Investment Castings

View A Video on Investment Castings - A Quick Introduction

Investment casting takes its name from the ceramic slurry that is used to form a shell around a wax pattern. A wax model is created and attached to a sprue. Several hundred waxes may be affixed to the same sprue forming an assembly called a tree. After the tree is dipped into the slurry multiple times it is sprinkled with coarse material such as sand. Once the investment is thick enough around the wax, it is inserted into an oven where temperatures can reach upwards of 1000°F. The wax melts and drips out while the investment maintains its size and shape. A metal is heated until molten and is poured into the hollow shell, traveling along the lines left by the sprue and filling the impressions left by the wax. When the metal is no longer glowing red hot, the shell is removed and the cast metal parts are cut off from the sprue. They may receive additional machining and finishing though they require less than other methods of casting.

The investment casting process results in parts with excellent surface finishes because of the smooth impression left by the wax models. There are no seams or lines since the part is made whole, not in multiple pieces that must be joined; this also cuts down on wasted material. Investment casting products also fit the exact specifications of the original design. Investment casting is also called lost wax casting or, simply, wax casting. Many manufacturers depend on the investment casting process to produce dimensionally accurate precision investment casting parts. Though it is more expensive than other types of casting processes, it provides design flexibility with greater detail and a wide selection of metal alloys to use for materials. It also produces lightweight parts. Precision investment casting also saves money because it does not require welding, assembly or finishing work. The quality of precision investment casting is determined by the strength and tensile ductility of the casting. Similar to investment casting is centrifugal casting, a manufacturing process that produces cylindrical parts with thin walls. This technique is generally used for stock parts made in standard sizes that require additional machining or working; the parts are typically not end products themselves. Centrifugal casting produces high quality parts with uniform wall thicknesses because it has tight control over the material.

Investment casting is done with a wide variety of metals. Aluminum investment castings are among the most common, along with stainless steel and copper, though almost any castable metal may be used. Stainless steel castings are used frequently around hazardous substances or in sanitary environments because it is easy to clean and resists corrosion. Brass castings are used in many applications used industrially, commercially and domestically everyday. Copper investment castings are lightweight, strong and are widely used in many different capabilities such as pipe fittings, machine tools, mixing equipment, lock parts and jewelry. Investment castings can be found in the aerospace, automotive, chemical, defense, food processing, electrical, railroad, mechanical, marine, electronic, textile and engineering industries. When ordering steel investment castings, the type of steel that will be used to produce the part must be identified. By specifying the test methods for an order, the requirements of the material can be guaranteed. Because steel is stronger than cast iron, wrought iron and malleable iron, steel investment castings are typically used in the manufacturing of parts that must endure shocks, wear or heavy loads. Titanium castings are very strong and lightweight and are used for turbocharger components, industrial tools and instruments and other applications in the defense, aerospace, transportation, power generation, marine and medical industries.

Investment casting has been used to produce metal parts for thousands of years. Originally honeycomb was used to create the wax patterns. As materials and technologies have improved, the general process remains fairly simple and has not changed dramatically. It is a straightforward concept: form a mold around a wax pattern that will melt away, leaving a hardened shell that can be filled with molten metal. Today's manufacturers are aided by improved equipment that can produce identical results time after time. As the investment is mixed, fans and blowers remove the dust which contains silica, a potentially harmful dust that should not be inhaled. A vacuum is often used to remove air bubbles from the investment to ensure that all the patterns on the tree are encased in the ceramic slurry without pockets of air. Automated ovens and furnaces control the temperature for a precise burnout while other heating equipment can melt the alloys to the appropriate temperature. Extensive knowledge regarding the chemical formulas and molecular composition of the materials makes it possible for manufacturers to choose specific attributes for the part, such as durability, magnetism or weight load while avoiding negative characteristics. Science and engineering have helped make the traditional process of investment casting even more successful in the modern age.

Images Provided by Ferralloy Inc.

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Investment Castings Types

• Aluminum investment casting is the process that produces industrial parts through the metallic replication of wax models. Molten aluminum is used to create parts that have a very smooth surface finish, high dimensional accuracy and acute detail without flash or parting lines. Aluminum and aluminum alloys are among the most commonly used metals for investment casting.
• Brass investment casting is the manufacturing process that creates parts through the metallic replication of wax models. Investment casting using molten brass results in a final product that is very smooth and detailed without flash or parting lines. This technique creates parts that are precise and lightweight.
• Centrifugal casting is a manufacturing process that is used to produce parts with thin walls. This technique is generally used for cylinders or for stock parts in standard sizes that are not finalized products.
• Copper investment casting is the process where parts are manufactured through the metallic replication of wax models. The process of investment casting uses liquid copper to manufacture parts that have a very fine, smooth finish and are intricately detailed.
• Ferrous investment castings are cast out of metals that contain iron, including 300- and 400-series stainless steels, carbon and alloy steels and cobalt and tool steels.
  
• Investment casting process is the method through which industrial parts are produced through the metallic replication of wax models.
• Investment casting products are parts produced through the metallic replication of wax models. The parts may be made of almost any castable metal including carbon, stainless steel, aluminum, copper, titanium and brass. Because the parts are made from a wax model made to the exact specifications of the intended design, there is a high degree of dimensional accuracy.
• Lost wax casting consisted of creating and plastering a wax model, replacing the wax with molten metal and removing the plaster after the metal cooled.
• Non-ferrous investment castings are formed from metals that do not contain iron, including aluminum and copper-based alloys.
  
• Precision investment casting refers to the highly precise and reliable method of metal casting also known as investment casting. Investment casting is used to produce complex metal parts in an assortment of sizes, shapes, and weights. Many manufacturers depend on the investment casting process to produce accurate dimensional, precision investment casting parts.
• Stainless steel investment casting is the process through which parts are manufactured by the metallic replication of wax models. Parts created by investment casting have very fine finishes that require little additional machining. There are no flash or parting lines because the part is made as one whole piece from molten stainless steel. High levels of accuracy and precision are attainable; investment casting can produce parts that are too complex for other methods.
• Steel investment castings are a type of ferrous investment casting in which casts are made out of stainless and carbon alloy steels. During steel investment casting, free-flowing liquid steel is shaped through the use of molds. Steel investment casting produces small, complex, or hard-to-machine shapes, eliminating the need for assembly, welding, and other finishing work.
• Titanium investment casting is the manufacturing process that produces parts through the metallic replication of wax models. The resulting titanium parts have a very smooth finish that requires little finishing after the process. Investment casting is effective at producing complex and detailed parts that other methods cannot. There are no joints or flash lines because the part is made as one whole piece.
• Wax casting is a process that uses a wax pattern to create metal parts. Due to the smooth surface of the wax pattern that is used to create a ceramic mold, the resulting parts have a very fine finish that requires little machining. Wax casting is effective at producing complex and detailed parts that other manufacturing methods cannot.

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Investment Casting Terms

Alloy - A compound consisting of either more than one metal or a metal and a nonmetal blended together.
 
"As Cast" Condition - Casting that is not heat treated after being removed from the mold.
 
Burnout - The process whereby excess water and wax is removed from a molding.
 
Casting - Forming and shaping a material substance, such as an industrial piece of equipment, by pouring liquid into a mold and allowing it to harden.
 
Cavity - The empty space within the mold into which the molten metal will be poured.
 
Centrifugal Casting - The process of filling a permanent mold cavity with molten material while rotating the mold. Centrifugal casting is often used during the investment casting process to increase the amount of pressure exerted upon the metal in order to reproduce mold details, such as lettering and holes.
 
Ceramic Mold Process - The process by which an investment mold is created from a pattern in order to be cast. Ceramic molds consist of solid molds and ceramic shells.
 
Ceramic Shell Process - The method of creating a ceramic mold through the immersion of a pattern into a slurry of liquid and covering the coated pattern with sand. The pattern is usually immersed and coated nine to ten times before the shell is completed.
 
Cocoon Process - The method of applying protective plastic to metal.
 
Cold Shut - Lines on the surface of a casting that are the by-product of the incomplete fusion of metal streams.
 
Concentricity - The sharing of a common point or axis of two or more surfaces of shapes, such as cylinders, cones, spheres and hexagons.
 
Core - A form inserted into a mold to create internal mold features.
 
Dirty Casting - A surplus of nonmetal substances within a casting.
 
Ferrous - Pertaining to or containing iron.
 
Forging - The method of reshaping metal through the application of heat and compression.
 
Gates - The openings in the gating system that transfer the molten metal from the runners into the mold cavity.
 
Gating System - The structure that transfers the molten metal into the mold. Gating systems are designed to uniformly transfer the metal into the mold cavity to create an even mold.
 
Inclusions - Small pieces of refractory materials, sand, slag or deoxidation products that have been trapped within the casting while it was solidifying.
 
Injection Molding - The injection of a molten substance, usually metal, under substantial pressure into a mold.
 
Lost Wax Process - An ancient process related to investment casting that consisted of creating and plastering a wax model, replacing the wax with molten metal and removing the plaster after the metal cooled.
 
Misrun - An incomplete casting.
 
Mold - A hollow container from which liquid substances can be formed into solid shapes.
 
Molten - A solid that has been converted into a liquid through the application of heat.
 
Non-ferrous - Not pertaining to or containing iron. Examples of non-ferrous metals include aluminum- and copper-based substances.
 
Pattern - A casting model created by injecting metal dye with wax. When the wax solidifies, it forms the pattern, which is then molded and cast.
 
Pattern Oversize Factor - A term that refers to the extent to which the pattern size must be increased to allow for metal shrinkage.
 
Plaster Mold Casting - The process in which a wax pattern is sprayed with plaster slurry to create a mold.
 
Permanent Mold Casting - The process of using metal, often iron, to create the mold. Permanent molds are cheaper than sand molds when a large quantity of parts is produced, as they can be used more than once.
 
Pouring Cup - The part of the gating system that pushes the molten metal into a mold.
 
Riser - An open space in a mold into which extra molten metal is trapped.
 
Runners - Horizontal mechanisms connected to the sprue that transfer the molten metal from the sprue to the mold.
 
Sand Casting - The process of pouring molten metal into a natural or an artificial sand mold to form large parts, often made of iron.
 
Shell Casting - The process in which sand and plastic are poured over a hot metal pattern, whose heat causes the sand and plastic to mold to the pattern. The mold is removed from the metal, filled with molten metal and broken once the metal hardens, forming a cast.
 
Shrinkage - The contraction of metal during the cooling process.

Shrinkage Allowance - The degree to which a pattern is enlarged to allow for the contraction of the metal during the cooling process.
 
Slurry - The thick, insoluble liquid made of substances like plaster and silica from which a mold is created by repeatedly covering the patterned wax with the liquid.
 
Solid Mold Process - The method in which a wax pattern is placed within a steel container called a flask. The flask is then filled with a liquid plaster, which hardens into a mold of the pattern.
 
Sprue - Vertical mechanism connected to the pouring cup that transfers the molten metal to the runners.
 
Straightness - A term that refers to the deviation of the cast axis from the true axis.
 
Vents - Small holes in the gating system through which air is pushed out of the mold as molten metal is poured into the mold.

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