Metal Casting Technologies : MCT MAR 2018 (1ST QRT)
36 www.metals.rala.com.au METAL Casting Technologies 1st Quarter 2018 37 accuracy of the 3D model, which provides more exact geometry than is usually considered. Another is that, by running a simulation, thermal effects such as heat saturation of cores are accounted for explicitly. It would also be possible to place chills on the casting model, and even gating for a mould-filling simulation which would accommodate the effects of heat loss during filling. All of these effects are difficult to account for explicitly when performing traditional design calculations. It is generally recommend that, after a rigging design is completed using this procedure, a verification simulation be run with the actual geometry of the calculated risers. However, it is expected that the percentage of “first-time” simulation successes would be considerably higher using this approach. As an example of how this approach might be applied, consider the casting model shown in Figure 1. Designing the risering for this casting begins with selecting the casting alloy and mould material, which in this case are ductile iron and green sand. A Finite Difference mesh is generated, and a simulation is run with no risers attached. The result of this simulation is shown in Figure 2 as a plot of solidification time throughout the casting. The next step is to apply the formula which converts the simulation data to modulus values. This is done simply by clicking a button, as shown in Figure 3. This operation calculates a modulus value for each point within the casting. After this calculation is performed, the pattern-recognition algorithm is then applied to the modulus values, so that individual feeding areas within the casting can be identified and the result is a display which indicates the number of suggested risers to produce this casting. In this case, the system has suggested a single riser in the position indicated in Figure 3. The modulus data and the volume of each of the feeding areas can be displayed, based on the results of the modulus and pattern calculations. Along with consideration of the BacktoBASICS solidification time of any arbitrary riser with the casting. The answer to this question was the development of a calculation which could convert the solidification times in the casting to equivalent thermal modulus values. This would allow the user to directly compare a riser with the casting, since a modulus value for a riser can generally be calculated easily. In order to develop such a procedure, it was necessary to devise a formula which would take into account the wide range of properties of the diverse array of casting alloys which are poured today, so that the resulting modulus values would be accurate no matter what alloy was being poured. Another question to be answered was whether, given an array of modulus values within the casting, a system could be devised which would recognise separate feeding areas within the casting and thus give advice as to how many risers would be required, and where they should be located. This has been accomplished by development of pattern- recognition software which is able to locate isolated areas of high modulus values, which are essentially “hot spots” in the casting which need to be fed. In some more rangy castings, there may be many such areas; therefore the system must be able to discriminate between very small areas which don’t need feeding, and larger areas which do. The level of discrimination can be set by the user, by adjusting a “slider bar” from a “Less Sensitive” position to a “More Sensitive” position. Once the individual feed areas are identified, an appropriate riser can be sized for each of these. Since the maximum modulus value and the volume of each feeding area is known, it is relatively simple to apply known rules (as discussed above) to calculate the correct size riser for each area. Also, by plotting the location of the maximum modulus values within each feeding area, we can pinpoint the required attachment point for each riser. Of course, the actual attachment point is subject to considerations such as parting line location, ease of removal, machine locating points and other practical issues. Using this methodology, what amounts to an almost automatic calculation of required risering for a casting is achieved. The starting point is a 3D model of the casting, which can be transmitted from a CAD system. Then, the alloy and mould material are selected and a simulation with no risers is run. With a few clicks of the mouse, the system then analyses the simulation results, calculates modulus values, and suggests the number and location of required risers. The details of each riser are then provided by calculations which embody riser design rules based on modulus and volume requirements. There are many reasons that such an approach provides more accuracy than traditional design calculations. One is the FIGURE 4. Riser size calculation based on 2 castings per riser. FIGURE 3. Plot of riser number and location. FIGURE 2. Plot of solidification time. FIGURE 1. Model of casting. FIGURE 5. Determination of gating requirements. FIGURE 6. Model of fully rigged design with 4 castings per mould. THERE ARE MANY REASONS THAT SUCH AN APPROACH PROVIDES MORE ACCURACY THAN TRADITIONAL DESIGN CALCULATIONS. ONE IS THE ACCURACY OF THE 3D MODEL, WHICH PROVIDES MORE EXACT GEOMETRY THAN IS USUALLY CONSIDERED.
MCT DEC 2017 (4TH QRT)
MCT JUN 2018 (2ND QRT)