Sand Casting Process
 |
| (a) Sand Casting Diagram |
What is the sand casting process?
Sand casting can be defined as the founding process that uses a mould made up of sand for producing castings in a factory called a foundry.
The majority of casting produced today all around the world is made by this process (about 65%).
The sand casting process is also known as the sand moulded casting process. Because refractory mould is made up of silica sand, clay and moisture.
The pattern is used to make a cavity in this refractory mould where molten metal is allowed to flow through the gating system and solidifies to form the final casting.
The sand-casting process is explained below with animation in the animation (b)
 |
| (b) Sand Casting Animation |
This is a type of expandable moulding process, which means after casting is solidified in the refractory mould the sand mould needs to be broken to take the final product (casting) out.
To understand this process better, engineers need to understand sand casting terminology first. This article discusses all the basic terms that are going to be used in the following article.
What is the sand casting process also called?
This process is also called sand moulding, sand casting or sand casting process.
I have explained the sand moulding process with diagrams, animation, a flow chart, a step-by-step process, type and differences between sand casting vs other casting processes and applications.
{tocify} $title={Sand Casting Table of Contents}
Sand Casting Step-By-Step Process
Explain the entire sand casting in 6 step-by-step processes with a diagram from making pattern, core, sand mould, pouring operation, solidification, shake out and secondary manufacturing operations?
Following is the sand-casting flow chart for making casting products in the foundry.
 |
| (c) Sand Casting Flow Chart |
The flow chart above (c) shows six steps involved in producing sand casting. I have explained all these steps in detail below, Let us learn the journey of producing casting from the pattern and core making (Step 1) to the secondary manufacturing processes (Step 6) below.
STEP 1: Making Sand Casting Patterns And Cores
a) Pattern Making
Why do we need patterns in the sand casting process and how do
we manufacture
patterns?
The pattern-making process consists of making a pattern tool that will be used to create a cavity in the sand mould. This is the same cavity where molten metal will be poured to manufacture the desired casting.
Pattern-making is a significant step that is done even before making the sand mould, pattern serves as a reference point for making mould cavities.
As shown below in diagram (d) pattern is been used to create a cavity in the mould. The cavity in the mould resembles closely the shape and size of the casting that will be dispatched to the customer.
 |
| (d) Sand Casting Pattern and Mould Cavity |
What pattern-making materials are available in the foundry and what machines are used to produce accurate patterns?
Patterns are made with various pattern materials such as wood, plastic, rubber, wax, plaster of pairs, metal and form (form for disposal pattern).
These patterns are made in a workshop having lathe machines, milling machines, drilling machines, grinding machines, slotting machines, planning machines, injection moulding machines and plastic moulding machines for making wood, wax, plastic and metal patterns.
It is also very interesting to see that a few metal patterns need to be sand cast to further produce patterns to create an impression.
These pattern pieces are made on machines mentioned above and are joined together by welding machines (for metal patterns) and different kinds of adhesives for wood and foam patterns.
These machines give the final shape to the pattern to create a negative impression in the sand where molten metal will be finally poured.
What care must be taken while making a pattern for casting?
The patterns used should have important characteristics and pattern properties to qualify to be a pattern in the casting process.
Depending upon the pattern material, geometry, withdrawal method, distortion and shrinkage various kinds of pattern-making allowances such as rapping allowance, shrinkage allowance, distortion allowance, draft allowance and machining allowance are applied to the pattern.
These patterns should be colour-coded for easy visualization during the moulding process for the pattern creator and sand mould-making operator.
b) Sand Core Making
 |
| (e) Sand Casting Core |
Why there is a need for sand cores in the sand-casting process?
Cores as shown above in diagram (e) are used in this process to create holes, recesses, passages and cavities in the final casting. Once the patterns are made it is time to make sand cores. Core-making takes place at the core-making unit in the foundry.
There are various types of cores depending upon the shape, size, position and core material that can be utilized for making required cavities in the casting.
Sand cores are made in core boxes made from wood or metal. Green sand, dry sand or synthetic sand are poured into these core boxes and a core of desirable shape is obtained. These cores are further cured and hardened before positioning them in the mould cavity to avoid erosion of the core sand when molten metal comes in contact with them.
Cores used in this process should have important core characteristics and properties to qualify to make cavities in the mould.
These cores are supported by chaplets (chaplets are metallic objects that support cores) and core prints (core prints are extensions of the core made from dry and green sand that help support the core ) during the casting process in the mould.
STEP 2: Sand Mould Making Process
What is the sand mould-making process and why is it necessary for the sand casting process?
 |
| (f) Sand Casting Mould |
The sand mould-making process is about making a mould using various types of sand, additives, binders, water and an oven for drying dry sand mould and skin-dried mould.
A typical mould used in this process is shown in the diagram above (f) which consists of a parting line which divides the mould into two sections such as cope (top part of the mould) and drag (bottom part of the mould), flask (which holds the mould together), a gating system such as a pouring basin, sprue, runner and riser.
The mould-making process takes place at the mould-making unit in the foundry. Sand mould making is done in two following ways:
- Hand moulding
- Machine moulding
The first is making sand mould manually called the hand moulding method which is a time-consuming manual way of making sand mould using simple low-cost hand tools.
I have explained the step-by-step process for making sand mould covering sand mould-making method by hand.
The second moulding method is through machines which is called machine moulding where the machine is employed to make green sand moulds automatically.
The mould-making process starts with making the drag side of the mould section first followed by the cope side of the mould.
Various types of moulding sand such as facing sand, system sand, green sand (for making green mould), dry sand (for making dry mould), parting sand, loam sand, core sand and backing sand are poured on the bottom board to make a complete sand mould.
Depending upon the design of the final casting one of the patterns among 17 pattern types is selected based on important pattern material factors for making a cavity in the sand casting process.
This entire mould-making process is carried out under the same roof in the foundry from sand testing, compacting/shaping and drying the mould. The sand used here should have specific sand properties making it suitable to be used for making mould.
Moulds that can be made from this process are green sand mould, dry sand mould and skin-dried mould used to produce casting from smize to large sizes.
Mould making is a crucial step done with sand mould-making materials such as binders and additives.
In the sand mold casting process, sand serves as an important element for making a sand mould. Sand has a high burning point, is cost-effective, readily available and easily mouldable to produce prototype casting to large-size sand casting.
Cores, core prints, chaplets, padding and chills are placed in the mould once mould making process is completed and before we start the pouring operation.
STEP 3: Melting Ingots In Furnace And Pouring Molten Metal Into The Sand Mould
Ingots are bars of desired metal that are melted in a furnace and cupola.
Melting and pouring take place at the melting and pouring unit which consists of furnaces (electric, induction, fossil fuel, metal pot, pit furnace and crucible), cupolas, ladles, cranes, degassing and inclusion unit equipment.
An electric arc furnace is used for heavy ferrous material cast iron and steel. Crucible furnaces which are gas flames used in small foundries. While induction furnaces are used for ferrous and non-ferrous metals.
Cupola is used for melting ferrous metals such as cast iron and steel while a furnace is used for melting non-ferrous metals such as aluminium, lead, zinc, tin, brass, bronze and cadmium.
This metal is heated while keeping in mind heat loss during the transfer of metal from the melting unit to the ladle and ladle to the sand mould. After this process inclusion and degassing of the molten metal is done.
Inclusion in the moulding process consists of the addition of silicon alloys to molten metal iron to improve mechanical properties, strength and graphite. Induction can be done in-stream, in mould or directly in the ladle.
Degassing in the moulding process consists of removing large amounts of dissolved gases like hydrogen which need to be removed to prevent defects such as porosity during the solidification process of metal. Inert gases such as argon are purged in the molten metal with a metal degassing machine to remove hydrogen out of aluminium alloys with argon bubbles.
A ladle (molten metal handling equipment) is used to pour metal into the pouring basin. Molten metal is transferred from the furnace or cupola to sand mould. Before metal is poured into a ladle to transfer the molten metal optical pyrometer is used to measure the temperature of the molten metal.
The pouring of molten metal into the sand mould is called the pouring operation.
Shank ladle and hand ladle are used for pouring molten to produce small casting products, while bottom pour ladle and teapot ladle are used for pouring molten metal into large-size casting.
Metal first comes in contact with the sprue reducing turbulence and the metal flows through the pouring basin to the vertical sprue towards the basewell.
Sprue regulates the flow of molten metal before it reaches the sprue base well.
When metal is poured into the pouring basin metal needs to be poured at a uniform controlled flow rate filling the pouring basin, sprue, runner, ingates, gates and riser keeping in mind there is no ersoin of mould sand displacement of cores in the mould.
The base well is cut in the mould to avoid sand erosion and turbulence as metal lands on it from vertical sprue at gravitational force.
Once the base well gets filled the metal starts flowing in the runners which is a horizontal channel that connects the gate which is the entry to the mould cavity.
The runner is a passageway through which molten metal reaches the gate.
A gate is an entry to the mould cavity. Once the molten metal reaches the mould cavity metal starts filling the cavity. The remaining metal fills the riser which will be further used during the liquid solidification process. The riser is a vertical section connecting the end of the mould cavity.
Static weight called pouring weight is kept on the cope to compensate for upward forces that are acting on the mould.
Cores are fixed by chaplets to overcome turbulence and metallostatic forces acting on them while metal flows.
Gases formed during this process escape from the mould's vent passages, reducing gas defects in the final casting. After this point solidification of metal takes place.
STEP 4: Solidification Of Molten Metal
Once the mould is full of molten metal, it is allowed to solidify. Solidification starts at the outer layer of the casting (near the mould walls) and starts solidifying towards the centre of the casting.
Two types of shrinkage take place during the solidification of casting.
- a) Liquid shrinkage.
- b) Solid shrinkage.
a) Liquid Shrinkage
When molten metal starts solidifying from the liquid stage to the solid stage at solidus temperature we call it liquid shrinkage in casting.
In order to manage liquid shrinkage, the riser is used.
The riser takes care of liquid shrinkage by supplying the required molten metal to the solidifying casting.
The riser is a gating element which solidifies late in the mould as it has the vital job to supply molten metal to the casting whenever required.
b) Solid Shrinkage
When the metal has solidified it starts losing temperature and metal shrinks, this shrinkage is called solid shrinkage.
In order to manage solid shrinkage, allowance is provided for pattern and casting.
Solidification always starts from the wall of the mould towards the centre of the casting.
It is important to solidify molten metal slowly and uniformly to avoid macrosegregation of the casting.
Entire casting cannot be of the same thickness, for this chills are used to achieve directional solidification at varied thicknesses.
Chills are available with various metals such as copper, steel, iron, graphite and ceramic which can be used in this process. If metal solidifies at the wall faster formation of chilled crystals takes place.
Metal grains that are present between the mould wall and the centre of casting are called equiaxed crystals. These crystals are in a zone called the mushy zone.
The crystals formed in the centre of the casting are called columnar crystals.
STEP 5: Shake Out And Break The Mould To Remove Solidified Casting
Once the metal has been solidified it is time to take casting out with the help of draw spikes.
At times water-jet or vibrations are used to break the mould before the pattern is lifted from the mould.
The mould is broken and the entire solidified casting along with the gating system is taken out.
Runner and gates are cut away by band saw from casting to obtain a final product.
Any excess sand on casting is blown away by the bellow device. It is basically a blower to blow sand stuck to the casting and gating system.
Sand can be reused again in the mould while the cut-out gating system will be re-melted in a furnace.
In large foundries material handling units such as robots, mono-rail hoists, cranes, automatic guided vehicles, elevators, conveyors, lifters and elevators are responsible for the transportation of goods such as final casting, raw material, moulding sand, binders, additives, ladle, chemicals, ingots, core sand, cores, molds, mould making equipment and other sand casting materials.
STEP 6: Casting Cleaning, Quality Control And Inspection, Secondary Manufacturing Operations For Producing Final Sand Casting
Casting Cleaning
This is an important step after casting is been taken out of the sand mould. Casting cleaning starts when casting is cooled and all metallurgical transformation has stopped around 400℃.
It becomes important to clean casting from sand, projection, fins and gating systems. The following methods are used in this process to clean sand casting parts. This entire process of cleaning casting is called fettling and is done in the foundry
- Hydrabolasting
- Tumbling
- Sand Blasting
Hydrobaslting: Process of separating sand cores, gates, riser, runners, fins and projections from final casting by just knocking the casting with an iron bar or vibrating the casting on the machine.
Band chippers, sawing, cutters, hack saws and hammers are used for a few brittle materials such as grey cast iron and non-ferrous materials such as aluminium, zinc and tin.
For non-brittle materials such as steel abrasive cutters, grinding machines and thick section welding flames are used to cut the gating system away separating the actual casting.
Tumbling: Casting products that do not have overhead projection and sharp counters are placed in a barrel and rotated around the horizontal axis up to 50 rpm. All fins, projection, gating system and sand attached to the final casting get separated leaving behind the final clear casting.
Sand Blasting: The process of cleaning casting by spraying air at high pressure and steel grit grains on the casting separating sand and casting is called the sandblasting process. This process is done in a controlled and enclosed environment protecting the operator from steel grit garins.
Inspection and Testing Of Casting
Inspection methods such as visual inspection, magnetic and magnetic particle testing, NDT, ultrasonic testing, eddy current inspection, geometrical dimensions, pressure testing, radiographic testing, metallurgical control and inspection and radiographic and fluorescent penetration are employed to
test the sand cast.
After inspection heat treatment processes such as annealing, normalizing and hardening are carried out on the casting.
Secondary Manufacturing Process
After casting is cleaned, inspected and tested secondary manufacturing finishing processes such as grinding on a grinding machine, turning, milling, lapping, honing, welding of casting components, repairing of segments of casting and finishing operation to remove any excess metal and burr is carried out.
CNC machining can be used for machining to achieve the required tolerance.
Powder coating, polishing, painting and finishing are done to give
the final touch to the casting before dispatching.
In small foundries, casting is outsourced for secondary operations and inspections but in major big foundries, everything is done under the same roof.
Sand Casting Problem With Surface Finish And Porosity
a) Surface Finish In Sand Casting
The surface finish of casting depends on many factors, grain size and facing sand play an important role.
Corser grain size sand gives a bad surface finish but simultaneously reduces porosity defects in casting.
Let us understand the relationship between facing sand and surface finish.
When metal comes in contact with facing sand or carbon sand, gas is produced that serves as a shield not allowing molten metal to get stuck to the sand in the mould fully.
This reduces the surface defects in the final casting giving a better surface finish to the end casting.
b) Porosity In Sand Casting
Sand grain size has a vital impact on final casting when it comes to porosity and blow holes.
Both fine and coarse grain size sand should be used in combination for sand moulding.
Finer grain-size sand produces a better surface finish but restricts escaping gases from the mould giving porosity defects.
If only course grains were used in the mould there would be better permeability (escape of gasses) but the final casting would have a rough texture giving a rough surface finish.
Sand Casting FAQ
Question: Why sand casting is preferred and widely used?
Answer: Sand casting is cost cost-effective manufacturing process used to produce products from prototype (using the solid pattern in green sand mould) to large-size (large casting can be produced with skin-dry mould at a lower cost) casting which is difficult to produce at the same price using another casting process such as die casting.
Question: Why is sand mixed with plaster?
Answer: Plaster is mixed with sand to make a sand-paster mould to produce casting at a faster rate. This is done with a plaster moulding process.
Question: What metal ingot materials are used in the sand casting process?
Answer: The metal used in the sand-casting process is cast iron, nodular iron, white iron, cast iron alloys, ductile iron, malleable iron and grey cast iron.
Aluminium and aluminium alloys such as aluminium-copper and aluminium-silicon.
Steel and steel alloys such as cast steel, stainless steel, heat resistance steel, plain carbon steel, duplex, cast steel, super duplex steel and plain carbon steel.
Magnesium and magnesium alloys for lightweight electrical and electronic components.
Other non-ferrous metals such as zinc, zinc alloys, copper, gold, silver, tin, brass, bronze, nickel, silicon alloy, titanium, lead and tin.
Question: There are how many types of steps involved in sand casting?
Answer: There are 6 steps involved in the sand casting process, which includes making pattern and core → Making sand mould → Pouring molten metal into mould (pouring operation) → Allowing the molten metal to solidify into sand mould (Solidification operation) → Removal of solidified casting out of sand mould (Sake out operation) → Secondary manufacturing processes (heat treatment process, finishing, machining, grinding and powder coating)
Question: What is history and when was sand mould casting invented?
Answer: The invention of this process goes back from 1500 BC to 2000 BC to produce casting in China, Egypt, Medieval, Greece, Rome and Europe. Small pieces of jewellery, pewter, bowls, tools, emblems, statues, symbols, decorative items, bronze sculptures, beautiful ornaments, bells and artistic architecture.
Question: What are the advantages of sand casting?
Answer: Advantages of sand casting compared to other types of casting are that, sand casting can produce casting with large sizes at low cost with both ferrous and non-ferrous metal for automobiles, inning, machine tools, manufacturing industry.
Question: What are the disadvantages of sand casting?
Answer: The disadvantage of sand casting compared to other types of casting is that sand casting has a low surface finish and dimensional tolerance as compared to other casting processes such as die casting and investment casting.
Question: What are the applications of sand casting?
Answer: The use and application of sand casting can be seen in the automobile industry to produce engine casing, the machine tool industry to produce lathe beds, the aerospace industry to produce valves, the military industry to produce external tank bodies, cookware, and electrical and electronic industries.
Sand Casting Conclusion
The sand moulding process has always been one of the most used casting processes for producing casting in higher volumes as compared to other casting processes such as special casting processes and die casting processes.
This process is great for making large-size castings up to 250 tonnes and castings where surface finish, surface texture, finish and production quality are not primarily important.
For better surface finish and dimensional accuracy other special casting processes such as die casting, shell casting and investment casting are employed to compensate for the drawbacks of sand casting such as blow holes, porosity, metal penetration, surface roughness, sand holes, runout defects, dirt, scabs, swelling, fusion of molten metal & mould and drop defects due to weak sand mould.
The dimensional tolerance achieved by this casting process is 0.008 cm on the lower side and 0.3 cm on the upper side.
For a high production rate, lower solidification time and better dimensional accuracy die die-casting process is used. But for large-size symmetrical longer length casting continuous casting is employed.
Sand moulding can produce simple shapes but for very complex shapes and structures investment casting is used.
3D printing patterns are a very promising and futuristic casting method that overcomes most drawbacks of the traditional sand mould casting process.
CNC-manufactured patterns, layer manufacturing and 3D-printed patterns are used to reduce the time for producing patterns and cores. These patterns and cores can be mass-produced by replacing time-consuming traditional sand cores which are made in core boxes manually.
3D printing patterns and cores have very close tolerance, surface texture, pattern tool life, reliability and mechanical properties giving better dimensional accuracy to the casting.
This sand moulding process has improved over the period where instead of hand moulding sand mould, automatic machine moulding is used.
There has been a huge improvement in past years with pattern material, synthetic binders, sand material, clay, mould-making machines, tools and types of equipment used making this process more efficient and productive.