Metal Casting Technologies : Whos who September 2012
46 www.metals.rala.com.au very last stages of cavity filling. The mechanical type of valve is highly efficient since it extracts air until the end of the cavity fill, but its weakness is in the complexity of the mechanical shut off mechanism, which can be prone to blockage in the harsh die casting environment. Conventional chill vents are also commonly used with vacuum, but their weakness is the small evacuation area which restricts the rate of air evacuation. Typically the gap in the washboard area is less than 1mm in depth and about 100mm in width. To increase its evacuation area, the chill vent has to be made wider. This will increase the die projected area, and as a consequence increase the risk of die flash. The simple vacuum system and the conventional chill vent used with vacuum offer low evacuation efficiency. Considering the additional cost and maintenance, die casters have limited incentive to adopt these vacuum systems. The mechanical valves offer lower reliability and productivity due to unpredictable machine stoppages. CASTvacTM The development of a new vacuum valve was an outcome of a challenge posed by industry to have a vacuum valve that was immune from the blockages they experienced with their mechanical shut-off valves. The aim of the development was therefore to design and construct a valve which was reliable and whose evacuation efficiency could match the mechanical valve. To achieve this reliability, an ideal design would be for a valve with no moving parts. The mechanism of the conventional chill vent was naturally considered (a so-called ‘valveless’ approach). However, increasing the venting area for a conventional chill vent can only be achieved by extending the width of the zigzag washboard since the gap size has to remain small in order to terminate the flowing molten metal. In addition, to avoid die modification, our industry partners specified the new valve would need to be compatible with the existing die pocket dimensions for the mechanical valve they were already using. An innovative concept was developed by constructing a folded washboard with its major chill faces at a right angle to the die parting face, instead of parallel to the parting face of the conventional chill vent. This would allow the new valve to have nearly four times the venting area of the conventional chill vent using the same die footprint. Such an increased venting area enables vacuum performance to meet that achieved by mechanical valves. This innovation has been patent protected. The product from this innovation was named CASTvacTM, as it was developed by CAST CRC Limited. A CASTvacTM valve is illustrated in Fig. 1 (the two halves side-by-side) and Figure 2 (two halves semi-closed). It can be imagined that a continuous venting pathway will be formed when the two halves are engaged. An additional benefit of CASTvacTM is that the venting area is increased without sacrificing the die projected area which is an important resource for the die designer. This is a result of the forces generated from the metal pressure acting on the major faces of the washboard, which are directed at right angles to the die parting face, counteracting each other. For instance, the machine clamping force required to hold the same venting area will be reduced from 360 tonnes for a conventional chill vent to 87 tonnes for a typical size of CASTvacTM valve (calculated from a metal pressure 80MPa and the available venting area). The valve has been implemented in production for six years and made over one million shots on 800-tonne to 2500-tonne TEcHNicAL fEATURE Fig. 1: The CASTvacTM fixed and moving half valves. Fig. 2: CASTvacTM in semi-closed position.
Whos who September 2011