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 is due to the destruction of the sand mold when removing the casting (the item produced). 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.
Let’s Get Started!
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. This casting is then allowed to cool at room temperature before the sand mold is broken and the object is removed.
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 moulding 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 moulds 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, medical, construction, and marine. This is because sand casting is used for heavy equipment.
Sand casting is most frequently used to create intricate shapes that would be challenging or expensive to create using alternative 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 Works?
Melted metal is poured into a sand mold that has a cavity in it, where it solidifies, to create sand casting. Sand particles bound together by an inorganic binding agent form the mold. The casting is taken out of the sand mold once the metal has cooled to room temperature.
Sand is used as the mold material in the metal casting technique known as sand casting, often referred to as sand molded casting. An object created via the sand casting procedure may also be referred to as “sand casting.” Foundries are specialized enterprises where sand castings are made. 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 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 involve in sand casting are pattern making, mold making, pouring stage, cooling stage, removing stage, trimming and grinding, and inspecting of 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 it 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 wrap and deform. It can wear quickly from the sand. Whist, patterns made with metal last longer and can be reused to create the same type of cavity, which helps to reduce tooling costs. But it is more expensive. In pattern making, an allowance is added to it for thermal contraction or shrinking.
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 is 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 resistance 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. The flask is coupled back together, leaving the shape 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. it is typically made out of sand as it covers the portions where the cores are located in the pattern. The core is produced in the mold before molten metal is poured. 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.
That is why the core can be supported with chaplets which help hold the core 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, metal is melted in a furnace at a certain temperature while the mold is already prepared. That is, it has been clamped and the riser and gateway have been cut. 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.
The metal is been melted in a crucible (high refractory material) to some extent. It is removed from the furnace using a crucible tong to the pouring shank which ensures better pouring. Though 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 the molten metal is poured into the cavity, it begins to cool and solidifies after some time. The molten metal takes the shape of the cavity and solidifies obtaining such a shape. The mold is a break after the cooling elapsed which can be estimated based on the thickness of the casting and the temperature of the metal.
At this stage 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)
The casting cools and is solidified at a predetermined time. The mold can simply be broken and the casting is obtained. This process is also called checkout. The breaking is typically done by a vibrating machine that shakes the sand and casts 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 so as the channel. The metal in the channels must be trimmed off and the casting must be trimmed to the required dimension. Casting can be trimmed manually via cutting sawing or 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, non-conforming castings have been identified during the cleaning and grinding stages. However, a final check is still necessary. Additionally, we must create a set of tools or through-stop gauges for inspection in order 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 great 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 cooling stage of the casting. Sand casting is used mostly in producing automotive products, such as engine blocks, casings, and housings. Some other advantages of the sand casting process include:
- Both ferrous and nonferrous metals are used for casting.
- Low cost of production.
- Low cost for post-casting tooling.
- 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