Deep Draw Metal Stamping for Strength and Efficiency
The deep drawing sheet metal forming process is a cold-forming metal fabrication process that is conducted in a room-temperature environment. A metal blank is stretched around a die cavity, eventually conforming to the shape of the plug, shape of the die, and the depth of the draw. This process is ideal for applications that require their depth to be greater than their radius, with excellent ductility, durability, and structural integrity. Because of work hardening and material thickening that takes place during material flow through the forming process, deep draw metal stamping is the ideal process for applications being placed in harsh environments or used for heavy-duty jobs. Common materials used for deep drawing are stainless steel, CRS, aluminum, copper, and brass. Typical shapes that result from the draw are housings, enclosures, cups, or domes. There are many benefits to the deep drawing process, two key items being the strength of the parts produced, and the efficiency of the process.
Deep Draw Stamping: Durable & Long-Lasting Parts
High precision deep drawn parts exhibit enhanced structural integrity due to work hardening, the process in which the metal’s structure is compressed and stretched during the forming process, where material flow is controlled to gradually form over the die and plug.
Deep drawn metal stamped parts formed seamlessly from a single metal blank, resulting in a finished shape with fewer potential failure points and without weak joints that may occur with secondary forming processes such as welding. Deep drawn parts may be long and narrow cylindrical housings, with length significantly greater than its outside diameter. Being seamlessly constructed enhances its structural integrity, giving deep drawn parts superior load-bearing capabilities, while making them ideal for heavy-duty applications for demanding industries like Oil and Gas, Aerospace, and Marine environments.
Deep drawn seamless shapes with no welds enhances smoother product flow and enables the option of adding water-tight or airtight seals. These characteristics make deep drawn parts advantageous in food processing, plastics, and material handling systems. Their seamless design enables a leak-proof performance and allows for easy cleaning and sanitization when required.
Work Hardening: How Deep Drawing Increases Strength
Work hardening is a process where the metal becomes stronger and harder through plastic deformation, specifically during cold forming processes like deep drawing. This occurs around room temperature, below the material’s recrystallization temperature.
The recrystallization temperature is defined as the lowest temperature at which new, defect-free grains form and replace the existing deformed ones within a specifed time period. For most metals, this temperature ranges from around 30% to 50% of the metal’s melting point.
Understanding work hardening is essential in deep drawing because the process truly changes the metal’s structure without altering its chemical composition. The applied mechanical stress of the forming process permanently deforms the material. Resulting in increased hardness, improved durability, also oftentimes results in an enhanced surface finish.
Deep Draw Efficiency: Streamlining Production
The Deep drawing process also typically eliminates the need for secondary forming operations, as the final geometry can be achieved with one or two strokes of the press. The process can be fully or partially automated by setting the press parameters for precise and accurate control. With high repeatability and consistency, producing high-volume or low quantities will meet exact requirements and specifications while minimizing defects like wrinkling and tearing. The efficiency of the process reduces material waste and scrap, contributing to cost-savings and supporting sustainability initiatives.
Because of their seamless build, many deep drawn parts can be used nearly immediately after forming. Components often require no welding or further forming, allowing them to be cleaned, inspected, and shipped. Overall, deep draw metal stamping simplifies assembly and accelerates throughput, an excellent process for a multitude of applications.
Capacity: Small or Large Production Runs
With the use of automated and semi-automated presses, deep draw operations can be performed continuously, streamlining production and improving cycle efficiency. Parameters can be set on the press to draw the exact same shape, for large or small production runs. Depending on the part geometry, some applications can be formed from a single metal blank in one stroke of the press, while others may need a couple re-draws. This efficiency reduces the need for additional fabrication or secondary metal forming operations, minimizing production time and shortening lead times.
Mechanical Properties and Formability of Deep Drawing Stainless Steel
Good – Lowest cost of high quality deep drawing stainless steel material. The high nickel content allows for good deep draw working. Elong = 60%, Yield 34/30 KSI, Tensile 85/75 KSI
Good – More corrosion resistant than 304 grade due to the addition of molybdenum with the same higher levels of nickel, thus allowing for it to be an excellent deep drawing stainless steel. Elong = 60%, Yield 30 KSI, Tensile 75 KSI
Low – 400 series, generally lower cost stainless steel than 300 series since there is typically very little or no nickel, is not ideal for extremely deep parts since the lack of nickel reduces stainless steels deep draw ability. Elong = 30%, Yield 65 KSI, Tensile 95 KSI
Low – 430 is similar to 410 with little less strength. Like 410, material is readily available and is less expensive than 300 series stainless steel. Elong = 30%, Yield 45 KSI, Tensile 75 KSI
Low – 444 stainless steel is similar in its ability to form compared to 400 series stainless grades. Uncommon to 400 series, this specialty stainless grade has Molybdenum added with Titanium and Niobium added to stabilize the microstructure. This allows the material to be similarly corrosion resistant to 316, without the cost of 316 due to the lack of Nickel. The low Nickel also allows for excellent resistance due to corrosion cracking to chloride induced stress, which is why SS 444 is frequently used for parts in contact with tap water. Elong = 20%, Yield 40 KSI, Tensile 60 KSI
Frequently Asked Questions about the Deep Drawing and Metal Stamping Process
Deep Draw forming with conventional tool and die technology is the stretching of sheet metal stock, commonly referred to as a blank, around a plug in either a hydraulic or mechanical press. The edges of the blank are restrained yet allowed to slide by a precise pressure between two tool surfaces; normally in a ring shape. One ring is the blank holder and the other is the forming die. The plug passes through the blank holder ring into the cavity of the die ring at the desired depth to achieve the end shape. The dimensions on the part are set based on the shape of the plug, the shape of the die, and how deep the part is drawn.
- Calculate the blank size and optimize
One of the most common hidden costs associated with deep drawing is the scrap generated from producing blanks. When parts are produced in large batches in the thousands, or ten thousands annually, custom coil sizes can be sourced to reduce waste. If not, at TMS, blanks are laser-cut from sheet metal in-house. By optimizing the blank diameter and shape, excessive engineered scrap or unnecessary drops can be reduced or eliminated, significantly impacting the overall cost of the part. It is key to take into account extra material, sufficient enough for trimming and work holding in the blank size.
2. Design to minimize the material needed for trimming & work holding
It is normal for the finished part to be a different size than the initial blank size used to form the part. This is because of the necessary method to hold and process the part from start to finish. It is important to keep in mind that the final part may require additional material to accommodate secondary processes. At Toledo Metal Spinning, examples of secondary services offered are adding interior and/or exterior finishes or welding inlets or fittings.
3. During the design phase, plan for thinning
Throughout the drawing or deep drawing process, material the material may endure significant surface tension and stretching. This typically causes thinning near the closed end or first contact surface on the punch. It also results in thickening near the flange or open end. Accounting for this variation during the design process will ensure proper fit, reduce scrap, and improve part performance.
4. Design to allow for a taper
Because the punch and die require adequate clearance to accommodate mill tolerance on material thicknesses, there is a taper in their side walls. The taper is influenced by the material type, thickness, part depth, and punch-to-die clearance, typically ranging from 0.005 to 0.010 inches per inch of depth.
To minimize taper:
- Minimize the part depth
- Use tooling designed specifically for the application
- Select materials with better formability that thin less
- Accommodate material flow with larger draw radius
If the taper is not acceptable, secondary forming processes can reduce and sometimes eliminate the taper. However, this oftentimes results in an increased cost per part.
5. Select material that is formable
Selecting the most formable material and/or temper for the deep draw metal stamping process is crucial. Material selection can significantly reduce or eliminate scrap during the draw process. While some applications require higher stiffness or strength, optimizing for formability often lowers the overall cost.
6. Use existing tooling when able
As many manufacturers do, at Toledo Metal Spinning, we maintain a large library of existing tooling that can be of use. Like TMS, a company that chooses to invest heavily in their own tooling can produce shapes very close to designed dimensions. Sometimes, minor design modifications can eliminate the need for new tooling or engineering charges. For a re-cut charge, repurposing inactive tooling is another cost-saving option when dimensions align closely with a new part design.
7. Hydro-mechanical drawing (hydroforming) requires less tooling
Hydro-mechanical deep drawing, also known as hydroforming, is a process that uses hydraulic fluid as either a punch that pushes the material into the cavity or a die to push the metal over a punch. Though it is similar to deep drawing, it differs because a rubber bladder acts as the die with thousands of PSI behind it. When comparing hydroforming with deep drawing, the rubber blader pushes the blank around the plug, instead of stretching the material. This results in less stress on the metal, and allows for greater reduction ratios and more elaborate or complex geometries.
8. Uniform metal thickness is achieved by hydro-mechanical deep drawing (hydroforming).
Hydro-mechanical forming applies lower forces on the material, allowing more uniform wall thickness throughout the part. This is especially helpful for difficult to draw, irregular or asymmetrical shapes, where a metal die wouldn’t allow the metal to flow or change to be forgiving of the metal’s needs.
9. Optimize for a minimum number of draws.
Each time a part is deep drawn, its diameter or length/width are reduced. The number of draws required for the final shape is dependent on the material’s elongation capacity. For example, 14-gauge stainless steel can typically be reduced:
- 45% in the first draw
- 30% in the second draw
- 15% in subsequent draws after annealing (this additional process would most-likely be required to further form the drawn part)
With this in mind, a 10-inch blank could be drawn to approximately 3.27” in diameter after three reductions. Minimizing the number of draws helps to reduce cost and processing time.
10. Add features that leverage the benefit of in-house secondary operations.
Forming is often followed by cutting, finishing, secondary forming, or joining, processes that are necessary to complete the part. A deep drawing facility typically offers these services in-house, such as TMS, and can streamline production and reduce total cost, depending on the part design.
During the prototyping phase, reputable manufacturers like TMS, will conduct a design for manufacturing and assembly, or DFMA, to review and identify opportunities for cost-savings, and features that may assist in overcoming design or cost barriers.
Take into account the variety of processes the manufacturer is offering.
At Toledo Metal Spinning Company, we offer deep draw metal stamping and hydroforming. These processes are used for different purposes, and having an understanding of when and why they are used, could lead to significant cost savings, depending on the shape and use of the final part. Hydro-mechanical drawing, or hydroforming, utilizes hydraulic fluid, rather than a punch to push the material into the cavity or die to push the metal over a punch. The difference between hydroforming forming and deep drawing is that a rubber bladder is used, rather than a die, with thousands of PSI behind it. The bladder pushes the material to shape it, instead of stretching, resulting in less stress on the metal and allowing for greater reduction ratios or complex shapes.
Is a polish or any other secondary processing needed?
It is important to keep in mind that the final part may require additional material to accommodate secondary processes. At Toledo Metal Spinning, examples of secondary services offered are adding interior and/or exterior finishes or welding inlets or fittings. If a sanitary polish is needed, a fitting or inlet needs to be added, make sure to account for a few extra steps in the fabrication process.
If possible, plan to use existing tooling.
Many manufacturers like Toledo Metal Spinning Company maintains a large library of tooling in-house that can product shapes very close to the designed dimensions. Sometimes, when minor design modifications are needed, re-cutting an inactive tool when dimensions align close enough with a new part design. This can lead to significant cost savings, rather than making a brand new tool.