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Metal Casting Technologies : March 2009
Back to B A S I C S Indirect external chills Indirect chills are those which are separated from the casting surface by a thin layer of sand. Their purpose is to reduce chilling severity or maintain casting surface finish or profile. Indirect chills work very slowly and are generally not recommended for light casting sections. They can be reasonably effective on heavy casting sections where the heat from the casting has time to saturate and transfer through the sand layer to the chill material. Internal chills Internal chills are usually used in isolated hot-spots where shrinkage is difficult to overcome by placement of feeders over the section or chilling with external chills. Internal chills are solid section of metal of similar composition to the casting alloy and they work by absorbing heat from the casting section hence accelerating solidification locally. The use of internal chills is generally more risky than either feeding or external chilling as there is usually a delicate balance between achieving complete fusion of the chill and adequate soundness of the casting section. Even relatively minor changes in chill section size or metal temperature can upset this balance. Problems most often encountered with internal chills include: 1. Blowing 2. Internal hot-tears 3. Inadequate fusion 4. Shrinkage Blowing is often caused by the use of dirty, rusty or damp chill materials. A high degree of care is required with the surface preparation of internal chills, surfaces should be shot-blasted prior to use and the chill inserted into the mould at the latest possible time to reduce the possibility of condensation on the surface. Surface coating of internal chills with nickel, copper or tin reduces the possibility of blowing but increases the cost of the chill. Foreign bodies such as slag and oxides can attach to surfaces of internal chills during casting; these foreign bodies can also be the cause of blow defects. Internal hot-tears are caused by the use of oversized internal chills. If the chill is too large to fuse properly, the chill material will expand inside the shell of metal solidified around it causing the shell to tear. Low pouring temperatures can also be the cause of internal tears again because of inadequate chill fusion. Inadequate fusion of the chill material caused by oversized chills or low pouring temperatures may be cause for rejection of the casting. This may not be the case if the chill is positioned in a boss or lug and removed during subsequent machining operations. 52 www.metals.rala.com.au Shrinkage can result if the internal chill is too small or the pouring temperature too high causing premature melting of the chill material rendering it ineffective. The use of multiple internal chills which are grouped together can also cause areas of shrinkage between the chills by cutting off feed channels. In order to combine adequate soundness and fusion the size and positioning of internal chills must be carefully considered. Generally, approximately 4-6% of the metal volume to be chilled is the correct size but this can be influenced by the position of the chill in the mould and the amount of metal flowing past the chill during casting. Tight control of metal temperature and fill rate is necessary to ensure consistent results. Chill aggregates Chill aggregates are materials, such as zircon or chromite sand, which have a higher thermal conductivity and thermal capacity than silica sand. Their effect as a chilling material is relatively small compared to metal chills but greater than that of silica sand (see table 1). Chill aggregates can be very useful materials as they are easily moulded to the profile of a casting section without many of the problems associated with external chills. Their effectiveness as a chilling material falls off sharply with increasing section size however, and they are relatively ineffective as a chill with casting sections greater than about 25mm. The thickness of chill aggregate materials should be at least equal to the casting section to be chilled. Because of their mild chilling action, chill aggregates are very effective in preventing hot-tears in unfed casting junctions and hot-spots. They are also effective as a buffer material between external metal chills and silica sand, or in pockets and corners where burn-on is likely to occur. ¦ Material Freezing time of 152mm sphere. (minutes) Copper Chill Steel Chill Steel Shot Chromite Sand Zircon Sand Olivine Sand Silica Sand 4.2 4.3 9.0 13.4 13.8 15.5 17.0 Relative solidification time for chills referred to silica sand 0.24 0.25 0.53 0.79 0.82 0.93 1.00 Table 1. Chilling effect of various materials (Locke & Briggs)