Investment castings can be made from an original wax model such as the direct method or from wax replicas of an original pattern that need not be made from wax such as the indirect method. The following steps describe the indirect process, which can take two to seven days to complete. The steps below are the steps involve in the investment casting: Firstly is produce a master pattern, an artist or mould-maker creates an original pattern from wax, clay, wood, plastic, or another material.
In recent years the production of patterns using 3D printing has become popular using either standard PLA filament or custom made ‘casting wax’ filament, in which case one goes directly to step 5.Secondly is create a mould. A mould, known as the master die, is made to fit the master pattern.
If the master pattern was made from steel, the master die can be cast directly from the pattern using metal with a lower melting point. Rubber moulds can also be cast directly from the master pattern. Alternatively, a master die can be machined independently without creating a master pattern. Third steps is produce wax pattern. Although called wax patterns, pattern materials may also include plastic and frozen mercury. Wax patterns can 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 pattern thickness is reached.
Another method involves filling the entire mould with molten wax and letting it cool as a solid object. 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, ceramic cores are removed after the product has hardened. Then, investment materials were applied. The ceramic mould, known as the investment, is produced by repeating a series of steps coating, stuccoing, and hardening until a desired thickness is achieved. Coating involves dipping a pattern cluster into a slurry of fine refractory material and then draining to create a uniform surface coating. Fine materials are used in this first step, also called a prime coat, to preserve fine details from the mould. Stuccoing applies coarse ceramic particles by dipping patterns into a fluidised bed, placing it in a rainfall-sander, or by applying materials by hand. Hardening allows coatings to cure. These steps are repeated until the investment reaches its required thickness usually 5 to 15 mm (0.2 to 0.6 in). Investment moulds are left to dry completely, which can take 16 to 48 hours. Drying can be accelerated by applying a vacuum or minimizing environmental humidity. Investment moulds can also be created by placing the pattern clusters into a flask and then pouring liquid investment material from above. The flask is then vibrated to allow entrapped air to escape and help the investment material fill any small voids. 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. 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 coats, zircon-based refractories are commonly used, because zirconium is less likely to react with the molten metal. Prior to silica, a mixture of plaster and ground up old molds such as chamotte was used. 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.Next steps of investment casting is dewax, once ceramic moulds have fully cured, they are turned upside-down and placed in a furnace or autoclave to melt out 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 as the wax is heated it expands and introduces stress. To minimize these stresses the wax is heated as rapidly as possible so that outer wax surfaces can melt and drain quickly, making space for the rest of the wax to expand. In certain situations, holes may be drilled into the mold before heating to help reduce these stresses. Any wax that runs out of the mold is usually recovered and reused. Next steps of investment casting is burnout preheating. The mold is then subjected to a burnout, which heats the mold to between 870 °C and 1095 °C to remove any moisture and residual wax, and to sinter the mold. Sometimes this heating is also used to preheat the mould before pouring, but other times the mould is allowed to cool so that it can be tested. Preheating allows the metal to stay liquid longer so that it can better fill all mould details and increase dimensional accuracy. If the mold is left to cool, any cracks found can be repaired with ceramic slurry or special cements.In additional, pouring steps is done in investment casting. The investment mold is then placed open-side up into a tub filled with sand. The metal may be gravity poured or forced by applying positive air pressure or other forces. Vacuum casting, tilt casting, pressure assisted pouring and centrifugal casting are methods that use additional forces and are especially useful when moulds contain thin sections that would be otherwise be difficult to fill. Due to its complexity and labor requirements, investment casting is a relatively expensive process ” however the benefits often outweigh the cost.Practically any metal can be investment cast. Parts manufactured by investment casting are normally small, but the process can be used effectively for parts weighing 75 lbs or more.Investment casting is capable of producing complex parts with excellent as-cast surface finishes. Investment castings do not need to have taper built in to remove the components from their molds because the ceramic shells break away from the part upon cooling. This production feature allows castings with 90-degree angles to be designed with no shrinkage allowance built-in, and with no additional machining required to obtain those angles.The investment casting process creates parts with superior dimensional accuracy; net-shape parts are easily achievable, and finished forms are often produced without secondary machining.Each unique casting run requires a new die to produce wax patterns. Tooling for investment casting can be quite expensive; depending on the complexity, tooling costs can run anywhere between $1000 and $10,000.For high volume orders, the time and labor saved by eliminating or decreasing secondary machining easily makes up for the cost of new tooling. Small casting runs are less likely to make up for the investment. Generally, investment casting is a logical choice for a run of 25 parts or more.It usually takes 7 days to go from a fresh wax pattern to a complete casting; the majority of that time is taken up by creating and drying the ceramic shell mold. Some foundries have quick-dry capabilities to produce castings more quickly.The time and labor-intensive nature of investment casting doesn’t only effect cost. Foundries have limited equipment and production capacity, so longer lead times for investment casting are common.
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