What’s Casting? Comprehensive Guide to the Casting Process and Methods

Metal casting is a process for manufacturing metal parts. It is known for its ability to create complex shapes, use a wide range of materials, and mass-produce parts. But what exactly does casting mean? The process involves pouring molten metals and alloys into pre-made mold cavities, which then harden. This ultimately transforms them into the desired functional parts. The mold cavities are therefore like a mirror image of the shape of the part to be manufactured.

There are many casting processes, each with its own advantages and disadvantages. These include sand casting, die casting, and investment casting. The type of raw material and the specifications of the part to be produced determine which method should be used. This article provides a comprehensive overview of the casting process, the technology, the available metals and alloys, and the advantages and disadvantages.

What is Casting?

Casting is the process of shaping molten metal to make a part of metal or alloy. The shape of the liquid metal after it has solidified is determined by the mold cavity. When the material is poured, it flows over every corner of the cavity (mold) and covers every detail. The solidified part is then pushed out of the mold or casting cavity.

The casting process is 7,000 years old and has been around since around 3200 BC. At that time, various useful objects were cast in copper in Mesopotamia and China. After 645 BC, sand casting became one of the most common methods of manufacturing tools and cookware.

In modern manufacturing, the metal casting process is used in a wide range of fields and its capabilities are constantly being expanded. It allows precise and complex parts to be produced cost-effectively, especially when large quantities are required. In a press, casting, or other type of mold, the cavities can be used to produce thousands of similar parts. Sand and investment molds, on the other hand, can only be used once. This cost-effective casting process is therefore often used for testing prototype projects. In addition, the cast parts retain their original physical and mechanical properties.

How Does the Casting Process Work?

There are two different methods of casting or molding: using a model to create the cavities and precise molds that can be used multiple times. Here are some terms related to the casting process:

• Model: This is a full-size replica of the desired part. It is usually made of metal, plastic, or wood and is used to create the mold.
• Core and Cavity: The cavity is the hollow part of the external contour of the casting. The core is used to cast internal features such as holes and channels.
• Pouring System: This consists of channels that guide and control the flow of liquid metal from the pouring furnace to the cavity. Parts such as sprues, runners, and gates are part of this.
• Riser: A reservoir in the mold that allows liquid metal to flow into the casting to compensate for the shrinkage of the metal as it hardens. This prevents holes and other defects in the casting.

Now, let’s take a step-by-step look at the casting workflow.

Step 1: Creating the Mould Cavities

To create the cavities, you need a model that serves as the shape’s body. You can use wax, plastic or wood to create a replica or design. Therefore, when designing and creating the model, factors such as dimensional adjustments to accommodate shrinkage allowances, draft angles for easy removal, and core imprints for hollow parts need to be considered.

After the model has been created, it is placed in a mould box, which is surrounded by a moulding material such as sand or ceramic. The mould then takes shape and leaves a space behind, which will leave the cavity. It should be noted that permanent mould cavities are usually produced using high-tech production methods. For example, they are made using CNC machining and electrical discharge cutting of aluminium or stainless steel.

Step 2: Pouring the Molten Material

First, the material is melted using an electric arc, induction or crucible, depending on the size of the casting. Large furnaces are required to melt the raw material for large castings, for example. In addition, the material needs to be stirred well before pouring.

Pouring can be done manually by manipulating the crucible or mechanically and hydraulically by tipping. Continuous casting machines, on the other hand, can automatically pour molten metal into a pouring bed as the mould moves along the production line. The temperature varies depending on the casting material being melted. An example is stainless steel casting, which can reach temperatures of 1250°F or 1000°F. The gate and pouring system then help the liquid material to flow into the cavity.

Step 3: Cooling and Solidification

After the liquid metal has filled all the details of the cavity, it needs time to cool and solidify. Channels and tunnels can be used in the cooling system to control the cooling rate. The time required for cooling depends on the size, complexity and material type of the casting.

As the metal solidifies, very small solid particles (called nuclei) come together and start to grow crystallographically. This ultimately forms the grain structure of the material. Through this mechanism, the original properties of the metal material can be restored.

Step 4: Removing the Casting

The final step is to remove the product from the mould. If the mould can expand (sand or investment casting), it is destroyed by the operator or a mechanical system. This allows the finished product to be removed. If the mould is reusable, an ejector is used to remove the metal casting. Afterwards, cleaning and post-treatment ensure that the part is dimensionally correct and has a smooth surface.

Now we know what casting is and how it works. So, let’s move on to the different types of casting.

Different Types of Casting Methods

As companies continue to seek more precise and complex castings, metal casting technology is also constantly being innovated. There are many casting methods, such as sand casting, silicon mold casting and die casting. Each casting process has its own advantages. You can choose the best method for your project based on their working principles, advantages, and how to use them. Here are a few of the most commonly used casting methods in manufacturing.

Sand Casting

Sand casting molds are made from fine-grained silica-based materials or sand grains that are packed together tightly. The mold consists of two parts: the upper and lower mold halves. The space between these two parts is then filled with molten metal. This mechanism also ensures that the shape is perfectly aligned, is easy to handle, and has a good gate.

The Advantages of Sand Casting:

• The mold-making and casting process is simple and cost-effective in terms of mold costs.
• Both ferrous and non-ferrous metals can be used for sand casting.
• It can cast parts of all shapes and sizes. In addition, it can cast parts weighing more than 200 tons.
• Although the dimensional accuracy is not high, the post-processing is simple and time-consuming.

Applications for Sand Casting: Engine crankcases, valves, bearings, electronic components, pistons, bushings, factory machinery, etc.

Investment Casting

The investment casting process is a little more complex than sand casting. It involves creating a mold from a wax pattern connected by a wax sprue. The wax pattern is then filled with a refractory material such as ceramic. The mold is then heated to melt the wax pattern within the refractory material and leave a smooth surface in the cavity. This is why the process is also known as lost wax casting. Liquid metal is then poured into this cavity. After the part has hardened, the mold is broken to release the part.

Advantages of Investment Casting:

• By using a wax pattern, hollow parts, undercuts, and internal channels can be created without the need to insert cores.
• The surface finish is better and the texture is more attractive.
• The dimensions are more precise, with tolerances that can be kept to ±0.1mm
• Suitable for manufacturing thin-walled parts with complex shapes

Applications for Investment Casting: Engine parts for military and civil aircraft, parts for plants and factories, engines and exhaust systems for cars, medical implants and instruments, etc.

Die Casting

For this casting process, permanent molds are made from sturdy materials such as hardened tool steel. It is commonly used for zinc, aluminum, copper, tin, and other metal materials. As part of the die casting process, a furnace is used to melt the raw materials. It should be noted that the furnace may or may not be connected to the die casting machine. The liquid is then injected into the mold by means of a hydraulic plunger or injection unit. At the same time, pressures of up to 250,000 psi can be generated.For more information on the design of die casting molds, then the Die Casting Mold Design Guide will provide you with more information.

Advantages of Die Casting:

• High precision, and consistent quality throughout the production cycle
• No extensive post-processing required
• Cost-effective for high-volume production

Applications of Die Casting: Can be used to produce a wide range of products, such as automotive parts, aircraft turbine blades and bodies, electrical housings, industrial products, machine tools, and household goods.

Centrifugal Casting

In centrifugal or rotational casting, liquid metal is poured into a cylindrical mold that rotates around a central axis. The overflowing metal is pushed against the walls of the mold by centrifugal force, forming a smooth, even layer. When the metal solidifies, it takes on the shape of the mold with a specific thickness.

Advantages of Centrifugal Casting:

• Since the liquid metal is continuously injected, there are no holes
• Centrifugally cast parts have a very low risk of blowholes and shrinkage cavities. This is because they solidify from the outside in
• Since there are no risers, less material is used in the pouring process
• Dense and homogeneous grain structure

Applications of Centrifugal Casting: This casting method is mainly used to manufacture symmetrical objects. Examples include hollow cylinders, bushings, pipes, pressure tanks, discs, etc.

Low-pressure Casting

In most cases, the pressure in the pouring or pressurizing furnace, which is connected to the casting machine, is between 0.02 and 0.07 Mpa. The pouring furnace is kept below the casting and the liquid metal is pushed upwards through a riser tube and then injected into the cavity. A constant pressure is required to fill the cavity. Once the mold is full, cooling channels keep it cool in a controlled manner until the entire mold has solidified.

Advantages of Low-pressure Casting:

• This casting process allows precise control of filling and eliminates turbulence. This reduces defects such as porosity and shrinkage.
• Low-pressure casting is very precise and meticulous.
• This casting method is suitable for a wide range of non-ferrous materials, such as aluminum alloys.
• The smooth filling also allows the casting of sharp-edged and complex shapes.

Applications of Low-pressure Casting: Cylinder heads and frames, dishes, custom hollow and complex profiles, fittings, electronic parts, etc.

Gravity Die Casting

In gravity die casting, metal is poured without pressure injection or a plunger device. Gravity die casting involves using gravity to move the material from the furnace or ladle to the die. There are no other forces influencing the flow of the liquid until the filling is complete. In addition, it is mainly used for non-ferrous metals with a low melting point, such as magnesium, copper, zinc, aluminum, and aluminum alloys.

Advantages of Gravity Die Casting:

• It results in better structure, mechanical properties, and external finish.
• The final part has a very low porosity because there is no airflow.
• It is easier to manufacture the tool than with other permanent molds because it does not require an injection unit.

Applications of Gravity Die Casting: This casting process can be used for many different products in many different industries. It can be used to manufacture car parts, industrial tools, aircraft engines and casings, decorative items, parts for household appliances, etc.

Vacuum Die Casting

In the vacuum casting process, liquid metal or plastic is injected into a vacuum. In most cases, a pump or similar expels all the air from the mold during the casting process.

Like polyurethane molds, silicone molds are used to shape rubber and plastic. But you can also use other types of molds to vacuum-cast different materials. In addition, this casting process has some similarities to the injection molding method. Moreover, this vacuum casting method can also be used together with conventional die casting to reduce the risk of air entrapment.

Advantages of Vacuum Die Casting:

• Very high precision and accuracy, especially suitable for projects that use 3D printing to create models.
• Fine casting with many intricate details
• Eliminates the possibility of air pockets forming inside the casting.
• Good for casting parts with thinner walls

Applications of Vacuum Die Casting: Functional plastic prototypes, medical implants and prostheses, consumer goods, body panels and other parts, food processing parts, etc.

Extrusion Die Casting

Parts are made by forcing liquid and semi-solid materials into a mold. First, liquid metal is poured into a heated open mold. The upper die is then closed and pressure is applied via a hydraulic plate or another suitable mechanism. When subjected to pressure, the part hardens and has better mechanical properties. It is also known as liquid metal forging.

There are two types of squeeze casting: direct and indirect. In the direct method, the mold is filled with molten metal and the upper part of the mold is closed. In the indirect method, the liquid metal is poured into the cavity and high pressure is applied using a punch or plunger.

Advantages of Squeeze Casting:

• Excellent surface finish with few pores.
• Short production cycle.
• Squeeze casting produces precise parts.
• Rapid heat transfer during the pressing process results in a fine microstructure.

Applications of Squeeze Casting: High-strength automotive parts, aerospace parts, industrial gears and hydraulic parts, medical equipment parts, etc.

Lost Foam Casting

The name “lost foam” comes from the fact that the final shape is made from a polystyrene foam model of the desired component. The mold is placed in a refractory material and then liquid material is poured into the mold. The refractory material then melts and, after solidification, forms the desired shape. Finally, when the mold breaks, the component is revealed.

Advantages ofLost Foam Casting:

• Complex shapes can be processed. The final component also has the advantage of a high surface quality and requires little subsequent processing.
• Lost foam casting can be used with a wide range of materials. These include metals such as iron, stainless steel, aluminum, and copper.
• This casting process simplifies the manufacturing process and makes production easier. This saves time, labor, and material waste.

Applications for Lost Foam Casting: It can be used to make rapid test prototypes, cylinder heads and car weldments, pipe fittings, valves, pump housings, etc.

Continuous Casting

In the production line, continuous casting involves continuously filling, cooling, and removing the parts. A ladle or furnace is used to set up and run the process. The material is fed into the mold cavity via a control system. The water channels in the mold then act as a cooling system, rapidly reducing the temperature. As a result, the molded parts can be kept at a lower temperature in this way. At the same time, they are brought to room temperature using additional water spray.

Advantages of Continuous Casting:

• Continuous casting does not require runners, sprues, or risers. The process is therefore simpler.
• The nominal yield is 100%, with no material wastage.
• Continuous casting technology enables smoother production, improving productivity and output.

Applications of Continuous Casting: Parts with regular shapes can be manufactured. Examples include beams, columns, rods, strips, tubes, etc.

Learn more: What is the difference between die casting and sand casting?

Materials Used for Casting

Metal casting processes can be used for a wide range of metal and non-ferrous metal alloys. The following table provides a brief overview of the available casting materials and their grades and properties.

Advantages of Metal Casting

Complex Shapes and Geometries

The casting process can be used to manufacture parts with complex shapes and features. These include internal channels and cavities, undercuts, thin walls, hollow sections, multi-part assemblies, asymmetrical geometries, etc. This is because the molten metal can flow into any small cavities and sharp corners.

Versatility of Materials

The casting process can be used with many metals and alloys, including ferrous and non-ferrous metals. Alloys made from iron, steel, zinc, copper, aluminum, and lead are just a few examples. This means you can freely choose the material that best suits your needs. In addition, casting metals that are difficult to machine or forge is also a straightforward process.

Cost-effectiveness

Sand casting and other expandable mold casting methods are inexpensive for small batches or prototypes. Casting processes using permanent molds are used for mass production. Because the mold costs are only paid once, they cover millions of production runs. Overall, casting is a low-cost manufacturing method.

Size Range

The casting process can manufacture parts of any size, from a few hundred grams to over 200 tons. They range from the engine blocks of small motorcycles to the housings and bases of turbines for large industrial machinery.

High Strength and Durability

The controlled cooling process used when the mold solidifies ensures a homogeneous grain structure in all directions. This ensures that the mechanical properties (strength and toughness) of the metal parts remain unchanged. In addition, advanced methods such as heat treatment and alloying can extend the service life of the product and provide better resistance to wear and corrosion.

Less Material is Wasted

You can remelt the waste material from gates, feeders, and runners for other cycles or batches. As a result, very little waste is produced in casting production. This reduces production costs and is good for the environment.

Disadvantages of Metal Casting

Upfront Costs and Time

The cost and time involved in manufacturing moulds are higher than the cost of tools used in other manufacturing methods. This changes the cost of small-scale manufacturing, especially for permanent moulds. In addition, the time from design to market is also affected by the lengthy setting and production process.

Surface Finish and Accuracy

For sand casting and refractory casting processes, due to the characteristics of the moulding material, the cast parts usually have a rough surface and inconsistent dimensions. Therefore, the cast products require further processing, such as trimming and polishing.

Defects and Quality Control

Metal castings can have defects such as pores, shrinkage, warping and inclusions. This can be caused by trapped gas, turbulent flow, uneven cooling, complex quality control processes, etc. These defects can weaken the structure and properties.

Complex Process Control

Another problem with the casting process is that it is difficult to control the entire process because of the different factors that need to be taken into account. These factors include temperature, flow rate, pressure, cooling rate and curing time. In addition, the metallurgical quality of the molten material needs to be taken into account, which adds to the complexity of the control. As a result, the number of faults and the subsequent amount of work that needs to be done may increase.

Different Applications of the Casting Process

The next thing we want to discuss is the diverse uses of the casting process. It can provide high-quality parts for a wide range of industries.

Automotive Industry

The die-casting process was originally invented to manufacture complex zinc parts for cars. This shows how versatile the casting process is in the automotive industry. Today, engine blocks, cylinder heads, and transmission parts are all manufactured using casting technology to produce parts that are both lightweight and durable. These parts help to improve fuel efficiency and performance.

With plug-in cores and custom tooling technology, the casting process can produce all the small details and complex functions of these parts. In addition, aluminum die-casting technology can produce lightweight items with a long service life:

• Aluminum castings make engine parts strong and lightweight
• Power transmission parts and chassis parts
• Air conditioning cooling parts
• Fuel intake systems, steering gears, engines, etc.

Aerospace

Lightweight materials such as nickel-based superalloys, aluminum alloys, and magnesium alloys can be manufactured using the casting process. They can be machined into complex and diverse aerospace components. At the same time, controlled cooling and solidification give them improved properties. It therefore helps the aviation industry to produce strong and lightweight components. This enables aircraft and spacecraft to use less fuel and have better performance.

Investment casting is therefore the most widely used in the aviation sector. In addition, molds, lost foam molds, sand molds, and other casting processes can be used. Thus, casting can be used to manufacture aviation parts with complex shapes and internal cooling channels. Examples include engine blocks, cylinder heads, and transmission parts. Other applications include:

• Engine parts, rotor blades, fuel system parts, landing gear parts, etc
• Intake and outlet volutes in pump housings
• Components for hydraulic systems
• Components and walls for control rooms

Consumer Goods

Casting is also used to manufacture a wide range of consumer goods. This is because it can be used to create complex designs. Examples include ornate decorative items, delicate cooking utensils, and durable cookware. These are manufactured using investment casting, pressure casting, lost foam casting, and sand casting. For these applications, extra attention is paid to fine details and aesthetics. The casting process therefore also ensures that the cavity walls have a smooth surface. Common consumer goods include:

• Decorative items such as picture frames, wall hooks, lamps, etc
• Hardware tools such as door handles, knobs, etc
• kitchen tools such as spatulas, bottle openers, sink taps, mixer bases, and stove burners.

Industrial Equipment and Machinery

Casting maintains the properties of the raw materials unchanged or even improves them by adding fillers and other additives. As a result, sand molds, press molds, and other types of casting molds can be used to create strong, rust-resistant alloys. Examples include aluminum, carbon alloys, and stainless steel.

In addition, casting is suitable for large and heavy shapes. As a result, casting can be used to create parts for heavy machinery and industrial equipment. These parts are strong and durable and will not wear out or rust. Examples include pumps, gears, valves, hydraulic cylinder parts, crushing jaws, conveyor belt rollers, custom housings, etc.

Medical Equipment

In addition, chromium, titanium, zirconium, and titanium alloys can be carefully cast using different methods. These are biocompatible materials that are particularly suitable for manufacturing medical devices. Investment casting produces parts with smooth, precise surfaces and many complex features.

These features are essential for medical implants and surgical instruments to ensure patient safety. In addition, equipment housings, parts for diagnostic equipment, and medical furniture are manufactured using sand, mold, and other types of mold. The following are more specific examples of applications

• Custom hip, knee, spine, and dental implants
• Surgical tools, forceps, and heart valve stents
• Surgical scissors and surgical trays
• Magnetic resonance imaging (MRI) machine housings and computed tomography (CT) machine frames

Energy Industry

Casting high-strength metals produces components and products that can withstand high stresses and harsh chemical and weather conditions. Magnesium, aluminum, stainless steel, and zinc alloys are used, for example, to produce parts for the energy industry. In addition, methods such as investment casting and die casting ensure that the end product is precise, consistent, and free of defects. This is important for the reliability of power plants.

• Wheel hubs, shaft sleeves, and main engines are cast in sand and investment casting processes. As a result, these components are very precise and robust.
• Turbine blades, vanes, and combustion chambers have fine details and tight tolerances
• Manufacturing parts for nuclear reactors such as pressure tanks, core shrouds, and steam generator components
• Solar screens and components for electric vehicles

Comparison of Casting and Injection Molding

Basically, the injection molding process also involves cooling molten material in a mold cavity. However, the mechanisms, dynamics, material compatibility, and other factors are different. Next, let’s take a closer look at what casting and injection molding are and the differences between them.

Injection Molding Process

This manufacturing method is mainly used to produce many plastic parts. It can also be used for some thermosetting plastics and metals. Under high pressure, liquid metal or plastic is injected into the mold cavity. The material then flows in the mold and molds itself to fit the hollow part after hardening. An ejector pin or other system then removes the part from the mold. In most cases, high-strength steel alloys are used to make injection molds.

Advantages of the Injection Molding Process:

• Consistent dimensions and quality in all batches
• Cost-effective in large production runs
• Short production times
• Can be highly automated
• Can accommodate secondary parts and other materials for manufacturing a single product

Disadvantages of the Injection Molding Process:

• For small production runs, the high price of tools and equipment makes it cost-prohibitive.
• Not as good as cast parts in terms of quality and structural integrity
• Only polymers and a few metal materials can be used
• Injection molding is not suitable for manufacturing large parts

Applications of the Injection Molding Process:

It can be used to manufacture automotive parts, precision hardware for spacecraft, factory machinery, reliable energy parts, heavy machinery, medical implants, etc.

Metal Casting Process

Casting, and especially die casting, is very similar to injection molding. You can think of a die casting mold as an injection molding mold. However, die casting and injection molding differ significantly in the way pressure is used. In the die casting process, pressure is maintained from the filling of the material to the solidification of the metal. In injection molding, however, molten plastic or polymer is forced under high pressure into the cavity, where it cools and takes on its final shape.

Advantages of Casting:

• Parts with a high-quality surface finish and good mechanical properties can be produced
• Complex shapes and features such as undercuts, recesses, deep grooves, and uneven contours can be produced
• Low tooling and equipment costs
• Large parts can be produced in series
• Aluminum, stainless steel, carbon steel, zinc, magnesium, and many other materials can be used.

Disadvantages of Casting:

• Cast parts may require further processing after they are made. This ensures that their dimensions and surface finish meet requirements.
• Compared to plastic molding, cast parts are more likely to have pores, gaps, and other defects.

Applications of Casting:

Goods that people buy, medical equipment, housings and parts for electronic devices, industrial gears and bushings, sporting goods, household tools, and storage containers.

Summary

We have discussed what casting is and other related topics. Such as materials, types, advantages and disadvantages, and uses. Overall, it is a common method for manufacturing complex metal parts. Because these parts are durable, structurally complete, do not rust or wear, and fit tightly.

Therefore, the casting process is suitable for many industrial environments. Casting processes such as sand, pressure, and lost wax casting can meet a variety of industrial needs. In addition, you need to choose the right metal casting process based on the type of material, complexity of the design, required accuracy, production scale, budget, and other factors.

At Yonglihao Machinery, we can provide you with professional metal casting services. Since we can manufacture our own molds, we can handle custom and unique molds. In addition, we place a strong emphasis on quality control and real-time tracking. This ensures that every part we cast is precise and consistent. As a result, our experts and foundry will help you better complete your project.