Deep Drawn Stamping Process
Deep draw is a highly specialized process in metal stamping requiring properly equipped press equipment and engineering expertise. At Larson Tool, we have built a reputation in the industry as deep draw experts for developing a proprietary process for deep draw stamping and customized products. As subject-matter experts, we feel a responsibility to share our expansive knowledge about deep draw stamping—which is why we’ve created this helpful guide on the topic. The more you know, the easier it will be for you to plan your next project and find the right metal stamping partner. Read along below in chronological order or jump to the section that interests you most:
Deep drawn stamping refers to the process of forming sheet metal under compressive and tensile conditions to into a cavity, producing a closed bottom, round, or irregularly shaped cup or cylinder. It should not be confused with stretch-forming. The material is actually forced into a plastic state as it is dragged over the die radius and down into the die. This compression process is done under calculated and very controlled conditions involving blank-holding pressures, punch and die radii, punch speed, and lubrication.
The end result creates a hollowed vessel, which can be cylindrical or square/rectangular in nature. Deep drawn products vary in size, from very shallow, requiring a single draw operation, to very deep, requiring many draw and re-draw operations. Deep drawn parts are very versatile and are used across a diversity of industries—from electronics to automotive and refrigeration, plumbing, and lighting. Items that have been deep drawn include assembly housings, industrial tanks, pressure vessels, and fire prevention devices. More “everyday” items that are deep drawn are appliance parts, containers, kitchen sinks, and fire extinguishers.
Deep draw stamping is optimal for products that require water- or air-tight enclosures (there are no seams), significant strength, and minimal weight. Often, it is the most effective way to form products that have unconventional or difficult geometries.
In terms production logistics, deep draw stamping provides many benefits:
- Increase speed. Draw speeds are limited only by the contact velocity required by the material. If multiple draws are required to produce the finished draw, they can be performed simultaneously in stations of a transfer or progressive die, optimizing cycle times. If you have large volume needs and want to keep costs down, these rapid cycle times are an efficient way to go.
- Lower costs. Along with the productivity savings mentioned above, the tooling costs for deep draw are less, typically requiring just one set of simple tool steel or carbide dies which are very durable and renewable, diminishing the need for re-tooling.
- Maintain tolerances. The deep draw forming process is extremely repeatable and once set, can produce hundreds of thousands of parts within a tight tolerance range.
- Exceptionally strong parts. Unlike casting or machining, rolled metals have an elongated and cohesive grain structure. Due to the nature of the draw forming process, that structure flows uninterrupted throughout a deep drawn part, which results in a very strong finished product.
- Metal diversity. A wide range of metals can be deep drawn, providing a number of manufacturing options. The most common metals include mild steel, stainless steel, aluminum, copper, and brass. A knowledgeable stamper should be able to offer assistance in choosing the best material or alloy for your application.
The two stages of a draw are cupping and drawing.
When the punch first contacts the blank, the nose of the punch initially embosses the material into the die. Some stretching occurs at this point and produces what is known as a “shock line.” This is a pronounced area of thinning around the radius at the bottom and just up into the straight wall of the shell. Depending on the shape of the bottom, the material may still be near original thickness across the bottom face (flat bottom) or thinned out by a stretching action (spherical bottom). As the blank is pulled into the die, the material at the circumference gathers and the wall progressively thickens. As the blank is pulled in to near shell diameter, the material thickens to as much as 10% over the original thickness. Clearance must be provided for this thickening to occur so that the material will not get bound up between punch and die. In addition, the punch must be tapered so that the finished shell can be stripped off. Therefore, a drawn shell will taper from bottom to top. It is possible to minimize this through subsequent sizing operations, but not eliminate it entirely.
The blank used to produce a shell is cut from rolled strip material with a grain structure elongated across the blank in the direction of rolling. Since this cross-grain does not pull into a drawn shape evenly from all directions, stresses are induced in the shell wall. Due to these uneven stresses, a drawn shell will not be perfectly round. A flange added to the top of the shell will minimize this, but the smaller the flange, the less strength it has to keep the shell round.
Since the original blank is so altered by the deep draw process, the wall thickness cannot be specified in terms of mill tolerances. Depending on application, there are three ways of specifying the thickness of material in a shell:
- Thickness of sheet or strip material to be used – In this case, the finished cylinder wall thickness will vary, typically +/- 10%.
- Minimum wall thickness – In this case, the sheet or strip material will be determined either by calculation or design development.
- Maximum wall thickness – As above, calculation or development will determine the starting material thickness, which in either case, will determine material use and cost.
Wall thickness can be specified in more detail, but only after development work has been done with the draw process. Since the material is formed around the punch, shells are typically dimensioned to the inside diameter, with taper allowed from bottom to top. Alternately, the shell can be dimensioned to the outside diameter with the maximum size found at the top or open end, and tapering down to the bottom.
If a straight shell with no flange is required, the shell will be “pinch-trimmed”—that is, trimmed flush with the outside diameter. Since the shell has a radius at the top, the remaining trimmed edge will have a partial radius from the inside, abruptly ending in a somewhat sharp outer edge. Also, since the die must have enough clearance to accept the shell, there will be a slight flare at the top of the shell. The bottom of a shell can be pierced out in a similar manner to produce a tubular part, but the same pinch-trim principles apply to the inside diameter. If a straight, cut-off edge is desired, it would require a secondary machining or cut-off operation and should be specified on the part drawing. You can view specifications of the deep draw presses and progressive die presses on our Facility page.
A leading manufacturer that designs and builds gerotors for use as pumping and motor elements contacted us to help in the development and deep draw stamping of parts.
Our customer encountered a potential problem after the first production run. There were concerns about the surface roughness of the inner cylinder failing. Working closely with their engineering staff and our material supplier, we were able to find a solution and satisfy the production needs of this customer and continue to do so by delivering this part to specification—every time.
So, what was the solution and how did it come about? Read this case study now.
For almost 100 years, Larson Tool & Stamping Company has been a valued supplier of high volume, precision metal stampings and assemblies to hundreds of U.S. companies. We offer a wide range of capabilities including forming, stamping, deep drawing, assembly, brazing, painting, coining, and more. Larson delivers high-quality, cost-effective solutions.
As industry specialists in deep draw, we have a massive capacity to produce deep drawn shells in progressive, as well as automated transfer equipment. Larson Tool works with our customers from the earliest stages of design to optimize your part design for the metal stamping process and determine the best materials, tooling, and process solutions for your product. We’ll leverage our years of experience in the industry to help you with all your metal stamping needs. Contact us today for help regarding your deep draw stamping initiatives!