Conventional blanking requires high press forces to shear the sheet metal. As steel strength increases, snap-through tonnage also increases, potentially leading to press and tooling damage. Over time, clearances may change, affecting edge quality. Shearing results in a work-hardened edge, and a shear affected zone know to decrease edge stretchability.
Blanking dies are often among the last dies completed prior to launch. Historically, this led to the use of hand-cut, steel-rule, or laser cut blanks during tryout. When laser cut blanks of AHSS are used during try out, the high hardness in the laser cut edges may damage the blank holder surface. Control the laser cutting parameters to reduce burrs and edge hardness. Deburr the edges of laser cut blanks prior to forming to minimize tool damage, especially when using soft tool materials like kirksite.
Using laser cut blanks during tryout offers product flexibility, since blank dimensions may change as part of the development process. However, upon completion of the blanking die, the dimensions cannot change without costly modifications.
Instead of using a blanking die, some companies are choosing to use laser blanking in production environments. Laser blanking eliminates the cost of blanking die construction and tooling maintenance. A dedicated blanking die for each part is no longer necessary – one laser blanking line can create blanks for multiple parts. Blank shape changes become another way to solve stamping formability problems. This approach also has the flexibility of incorporating blank size optimizations as part of the cost savings efforts targeted through the life of a stamping. Simple programming changes optimize nesting to take advantage of coil width capability improvements made by the sheet metal supplier.
Curved sections on blanking dies present design challenges related to cutting tool steel placement to avoid the creation of a notch at the mating point between two adjacent sections. Maintenance of sharp edges and consistent clearance is another challenge–advanced steel grades damage even the optimal cutting tool steels over time. Laser blanking avoids these concerns.
Multiple heads within the laser blanking system allow for faster processing speeds. According to Citation F-6, typical outer body parts can run 12 to 25 strokes per minute (SPM) on a two-head configuration, while complex body sides tend to run 4 to 6 SPM. Thicker inner structural parts typically run in 6 to 10 SPM area, but configurations with six laser cutting heads increase this rate to 30 to 40 SPM. Daimler achieves the blank production rate of 40 hoods per minute.H-9 On a continuously moving coil, laser heads reach a linear cut speed of 100 meters per minute.S-27 These figures continue to improve with advances in laser technology.
Similar to a shear-affected zone, laser-cut blanks have a heat-affected zone (HAZ) comprising the area from the laser-cut edge into the blank. The typical HAZ distance is under 0.2 mm on steel up to 2 mm thick, which is less than a comparable shear cut edge. In addition, the hardening increase is lower, and edge quality is more consistent than seen in sheared edges.F-6
Note that this discussion relates to the creation of first-operation blanks directly from coil feedstock. For a discussion on Laser Welded Blanks, visit the pages related to Tailor Welded Blanks.