Metal Casting Technologies : MCT-2NDQRT-2017
34 www.metals.rala.com.au Figure 2 shows a number of castings at mould knock-out. In every case, only one of the two top feeders has piped. This indicates the casting is a single feed area and as such only one feeder should be used. As a general rule, if a feeder doesn’t pipe it doesn’t need to be there and can be safely removed. Traditionally, the determination of feed areas has been established by a lengthy and expensive trial and error method potentially involving a number of tooling changes. Some experienced foundry engineers are able to identify the number of feed areas based on the calculation of a geometric modulus value for various parts of the casting. Once the geometric modulus values of potential feed areas has been calculated, a “Transfer Modulus” value is determined for the connecting sections to establish if it is possible to feed one section through the connecting section or if individual feeders need be used. The Transfer Modulus is influenced by such factors as metal chemistry, cooling rate, mould rigidity and inoculation practice and as such can be very difficult to determine, particularly on complex casting sections. Increasingly, casting simulation is being used to determine feed areas quickly and accurately by analysing a 3D model of the casting. The following is an example of this using a model of the above cast iron Bracket. The thermophysical data is assigned to the model and the casting “solidified” as shown in Figure 3. Based on the thermal analysis of the casting, accurate thermal modulus values are determined for every part of the casting. Next, with consideration of the chemistry, cooling rates, temperatures, metallurgical quality and mould rigidity, erhaps one of the most challenging aspects of designing casting method systems for grey and ductile iron castings is the need to determine the number and position of feeders. Unlike steel castings where individual feeders need to be positioned over isolated sections of longer freezing time to adjoining sections, with graphitic irons it is often possible, and if fact desirable, to use only one feeder per feedable zone. Due to the self-feeding effect of eutectic expansion, it is generally possible to achieve soundness in an isolated “heavy” section by feeding through adjoining sections of shorter freezing time. Typically, if the adjoining “thin” section has a geometric modulus greater than about 60% that of the heavier section requiring feeding, this is sufficient to ensure soundness. Placing two or more feeders in a single feedable zone usually results in only one of the feeders piping, generally the one which might be favoured by hotter metal from the gating system. Once one feeder pipes, pressure within the casting is equalised to that of atmospheric and hence the pressure differential required to puncture feeders is lost. Those feeders that don’t pipe will often have a region of porosity at or near the feeder contact. If a graphitic iron casting has multiple feeders and one or more of them don’t pipe, invariably those that don’t show pipe are not required. The pattern plate shown in Figure 1 has two grey iron Bracket castings mounted. They are gated into the bottom of the castings and there are two “cold” feeders off the top of each casting. See Figure 1. FIGURE 2. Castings at mould knock-out, in each case only one of the two feeders have piped. METAL Casting Technologies 2nd Quarter 2017 35 BacktoBASICS Determination of Feed Areas in Graphitic Iron Castings J. F. Meredith Casting Solutions Pty Ltd P FIGURE 1. Pattern plate for cast iron brackets. Casting Solutions Pty Ltd PO Box 131, Moorebank, NSW, 2170 Australia Tel: +61 2 9792 3782, Fax: +61 2 9792 3782, Email: firstname.lastname@example.org, Mob: 0412 178 895 Contact: Jeff Meredith – Director Category: Foundry Consultants Products: SOLIDCast, FLOWCast, OPTICast simulation software. www.finitesolutions.com Services: l metallurgical l solid modelling l flow, solidification and optimisation modelling of castings l casting methoding service l defect analysis l training “AS A GENERAL RULE, IF A FEEDER DOESN’T PIPE IT DOESN’T NEED TO BE THERE AND CAN BE SAFELY REMOVED.” SOME EXPERIENCED FOUNDRY ENGINEERS ARE ABLE TO IDENTIFY THE NUMBER OF FEED AREAS BASED ON THE CALCULATION OF A GEOMETRIC MODULUS VALUE FOR VARIOUS PARTS OF THE CASTING. FIGURE 3. Model of cast iron Bracket is solidified.