Sand Casting Process
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| (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)
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| (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.
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| (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.
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| (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
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| (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?
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| (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 Types
What are the types of sand casting processes?
- Green sand moulding.
- Dry sand moulding.
- Skin-dry moulding.
- Co2 sand casting process.
- Sodium silicate sand casting.
- Cement sand casting.
- Dicalcium silicate moulding.
- Ferro-silicon moulding process.
- Shell moulding process.
Green Sand Moulding
Dry Sand Moulding
Skin-Dry Moulding
Carbon Dioxide Moulding (`CO_{2}` Sand Casting )
This process is called carbon dioxide moulding or `CO_{2}` sand casting because of the use of carbon dioxide to create a strong bond between fine sand and water glass to make a strong mould without the use of clay.
GFN (grain fineness number) for dry sand in this process is higher allowing the process to produce higher surface finish casting.
The process of making `CO_{2}` sand moulding process starts with mixing fine sand, sodium silicate (up to 5%), a little moisture, adding additives (coal dust, saw flour, dextrin, molasses and husk) and curing the mixture under `CO_{2}` gas until sodium silicate gel gets hard and binds fine dry sand together.
The process of making a `CO_{2}` sand core is similar to the sand mould described above. The bench life and strength of core and mould are limited for a longer time, bonds do not stay strong between sand and sodium silicate.
The disadvantages of the `CO_{2}` sand moulding process are that moulds have difficulty in shakeout property during pattern removal and collapsibility after casting is solidified.
Sand cannot be reused again and the process becomes costly because of `CO_{2}` gas consumption and sodium silicate as compared to us of clay and water to bind sand particles.
The advantage of using this process is the process of making `CO_{2}` sand moulds requires the same equipment, mould-making hand tools, floor space and labour except `CO_{2}` gas equipment.
This process is faster than the traditional way of making a sand mold, eliminating baking core and mould in ovens. Reducing labour cost and time for mould production but giving better surface finish and dimensional accuracy output.
Cement Sand Moulding
The cement sand moulding process is done by a slurry mixture of fine sodium silicate (2.5 to 4.5%) sand and cement to form a mould where molten metal is poured into the mould cavity to produce casting products.
Other binding agents such as sawdust, cereals, molasses and husk are added to the mould to improve the strength of the cement mould.
The advantage of cement sand moulding is that a better surface finish is obtained at the end of the casting process. The flowability of cement sand mixture is better than traditional sand, clay and moisture reducing ramming and mould-making operations.
Cement sand casting has fewer surface defects, fewer mould defects and better dimensional accuracy as there is better permeability. This process is best suited for brass and bronze casting.
The disadvantage of cement sand mould compared to traditional sand casting mould is that removing the final casting from the mould becomes difficult as a hard mixture of cement and sand needs to be broken. This mould has lower collapsibility properties as compared to dicalcium silicate mould.
If the same process was carried out with dry sand moulding, dry sand moulds had to be dried completely before which was more time-consuming.
Dicalcium Silicate Moulding
The dicalcium silicate moulding process starts with mixing sand with dicalcium silicate (2-3.5%), sodium silicate (5-6%) and additives to the moulding sand mixture's improving surface finish, hardness and flowability.
Dicalcium silicate, sodium silicate and binding chemical agents mixture is directly poured into the flask to form a sand mould. There is no need for ramming and compacting required for this mould. Chemicals create a strong bond between sand making a solid mould.
Dicalcium silicate moulds have better collapsable properties as compared to cement sand moulds. This process can produce small to large casting porosity defect-free casting for ferrous metals such as steel, grey cast iron and cast iron.
Ferro-Silicon Moulding Process
This process starts with mixing sodium silicate and ferro-silica con together in a 2:1 ratio and mixing it with silica sand. This mixture forms an exothermic reaction between these chemicals at room temperature forming a strong bond between them making sand mould stronger and having high GFN.
The process takes place without baking, external heating elements for curing mould sand or any `CO_{2}` gas.
Suction Moulding
The suction moulding process is where negative pressure is created around moulding sand sucking all sand towards the flask producing a cavity where molten metal is poured for producing sand casting.
This process starts with pouring sand on the pattern and creating pressure in the moulding sand for the ramming and compacting operation. The suction moulding process is suitable for large ferrous and non-ferrous castings for creating an accurate mould cavity producing high dimensional tolerance casting.
Shell Moulding
The shell moulding process consists of making a sand mould by heating a metal pattern and forming a mould made from a mixture of thermosetting resin and fine sand.
In the shell moulding process a shell mould is created in which molten metal is poured to make highly dimensional accuracy and surface finished casting because of fine grain sand (Sand used in this process has GFN up to 150).
The shell mould mould-making process starts with pouring the mixture on metal and curing it for up to 60 seconds up to 150℃.
Sand Casting Process Cost
Labour, Machine and Equipment Cost
- For manufacturing casting in small sizes and quantities with lesser cost hand moulding is done but in larger foundry machines moulding is used to automate the process.
- In hand, moulding sand moulds are made by hand which requires a higher labour cost and metal is poured in the mould by ladle. In hand moulding time required for mould is higher but the cost of producing sand mould is less as no machine is required.
- In the machine moulding process initial cost of the machine is high and is justified only when there is a high volume of orders for producing casting. The time taken to produce sand casting using machine moulding is lower than the hand moulding process.
- Machine moulding is more accurate than hand moulding and the process can be done using a semi-skilled operator while in hand moulding skilled operator is necessary.
- Machine and equipment cost includes inspection and quality checkup cost that increases with modern equipment such as X-ray machines in the foundry.
Tooling Cost
- Tools used in sand casting processes are patterns, core boxes, core prints, padding, chills and cores.
- The cost of the pattern depends upon the size of the final casting, the material used and the allowance given on the pattern.
- Wood as pattern material is cost-effective but comes with the drawback of change in dimensions when it absorbs moisture. Metal provides higher accuracy but is unsuitable for low-volume production as the cost is unjustified.
- Padding and chills are used in the sand-casting process to provide directional solidification while casting is cooled. The more the difference in thickness in the final casting more there will be a need for padding and chills to have proper directional solidification.
- More cavities and holes present in the casting will be the need for cores and core prints in the sand moulding process. This will overall increase the tooling cost for producing recess and cavities for casting with a higher number of cavities.
- To improve the strength, toughness, and durability of cores, cores need to be cured.
Production Cost
- Production cost will increase with the number of sand casting to be manufactured, the material used, the type of furnace used in the foundry, the efficiency of the furnace, heat loss during melting operation, the number of cores and core boxes used, type of material handling equipment used in the sand casting process, the energy required to melt, transport and cool material.
- Production cost is directly proportional to the variables mentioned above.
- Production costs will decrease when process parameters are taken care of and sand casting defects are reduced.
- Material cost during the process is reduced when the final product is defect-free and passes through proper quality parameters to qualify as sound casting.
- Production cost includes all costs associated with making core boxes, cores, core prints, mould making, cost of sand, melting metal, secondary manufacturing processes such as grinding, machining, passivation process, surface finishing operation, core curing, tumbling, sandblasting, sand curing, heat treatment, polishing, masking, anodizing, anti-rust coating, powder coating, sub assembling, zinc plating, spray-painting casting products, impregnating and coating.
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 Vs Investment Casting
| Sand Casting | Investment Casting |
|---|---|
| 1) Limitations over complex shapes and designs can be produced. | 1) More complex shapes such as gas turbine casting can produce as compared to sand casting. |
| 2) Requires more machining to the finished parts as compared to investment casting. | 2) Requires less machining of the finished parts. |
| 3) It is a non-precession casting process. | 3) It is a precession casting process. |
| 4) The steps required for producing the final pattern are less. As the only pattern used | 4) The steps required for producing final investment casting are more. |
| 5) The steps involved in producing sand casting are the production of sand mould, pattern and final casting. | 5) The steps involved in producing investment casting die mould production, wax pattern making, ceramic mould making and then final casting. |
| 6) Sand casting has lower accuracy, surface finish and dimensional tolerance. | 6) Investment casting has higher accuracy, surface finish and dimensional tolerance. |
| 7) The surface finish obtained is of low quality and has some open pores because of gases trapped. | 7) A higher surface finish is obtained using this process giving better surface quality. |
| 8) No limitation over the size of the casting product, large-size casting can be easily produced using skin-dried and dry moulding at low cost. | 8) There are limitations over the size of the casting product, larger-size castings are difficult to produce. |
| 9) Difficult to mass-produce casting in higher volume. | 9) The rate of production of casting is higher than sand casting as mould can be made in a short cycle time. |
| 10) The casting yield is less for this process as the runner, riser and sprue sections have higher metal content. | 10) Casting yield is more for this process. |
| 11) Heavier secondary manufacturing processes are required after casting is taken out of the sand mould. | 11) A secondary manufacturing process required is less for final casting because the refractory ceramic mould gives better quality casting than sand mould. |
| 12) Final casting has a non-accurate net shape. | 12) Net shape casting is accurate. |
Sand Casting Vs Die Casting
What is the difference between sand casting and die casting?
| Sand Casting | Die Casting |
|---|---|
| 1) Best for producing medium to large-size casting. | 1) Best for manufacturing small-size casting. |
| 2) Lower dimensionally accurate casting with inaccurate tolerance can be produced. | 2) Greater dimensionally accurate casting with close tolerance can be produced. |
| 3) Cost-effective mould-making process as low-cost sand is used. | 3) Making a mould is costly and only justified if products are to be mass-produced. |
| 4) The conventional process is more labour-intensive and the end casting result still depends upon the skill of the operator. | 4) The non-conventional process is less labour-intensive and end-casting results do not depend upon the skill of the operator. |
| 5) Difficult to produce products with less thickness and higher draft angle. | 5) Easy to manufacture products with less thickness and higher draft angle. |
| 6) The surface finish achieved is lower than die casting because of course sand. | 6) The surface finish achieved is better than sand casting as metal is used as a die. |
| 7) Suitable for small volumes of casting products as the production cycle time is longer. | 7) Suitable for large volumes of precise casting products as the production cycle time is less. |
| 8) Mould is made from sand, silica and moisture is prone to casting defects such as blowholes, mould wash and mould drop defects etc. | 8) Casting defects related to moisture are negligible in die-casting. |
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9) Higher secondary manufacturing processes are required because of sand moulds. |
9) Less secondary manufacturing processes are required as die mould gives better castings. |
| 10) Sand casting has downtime between casting production as mould needs to be broken and made again. | 10) Downtime here is less as die mould can be used for multiple cycles as compared to the sand casting method. |
| 11) External pressure is not used in this process, molten metal flows in all sections of mould through gravity. | 11) External pressure is used to force the metal into the die mould cavity and all sections of the die. |
| 12) Suitable for production casting components from ferrous and non-ferrous metals. | 12) Best for non-ferrous metals only. Die cannot handle high-temperature ferrous metal. |
Sand Casting Vs Continuous Casting
| Sand Casting | Centrifugal Casting |
|---|---|
| 1) The mould is static and does not rotate during the sand moulding process. | 1) Mould is dynamic and rotates around its own axis forcing metal to the circumference of the mould. |
| 2) As mould does not rotate slag, inclusion, oxides and dirt remain in the casting. | 2) As mould rotates around its own axis slag, inclusions, dirt, oxides and foreign particles accumulate at the centre of the casting components. |
| 3) Sand moulds are made from green sand, dry sand and a combination of both sand to produce mould. | 3) Mould is made from a metal material such as tool steel. |
| 4) Molten metal is poured and flows through passages and a gating system through gravitational force. | 4) Mould is rotated and molten metal is cooled while centrifugal force pushes metal towards the circumference of the dies. |
| 5) Sand casting products produced with sand casting are not dense compared to centrifugal casting. | 5) Casting products produced from centrifugal casting are dense compared to sand casting. |
| 6) Difficult to produce hollow, denser and longer casting products such as pipes and tubes. | 6) Easy to produce continuous castings such as pipes, tubes, bars and small accurate casting with true centrifugal casting. |
| 7) The cost of manufacturing casting products is low as the tooling cost is lower. | 7) The cost of manufacturing casting is high as the mould is made from metal and needs to be rotated about its own axis. |
| 8) Sand cores are required to produce concentric holes and cavities in the casting parts. | 8) No cores are required for casting components as metal rotates in the mould and due to centrifugal force cavity is produced in the mould. |
Sand Casting Vs Permanent Mould Casting
What is the difference between the sand casting and the permanent mould casting process?
| Sand Casting | Permanent Mould Casting |
|---|---|
| 1) Expandable sand mould is used in this process which is destroyed for every casting cycle. | 1) Permanent mould is used in the permanent mould casting process which is used for 100000 casting cycles at least. |
| 2) Larger-sized castings can be produced using the dry sand moulding process using dry sand. | 2) Large-size casting cannot be produced due to limitations over the size of the metal mould. |
| 3) Manufacturing casting at any volume is possible as this is a cost-effective method for manufacturing casting. | 3) The cost of manufacturing casting with lesser volume is not possible as metal moulds are costly. |
| 4) Solidification of molten metal takes place without any external cooling elements and system. | 4) Molten metal in permeant moulds is cooled with automated cooled water channels passing through the metal die. |
| 5) Mass production of casting of the same shape and size is not possible because of solidification time is low, the pouring rate is slow and expandable moulds need to be broken after every casting cycle. | 5) Mass production of casting products is possible as mould need not be broken after each production cycle. The pouring rate and solidification rate of molten metal are fast. |
| 6) This process is prone to mould material defects such as drops, sand holes, cuts washes of sand mould, dirt, sand mould shift, scabs, mould swelling and runout defects. | 6) Mould in this process is not made from sand giving advantages of reduction in casting defects such as runout, mould shift, scabs and cuts washes of mould. |
| 7) Labour cost is high in small foundries for sand mixing, drying, sand testing, pattern production, sand core production, mould production and breakout of mould. | 7) Once the permanent mould is made from metal multiple casting can be mass-produced with fewer operators and low-skill labour, reducing the labour cost. |
| 8) The floor space required for this process is more of a setup for making green, skin-dry and dry sand moulds takes space. | 8) The shop floor space required for this process is smaller as the sand mould-making unit is not required. |
| 9) Not able to produce highly accurate casting even after using dry sand moulds as compared to permanent mould casting processes. | 9) Able to produce highly accurate dimensionally sound casting as compared to sand casting. |
Sand Casting Vs Shell Moulding
| Sand Casting | Shell Moulding Casting |
|---|---|
| 1) Mould is made from sand, clay, moisture, binders and additives. | 1) In the shell moulding process clay and additives are not used mixture of sand and thermosetting resin is used to make sand-resin mould. |
| 2) Metal pattern is not heated to cure sand, clay and moisture to form sand mould. | 2) The metal pattern is heated after pouring fine sand and resin together. |
| 3) Dimensional accuracy and section thickness of casting are less than shell moulding. | 3) Dimensional accuracy, edges and section thickness of casting are better than sand casting. |
| 4) This process requires more foundry floor space for making mould, casting and curing sand moulds. | 4) This process requires less foundry floor space. |
| 5) Silica sand used in this process is reusable, environmentally friendly and cost-effective. | 5) The same sand used is mixed with resin and cured. Practically it is difficult to recuse and recycle as sand is not environmentally friendly. |
| 6) The permeability property of sand during the casting process is slowly increasing gas defects. | 6) The permeability property of sand during the casting process is high exhausting all gases and vapours from the mould and reducing gas defects. |
| 7) Mouldmaking is time-consuming and reduces the productivity of the foundry to produce casting in large volumes. | 7) Shell moulds can be produced at a faster rate on the machine with the heated metal pattern. |
Sand Casting Design
- The shape of casting should be simple reducing mould, core and pattern-making production costs and time. Mould, casting and core should be designed along a straight parting line of the mould.
- The parting line should be along a parting plane. The parting line should be sharp, clear, regular and straight. The parting line should be low for aluminium-based alloys and for steel alloys parting line should be high.
- The core location in the mould should not interfere with the flow of molten metal but should help molten metal flow uniformly through the mould cavity.
- Small-size casting should be manufactured on the drag side of the mould only without making a cope section of the mould.
- Patterns should be designed for easy removal without damaging the mould cavity during pattern withdrawal reducing shake allowance.
- Machining allowance should be provided for secondary operations such as drilling, boring and reaming to completely avoid any complex design of the pattern, cores and final casting.
- Green sand moulds should be used where surface finish of casting is not of primary importance at the same time surface finishing secondary operations can provide better surface finish without using expensive dry sand moulds.
- Complex casting parts should be manufactured separately and welded together reducing the overall production cost of making sand mould especially for producing multiple-piece patterns.
- Bosses, pads, lugs and undercuts should be avoided in the final casting.
- The use of cores should be avoided wherever necessary. This reduces the production cost of mould sections, cores and chaplets.
- All sections of casting should cool uniformly without any stress concentration on the solidifying casting. Casting should be designed keeping in mind the solidification properties of molten metal and shrinkage allowance.
- Casting should be designed to easily break off the core and separate the sand core from the casting.
- Casting designs are connected to the gating system, gating system in casting should be designed to facilitate proper feeding of molten metal to make a sound casting.
- While designing casting care must be taken that all sections should be of uniform thickness to avoid concentration of molten metal and shrinkage defects.
- Casting should not have sudden changes in thickness and isolated thickness to avoid hotspot defects.
- Care must be taken while designing casting to provide pattern allowances such as machining allowance, shrinkage allowance, draft allowance and rapping allowance should be properly given for sound casting.
- Casting should be designed to eliminate adjacent sections of casting with different thicknesses to reduce warpage defects.
- Proper venting should be provided while using green sand moulding for the escape of gases, water and vapour from the mould as molten metal comes in contact with green sand. This eliminates gas defects such as blowholes, and pinhole defects.
- The right combination of round, semi-angular and angular sand grain should be used to give a proper surface finish and at the same time allow the escape of gases out of the mould.
- Casting should be designed keeping in mind cleaning, inspection, testing, service and repair. All casting sections should be accessible to the cleaning, inspection, testing service and repair.
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.