Types of Casting Processes

Introduction

This Article takes an In-depth look at Casting Processes

You will learn more about topics such as:

• History of Casting
• The Casting Process
• Types of Casting Processes

History of Casting

The casting process is an ancient art that goes back several thousand years to the beginning of written history. The archeological record has finds that document the use of the casting process over 6000 years ago around 3000 BC or BCE. The supposition is that the molds of that time were two pieces of pottery that were tied together with a rope and had a hole in them to be able to pour in the molten metal. Early weapons and hunting tools are presumed to have been formed in this way

The ancient techniques were used by the Egyptians to plaster the heads of mummies, which was part of the spiritual beliefs of the Egyptian culture. Included in this ceremonial process was the molding of jewelry and other items. At the same time the Egyptians were perfecting the art of casting, eastern cultures were using the method.

By the time molding reached the Greeks and Romans, it had become an artform used to cast bronze statues using a hollow wax casting. Each part of a piece was cast separately. The core of the mold was made of clay and covered with wax followed by a layer of clay that was heated to melt the internal wax, which was heated a second time to burn out the remaining wax. Once the mold was stable and prepared, the molten metal was poured into the area where the wax had been removed, a method that is similar to modern day investment casting.

Artisans of the Renaissance period were fascinated by the works of the Greeks and Romans. They continued and perfected the casting process with improved molds made from wood, terracotta, or plaster. The most difficult part of the process was the creation of the mold to produce a correctly proportioned form.

The present use of casting to produce tools, bowls, and other practical items was begun in China around 1000 BCE. Using iron, the Chinese mass produced farm tools and weapons. The technique did not reach European cultures until several centuries later and was used to make cannon, cannon balls, and bullets. With the advent of the industrial revolution in America and Europe, casting became a standard manufacturing process much like it is today. As new metals were discovered and techniques improved, the products produced were of higher quality and endurance. Today, a variety of casting methods are used to make everyday items for commercial and industrial use.

With the advent of the industrial revolution in America and Europe, casting became a standard manufacturing process much like it is today. As new metals were discovered and techniques improved, the products produced were of higher quality and endurance. Today, a variety of metal casting methods are used to make everyday items for commercial and industrial use.

The evolution of metal casting took place over several thousand years. As methods for heating and melting metal improved, casting processes rapidly improved. The first metal to be cast was gold, due to its malleability and low melting point. Of the many original metal casting, the oldest to survive is a copper frog from 1500 BCE. China added to the advancement of casting with the discovery of sand casting and iron or pig iron as it was known.

Several events during the first industrial revolution significantly impacted the growth and development of casting. In 1809, A G Eckhart introduced centrifugal casting as a method for evenly distributing the molten metal by rotating the mold. In 1837, Jarvis Adams introduced the first casting machine, which was used to produce letters for printing presses. In 1856, Sir Henry Bessemer discovered that blowing oxygen into molten metal removed excess carbon and impurities to make pouring molten metals easier.

Foundries

Foundries go back centuries to a time when casting was completed using primitive equipment and relied on manual labor. They have been a part of casting since the process moved from local businesses into the industrial world. Foundry casting produces castings by melting metal and pouring it into molds. Modern foundries are technologically advanced and heavily mechanized to reduce the emphasis on labor. They include furnaces, ladles, forklifts, cranes, conveyors, and transfer containers that are designed to perform in exceptionally high temperatures.

Foundries are differentiated by the metals they process since the type of equipment in a foundry is designed to handle certain types of metal. Electric arc furnaces are used in steel foundries while induction furnaces are used in copper foundries. Additionally, the size of the foundries equipment can range from small table top units up to equipment weighing several tons with production quantities ranging from ounces to tons. Foundries are hazardous and dangerous due to their atmosphere and temperatures at which the metals have to be formed and melted.

Metals Used in the Casting Process

In the metal casting process, different metal parts have different requirements and standards, such as strength, durability, appearance, and complexity. The list of specifications requires that the correct metal be chosen for a part to perform properly. The selection process necessitates an understanding of the properties and characteristics of certain metals.

Aluminum

Alloys of aluminum are very castable with a high level of machinability and lower cost. The natural properties of aluminum make it ideal for a wide range of applications. Aluminum is cast using all of the different casting methods with the parts being produced used for industrial and commercial products.

Carbon Steel

Carbon steel has a high level of machinability and weldability but always retains its toughness. Castings made of carbon steel are able to operate in conditions with high pressure where wear resistance and strength are required. The mechanical properties of carbon steel are determined by the amount of carbon it contains with higher percentages making the steel harder and stronger. Carbon steels with low to medium carbon are cast using investment casting.

Copper Based Alloy

Copper is a highly malleable metal that has been cast for centuries. It is never cast in its pure form since pure cast copper can have porosity problems. Common copper alloys that are cast are C80000 up to C9999, which are chosen for their tensile and compressive strength, wear resistance, machinability, conductivity, and corrosion resistance. Copper alloys are placed into three groups according to their solidification or freezing range. The most popular copper alloys are brass, which is made up of copper, zinc, and bronze, and bronze, made up of copper and tin.

Magnesium

Magnesium is widely used as a base material for numerous alloys with magnesium alloy AZ91D being the most common alloying form. The use of magnesium as an alloy is due to its lightweight, durability, good castability, and exceptional toughness. Magnesium is 75% lighter than steel but with steel’s strength. What makes magnesium ideal for casting is its ability to be shaped into complex parts with thin walls and superior dimensional stability. Magnesium is cast using injection molding where the mold is immersed in a reservoir of molten magnesium and a piston forces molten magnesium into the mold.

Nickel Based Alloy

Nickel is another metal that was used in ancient times for casting. Monel is a name for alloys composed of nickel and copper with small amounts of iron, manganese, carbon, and silicon. It is stronger than pure nickel but difficult to machine once it has been cast. Inconel is an austenitic nickel chromium alloy that is resistant to oxidation and corrosion. Nickel alloys are cast using investment casting and produce components for high temperature or corrosive environments. The right combinations of nickel and its alloys can have the tensile strength of carbon steel with good ductility and wear resistance.

Iron

As with nickel, gold, and copper, iron has been cast for many centuries and originally was in the form of the very brittle metal, pig iron. Iron comes in several forms, which include gray iron, white iron, malleable iron, ductile iron, and graphite iron. Each of the various types have properties and characteristics that are appropriate for certain applications. Cast iron has a carbon content of 2% to 4% and other alloys including 1% to 3% of silicon. The high carbon content of cast iron means it solidifies as a heterogeneous alloy with a single microcrystalline structure.

Stainless Steel

Stainless steel is a general term used to describe a family of metals that contain chromium and are known for their resistance to corrosion and bright silvery appearance. The most common grades used for the casting process are the 300 series, 400 series, 14-4 PH series, 15-5 PH series, 17-4 PH series, and series 2205. The main differentiation between stainless steel grades is determined by their carbon. Stainless steel grades with low carbon content are highly resistant to corrosion while grades with a high carbon content are heat resistant. These two factors determine which grade will be used for casting with investment casting being the most common process.

Zinc

Zinc is a very popular metal for casting due to its lower tooling cost. Due to zinc's low melting temperature, zinc casting dies can last ten times longer than aluminum die casting dies and five times longer than magnesium dies. Zinc is cast using fast cycling hot chamber die casting where the molten zinc enters the die through a gooseneck that is connected to a tank of molten zinc. A plunger draws zinc from the molten tank and forces it under pressure into the die. Parts rapidly solidify and are ejected.

The Casting Process

In many ways, the process of casting has changed very little since its founding thousands of years ago. Even with today‘s technological advances and progressive methods of production, the casting remains unchanged and still employs a mold and molten metal. The evolution over the many centuries has seen the development of a more exacting and automated method for the production of high quality products.

There have been great strides in casting, which have increased efficiency and production. Unlike the Egyptians, Greeks, and Romans, modern engineers can design a part and have it in production rather easily since much of the development of molds and casts is completed through automation and electronics. An innumerable number of products are produced each day that fill store shelves and serve as parts of cars, planes, and spaceships.

Every device we use is produced from the casting process. The biggest difference between present day casting and the process of a hundred years ago is the amount of planning, precision, design, and tolerance achieved through computerization and automation. Cores and molds are more detailed and precise down to the smallest detail and part.

Casting begins with the designing of the pattern, which is a model for the item to be cast. Patternmaking is a complex process of shaping the mold cavity with accurate dimensions. Once the item is set and cold in the mold, provisions have to be made for it to be extracted from the mold without breaking, which means making allowances for shrinkage during solidification as well as possible distortions. The pattern must include a method of feeding liquid metal into the mold. Any errors in the development of the pattern can lead to flaws and a failed casting.

Core making applies to parts that will have an internal cavity and does not apply to all casting processes. It is most commonly used in sand casting, die casting, and injection molding. When a casting is going to be hollow, sand or metal, called the core, shapes the interior of the form. Cores are strong but collapsible and are easily removed at the end of the casting process. The use of cores allows for the creation of complex designs such as holes or special chambers. When molding an automotive engine, five cores are required to produce the necessary chambers for an internal combustion engine.

Molding is a process for making a cast of a pattern. In casting, the mold is held in place in a frame called a flask. A type of sand is forced into the flask surrounding the pattern creating the mold. Once shaped, the pattern is removed leaving the casting. After completing the mold design, it can be fired, depending on the material, which hardens it and prepares it for the molten metals.

The next step is to melt the metal for pouring into the mold through a channel or hole called a sprue. Once the molten metal hardens, the mold is shaken or vibrated to remove sand from the casting, which is collected to be reused.

The final step in the process is cleaning the product. Excess molding material is removed as well as deformities and jagged edges. The product is worked to its final form and shaped. It may be burnished or polished depending on its specifications.

Aluminum Casting

All forms of metals can be cast and formed using the casting process, which range from hardened steel and stainless steel to copper and zinc. Of the wide selection of metals, the one that is used the most is aluminum due to its exceptional strength to weight ratio, resistance to corrosion, and bright finish. Every form of casting can be used to shape and mold aluminum with die casting, sand casting, and permanent mold casting being the main choices.

• Die casting uses pressure to force molten aluminum into a steel die. It is used for the production of high volume aluminum parts that require minimal finishing and machining. The main cost of aluminum casting using a die is the tooling and shaping of a die that has longevity due to the steel from which it is shaped. The pressurized system of die casting has rapid cycle times to form high strength exterior skins on aluminum parts with interiors that are weaker than those formed by permanent mold casting.
• Permanent mold casting, also known as gravity die casting, has molten aluminum poured into a mold made of steel. The process is used to produce aluminum castings that are consistently shaped and rapidly cooled for better microstructure and improved mechanical properties. The inner surface of the mold for permanent mold aluminum casting is sprayed with a protective coating to protect the mold and assist in solidification. Permanent mold casting produces aluminum castings with a high degree of structural integrity, internal soundness, and superior mechanical properties.
• Sand casting of aluminum involves packing sand around a pattern that has the configuration of the desired part. The pattern is slightly larger than the final product to allow for shrinkage of the aluminum casting during the cooling process. Sand casting is an economical process that is effective in creating large aluminum castings with detailed designs and intricate shapes. Unlike permanent mold casting and die casting, upfront costs are low due to using sand as the mold. On the flip side, per part cost is higher due to the nature of the process. Sand casting is not used for high volume mass production of aluminum castings.

Types of Casting Processes

The basic methods from ancient times have been transformed into a vast array of casting techniques designed for specialized and specific purposes. Each of the different processes can produce quality parts and have manufacturing benefits. An understanding of the advantages and disadvantages of each method can help in choosing a method designed to meet individual production requirements. Some of the popular kinds of casting processes include sand, die, investment, and plaster. The basic principles of each method may seem similar. How the processes are completed and the quality of what they produce differs greatly.

Die casting

Die casting forms parts or designs by injecting molten metal into a die or mold using high pressure. An extinct method of printing called linotype used the die casting method to produce printing plates for large printing presses. Its development replaced or added to the gravure process that preceded it. With the development of the computer, linotype machines disappeared, replaced by efficient technical methods.

There are two types of die casting – cold chamber and piston or gooseneck. These methods vary according to how the molten metal is injected into the die. Understanding the difference between the two processes can help in deciding on the production method for the design of a part.

Cold chamber die casting is used with metals that have a high melting point. Common materials used in this process are metal alloys such as aluminum, brass, and copper. The cold chamber process requires the use of a furnace and ladle for pouring molten metal. There are two methods of introducing the molten metal to the die in the cold chamber process – ladling or by a high pressure plunger. Cold chamber die casting requires much higher pressure than other die casting methods but takes a few minutes for the molten metal to solidify. Also, the dies can have multiple chambers making it possible to produce several parts at the same time.

In the piston or gooseneck process the piston is removed and the die is submerged in the molten metal. When the die is completely immersed and the gooseneck is full, the piston forces the molten metal out of the gooseneck into the die. The piston process has a rapid cycle time of approximately 15 minutes making it possible to produce parts quickly and efficiently. The method is restricted to metals with a low melting point and can not be used with aluminum, which sticks to the sides of the die.

The first step in the die casting process is the creation of the two sections of the reusable steel mold. To ensure ease of removing the casting from the mold, a lubricant is applied to help regulate the temperature as well as assist in removal when the die is separated. The two sections are then firmly clamped together, and molten metal is injected. Die casting has the flexibility of producing highly complex and intricate parts or very simple ones but is restricted to non-ferrous metals.

There are four basic categories of dies: single cavity, multiple cavity, combination, and unit. As the name implies, single cavity dies have one single chamber while multiple cavity dies can have chambers that are similar or different depending on the process. Multiple cavity dies with different cavities are referred to as combinations. Unit dies have several cavities connected by a sprue and can produce several parts in one casting.

Regardless of the restriction of using only non-ferrous metals, die casting has the advantage of producing parts in the correct size with an excellent shape tolerance. The ability of having dimensional consistency and uniform design are two qualities that have made it popular for many years. As with some of the other casting techniques, die cast parts require little machining post casting.

The biggest drawback to die casting is the expense of the process, which is mainly related to the creation and tooling of the die. Though they can be designed and engineered using computer software, they are produced using molten steel restricting the ability to experiment and make prototypes. Since dies can be stored and reused, it is an excellent way to produce large quantities of parts, which lowers the cost of the initial investment. It should not be considered for single parts, prototypes, or small runs.

There is a limitation to the mechanical properties of die cast parts. They are seldom considered for use as components and do not function as structural parts. Items that are die cast are designed for immediate use such as an engine block.

Gravity Die Casting

With gravity die casting, the molten metal is poured directly into the mold cavity using a ladle or other form of container. The fundamental principle of gravity die casting is to use gravity as the force to fill the cavity of the mold. The goal of the process is to have the filling process produce minimal turbulence to reduce oxidation and foaming, which further minimizes porosity and inclusions to give optimum characteristics to the complete product.

Tilting the mold in gravity die casting produces denser, high quality castings that have high strength and stiffness, making the process beneficial for the production of brake systems and suspension systems. The process is ideal for high production runs and automated production.

Pressure Die Casting

Pressure die casting forces the molten metal at high speed and pressure into a closed die. One half of the die is stationary while the other half is moveable, both of which are mounted on the platen of the casting machine. A sprue, connected to the stationary portion of the die, receives the molten metal that is injected by a hydraulic driven piston that creates the pressure. Toggles and hydraulics are used to absorb the injection pressure and keep the die tightly closed. In a few seconds, the molten metal is turned into a solid cast part.

The metals used in pressure die casting are aluminum, zinc, and magnesium. Pressure die casting can produce high volumes of light alloy products with exceptional speed and efficiency. Once a product is cast, it requires little to no after process secondary finishing.

Miniature Die Casting

Miniature die casting is used to produce small, intricate, and complex components using dies and die casting machines. The process of miniature die casting is the same as die casting processes used to produce larger parts but with faster cycle times, which gives the die casting tools a longer useful life. Aside from lower tool costs, a main advantage of miniature die casting is the exceptional precision with tolerances of +/- 0.001 and wall thicknesses of 0.020 in parts weighing 0.75 lbs (0.34 kg).

Parts produced using miniature die casting include control units, computer hardware, telecommunication parts, and various electronic devices. As with pressure die casting, miniature die cast parts and products require little to no after casting finishing.

Aluminum Die Casting

Aluminum is the most common non-ferrous metal that is used in the casting process. Casting of aluminum produces highly durable lightweight parts without damaging the properties or characteristics of the aluminum. Aluminum die cast parts have surface finishing options with the advantage of being able to withstand higher operating temperatures than other non-ferrous metals. Much like high pressure casting and miniature casting, aluminum casting has rapid cycle times that allows for high volume production.

Die casting of aluminum is able to produce highly complex shapes with intricate features and is a better process for achieving exceptional tolerances and better part finishes, which eliminates the need for surface finishing. Aluminum is mainly cast using one of its many alloys with alloys A380, 383, B390, A413, A360, and CC401 being the most used. The choice of an alloy is based on the final use of the cast part. Each alloy has properties and characteristics that make it ideal for a specific application.

Sand casting

Sand casting uses sand molds to form and shape castings. It is a common production method for the manufacture of metal parts of varying sizes and weights and can produce complex detailed parts using any type of metal alloy. Though sand casting is a cost effective and economical method, it is capable of efficiently producing high quality parts. All of the materials used in the process are reusable and recyclable, which adds to its low cost.

The sand casting method is one of the few processes to be used with metals that have a high melting point such as certain types of steel, nickel, and titanium. The flexibility and heat resistance of sand casting as well as its low cost has made it the most widely used casting process.

Castings are made by pouring molten metal into the mold cavity. The sands used to make the castings have a special bonding material that increases its resistance to heat and ability to hold its shape. For many years, Green sand has been mostly used to create the castings, which is a mixture of sand, coal, bentonite clay, and water. Recently, silica (SiO2) has become more widely used than Green sand.

There are several characteristics of sand molding, aside from low cost, that have made it a popular process. Sand molds retain their shape under mechanical stress but are permeable enough to release gases and steam. When sand is applied to the pattern, it can fill small recesses to create a precise mold of minute details. Though molding of large heavy parts is a difficult process, sand casting easily adapts and adjusts to produce parts of any size and can cast ferrous and non-ferrous metals.

Regardless of its popularity, sand casting has certain drawbacks and limitations such as poor dimensional accuracy and the inability to produce parts that require a high tolerance. Also, parts produced by sand casting tend to have a rough or coarse finish.

Though it has these disadvantages, it is still one of the most popular and profitable methods for part production.

Gray Iron Casting/Grey Iron Casting

Gray iron casting involves the use of molten iron that is poured into a mold cavity and allowed to harden and solidity. Of the various types of casting methods, iron casting is the oldest and has been used for centuries to produce weapons, cookware, tools, and utensils. The types of alloys added to iron determines the type being cast. A differentiation between the types of gray iron is the amount of carbon they contain, which determines their melting temperature, weldability, and machinability.

Gray iron casting, or grey iron casting, uses smelted gray iron, which is an alloy containing iron and carbon with traces of phosphorous, sulfur, silicon, and manganese. The materials for gray iron casting have a graphitic microstructure, which is a predictor of the strength and impact resistance of iron castings.

A common part of the production of gray iron castings is the use of heat treatments to improve the mechanical properties of the casting and increases its thermal conductivity, strength, durability, machinability, and cost. Finished gray iron castings are subjected to a variety of finishing processes to reach the required tolerances.

Uses for gray iron castings include valves, engine blocks, brake drums, pump housings, and cookware. The methods to produce gray iron castings include lost foam, mold, and sand casting.

Investment casting

Investment casting uses a wax pattern coated with a ceramic material, which hardens to the shape of the casting. Once the ceramic sets, the wax is melted away and molten metal is poured into the emptied cavity. When the metal solidifies, the casting is broken to release the metal part. Also known as lost wax processing, it is a method that has existed for over 5000 years and dates back to the time of the ancient Egyptians and Chinese.

The first step in the investment casting process is to produce a wax pattern, which can be made from plastic but is most often made from wax. The mold can be cast or machined with its dimensions carefully calculated and engineered to avoid shrinkage. Since the process requires precise measurements, several trials may be necessary to reach the proper proportions, which makes investment casting molds expensive.

Investment casting is used to produce precision parts from several alloys or metals, including aluminum, stainless steel, carbon steel, brass, and bronze. The parts produced are found in several industries including fluid power, oil and gas, food and dairy, military, firearms, aerospace, and aviation as well as agriculture.

Investment casting parts have excellent dimensional tolerances with a higher degree of accuracy and require little finishing or machining and can produce complex shapes with intricate designs. As with sand casting, investment casting has little waste since the ceramic material can be reused and is able to produce parts from several different alloys.

Though investment casting is an expensive process compared to sand casting, the quality of the parts it produces makes its use appealing. Parts have an excellent finish and require very little machining or finishing, which can compensate for the added initial cost.

The two methods of investment casting are gravity fed and vacuum. With the gravity fed process, the molten metal flows into the mold by the force of gravity without the use of pressure or other means. Vacuum casting is a precision casting process used for casting aircraft parts and involves inserting the molten metal into the mold under negative pressure.

Vacuum casting varies from normal investment casting in that it relies on a vacuum to insert the molten metal. The process begins with a two piece mold placed in a vacuum chamber where the molten metal is drawn into the mold by negative pressure. While investment casting allows the workpiece to cool in the sand mold, vacuum casting solidifies a casting in an oven.

Permanent mold casting

As the name implies, permanent mold casting uses reusable molds much like die and centrifugal casting and has a variety of applications for jobs that require mass production or duplication. Though it is more expensive than the other forms of casting, it is ideal for the production of parts for major industrial operations.

As with die casting, molds for permanent mold casting consist of two pieces made of metals with a high melting point such as steel, graphite, bronze, or cast iron. The parts fit tightly together with an opening at the top for the molten metal to enter. As with die casting, when the molten metal cools, the two sections are separated to release the finished part.

The permanent molding process begins by heating the mold to remove any moisture and prevent damage to the mold from thermal expansion when the molten metal is inserted. Also, preheating helps keep the molten metal from cooling during the casting process.

There are different methods for introducing the molten metal into the mold include gravity, pressure assisted, vacuum assisted, and slush casting. With the gravity method, the molten metal is simply poured into the mold. It is the least expensive method. When a mold requires fine details, low pressure is used to force the molten metal into the mold. With the vacuum method, air is removed from the mold creating a vacuum that sucks the molten metal into the mold. The use of low pressure and vacuum is for parts with small spaces and fine details. In the slush method, the molten metal is poured into the mold and allowed to harden against the outer surface of the mold. Once the surface material is solidified, the remaining molten metal in the center is poured off leaving a hollow casting. The slush method is used to make hollow chocolate Easter bunnies.

Lost foam casting

In the lost foam casting process, the casting mold is made of polystyrene foam that is formed from a block of foam or made using the injection molding process. Lost foam casting is another form of investment casting where foam replaces wax to create the mold. The process was introduced in 1958 by H. F. Shroyer who received a patent to use polystyrene in green sand to form a foam pattern.

The tooling for lost foam casting includes a split cavity aluminum die where the foam pattern is produced. Lost foam patterns are very similar to permanent die casting molds and require the same amount of expertise and experience in their tooling. Aluminum dies for lost foam casting are very durable and have a very long life cycle.

The pattern making process for lost foam casting involves the production of a foam pattern with a gating system that is produced by a foam press. Included in the pattern making process is the addition of risers and gates. Pattern making is critical to the quality and value of the piece being produced and requires close attention to details and precision accuracy. Patterns are made using a closed die or shaped from a solid piece of polystyrene.

One or multiple parts can be produced from a lost foam pattern. The gating system and pattern are referred to as a cluster, which has to be coated with a permeable ceramic refractory coating. The processes for coating the foam pattern include dipping, brushing, spraying, or flow coating. The purpose of the coating is to create a barrier between the surface of the foam and sand. Additionally, the coating controls permeability and allows the gas from the vaporizing foam to escape into the sand.

The cluster is allowed to dry and harden prior to being placed in a foundry flask with loose, unbounded sand. In order to form a tight compact seal around the pattern, the flask, sand, and pattern are vibrated to impress the shape and pattern of the part in the sand. A special type of sand is used in the lost foam process referred to as green sand, which is a mixture of sand, clay, sludge, anthracite, and water. It is called green sand not for its color but for the fact that it is not set.

Once the sand is tightly pact and the impression of the pattern is encased in the sand, the molten metal is poured into the top of the gating system. The molten metal causes the polystyrene foam pattern to vaporize as the pattern is filled. The amount of molten metal is carefully calculated and measured before it is poured into the gate system. Air vents in the sides of the flask allow the vapor from the foam to escape.

The solidification of the pattern varies according to the type of metal being used in the casting process. Cooling begins immediately after the completion of the pouring and can take a few minutes. As the temperature drops, the molten metal begins to form crystals near the walls of the sand mold, which continues until the entire pattern is solidified.

After the casting is sufficiently hardened, the cooled metal is removed or broken from the sand mold. In most cases, the sand is removed by being shaken off or out of the flask. Once out of the sand, the gating system has to be removed leaving the completed parts.

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The fully shaped and formed part is now ready for the many varieties of post treatments designed to perfect and enhance the cast piece. The different types of post treatments include removing of the gates, risers, and runners and sandblasting or grinding the metal workpiece to achieve the necessary smoothness, tolerance, and shape. As with all die cast parts, there are a wide variety of machining processes that are used to perfect the final component.

Centrifugal casting

Centrifugal casting, also known as the deLavaud process, uses a spinning mold to produce lengths of pipe through the use of G force created by rapidly rotating the mold. The concept was invented by French engineer Dimitri Sensaud deLavaud as a more efficient method of producing iron pipe.

The centrifugal process consists of a spinning steel mold enclosed in a jacket of water or water spray. Molten metal is injected into the casting by a ladle through a trough, which rides on a movable carriage or platform. As the molten metal enters the casting, it stretches to the full length of the mold. The molten metal is first ladled into a bell from which it enters the casting and continues to enter the mold until the full length is full to the spigot end. The centrifugal movement forces the iron to the wall of the mold where it solidifies to a seamless pipe. Joints are created by a resin coated core of sand of the correct dimensions for the mold, which prevents molten metal from escaping.

To increase the adhesion of the mold, it is peened to improve surface friction and enhances the life of the mold. Also, peening helps sprays stick to the walls of the mold to make removal of the casting more efficient. During the casting process, the die can be spinning vertically or horizontally depending on the configuration of the part to be produced where ring and cylinder parts are shaped vertically, and tube shapes are made horizontally.

The centrifugal force of the process removes less dense materials such as impurities and "floats". Solidification happens under the pressure of the spinning force creating a defect free part without cavities or gas pockets.

Aside from pipes, centrifugal casting can be used to manufacture flywheels, cylinder linings, and axi-symmetric parts. The high quality of cylinder liners and sleeve valves from the centrifugal process cannot be produced using any other method of casting.

Pressure casting, a form of centrifugal casting, is used for asymmetrical parts that cannot be spun around their own axis. The method is quick and cost effective for the production of high volume parts with a tight tolerance. A molten metal alloy is injected into a steel mold under high pressure and solidifies almost immediately to be extracted. This method can be used for large gear rings and other such items.

Plaster casting

Plaster casting is a process used to manufacture non-ferrous alloy parts with a smooth, even finish. Precise detailed parts with dimensional accuracy are normally produced using this plaster casting. As with many modern designs, the pattern for the casting is created in CAD or some type of software and includes allowances for shrinkage.

The process of making the mold begins with plaster composed of gypsum or calcium sulfate, which is mixed with talc, asbestos, sand, sodium silicate, and water. The mixture of these elements forms a slurry that is sprayed on the pattern, which has been sprayed with an anti-adhesive to avoid plaster sticking to the pattern. Molds form in a few minutes, are removed from the pattern, and dried. The cores and mold are then assembled prior to having molten metal poured into them. As with investment casting, when the metal cools and hardens, the mold is broken away to release the part.

There are limitations with plaster casting since the process is complicated and takes time, which increases its cost. Its greatest success is with materials that have a low melting point such as aluminum, copper, magnesium, or zinc. Since it takes little time to produce a mold, it is an excellent method for taking a CAD rendering and making a prototype.

Final products produced using plaster casting have smooth even surfaces with excellent details. Unlike other casting methods, the process precisely replicates the intricate and complex details of parts that have thin walls. As with sand casting, it can form large parts from non-ferrous metals with low melting points.

Vacuum Casting

Vacuum casting, also known as urethane casting or polyurethane casting, is unlike permanent mold casting or die casting in that it uses silicone molds to produce plastic and rubber components. The silicone mold is created using 3D molding and follows the traditional injection molding process. The pattern is fitted with cores, inserts, and gates after which it is placed in the casting box. Risers are added to allow air to escape from the mold.

Silicone is poured over the master pattern under pressure into the casting box where it completely covers the pattern, filling in every single detail. Once completed, the pattern and its silicone covering are cured at 40o C (104o F) for 8 to 16 hours. How long the mold cures is dependent on its size. After curing, the box and risers are removed.

The cured mold is cut using a wavy pattern to expose the negative cavity for the part. The wavy pattern is to ensure the proper alignment of the mold halves during production. The resin for the part is prepared and mixed in exact proportions that may include colors and placed in a pouring bowl. To prevent the possibility of air pockets or voids, the resin mixture is poured into the mold under a vacuum, which ensures a bubble free mold and removes resistance to the flow of the resin.

Once the resin has been cast, the mold is cured in a heated chamber, after which it is removed from the mold. The finished part has the gates and risers clipped off the casting and has any imperfections or rough surfaces treated and finished.

Vacuum casting can be used to produce small batches of high quality parts for prototyping and low volume production. A wide selection of resins are available to fit the needs of various applications including clear, rubbery, flame retardant, and colors. Resin types can be easily switched without having to change tooling, which provides the opportunity to test different resins to better match the needs of an application.

Squeeze Casting

Squeeze casting is a combination of casting and forging where molten metal solidifies, under pressure, in a closed die that is positioned between the plates of a hydraulic press. In the direct method of squeeze casting, molten metal is poured into one half of a preheated die, and the upper half is closed over it to cause the molten metal to fill the die. Pressure of 70 MPa up to 140 MPa or more is applied using a hydraulic press during solidification. With the indirect method of squeeze casting, liquid metal is injected into an indirect squeeze casting machine through a shot sleeve and further injected into the die chamber through a gate.

As the molten metal begins to lose its liquid state, the upper half of the die is forced under greater pressure into the bottom half of the die until the casting solidifies. Pressurization ensures that the molten metal is equally distributed to avoid the metal part from being broken or separating. Once the metal has solidified, the ram of the hydraulic press withdraws, and the casting is ejected. The pressure created by the hydraulic press, during the process, is far less than that which is produced during forging. The result of the process is improved non-uniform metal structures and exceptional mechanical properties.

Continuous Casting

Continuous casting involves introducing molten metal into a mold that can rapidly solidify the molten metal, which ensures a fine, uniform grain structure and excellent physical properties. With continuous casting, molten metal is fed directly from an induction furnace into a mold through a series of holes in the top of the die. A water cooled jacket surrounds the mold that causes the molten metal to solidify rapidly. Molten metal above the die acts as a riser to keep the die filled to prevent shrinkage.

Once the molten metal solidifies, it is ejected through the bottom of the die by a mechanical device. The exit of the metal from the die is carefully controlled until it reaches the desired length and is cut off by a saw. The low casting temperature, chilled mold, and directional solidification produces castings with homogenous, fine grain structures and high density.

The two methods of continuous casting are horizontal and vertical. In the horizontal method, the equipment stretches out horizontally on one level. The process produces hollow tubes using gravity. Both methods produce rectangular, hexagonal, and square shapes as well as gear teeth and several other shapes. Continuous casting is ideal for producing small inside diameters. The main benefit of continuous casting is its ability to produce long tubular forms that are difficult for other casting methods.

Shell Molding or Mold Casting

Shell molding is similar to sand casting but is different in that it has a hardened shell of sand that forms the mold cavity using the pattern of the part. The sand in shell molding is finer and is mixed with a resin that is heat treated to form a shell around the metal pattern. After the sand and resin are mixed, the created sand is poured into a heated mold that reaches temperatures of 204o C to 371o C (400o to 700o F), which triggers a reaction with the resin covered sand. When the sand makes contact with the pattern, a shell is formed on its inner surface to create the mold.

The mold for shell molding is a metal piece in the shape of the desired part, which is used to form shell molds. The pattern is reusable while the formed molds are disposable like sand casting molds. The process can be used with ferrous and non-ferrous metals, which are most commonly cast iron, alloy steel, stainless steel, aluminum alloys, and copper alloys. Parts cast by shell casting are gear housings, cylinder heads, connecting rods, and level arms. The pattern for shell casting has two pieces of different metals. Although steel and iron are commonly used to produce patterns, aluminum is also used for low volume or graphite casting.

Top Five Casting Machines

Waupaca Foundry, Inc.

Waupaca Foundry makes ductile iron and gray iron castings using the most advanced technological techniques. The company supplies castings to leading automotive, agricultural, construction, and industrial markets. Dedicated to reducing its environmental impact and increasing efficiency, Waupaca has increased its energy efficiency by over 13% with its compressed air optimization project.

Shibaura Machine Company of America

Shibaura is the leading supplier of HPDC, high pressure, cold chamber, semi solid aluminum and magnesium die casting machines with capacities of 1350 kN tons up to 35000 kN tons. The company’s full line of die casting machines includes small, medium, large, and E-series that combines the speed of an electric servo motor with three platen toggle clamping.

Birch Machinery Company (BMC)

BMC manufactures aluminum and zinc die casting machines and trim presses. Additionally, the company rebuilds and remanufactures die casting, injection molding, blow molding, and trim press machines. BMC has a full line of hot and cold chamber machines that are exceptionally reliable and maintenance free. The company’s double pump provides continuous high flow and high pressure for its die actuating system.

RDO Induction, Inc.

RDO manufactures vacuum induction casting machines and induction heaters for casting prototypes. The company’s CS casting system is used for high volume small parts and low volume large parts. RDO’s supercast pro casting system is used at foundries, metal recyclers, and large casting manufacturers. It is an investment casting system that performs the same functions as old rotocasting systems. RDO’s CS1, CS2, and CS3 are designed for use with all types of metals and alloys.

Gesswein

Gesswein manufactures induction heated, pressure over vacuum casting machines for the manufacture of dense castings. The company’s line of casting machines includes the Galloni G1, Galloni G3, Galloni Heavy Duty, and the Galloni Pressovac Dual Touch. Gesswein’s high production casting machines cast platinum, gold, silver, and steel in cycle times of 3 to 4 minutes for gold and platinum. Cast pieces produced by Gesswein vacuum casting machines have exceptionally smooth surfaces with a dense molecular structure.

Conclusion

An understanding of the various casting methods is critical to making the decision of how to produce a conceptualization. Each of the different types has their advantages. The major considerations are the cost of production and the number of parts to be produced. Casting manufacturers specialize in one of the varieties of approaches. There are a few producers who will offer a variety of production methods. Carefully reviewing the qualifications of each producer as listed in the IQS Directory can assist in selecting the proper company for the job.

Here we list down and discuss 6 different types of casting processing commonly applied in metalworking foundries. Check here to specify the different types of casting process, pros & cons, and applications of each casting technique.

Casting is a metalworking process in the foundry that used to manufacture casting parts severing a range of industries, from mechanical engineering, automotive components, aerospace parts to everyday household products.

The casting processing works on melting metal, pouring into the molds, and waiting for solidification. The collected roughcasting either can be put in use directly or has to go through the further machining steps.

Metal casting is a highly flexible process and can fabricate complex-shaped parts regardless of the metal hardness, but only on the melting temperature of the metal. It can be said that any metal that can be melted can be cast.

There are different techniques to process the metalworking and each foundry invests and specializes in certain types of casting processing. In this article, we introduce the 6 different types of casting process commonly used in metal foundries. You will seek out the answer to how casting parts are processed by different types of metal casting methods and the pros & cons of each technology.

1. Green sand casting

What is green sand casting?

The green sand technique has such a long tradition in the casting industry but is still widely used today due to its effectiveness. The green sand casting is among the different types of casting that most traditional and preferable in the casting foundry.

In this casting processing, the sand mold is mass production. Each sand mold is used once and being broken to collect the roughcasting.

Different from the resin sand casting, green sand features wet-content. The term “green” not about its color, but moisture texture in the sand. It and has the ability to bond naturally mostly due to clay agents.

Green sand mold composition

Green sand casting mold

To make a green sand mold, the metal foundry combines these following components:

• Sand: Silica sand (SiO2), Chromite sand (FeCr2O4), or Zircon sand (ZrSiO4) (about 75 to 85%)
• Bentonite Clay: (5 to 11%)
• Water (2-4%)
• Others (3-5%)

The green sand casting process

The green sand casting are processed by following these steps

• Step 1: Mixing the sand according to a ratio. It is very important that determines the bonding of the sand mixture. Each metal foundry follows the above ratio but they will have a secret mixing formula by adding other agents.
• Step 2: Loading sand into the mold maker and press the pattern to create the mold cavity. Today foundry uses a cope and drag device to make the sand mold.
• Step 3: Removing the pattern, a mold cavity that has a similar shape to the intent casting is created.
• Step 4: Applying a coating layer to the mold surface to increase the surface gloss and heat resistance.
• Step 5: Pouring the molten metal into the mold cavity and cooling. Collect the roughcasting and go further machining steps (if needed).
• Step 6: Repeating these steps to make enough mold for mass production.

Green sand casting characteristics

Advantage:

• It is economical and inexpensive production, particularly in low volume run
• Do not require very high tech investment in the factory facility
• Apply for medium and high volume casting order
• Allow any alloys to be poured into the mold (both ferrous and non-ferrous materials). Plus can use for almost pattern and design
• The sand can be reused after the casting is collected. It saves input material cost for the foundry.

Limitations

• Limit in casting size. It can make castings in a range of 1 to 500 pounds weight. For the larger, it should be considered other casting methods.
• Not preferable to cast intricate casting details
• Large tolerance that would need more machining treatments
• Create concerns on casting defects such as blowhole, porosity, etc. but can control by foundry’s technical tips.

Green sand casting applications

Pump housing casting by green sand

Green sand casting is very popular with a range of applications. In America, 42% of casting parts are made by the green sand process while no-bake casting, in comparison, takes 40% (the second popular casting method).

You can find the green sand casting parts, from outdoor decoration pieces such as lamp post, bench, litter bin; engineering details, automotive parts, aerospace, pump housing to marine buoy weight, etc.

Read more about green sand casting here.

2. Furan resin sand casting

What is furan resin sand casting?

Other common different types of casting process in metal foundries are furan resin sand casting. Unlike the green sand that provides natural bonding, the furan resin sand casting relies on some catalysts and binders to bonding the sand. In this casting method, the sand, furan resin, and Catalystsare mix together and the sand mixture is self-harden by the chemical reaction.

The roughcasting collected features good smooth surface finish and high precision. 

Furan resin sand mold composition

Furan resin sand mold for lamp post base

In the furan resin sand casting, the mold is made by this following formula:

• Sand: Silica sand (SiO2) (40-60%)
• Furan resin: 0.7-1.3% depending on the casting mass. It is required less or without nitrogen accordingly to casting requirements and casting parts structure.

Furan resin play the important role in this casting method. It is a polymer compound with 75% Furfurylalcohol + 11% Formaldehyde + 9% Ure + 5% Water. The ratio of Formaldehyde and Urea affects the solidification time and the strength of the resin mixture, while Furfurylalcohol (FA) affects the heat resistant properties of the mixture.

• Catalyst: solidification agent and annexing agents (30-50%)

The furan resin sand casting process

The furan resin sand casting processes are explained by the following chart:

Furan resin sand process

Furan resin sand casting characteristics

Advantage

• Tight tolerance and high precision
• It provides a great smooth surface casting finish in comparing to other casting methods
• Minimize the casting defects such as sand holes, air holes, and shrinkage
• Flexible in casting size. Furan resin sand casting can make large pieces range from a dozen kilogram to several tons of weight

Limitation

• It is a more expensive method than other casting processes
• Strict requirement for raw material
• Creates environment concerns

Furan resin sand casting applications

Casting lamp post pole by furan casting

With its excellent advantage, the furan resin sand casing is often preferred to cast intricate and detailed parts that require high precision and tight tolerance such as automotive parts, agricultural machinery details, aerospace parts, ship engines, etc.

Read more about furan resin sand casting here.

3. Lost foam casting

What is lost foam casting?

The lost foam casting is a kind of evaporative pattern process that similar to investment casting. Instead of wax, the pattern is made of foam in the lost foam method.

Unlike the sand casting, that pattern can be removed and reused to make other molds, lost foam pattern can be used once and is evaporated leading to form the casting parts.

Not as popular as sand casting, but the lost foam casting is one of the different types of casting process that remains pretty awesome features that suits to mass production of small and medium detail casting parts.

The lost foam casting process

The lost foam casting process follows these steps:

• Step 1: pattern making. The lost foam pattern can be made by cutting machines or by polystyrene beads injection molding method. The pattern can be a competed pattern (simple design) or a few sections then being glued together to form a replica of intent casting. 
• Step 2: insulation paint coating to enhance the durability of the mold surface and protect from erosion and broken
• Step 3: the foam pattern is placed into a flask surrounding the unbound sand and being compacted.
• Step 4: pouring the molten metal, evaporating the foam pattern, and forming the roughcasting
• Step 5: cleaning the remaining sand and further machining if needed.  

Foam pattern

Lost foam casting characteristics

Advantage

• Flexible design
• Cast complex shape that unthinkable in other casting processes
• High precision and tight tolerance  
• No core defects, no mold shift, excellent smooth surface finish
• Reduce the processing time
• Reduce production and investment cost

Limitations

• It is more expensive than other methods
• It will be very costly for set up or small orders

Lost foam casting applications

Kettlebell casting by lost foam process

The lost foam casting does not limit to any design and type of alloy. It is often choosing to cast complex casting parts that other methods unable to work. For example automotive detail parts, high precision machinery elements, intricate core products…

Read more about Lost foam casting here.

4. Investment casting

What is investment casting?

Investment casting or lost wax casting is an advanced casting method used to cast complicated and thin-wall shapes. This process is similar to the lost foam casting. Instead of the foam pattern, the disposable wax pattern is formed by the injection method then coated with several layers of refractory material.

By melting the wax pattern, a mold cavity is created and ready for the metal liquid being poured into. Note that the pattern is used only once and that each casting needs a pattern.

This method has the characteristics of casting details with high accuracy, so it is often preferred to cast products for the machine manufacturing industry, especially the production of small details, high precision, without machining or cannot be machined.

The investment casting process

Investment casting Process

• Step 1: Making the master pattern and mold
• Step 2: Inject the wax into the mold to make wax patterns.
• Step 3: Assembling pattern sections
• Step 4: Shell making
• Step 5: De-waxing by heat
• Step 6: Pouring the molten metal
• Step 7: Shattering
• Step 8: Cutting off
• Step 9: Finished casting parts

Investment casting characteristics

Advantage

Investment casting is a modern casting approach with many advantages.

• Compared with sand casting, lost wax casting can make better complex details, thin walls, high surface quality, and significantly reduce machining works. Many finished casting parts can be used immediately.
• It can make products weighing from a few grams to a ton.
• Can cast hard-to-melt alloys such as stainless steel, thin steel
• The casting surface is high precise and gloss
• The casting is less likely to crack or warp

Limitations

• High labor intensity
• Long production cycle
• Mold manufacturing costs are high

Investment casting applications

Complex castings, or in mass production to fabricate gears; bicycle trunks; moto disc; spare parts in blasting machine … Casting with weight from 0.02 ÷ 100kg; thickness to 0.3mm and hole diameter to 2mm.

5. Die casting

What is die casting?

Die casting is the molding material method under high pressure and applied to cast non-ferrous metals and alloys.

It is used in casting parts for bulk orders because the advantage of this casting method is its high productivity and simplified process thanks to the application of a fully automated line. The roughcasting collected has tight tolerance, high precision, and a nice gloss surface finish.

The die casting process

Die casting process

• Step 1: Melting the metal into fluid
• Step 2: Pouring the molten metal into the chamber
• Step 3: High-pressure piston injection system pushes the fluid into the mold
• Step 4: Wait until the casting has solidified, then remove the core
• Step 5: The latch will eject the object out
• Step 5: Remove the scrap materials and continue the next segment

The entire casting process of the die casting method requires the use of high-pressure pistons and tight presses to ensure the durability of the casting. With the automatic line process, the quality of the casting depends on many factors such as molten metal composition, machinery system, casting pressure, etc.

The die casting characteristics

Advantage

• Tight tolerance and high precision
• Able to cast thin-wall casting even 1mm
• High gloss casting surface
• High consistency on dimension and uniform design
• Reduce machining work
• High volume production efficiency and suit for bulk up order
• Completely automatic lines that save the labor cost

Limitations

• Expensive tooling cost
• Requires advance machinery investment
• Limit in molding design

Die casting applications

Die casting is often used in the casting of small parts, bulk orders such as pistons, crankshafts, gearboxes, etc. It can produce the casting that often weighing up to 5kg, but there is also a casting case for parts weighing up to 50kg but the price is very high.

The alloys used in die casting need to be uniform, have good dilute properties when melted and the composition is stable to not adhere to the mold, and have a plastic strength when heated at high temperatures. Copper, aluminum, and stainless steel are often preferred in this casting technology.

In die casting, aluminum alloys are used the most compared to all other alloys. In the US alone, aluminum die casting is worth up to $ 2.5 billion annually. Particularly, aluminum die casting accounts for twice as much as all other methods combined.

6. Permanent mold casting

What is permanent mold casting?

Permanent mold casting, also known as metal mold casting is a casting process in which a mold is made of metal like die casting. It is used to produce a large quantity of castings by using a single reusable mold. The mold can be reused many times hence the so-called permanent molding method.

Permanent mold casting is suitable for larger castings than die casting, about 10kg, of course, it can be higher, 20kg even 50kg, and the cost will be higher.

The permanent mold casting process

The permanent mold casting process is simply included these steps:

• Step 1: pouring molten metal into the mold
• Step 2: allowing for cooling and solidifies
• Step 3: opening the mold and collecting the casting;
• Step 4: then continuing these above processes to make as much casting as required.

Notably, the mold in this process is made from a high-temperature metallic material such as cast iron to be able to bear the continuous heating and cooling involves while taking big volume.  

Permanent mold casting characteristics

Advantage

• High precise and tight tolerance casting result
• Nice surface finish with less casting defect as sand casting does
• The casting is a high strength, toughness, and ductility  
• The durability of the mold is high.
• Due to saving mold making time, high productivity reduces production costs.

Limitations

• It is difficult to cast objects that are too complex, with large walls and weight
• Mold manufacturing costs are high.
• There is a need for a mechanism to push the casting out of the mold, so they sometimes form a dent in the casting.

Permanent mold casting applications

This permanent casting mold is most commonly used to cast aluminum, magnesium, copper alloys, and gray cast iron because of its low melting point. Suitable for mass production with simple and small or medium-sized castings such as pistons, gear billets, kitchen utensils, details in machine tools, aircraft wheels, pump parts, etc., that is not heavier than 25kg. For objects with a complex internal shape, it is recommended to use a sand core.

Summary

The above content has listed down and analyzed the 6 different types of casting process commonly used in the metal foundries manufacturers. Overall, each of these casting technique maintains both strengths and limitations. Depending on the casting requirements and project demand, the foundries will consider the casting process that suits the application.

As one of the leading casting companies and wholesale to the world market, VIC is the sand casting foundry with three main casting techniques included green sand casting, furan resin casting, and lost foam casting. With these three different types of casting, we are able to manufacture any casting parts, from the small, medium, large size to simple, intricate shape design. Contact VIC foundry for an OEM casting part project via email [email protected].