AHSS applications are growing, where the high strength body components help meet crash requirements, and thinner sections achieve weight reduction for improved fuel economy and lower overall emissions. Both higher strength and thickness reduction contribute to lower overall formability, higher forces, greater temperatures and accelerated die wear, and each of these outcomes reduce the size of the manufacturing window. To manage all of these challenges, the pressroom must implement advanced process control measures in order to minimize normal process variation, as the process is now less tolerant of this variation. The same holds true for die and recipe maintenance.
Understanding this reality, it’s important to make process decisions that enhance the forming window. This includes better die materials, surface treatments, surface coatings and data-based lubricant specification. Cross-functional communication is required to create awareness among employees in the level of process discipline critical to success. Controlling the inputs of the stamping process become vital to achieving predictable outputs, therefore process recipes should be defined and fixed across all shifts. This is the spirit behind process control, which defines the relationship between key inputs and outputs of the stamping process. Figure 1 shows critical stamping inputs, such as the steel, lubricant, press and die conditions. All of these contribute to final part outputs, and thus need to well-documented, maintained and understood for process reliability and repeatability.
Reference Panels are ideal for AHSS processes, and effectively establish process control of the forming operation. A reference panel is a draw shell that documents input settings and process outputs (forming strains, draw-in lines, trim scrap) at the time the panel was formed. It thus serves as a useful reference for each die-set, to compare to the current operational panel and process, ensuring that all inputs are stable. Visual changes in panel appearance or measured changes in strains or draw-in lines allow rapid detection of input variation, and process adjustments to be incorporated prior to incurring unacceptable panels (characterized by scoring, distortion, buckles or splits).
The use of reference panels should be part of a Formability System, characterized by systematic panel reviews, recipe confirmations, and die improvement / die maintenance planning. As an example, production realities are such that reference panels won’t be used if they are not stored in a protective rack, near the floor (so access is easy). Figure 2 shows an example reference panel rack system in a North American stamping plant.
All affected employees need to be trained on the importance of this tool, so panels are preserved and not discarded during housekeeping events. And a disciplined panel review process aligns production and trade personnel to ensure that the reference panels are used during each part run. Draw-in or thickness templates allow for formability measures on panels in very precise and consistent locations. Measuring these outputs on a regular basis (every part run) enables a determination of process stability and trends, which becomes important towards scheduling PM. Examples of draw-in templates are shown below in Figure 3.
Because AHSS products are susceptible to edge shearing from trim and pierce operations, these operations require advanced tooling and more frequent PM intervals. All die maintenance will be critical, requiring more frequent polishing, insert replacement (or reconditioning), surface treatment assessments to plan repair or reapplication, etc. Finally, tool and die personnel should attempt to define the size of the forming window with split-buckle analysis, varying shut height while maintaining all other variables constant to determine when unacceptable buckles or necking initiate. This information is then used to develop input (recipe) settings that achieve production stability and robustness.