What is Casting?
Casting is a manufacturing process in which a liquid material is usually poured into a mold that contains a cavity of the desired shape and then allowed to solidify. The solidified part is also known as the casting, which is ejected or broken from the mold to complete the process.
Casting materials are usually metals or various time-setting materials that harden after mixing two or more components together; examples are epoxy, concrete, plaster of Paris, and clay.
Metal casting is a 7,000-year-old process used in both manufacturing and fine art. The first known cast object is a copper frog that dates back to 3200 BCE, found in present-day Iraq. During metal casting, molten metal is transferred from a crucible into a mold to create a positive metal cast object. The metal and mold are cooled, and the metal object is removed and finished.
Traditional metal casting techniques include lost-wax casting, plaster mold casting, die casting, and sand casting, to name a few. These metal casting processes may be completed in a foundry or a jewelry studio.
Metal casting processes have been known for thousands of years, and have been widely used for creating sculptures, jewelry, transportation, weapons, and tools.
Casting is most commonly used to create complex shapes that would otherwise be difficult or uneconomical to make using other methods. Heavy equipment such as machine tool beds, ship propellers, etc. can be easily cast to the size required instead of having several small pieces joined together.
Related Posts: What is Metal Casting?
What is the Casting Process?
Casting processes involve the use of molten material, usually metal. This molten material is then poured into a mould cavity that takes the form of the finished part. The molten material then cools, with heat generally being extracted via the mould, until it solidifies into the desired shape.
Metal casting comes in two main categories: processes with reusable molds and processes with expendable molds. In both processes, the caster melts the metal material in a crucible, pours it into a mold, then removes the mold material or the casting once the metal has cooled and solidified.
The basic metal casting process involves creating a pattern and a mold, then pouring molten metal into the mold. You will then extract the solid metal casting and finish your piece. This process is customizable for different types of metal casting, along with shapes, sizes, and more.
Step 1: Create the pattern
Before you make your mold, you must create a pattern to determine the mold’s shape. The pattern can be a 3-dimensional model of your final cast. It may be shaped in wax, sand, plastic, or even wood.
Some casters use molds made of plaster or silicone, which are materials that could not withstand a molten metal cast but allow the caster to mass-create wax multiples to use in expendable mold casting.
When you are shaping your pattern, make sure your account for any anticipated shrinkage when the metal cools. Patterns may also be gated with sprues to allow the molten metal to flow into the mold.
Step 2: Make the mold
After you have created a pattern, it is time to make your mold. As we mentioned above, you may choose to make a reusable mold, which is typically made from metal, or a single-use mold, which may be made from sand, plaster, or ceramic shell.
Each of these methods for making molds are optimized for different casting metals and various levels of pattern complexity. If you are working with a wax or plastic pattern, you can burn out the pattern inside of a kiln.
Step 3: Choose the metallic alloy
All metal castings are produced from either ferrous or non-ferrous alloys. Alloys are a mixture of elements that provide the best mechanical properties for the final cast’s use. Ferrous alloys include steel, malleable iron, and gray iron.
Non-ferrous alloys that are most commonly used in casting are aluminum, bronze, and copper. If you are working with precious metals in a jewelry studio, you may work with silver, copper, gold, and platinum.
Step 4: Melt the alloy
Melting processes vary between alloys because each alloy will have a different melting temperature. Essentially, melting consists of placing the solid alloy in a crucible and heating it over an open flame or inside of a furnace.
Step 5: Pour into the mold
Pour the molten metal into the mold cavity. If it is a small casting, you may simply pour from the crucible where the metal was heated directly into the mold. A larger casting may require a small team to support heating the metal inside of a furnace, and transferring the metal into a larger crucible or ladle before being poured into the mold.
Make sure to follow all recommended safety guidance when pouring molten metal. Make sure you wear protective clothing, including natural fiber clothing, long pants and sleeves, insulated gloves, and safety goggles.
Work in a well-ventilated space to avoid any risks from dangerous fumes. Make sure you have a chemical fire extinguisher nearby and keep your walkway between the furnace and the mold clear. Allow the mold to solidify before moving on to the next step.
Step 6: Remove the casting from the mold.
When the metal has cooled and solidified, you can remove it from the mold. If you cast into a single-use mold, you can break away the mold from the casting. If you used a plaster investment, you will want to quench the plaster in water after the metal has solidified. The water will help break away the mold. For reusable molds, you may use ejector pins to extract your casting.
Step 7: Finishing
File and polish your solid metal cast! This may involve cleaning your cast metal object, like scrubbing away excess mold material in water, breaking off the casting gates with clippers for small objects, or even an angle grinder for large pieces.
Different Types of Casting
Different Types of Casting Process:
- Sand casting.
- Investment casting.
- Die casting.
- Low-pressure casting.
- Centrifugal casting.
- Gravity die casting.
- Vacuum die casting.
- Squeezing die-casting.
- Lost Foam Casting
- Continual Casting
1. Sand Casting
Sand casting is one of the most popular and easiest types of casting and has been used for centuries. Sand casting enables smaller batches than permanent mold casting and at a very reasonable cost. Not only can manufacturers use this method to produce products at a low cost, but sand casting also offers other advantages, such as very small operations.
The process allows castings that are small enough to fit in the palm of the hand to castings or large enough like train beds. Sand casting can also cast most metals, depending on the type of sand used to make the molds.
Sand casting is typically based on silica-based materials, such as synthetic or naturally bound sand. Cast sand generally consists of finely ground, spherical grains that can be tightly packed together to form a smooth mold surface.
The casting is designed to reduce the risk of cracks, tears, or other defects by allowing a moderate amount of flexibility and shrinkage during the cool-down phase of the process. The sand can also be strengthened by adding clay, which will make the particles more closely bond. Automotive products such as engine blocks are made by sand casting.
2. Investment casting
Investment casting is also known as lost wax casting, it uses a disposable wax pattern for each cast part. In this type of Casting, wax is directly injected into a mold, removed, and then coated with refractory material and binding agent, usually in several stages, to build a thick shell.
Several samples are put together to form common sprues. Once the shells have hardened, the patterns are inverted and heated in ovens to remove the wax. These patterns require extreme care as they are not strong enough to withstand the forces involved in making molds. One advantage of investment casting is that the wax can be reused.
Investment casting is widely used to make parts for the automotive, power generation, and aerospace industries, such as turbine blades. These castings ensure that high-quality components are made with key benefits of accuracy, repeatability, versatility, and integrity.
3. Die Casting
Die casting is a method of molding materials by forcing molten metal under high pressure into a mold cavity. Most die castings are made from non-ferrous metals especially zinc, copper, and aluminum-based alloys. However, iron metal die-cast parts are possible.
The die-casting process is particularly suitable for applications in which many small to medium-sized parts with good details, fine surface quality, and dimensional accuracy are required.
4. Low-pressure casting
In low-pressure casting, the die is filled with metal from a pressurized furnace, the pressures typically being around 0.7 bar. The holding furnace is located in the lower part of the vertical die casting machine, with the molten metal being injected straight up into the bottom of the mold. The pressure holds the metal in the die until it solidifies.
One of the main advantages of this process is precise control of the die cavity filling. Molten metal flows quickly and smoothly through the feed lines, reducing oxide formation and preventing porosity.
This process was developed for the production of axially symmetrical parts such as car wheels. By using sand cores in the die, however, it is also well suited for the production of parts with hollow profiles and complex geometries.
5. Centrifugal Casting
Centrifugal casting is used to make long, cylindrical parts like cast iron pipes by relying on the G-forces developed in a spinning mold. Molten metal introduced into the mold is flung against the inside surface of the mold, creating a cast that can be void-free.
This types of casting were originally invented as the de Lavaud process using water-cooled molds, the process is applied to symmetrical parts such as soil pipe and large cannon barrels and has the advantage of making parts with a minimal number of risers.
In the case of asymmetrical parts that cannot be rotated around their own axes, a variant of centrifugal casting called pressure casting arranges several parts around a common sprue and rotates the molds around this axis.
A similar idea is used in casting very large gear rings, etc. Depending on the material being cast, metal or sand molds can be used.
6. Gravity Die casting
Gravity Die Casting is a permanent mold casting process, where the molten metal is poured from a vessel or ladle into the mold. The mold cavity fills with no force other than gravity, filling can be controlled by tilting the die.
Undercuts and cavities can be machined into the component shape using sand cores. This type of casting offers a better surface quality than sand casting as well as better mechanical properties, both due to the rapid solidification.
In addition, this process has a higher casting rate than aluminum sand casting, but metal molds are more expensive than sand. The advantages of this process include the possibility of low gas porosity and fine grain sizes can be achieved.
Compared to sand casting, this process requires less post-processing and cleaning, and gravity dies casting tends to result in a higher quality product. The gravity dies casting manufacturing process is generally less cost-effective in making tooling compared to sand casting.
7. Vacuum Die Casting
Vacuum-assisted die casting is an important process capability at Kennedy Die Casting. The vacuum evacuation of the die cavity reduces gas entrapment during metal injection and decreases porosity in the casting. The result is die-casting with a higher level of quality.
Vacuum systems are only a supplement. They do not substitute for good die-casting design practice in the engineering of the die cavity, runners, gates, and overflows.
8. Squeezing die casting
Squeeze casting, also called liquid forging, is a hybrid metal forming process that combines permanent mold casting with die forging in a single step where a specific amount of molten metal alloy is poured into a preheated and lubricated die and subsequently forged and solidified under pressure.
9. Lost Foam Casting
Lost-foam casting (LFC) is a type of evaporative-pattern casting process that is similar to investment casting except foam is used for the pattern instead of wax. This type of casting takes advantage of the low boiling point of polymer foams to simplify the investment casting process by removing the need to melt the wax out of the mold.
10. Continual Casting
Continuous casting is a refinement of the casting process for the continuous mass production of metal profiles with a constant cross-section. Molten metal is poured into a water-cooled, open-ended mold that allows a “skin” of solid metal to form over the still-liquid center, gradually solidifying the metal from the outside in.
After solidification, the strand, as it is sometimes called, is continuously withdrawn from the mold. Predetermined lengths of the strand can be cut off by either mechanical shears or traveling oxyacetylene torches and transferred to further forming processes or to a stockpile.
Cast sizes can range from the strip (a few millimeters thick by about five meters wide) to billets (90 to 160 mm square) to slabs (1.25 m wide by 230 mm thick). Sometimes, the strand may undergo an initial hot rolling process before being cut.
Continuous casting is used due to the lower costs associated with continuous production of a standard product, and also increased quality of the final product. Metals such as steel, copper, aluminum, and lead are continuously cast, with steel being the metal with the greatest tonnages cast using this method.
Advantages of casting process
Casting has the following advantages over other manufacturing processes.
- It can create any complex structure economically.
- The size of object doesn’t matter for casting.
- The casting objects have high compressive strength.
- All structure made by casting has wide range of properties.
- This can create an accurate object.
- All material can be cast.
- It creates isotropic structure.
- It is cheapest among all manufacturing processes.
- Composite component can be easily made by casting.
Disadvantages of casting process
The disadvantages of the Casting Process are as follows.
- Dimensional accuracy: Because of shrinkage in the casting, the dimensional accuracy might be at risk. So, the designers must take care of providing the allowance to the product (Pattern) before pouring the molten metal.
- Low strength: Due to high porosity compared to a machined part.
- Post-processing: It requires Secondary machining operations in order to improve the surface finish.
- Lower Melting Point: Generally limited to metals of the lower melting point.
- Not suitable for low-volume production.
Applications of Casting process
- Transport: automobile, aerospace, railways and shipping
- Heavy equipment: construction, farming and mining
- Machine tools: machining, casting, plastics moulding, forging, extrusion and forming
- Plant machinery: chemical, petroleum, paper, sugar, textile, steel and thermal plants
- Defense: vehicles, artillery, munitions, storage and supporting equipment
- Electrical machines: motors, generators, pumps and compressors
- Municipal castings: pipes, joints, valves and fittings
- Household: appliances, kitchen and gardening equipment, furniture and fittings
- Art objects: sculptures, idols, furniture, lamp stands and decorative items.