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Metal Casting Technologies : June 2006
34 METAL Casting Technologies June 2006 BacktoBASICS T From "Naked" to "Dressed" -- A Case Study of Casting Rigging Via Computer Simulation ABSTRACT his paper focuses on the practical approach of rigging steel castings using computerized casting simulation. While software has been commercially available for over 20 years to simulate the pouring and solidification of castings, it has mainly been used to 'test' rigging systems that have been developed using traditional shop floor trial and error methods. Only recently has software developed to the point where simulation can be actually used to design the methoding, not just test it. This process involves running an initial simulation on a 'naked' casting, that is, a part without any gating or feeding system. Results from this analysis are then used directly by the system to determine feeding zones on a part and design risers to supply the appropriate amount of feed metal to prevent shrinkage in those feeding zones. Casting alloy, mould material and feeding practice are all taken into account. Once risering is designed, the gating system is also developed; using actual simulation results, so that very little operator time and effort is required. Appropriate pouring times are determined, along with the dimensions of the downsprue, runner(s) and gate(s), so that the castings will fill in an optimum fashion, based on casting size and critical section thickness. If a bottom pour ladle is used, the system can also supply critical information to insure proper filling time and speed. The fully rigged model is then built and verified using full CFD-based fluid flow analysis and combined thermal-volumetric solidification analysis. Verification simulations assure that the rigging system will perform as advertised; that is, fill the mold in an appropriate way and provide good temperature gradients for directional solidification and a sound casting. If desired, the verified model can also be submitted to automatic casting/ process optimisation analysis to maximise the efficiency of the method, without sacrificing casting quality. A detailed case study will be used to demonstrate how this process is successfully applied on a daily basis in the foundry. INTRODUCTION Correct design of gating and feeding components for castings is essential for a foundry to be successful in the production of high-quality castings. In today's environment where customers demand that the lead time for new parts be as short as possible, foundries who can produce sound castings from the very start have a distinct advantage; proper rigging system design is the key to making this happen. Design of efficient gating and feeding systems for steel castings has been difficult for foundry engineers due to a number of factors. Chief among these is the complex geometry of many commercial castings; while there have been well- established design rules for a number of years, the application of these rules to a variety of commercial casting shapes typically involves cumbersome calculations that, when performed manually, require a number of simplifications to reality. These approximations can reduce the accuracy of the resulting designs. Even when rigging calculation methods are used, if the work is not integrated with a simulation tool, extra effort is needed to perform the calculations and the data used is not nearly accurate as simulation results themselves, which take into account such things as casting alloy, moulding materials and the like. With the advent of sophisticated software simulation systems in recent years, it has become possible to synthesize a number of the elements of good rigging design into a general method that is fast, thorough and highly accurate. This method overcomes many of the difficulties listed above. In addition, because of the automation involved, this method allows foundry personnel who may have limited experience (e.g., new foundry engineers) to effectively design casting process methods. In the past, some foundries have experienced major problems when experienced designers retired from the workforce; an automated design method lessens the impact of such an occurrence. David C Schmidt Finite Solutions Inc Slinger, WI USA J. F. Meredith Solutions Pty Ltd - Casting Solutions Pty Ltd STL model of a valve body casting.