Stamping, also known as pressing, is a process in which metals are formed into a desired shape. Stamping includes a variety of metal forming processes such as blanking, embossing, bending, flanging, and coining. Stamping is an economical way to form metal components with a variety of characteristics including strength, durability, and wear resistance. In this article, we look at the elements that are taken into consideration during the stamping process.
Examples of various types of metals (Picture source: www.garudapromo.com)
Ferrous metals are the most widely used material for general stamping and is known for having low carbon content which ranges between 0.05 and 0.20%. The low carbon content also makes this category of materials the least expensive to be used in stamping. Non-ferrous metals are metals such as magnesium, zinc, and titanium. Non-metallic materials are commonly used in blanking and piercing processes. Examples of materials in this category are fiber boards, papers, leathers, rubber and wood-based composition board. Various types of plastics are also subjected to this category of materials.
Example of a burred edge on a metal (Picture source: www.fourwheeler.com)
Burrs are produced as a by-product of the stamping process. In the manufacturing of plastic products, burrs are referred to as parting lines. In common manufacturing principles, burrs are not acceptable. This is usually due to safety reasons, either for handling of the product during manufacturing/assembly or for product safety. It is important to observe the difference between the two sides of a blanked or sheared metal sheet. This will help manufacturers decide the face over sharp edge needed and on which side the rounded or pulled down edge is to be made. The sharp edge of the metal sheet should be handled with extreme caution to avoid damage it cause to the other parts through its sharp edges. Generally, bent edges should be provided so keep the burr side on the interior of the bend.
In terms of aesthetics, burrs can cause discoloration and unevenness on the surface of the material that is being stamped. Hence, it is important for materials with burrs to go through another step called the de-burring process. In this process, the material goes through mass finishing processes or secondary operations to smoothen out the burred area.
Long, narrow projections must be avoided as narrow sections can cause the die punch to be thin and fragile. Besides that, narrow sections make it easier for the metal to be distorted, thus reducing product quality.
To save cost, the designer should recommend shapes that can be produced using standard existing universal bending dies. Creating new dies is not only difficult but is also a very time-consuming and expensive procedure.
Using connectors such as springs and other flexible sections, snap-fit elements, folding tabs, and press fixable designs can be incorporated into the design of the product to eliminate the need for other parts to assemble the metal, including separate screw fasteners or other fasteners. Other than that, welding methods such a projection welding and spot welding can be implemented to reduce the number of parts in an assembly and make the stamped metal itself easy to assemble. This will save cost and save time during the assembly process.
Figure showing sharp edge of a metal (Picture source: www.worldofsteel.com)
Presence of sharp external corners in punching tools or dies can often lead to premature breakage, causing larger burrs or rougher edges of the material. Furthermore, sharp interior corners of punches and dies are the point of stress-concentration. Higher points of stress-concentration can potentially cause cracking and failure due to heat treatment or through usage (wear and tear). This can lead to low quality standards and short product life, which is why both internal and external sharp corners must be avoided as much as possible.
Example of a punching tool that is used to inject holes onto the metal sheet (Picture source: www.thefabricator.com)
Many parts require holes, slots, or indentations for assembly purposes or sometimes, aesthetic purposes. In order to achieve this, the stamping process usually involves driving a sharpened punching tool through the sheet or strip material and into a die cavity. The holes that are pierced on the metal should be done in the same operation. This step is vital as it ensures hole-to-hole tolerance and part repeatability. The diameter of hole should not be less than the stock thickness of the metal used. The spacing between the holes should be a minimum of two times (x2) the stock thickness. The minimum distance from the edge of a hole to the adjacent edge of the blank should be at least equal to the stock thickness of the metal. If the spacing is too small, bulges will be resulted in the edge area adjacent to the hole. If this minimum distance is not observed, distortion of the hole will occur. A non-functional opening, either square or rectangular shaped, is pierced directly beneath the desired hole, if the design requires the lowest edge of the hole to be closer than the recommended minimum. Round holes are recommended as irregular shaped holes will incur additional costs due to the tooling cost.
In order to take a desired shape, the materials will need to go through a bending process so that it can be angled. However, in certain cases, distortion may occur during this stage. The material on the interior of the bend is compressed, while the material on the exterior of the bend is stretched. On thicker materials and bends with relatively small interior bend radii (x2 the metal thickness or less), thinning of material may occur from the result of the bending process. In addition to that, the compression the metal goes through during the bending process forces excess materials to exit at either end of the bend radius, creating what is called bend bulges. In a similar situation, when two adjacent sides of the metal are folded upwards (as in forming a box), some form of relief is needed at the base of the bend to avoid “pinched” corners. Usually, this would be in the form of a round hole placed at the convergence point of the sides. However, the bulges can be removed through contouring.
During the stamping process, the metal will be placed under extreme impact forces on the body of the punch. The punch tooling can fail if there is not enough cross section area to support this force. Hence, it is best to design the perforations with a cross-section area to allow the force to dissipate equally rather than being concentrated at one particular area.
Hence, when engineers and designers converge to manufacture a product, the thought-processes should not only be considered upon the aesthetics of the product but also the practicality and functionality of the product. This will make the manufacturing process easier while providing consumers a 2-in-1 benefit – a product with good looks and a purposeful function.