Free Metal Casting & Defects in Fabrication Essay Sample
Casting refers to a manufacturing process that involves melting a solid, heating it to proper temperature and then pouring the molten liquid into a cavity or mold which contains it in the proper shape during solidification. This way simple or complex shape can be made from any metal that can be melted. Cast parts differ in size. Some are as small as a fraction of and a fraction of an once such as an individual teeth to over 30 feet and many tons such as the huge propellers and stern flames of ocean liners. Casting is very efficient in the production of complex shapes and parts having hollow sections or parts that contain irregular hollow sections or surface curves. Because of its usefulness in these processes, casting is among the most important components of industrial processes. Today virtually any design can be produced using metal casting processes. There are various processes that are used in casting share the experience that materials solidify in a manner that would maximize the metal properties while at the same time preventing defects such as shrinkage voids, gas porosity and trapped inclusions.
Sand casting is an old method whose practice is responsible for much of the industrial growth of the world. At the beginning, the sand caster played the role of smelter and refinery. The sand caster made his sand molds and then proceeded to make sand molds. He would then smelt virgin ore in a variety of home made smelters or furnaces and did the casting. As demand for metals increased the foundry man found out that he needed to devote more time to his new venture. This left him with no time to use in collecting and smelting. The result was a smelter who concentrated his time and energy to the efficient production of pig iron, bronze ingots which he then sold to the burgeoning number of sand foundries. There has been much development in the sand casting methods but the basics remain the same. The liquid mold is poured into a sand mold that was already prepared and when it solidifies, the mold is broken up and the casting is realized. Development have been taking place in sand casting that have led to improved methods of producing sand molds, mold handling equipment, automatic pouring of molds, automatic sand conditioning, automatic shake out and better melting equipment.
1. Fabrication of the desired pattern out of wood usually and split it in two.
2. The bottom half of the pattern which is called the drag is placed in a box referred to as a flask.
3. Apply a release coating to the pattern, then fill the flask with sand before ramming it to compact the sand.
4. Turn the drag half of the mold over and place the top half of the flask on top of it. The top is then placed over the drag and release coated.
5. Top half of the pattern also called the cope is installed with risers and a sprue. The sprue is where liquid metal enters the mold.
6. The cope half is then packed with sand and rammed.
7. The two halves are then separated and the patterns removed. If hollow sections are required a sand core will be placed in the drag half of the mold. A gating system is then cut into the sand on the cope half of the mold.
8. The two halves are reassembled and clamped or bolted shut for casting.
The advantages of sand casting are low cost of equipment, design flexibility, ability to cast complex patterns, ability to produce large castings, it can be done with a wide range of casting alloys and it is quite economical for small quantities.
Disadvantages are that the process does not permit close tolerances and it produces rough surface finishes. It also requires high labor cost per part.
Die casting can be described as a precision manufacturing process where molten metal is injected at high pressure and velocity into a permanent metal mold. Die casting has two basic processes; the hot chamber process and the cold chamber process. In the hot chamber process, a metal injection system is immersed in molten metal. The hot chamber process is used for alloys with low melting points only like lead and zinc.
The hot chamber process has these steps:
1. Hydraulic cylinder is used in applying pressure on the plunger.
2. The plunger pushes the metal from the sleeve through the gating system into the cavity.
3. High pressure is maintained through the solidification process.
4. The die opens after solidification is completed
5. The part is injected from the cavity.
The cold chamber process is used for alloys with high melting points like aluminum and brass. The cold chamber die-casting force follows the following steps:
1. Molten metal is ladled into the shot sleeve
2. The hydraulic cylinder applies pressure on the plunger
3. The plunger pushes metal from the sleeve through the gating system into the cavity.
4. High pressure is to be maintained during the solidification process.
5. After completing solidification is complete, the die opens.
6. The part is then injected to the cavity
Advantages of die-cast
High mechanical properties in combination with light weight. High thermo conductivity, good mechanibility
The parts do not require machining after production other than polishing.
The parts can be cast with close dimensional tolerances.
Low scrap rate.
Disadvantages of die casting
Only a limited number of metal alloys can be used. These are zinc alloys, and to a lesser extent aluminium , magnesium, copper-base alloys, tin based alloys and that is lead.
The maximum size is limited by the die sizes available and the capacity of the die casting machine available.
The cost of dies is high and die casting machines requires large volume of work to become economically
Defects of casting
The most commonly encountered defects are:
Blow; this a fairly large well-rounded cavity produced by gases which displace the molten metal at the cope of a casting. Blows can be avoided by having a proper venting is therefore essential to make room for this expansion and to avoid the buckles.
Scab: this is a thin layer of metal protruding above the casting surface on top of a thin layer of sand. It results when the up heaved sand is separated from the mould surface and the liquid metal flows into the space between the mould and the displaced sand.
Rat tail; this is a long shallow, angular depression normally found in a thin casting. Instead of the expanding sand up heaving, the compressed layer fails by one layer, gliding over the other.
Swell; this is found on the vertical surfaces of a casting if the moulding sand is deformed by the hydrostatic pressure caused by high moisture content in the sand.
Misrun; this occurs when the liquid metal starts freezing before reaching the furthest point of the mould cavity due to insufficient heating.
Lost foam casting
According to Black, lost foam casting is a casting process that was invented in 1958. However, the use of this process was not immediate because of a number of restrictions that that were based on tis patents. Presently, this is a casting process that has had one of the most significant commercial successes in the casting industry.
Process of foam casting
1. The firs process is consist of producing foam which has a pattern that is similar to the product to be produced at the end of the process.
2. The different parts of the foam pattern are then held in position into a foam tree whose size is dependent on the pattern being produced.
3. The pattern is then left for a while to give it time to stabilize and then it is dipped into a refractory material which creates a thin coat on the foam pattern.
4. The foam is then removed and suspended after it has been dried and sand poured around it in order for the foam to have some stability
5. The above process is then followed with the pouring of molten metal into the mould. This will result in the burning of the foam used and the molten metal duplicating that of the initial pattern
6. Once solidification is complete, the container is turned upside down and sand which did not bond allowed running out and the casing removed.
This process is suitable for ferrous and non-ferrous metals and does not need the use of cores or parting lines. In addition, the process support production of more units of pattern even with little tolerance to miniature dimensions.
Excessive cooling makes this process unsuitable for use for process that may increase the cost of the casting work. In addition, this process does not provide a means of casting carbon steels.