Types of casting processes
Where did casting begin?
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.
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 cast. 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.
The different kinds and methods of casting
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 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.
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.
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.
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, and dimensions are carefully calculated and determined 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 and plaster casting, the quality of the parts it produces makes its use appealing. Parts produced in this manner tend to have an excellent finish and do not require machining or finishing, which can compensate for the added initial cost.
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.
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.
The final word
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.