Deep draw forming
Understanding the art and science of deep drawing of steels | Sheet metal working is rather an expansive field, where there are more than 20 sub-topics, each seems to be a science and art by itself e.g. bending, straightening, nibbling, punching, cutting, slitting, slotting, embossing to name but a few. Amongst these operations, deep drawing of steel sheets is an exalted art and science too – we are all familiar with rather attractive products like kitchen sinks or fuel tanks in motorcycles, which are usually deep drawn. This article examines the process, benefits, limitations and enhancement of this beautiful process that stretches specified steels to their limits and offers durable products.
Deep draw metal forming is a complex process in which flat, sheet steel is formed into simple or more complex components. A circle or disc of sheet metal is first blanked from the coiled stock in the first station of the die. Then the flat blank is cupped around the nose of a punch and pressed into a die to form a cup.
During the drawing process, the metal is formed under pressure. Forces of tension (in the steel wall) and compression (in the shell flange) are induced as the steel is stretched by the “nose” of the punch into the die. These forces tend to thin the shell wall and thicken the shell flange. Depending on the length to diameter ratio, several draws may be required to create the basic shape of the part. In the drawing process several stepped diameters may also be created in the part. A wide variety of in-die stations can be developed to provide additional features. These may include coining, bottom piercing, multiple side piercing, extruding, embossing, marking, burnishing, ironing, chamfering, flange forming, shaped flange trimming, beading, reverse drawing, and rib forming.
The deep drawn metal stamping process gives deep drawn parts numerous advantages over traditionally machined, cast, or moulded parts. It can reduce costs and waste, lower assembly costs and time, and improve metal structure to produce a stronger finished part. It also provides a net shape that minimises waste. This means your part uses the minimum amount of metal required to deep draw the part, and you aren’t wasting money on metal that’s tossed in the scrap bin.
Processes that deep drawn metal stamping can replace are machining, screw machining, cold heading, die casting, investment casting, metal injection moulding, etc. Deep drawn stampings are generally produced from stainless steels, low carbon steels, aluminium and alloys, copper, brass, etc. Industries for which deep drawing technique works is automotive, lock hardware, power storage, HVAC, plumbing/builder’s hardware, power tools, small engines, indoor lighting, appliances, lawn equipment, fire protection, electronics, ammunitions etc.
As deep drawing is a process for forming sheet metal between an edge-opposing punch and a die (draw ring) to produce a cup, cone, box or shell-shaped part, it induces powerful stresses. It is often more cost-effective than spinning when the run quantity is very large and a high production rate is required. It is one of the most popular metal forming methods as it involves the use of metal dies to form blank sheets of metal into a desired shape. Specifically, if the depth of the item created is equal to or greater than its radius, then the metal forming process can be called deep drawing.
This process begins with metal blanks. Typically, single blanks are used in order to facilitate the creation of parts or products with deeper shapes. Sometimes, these metal blanks will be placed on a reel to enable the metal to form efficiently. At each step in the deep drawing process, the metal blank is shaped through pressure applied by a metal die. Though deep drawing is similar to metal stamping, the terms are not interchangeable. Stamping does not leave a single machine until the metal has completely formed. In general, deep drawing is used to fabricate parts and products that are deeper than metal stamping can accommodate, which is done in several steps.
The formability limitations of conventional deep drawing pose a barrier for some industrial uses. Radial drawing stress and tangential compressive stress are a common concern that can result in wrinkling, fracturing or cracking in some applications. Numerous unconventional deep drawing techniques have recently been implemented that have helped increase the industrial uses of deep drawing. These processes include hydro-forming, hydro-mechanical deep drawing, aqua-drawing, hydraulic deep drawing, etc.
Almost all industries can benefit from deep drawing manufacturing processes. It is most useful for manufacturing small component parts such as electronics relays, solenoids, and assembly housings. Products of all shapes and sizes can be economically created through the process, including items such as aluminium cans, cookware, and kitchen sinks.
Deep drawing process is especially beneficial when producing high volumes, since unit cost decreases considerably as unit count increases: once the tooling and dies have been created, the process can continue with very little downtime or upkeep. Tool construction costs are lower in comparison to similar manufacturing processes, such as progressive die stamping, even in smaller volumes; in these situations deep drawing may also prove the most cost-effective manufacturing solution.
When considering the functionality of the end product, deep drawing poses still more advantages. The technique is ideal for products that require significant strength and minimal weight. The process is also recommended for product geometries that are unachievable through other manufacturing techniques. Deep drawing is perhaps most useful for creating cylindrical objects: a circular metal blank can easily be drawn down into a 3D circular object with a single draw ratio, minimising both production time and cost. Production of aluminium cans is one example of a popular use of this method. Squares, rectangles and more complex geometries may create slight complications, but are still easily and efficiently created through the deep drawing process.
Typically, as complexity of the geometry increases, the number of draw ratios and production costs will increase. Deep drawing may be a viable production solution for any manufacturing process that requires one or more of the following:
• Seamless parts: deep drawn parts are created from a single sheet of metal
• Rapid cycle times: large quantities of products are easily manufactured through deep drawing
• Complex axi-symmetric geometries: deep drawing delivers exceptional detailing and accuracy
• Reduced technical labour: precision deep drawing can deliver similar results as technical labour in quicker time frames
If the manufacturing costs rise, the subsequent attractiveness of deep drawing could reduce. More complex products will obviously increase maintenance, labour and production costs. When considering the costs of deep drawing, factors such as number of part features, location of part features, direction of part features, protruding part features, etc. will likely increase expected cost.