In this article, we’ll dive into the modern sand casting process, examining how it works and why it remains one of the most widely used metal casting techniques even thousands of years after its inception.

What Is Sand Casting?

Sand casting, or sand moulded casting, is a metal fabrication process that uses dies made from sand to create metal products. Sand is easily mouldable with common objects. After shaping it as desired, hot molten metal is poured into the mould. As the metal cools, it takes on the shape of the mould as it solidifies. The final metal product is then extracted from the sand mould and undergoes finishing before being put to use.

Using the sand casting technique, products of all sizes and complexity levels can be manufactured. It can be used to cast items as small as pins or as large as railway locomotives. Additionally, it is compatible with a wide range of metals, including cast iron, malleable iron, steel, nickel alloys, copper alloys, aluminium alloys, and magnesium alloys.

Its versatility makes sand casting the most widely-used casting process in the world, with over 60% of all cast metal products being made using this method.

Properties of Moulding Sand

While there are many different types of sands to choose from, not all of them are suitable for the casting process. Each of them exhibits certain traits that may or may not align with the project requirements.

Depending on the casting process and metal, the right combination of properties in the moulding sand is needed. To enhance cohesion, special additives and binders can be added to the basic sand, clay, and water mixture.

These binders, which can be organic or inorganic, include cereals, ground pitch, sea coal, gilsonite, fuel oil, wood flour, silica flour, iron oxide, and pearlite among others. Some of these binders have contradicting effects but trade-offs are often necessary to balance the properties for specific casting applications.

The four most common types of sand used for casting are green sand, dry sand, sodium silicate and resin sand:

1. Green sand is called so because it is essentially raw sand mixed with water. It exhibits a clay-like consistency. Green sand offers a low-cost option for casting sand with good reusability and is compatible with high production rates. However, its low malleability can cause the mould to collapse under certain situations. It also has a propensity to create surface defects.

2. Dry sand, on the other hand, is a moulding sand that uses clay and contains very little water acting as the binding agent. It holds its shape quite well once rammed into place. A benefit of dry sand is that it creates very little steam during the filling stage, which in the case of more moist sands, can lead to mould fracturing if left unchecked.

3. Sodium silicate, aka water glass, is a special sand that uses a sodium silicate binder to integrate the sand. Sodium silicate can create more intricate or complex shapes than green and dry sand.

4. Resin sand, aka Furan sand, is another popular sand used for sand casting It uses a furfuryl alcohol resin to make a coherent mass out of the sand. The robust and highly stable compound has very little tendency to break down and provides excellent dimensional accuracy and surface finish. However, it is the most expensive of all sands. It also has a higher lead time.

Casting engineers choose the type of sand based on the metal, product geometry and tolerance limits. But no matter the specifications, they always screen a sand type for the following properties:

Refractoriness

Refractoriness is a material’s property that allows it to withstand thermal shocks and burning when subjected to high temperatures. The sand serves as a refractory material for molten metal and thus needs to have a high melting point. The selection of sand for casting is largely influenced by the process temperature. If the refractoriness is insufficient, the sand will melt and fuse to the casting.

For many metals, such as aluminium, the sand only needs to withstand a temperature of 700 degrees Celsius. However, sands for steel castings need to withstand at least 1500 degrees Celsius as the melting point is higher.

Permeability

The sand must vent steam and other gases generated during the casting process. Each unit of water in the mould produces 1600 units of steam.

If the gases and steam get trapped, it can cause defects such as blowholes and gas holes in the final product. Their removal is crucial to obtain a good quality finish. Thus, permeability is a crucial property in sand for achieving a high-quality finished product.

The properties of moulding sand are influenced by the grain size and distribution: small grains increase mould strength but reduce permeability, while larger grains enhance permeability. Fine grains yield a smooth surface finish on castings but have low permeability, whereas coarse grains provide higher permeability but a rougher finish.

High Thermal Conductivity

Thermal conductivity is the property of a material that allows heat to flow through it. Efficient heat flow leads to quicker cooling of the molten metal. Sands with high thermal conductivity are required to transfer the heat from the molten metal to the atmosphere.

Low Thermal Expansion

Thermal expansion is the increase or decrease in the volume of a material when heat is added or removed, respectively. For the sand casting process, sand with a very low thermal expansion is needed. This type of sand will maintain its shape during various stages of the casting process, including the pouring of liquid metal and subsequent cooling.

Flowability

The ability of sand to flow into smaller areas, such as cavities, depressions, and deep channels of the pattern, determines whether we can use it for casting complex shapes. Sand with higher flowability will fill all the features of a pattern without requiring external pressure.

A highly flowable sand is not needed for simple shapes, such as a manhole cover, but it may be required for something more complex like a sculpture. The level of flowability needed may therefore be determined on a case-by-case basis.

Cost

The cost can also vary significantly from sand to sand. Silica sand, for instance, has high availability but also issues like dimensional instability and the tendency to burn or even explode to form sub-micron-sized particles under thermal shock. Silicosis is also an extremely concerning issue with workers being exposed to silica for extensive periods. Olivine and chromite sand, on the other hand, are more expensive and less available but do not have the above-mentioned issues.

Sand Casting Process

Sand Casting Process

The casting process is straightforward for simple products. A cavity with the desired shape is created, and molten metal is poured into it through a vertical channel. Once the metal solidifies, it is extracted by removing the mould sand around it.

However, with complex shapes, aspects such as how to fill the mould, remove excess metal, achieve a high-quality surface finish, and ensure the optimum temperature and velocity of the molten metal for uniform filling must be carefully considered.

Let’s explore a typical sand casting process in more detail:

1. Mould Creation

The mould is created using a pattern, which is a replica of the desired product. The pattern may be made from a different material or process, such as machining or additive manufacturing, but it has the same shape and dimensions as the final product. Multiple moulds can be created using a single pattern.

The pattern is separated into two parts along the centreline of the shape. Each half is then placed in separate sandboxes, and the sand mixture is packed around them to create impressions of the patterns in the sand.

Once the patterns are removed from the sand, the two sandboxes are placed on top of each other, securing them to form the complete mould. The top half and bottom half are referred to as the cope and drag, respectively. The mould cavity thus generated has the negative shape of the product. For example, a protrusion on the final product appears as a depression in the mould.

2. Gating System Creation

The gating system provides a pathway for the molten metal to flow into the cavity. A well-designed system can be crucial for the quality of the product. An effective gating system is simple, efficient, and minimises turbulence. It also traps any gas or slag present in the molten metal.

Key components of the gating system requiring careful design include:

• Pouring Basin and Sprue – The pouring basin serves as the funnel-shaped entry point for the molten metal. The metal flows into the sprue from the pouring basin, moving under gravity towards the cavity.

• Runners – Runners are horizontal channels for the molten metal to reach the die cavity from the sprue.

• Gates – Gates are the entry points of the die cavity, ensuring a smooth flow of the molten metal into the die cavity, maintaining pressure, and preventing flashing. They also help restrict any slag from entering the die cavity.

• Risers – To counteract shrinkage as the metal cools, risers serve as reservoirs in the gating system, storing extra molten metal. As the metal begins to shrink, risers provide additional material to the cavity, preventing metal shortages.

• Cores – When a product requires a hollow section or cavity, a solid sand insert called a core is placed inside the die to form the intended cavity. The molten metal fills around the core, creating the cavity in the final casting. Chaplets hold the core in place during casting.

3. Pouring the Molten Metal

The molten metal is poured from the top into the cavity. The volume of metal used slightly exceeds that of the product to ensure that some excess material is stored in the risers.

4. Solidification into Casting

Once the die cavity is filled, the casting is allowed to cool. Natural or forced cooling may be applied depending on the metal and process. Proper cooling is essential for achieving the desired properties and maintaining dimensional accuracy in the final product.

5. Shakeout and Sand Recovery

Shakeout is the process of removing the sand castings from the mould. The sand is shaken and shifted to separate the solidified casting, which is then sent for final finishing operations, while sand recovery begins.

Since not all of the sand can be recovered, some of the old sand is replaced with fresh sand for future sand casting processes. To restore the sand’s properties for reuse, mechanical, chemical, and thermal treatments may be applied, along with adjustments to its composition by compensating for lost moisture and additives.

Advantages of Sand Casting

Despite the development of many advanced technologies in recent decades, sand casting remains one of the most popular metal fabrication processes. Its unique advantages make it indispensable in the modern manufacturing landscape. Today, sand casting is often automated, enhancing its adaptability and efficiency to meet the demands of contemporary production environments. Here are some key advantages that make sand casting such a valuable process.

Suitable for all types of production

Sand casting is compatible with various production setups, allowing for single-piece, batch, or mass production. This method is extensively used in industries like automotive, defence, agriculture, mining, construction, healthcare, and renewable energy.

In the automotive industry, sand casting is used for developing prototypes, engine blocks, cylinder heads, and various IC engine parts. Its application has also expanded to EV manufacturing, where sand casting is employed to produce structural components, motor housings, battery casings, and more.

There is almost no limitation on size as well. Products ranging from a few grams to a few tons in mass can be cast with mathematical precision.

Cost-efficient

One of the primary factors that drive various sectors to adopt sand casting technology is cost. Sand casting is significantly more cost-effective compared to other casting processes. The sand can be reused for future castings with minimal processing.

The automation in high-volume production also reduces the cost per piece while ensuring consistent and reliable quality. Sand casting also incurs low tooling costs compared to other casting methods.

Suitable for all metals

Sand casting can work with almost any ferrous or non-ferrous metal alloy. Examples include gold, iron, lead, and titanium. The only requirement is that it should melt and flow as a liquid metal into the mould. This flexibility allows foundries to work with a wide range of metals without requiring significant process or equipment modifications.

Fast process

Although processes like centrifugal casting can be more efficient for specific shapes, sand casting offers quick turnaround times both for prototyping and large-scale applications.

Limitations of Sand Casting

All manufacturing processes involve trade-offs. In the case of sand casting, however, these trade-offs are minimal compared to the benefits. Let’s examine some of those limitations:

Poor surface finish

Poor sand quality and process deviations can sometimes result in a poor surface finish in sand castings. Any breakdown in the sand mould can lead to a rough surface. For products needing a high-quality surface finish without extensive post-processing, die casting and investment casting are often preferred, but sand casting can also produce refined finishes with the right sand and processes.

Using high-quality, finer sand provides a much better surface finish, but it increases the cost of the process. Proper ramming or packing of sand particles also helps to improve the surface finish. To rectify surface defects such as an overly rough finish, sand castings usually undergo machining operations like turning, grinding and shot blasting.

Loose control of cooling rates

While cooling rates can be controlled more effectively in many metal fabrication processes, the same cannot be said for sand casting. Sand, being an insulator, exacerbates the challenges related to the cooling stage. The cooling stage is considerably prolonged in sand casting, impacting lead times and production rates.

Some variation in microstructure anisotropy can occur due to varying cooling rates. Anisotropy refers to the changes in mechanical properties resulting from alterations in the grain orientation of the material.

Forming internal cavities can be difficult

The sand casting method requires the placement of cores in the mould cavity to form internal cavities. For instance, when manufacturing an engine block, empty cavities for the engine cylinders need to be created during the casting process.

Creating internal chambers, holes, or spaces within the mould cavity is challenging as they must be positioned correctly and adequately supported to withstand the casting process. The more complex these cavities get, the more challenges need to be overcome.

Skilled labour is required if the process is not automated

Automated processes can be costly. However, without automation, the process relies on skilled sand casting experts to craft patterns, moulds, and oversee the process to ensure a high-quality product. Without skilled labour, the outcomes may fall significantly below acceptable standards, resulting in reputational and material losses.

Sand disposal can be a challenge

Foundries recycle moulding sand as much as possible, but after a specific number of cycles, it is no longer fit for reuse. However, waste sand from foundries is a harmful contaminant that can lead to environmental issues.

Construction is one of the sectors where waste foundry sand is now being diverted for disposal. It is being used as a substitute for fine aggregate (sand) in concrete. This practice not only addresses the issue of sand disposal but also lowers the cost and carbon emissions linked to concrete production.

But apart from this, most of the waste foundry sand ends up in a landfill, which is certainly not ideal. The options for recycling casting sand are limited.

Conclusion

Despite traditional sand casting being an ancient process, it has undergone remarkable refinements, making it highly relevant and useful in today’s manufacturing landscape. Its popularity is partly attributed to its versatility across various metal types and product sizes, and partly to the cost efficiency compared to other casting processes.

Ongoing innovations in material science and process automation further expand sand casting’s significance in industries ranging from automotive to construction. As industries evolve, sand casting will persist in shaping the industrial landscape, blending tradition and innovation in metalworking techniques.

The post Sand Casting – Process, Sand Properties, Pros & Cons appeared first on Fractory.

In this article, we’ll look into centrifugal casting process to understand its operation in detail, its types, and the benefits it offers.

What Is Centrifugal Casting?

Centrifugal casting is a metal casting process that uses a spinning die to produce cylindrical products like pipes and tubes. During this process, molten metal is poured into a spinning die. As the metal flows into the spinning die, centrifugal force distributes it uniformly along the inner diameter of the die.

The thickness of the casting can be accurately controlled by regulating the amount of metal poured into the cylinder. The liquid metal starts cooling and solidifying from the outside surface in contact with the cylinder.

While cylinders can be cast through other methods, they would have limitations on size, tolerances, seam strength and corrosion resistance. The formation of internal cavities would require cores and the final part may exhibit issues such as gas porosity, oxides and non-metallic inclusions.

Centrifugal casting, on the other hand, has no limitations on the length of the finished part. It allows the casting of very long parts with remarkable accuracy and consistency. Most machines can produce pipes up to 15 meters (49 ft) in length with a diameter of up to 6 meters (20 ft). The wall thickness can vary from 2.5 mm (0.1 in) to 125 mm (~5 in).

The process works best for thin-walled cylinders but it can also be used for thick-walled applications such as gear blanks, ball bearings, railway carriage wheels, and more. Centrifugal casting can also produce non-circular shapes with a near-constant radius.

Multi-layer casting is another unique feature of centrifugal casting. In this casting technique, two dissimilar materials can be combined to form a multi-layer centrifugal casting. This finds many applications in several industries. For example, steel pipes can be coated on the inside or outside with concrete to enhance their corrosion resistance and compressive strength.

Common applications of the centrifugal casting process are in piping, aerospace, automotive and military sectors. Some common products made using this process include engine cylinder liners, jet compressor cases, metal pipes, railway carriage wheels, bearings and many other tubular parts in high-reliability applications.

Centrifugal Casting Process

Centrifugal casting is a fairly straightforward process. Most often, the process is used to create standard-sized products rather than specific ones. Standardisation allows the process to take advantage of economies of scale and reduce part costs.

Let’s explore a typical centrifugal casting process, its stages and their significance. The four stages are as follows:

1. Mould preparation

2. Pouring of molten metal

3. Directional solidification

4. Removal and finishing operations

Centrifugal Casting Process

1. Mould Preparation

The mould preparation begins with cleaning the die’s inner surface to remove contaminants and debris from previous use. Once it is clean, the motors responsible for spinning the die are powered up.

The amount of force applied to the molten metal is proportional to the die’s diameter and spinning RPM. Typically, a die has a rotational speed between 300 and 3000 RPM. The centrifugal force produced by these high speeds can reach up to 100 times the force of gravity.

Then, it is time for preheating and applying a ceramic slurry to the inner surface of the die. The heat dries and cures the ceramic slurry, ensuring that the refractory lining adheres to the die’s surface.

The ceramic slurry is necessary to protect the die and facilitate the removal of the casting upon solidification. An ideal slurry also provides a smoother surface finish for the part.

2. Pouring of Molten Metal

The metal is melted in an external furnace and poured into the pouring basin of the casting machine using a ladle. The pouring basin then transfers the metal to the centre of the die through a spout. The high centrifugal forces ensure that the final product is free of cavities or gas pockets.

The centrifugal force also separates the low and high-density components in the molten metal. All the impurities float towards the inner diameter of the molten metal due to their lower densities. This results in an increased density of the molten metal at the outer diameter compared to the inner diameter.

3. Directional Solidification

The metal casting rotates continuously during the rapid cooling stage until its temperature is below the transition range. Cooling in centrifugal casting begins from the outside diameter towards the inside diameter. This type of controlled solidification in castings is also known as directional solidification.

Directional solidification offers advantages such as improved grain structure, lower concentration of impurities and the development of beneficial microstructures. Solidification shrinkage always occurs at the inner wall of the finished product in centrifugal casting.

4. Removal and Finishing Operations

Once solidification is complete, the casting is removed from the die using extraction equipment if no heat treatment is needed. In some cases, the casting is subjected to annealing before the steel mould is removed.

It is then sent for finishing operations. The impurities and lighter material that floated inward during the second stage are now machined, leaving a strong, defect-free part with excellent mechanical properties. The outside is shot blasted to remove any traces of the refractory material.

Visual and dimensional inspections are also carried out at this stage.

Types of Centrifugal Casting

Centrifugal casting processes can be classified in different ways. One method is based on the nature of the casting process, where they are divided into true centrifugal casting, semi-centrifugal casting and centrifuging processes. A second, more common method is to classify them based on the die’s orientation, i.e., horizontal and vertical centrifugal casting processes.

The processes between the two classification methods can overlap. For instance, a true centrifugal casting may involve either a horizontal or vertical centrifugal casting process.

Classification Based on the Nature of the Process

Animation of Centrifugal Casting Types

True Centrifugal Casting

The true centrifugal casting process is the most popular type of centrifugal casting process. This is the same process described in the previous section. Unlike other processes, this method does not involve a gating system. The material is poured directly into the cylindrical mold, where it contacts the mould’s internal surface and spreads uniformly.

Semi-Centrifugal Casting Method

The semi-centrifugal casting method is similar to the true centrifugal casting process, except that the former creates solid castings, unlike the latter process which produces only tubular shapes. It handles the production of solid components such as spokes, gear blanks and pulleys.

The material away from the rotational axis is denser compared to that around the axis. Impurities, due to their low density, also accumulate around the axis. After solidification, the material around the axis is machined away to make room for the axis on which the produced component will spin.

Semi-centrifugal casting provides us with a product that has excellent mechanical properties and is free of impurities.

Centrifuge Casting

Centrifuging is a unique form of centrifugal casting method that uses centrifugal forces to fill conventional moulds instead of applied pressure. In this process, we use a rotating mould with a central sprue at the axis. The sprue channels the molten material into the rotating mould as it spins.

The centrifugal force from the rotations pushes the molten metal outward and into the cavities spread along the circumference. This process includes all the components of the gating system, such as sprue, runners, gates and risers.

The spinning motion of the die pushes the material into the die until complete distribution and solidification occur. The centrifuging process is used for small-sized products such as jewellery, sleeves and bushings.

Classification Based on Die Orientation

When designing a die for centrifugal casting, we can choose to align it either vertically or horizontally. Each orientation has its benefits and limitations:

Horizontal Centrifugal Casting

Horizontal centrifugal casting refers to the casting process in which the mould’s spin axis is parallel to the ground. The horizontal axis arrangement enables the production of castings with a very high length-to-diameter ratio. Horizontal centrifugal casting is a cost-effective method that delivers high-quality results.

Some common products made using the horizontal centrifugal process include ductile iron pipes, reformer tubes, furnace rolls, sleeves and roller shells.

Vertical Centrifugal Casting

In vertical centrifugal casting, the mould spins along the vertical axis, perpendicular to the ground. This technique is more suitable for products with a short length-to-diameter ratio. Typically, the length should be less than twice the product’s width for optimal results in vertical centrifugal casting.

The effect of gravity needs to be accounted for in vertical centrifugal castings, as it can lead to distortion in the absence of sufficient centrifugal force.

Some examples of products made with this manufacturing process include ball bearings, gear blanks and pulleys.

Advantages of Centrifugal Casting

Centrifugal casting is a special type of casting process that offers unique advantages compared to other casting processes:

• Unrivalled rotational symmetry in parts

• Fine-grained structure

• Ability to seamlessly integrate two materials

• Can work with a variety of materials besides metals, such as glass and concrete

• Eliminates cores and gating system

• Lower manufacturing costs and reduced machining requirements

Limitations of Centrifugal Casting

Each manufacturing process has its limitations, centrifugal casting is no different:

• Restrictions on complex shapes: Centrifugal casting is limited to producing cylindrical castings and, to some extent, other shapes. Even within cylindrical castings, the complexity achievable through this process is restricted. For applications needing more flexibility in shape or size, other methods, such as sand casting is often chosen, offering adaptability across a broad range of geometries and product sizes.

• Inaccuracies in the inner surface diameter are fairly common. However, it is possible to generate net or near-net shapes with the right controls.

• The process requires a high initial investment. However, similar to die casting, it boasts low per-part costs, making it cost-effective in mass production scenarios.

• Parts with small inside diameters are difficult to cast.

• Operational safety can be a concern with this process because the die operates at high rotational speeds along with other rotating and moving parts.

• The centrifugal process is not particularly effective for small-sized products.

The post Centrifugal Casting – Working Principle, Types & More appeared first on Fractory.

What Is Die Casting?

Die casting is a metal casting process that uses metal dies to create metal parts. Molten metal is poured or injected into the metal dies and allowed to solidify. Once solidified, the parts are removed from the mould and put to use after minimal finishing operations.

The die casting process is primarily used for casting non-ferrous metals, such as aluminium, magnesium, zinc and their alloys, but it can also be used for other metals with comparable melting points.

Die casting is essentially the same as injection moulding, as the primary difference lies in the materials used – molten metal vs molten plastic. Additionally, die casting requires higher temperatures and pressures due to the properties of metals, and the moulds are typically made from high-strength steel to withstand these conditions, whereas injection moulding can use moulds made from a variety of materials due to the lower temperatures and pressures involved in processing plastics.

Applications of Die Casting

Die casting is widely used in various industries to produce parts such as:

• Automotive components (engine blocks, transmission cases and wheel rims)
• Consumer electronics (housing for laptops, smartphones and cameras)
• Appliances (handles, brackets and heat sinks)
• Aerospace components (aircraft parts and fittings)
• Industrial equipment (pumps, valve bodies and enclosures)

Die Casting Process

The die casting process can have different mechanics depending on the metal, application and the process employed. For instance, the molten metal could be poured via gravity or injected into the die under high pressure.

Let’s explore a typical die casting process, which involves six main stages:

1. Mould Creation

2. Mould Preparation

3. Metal Injection

4. Cooling

5. Casting Removal

6. Finishing Operations

1. Mould Creation

The mould in die casting is a metal die with an internal cavity the same shape as the product. Nowadays, CAD/CAM and CFD (Computational Fluid Dynamics) software are used to design moulds. These programs can accurately simulate the smallest details about a mould such as internal friction, temperature distribution, flow rates, heat dissipation rate and lubrication efficiency.

After successful simulations, the engineers share the drawings and specifications, such as tolerance and tonnage, with specialist toolmakers. These specialised toolmakers create the mould through CNC machining and run trials to ensure that the final product meets the specifications. In addition to the cavity, an industrial die also contains channels for metal flow, a cooling system and ejector pins.

This is one of the critical stages in die casting. A high-quality mould can eliminate many issues downstream, such as surface defects, material wastage, shrinkage, warpage and incomplete filling.

2. Mould Preparation

The mould preparation stage involves setting up the mold in a way that is most favourable to producing high-quality castings.

The mould is made up of two halves: the fixed die half, which is connected to the injection system, and the removable die half, which separates to remove the casting.

Engineers clean and heat the mould before pouring molten metal into it. They also spray the inner surfaces of the mould with lubricant or refractory material. Of course, these processes can be automated as well.

The lubricant serves multiple functions, including reducing internal friction, aiding in maintaining the die’s temperature, preventing premature solidification of the molten metal in the die, and facilitating the removal of the cast from the die once the metal has solidified.

Finally. the mould halves are closed and secured together using high pressure before pouring the metal.

3. Metal Injection

In this stage, molten metal fills the die uniformly. This action can be performed in two ways. One method is gravity die casting, where the force of gravity is utilised to fill all the internal cavities in the mould. This process involves pouring the molten metal from a ladle into a downward sprue that leads to the mould cavity.

The second method, also known as high-pressure die casting, fills the mould by injecting the molten metal into it under high pressure. The pressures can range from 100 bars (1,400 psi) to 1800 bars (26,000 psi). This method offers several advantages over gravity die casting, such as faster and better filling of the mould, the capacity to create intricate structures with thin walls, high dimensional stability, smoother surface finish and the minimisation of finishing operations downstream.

4. Cooling

Once the molten metal has completely filled the mould, the cooling stage begins. It is important to ensure a uniform and controlled cooling rate to obtain the desired microstructure in the final product.

Molten metal must be given enough time to cool to prevent surface or internal defects from forming in the part.

5. Casting Removal

Once the cast is set, the mould halves are unclamped to facilitate the removal of the casting. This may be done manually or automated using ejector pins. The ejector pins are built into the mould to safely and automatically separate the cast when the mould halves are separated.

6. Finishing Operations

Finishing operations are necessary to remove the sprue, runners and other gating system components from the casting. These components are then remelted in the furnace for reuse.

The casting may also need finishing operations, such as surface grinding and machining before it is ready for handover to the customer.

High Pressure Die Casting Process

Types of Die Casting

Die casting processes can be carried out under different conditions depending on the specifications of the process and the final part.

Among the many specialist processes in die casting, two are worth elaborating on: the hot chamber die casting and cold chamber die casting processes. Let’s have a look at both of these processes to gain a better understanding.

Hot Chamber Die Casting

The hot chamber in the hot chamber die casting process refers to an internal furnace within the casting machine. The furnace is connected to the die cavity through a feeding system known as the “gooseneck”. As the metal is melted within the casting machine itself, this process offers faster cycle times of about 20 minutes, making it a worthy contender for mass production.

The hot chamber, however, cannot reach very high temperatures. Therefore, the process is most suitable for low melting point metals such as lead alloys, zinc alloys and magnesium alloys. As the metal melts, it is forced into the die through the gooseneck by a plunger.

The plunger withdraws when the metal in the die solidifies. The mould opens and the casting is removed. Then, the mould closes once again, the plunger transfers the hot metal into the die cavity, and repeats the process.

The hot chamber die casting process is popular for its speed, accuracy and consistency. Additionally, it offers a longer die life and improved corrosion resistance.

Cold Chamber Die Casting

The cold chamber die casting process is similar to the hot chamber process, except that the metal is not melted in the casting machine.

The metal is melted in an external high-temperature furnace and transferred to the cold chamber die casting machine via a ladle. In the cold chamber machine, a hydraulic piston forces the metal into the metal die for casting.

This process is suitable for metals with higher melting points such as copper, magnesium, aluminium and their alloys. Magnesium alloys start melting at about 360°C (680°F) and go up to 650°C (1200°F). Therefore, some magnesium alloys can be cast using hot chamber die casting, while others can be cast using cold chamber die casting.

Metals with higher melting points offer greater strength, durability and hardness compared to low melting point metals. Therefore, cold chamber die casting is a better choice for demanding applications.

Advantages of Die Casting

The die casting process is a modern casting method with numerous unique advantages. Although it has been around for many centuries, it has garnered popularity in recent decades. Let’s explore the advantages of die casting to comprehend why it is a favored choice in the casting industry today:

Complex shapes are a breeze

The die casting industry can create highly complex shapes through this process. It is one of the few processes that can consistently produce high-quality parts at a rapid rate while maintaining narrow tolerances. The high pressures involved are capable of pushing molten metal into the thinnest channels with extreme reliability.

Master at mass production

The quick cycle times, high potential for automation, high material efficiency, and usage of low melting point metals make it highly effective for high-volume manufacturing. The industry uses the process in most serial production applications.

Made for the 21st century

The die casting process can seamlessly incorporate all the latest technology. It is highly compatible with automation and other supporting technologies such as 3D printing and simulation, CAM and CAD software.

First-rate low-cost operation

The cost per piece decreases significantly as the volume increases. Although the initial investment in mould creation and automation integration may be higher than other processes such as sand casting, it more than compensates for it by having one of the lowest operating expenses among all the various types of casting processes.

Buttery smooth finish

Die casting is popular for delivering one of the best surface finishes among casting processes. In many cases, finishing operations may not even be needed, and the product can be put to use after trimming off the excess material from the gating system components.

Additional finishing operations such as grinding, machining and powder coating are only necessary when the application requires a high-quality polish or specific textures.

Limitations of Die Casting

It is equally important to be aware of the limitations and shortcomings of a manufacturing process. Die casting limitations are as follows:

High initial investment

The die casting process involves a significantly higher initial investment compared to other processes. This aspect of die casting can deter interested manufacturers who only have a small number of parts to produce. The economic advantage of die casting lies in mass production. It is not suitable for manufacturing prototypes or small batches of products.

Turbulence can be an issue

The high-pressure injection of molten metal is likely to generate turbulence, which can result in defects like porosity. To address this, most foundries now use flow simulation during mould design. By optimising the mould design and casting process parameters, turbulence can be minimised in the majority of cases.

For example, the metal is initially pushed into the mould with a slow initial velocity to minimise turbulence and the formation of gas bubbles.

Size and weight limitations

Die casting is most suitable for small to medium-sized applications. There are practical limitations on the weight of die-cast parts, as larger castings will naturally require larger dies and related equipment. Process parameters, such as the clamping force and heat dissipation capacity, cannot exceed a certain limit.

As the size of the casting increases, the cost of the process also increases at a greater rate, rendering the process unfeasible beyond the inflection point. In such cases, alternative methods like centrifugal casting may be more appropriate, as they are better suited for larger or cylindrical parts, where uniformity and precision are critical.

To Sum It Up

The die casting process is a modern marvel that can produce the most delicate and complex shapes effortlessly. It surpasses other casting processes, such as sand casting and investment casting, in terms of speed, accuracy, consistency and surface finish. Aluminium die casting, in particular, has been widely utilised in manufacturing to create lighter vehicles while maintaining their original strength.

However, die casting’s disadvantages, such as high initial cost and limitations on size and weight, restrict its use to a small group of applications. Nevertheless, advancements in simulation and automation are enhancing die casting’s capabilities, ultimately expanding its scope within modern manufacturing.

The post Die Casting – What Is It, Process, Types and Benefits appeared first on Fractory.

The founding principles of investment casting have remained the same over thousands of years. However, adding supporting technologies such as CAD/CAM, additive manufacturing, and real-time monitoring capabilities has significantly improved the final part quality, consistency, and affordability of this industrial process.

In this article, we shall explore the workings of modern investment casting processes, along with their advantages, disadvantages and use cases.

What Is Investment Casting?

Investment casting, also sometimes known as precision casting or lost wax casting, is one of the most popular metal forming techniques. The term ‘investment’ originates from the archaic meaning of ‘invest,’ which refers to clothing or surrounding an object with a layer of material.

In this process, a wax pattern is encased in refractory material to form a ceramic shell mould. The pattern is slightly larger in dimensions than the final product to accommodate shrinkage when the material cools.

Since the shell mould has a hard surface, investment casting delivers smoother surface finishes than sand casting. The process can work with most metals. Some popular metals that are highly compatible with investment casting are bronze, magnesium, stainless steel alloys, glass, carbon steel, brass, aluminium and aluminium alloys.

Investment Casting Process

Investment Casting Process Walkthrough

The investment casting process is more intricate than the widely used sand casting process, thus involving more stages. Investment casting involves 8 stages and these are as follows:

1. Creation of the master pattern

2. Creation of the master die

3. Creation of the wax pattern

4. Creation of the shell mould

5. Removal of wax

6. Pouring of molten metal and subsequent cooling

7. Removal of casting from the shell mould

8. Post-processing operations

Let’s look closer at what is happening in each of the stages:

1. Creation of the Master Pattern

The master pattern is an exact duplicate of the product required. It may be made from wood, metal, plastic, clay, wax or other specialist alloys. Over the years, manufacturers have shifted to 3D printing master patterns due to their compatibility with CAD/CAM and improved dimensional accuracy.

2. Creation of the Master Die

The master pattern is used to create a master die. The master die’s creation technique depends on the master pattern’s material. For instance, if the master pattern is from steel, it can be placed in molten aluminium, since aluminium melts at 660°C (1,220°F), whereas steel melts at around 1500°C (2,732°F). Thus, the molten aluminium will not dissolve the steel. Upon solidification, it will create an aluminum-based master die.

3D-printed master dies are increasingly being used because they offer a cost-effective alternative to metal dies when only a few castings are needed.

3. Creation of the Wax Pattern

There are several ways to produce wax patterns from master dies. One method involves filling the die with a small amount of wax and shaking it until it evenly coats the inner surface of the die and solidifies. This process is repeated until the desired thickness is achieved, resulting in a hollow wax pattern.

The second method involves filling the master die completely with wax. The wax may be fed directly or through high-pressure injection into the die. The high pressures enable the wax to fill all of the cavity’s features in the case of complex components. The wax patterns thus produced are solid.

During this stage, cores can be added to create internal cavities in the wax pattern. Common materials for cores include soluble wax or ceramic. The soluble wax is removed in stage 5, while ceramic cores are removed after the final product has hardened.

4. Creation of the Shell Mould

Wax patterns cannot withstand the high temperatures of molten metal. Therefore, ceramic moulds are created from wax patterns to endure the heat of molten metal. The ceramic mould is formed by immersing the wax pattern into a slurry of refractory material and allowing it to solidify.

The process is essentially the same as coating vanilla ice cream with a layer of chocolate. When the cold vanilla ice cream comes into contact with hot melted chocolate, the chocolate sticks to the ice cream and solidifies almost immediately, creating a thin shell of chocolate covering the ice cream.

Similarly, when the ceramic slurry comes into contact with the wax pattern, it adheres to it and solidifies. Several passes of the wax pattern are made, starting with a finer slurry and then a coarser one to achieve the desired thickness of the ceramic mould. The mould is then left to cool down.

5. Wax Removal

Once the slurry has hardened, it is placed in a furnace to remove the wax and for further sintering of the ceramic mould. Sintering is the process through which items are hardened through pressure and heat but without liquefaction. As the temperature rises, sintering hardens the mould while the molten wax is reclaimed for future use. This stage gives us the solid ceramic mould for the actual casting process.

6. Pouring of Molten Metal and Subsequent Cooling

After the wax removal stage, the mould is cooled for testing. If any cracks or other imperfections are detected, they can be rectified using ceramic slurry or special cement.

Upon successful test results, the mould is heated again before pouring liquid metal into it. This preheating ensures that the molten metal remains in the liquid state for a longer period until it uniformly fills the mould.

To pour the metal, the ceramic mould is inverted and placed in a sand-filled container. The molten metal is poured into the mould under gravity or external pressure.

Vacuum-assisted filling may also be used. It comes at a higher price point but offers significantly better resource efficiency compared to gravity and pressure-based filling.

The vacuum pulls the molten metal into the mould, and once all the crucial parts solidify, the vacuum is released, allowing the unused melt to drain out. This process minimises material solidification in the sprue and gates, resulting in up to a 95% material yield, compared to 15-50% in gravity pouring.

The material is then allowed to cool and solidify until it is hard enough to withstand separation from the ceramic shell mould.

7. Removal of Casting From the Mould

The divesting is typically carried out by hammering the mould to release the casting. Other methods to separate the metal casting from the mould include waterjetting, media blasting, vibration and chemical dissolution of the mould. The sprue, runners and other gating system components are then separated and recycled.

8. Post-Processing Operations

The investment casting process generally does not require subsequent machining. However, if the results are not as expected, the casting may need surface finishing operations before use. Typically, surface grinding with machine tools is adequate for polishing and trimming any surface defects. Nevertheless, depending on the situation, other finishing processes such as hand tooling, welding, and hydraulic straightening may be necessary.

Advantages of Investment Casting

The investment casting process offers the following advantages:

• Excellent dimensional accuracy even for complex components

• Smoother surface finish

• Works with a wide range of metals

• Parts are created as a single casting with no parting lines

• Compatible with low- and high-volume manufacturing

• Minimal wastage of resources

• Parts can be combined, eliminating the need for assembly operations downstream

• 90-degree angles can be cast without worrying about shrinkage allowance

• More environmentally friendly than other metal fabrication processes

Limitations of Investment Casting

The investment casting process also has certain limitations:

• A very high number of variables that can affect part quality. A high level of process control is essential for producing good-quality parts

• There is an upper limit on the size of investment castings, which is lower than that of some other casting processes such as sand casting and centrifugal casting

• Casting objects with internal cavities can be more challenging than other casting processes

• Longer production cycles and lead times

• Investment casting techniques are more expensive than other processes, especially when the quantity is low and the manufacturing process involves the use of permanent tools

Applications of Investment Casting

The industry uses investment casting to produce the following products:

• Turbocharger rotors and turbine blades

• Firearm components such as triggers, hammers and receivers

• Power generation and cooling system parts such as pumps, valves and combustor components

• Intricate jewellery with delicate components and tight tolerances

• Automotive parts including engine components, transmission components, brakes, door handles, gears, housings, brackets, rods and more

When to Choose Investment Casting

After learning about the investment casting process, let’s summarise our knowledge by noting the five scenarios where investment casting provides the best ROI:

Medium size range – Investment casting gives better results when the weight of the product ranges from a few grams up to 1,000 kg. Parts weighing more than 1,000 kg are better suited for the sand casting process.

Extremely complex geometry – Investment casting can create super complicated shapes such as turbine blades, medical equipment, dental crowns and inlays, power generation equipment, aerospace components and more.

High number of parts – Investment casting is not cost-effective when a small number of items need to be manufactured, as the process becomes more expensive. However, as the quantity of items increases beyond a certain threshold, the cost per piece can decrease below that of other methods, including sand casting.

Need for a high surface finish – Investment casting uses a hard mould with a very fine surface, allowing the process to achieve an exceptional surface finish.

Need for high dimensional accuracy – It is possible to cast net or near-net shapes through investment casting. The need for post-processing is also minimal with this process.

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The process of casting metals dates back over 7000 years. Over the centuries, the casting process has been continuously developed and refined. Today, it enables the repeatable production of highly complex shapes, some of which would be impossible to achieve through any other method.

It is now used so extensively that it is impossible to avoid cast products, no matter what type of environment you are in. Some examples of cast metal products include engine blocks, fire hydrants, motors, tools, traffic lights, manholes, pipelines, valves, and various fittings.

In this article, we’ll explore the metal casting process to understand what it is, explore its types and how they work.

What Is Casting?

The casting process enables the production of metal parts with desired shapes by heating the metals over their melting temperature and pouring the liquid metal into moulds. The molten metal flows into the empty cavities, conforming to the mould’s shape. Once the metal cools and solidifies, it is removed from the mould and sent for post-processing. In post-processing, the surface finish and appearance are enhanced and any necessary protective coatings are applied.

Metal casting is only limited by the manufacturer’s imagination. It can form highly intricate shapes.

Casting is preferred for complex geometries because it is more cost-effective and the process is straightforward compared to, for example, CNC machining in such instances. But it is also widely used for the simplest shapes because of its quick turnaround time and large production capacity.

Modern methods of casting can produce near-net shapes, significantly reducing the necessity for deburring and other post-processing operations.

Metal Casting Process

The reason the metal casting process became so popular was its inherent simplicity. There was no need for modern machinery or complex processes to obtain finished products.

Hence, even ancient armies were able to leverage it for weaponry and tool production. But over the years, the advances in casting have added some steps to the process to ensure that the final products have the best quality possible. Let us look at the typical metal casting process used in industries today.

Step 1: Mould Creation

The mould creation is arguably the most crucial step in the metal casting process. The quality of the mould directly impacts the quality of the finished product. The mould creation method varies depending on the type of metal, part geometry, production capacity, and the desired finish. Once these specifications are decided on, the appropriate materials and the method can be determined.

The mould is created as a negative impression of the desired part. It is made of a material that can not only withstand the high temperatures of molten material but also separate easily from the solid metal part after cooling. Some materials used for mould include sand, plaster, refractory slurry and ceramic liquid. Some moulds can only be used once, while others may be used repeatedly.

Step 2: Melting and pouring of metal

The casting metal is heated above its melting point to create a free-flowing liquid. The liquid must be free of contaminants and dross as much as possible. Arrangements can be added to the mould to prevent the entry of dross into the casting. The molten metal is poured into a pouring basin at the top of the mould. The liquified metal then enters the sprue (vertical channel) and is distributed into the mould cavity through runners (horizontal channels) and gates (entrances for the cavities).

Less viscous metals can flow easily into the casting and form finer patterns. Metals with high viscosity exhibit non-turbulent flow, which helps eliminate defects like porosity. The solidification rate and driving force are also influenced by the viscosity of the molten metal.

Step 3: Removal of the metal casting from the mould

In the case of a single-use mould, the mould is broken to retrieve the metal casting.

For permanent mould casting, there are various opening arrangements on the mould to access and retrieve the casting. The mould can then be reused on the next batches.

Step 4: Finishing and post-processing

The casting retrieved from the mould cannot be used in its current state. When the molten metal is poured into the mould, the liquid metal in the sprue, runners, risers and vents also solidifies. All of these appendages are cut off. Surface finishing may also be necessary.

The part is then sent for cleaning and coating if the application requires it.

Types of Casting Processes

Castings can be created from various metals like aluminium, zinc, steel and iron. Each metal possesses unique characteristics, necessitating adjustments in the casting technique to achieve the best possible product.

The casting technique also depends on parameters such as alloy composition, part geometry, cost, desired final quality, production run size, and performance criteria.

The casting processes are divided in two ways: the first is based on the lifespan of the mould (expendable and non-expendable mould casting) and the second is the method of alloy injection (gravity, pressure or vacuum casting). In this article, we will categorise the casting processes into expendable and non-expendable mould castings.

Expendable Mould Casting Process

Expendable mould casting processes use moulds that can only be used once. These moulds are used in various casting processes such as sand casting, shell mould casting, and investment casting among others. It must be noted, however, that the these moulds may not always be discarded after use. The manufacturers may try to salvage the moulds when mould reform is possible.

Let’s have a closer look at some of the processes using non-reusable mould castings:

Sand Casting

Sand Casting Process

In sand casting, sand is used as the mould material. Sand is cheap, easily available, pliable and can withstand high heat better than most materials. Green sand (moist) and dry sand are the most suitable materials for the casting process.

A sand mould in the negative shape of the desired part is created and then the molten metal is poured into it. Once the metal solidifies, the sand is removed to retrieve the casting.

Sand casting is one of the oldest casting methods. It remains very popular to this day due to its simplicity, versatility and affordability.

Sand casting can produce products ranging in size from a few centimetres to tens of metres. Products can weigh as little as 75g and go up to several tons.

However, sand casting does not provide the best surface finish and is therefore more suitable for larger products that can be refined for a smoother finish later.

Plaster mould casting is a type of sand casting method where plaster is used to build the mould instead of sand.

Shell Moulding

Shell Mould Creation Process

Shell moulding is a type of sand casting process. However, instead of loose sand, metal casting occurs inside a thin-walled, hard sand shell.

To create this shell, a pattern must be created first. A pattern is an object that has the shape of the desired part. The pattern is heated and placed into the sand mixed with a thermosetting resin binder. The sand around the pattern takes on the shape of the part, and the resin hardens it. The shell is further cured in the oven. Once the shell is ready, it is placed in the baking material and molten metal is poured into it. The casting is retrieved upon solidification.

Shell moulding offers advantages such as lower manpower requirements (when automated), increased production capacity, and improved dimensional accuracy.

However, shell moulding is slightly more expensive than sand casting due to the resin used. It may also have lower material strength and high porosity issues. The labour costs can also end up being quite high if the process is not automated.

Investment Casting

Investment Casting Process

Investment casting, also known as lost-wax casting or precision casting, uses wax to create accurate castings consistently.

The process starts with a wax pattern obtained through metal dies. Several wax patterns are assembled with gates, runners and a sprue. This allows to cast several parts simultaneously.

The assembly of wax patterns is dipped or “invested” in a refractory slurry that takes on the shape of the assembly.

The wax pattern, along with the slurry shell, is then heated. The wax flows out of the shell and can be reclaimed for reuse.

The slurry shell then forms the mould for the casting process. Molten metal is poured into the mould, where it solidifies into the required part.

Investment casting is highly precise. It is used for complex parts that require accurate parts, such as turbocharger rotors. It also provides a very good surface finish.

On the other hand, the process is more expensive than sand casting and shell moulding. It has limitations on some features, such as the size and depth of holes. Investment casting can be challenging to execute when cores are involved.

Non-Expendable Mould Casting Process

These casting techniques use permanent moulds that can be reused multiple times. There is no need to prepare a new mould for each casting, as is the case with expendable mould casting. This can be advantageous for castings that require a fine surface finish, high dimensional accuracy, and complex shapes.

The following casting processes can be classified as non-expendable:

Die Casting

High Pressure Die Casting Process

The die-casting process uses a metal die to cast other metals. The die is usually made out of non-ferrous alloys of aluminium, copper and zinc, but ferrous metal dies are also used.

The metal dies are expensive to make but they last a very long time. This makes die casting more suitable for high-volume production.

Die-casting, compared to other metal casting techniques, is more suitable for small to medium-sized parts that require a high-quality surface finish.

Die casting processes are of two main types: gravity die casting and pressure die casting. Gravity die casting uses the force of gravity to fill the mould with molten metals. The channels and the mould cavities are designed in a way that allows the metal to flow naturally into the pockets.

Pressure die casting injects molten metal at high pressure to fill the mould. This process offers greater flexibility in mould design but is more expensive and skill-based.

Pressure die casting is more suitable for complex geometries, while gravity die casting offers greater value for simpler designs.

Centrifugal Casting

Centrifugal Casting Process

Centrifugal casting uses centrifugal force to cast circular parts. The process is generally used for creating standard-sized castings, such as pipes, and not finished products.

As the process leverages gravity and pressure, it creates very fine designs with remarkable accuracy.

The molten metal is fed through the centre and the spinning motion of the mould pushes the metal to the edge, forming an annular shape. By adding various materials successively, it is possible to produce pipes with multiple layers. For instance, centrifugal casting can create cast iron pipes with a cement lining on the interior to prevent corrosion and subsequent tuberculation.

The centrifugal force can also push viscous molten metal into extremely fine cavities. The metal can be moulded into intricate shapes and form complex designs. Thus, this concept is commonly used to create jewellery as well.

The casting machines may be horizontal or vertical. Horizontal machines are suitable for long products like pipes, whereas vertical axis machines are more appropriate for shorter products such as bearings.

Centrifugal casting offers advantages such as not requiring cores, producing excellent grain structure, and having no limitations on dimensions like length, wall thickness and diameter.

Centrifugal casting is the best alternative for specific applications, such as thin-walled cylinders. However, it has disadvantages such as shape constraints, inability to work with smaller diameters and compatibility with limited alloys. Impurities gather at the centre and are thus generally removed by machining.

Continuous Casting

Continuous Casting Process

Continuous casting, also known as strand casting or slab casting, produces a continuous casting of constant cross-section.

Molten metal from a furnace is carefully poured into a tundish, from which the metal is transferred to a water-cooled mould with the desired cross-section. The cooling speeds up the solidification of the molten metal.

While the metal solidifies, it is also pulled out of the mould through rollers, where the casting may be cooled and shaped further to final dimensions. Once the solidification is completed, the casting is cut into standard dimensions.

Continuous casting offers a high production rate, increased accuracy, reduced waste, and enhanced grain structure.

However, the process requires a high initial investment, and the final product may have internal defects and poor surface quality.

Summary

Metal casting is a proficient and cost-effective method for producing dependable metal products. It offers a wide range of processes. Each process has pros and cons that make it suitable for specific applications.

For instance, sand casting is perfect for one-off complex pieces, whereas die casting is more appropriate when intricate products are needed at a high production rate.

Moulds used in casting are categorised as expendable moulds and non-expendable moulds. Expendable moulds are destroyed after each use, such as in sand casting, while non-expendable moulds are more permanent and can last from a few days to a few years.

The versatility of metal casting makes it an essential metalworking technique for various industries, from automotive to aerospace, ensuring the high accuracy and repeatable production of complex parts.

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