- Education and Science
Sand Casting and Its Processes
What is sand casting? Sand casting is the process of producing objects called castings with the use of sand as the molding material and molten metal which is obtained after heating to high temperature as the casting liquid. This write-up is based on my experience at National Metallurgical Training Institute where I did my six months industrial training. Casting is carried out in a place called foundry. It is a place set apart for casting of metal of various shapes. The processes involved in metal casting include placing pattern into a molding box, filling with molding sand, pouring of the molten metal and solidification, which results to the formation of castings.
Pattern to be used for casting is placed inside a molding box. A pattern is a structured or shaped object which forms the cavity that results to real casting after pouring of the molten or liquid metal. Let me give an advice, do not enter foundry workshop without taken all safety measures into consideration. Foundry is not a place for playing or unserious business. Patterns can be made of certain materials. There are plastic, wooden and metal patterns. Most widely used of all these three is wooden pattern. The pattern I made use of during my casting process was made of wood. Flat part of any pattern is the one that is placed on the floor of the foundry which is covered by a molding box. In the other words, the part of the pattern that is in close contact with the floor is the flat portion. This is done to enhance the stability of the pattern. Note that the two parts of pattern are cope and drag. Cope forms top part of a mold or pattern and drag is bottom part of a pattern.
Filling of Molding box
There should be proper mixing of the molding sand before filling takes place. The sand has good plasticity property. This property prevents it from collapsing when it is raised up after compacting in the molding box. When you are mixing the molding sand with water, be careful not to make the sand waterlogged. I said this because molding sand with excess water will easily collapse from the molding box. When you measure out the proportion of the molding sand to be used, measure out the appropriate quantity of water as well. After proper mixing of the two, spread parting powder on the top of the already placed pattern. Parting powder is spread so that the pattern can easily be removed from the mold. The box containing the pattern is finally filled with the mixed molding sand. The filled sand is rammed. Ramming is the process of compacting molding sand in molding boxes by the use of a tool called rammer. Rammer is a tool for compacting process which has rectangular or square shaped head. Some molding sand is added after the ramming until the molding sand is in equal level with the molding box. Scraper is used to scrape out the remaining or excess molding sand to maintain equal level with the molding box. The box is inverted to form the second part of the mold. The second part of the molding box is placed and small hollow pipes that form the runners and risers placed vertical straight. You then lock the molding box with location pins. Location pins are used to situate the two halves of the molding box in good position. After filling and ramming of the molding box, vents are made from the top mold down to the bottom half. These vents are made for easy emission of gases when molten metal is poured into the mold cavity. When the upper part of the mold, risers and the runners have been formed, the location pin is now removed and the two halves separated. The pattern is removed and ingate system created. Ingates are channels through which molten metal flow round the mold cavity. The mold is left for some time to solidify. The solidification of the mold can take day or days. The time taken for the solidification is dependent on the size of the mold.
The molten metal I used during my industrial training was the one obtained from Aluminum. The Aluminum scraps are melted in crucible furnace. The more the scraps melt; the more scraps are added in order to generate large quantity of melts. The melting of the Aluminum takes place when the scraps have been heated to a temperature of about 650 degree Celsius. The source of heat to the crucible furnace can be lumps of coal. I conducted my own melting in chimney hood with coal as the source of heat supply. Dross can form on top of the molten metal. Dross is impurities that are visible when metals are melted and they settle at the top of the molten metal. The dross is removed with a tool called skimmer.
Before pouring takes place, the mold must have solidified and it is covered and locked with location pins. Molding boxes that do not have location pins are joined together with a type of clay called fire clay. The molten Aluminum is poured through the runners. The identification that the mold cavity is filled is noticed through the risers. Once the molten Aluminum metal wants to start running out through the risers, stop pouring the melt. Risers perform three basic functions during casting process. They indicate that casting is done, help in emission of gases during pouring of molten metal and show that mold is filled. Flash can occur after pouring and solidification. It is an unwanted part of casting which results when a mold is not tightly covered. Finally, the casting that is formed at the end is machined and ready for use.
The above picture is a casting made through sand casting process. The casting metal used during the work was aluminium. The aluminium was heated to its melting point before pouring. When looked at critically, it is observed there are holes on the casting which resulted due to impurities in the sand. It is one of the disadvantages of sand casting process.