CAST IRON CASTING MANUFACTURING

Casting as an old technique is the quickest link between engineering drawing 2nd manufacturing. It provides us with the possibility of forming wide range of shapes with wide range of materials. Sand Casting is simply melting the metal and pouring it into a preformed cavity, called mold, allowing (the metal to solidify and then breaking up the mold to remove casting. In sand casting expandable molds are used. So for each casting operation you have to form a new mold. Basic Requirements for metal casting: 1. A mold cavity, 2. Melting process, 3. Pouring technique, 4. Solidification process, 5. Removal of casting, 6. Finishing draft allowance. Sand Casting is the most important and mostly used casting technique. To perform sand casting we have to form a pattern (a full sized model of the part), enlarged to account for shrinkage and machining allowances in the final casting. Materials used to make patterns include wood, plastics, aluminum, fiberglass, cast iron and some other metals. Wood is a common pattern material because it is easily worked into shape. Its disadvantages are that it tends to warp and the sand being compacted around it abrades it, thus limiting the number of times it can be reused (used for a small number castings). Metal patterns are more expensive to make, but they last much longer. For example aluminum is the most common metal to be used if” many castings are to be made by the same pattern. So selection of the appropriate pattern material depends to a large extent on the total quality of castings to be made. The size of the pattern depends upon the shrinkage during cooling and the Finishing allowance. Some special coating to prevent their destruction should coat patterns. Patterns have also some identifiers such as colors on them, each of which has different meaning that represent different treatments and requirements for the patterns. The color-coding for patterns in sand casting is as follows: 1) Red indicates that the surface of the material should be left as it is after casting. 2) Black indicates that the surface needs core and shows the position of the sand core. 3) Yellow indicates that the surface needs machining. The casting will be missing. To ensure that cores retain the correct arrangement core prints are placed into the mold. Some metal springs called densiments are placed into the mold Io provide uniform solidification of the metal throughout the mold. Nails are inserted into thin parts of the mold to reinforce them. After forming the mold cavity, an alcoholic liquid is sprayed aver the cope (The upper part of the mold) and heated with flames to harden and to dry the surface. Filling a metal box having two halves, which is called the flask forms mold. So mold is also made up of two halves, which is separated by a parting line. The reason for this is to remove the tasted part easier from the mold. The upper part of the mold is called the cope and the lower part called the drag. The cope and drag are prepared separately and when they are ready they unites and metal is poured into it through a canal called sprue, which transmits the molten metal via runner into the mold cavity. The runner should not be big because it will increase the amount of the waste metal. It should not be small because this enhances rapid solidification in the runner causing a blockage. At the bottom of the sprue there is a gap called well for the collection of the unwanted sand, which comes with the flowing metal. There is also a riser system, which acts as an inventory of molten metal when the mold cavity is fulfilled with the metal and feeds automatically the cavity of the part that we want to get. This system is essential because as the molten metal cools down it shrinks so the amount needed to replace the shrinked metal comes fom the riser itself eliminating shrinkage cavities. A casting may show microporosity. This can be eliminated with directional solidification either by incorporating a metal chill into the mold or by tapering the thinnest section of the runner. Chills are also used around thicker parts of the casting to provide uniform cooling of these parts with the thinner parts to prevent cracks. Chills, by this way, preserve the mechanical properties of the whole casting. The steel is melted in electric-arc furnaces. The advantage of electrical furnace, the scrap steel, which was used before (metal left In risers and runners) can be melted in these furnaces and used again. When the furnace reaches the suitable temperature, it is turned off. The molten metal is filed into the portable reservoir called table and then table is moved to just above of the mold and metal is poured into the mold’s pouring basin. A powder is added to the mold’s surface to prevent metal’s rapid cooling during pouring. Another powder is sprayed aver the mold to form a blanket of inert gas to prevent the oxidation of the molten metal. The steel is melted in electric-arc furnaces. The advantage of electrical furnace, the scrap steel, which was used before (metal left in risers and runners) can be melted in these furnaces and used again. When the furnace reaches the suitable temperature, it is turned off. The molten metal is filed into the portable reservoir called label and then table is moved to just above of the mold and metal is poured into the mold’s pouring basin. A powder is added to the mold’s surface to prevent metal’s rapid cooling during pouring. Another powder is sprayed aver the mold to form a blanket of Inert gas to prevent the oxidation of the molten metal. The completed casting is left for cooling and when it completely cools down the whole flask is taken to a vibrating platform to remove the casting from the mold. Excess parts are cut either by oxygen if the casting is of steel (hard), or by hammering if the casting is of cast iron (brittle).