The Proper Casting Processes
The sand casting of any object in foundry workshop starts with the pattern making process. The pattern can be made of woods, plastics or metals. In Nigeria, the most widely used pattern by students in Metallurgical Engineering department is those made from wood. The question is: whose function is it to make these patterns? Who makes pattern used in foundry workshops. The answer is: the pattern makers make patterns. It is when these patterns are made that casting will proceed. Remove pattern from sand casting and precision casting in engineering seizes to exist. In the other words, without pattern, there will be no casting process.
The already produced pattern is placed into the moulding box in the first place. A moulding box is a constructed material or device which is used in the casting process. Moulding boxes can be rectangular or square in shape. This is usually made of two halves. It contains a location pin which is used for position before the proper casting process.
Sand which is used for casting is called the moulding sand. Appropriate quantity of water is used to mix the sand to avoid the sand from being waterlogged. In mixing the sand with water, the water is sprinkled on the dry moulding instead of pouring. When the water is poured, it can result to excess.
The pattern is placed in a moulding box. Note that the part of the pattern to be placed on the floor is the part that is flat. This is because the flat portion is more stable on the floor. A powder called parting powder is sprinkled for easy removal after moulding. The moulding box is filled with moulding sand. First, when the mould is filled to some extent, it is compacted with a casting tool called rammer. The compression is to avoid the mould from collapsing when it is raised up. After final ramming, the extra compressed moulding sand is scrapped out using scrapper. The lower part of the mould which is first made contains the drag.
For the formation of the top part of the mould, the mould which is first formed is then inverted and the second half of the moulding box placed. Pipes or hollow shapes which form the risers and runners are placed before filling again with moulding sand. These pipes that form risers and runners are kept erect and straight at the suitable positions. The moulding is filled with moulding sand and then rammed very firmly. Vents are made for emission of gaseous materials from the yet to be poured molten metal. The pipes which form the risers and runners are removed and proper ingates created.
Note that the runners are larger in size than the risers. What are the functions of risers in sand casting? In sand casting, the risers perform three principal functions:
- Channel for emission of gases;
- It indicates that casting is done when melt is poured through the runners;
- It indicates that the mould cavity is filled.
The pattern used in the mould is removed before solidification process. This is easily removed because of the parting powder poured on the drag and the cope before formation of the mould. Addition of parting powder can also be called dusting which is applying of powder to mould surface such as graphite to the mould surface (How to Cast Small Metal and Runner Parts by William Cannon) The next step that follows after this is to leave the mould to dry. The formed impression by the pattern is called the mould cavity. This is what forms the final output called the casting.
The molten metal is formed by heating the scraps to the melting point. Many metal types can be used, but Aluminium scrap is what is used in this write-up. The scraps are added into crucible and heated to the temperature of about 660.3 degree Celsius. This is the melting point of Aluminium. In some cases, dross is formed on the top of the molten Aluminium.
Dross is the impurity found on the surface of molten metal. The dross is removed with a tool called skimmer.
The pouring stage takes place when the mould solidifies after some days depending on the size of the mould or the
weatherof the day. The cope is placed on the top of the drag. They are joined firmly with fire clay. The molten metal is poured through the runner. The question that can come into play in this stage is: how do you know that the mould is filled? An indication that the mould is filled is identified through the riser. Once the melt starts coming out from the riser, it is an indication that the mould cavity is filled. The change from molten metal to cool solid casting takes place in three main steps (Metal Casting: A Sand Casting Manual for the Small Foundry, Volume 2 by Steve Chastain). These three stages are cooling of the molten metal from pouring temperature to the solidification temperature, removal of heat of fusion, the third and the last is cooling of the solid metal (casting) to the room temperature.
The sand casting is removed after cooling. It is then machined to obtain the final and original casting. Flash is at times formed when pouring; this is removed through the process known as fettling. The flash is formed as a result of weak contact between the drag and the cope. Fettling can be conducted by clamping the casting on a vice and then remove the flash with saw.