This article takes an in depth look at investment casting.
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Investment casting is a metal working process that uses a ceramic shell built over a wax pattern to produce parts with extraordinarily uniform and smooth surfaces. The wax pattern is produced from an aluminum die.The final products of investment casting are without parting lines, mold half marks, or other deformities. Every part is seamless and flawless with an exceptionally fine finish.
The process of investment casting produces parts, components, and pieces with minimum waste, use of energy, or need for after casting finishing. The most distinguishing feature of investment casting is the accuracy and exceptional tolerances of the completed parts.
Investment casting, also known as the lost wax process or cire perdue in French, has been used as a metal working method for thousands of years. It originated in China and was ignored by modern industry until the 20th Century when the technology that made it easier to remove the wax from the casting was developed.
During World War Two, investment casting saw rapid growth as a means of providing exceptionally precise and flawless parts that could not be shaped using traditional methods. After the war, it became the most used process for industrial applications that required complex and intricate designs.
Tooling for investment casting refers to the wax injection dies used to create the wax patterns that form the basis for the process. The critical factor in the tooling is the required part to be produced, a distinction that is determined by the user. Tooling is part of the design function and includes Advanced Product Quality Planning (APQP), a method of design planning developed in the 1980s.
At the center of APQP is an examination of production and assessing of each of its stages to prevent errors and unnecessary repetitions. During APQP, every aspect of the end product is evaluated, engineered, and discussed such that the tooling, which includes the patterns and cores, are precision designed.
The die is the result of the APQP process. Wax injection dies are made from aluminum because of aluminum‘s thermal properties that dissipate heat quickly and reduce cycle times. Since aluminum is malleable and pliable, it can be easily shaped and formed. The die cavities of aluminum do not suffer wear from the wax injection process; this increases their lifespan.
Once the die is fabricated, it is checked for leaks, cracks, holes, or other imperfections that can affect the quality of the final part. Old dies are stored in plastic bags in a humidity controlled environment and are checked prior to being reused.
Several types of waxes are used to create the wax pattern. The type that is selected is dependent on several factors such as flow properties, whether it can be reclaimed, dimensional consistencies, surface finish, and the needs of the application. Common waxes that are used include filled pattern, non-filled pattern, runner, water soluble, and sticky.
Filled pattern waxes have fillers added that add properties to the casting that ensure strength, dimensional stability, lower thermal expansion, and limited shrinkage. Fillers include bisphenol, organics, terephthalic acid, and cross linked polystyrene.
Non-filled waxes contain fillers but not as much as filled waxes. They provide exceptional mechanical properties and thermal performance. Non-filled waxes are used for complex geometries and defined patterns.
Runner waxes are used for castings that require excellent mechanical strength with lower viscosity. They have a low melting point and drain quickly from the ceramic mold.
Water soluble wax is used when a part has complex and intricate internal patterns or designs where an intricate core is placed inside the pattern. Once the core is completed, the water soluble pattern is placed inside. As the pattern cools, the wax dissolves.
Sticky wax bonds pattern waxes together during pattern assembly and creates a long adhesion to prevent errors during constant handling.
Wax is injected into the die or mold to create the pattern. The dimensions of the wax pattern are slightly larger than the final part to account for the contraction that takes place in the ceramic mold. The die is clamped shut and an injection nozzle is aligned with the sprue of the die. The sprue is the path the wax follows as it enters the die cavity.
Wax pellets are melted in a holding tank connected to the injection press. The holding tank constantly agitates to keep the mixture homogeneous. A hydraulic powered cylinder pushes the wax through a heated hose into the sprue, filling the die cavity.
Investment casting produces high precision and finished parts in large quantities by assembling the individual patterns on a wax runner to which the patterns are affixed. Aside from its function as a method of holding the patterns, the wax runner serves as the metal feeding system or tube through which the individual parts will be fed molten metal during casting.
Wax runners are created with exactly the same method used to produce the patterns. A metal element is located at the end of the runner around which the wax is injected. A pin connected to the metal element sticks out and will be connected to a hanger plate later in the process. A ceramic cup is also added to act as a funnel when the molten metal is poured into the runner and patterns.
The gates of the patterns are connected to the runner by melting its end on the surface, dipping it in a hot melt adhesive wax, such as the sticky wax, and pressing it to the runner. As the melted wax cools, it locks to the runner and is welded by a small torch to smooth the connection.
Once the casting pattern is assembled and set, the slurry is applied by being dipped in the slurry, which is made up of fine grain silica, water, and some form of binding agent. The combination of these elements creates a ceramic coating that is applied multiple times to achieve the desired thickness.
Following slurry dipping, the pattern assembly is coated with stucco, which was determined during the APQP. The stucco process starts with a thin coating and becomes progressively rougher with each application.
The shell coating is an essential part of the process. The strength of the shell has to be sufficient to endure all of the casting procedures. The multiple dippings and stucco coating are necessary to ensure the shell‘s stability and permanence.
There are various forms of heating used to remove the wax from the hardened shell. A common modern practice is the use of an autoclave—a steam heating device. The temperature of the autoclave has to be sufficient to melt the wax and preheat the mold for casting.
Typically, investment casting molds are fired to 1800° F (982° C). The steam in the autoclave removes any potentially volatile materials. Once the molds are sufficiently cleared and heated, they are ready for the pouring of the metal.
Casting involves pouring molten metal into the preheated mold cavities. At this stage, the key to the quality of the casting rests in time and temperature. After the molten metal has been poured, a vibrating machine gently shakes the ceramic shell for five to seven seconds. The metal from ingots has been melted to a molten state. Once the shell is filled, it is allowed to cool at room temperature.
Once the shell has cooled and the metal has set, the shell material is removed. There are various methods used to remove the shell, including a hammer, high pressure water blasts, a vibratory table, chemicals, or a specially designed knockout machine. During the knockout process, the shell is tightly clamped and held in position to ensure uniformity of the process.
The knockout process can be especially difficult, particularly for parts that have intricate and complex sections.
Cut off involves removing the individual parts from the sprue or runner. Once the part has been disconnected from the runner, the remaining portions of the gates are ground away. There are various methods for removing the parts from the sprue, including the use of a chopping saw, torch, or laser. In highly technical or high production operations, parts may be cut off using a programmable cutting saw.
The purpose of heat treatment is enhancement of the mechanical characteristics and properties of the component. The casting process reduces the strength, durability, and toughness of a metal. Heat treatments eliminate internal stress. The types of heat treatments used for investment casting include vacuum solution annealing, hardening, tempering, and precipitation hardening.
The purpose of vacuum solution annealing is to remove precipitative material and change the workpiece to a single phase structure. After the completion of annealing, the workpiece is soft and ductile, ready to be hardened. At this stage, the workpiece is workable, machinable, weldable, and has dimensional stability.
Hardening includes heating the metal until it reaches its austenitic crystal phase. After this, it is cooled very quickly. The process increases the strength and wearability of the workpiece.
Tempering heats the workpiece to a temperature just below its critical range, holds it there, then cools it. The process of tempering reduces brittleness and requires precise control, so it does not affect hardness.
Precipitation hardening, or age hardening, makes the workpiece harder and is performed in a vacuum at temperatures that range between 900° F (482° C) and 1150° F (621° C). The process includes heating the workpiece, treating it with a solution, cooling, and heating it again before rapidly cooling it.
A final step in the investment casting process is finishing, which can take many forms depending on the requirements and specifications of the part‘s design. A typical finishing process is grinding that is used to further remove any deformities or remainders of the gate. Though the surface of the part is very smooth, further polishing may be necessary to enhance and perfect it.
Finishing may be completed using sandblasting, shot blasting, or other machining methods.
The range of surface treatments for investment cast products include rust protection and corrosion resistance enhancing polishing and chemical treatments. The surface of an investment cast part can vary according to the grade of the alloy and product. The types of treatments include:
Though investment casting, or lost wax casting, has been used for thousands of years, there have been innovations that have added to the effectiveness of the process. The variations are designed to enhance the method for developing the pattern and addressing the use of wax. The overall term used for lost wax casting is evaporative pattern casting since the material used to create the pattern is removed or evaporates.
Though these alternate methods create patterns in alternative ways, they have similarities to investment casting and can be considered as offshoots or variations. The main differences in the variations are the materials used to create the pattern or the formation of the pattern.
Lost foam casting has gained popularity as a replacement for investment casting due to its ability to fit into mass production and automated processes. The method of lost foam casting is a recent addition to evaporation casting. It was developed by H. F. Shroyer in 1958 as a process that uses polystyrene foam placed in casting sand.
Like in investment casting, an aluminum die is used to create the pattern. Polystyrene beads are placed in the mold or die and are steam heated; this causes the beads to melt and take the shape of the mold. As the beads are heated, they expand and assume the contours and dimensions of the die.
The individual patterns are attached to a sprue or runner and sprayed with a refractory coating of ceramic material. The coated mold is placed in a vented container, which is packed with sand to hold it in position. As molten metal is poured into the container, the polystyrene evaporates, making room for the molten metal.
In some cases, patterns do not have to be shaped in a die but can be hand carved. Using a machine or shape tool, polystyrene can be cut, formed, and configured to the desired dimensions of the workpiece. This type of pattern making is used for one-off parts or prototypes.
Lost foam casting is a manufacturing process used to create ornate, decorative, and complex metal configurations, shapes, and designs and can be used by engineers to create three dimensional renderings of their conceptualizations.
Direct investment casting differs from traditional investment casting, which is referred to as indirect investment casting, by how the pattern is created. With indirect casting, the pattern is formed in a die to create a wax representation. Several wax duplicates are attached to a sprue or runner, dipped in a ceramic solution, dipped in stucco, and dewaxed for the pattern to be filled with molten metal to form several versions of the component.
Direct casting varies in the way the pattern is formed and preformed using a variety of techniques. The first of these techniques is carving the pattern by hand or machine to create a one up version that is processed using the lost wax method. This process is used for producing a prototype, assessing dimensions, or for short runs of finished parts.
The introduction of computer assisted drafting (CAD) has led to a technological method for creating a pattern. With the use of CAD, a three dimensional representation of the workpiece is engineered and designed. Much like with a CNC machine, the design is programmed into a stereolithography (STL) optical fabrication machine that creates a three dimensional representation of the pattern using the input data.
In essence, STL is a method for fabricating a solid formed shape using a photosensitive liquid polymer and directed laser beam. Fabrication is accomplished in layers, with one layer added onto the previous layer to gradually and slowly build and shape the three dimensional geometric design. A representation of STL can be seen in the diagram below.
Water glass investment casting is a process that is commonly used in China. In water glass investment casting, water glass is used as the binding agent for the shell instead of ethyl silicate. The process originated in Russia in the 1950s and has the advantages of material costs and production cycle.
The surface finishes from water glass investment casting are comparable to casting that uses silica sol casting technology since it avoids defects that are found in traditional shell technology. The process, operation, and parameters of water glass investment casting are less complicated and can be completed by untrained, general workers; this improves production and efficiency.
Investment casting is a very versatile metal working process that is used to shape pipe fittings, automotive parts, marine hardware, and food machinery. There are a wide variety of metals that can be used for investment casting that have differing properties to benefit a varied range of applications.
All ferrous and non-ferrous metals can be shaped and configured using investment casting. Of the varieties of ferrous metals, ductile iron, carbon and alloy steels, and selected grades of stainless steel are the most used. Non-ferrous metals, such as copper alloys, magnesium, and aluminum, can be used, with aluminum being the most popular.
Aluminum alloys for investment casting have a density of 2.7 g/cm3 or slightly higher. The types of parts made of aluminum from investment casting include aircraft and engine parts. Aluminum alloys A-356, A-357, C-355, and F-357 that contain silicon, magnesium, iron, and zinc are the most used alloys. Components made from aluminum have corrosion resistance and weldability, and some have exceptional strength.
Stainless steel is a ferrous metal that contains chromium that provides added protection against stains and corrosion. There are several types of stainless steel with each type or grade having beneficial properties. The variations in stainless steel are due the chemical composition of its alloys. Stainless steel is an ideal metal for parts that are exposed to environments with high temperatures or liquids.
The main grades of stainless steel used for investment casting are the 300 and 400 series. Austenitic 300 series stainless steel has excellent corrosion resistance but does not gain strength through heat treatment. Martensitic 400 series stainless steel has exceptional strength and machinability and can be hardened through quenching and tempering, which also increases its strength.
Carbon steel is one of the better choices for investment casting products since it can operate in high pressure conditions, is wear resistant, and has exceptional strength, toughness, and hardenability. The properties of carbon steel are determined by the amount of carbon it contains that increases its hardness and strength during heat treatment.
Mid and low carbon steels are the types that are used the most for investment casting. Mid carbon steel has ductility, strength, and wear resistance and can be hardened and tempered by heat treatment. Low carbon steel can easily be shaped but is not strengthened by heat treatment.
Nickel alloys have high strength and are resistant to heat, corrosion, and wear. They can be welded and fabricated and are resistant to cracking or stress corrosion. The main use of nickel alloy investment castings is under conditions where there are high temperatures and corrosive elements.
The popularity of nickel alloy investment castings is due to their tight tolerances and exceptionally smooth finishes as well as their ability to be processed in complex and intricate shapes. Of the various investment casting metals, nickel alloys are a cost effective solution.
Copper alloys have corrosion resistance, thermal conductivity, and toughness. They are used in investment casting due to their easy castability. Copper alloys are machinable with excellent mechanical properties as well as friction and wear resistance. The types of copper alloys used for investment casting include series C-84500, C-85800, C-86000, C-87000, C-90000, and C-95000. The wide range of alloys provides a sufficient selection to choose the correct alloy for any application.
Cobalt alloys have high strength and heat and wear resistance. They have a natural resistance to oxidation with an exceptionally high melting point that makes them ideal for corrosive and chemically charged environments. Cobalt alloys have creep resistance and resistance to thermal fatigue for high temperature applications.
The various cobalt alloys contain combinations of chromium, nickel, tungsten, and molybdenum; this changes its properties and type of resistance. The cobalt alloys used for investment casting include numbers 6, 21, 25, 31, and 93.
For several years, it was difficult to cast magnesium using investment casting due to the fact that molten magnesium reacts with the silica mold shell. Recently, an inhibitor has been introduced; this allows the use of magnesium in investment casting.
Magnesium is lightweight and has an excellent strength to weight ratio. It is versatile and comes in a wide array of alloys, which include AZ91D and AM60B with alloys AZ81, AM50A AM20, AE42, and AS41B used for their creep resistance and high temperature applications.
Investment casting is an extremely popular method for the production of a wide range of parts and components. The process of investment casting allows for design flexibility for the creation of intricate and complex components from a huge selection of metals and alloys.
The simplicity of investment casting allows for high production runs with exceptionally accurate dimensional consistency. The original reason investment casting was pulled from the dark ages into the 20th Century was the development of the jet engine in the 1940s that would not allow for any inconsistencies or imperfection in its components. It is that aspect of investment casting that has made it an essential part of 21st Century manufacturing.
Aerospace was the first industry to rely on investment casting as a method for producing parts with exceptional tolerances and finishes. There is a wide assortment of metals used to make flight components, and investment casting provides the necessary selection of metals. Using any metal, investment casting produces precision parts with minimal materials and limited energy waste.
Aerospace components have to withstand extreme weather, fluctuating pressure, and various forms of operational wear; this requires they have superior durability. The process of investment casting has the necessary consistency, precision, and tensile strength to meet and exceed the requirements. Its main benefit for aerospace is its precision, which allows interlinked parts to match up quickly and easily.
Firearm manufacturers rely on investment casting due to the fact that it allows them the freedom to develop and implement unique designs. The manufacture of firearms demands precision and accuracy, and investment casting parts provide the net shapes that can be fabricated from a selection of alloys.
Investment casting minimizes the amount of metal that has to be removed during the finishing process. Using CNC machining, producers are able to make uniform parts with little variation at a low cost.
The medical and dental fields require instruments and components with the greatest amount of precision to meet tight tolerances and dimensional requirements. Surgical tools, implants, machines, stretchers, and wheelchairs are all produced using investment casting.
The lifesaving potential of investment cast parts makes their proper production critical. Every piece of equipment must be of the highest quality.
The main requirement for locks is that they be durable and resilient. Investment casting allows for the production of specialized locks as well as ordinary locks for domestic use. The need for locks to mesh precisely requires their casting be accurate down to the most minute detail.
A wide assortment of equipment is used to produce all the food we eat. Those huge machines rely on investment casting for the manufacture of their components and parts. Components for the food industry are made from stainless steel or specialty alloys due to the need for precision and durability. Meat slicers, poultry processing equipment, ice machine parts, and grills and warming machines have parts and pieces made from investment casting.
Fluid power transmits power through use of gases or fluids. The process includes the use of hydraulic and pneumatic equipment that converts power into a usable form. The types of components that are required include ball valve parts, steam trap parts, impellers, needle valves parts, compressor parts, and pump components. Like the food and dairy industry, the fluid power industry uses stainless steel as well as aluminum and some specialty metals.
There are many choices for the production of metal parts. Each of the choices has its benefits regarding production, quality, and accuracy. Of the processes available, investment casting technology has become the process of choice for the production of precision parts with excellent finishes. With low cost, design freedom, and unlimited quantities, investment casting is the ideal process for modern part manufacturing.
The many advantages of investment casting has made it the number one metal fabricating process for industrial and commercial products.
Design freedom is especially important for complex and intricate parts that may have multiple internal and external shapes. Investment casting is not limited by size, thickness, or configuration. It has the ability to adapt and shift to meet any challenge.
Of the many advantages of investment casting, tighter tolerances are the most important. When a part is engineered, it has to be manufactured to exactly meet the specifications of the design such that it will easily integrate with other parts. The majority of investment casting parts meet casting tolerances (CT) of five or six.
Investment casting produces superior finishes that require little after production finishing. The types of finishes the process produces is a feature for which it is famous. The quality of surface finishes far exceeds those from other casting processes. No other method of production can match the combination of tight tolerances and exceptional finishes found in investment casting.
Every production manager will tell you that defects are at the heart of production delays and labor costs. Defects produced in a production process create waste, require extra machining, and slow production runs. These disadvantages are not a part of investment casting. Every piece produced by the process is flawless and does not require any form of secondary processing.
When a part is extracted from the shell, it has a smooth, uniform surface and is ready to be packed and shipped.
Another major expense that is included in every manufacturing process is waste, the material that is left over after the completion of production. Investment casting requires very little after production finishing, which drastically limits the amount of waste produced. The lack of waste has multiple benefits, including lower production costs, labor costs, and faster turnarounds.
Included in the lack of waste is the removal of the need for specialized equipment such as deburring machines, heavy duty grinders, and various cutting tools. An additional factor is lower energy costs, increased efficiency, and exceptionally economical company performance.
There isn‘t any limitation on the amount of parts that can be produced using investment casting. From very small runs to those that go into the thousands, investment casting can produce parts rapidly and flawlessly. Parts that weigh 0.1 kg or those weighing hundreds of kilograms can be produced and finished using investment casting.
Every form of metal and alloy can be shaped and processed using investment casting. It is a feature of the process that has made it the number one method for producing equipment components. Unlike other casting methods, investment casting can work with any form of molten metal to create a reliable and useful part.
It is difficult to find a casting process that can produce components with minute and miniature details. This particular attribute of investment casting is the reason it was chosen to support the aerospace industry and was used in the development of the first jet powered aircraft. As society moves deeper into the age of technology, flawless parts and accurate dimensional tolerances will become more and more important.
From the initial pattern to the ceramic shell to the cut off of parts, every step of the investment casting process is designed to produce intricate and precise details with accuracy.
One of the key principles motivating modern business is sustainability. This is an evident benefit in the investment casting process. Patterns, wax, ceramic slurry, and stucco coatings can be used over and over again without the creation of waste. It is this aspect of the process that has made it such a popular method of production.
There are several factors that influence the quality and accuracy of investment cast parts. The main considerations are the structure of the part, casting material, molding, shell making, and pouring. Any error in the process can have an effect on the shrinkage rate, which would lead to deviations in the dimensions.
The first influential step in the process is the creation of the wax pattern; it has to be produced with a great amount of precision and accuracy.
Casting structure is influenced by the thickness of the part‘s walls. If they are too thick, they can increase the rate of shrinkage. If wall thickness is too low, the opposite effect is produced. A free systolic rate that is too large can block the shrinkage and make it smaller.
As with every form of casting, the material has an important influence on the results of the casting. Low carbon content decreases the rate of shrinkage.
The injection pressure and temperature are two very obvious factors that will influence the results of the casting process.
The type of material chosen to make the shell can have an influence on the shrinkage rate. Certain materials, such as zircon sand, have a small expansion coefficient and are ideal for the process.
Improper heating of the shell can have a negative effect and result in small shell expansion.
The pouring temperature is the temperature at which the molten metal enters the gates. If the temperature is too high, it will produce defects such as coarse grains on the internal structure. A low temperature influences the fluidity of the molten metal. The recommended temperature is 1650° C (3002° F).
The main problem with improper pouring temperature is shrinkage. The pouring temperature should be kept constant to reduce shrinkage. A higher temperature will not require more energy but will produce more precise and accurate parts.
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