metal stamping process
Sheet metal parts are all around us: motor-parts, metal desks, LED parts, building-hardware, 3C-products, auto parts, aircraft fuselages and so on. They are lightweight, strong, and can take on complex shapes. The sheet metal stamping process is characterized by very high production rates, low labor costs, but high equipment and tooling costs Thus, this process is ideally suited for high-volume production.
The sheet metal blanks used in stamping are typically made of low-carbon steel, because of its low cost, good strength and excellent formability. The formability of various sheet metals is typically determined by marking the sheet with a grid of small circles and then stretching it over a punch. The deformation of the circle's ins measured in regions where tearing has occurred and used to construct a forming-limit diagram.
In applications where lightweight is important, aluminum, or alloys of steel with magnesium and titanium are also used, these lighter-weight materials are typically more expensive and less ductile and harder to from. Thus, developing tooling to produce defect-free parts using these lightweight materials is difficult.
A typical stamping press consists of a punch (upper of stamping mold), a stamping mold, and blank holder (binder), which holds the sheet metal in place during the punch stroke (i.e., while the punch is lowered into the stamping mold). The stroke (e.g., 50~80 mm) depends on the desired part geometry.
The stamping mold design will often include appropriately placed draw beads to help regulate the material flow into the stamping mold.
Commercial stamping operations are typically done at high pressure and high speed, thus, leading to short duration. The sheet metal material is plastically deformed, and flows into the stamping mold cavity and conforms to its shape. Proper design of the stamping mold allows complex shapes to be produced rapidly and cost effectively. Blank holder force and punch force must be properly selected to hold the sheet metal blank in place, through friction forces, while still allowing the sheet metal to plastically deform and flow into the stamping mold cavity.
Typically, stamping mold design, and selection of nominal process parameters is based on a finite element analysis (FEA). A variety of presses (e.g., mechanical or hydraulic) can be used, and nitrogen cylinders are typically placed under the stamping mold as a cushion to absorb the energy of the punch stroke and provide blank holding forces. For complex parts, a stamping line with several presses and stamping molds is used. Stamping molds can be quickly removed from the press to enable the production of a variety of different parts on the same press line. A more detailed discussion of the stamping process and equipment used.
Typical quality problems in sheet metal stamping include wrinkling (due to compressive stresses), tearing (due to tensile stresses) and spring back (due to elasticity). If the binder force is too high, locally in a particular area of the stamping mold, then the flow of material into the stamping mold is restricted and tearing is likely to occur in that region. If the binder force, again locally in a particular region, is too low then excessive material flow can lead to wrinkling. Spring-back can be accounted for in the design of the stamping mold, as well as by varying the punch force during the stroke to set the part geometry.
Mechanical presses typically allow the operator to set the desired stroke, speed, and blank holder force. The blank holder force typically cannot be adjusted around the stamping mold, and it is not possible to control the binder or punch force during the short time in which the part is formed. Hydraulic presses, depending on the design, do provide some additional flexibility in setting the blank holder and punch forces during operation. Thus, the control capabilities of stamping presses are limited, and they require significant trial and error during the stamping mold tryout process to establish the best settings for use in a production run. Furthermore, during production, factors such as blank material thickness and formability variations, and changes in lubrication, can act as disturbances and lead to quality problems