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Metal Casting Technologies : March 2006
45 APPLICATION LIMITATIONS: Problems associated with knock-off cores are generally caused by failure to heat the core sufficiently either because the core is too thick or the metal temperature too low. The core must absorb heat from the surrounding metal in order to reach a temperature greater than the liquidus before the alloy reaches that temperature. The heat required for this must come from the superheat of the steel and not its latent heat. It is important to ensure there is sufficient superheat to achieve this requirement and the core is of the correct thickness. Shrinkage is often encountered under knock-off cores when they are positioned on casting sections which are less than the feeder diameter, again the problem is caused by inadequate heating of the core. If knock-off cores are to be used on thin casting sections, then the section should be padded to ensure adequate heating on both sides of the core. Segregation of carbon and other alloys is sometimes encountered at the contact of knock-off cores at a greater level than is normally experienced with full contact feeders. This problem can usually be minimised by increasing the orifice diameter to 3.0(Mc). Carbon segregation is often accentuated by the use of carbon containing anti-piping compounds and the use of such materials should be avoided in alloys where this could be a problem. ● Fig. 2. Application of a knock-off core on a steel casting. Fig. 3. Application of a knock-off core on a steel casting. Fig. 4. Application of a knock-off core on a steel casting. METAL Casting Technologies March 2006