Investment casting is an industrial process based on and also called lost-wax casting, one of the oldest known metal-forming techniques.[1] From 5,000 years ago, when beeswax formed the pattern, to today’s high-technology waxes, refractory materials and specialist alloys, the castings allow the production of components with accuracy, repeatability, versatility and integrity in a variety of metals and high-performance alloys. Lost-foam casting is a modern form of investment casting that eliminates certain steps in the process.
The process is generally used for small castings, but has been used to produce complete aircraft door frames, steel castings of up to 300 kg (660 lbs) andaluminium castings of up to 30 kg (66 lbs). It is generally more expensive per unit than die casting or sand casting, but has lower equipment costs. 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.
Process[edit]
Casts can be made of the wax model itself, the direct method; or of a wax copy of a model that need not be of wax, the indirect method. The following steps are for the indirect process which can take two days to one week to complete.
- Produce a master pattern: An artist or mould-maker creates an original pattern from wax, clay, wood, plastic, steel, or another material.[2]
- Mouldmaking: A mould, 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 moulds 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.[2]
- Produce the wax patterns: Although called a wax pattern, pattern materials also include plastic and frozen mercury.[2] Wax patterns may be produced in one of two ways. In one process the wax is poured into the mold and swished around until an even coating, usually about 3 mm (0.12 in) thick, covers the inner surface of the mould. This is repeated until the desired thickness is reached. Another method is filling the entire mould with molten wax, and let it cool, until a desired thickness has set on the surface of the mould. After this the rest of the wax is poured out again, the mould is turned upside down and the wax layer is left to cool and harden. With this method it is more difficult to control the overall thickness of the wax layer.[citation needed]
If a core is required, there are two options: soluble wax or ceramic. Soluble wax cores are designed to melt out of the investment coating with the rest of the wax pattern, whereas ceramic cores remain part of the wax pattern and are removed after the workpiece is cast.[2] - Assemble the wax patterns: The wax pattern is then removed from the mould. Depending on the application multiple wax patterns may be created so that they can all be cast at once. In other applications, multiple different wax patterns may be created and then assembled into one complex pattern. In the first case the multiple patterns are attached to a wax sprue, with the result known as a pattern cluster, or tree; as many as several hundred patterns may be assembled into a tree.[3] Foundries often use registration marks to indicate exactly where they go.[citation needed] The wax patterns are attached to the sprue or each other by means of a heated metal tool.[2] The wax pattern may also be chased, which means the parting line orflashing are rubbed out using the heated metal tool. Finally it is dressed, which means any other imperfections are addressed so that the wax now looks like the finished piece.[4]
- Investment: The ceramic mould, known as the investment, is produced by three repeating steps: coating, stuccoing, and hardening. The first step involves dipping the cluster into a 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 is stuccoed with a coarse ceramic particle, by dipping it into a fluidised bed, placing 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.[2][5]
- 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 sometimesquartz is used because it is less expensive. Aluminium silicates are a mixture of alumina and silica, where commonly used mixtures have an alumina content from 42 to 72%; at 72% alumina the compound is known as mullite. During the primary coat(s), zircon-based refractories are commonly used, because zirconium is less likely to react with the molten metal.[5]Chamotte is another refractory material that has been used.[citation needed] Prior to silica, a mixture of plaster and ground up old molds (chamotte) was used.[6]
- The binders used to hold the refractory material in place include: ethyl silicate (alcohol-based and chemically set), colloidal silica (water-based, also known as silica sol, set by drying),sodium silicate, and a hybrid of these controlled for pH and viscosity.
- 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.[7]
- Burnout & 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 used 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.[7] 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.[8]
- Pouring: The investment mold is then placed cup-upwards into a tub filled with sand. The metal may be gravity poured, but if there are thin sections in the mold it may be filled by applying positive air pressure, vacuum cast, tilt cast, pressure assisted pouring, or centrifugal cast.[8]
- Removal: The shell is hammered, media blasted, vibrated, waterjeted, or chemically dissolved (sometimes with liquid nitrogen) to release the casting. The sprue is cut off and recycled. The casting may then be cleaned up to remove signs of the casting process, usually by grinding.[8]
Advantages of Investment casting[edit]
- Intricate forms with undercuts can be cast.
- A very smooth surface is obtained with no parting line.
- Dimensional accuracy is good.
- Certain unmachinable parts can be cast to preplanned shape.
- It may be used to replace die-casting where short runs are involved.
Disadvantages of Investment casting[edit]
- This process is expensive, is usually limited to small casting, and presents some difficulties where cores are involved.
- Holes cannot be smaller than 1/16 in. (1.6mm) and should be no deeper than about 1.5 times the diameter.
- Investment castings require very long production-cycle times versus other casting processes.
- This process is practically infeasible for high-volume manufacturing, due to its high cost and long cycle times.
- Many of the advantages of the investment casting process can be achieved through other casting techniques if principles of thermal design and control are applied appropriately to existing processes that do not involve the shortcomings of investment castings.
Counter-gravity pouring[edit]
A variation on the pouring technique 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.[9]
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.[9]
Vacuum pressure casting[edit]
Vacuum pressure casting (VPC) uses gas pressure and a vacuum to improve the quality of the casting and minimize porosity. Typically VPC casting machines consist of an upper and a lower chamber. The upper chamber or melting chamber housing the crucible, and the lower casting chamber housing the investment mould. Both chambers are connected via a small hole containing a stopper. A vacuum is pulled in the lower chamber, while pressure is applied in the upper, and then the stopper is removed. This creates the greatest pressure differential to fill the molds.[11]
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