05-11-2012, 12:19 PM
Investment Casting
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INTRODUCTION
Investment casting is also known as the lost wax process. This process is one of the oldest manufacturing processes. The Egyptians used it in the time of the Pharaohs to make gold jewellery (hence the name Investment) some 5,000 years ago. Intricate shapes can be made with high accuracy.
A pattern made of wax is coated with a refractory material to make mould, after which wax is melted away prior to pouring molten metal. “Investment" comes from a less familiar definition of "invest" i.e.-“to cover completely" this refers to coating of refractory material around wax pattern. It is a precision casting process capable of producing castings of high accuracy and intricate detail. It can produce complicated shapes that would be difficult or impossible with die casting, yet like that process, it requires little surface finishing and only minor machining.
In addition, metals that are hard to machine or fabricate are good candidates for this process. It can be used to make parts that cannot be produced by normal manufacturing techniques, such as turbine blades that have complex shapes, or airplane parts that have to withstand high temperatures. The process is generally used for small castings, but has produced complete aircraft door frames, steel castings of up to 300 kg and aluminium castings of up to 30 kg. It is generally more expensive per unit than die casting or sand casting but with lower equipment cost.
Investment casting is one of the oldest manufacturing processes, dating back thousands of years, in which molten metal is poured into an expendable ceramic mold. The mold is formed by using a wax pattern - a disposable piece in the shape of the desired part. The pattern is surrounded, or "invested", into ceramic slurry that hardens into the mold. Investment casting is often referred to as "lost-wax casting" because the wax pattern is melted out of the mold after it has been formed. Lox-wax processes are one-to-one (one pattern creates one part), which increases production time and costs relative to other casting processes. However, since the mold is destroyed during the process, parts with complex geometries and intricate details can be created.
CASTING
Casting is a manufacturing process by which a liquid material is usually poured into a mold, which contains a hollow cavity of the desired shape, and then allowed to solidify. The solidified part is also known as a casting, which is ejected or broken out of the mold to complete the process. Casting materials are usually metals or various cold setting materials that cure after mixing two or more components together; examples are epoxy, concrete, plaster and clay. Casting is most often used for making complex shapes that would be otherwise difficult or uneconomical to make by other methods.
MOULDMAKING
A mold, known as the master die, is made of the master pattern. The master pattern may be made from a low-melting-point metal, steel or wood. If a steel pattern was created then a low-melting-point metal may be cast directly from the master pattern. Rubber molds can also be cast directly from the master pattern. The first step may also be skipped if the master die is machined directly into steel.
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INVESTMENT
The ceramic mold, known as the investment, is produced by three repeating steps: coating, stuccoing, and hardening. The first step involves dipping the cluster into slurry of fine refractory material and then letting any excess drain off, so a uniform surface is produced. This fine material is used first to give a smooth surface finish and reproduce fine details. In the second step, the cluster isstuccoed with a coarse ceramic particle, by dipping it into a fluidised bed, places it in a rainfall-sander, or by applying by hand. Finally, the coating is allowed to harden. These steps are repeated until the investment is the required thickness, which is usually 5 to 15 mm (0.2 to 0.6 in). Note that the first coatings are known as prime coats. An alternative to multiple dips is to place the cluster upside-down in a flask and then liquid investment material is poured into the flask. The flask is then vibrated to allow entrapped air to escape and help the investment material fill in all of the details. Common refractory materials used to create the investments are: silica, zircon, various aluminium silicates, and alumina. Silica is usually used in the fused silica form, but sometimes quartz is used because it is less expensive.
DEWAX
The investment is then allowed to completely dry, which can take 16 to 48 hours. Drying can be enhanced by applying a vacuum or minimizing the environmental humidity. It is then turned upside-down and placed in a furnace or autoclave to melt out and/or vaporize the wax. Most shell failures occur at this point because the waxes used have a thermal expansion coefficient that is much greater than the investment material surrounding it, so as the wax is heated it expands and induces great stresses. In order to minimize these stresses the wax is heated as rapidly as possible so that the surface of the wax can melt into the surface of the investment or run out of the mold, which makes room for the rest of the wax to expand. In certain situations holes may be drilled into the mold beforehand to help reduce these stresses. Any wax that runs out of the mold is usually recovered and reused.
BURNOUT AND PREHEATING
The mold is then subjected to a burnout, which heats the mold between 870 °C and 1095 °C to remove any moisture and residual wax, and to sinter the mold. Sometimes this heating is also as the preheat, but other times the mold is allowed to cool so that it can be tested. If any cracks are found they can be repaired with ceramic slurry or special cements. The mold is preheated to allow the metal to stay liquid longer to fill any details and to increase dimensional accuracy, because the mold and casting cool together.
Counter-gravity pouring
A variation on the pouring technique is to fill the investment upside-down. A common form of this is called the Hitchiner process, which is named after the Hitchiner Manufacturing Company that invented the technique. In this technique the investment shell is placed in a vacuum tight mold chamber and then lowered into a pool of molten metal. A vacuum is then created, which draws the metal up into the investment shell. After the casting has solidified the vacuum is released, which allows any remaining liquid metal to flow back into the pool.
This technique is more metal efficient than traditional pouring because less material solidifies in the gating system. Gravity pouring only has a 15 to 50% metal yield as compared to 60 to 95% for counter-gravity pouring. There is also less turbulence, so the gating system can be simplified since it does not have to control turbulence. Plus, because the metal is drawn from below the top of the pool the metal is free from dross and slag, as these are lower density (lighter) and float to the top of the pool. The pressure differential helps the metal flow into every intricacy of the mold. Finally, lower temperatures can be used, which improves the grain structure.
LOST WAX-
The wax used to make moulds for investment casting requires special properties, it needs to be neither to soft or pliable nor too brittle, it needs to be solid at room temperature(this part is tricky)depends entirely on where you live.
Some types of wax will appear to have a memory i.e.; after forming; it may tend to return to its original shape. It is important to be able to work the Wax, e.g. (carve model, cut and twist, without it breaking or cracking). It must also be able to stick to itself; especially if you want to build up your model with several layers of wax you do not want it to delaminate. Another property required of this wax material is colour, don't be fooled into making a nice clear wax…you need an opaque look about it, so that as you model and work with it you can see what you are doing.
SLURRY-
The materials used for the slurry are a mixture of plaster of Paris, a binder and powdered silica, a refractory, for low temperature melts. For higher temperature melts, sillimanite an alumina-silicate is used as a refractory, and silica is used as a binder. Depending on the fineness of the finish desired additional coatings of sillimanite and ethyl silicate may be applied. The mold thus produced can be used directly for light castings, or be reinforced by placing it in a larger container and reinforcing it more slurry.
PATTERN-
Beyond good design practices, the key consideration is pattern modification to prevent shell Cracking and minimize residual ash. Ceramic shells have a very low coefficient of thermal expansion, so any expansion of the pattern during the burnout cycle may cause the shell to crack. Adding foundry-defined gating and vents to the CAD model and constructing it as an integral part of the pattern. To facilitate shell washing, gates are added to opposite ends of the pattern. This gate configuration provides a flow path for water to flush out the residual ash from within the shell.