What is Sand Casting, its Applications & How it Works?

Sand casting is the oldest casting process that exists. It uses patterns made of metal, wood, or wax to create models of the desired object. The sand casting process is complicated and has a slow production rate.

This slow production rate is because the sand mold must be destroyed to remove the casting. The process involves the use of patterns, sand molds, furnaces, and liquid metal.

In this reading, I’ll be discussing what sand casting is, its applications, and how it works. I will also explain the advantages and disadvantages of sand casting.

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Table of Contents

What is sand casting?

Sand casting is a manufacturing process in which molten metal is poured into a sand mold containing a hollow cavity of the desired shape. The mold is made of sand particles held together with an inorganic binding agent.

We then allow this casting to cool at room temperature, break the sand mold, and remove the object.

Sand casting is a process where molten metal is cast in a mold made from a sand mixture. Previously cost-effective for small-volume production, it is now suitable for high-volume production due to automated equipment. Sand castings typically have a rough surface, with surface impurities and variations.

Two main types of sand used for molding are green sand and dry sand. Green sand consists of silica sand, clay, moisture, and additives, while dry sand is a mixture of sand and fast-curing adhesive. Air-set molds produce castings with smoother surfaces.

Some of the materials involved in this process include metal, concrete, epoxy, plaster, and clay. Some of the tools used during the casting process are hand riddle, shovel, rammer sprue pin, mallet, etc.

Applications Sand Casting

Despite being an old process, sand casting is still one of the most common and widely used. This is because it still offers great benefits in various fields, such as automobiles, agriculture, medicine, construction, and marine. This is because sand casting is used for heavy equipment.

Most frequently, people use sand casting to create intricate shapes that would be difficult or expensive to create using other techniques.

Instead of constructing heavy equipment by combining numerous small pieces, large pieces of heavy equipment, such as machine tool beds and ship propellers, can be conveniently cast in the needed size.

Diagram of Sand Casting

How Does Sand Casting Work?

To create sand casting, pour melted metal into a sand mold with a cavity, where it solidifies. Sand particles bound together by an inorganic binding agent form the mold. Once the metal cools to room temperature, we remove the casting from the sand mold.

The metal casting technique known as sand casting, often referred to as sand-molded casting, uses sand as the mold material. An object created via the sand casting procedure may also be referred to as “sand casting.”

Foundries are specialized enterprises that manufacture sand castings. The process of sand casting is used to create more than 60% of all metal castings.

The primary benefit of sand casting over permanent mold casting techniques is the lower cost of the mold.

The method works well for producing complexly shaped castings in small quantities, but it cannot achieve tight tolerances, and the slow cooling rate causes the casting to have a coarse grain structure and poor mechanical qualities.

Despite the advancement of the sand casting process, the process remains the same, which includes pattern making, molding, melting, pouring, ejection, cleaning, fettling, and inspection.

Although defects are inevitable, some of them could even damage the entire casting. This is why all foundry workers are highly skilled, as the process also involves serious safety.

Steps Involve in Sand Casting

The major steps involved in sand casting are pattern making, mold making, the pouring stage, the cooling stage, the removing stage, trimming and grinding, and inspecting the object.

Step 1: Pattern Making

The first stage involved in sand casting processes is pattern making. Pattern-making seems to be tedious and intelligent work, as is the replica of an item to be produced.

It can be made of different materials such as wood, metal, synthetics, etc., depending on the volume and tolerance of the casting. Wood is the most common because it is less expensive and easy to shape.

However, wood patterns easily warp and deform. Wood patterns can wear quickly due to sand exposure. Whist, patterns made with metal last longer and can be reused to create the same type of cavity, which helps reduce tooling costs. But it is pricier.

The process of pattern making incorporates an allowance for thermal contraction or shrinkage.

Step 2: Mold making

In this stage of the sand casting process, a refractory material (sand is widely applicable) is loaded or packed on the flask, packed around the pattern still fully compacted. The pattern is then removed, leaving the shape in the mold cavity.

The sand used to make the mold is strong enough to hold the weight of the molten metal when it is poured and should be brittle enough to be broken when the casting cools and solidifies.

Clay and some chemical bonding agents are used to strengthen the mold in order to withstand the pouring.

If the sand is packed in the cope and drag while the pattern is in it, the cope and drag are separated so that the pattern can easily be removed.

A refractory coating (heat-resistant material) is added to the surface of the cavity to produce a better surface finish and to allow the mold to withstand the poured metal. We couple the flask back together, ensuring the shape remains in the cavity.

Core Making

Let me use this opportunity to elaborate more on core making in casting. Cores are internal holes and passages in a casting. Sand typically covers the portions where the cores are located in the pattern.

Before pouring molten metal, the mold produces the core. Core print is an inset in a pattern that allows the core to be held in place inside the mold. However, the core may lose position due to the buoyancy of the molten metal.

Chaplets can support the core and help hold it together with the mold. In this situation, the chaplets used must have a higher melting point than the poured molten metal. Chaplets are cut off at the end of the casting process.

Step 3: Pouring stage

At this stage, a furnace melts the metal at a specific temperature while preparing the mold. This involves clamping the mold and cutting the riser and gateway.

Gateway and riser are designed to allow the molten metal to smoothly or freely flow into the cavity. It also helps to eliminate turbulence, which prevents oxides and casting defects.

We have partially melted the metal in a crucible (high refractory material). It is removed from the furnace using crucible tongs to the pouring shank, which ensures better pouring.

Although pouring can be done manually or by an automated machine, Enough metal should be melted to fill the entire cavity and all channels in the mold; else there will be an unfilled portion in the casting.

Step 4: Cooling stage

After pouring the molten metal into the cavity, it starts to cool and solidifies over time. The molten metal takes the shape of the cavity and solidifies, obtaining such a shape.

The mold is a break after the cooling elapses, which can be estimated based on the thickness of the casting and the temperature of the metal.

At this stage, a defect likely occurs. If some part of the molten metal cools too quickly, it may exhibit cracks, shrinkage, or incomplete sections, so care should be taken when making the mold and when pouring the molten metal.

Step 5: Removing stage (breaking)

At a predetermined time, the casting cools and solidifies. You can simply break the mold to obtain the casting. We also refer to this process as checkout. Typically, a vibrating machine performs the breaking process, shaking the sand and casting it out of the flask.

The casting will likely contain some sand and oxide layers attached to it. The sand can be removed using shot blasting, mostly from the internal surface, to reduce the surface roughness.

Step 6: Trimming or Grinding

Remember the molten metal is poured through a channel (gateway and riser) to the cavity, which fills all portions in the mold, as does the channel.

Trim the metal in the channels and adjust the casting to the required dimensions. You can manually trim the casting using a cutting saw or a trimming press. Trimming time can be determined by the size of the casting envelope.

A big casting may require a longer trimming time.

Step 7: Inspecting

Usually, the cleaning and grinding stages identify non-conforming castings. But one last check is still required.

Additionally, we must create a set of tools or through-stop gauges for inspection to make sure there are no assembly issues for castings that have assembly requirements.

Advantages

Sand casting has advantages over other casting processes, even though several defects may occur. Sand casting offers outstanding strength when clay is added to the mold, making the sand strongly bond.

Sand casting is designed to reduce the potential for cracking, tearing, and shrinkage during the casting’s cooling stage. Automotive products like engine blocks, casings, and housings mostly use sand casting. Some other advantages of the sand casting process include

Both ferrous and nonferrous metals are used for casting.
The product has a low cost of production.
The cost of podcasting tooling is low.
Complex shapes can be produced.

Disadvantages

Below are the limitations of sand casting in their various applications:

A lower degree of accuracy
It is a tedious process
Rough surface finish
Mold can only be used once
Slow production rate
Labor intensive

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