow. This sets the cup diameter and the wall.

3. 03

   Redraw

   The cup is drawn to a smaller diameter. The wall may thicken slightly and the height increases.

4. 04

   Trim

   The uneven top edge is trimmed. Earing makes the rim wavy, so trimming is usually needed.

5. 05

   Form details

   Holes, bosses, ribs, threads or a flange are added after the main draw, when the wall is stable.

6. 06

   Finish

   The part is cleaned, plated or coated, then inspected for wall thickness and cracks.

Design rules that keep the draw practical

The first rule is to keep the depth in proportion to the diameter. A cup that is much deeper than its diameter needs more stages and a larger press. The second is to give the punch a generous corner radius. A tight punch radius concentrates strain and can tear the wall at the bottom. As a starting point, the punch radius should be at least the material thickness, and often two to four times the thickness for a deep draw.

The die radius also matters. Too small a die radius restricts the material flow and raises the draw force. Too large a radius lets the flange wrinkle. A common starting point is four to eight times the thickness, adjusted for the material and the blank holder force. The flange should be as narrow as the function allows, because a wide flange needs more blank holder force and more material.

Wall thickness is set by the blank and the draw, not by a free choice. A deep drawn wall is usually thinner than the blank at the bottom radius and slightly thicker at the top. If the part needs a specific wall, say so and allow the toolmaker to adjust the blank and the stages.

• Keep the depth within the draw ratio for the material
• Use a punch radius of at least one thickness, more for a deep draw
• Use a die radius of about four to eight times the thickness
• Keep the flange narrow and the corner radii generous
• Put holes and bosses after the main draw
• State the functional wall and the allowed thinning

The failure modes to watch

Deep drawing fails in three main ways. Wrinkling happens when the blank holder force is too low or the flange is unsupported, so the material buckles instead of flowing. Tearing happens when the draw force exceeds the strength of the wall, usually at the punch radius or the die radius. Earing happens because the sheet has different properties in different directions, so the rim comes out wavy after the draw.

The first trial should check all three. A wrinkle that is pressed flat in a later stage can leave a mark or a thin spot. A tear that starts at the punch radius may be visible as a hairline crack or may only show up in a leak test. Earing shows as a rim that is higher at four points, which means more trimming and more scrap.

Materials, lubrication and tool wear

Deep drawing grades are chosen for ductility and a low yield strength relative to the tensile strength. Mild steel deep drawing grades, aluminium 1050 and 5052, copper C11000 and some stainless grades are common. The material should be annealed or in a condition that can flow. A hard temper will crack before it draws.

Lubrication is part of the process, not a shop habit. The lubricant has to stay on the blank through the draw and be removed before plating. A poor lubricant causes galling, which picks up material on the die and scores the wall. Tool coatings and polished die surfaces help, but they do not replace the right lubricant. For stainless and aluminium, the tool material and the coating matter as much as the press settings.

Tool material follows the same logic. A deep draw puts high contact pressure on the die radius and the punch nose, so those areas need a hard, polished surface. A coating such as titanium nitride or a diamond-like carbon can reduce friction, but it must be compatible with the material being drawn. On aluminium, a polished carbide tool and a dedicated lubricant often work better than a hard coating that can pick up.

Inspecting a deep drawn part

The inspection plan should measure the features that the draw can change: the wall thickness at the bottom, the middle and the top, the height, the diameter, the flange width and the corner radii. Check for cracks at the punch radius and for wrinkles on the flange. If the part is leak-tight, add a pressure or bubble test.

A cross-section through the wall is more useful than a single thickness reading. It shows the thinning at the radius and the thickening at the top, which tells you whether the blank and the stages are right. On a plated part, check the coating thickness at the wall and the bottom, because plating throws differently on a curved surface.