What is Sand casting?
Sand casting, also known as sand molded casting, is a metal casting process characterized by using sand known as casting sand as the mold material.
The term “sand casting” can also refer to an object produced via the sand-casting process. Sand castings are produced in specialized factories called foundries.
Sand casting is a casting method that uses sand molds to shape and produce metal objects and components. Sand casting is one of the oldest known casting processes and is valued for its flexibility, cost-effectiveness, and ability to create complex shapes.
Sand casting offers great versatility in terms of the size and weight of the castings it can produce. It is suitable for small-scale custom productions and large-scale industrial applications such as automobile parts.
This process is particularly useful when the desired outcome needs to have specific features or internal cavities.
How Sand Casting Works?
Sand casting is a metal casting process that uses sand as the molding material. The sand will contain the mold and cavity into which the metal is poured using various pouring methods.
A 3D “die” is created and compacted with sand leaving a negative mold that will then be filled with molten metal.
Greensand or silica sand is tightly pressed into two match plates. A master pattern design is created using a 3D machined tool and this tool is used to create the desired cast shape, leaving a negative imprint. These match plates create the two halves known as the cope and drag.
Then the sand is left to harden. Once the molds are ready, the cope and drag are removed from the match plate and sealed.
The match plates are sealed then filled with liquid motel metal. Once the metal has cooled to the appropriate temperature the casts are broken out of the mold and additional machining, coatings, or testing can now be done.
The general process is described below:
#1. Position the Mold Pattern Within the Sand.
The drag and cope are two open frames within which the cavity is formed. Together they make a casting box referred to as a flask.
Place the drag (lower box) on a stable and robust work surface and place the pattern onto the work surface in the center of the drag, flat face down.
Dust the pattern and the work surface with release powder such as talc to enable decoupling of the work surface and the pattern later.
Cover the pattern and the inside of the drag with sand that has been riddled (shaken through a coarse sieve or riddler to remove clumps). Carefully ram this sand into the drag to form an integrated mass that closely fits the pattern.
Complete the fill with sand and further ram this to integrate the whole. Be careful not to disturb the position of the half pattern, but ensure that the mass is well integrated.
Use a strike bar to cut any bulge of sand from the face of the drag, to make a flat and smooth surface. Any stiff straight edge will suffice to cut this sand, but make sure the drag is filled with packed sand.
#2. Establish the Gating System.
With the drag filled, invert it to expose the upper face and the pattern. Decide where the conical feed and vent sprue positions should be placed.
Trim a small receiver well and vent reservoir at each position, using a finisher tool (blunt knife or spoon). These features are small depressions that should be large enough for the sprue patterns to seat into.
Cut channels from the receiver and vent wells to the faces of the pattern. This will allow the fill to flow from the wells to the cavity and the gas to escape as the second well fills.
Liberally dust the entire drag surface with parting powder to allow the next stage to disengage when formed.
#3. Position the Cope and Sprues and Complete the Fill.
Place the cope onto the drag and position the sprue cones into the two wells you cut in the drag surface. Re-dust the surface with parting powder.
Begin to riddle sand into the open top of the cope, making sure not to disturb the sprue patterns. Ram this new sand with care to not damage the drag setup but ensure that the sand is well integrated.
Complete the fill of the cope and ram the sand, so that the fill forms an integrated and robust whole. Strike the upper face of the cope to produce a flat surface.
When this is complete, remove the sprue patterns, being careful not to disturb the packed fill in the cope. Don’t worry about sand that falls into the hole, you can clear this later.
#4. Extract the Mold Pattern.
Carefully lift the cope off the drag and place it inverted next to the drag. Use a slick tool (precise repair and sand manipulation tool) to remove any upstanding features from the face of the cope fill.
This is usually achieved by screwing one or more hooks into prepared holes in the flat surface of the pattern to lift it.
More traditionally, draw spikes are hammered into the pattern to provide the lift “handle”. Where you cut the receiver/vent wells larger than the sprue patterns that were socketed into them, the space will have filled as you rammed the cope with fill.
Cut away this excess and clean up any detritus by blowing it away. Without damaging the formed cavity, lift the pattern out of the cavity in the drag. Clean up any residues and make sure the feed channels are open and clear.
#5. Vent the Cope, Reassemble the Flask, and Prepare for Fill.
Using a vent rod, form small holes from the cavity to the topside of the cope. This will allow extra gas passages for venting when the cavity is filled.
Make sure that the damage to the cavity is minimal. Vent the “high” points of the cavity. Try to avoid/minimize any cosmetic damage to the cavity face.
Clean up any detritus by blowing it away. Clean up damaged edges of the sprue holes using a slick. Place the cope back onto the drag and lock the two halves together.
Move the completed flask to the casting floor, ready to fill. A llow sufficient time (for green sand casts) for the moisture content to fall to an acceptable level. Other systems such as dry sand or furan-resin-bonded sand require less wait time.
#6. Introduce Molten Metal Into the Mold Cavity.
This description assumes melt has been completed and degassed. Poor degassing will result in a failed and porous casting. The crucible must be de-slagged and de-gassed completely, ready for the charge to be used to fill the cavity.
To complete this process, lift the crucible out of the furnace using crucible lift tongs. Place the crucible into the carrier tongs. Perform a final slag removal to make sure the surface of the melt is clean and smooth.
Lift the crucible with the appropriate tongs that allow you to carry and tilt it with reliable precision. Carry the crucible to the flask and carefully commence the pour.
Do not rush this stage; there will be steam and smoke, and a rushed pour will be less accurate, more hazardous, and more likely to erode the cavity faces and “include” sand detritus in the finished cast.
Fill until the sprue holes are visibly full this not only confirms fill but maintains some pressure to flow material into the cavity from these two cones, to compensate for initial shrinkage as the fill cools.
#7. Allow Sufficient Time for the Metal To Cool.
A rushed unpacking of the cast is hazardous and can result in distortion if the fill remains soft.
#8. Crack Open the Mold To Extract the Metal Casting.
Once cooled, unlock the cope from the drag and break out the casting and sand manually or using a shaker table. Sand is returned for re-use.
Remove the casting features that are not part of the required component (sprue or feeder) by cutting with a saw or grinder.
Fettle (clean up and improve) the finished part with a die-grinder or file to remove bumps from vent features, fill channels, and any sand that broke away from the cavity parts before casting.
Parts of a Sand Casting Mold
Cope and drag in Sand Mold.
Cope and Drag are the two parts of the casting flask. Cope is the upper part and drag is the lower part. Even if the casting process is flaskless, the same terms are used for the upper ans lower parts.
Generally, the flask is made of wood or metal. It contains molding sand. When metal is poured into the mold cavity the flask supports the mold.
Gating system.
If the molten metal is poured directly from the ladle, it will erode the bottom of the mould cavity. So molten metal is poured from the ladle to the cavity through a gating system.
The gating system in casting creates a series of channels through which molten metal reaches the cavity. Gating system has:
- Pouring Basin: It receives the molten metal from the liquid metal container.
- Sprue: Pouring cup is attached to the sprue. It is vertical in shape. On the other part of the sprue there is part called runner.
- Sprue Base: It’s the base of the sprue
- Runner: It is the horizontal part of the gating system. It connects the spues with the gates.
- The next part is the choke.
- Then comes the skim bob
- Gates and ingates: It controls the movement of the metal from the runners into the cavity.
- Riser
Riser.
The risers are also known as feed heads. When the metal solidifies it starts to shrink. And then risers come into play. These feeder supply metals to the cavity when shrinking of the metal starts.
Core.
Cores are required to create the castings with holes. It can be made of refractory materials. Most often core sand is used to make it. Metal cores are also available but less frequently used.
Chaplets.
Chaplets are the supports for the cores. These are needed particularly when the cores are very big. Usually, metal pieces are used to support the core.
Without chaplets the core can be displaced and the casting can be spoiled. These chaplets are set-up between the core and mold surface.
Caution should be taken while placing chaplets. Clean, oil and moisture free pieces should be used as chaplets.
Chills.
These are huge metal pieces used to reduce the effect of shrinkage. These increases the thermal conductivity and heat capacity. It helps in speeding up the cooling process. So thick metal parts are cooled quickly. They can be used along with the risers.
Types of Sand Used for Sand Casting
For over a half-century, Ermak has worked to master and perfect the art of sand mold casting. Sand casting is a popular method of producing non-ferrous alloy casts.
There are three types of sand used in casting, green sand, water glass or sodium silicate, and resin sand.
#1. Green Sand.
Green sand castings are formed using sand molds formed from wet sand that contains water and organic bonding compounds, also known as clay.
The term Green Sand refers to the sand mold not being “set” or “green” or “uncured” when the metal is poured into the mold.
Because the sand can be reused, sand casting with green sand is easy and inexpensive. The disadvantage is that sand is a soft mold that can collapse or shift during casting.
#2. Water Glass or Sodium Silicate.
Sodium silicate can be used to create sand mold castings. This process is beneficial in sand casting, where a cavity is required.
Sodium Silicate can quickly go from a liquid to a solid by passing carbon dioxide through it. This causes the sodium silicate to dehydrate.
Sodium silicate must be mixed with other materials so the core can be removed during product break down. If the mixture isn’t correct, the core will become stuck within the casting.
#3. Resin Sand.
The molding material for resin sand casting is a blend of quartz sand and resin sand. When resin sand is mixed and heated, it hardens into a solid, smooth mold.
A solid mold produces fewer faulty castings, but it comes at a greater cost and a slower production rate. Resin sand molds take longer to make because each one must be mixed and burned individually.
What metals are commonly used in sand casting?
Sand casting is a highly adaptive process that can form any metal alloy including ones with high melting temperatures, such as steel, nickel, and titanium.
The most common types of metals are aluminum, brass, cast iron, and cast steel. The choice of metal for casting determines the design functionality of the completed part and affects the casting’s quality, performance, and properties.
Factors such as production deadlines, quantities and volumes of parts, and varying cooling and solidification rates influence the choice of metal.
Important properties considered in the selection process include corrosion resistance, thermal conductivity, hardness, and the metal’s response to temperature changes.
Non-Ferrous Metals
1. Aluminum.
Aluminum encompasses a variety of alloys that are lightweight, machinable, and offer strength comparable to mild steel while being less dense.
Its favorable properties for casting include excellent castability, low density, and high corrosion resistance.
The base sands used for aluminum casting include silica, olivine, chromite, zircon, and chamotte, which are mixed with binders such as clay, oil, resin, and sodium silicate.
2. Bronze.
As with aluminum, bronze is a term used to describe several alloys of copper and tin that are altered by the percentage of copper, the percentage of tin, and the addition of other alloys such as aluminum, zinc, nickel, and iron.
The three types of bronze used for sand casting are aluminum bronze, manganese bronze, and silicon bronze.
- Aluminum Bronze consists of 9% to 12% aluminum and 4% to 6% iron and nickel with the remaining percentages being bronze. As is characteristic of bronze, aluminum bronze is corrosion and wear resistant with exceptional strength and toughness.
- Manganese Bronze has 55% to 65% copper, 20% to 25% zinc, 1% to 5% manganese, and 1% to 5% iron and has a resistance to corrosion, excellent strength with wear resistance, and extraordinary mechanical properties.
- Silicon Bronze is a bronze alloy with 96% bronze and 4% silicon, which makes it easy to machine and cast. It is normally used to produce ball bearings, bearing cages, spacers, gears, and parts of valves.
3. Brass.
Brass, an alloy of copper and zinc, varies in properties based on the proportions of these metals, affecting its characteristics and appearance.
Its resistance to rust and corrosion is attributed to the absence of iron or iron oxide, combined with the zinc and copper content.
Brass’s high copper content provides excellent conductivity and tensile strength, making it both easy to bend and form.
Its ability to maintain exceptional strength after molding contributes to its popularity in casting applications.
4. Zinc.
Zinc sand casting enables the production of components with thinner walls, fewer draft angles, and long, narrow holes.
Zinc is alloyed with copper, aluminum, and magnesium to enhance its strength, rigidity, castability, and toughness.
While zinc is easier to work with than aluminum, it is approximately two and a half times heavier. Like aluminum, zinc offers excellent corrosion resistance.
Zinc’s high hardness allows for the creation of parts with thin walls and complex shapes that maintain tight tolerances and long-lasting utility.
5. Lead.
Lead has been used in sand casting since Roman times. It is a heavy metal known for its corrosion resistance.
However, the use of lead in sand casting requires careful handling due to health risks associated with inhalation. Many countries have established standards to limit the amount of lead used in manufacturing.
Despite its health concerns, lead is still employed in sand casting today, primarily for producing small to medium-sized parts such as plumbing fittings and decorative items.
6. Copper.
Copper is typically alloyed with other metals to improve its mechanical and physical properties.
While stronger and more expensive than aluminum, copper offers high tensile strength and excellent electrical and thermal conductivity.
Its resistance to corrosion makes it suitable for a wide range of products.
Like lead, copper has been used for thousands of years to manufacture various home and industrial products.
Sand casting remains an economical method for producing copper items in large quantities, such as plumbing fixtures and hardware.
Ferrous Metals
1. Iron.
Iron is well-suited for casting due to its fluidity, low volume shrinkage, and minimal linear shrinkage. Despite its poor mechanical properties, with compressive strength significantly higher than tensile strength, iron is favored for creating complex shapes, asymmetric structures, and intricate parts.
- Gray Iron Castings. Gray Iron has excellent castability and machinability and is known for its low production cost and compressive strength. As with various non-ferrous metals, cast iron has exceptional thermal conductivity and retains its dimensional stability at high temperatures. It is a fragile metal with low impact and tensile strength.
- Ductile Iron Castings. Ductile iron has good fluidity with large shrinkage, which makes it susceptible to shrinkage cavities and porosity. Unlike gray casting iron, ductile casting iron has good mechanical properties with abrasion and impact resistance and fatigue strength.
- Malleable Iron. Malleable iron is white cast iron that has been annealed, which transforms the brittle structure of white cast iron into a malleable one. As its name indicates, malleable iron has excellent ductility, machinability, toughness and corrosion resistance. Like most iron, malleable iron rusts but is used to produce hand tools, pipe fittings, brackets, and electrical fittings. Aside from its susceptibility to rusting, malleable iron has a poor strength to weight ratio.
2. Mild Steel.
Mild steel is a low carbon steel composed of iron, carbon, and other elements. With a carbon content ranging from 0.15% to 0.30%, it is highly malleable and ductile.
Increasing the carbon content enhances its hardness, strength, and hardenability.
Widely used in sand casting due to its affordability and ease of work, mild steel can be machined, forged, and welded, making it suitable for various engineering projects.
However, common issues with mild steel include sand inclusions, air holes, cracks, and shrinkage.
3. Stainless Steel.
Stainless steel is a popular choice for sand casting due to its exceptional properties, including resistance to corrosion, durability, and strength.
Its low coefficient of thermal expansion makes it ideal for applications requiring exceptional accuracy, high tolerance, and dimensional stability.
Despite its toughness and strength, stainless steel has a density of 7.8 grams per cubic centimeter, making it a relatively lightweight metal for casting essential parts.
However, it is expensive due to its chromium and nickel content. Additionally, the high strength and stiffness of stainless steel can make it challenging to eject parts from the sand casting mold.
4. Alloy Steel.
Alloy steel is produced by combining carbon steel with elements such as cobalt, chromium, manganese, nickel, tungsten, molybdenum, or vanadium.
These alloying elements modify the steel’s strength, hardness, and corrosion resistance.
Alloy steel generally exhibits excellent ductility, wear and shock resistance, strength, and toughness, though it can be more challenging to machine, form, and weld compared to carbon steel.
Alloy steel is used extensively across various industries to manufacture parts that must withstand significant stress, including automotive components, structural supports, pipelines, and ship components and structures.
Its cost-effectiveness, being lower than that of stainless steel, contributes to its wide usage.
Applications of Sand casting
The applications of sand casting are:
- Pump bodies
- Bearings
- Bushings
- Air compressor pistons
- Impellers
- Electronic equipment
- Engine crankcases
- Fittings
- Engine oil pans
- Gears
- Flywheel castings
- Gas and oil tanks
- Machine parts
Advantages of Sand Casting
Sand casting, despite being an ancient process, offers advantages that render it highly relevant in high-technology production. Such benefits are:
- Versatility.
- Low cost.
- Simple tooling.
- Adaptability.
- Easy scale-up, but great for prototyping.
- Fast turnaround.
- Few design limitations, shape complexity.
- Extremely wide range of materials with few/no process changes.
- A wide spectrum of part sizes from grams to tons.
- Lower ecological footprint than most alternatives.
Disadvantages of Sand Casting
There are various disadvantages and limitations to this casting method. These are:
- The surface finish cannot be particularly fine.
- Dimensional accuracy and repeatability can be variable.
- Contaminated sand waste represents a disposal issue.
- Patterns can degrade as production scales.
- Cooling rates are uncontrolled, so there is often variability in microstructure anisotropy.
- Complex sand cores for internal spaces can be hard to handle.
- Non-automated production requires skilled labor.
- Automated production can require moderate to high costs to establish.
- Some processes offer low sand reusability and difficulty in restoring the sand condition.
Examples of Sand Casting Products
Examples of sand cast products abound in almost all market sectors, particularly in low-cost products and in heavy industries are listed below:
- Medium-speed marine diesel flywheels can be several tons in weight.
- Through-hull nickel-aluminum bronze skin fittings in boat/ship hulls.
- Barbecue grills and burner components.
- ICE engine blocks.
- Vehicle brake discs.
- Door furniture.
- Decorative end pieces for aluminum extrusions.
- Tools used in heavy machinery and oil fields.
- Jewelry in various precious and common metals.
Different Sand Casting Techniques
1. Green Sand Casting.
Green sand casting is the most common and traditional sand casting technique. This process uses a mixture of sand, clay, and water to create the molding material.
The name “green” refers to the sand’s moisture content, which provides cohesiveness and allows for easy mold formation.
Green sand casting offers flexibility in terms of mold adjustments, easy pattern removal, and good dimensional stability.
It is widely used across industries such as automotive, aerospace, and in many general manufacturing facilities.
2. Cold Setting / No Bake.
In cold setting or No Bake sand casting, a chemical binder is used to harden the sand mold at room temperature.
This technique eliminates the need for high-temperature curing and allows for faster mold production.
Cold setting is advantageous for applications that require quick turnaround times and where dimensional accuracy is critical.
3. Shell Molding.
Shell molding is a sand casting technique that involves the use of pre-coated sand molds.
The molds are created by heating the pattern and applying a resin-coated sand mixture to form a thin shell. This process offers improved dimensional accuracy and surface finish.
Shell molding provides excellent surface quality due to the smooth and precise mold cavity.
This method c is especially effective when creating high precision castings like medical tools, aerospace components, and more. This technique produces a similar result to that of die casting.
Sand Casting vs. Other Types of Casting
All casting forms a cavity into which the melt is poured. The variation is in the methods by which the cavity is formed.
Die casting uses a machined metal cavity for the part forming, with various approaches to introducing fill material and mechanization of the process.
Investment casting uses a plaster cavity cast over a then volatilized pattern or master. Various options exist in pattern material and fill technique, from simple hand pour to full automation.
Meanwhile, metal injection molding uses polymer-bonded, powdered metal feedstock, which is molded in machined steel cavities, processed, and finally sintered.
Sand casting uses a variety of techniques from entirely manual to fully automated, and even automated core production/placement to form cavities using bonded sand which is then machine-charged and de-molded.