Modelling of Magnetorheological Fluids for Blank Holder Control in Stamping

Given the increasing demand for precision and geometrical accuracy in sheet metal forming processes, the control of process parameters guarantees product soundness and the meeting of dimensional and geometrical specifications, avoiding defects such as wrinkles and tears. In recent years, the development of control and actuation systems for stamping processes have been concerned mainly with the blank holder (BH), the device that regulates the flow of material by holding the sheet flange during forming. The choice of BH actuation type depends on process characteristics (e.g. available space, required load, available working stroke, etc.), and its implementation often results from a trade-off between process requirements and actuator characteristics. Indeed, conventional BH actuators suffer from intrinsic limited re-configurability in the case of small batch production or drift in process parameters. In this context, Magneto-Rheological (MR) fluids represent one of the most versatile and promising solutions for the development of rapidly configurable and controllable closed loop systems. Technologies based on MR fluids have been applied mainly in automotive and civil anti-seismic systems, although recent research has shown promising applications in vibrations damping systems for metal cutting and blanking. The main objective of this work is to apply innovative MR fluid-based devices to BH control in sheet metal forming processes, with the aim of exploiting the advantages of these materials and overcoming the limitations of conventional actuators. A new semi-active actuator for BH control is designed and considered as a reference case for the investigation of MR fluid behaviour. A new approach is introduced, based on numerical modelling in a multi-physics environment, with the definition of a viscosity model able to describe MR fluid behaviour. Open-loop and closed-loop algorithms are designed to control MR fluid behaviour, with the aim of fine-tuning the actuator load in respect of process measurements. The numerical model and algorithms are then experimentally tested and validated. Building on these bases, the application of MR fluids and evaluation of relative performance is extended to an experimental test, which reproduces the deep drawing process. In this scenario, the whole system, composed of the MR fluid actuator and the control algorithms, is implemented to control the BH, demonstrating good capabilities in controlling metal flow and improving the geometrical accuracy of the stamped parts.