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Lost wax casting is a casting process that uses a wax pattern to create a ceramic mold for creating a part or product design. It has been known over the years as lost wax or precision casting due to its accuracy in recreating parts with precise tolerances. In modern applications, lost wax casting is referred to as investment casting. The original process was named lost wax casting but is presently used interchangeably with investment casting.
The process that makes lost wax casting unlike any other casting method is the use of a wax pattern to create the initial mold, which can have intricate and complex designs.
Lost wax casting, or investment casting is used to produce parts that require tight tolerances that can have thinner walls with surface finishes that require little after process finishing. A unique feature of lost wax casting is how it recreates a CAD design using wax to create the pattern of the piece to be manufactured.
The casting process for lost wax casting involves multiple steps that begin with the creation of the pattern, or master, from aluminum. The method for creating the pattern determines its replicability.
The initial step in the lost wax casting process begins with the creation of a 3 dimensional CAD rendering of the part to be produced, which will be used to create the aluminum die.
The die is created using the design from the CAD rendering. It is a negative relief of the part to be cast.
Wax, in a semi-liquid state, is poured into the die to form the wax pattern, which is adjusted to allow for shrinkage. This process can be repeated as many times as necessary depending on the number of parts to be cast.
The wax patterns are connected along a runner to form the sprue, which may be connected to other pattern groups to form a cluster. The sprue, runner, and wax patterns are referred to as a tree.
To build the shell, the pattern is dipped in ceramic slurry, which coats the pattern to form a hard exterior shell around the pattern. One end of the wax tree is left exposed for removal of the wax.
The hardened ceramic shell will be where the molten metal will be added to form the final part. To accomplish this, the wax on the interior of the ceramic shell has to be removed, which is done by placing the ceramic shell in an autoclave or oven. As the ceramic shell is heated, the wax melts and runs out of the shell. It is this part of the process that gives lost wax casting the name "lost wax".
Though the mold has undergone the dewaxing process, there still may be residual wax and moisture in it. To remove these extraneous materials, the mold is subjected to a burnout process, which heats it to over 1037° C or 1900° F. Also, this step helps to solidify and harden the ceramic mold to prepare it to receive the molten metal.
The ceramic mold is placed with the open side up to have the molten metal poured in. This process can be completed by simply allowing gravity to distribute the metal in the ceramic mold or have it forced in by a form of pressure. The method that is chosen depends on the size of the mold and the type of molten metal.
Regardless of the descriptor, the ceramic material that forms the mold has to be removed. This can be accomplished using a variety of methods, which can include simply hammering the ceramic, blasting, high pressure water, or the use of some form of chemical that may include liquid nitrogen.
The finished part has to be separated from the gates and runners once the ceramic mold has been removed. This is normally performed with a grinder and the waste material is collected for reuse.
Though the part is fully molded, it will need to be sandblasted to remove scales and residual ceramic to enhance its finish. This can be completed in a variety of ways that include shot, small metal balls, or sand blasting.
There are certain parts that require extra protection from rust, corrosion, and weather damage. This added coating is applied by dipping the part in anti-rust solution or oil. Other surface treatments include painting and galvanizing.
Though the process described above is one of the processes used to manufacture lost wax castings, it is not the exclusive process.
Lost wax casting uses a wide assortment of metals to create parts with exceptional accuracy and tolerances. The types of metal chosen for a casting depends on the requirements of the design and the type of part. One of the reasons that lost wax casting is so popular is the limitless number of metals that can be applied to the process.
The main alloys of stainless steel are chromium, nickel, and molybdenum, which determine the grain and mechanical properties of the casting. How the alloys are combined decides how the casting will be able to endure heat and resist corrosion. Since stainless steel is 10% chromium, it is resistant to liquid corrosive conditions and oxidation.
Carbon, low alloy, and tool steels are used in the aerospace, agricultural, medical, and firearm industries, to name a few. Steel is chosen for lost wax casting because it is economical, available in several grades, and can be heat treated to adjust its ductility.
Ductile iron castings have exceptional surface hardness, elasticity modulus, corrosion resistance, and a strong strength to weight ratio. The greatest benefit of ductile iron is that it is cost effective and offers a wide range of design freedom.
Beryllium copper, after heat treatment, has exceptional thermal and electrical conductivity with hardness that is greater than steel and thermal conductivity greater than aluminum. Castings made from beryllium copper have good ductility, are easily welded, and can be easily machined. Copper that is made with 2.0% to 2.5% beryllium has excellent wear, toughness, and is anti-galling.
Aluminum is an ideal metal for lost wax casting due to its machinability and corrosion resistance. When it is alloyed with other metals and heat treated, it develops exceptional strength that is comparable to low carbon steel. The fluid nature of aluminum alloys allows for parts to have thin walls.
Cobalt is naturally resistant to oxidation with a hard and lustrous finish. It is protected from oxidation by a passivating oxide film. Cobalt is found in nature combined with other alloys and is separated using a smelting process. The corrosion, heat, and wear resistance of cobalt makes it ideal for lost wax casting and is used to produce parts for aerospace, the automotive industry, and for military use. It can be found in orthopedic implants for its wear resistance.
Brass, a copper alloy known as red or yellow brass, lost wax castings are used for plumbing fixtures and door hardware. The main characteristic of brass is its appearance as well as its finishing and polishing qualities. The one drawback to the use of brass is its lead content, which has restricted its use.
The nature and quality of bronze depends on the alloys with which it is combined. The added elements of bronze are what enhances its performance and quality. Tin increases its strength, while lead decreases its strength but increases its lubricity. To create corrosion resistance and increase strength, manganese and aluminum can be added.
Nickel contains nickel, chromium, and molybdenum and has exceptional strength as well as resistance to heat, wear, and corrosion. It is chosen as a casting metal because it can be easily welded and fabricated with resistance to cracking and corrosion. Nickel alloy castings are used in applications where there is extreme heat and corrosion, such as aerospace, marine environments, and chemical factories. Common Inconel nickel alloys are Inconel 600 and 625.
There are a wide variety of parts, products, and components produced using lost wax casting or investment casting. The precision and accuracy of the processes have made them an ideal method for producing parts with thin walls, intricate details, and close tolerances. A high percentage of investment cast parts are general hardware, such as brackets, valves, and pump parts.
Some of the industries that have their parts produced by lost wax casting include aerospace, medical, robotics, military manufacturers, and the automotive industry.
Lost wax casting is essential in the construction of airplanes and spacecraft. Parts produced by lost wax casting are found in every portion of space and commercial flight vehicles from the bearings that open the doors to essential parts of engines.
The automotive industry relies on lost wax casting for the production of engine components, compressor parts, and gearbox components. Aluminum lost wax castings are used for space frames and suspension systems, while stainless steel castings are used for fuel filler doors. Other automotive components include drive and power train assemblies, fuel systems, and convertible tops. Parts are made from ferrous and non-ferrous metals to specifically fit the application. One of the benefits of using lost wax castings for automotive construction is excellent metal to metal lubrication and long wear.
The fuel pump housing, injector clamp, turbo wastegate, oil supply bend, and shifting finger, highlighted on the engine below, have been produced using lost wax casting.
Lost wax casting parts used for the oil industry must have unique qualities due to the nature of oil and gas drilling. The oil and gas industry operates in some of the most hostile environments in the world, which place a high demand on required parts. Components produced using the lost wax casting process are required to have exceptional wear and corrosion resistance. Gate valves, tube handling devices, elevator parts, valve brackets, slip linkages, and locking levers are all manufactured using lost wax casting. The main metals are aluminum, stainless steel, and alloy steel.
The biggest issues faced by the chemical industry are corrosion and erosion, which significantly impact the mechanical properties of the components it uses. Parts produced for the industry have to be able to endure extreme temperatures and pressure conditions for the safety of workers. Parts produced using lost wax casting must be able to endure temperatures that range between 1700° F and 2000° F. Typical products include bullhead tees, catalyst tubes, elbows, ethylene coils, heater hardware, and static cast fittings.
The applications for components for the food industry vary from harvesting wheat and raw food materials to processing and canning foods or the bottling of soft drinks. Some of the components produced using lost wax casting include meat slicers, various poultry processing tools, parts for ice machines, and racks and grills for preparing foods. Products for the food industry are designed to withstand acidic chemicals from foods and exceptional strength for reliable performance.
One of the uses for lost wax casting in the medical industry is the fabrication of medical implants. Castings have to meet exacting standards and be produced to regulatory specifications. Products produced using lost wax casting have exceptional density and mechanical properties. Several value added processes are used to ensure the life of the implants and remove porosity and other fatigue factors. Other products for the medical industry include surgical tools such as forceps. The lost wax cast is the preferred method for the manufacture of medical instruments since parts do not need machining and can easily be sterilized.
There are increasing demands on the power industry to improve its methods and use of parts that are long lasting and wear resistant. Components have to endure abrasion, erosion, cavitation, fatigue, and corrosion at exceptionally high temperatures. To manufacture parts for the demanding conditions of the power industry, lost wax part producers use cobalt, nickel, and stainless steel to combat the demands of power industry applications. Parts are required to be precisely machined and coated using a variety of processes to avoid wear issues.
Dental prosthetics are manufactured using cobalt and nickel alloys, which are more economical and cost effective than silver or gold. The mechanical properties of those metals ensure high performance, long wear, and proper appearance.
Lost wax casting for the tool industry uses steel alloys with exceptional tensile strength, hardness, and elasticity, which makes tool components durable, able to withstand shock and increased mechanical loads. The choice of lost wax casting is its ability to recreate complex geometries without the need for after casting processes, which is further supported by the dimensional accuracy of the final products.
As with the medical industry, products for the pharmaceutical industry must be able to undergo the sterilization process. Parts and instruments are produced using high grade stainless steel, which is corrosive resistant, able to withstand high temperatures, and able to meet hygiene requirements. A main characteristic of lost wax cast pharmaceutical tools, and the reason lost wax casting is used, is the smooth finish of components making it difficult for bacteria to be collected.
For the hoisting industry, the main concern in the manufacturing of parts is safety since parts are used vertically and horizontally for lifting and transport systems. Steel alloy is the most common casting metal for its tensile strength and elasticity. Lost wax casting is chosen to produce hoisting components due to its freedom of design and flexibility as well as the precise adherence to design parameters. Complex and intricate shapes can be produced without the need for mechanical post casting processing.
The process of lost wax casting has rapidly grown due to its many benefits and advantages. There are very few restrictions on the types, shapes, dimensions, and design of lost wax casting products. Engineers choose lost wax casting, or investment casting, because it allows them the flexibility to develop intricate and complex designs without having to account for shrinkage and post process finishing.
Since the molds from lost wax casting are made from ceramics, produced products are ready to ship or attach after being cast. The roughness average (RA) of a lost wax cast part averages around 125, which is the average of the peaks and valleys on the finished surface.
One of the critical factors in modern manufacturing is turnaround times since products have to be mass produced in a short time frame. Since lost wax cast parts do not require finishing, parts can be rapidly produced and shipped, which is further assisted by the reduction in handling and labor.
The initial expense for any manufacturing process is equipment and machinery. Lost wax casting uses less expensive equipment, which makes it less dangerous. The basic process employs the use of four machines though other equipment can be added.
The biggest advantage of lost wax casting is the tight and accurate tolerances that have a standard of ± 0.005. CAD computer designs are accurately and precisely recreated to exactly fit the final application.
There are very few limitations to the types and kinds of metals that can be used in lost wax casting. The types of metals include bronze, stainless steel, alloy steel, iron, aluminum, and copper, to name a few. The wide selection gives engineers a varied array of choices when designing parts. It also makes the process adaptable to any industry.
As there is little limitation on the types of metals used in lost wax casting, the same applies to the size of parts to be formed. The range of sizes starts with small dental implants up to complex aircraft engine parts weighing thousands of pounds. The size and weight of lost wax cast parts is dependent on the mold handling equipment.
Since lost wax casting requires less manpower, labor costs are significantly lower. Lost wax requires fewer resources and less time, which are major cost factors. The fact that parts do not need post processing further lowers the cost. These essential factors assist in keeping costs down for high quality parts.
Another factor that keeps the cost of lost wax casting down is the lack of waste. Most of the materials that are used in the process are saved and reused, which eliminates waste. In other production processes, post processing leads to waste of materials since the removed burrs and excess metals are unusable. Since lost wax casting does not require finish or post processing, waste is radically reduced.
Lost wax casting maximizes the use of resources and reuses the wax and ceramics for future projects. This eliminates waste materials being dumped into a landfill. Manufactures are able to reuse dies, molds, and wax multiple times without need for replacement.
Lost wax casting parts are lightweight, have complex features, and fine details. Engineers have a great deal of design flexibility from the types of metals they choose to the complexity of their designs. Parts are produced as one unit without joints or flash lines, which makes the parts more durable and less susceptible to breakage or poor performance. Every part has the same dimensions and configuration from the first one to the last one produced.
As with all production processes, there is always the potential for errors and flaws. In lost wax casting, defects occur due to operation errors. These mistakes can be avoided by careful examination of procedures during the casting process.
Gas holes can be internal or external and come in a variety of shapes. External gas holes are easily identifiable and can be caught by visual inspection. Gas holes are caused by poor preheating of the mold, lack of exhaust design, or pouring.
Each of these factors can be eliminated by careful attention during the molding process with simple attention to procedures such as storing materials properly, tilting the mold during pouring, and adherence to proper pouring temperature.
Shrinkage cracks are holes in the casting surface and are caused by improper mold working temperature, poor control of the coating layer, or improper design. It can be corrected by improved mold temperature, adjustment of the coating layer, designing pressure devices for the riser, and proper control of the cooling speed.
Slag hole is a hole in the casting that is filled slag and is found at the bottom, around the runner, or in a corner. It is caused by a poor pouring process or improper gate design. It can easily be prevented by proper gate design, tilting the mold during pouring, and proper control of the flux.
Thermal cracks are dark grey or black because of oxidation. Cracks are easily seen during visible inspection and are related to shrinkage or slag inclusion and occur inside of casting angles, thickness junctions, and sections between the pour riser and casting. The cause of cracking is a high cooling speed, which increases internal stress in the casting. Careful examination of wall thicknesses and wall transitions as well as adjustments to coatings can prevent this flaw.
Cold shuts are surface cracks with round edges with scale in the middle. It appears in the top wall, thin horizontal or vertical plane, or joint area of a wall. Venting, low temperature, improper coating, positioning of risers, and speed of pouring are all possible causes of cold shut. This flaw can be corrected in the design phase with proper runners and an exhaust system. Tilting during pouring is another method.
Misruns happen when the mold cavity is not completely filled, which is caused when the shell is too cold, the metal is too cold, or with a slow fill rate.
Inclusions are craters of varying sizes due to non-metal particles in the casting. They can also appear as cavities or irregular shapes with refractory materials. They can be caused by cracks in the wax mold that allow ceramic into the mold cavity.
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