Sand Casting | Metal Casting

Sand Casting:

Sand casting, also known as sand moulding casting, is a casting-based manufacturing method that makes use of a sand mould to create the final product. A wide range of metal goods and components of various sizes and shapes are produced using this process. To put its widespread use in context, figures reveal that sand casting is used to manufacture more than half of all metal castings, or around 60% of all metal castings. You will learn more about the six major phases of sand casting in the sections that follow.

What is Sand Casting?

Sand Casting is a solidification process. In the casting process, sand is used to build a mould, following which liquid metal is poured into the mould to form a component. Therefore, the microstructure can be finely tuned, such as grain structure, phase transformations and precipitation.

However, defects such as shrinkage porosity, cracks and segregation are also intimately linked to solidification. These defects can lead to lower mechanical properties. A subsequent heat treatment is often required to reduce residual stresses and optimize mechanical properties.

Sand casting uses natural or synthetic sand (lake sand)  which is mostly a refractory material called silica (SiO2). The sand grains must be small enough so that it can be packed densely; however, the grains must be large enough to allow gasses formed during the metal pouring to escape through the pores. Larger sized molds use green sand (mixture of sand, clay and some water). Sand can be reused, and excess metal poured is cutoff and re-used also.

The process is fairly straightforward: you make a pattern of what you want to cast, then use the pattern to make a sand mold, and then pour molten metal into the mold. After the metal freezes you end up with the piece that you want.

The sand used for green sand molding is critical and determines the favorable or unfavorable outcome of the casting.  It controls the tolerances, surface finish and the repeatability while in production.  Remembering that the tolerances on sand castings are usually wider than the other casting methods.

Ex: Gears, Pulleys, Crankshafts, Connecting Rods, Propellers, heavy Machine base etc.

The most common metals are Iron, Steel, Bronze, Brass and Aluminium. The process is to make medium to large parts like Valve bodies, Locomotive components and Construction Machinery. Likewise small parts of Buckles, Handles, knobs, and Hinges.

The sand casting process involves the use of a furnace, metal, pattern and Sand mould. The metal is melted in a furnace and then ladled and poured into the cavity of the sand mould, which is formed by the pattern. The sand mould separates along a parting line and the solidified casting can be removed.

Sand casting has certain characteristics.

• A sand casting method is used to manufacture more than 70% of all metal castings on the market.
• Sand casting may be used to create a broad variety of objects ranging from little sculptures to sections weighing more than 100 tonnes. Even the most intricate parts can be manufactured quickly and in one piece using this method.
• Dimensional accuracy is poor.
• Surface finish is poor
• It is very flexible and has the potential to be employed in large-scale manufacturing.

Sand Casting Has a Long and Proud History

Sand casting, the earliest known casting method, may be dated back to more than 1000 B.C., making it the oldest known casting process. Process controls, material alternatives, tolerance capabilities, the capacity to make intricate components, and a wide variety of size ranges are all things that have progressed significantly, of course. The fundamentals of the metals industry, on the other hand, have remained basically constant. Create a hollow in the form of the component you want to make, and then pour molten metal into it to complete the construction.

As a consequence of decades of evolution, sand casting has emerged as the most versatile and, thus, the most extensively utilised metal casting technology available today.

Materials for Sand Casting

Sand casting may be used with practically any alloy because of its versatility. The capacity to cast materials with high melting temperatures, such as steel, nickel, and titanium, is a significant benefit of sand casting. The melting temperatures of the four most frequent materials used in sand casting are listed here, along with the melting temperatures of the materials themselves.

Materials	Melting Temperature
Aluminium alloys	660 °C
Brass alloys	1082 °C
Cast iron	1088-1260 °C
Cast steel	1371 °C

The Steps in the Sand Casting Process

A. Manufacturing the Pattern – Preferred End-Result

The procedure makes use of a reusable pattern that has the same details as the end product that is wanted. Thermal contraction, often known as shrinkage, is accommodated in the design.

B. Gates And Risers: Metal Delivery System

It also comprises the metal channels that will feed the required cast product design with gating and risers. Thus, it directs the unavoidable thermal contraction to suitable places (which may be different from the actual intended completed product), regulates the metal flow, as well as provides for the necessary gas venting.

Materials used to create patterns include wood, metal, synthetics, expandable polystyrene (EPS), and other materials, which are chosen based on the volume and tolerance requirements of the project.

C. Creating The Mold

A ring of refractory material (in this case, sand) is produced around the design to ensure that it remains stable at high temperatures. After being cast, the material must be strong enough to support the weight of the liquid metal while remaining resistant to interaction with the metal, but it must also be fragile enough to be easily separated from the solidified metal after the casting has cooled.

Molds may be made from a range of sand materials, depending on their composition. The sand is generally mixed with additional elements such as clay or a chemical bonding agent to make it stronger so that it can withstand the pressure of the pouring procedure.

Alternatively, the mould may be made by cutting the necessary form cavity straight into a block of sand and then casting the resulting part. In product development, the approach is commonly utilised since design modifications can be managed and executed rapidly, or for components that are only used infrequently to avoid the storage and maintenance of a physical pattern. It is also widely used in manufacturing.

The mould is normally made up of two parts: the top half, known as the “cope,” and the bottom half, known as the “drag.” Once the sand has hardened (using the traditional/non-machined procedure), the pieces are separated and the pattern is scraped off the surface. A refractory coating is applied to the mould to improve the surface quality and to protect it from the turbulence created by the poured metal during the casting process. The pieces are reassembled with a hollow in the form of the pattern created by the pattern’s design.

Cores may also be included in the mould, which is a technique for creating internal pathways in the finished product that are specifically wanted.

D. Pouring the Metal Into the Mold

The static mould is filled with molten metal that has been poured straight into it. It fills the cavity that defines the final portion as well as the risers on each side of the finished part. The risers provide a continuous supply of liquid metal to the casting. Due to the fact that they are intended to cool and solidify last, the shrinkage and possible void are focused in the riser rather than in the desired component.

“Tilt pouring” is a term that refers to numerous different techniques. In order to reduce turbulence in the casting, this procedure is meant to enable metal to flow more smoothly into the mould during casting. Reduced turbulence may aid in the prevention of the formation of oxides and the formation of casting flaws.

This method has the capability of producing almost any alloy. When working with materials that are extremely reactive to oxygen, a method such as argon shielding may be used to keep air away from the molten metal during the melting process.

E. Shakeout

This occurs when the casting hardens and cools, including both the targeted portion and any extra metal required to complete the project. During the shakeout process, the sand is split up and moved about. It is possible to catch and recycle most of the sand that was used in the mold-making process

F. Final Operations.

The gates, runners, and risers are cut from the casting, and if required, further post-processing steps such as sandblasting, grinding, and other procedures are carried out to complete the casting dimensionally and structurally. Sand castings sometimes need at least some further machining in order to achieve the final dimensions or tolerances desired.

Heat treatment may be used to enhance the dimensional stability and mechanical qualities of parts. Additionally, non-destructive testing may be carried out. This might involve examinations using fluorescent penetrants, magnetic particles, radiographic images, or other techniques. Prior to shipping, final dimensional inspections, alloy test results, and nondestructive testing (NDT) are performed.

Which sand is used for the sand casting?

Green Sand

Green sand refers to the sand moulds that are made from wet sand, and it is also referred to as clay in certain circles. As the metal is being poured into the mould, the sand mould is still in an uncured condition. Because green sand may be reused, sand casting using green sand is a rapid and affordable method of casting.

The disadvantage of using sand as a mould is that it is a soft material that might collapse or move during the casting process, resulting in an unsatisfactory cast. The technique, on the other hand, is sufficiently dependable that it has endured for centuries and is still in use today.

Sodium Silicate or Water Glass

Mold casting may also be done using sodium silicate, which is a chemical compound. By flowing carbon dioxide through sodium silicate, it may swiftly transition from a liquid to a solid state. The sodium silicate becomes dehydrated as a result of this. When a cavity is needed inside a casting, this procedure is incredibly beneficial.

Even though sodium silicate may be used to form a solid core for a casting, it must be combined with other materials in order for the core to be removed once the casting has been completed. It is possible that the core will get trapped inside the casting if the combination is not perfect, rendering the casting ineffective.

Resin Sand

After being blended and heated, resin sand solidifies and forms a flat surface on the mould. A solid mould produces fewer faulty castings, but the drawback is that it is more expensive and produces castings at a slower pace. Whereas green sand moulds may be made in a short period of time, resin sand moulds take longer to make since each must be combined and then burnt in order to produce the right mould.

The cost of resin sand is much higher, and even though the resin may be reused, it must be replaced on a continuous basis. As a result, the procedure becomes more expensive.

Tooling in Sand Casting

The pattern that is utilised to form the mould cavity is the most important piece of tooling for sand casting. The pattern is a full-scale model of the component that is used to create an imprint in the sand mould.. As a result, certain internal surfaces may be excluded from the pattern since they will be formed by distinct cores and so will not be included in the pattern.

A little bigger pattern is created than the actual component because the casting will shrink within the mould cavity as it cools and solidifies. Additionally, various similar patterns may be utilised to generate several impressions in the sand mould, resulting in multiple holes that will yield as many components in a single casting as there are identical patterns used to manufacture.

The fabrication of a design may be accomplished using a variety of various materials such as wood, plastic, and metal. While wood is often used because it is affordable and simple to shape, wood may warp and bend if exposed to extreme heat or humidity. Wood will also deteriorate more quickly as a result of the sand. Although metal is more costly than plastic, it will endure longer and have greater tolerances.

The template may be reused to construct the cavity for several moulds of the same component, which saves time and money. As a result, a pattern with a longer life span will result in lower tooling costs. In order to create a pattern for a part, there are many options available, which are divided into the following categories:

Solid pattern

A solid pattern is a model of the component that is constructed from a single piece of material. Although it is the simplest to construct, it might provide some challenges when it comes to creating the mould. It is necessary to identify the dividing line and runner system independently. When it comes to geometrically basic items that are manufactured in small numbers, solid patterns are frequently employed.

Split pattern

A split pattern represents the component as two distinct parts that come together at the parting line of the mould to form a single piece. When two parts are used, the mould cavities in the cope and drag may be created independently and the separating line can be selected before the moulds are assembled. The usage of split patterns is normally reserved for pieces that are geometrically difficult and manufactured in small to moderate numbers.

Match – Plate pattern

When a match-plate pattern is used, it’s quite similar to a split pattern, with the exception that it’s affixed to the two opposing sides of a single plate rather than two separate plates. The plate is often constructed of wood or metal. This pattern design guarantees that the mould holes in the cope and drag are properly aligned, and the runner system may be integrated on the match plate if desired. If you need to produce big numbers of anything, match plate designs are a good choice. They are also popular for automated processes.

Cope and Drag Pattern

Mold halves are formed independently of one another in a cope and drag pattern, which is identical to a match plate design except that each half of the pattern is affixed to a distinct plate and the mould halves are made separately of one another. It is similar to a match plate pattern in that the plates are used to assure accurate alignment of the mould cavities in the cope and drag, and they may also incorporate the runner system in the design.

Cope and drag patterns are often desired for bigger castings, when a match-plate design would be too heavy and cumbersome to be practical. Furthermore, they are often employed when the process is mechanised and for bigger manufacturing numbers.

A core-box is another piece of tooling that is utilised in the process of sand casting. This kind of box, which is similar to a die, is used to generate sand cores if the casting demands them. It may be made of wood, plastic, or metal, and it is used in the same way as a pattern to create the cores. The core-boxes may also have numerous cavities, allowing for the production of many identical cores at the same time.

Sand Casting Moulds are available in a variety of shapes and sizes.

Variations of this fundamental procedure exist based on the pattern, flask type, and amount of automation used. For example,

Bench Moulding

Bench moulding is the method of choice for modest works. The whole procedure is carried out on a bench that is at a suitable working height.

Floor Moulding

Floor moulding is utilised for medium and large-scale construction projects. The mould is placed on the floor before the concrete is poured, as the name implies.

Machine Moulding

The process of machine moulding is used for mass manufacturing. Because machine moulding reduces time and labour while also providing improved precision and consistency, it is feasible to maintain tight tolerances while producing parts at a rapid pace. The basic processes carried out by moulding machines are the ramming of the moulding sand, the rolling over of the mould, the formation of the form gate, the rapping of the pattern, and the withdrawal of the pattern.

Defects that might occur in Sand Casting

Defects	Causes
Sections left unfilled	There is insufficient material
	Pouring temperature is rather low
Porosity	The melting temperature is very high
	The pace of cooling is not unifor
	Sand has a low degree of permeability
Hot tears	The pace of cooling is not uniform
Surface projection	Erosion of the sand mold’s internal surface
	A sand mould crack,
	Shift in the mould halves

Advantages of Sand Casting:

1. Low cost of mould materials and equipment

2. Large casting dimensions may be obtained

3. Wide variety of metals and alloy (Ferrous and Non-Ferrous) may be cast

 Disadvantages of Sand Casting:

1. Rough surface

2. Poor dimensional accuracy

3. High machining tolerances

4. Coarse grain structure

5. Limited wall thickness (2.5 – 5 mm)

Conclusion

The sand casting method is a flexible and well-understood manufacturing technique. Sand casting has become the technique of choice for applications in a wide range of advanced manufacturing industries as a result of process advancements that have occurred through time.

As with other materials and design choices, consulting with a metals specialist may assist in making the optimal metals choice. That is a dialogue that should take place as early as feasible in the design process in order to maximise manufacturability. However, even applications that have been using metal components in the same manner for a long time may be reviewed to see whether a greater performance or a more cost efficient conversion to a new method, such as sand casting, can be achieved by changing the manufacturing process.

Reference:

1. Gravity Die Casting
2. Hobby Metal Casting