During deep drawing, a part is formed by placing a selected material into a forming cavity where a blanking or forming punch forces the material into the shape of the mold. The resulting product has thin walls that cannot be achieved by other processes such as spinning, stretching, injection molding.
Because deep drawing can form thin-walled parts, it is often used in the manufacture of beer cans, shell casings for small-caliber ammunition, oil cans and many other products. It also provides higher stiffness and does not require additional thickness. In short, it can form inherently strong thin-walled components.
Deep drawing emerged in the Indus Valley Civilization around 1400 BC, making it one of the oldest metalworking methods in use today. It is mainly used to make flat round discs out of copper. The technique spread and was widely adopted in the first millennium AD. It is widely used in weapons production, including hammering iron directly against the hardened edge of forming punches, shaping iron into tools and weapons. In recent decades, deep drawing has expanded to include sheet metal forming processes, such as pressing a tubular shape onto a die.
Deep drawing with stainless steel offers several advantages over other materials. It is corrosion resistant, can form complex shapes, and can withstand high temperatures without losing its shape or strength. However, stainless steel has several disadvantages that need to be considered when designing deep drawn parts. Chief among them is that it is difficult to form and prone to warping or cracking under pressure. Various solutions have been developed to overcome these problems, allowing the production of high-quality parts that meet the requirements of a given application.
reduce wall thickness
Reducing wall thickness is the easiest way to overcome the problems associated with deep drawing of stainless steel, making the part easier to form and less prone to cracking or warping. However, reducing the wall thickness may also reduce the strength and stiffness of the part, so don’t overdo it. A good rule of thumb is to keep the wall thickness around 1/8″ (3mm).
Thick-walled parts are difficult to form using deep drawing. Deep drawn parts typically have wall thicknesses of about 0.007 inches (0.18 mm) or more. Few parts require a greater wall thickness than this.
This solution is simple and has the added advantage of low cost.
Note that part strength and durability may be compromised if the wall thickness is reduced. This option is not recommended for parts that require high stability.
Change the design of the part
In many cases, stainless steel deep drawing problems can be overcome by modifying the part. For example, you can reduce its overall size and/or complexity. This may include streamlining internal details, reducing bolt bosses, other protrusions, reducing the size of ribs and flanges.
The smaller and more streamlined the design of the part, the easier it is to form, and the less likely it is to warp or crack under pressure. It also means less material must be removed from the workpiece before deep drawing, reducing costs and waste.
This solution is relatively simple and low-cost, and it can be used without major changes to the part design, which is very helpful.
The downside is that it may not be suitable for all applications, especially those requiring high strength and durability.
Lubricants prevent metal from sticking to the mold walls, allowing the part to exit the mold smoothly. This minimizes surface defects on the part. While many deep-drawn parts can benefit from lubrication, special care is required for small parts with thin walls. Use organic lubricants such as oils, greases, waxes (including PTFE-based compounds) and inorganic lubricants, including graphite, molybdenum disulfide, and boron nitride.
You can use almost any lubricant as long as it is compatible with stainless steel and does not cause any adverse effects such as corrosion.
The advantages of lubrication are that it is inexpensive, easy to implement, and requires no design changes.
The downside is that some lubricants can compromise the quality of the parts produced, so be careful to choose the right lubricant for the application at hand. Cycle time increases because it takes time to lubricate and clean the mold after each part is struck.
Choose a softer grade of stainless steel
Using softer grades of stainless steel can overcome some of the problems associated with deep drawing. Softer stainless steel is easier to form and less likely to warp or crack under pressure. However, softer stainless steels are less durable and less resistant to corrosion than harder stainless steels.
The most common grades of stainless steel used for deep drawing are 304 and 316. The former is more complex and more resistant to corrosion than the latter, but it is also more challenging to shape.
The advantage of using a softer grade of stainless steel is that it is cheaper and easier to form, which means shorter cycle times.
The downside is that the part has less durability and corrosion resistance than parts made from harder grades.
The annealing heat treatment process softens the metal, making it easier to deform. Annealing can overcome problems associated with deep drawing stainless steel parts, but must be performed correctly to be effective. Annealing can make things worse if done incorrectly.
The best way to anneal a stainless steel part is to heat it to around 1000 degrees Fahrenheit (538 degrees Celsius) and then cool it slowly in a furnace or other controlled atmosphere. This will cause the metal to recrystallize and become softer.
The benefits of annealing stainless steel include making the metal easier to form and reducing warping or cracking under pressure.
The downside is that this is a relatively expensive process and some parts cannot be successfully annealed due to their design or geometry.
Use a forging process
Forging is another process that can be used to overcome the problems associated with deep drawing stainless steel parts. It involves compressing metal into a mold under high pressure, causing it to plastically deform and become stronger.
While forging can produce parts of any shape, round or square are most commonly used and require specialized equipment.
The benefits of forging stainless steel parts include making the metal stronger and easier to form, which reduces warping or cracking under pressure.
The downside is that it is a more expensive process than deep drawing and requires specialized equipment.
Use a vacuum mold
Vacuum dies can overcome some of the problems associated with deep drawing stainless steel parts. It is a specially designed mold that has been evacuated to create a vacuum inside the mold cavity. This causes the metal to plastically deform and become stronger.
Vacuum molds are most often used for thin-walled small parts. Typical applications include medical equipment, electronic components and semiconductor components.
Using a vacuum mold has several advantages over other methods such as hydroforming and stamping. The metal is not in contact with any liquid, so there is no risk of corrosion. Lubricants are also not required, which reduces cycle time and waste.
The disadvantage of a vacuum mold is that it can only be used for small, thin-walled parts.