A foundry is a place or factory where the production of casting is produced by the used of melted metal poured in a mold in order to solidify and produce a new shape. Foundry work is a process whereby casting is produced. In the engineering world, foundry work is highly specialized and recommended. It embraces many skills, including those of molders, pattern-makers, furnace men, metallurgist, and core maker.
Foundries are used in producing metal products for the railroad, motor parts, pipes, engines components and other heavy machines are made of casting. Almost all metal parts are made of casting.
Foundries are a facility that melts metals in special furnaces and pours the molten metal into molds to make a product. Foundries are usually specified according to the type of metal dealt with, i.e. Iron foundry, Brass Foundry, Bronze foundry, etc.
Casting is produced in iron, steel, copper, aluminium, bronze and some other alloy. Foundries usually specialize in one or more type of metal. Manufacturing of castings is obtained by pouring molten metal into sand molds known as gravity die casting.
There is another casting method used, a method like pressure and gravity die casting and centrifugal die casting. In die casting, molded metal is used in making pipes and other similar items. The liquid metal running into a rotating mold which causes the metal to make outwards and spread over the profile of mold.
Also check: Different types of Foundry tools and equipment
TYPES OF PATTERN USED IN FOUNDRY WORK
The pattern is the first stage in the production of casting. Development of pattern in foundry work is usually made with wood in order to form a cavity at the desired shape in the sand which is firmly boxes or flasks. The pattern is removed from the sand after forming the mold with some certain preparations. Liquid metal is run into the cavity to form the required casting.
working principles of casting
The pattern makers should consider the way in which the pattern is to be molded and how the casting will be done. In order for this to be archived, it will be done in two flasks. This first decision is on the location of the joint line. Sometimes it is readily located but there are no hard and first rules, each case being decided by consideration of the external shape of the object, or because of overhanging and projections. The position of the pattern in the flasks will often be influenced by the fact that the metal at the base of casting will always be the lens affected by impurities or by sand inclusion or by gases which will always rise to the top. Patterns are therefore made in such a way that important surfaces for machining can mold face downward.
Molding falls in two different divisions, green sand and dry sand molding, and foam molding. The loam moldings are made in a special sand mixture. They are used in certain kinds of heavy iron casting and extremely differ from green and dry sand moldings. The loam molding is built up with the loam often coated over brickwork.
- GREEN SAND MOLDING: it is so-called because the sand is damp and it had nothing to do with the color. Green sand molding can be opened or covered. In the open sand molds, the top surface of the casting is been exposed.
It is done when it is not necessary for the top casting to have a smooth surface. Whilst covered sand mold are made entirely within two or more molding flasks. it can also be done with part of the mold form on the floor and covered with a molding box.
- DRY SAND MOLDING: it is slightly different from green sand In the aspect of the different mixture is used and, mold bake is quite dry when molding is completed. The dry sand mold produces very clean castings and, a small gas is generated. With this effectiveness, dry sand molding is a safer method especially castings in steel at very high temperatures.
TYPES OF FOUNDRY SAND
1. GREEN FOUNDRY SAND: these type of foundry sand is expected to fulfill certain conditions:
- it must have a good green bond, i.e., to restrain the molded shape whilst moist.
- Highly refractory, i.e., resistant to heat, and
- They must not contain impurities which might cause scabbing on the casing surfaces.
Such sand will contain a high proportion of silica and its bonding quality will depend on the presence of some kind of clay material (bonding agent to the soil grains). Natural molding sands are those which are suitable as found and synthetic sands are sharp sands (clay-free) to which clay materials are added.
The green sand mold is of the utmost importance since an excess of moisture will make the sand impermeable, creating a dangerous situation when the molten metal is run into the mold. The steam generated has no escape zone through the sand, it then causes the molten metal to spurt back up the runner.
2. CORE FOUNDRY SAND: core sand is specially selected sands used in making cores of all shapes which are used inside molds to form holes or recesses in castings. Core sands must have the following qualities:
- Green bond
- Dry bond, the properties of holding shape after drying completely
- Permeability, i.e. it must allow gases generated to pass away rapidly
- It must collapse readily after casting so that core can be removed easily.
3. SILICA SAND: Silica sands have no natural bond, they are used in making oil sand core, dextrin and water being added to give a green bond. Linseed oil is also added, and whilst this gives no green bond, it oxides and gives a dry bond when the core is baked, leaving the sand permeable and not so firmly bonded that it will not give a little pressure as the casting contracts on cooling, or so hard that it is difficult to remove later.
4. FACING FOUNDRY SAND: A number of different materials used to protect the faces of molds against the action of molten metal and to help produce and smooth faces on the castings. Molding sand is, of the cause, highly refractory but this can often be improved and metal penetration of the mold prevented by:
- Adding small proportions of such thing as pitch, coke or coal dust to the facing sand which comes into contact with the metal.
- By dressing the face of the mold after the pattern is removed, dusting on talc (French chalk) or by spraying or brushing on plumbago or zircon in the form of a liquid suspension.
As mentioned earlier, the general step involves in casting are, pattern, molding, pouring, ejecting, cleaning, fettling, and inspection. The main purpose is to obtain a perfect required shape at the end. Molds obtained their shape from a pattern made of wood or metal replica of the object to be cast. After the pattern is been prepared, the furnace is charged above the metal melting point, in order to melt the metal to some specific temperature that the metal has totally liquidized. It is then tapped from the furnace through a spout into a refractory-lined steel pouring ladle which is used to transport the molten metal into a mold cavity. Impurities and slag from the top of molten metal should be removed before it is been poured into the mold cavity. After the mold is cooled and solidifies, the cast is them ejected from the mold. The casting is cleaned by removing the excess material from the casting, this process is called fettling. Fettling is also done to meet the required dimensions for the finished product.
Advancement in technology has Improved, modern foundries are heavily mechanized in the sense that one machine contains all equipment and can perform multi operations including, pattern and core making, casting and molding. These components include a large melting furnace, cranes, forklifts, ladies, conveyors and transfer vessels. There are different types and a large number of tools and equipment used in foundry shop for carrying out operations such as sand preparation, molding, melting, pouring and casting. These tools are broadly classified as hand tools, sand conditioning tool, flasks, powered operated equipment, metal melting equipment, and fettling and finishing equipment. Hand tools used in foundry shops are:
- Hand riddle
- Rammer (hand rammer, peen rammer, floor rammer, pneumatic rammer.)
- Sprue pin
- Strike off bar
- Draw spike
- Vent rod
- Spirit level
- Gate cutter
- Clamps, cotters and wedges
Safety in foundries is considered from skilled foundry workers with excellent situational awareness, strength, and endurance because foundry is such a dangerous uncomfortable work for a human.
Foundry work is very hazardous due to it working process that involves melting of metal at a very high temperature of 2600 degree Fahrenheit (1425 degree Celcius) and the unpredictable nature of chemical reactions at extreme temperatures. Steam explosion is very common if proper cooling process is not involved.
Loading metal in a charged furnace for melting is considered as one of the dangerous operations in the foundry, the loading should be done by experts who have developed necessary skills and understanding of metals and their properties, their melting temperatures, and safe procedures of work. A single mistake can lead to the destruction of equipment and causes injury to the workers.
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Foundries are hot and sweaty places to work. Workers are expected to wear hard hats, eye protection, face shields, thick aluminized clothing etc. foundries are designed with high roof in order to allow fresh air to circulate. The reaction of drop melted metal and concrete floor can lead to cracks, so foundries floor should be constructed of non-reactive or flame radiant materials.
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