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Metal Casting Technologies : March 2006
44 www.metals.rala.com.au BacktoBASICS O "Knock-off " Cores for Steel Castings ne of the most time consuming and costly operations in the processing of steel castings is the removal of feeder heads by thermal cutting followed by washing and grinding of the contact area. The use of "knock-off" cores, sometimes also known as "breaker" or "Washburn" cores, can greatly relieve this dressing room bottleneck by allowing feeders to be broken off or at least greatly reducing the contact area to be cut of. A typical design of knock-off core is shown in figure 1. Knock-off cores are generally rammed-up as part of the mould making process or inserted into a core-print after stripping the pattern. A typical application is shown in the following figures 2, 3 and 4. Despite the obvious benefits associated with the use of knock-off cores, many foundries tend to limit their use due to variable results. For successful application of knock-off cores, the foundry must consider: ■ The grade and type of sand to be used. ■ The type and quantity of sand binder. ■ Dimensions of the core. ■ Shape of the aperture. ■ Application limitations. SAND: The extreme conditions knock-off cores are subjected to dictate the sand must be resistant to penetration and sintering. Therefore, a high purity, fine grain silica sand is recommended, typical fineness is within the range 65 to 110 AFS. Special refractory sands such as zircon or chromite are recommended for knock-off cores on large feeder heads, perhaps greater than 200mm diameter. BINDERS: The binder system must be capable of producing a strong, rigid, non-friable core, which is able to withstand the rigorous casting conditions and burn out completely after use. Organic binders are generally favoured for these reasons and most heat or gas hardened processes are suitable. Addition rates are generally higher than for normal core applications because of the high strength requirement of the thin wafer core and the need for slow burn out of the binder. Sodium silicate is not a suitable binder for knock-off cores as the bond is plastic at elevated temperatures causing deformation of the core. Also, sodium silicate tends to lower the refractoriness of silica sand often resulting in sintering problems and poor surface finish. Additives such as bentonite and iron oxide should be avoided, as they will also tend to lower the refractoriness of silica sand. DIMENSIONS: Knock-off cores must be dimensioned so as to not cause premature solidification of the feeder contact. To achieve this objective, it is important the thickness of the core wafer and orifice size is calculated based on the solidification time (or thermal modulus) of the casting. The following relationship between core thickness, orifice diameter and casting modulus has proved satisfactory under most conditions. Orifice diameter = 2.34(Mc) Core thickness = 0.56(Mc) Mc = casting modulus Knock-off cores are also commonly dimensioned based on the diameter of the feeder head they are used with; typical core thicknesses 10% to 12% of the feeder diameter and orifice 40% to 50% of the feeder diameter are used. This approach will sometimes result in cores which are too thick and orifice diameters too small for the casting section to which they are attached and hence cause premature freezing of the contact resulting in porosity in the casting. APERTURE SHAPE: The shape of the knock-off orifice can affect the efficiency of feeding and ease of removal of the feeder. Generally, round necks are used and give good results in terms of soundness and ease of removal. The use of shapes having sharp re-entrant angles such as stars and crosses can aid the breakability of the contact and for this reason are often used on alloy steels. J. F. Meredith Solutions - Casting Solutions Pty Ltd Fig. 1. Typical design of knock-off core.