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This article will take an in-depth look at sand casting.
The article will bring more detail on topics such as:
Sand Casting, its Construction, and Methods
Types of Casting Sand
Applications and Advantages of Sand Casting
And Much More…
Chapter 1: Sand Casting Construction and Methods
This chapter will discuss what sand casting is and the construction of the sand casting mold.
What is Sand Casting?
Sand casting is a manufacturing process in which liquid metal is poured into a sand mold, which contains a hollow cavity of the desired shape and then allowed to solidify.
Casting is a manufacturing process in which a liquid material is usually poured into a mold, which contains a hollow cavity of the desired shape, and then allowed to solidify. Casting materials include metal, concrete, epoxy, plaster, and clay. This article will focus on sand casting.
The versatility of the sand casting process is well recognized. Sand castings can make castings in a wide range of sizes and weights with extremely complex geometries utilizing a wide range of metals. The use of sand as the molding media is the most distinguishing feature of the sand casting process.
The huge cost savings gained by using sand instead of other materials to make molds is a big benefit. The mold production costs account for a large portion of the costs associated with alternative casting procedures. However, because of the nature of sand, the molds utilized in the technique are disposable and non-reusable.
When removing a casting, it is impossible to maintain the sand mold intact. Sand casting, on the other hand, is appropriate for metals with high melting temperatures, such as titanium, steels, and nickel. It is the only process of casting that can work with these materials. Thus, the technology is the top choice in the aerospace and automotive industries for producing low-cost small-series parts.
Components of a Sand Casting Mold
The making of the sand casting mold usually requires four components which are:
The base sand is the sand that is utilized to create the mold in its purest form. A binding agent is necessary to keep it together. The core is also made of base sand. The following are the most prevalent varieties of base sand:
Binders or Binding Agents
The binding agents are the glue that holds the sand particles together. The following are the most frequent types of binders:
Clay and water
Improvement Through Additives
Additives are used to improve the mold surface finish, its strength, refractoriness, and cushioning.
This can be a fine powder or liquid used to facilitate pattern removal from the mold.
Methods of Making Sand Castings
Various methods are thus used in the construction of sand casting molds. These include:
The 'bedding-in approach' can also be used to form the solid cylindrical design. The drag is partially filled with molding sand and rammed in this technique. The pattern is driven into the sand after enough pounding. To ensure accurate sand ramming, the sand near the pattern is tucked and slammed tightly. The pattern is sometimes removed and the sand is examined for soft patches on the surface. If there are any soft patches, ramming with more sand is done until the sand is tightly packed. To ensure a well rammed mold chamber, the pattern is forced downwards once again.
The dividing line should be level with the surrounding smooth sand surface when bedding-in. The drag does not need to be rolled over when a pattern is bedded in. When employing pit molding to make larger molds, bedding-in can be used.
False Cope Method
Another method of molding the solid cylindrical design is the false cope technique. In this process it is unnecessary to ram the sand tightly beneath the pattern unlike in bedding-in. The design is first bedded into the coping without regard for sand ramming beneath the pattern, resulting in a smooth parting surface. After dusting the cope and pattern with separating sand, the drag section of the flask is placed on top of the cope. After that, the standard ramming procedure is followed. On a sand bed, the complete assembly is gripped and rolled over. The clamps are removed, as well as the cope and cope bottom board, which are then destroyed. After that, the empty cope is placed on the drag and the standard ramming procedure is followed. It should be noted that the cope is a dummy block that is used to create the drag correctly. This is known as 'false cope.'
Flat Back Method
This can be done using a flat back design. The mold cavity is either on the drag side, the cope side, or both after it is completed. The molding sand creates a hole. The dividing line is the starting point for the draft along the flat backs outside edge. The core was obtained using a dry sand core, and the pattern is split. In the first scenario, the axis of the hole (and core print) is vertical. The second situation is identical to the first, but the whole axis is horizontal.
The solid cylindrical pattern is rammed and rolled over on the molding board. Some of the sand is removed and smoothened to remove the pattern from the sand, as illustrated. A new splitting surface is created as a result of this. As a result, a parting line is created that connects the separating lines around the pattern. Coping down is the process of removing sand and creating a new parting surface. The mold is finished by pushing up the coping in the traditional way.
How Sand Casting is Done
When sand casting, several steps are followed which include:
Producing the Pattern – Desired Product
A reusable pattern with the same details as the desired completed product is used in the process. A pattern is always made larger than the final part to give an allowance for thermal contraction or shrink. Shrinkage allowance will account for the contractions that occur as a casting cools to room temperature.
Liquid shrinkage is a reduction in volume that occurs when a metal transitions from a liquid to a solid form. To compensate for this, the mold has a riser that feeds liquid metal to the casting. Solid Shrinkage: When a metal loses its solid state temperature, it shrinks in volume. To account for this, shrinkage allowance is included in the patterns.
The machining allowance will cover the extra material that will be eliminated in order to produce a completed product. The rough surface of the cast product will be eliminated in this process. The size, material properties, distortion, finishing accuracy, and machining method all influence the machining allowance. To ensure that the pattern is removed safely, all surfaces parallel to the pattern removal direction are tapered slightly inward. This is known as draft allowance.
Pattern Creation – Gates and Risers: Metal Delivery System
The metal channels that will feed the required cast product design with proper gating and risers are also included in the pattern. This regulates the metal flow and requires gas venting while driving the unavoidable thermal contraction to acceptable places (other than the actual desired finished product).
Depending on the volume and tolerance required, patterns are manufactured of a variety of materials, including wood, metal, synthetics, expandable polystyrene (EPS), and others. In other circumstances, such as pipe fittings, the component’s interior must be hollow. In such circumstances, extra patterns known as cores must be created.
Cores are a separate portion of the mold that prevents the liquified material from filling in any gaps. They're utilized to make interior cavities and other things that the mold can't produce. A core box is the tooling used to build the core, which is just another name for the mold that makes the core.
The materials used to make the core must meet certain criteria:
It must be strong enough to withstand the turbulence caused by the molten material
It must have a good hardness
It must have a high permeability so that the gasses formed during the casting process can easily escape
It must be able to withstand the high temperatures caused by the molten material
Smooth surface finish
Minimal gas formation when exposed to the molten material
It must be weak enough to break while the molten material cools and shrinks, allowing it to be removed after solidification.
Creating the Mold
Around the design, a refractory substance that is stable at high temperatures (in our case, sand) is created. The material must be strong enough to support the weight of the liquid metal during casting. It should also be resistant to metal reaction but fragile enough to be separated after cooling of the casting.
The mold can be made out of a variety of different sand materials. Other elements, such as clay or a chemical bonding agent, are usually added to the sand to make it stronger so that it can withstand the pouring operation. The mold can also be made by drilling the necessary shaped hollow straight into a block of sand. Because design changes may be handled and applied quickly, the technology is extensively employed during product development, or for portions with infrequent usage to avoid the storage or maintenance of a physical pattern.
The top half of the mold, known as the "cope," and the bottom half, known as the "drag," are usually made in two sections. The parts are split and the pattern removed once the sand has set (using the traditional/non-machined procedure). To improve the surface finish and protect the mold from the turbulence of the poured metal, a refractory coating is applied. The halves are reassembled, resulting in a cavity in the pattern's form. Cores, a means of producing appropriate internal pathways in the final product, may be included in the mold.
Pouring the Metal into the Mold
Molten metal is injected into the static mold directly. It defines the finished portion and the risers by filling the void. A continuous liquid metal supply comes from the risers to the casting. Because they are meant to cool and solidify last, the shrinkage and potential void are concentrated in the riser rather than the targeted section.
Liquid metal can thus flow into the casting smoothly with less turbulence. Reduced turbulence can aid in the prevention of oxide formation and casting flaws. This method can be used to make almost any alloy. Almost any alloy can be made using this method. For extremely reactive materials to oxygen, an argon shielding process can be used to keep air away from the molten metal.
The casting hardens and cools, containing both the desired item and the additional metal required to manufacture it. In a shakeout operation, the sand is split up. The sand used to make the mold is recovered, reconditioned, and reused in large quantities.
Casting Final Operations
The gates, runners, and risers are cut from the casting, and final post-processing such as sandblasting, grinding, and other methods are used if necessary to finish the casting dimensionally. To achieve final dimensions or tolerances, sand castings may require extra machining.
Heat treatment can be used to improve the dimensional stability or characteristics of parts. Non-destructive testing is another option. Fluorescent penetrant, magnetic particle, radiographic, and other inspections are examples. Prior to shipment, final dimensional inspections, alloy test results, and NDT are validated.
Chapter 2: Types of Casting Sand
The various types of casting sand include:
These varieties of molding sand are made from natural sand that has been moistened. It comprises roughly 15 to 30 percent clay, 8 percent water, and silica. The clay and water act as binding ingredients, giving the mold its strength. It's solely used for basic and crude casting. Both ferrous and non-ferrous metals are utilized in it.
When the moisture in green sand is removed, this casting sand is obtained. Because the molding sand provides increased strength, rigidity, and thermal stability to the casting, it is employed for large and heavy casting.
Face sand is used to face the mold, as the name implies. Silica sand and clay are typically used to make face sand. That is, no sand from previous projects is used. It's placed just adjacent to the pattern's surface. Because it comes into direct touch with hot molten metal, facing casting sand must have high refractoriness and strength. Molding sand of this sort produces an extremely fine grain in the mold.
Silica sand is blended with core oil (linseed oil, resin, and mineral oil) and other binding elements including dextrin, cornflour, and sodium silicate in core molding sand. It is utilized to manufacture cores because of its great compressive strength.
Loam sand is made up of an equal amount of sand and clay, and enough water. It's also utilized to make big, heavy things like hoppers and turbine parts.
Pure silica sand is put on the faces of a design before molding with this sort of molding sand. Before the pattern is embedded in the molding sand, parting sand is sprinkled over it. This sand is also strewn across the contact surfaces of the cope, drag, and cheek.
Backing and Floor Sand
This casting sand, which is referred to as floor sand, fills the volume box and backs up the facing sand.
This molding sand is suitable for large mechanical castings. It possesses a high degree of refractoriness, permeability, and strength. Machine molding is done with system sand to fill the flask completely. This molding does not utilize facing sand because it has been cleansed and has unique additives.
This molding sand has molasses as a binding agent and is typically used to make the core and sometimes intricate shape casting.
Chapter 3: Sand Casting Considerations
When it comes to sand casting design considerations, it's crucial to know what the casting will be used for, as well as any further processes it will have to go through after it's poured.
If a casting is going to be visible, it might need to be machined or coated to get a smooth finish. Heat treatment services, on the other hand, may be necessary if the casting will be utilized as a part of a structure or assembly that requires good stiffness.
In order to reach the intended final results, most castings must be machined or treated in some way. The following aspects are essential considerations:
Detailed Prints and Drawings
The most significant feature of sand casting mold design is providing extremely detailed prints and drawings - a sand foundry needs a drawing for both the casting and the machined completed product.
Detailed prints are an important communication tool in the sand casting design process, as they describe the designer's expectations and requirements for the finished product. The following information should be included in your sand casting product design:
If the casting requires cast-in identification markings, such as a component number, foundry code, or heat lot, make sure to specify the size and placement in your detailed prints.
Sand Casting Draft Angle
The sand casting draft angle is a perpendicular angle to the model that allows the contents to be easily removed from the delicate sand mold without destroying the outside wall. The product's molding process, how we design the casting, and the depth of the pattern inside the mold are all used to determine the sand casting draft angle. Nonetheless, many sand casting designers ignore the significance of the sand casting draft angle. The utility of the tapered surface in sand casting design can be improved by selecting a suitable sand casting draft angle.
In addition, because of the high metal flow, it can lower processing costs. As a result, the typical sand casting draft angle shall meet the ISO standard and will not influence the sand casting design's operation. As a result, your organization will be able to make more sand molds at a lower cost while maintaining consistent quality. The normal draft angle for sand casting is 2 degrees. With external and interior features, the minimum draft is roughly 1 degree.
Rounds and Filet
Allowing for ample rounds and filets is an important aspect of the sand casting design process. Generous rounds and filets enhance the appearance of a casting while also assisting in the distribution of strains and reducing cast stresses. Generous and appropriate corner filets also aid in the component’s pouring by avoiding turbulent flow occurrences and allowing the molten material to flow and fill the casting properly.
In sand casting design, determining the position of the separating line is critical. It is the dividing line. We rely on it to determine whether or not we should shift course. Because it impacts the final production cost and quality of castings, the engineering designer must grasp and record the parting line placement on the casting drawing. When it comes to placing the separation line, relying exclusively on the practical experience of metal foundry workers is insufficient.
The dividing line should be as low as feasible, and wide, short, and flat. If the parting line and the seam burrs do not match, the extension margin between them should not exceed 0.020.
Furthermore, the separation line we chose has a maximum seam flash extension material of roughly 0.015. If the parting line's position changes, pay notice. The use of the core, pouring position, weight of the casting, and dimensional correctness will all be affected.
It's crucial to pick the correct alloy for your casting. The alloy you choose can have a big impact on qualities like:
Undercuts and Cores
The undercut is used in the sand casting design to keep the model from being removed during the mold production process. The usage of the core will lengthen and raise the cost of casting. As a result, we should decrease or eliminate the use of core sand portions in our sand casting designs. In fact, the early definitions of the parting line provided by specialists were beneficial in understanding the product's properties and avoiding the undercutting problem. The balance and interplay between the many elements are not evident now that it has been refined. As a result, we must learn the norms and standards of sand casting design.
A consistent cross-section or wall thickness is often advantageous. However, it should not be used in sand casting design because many casting products do not allow for sudden section changes. In theory, only the thicker portion of the casting should not be cooled in isolation, and the cross-section can be regarded as effective. This occurs because thicker sections take longer to cool down.
They are unaffected by the solidification of the surrounding metal. The thicker section will solidify next, but it will not be able to be removed from the environment. It will result in problems in the casting, such as porosity or ripping. As a result, before executing a sand casting design, it is important to consider the thickness limit of the product.
It is critical in sand casting design to achieve consistent model solidification and avoid cavity formation. So, what's the best way to go about it? This has to do with the volume to surface area ratio of the sand mold. The solidification rate of castings is usually required to be proportionate to the square of their ratio.
This is because the part with a smaller volume-specific surface area hardens faster during product casting than the part with a larger volume-specific surface area—and vice versa. To remedy this difficulty, many metal foundries will increase the overall thickness or add certain materials in the load-bearing position. The correct approach, on the other hand, is to utilize stiffeners and gussets. Because, while the former enhances strength, the latter reduces the thickness of the local thick wall.
Corners and Angles
The cooling properties of the material used to manufacture sand molds, as we all know, have a significant impact on the quality of sand castings. As a result, when designing sand castings, this issue should be taken into account. When the cooling effect of the casting or the sand mold is poor, heat will be generated locally at the intersection of the sharp corner and the corner of the component.
As a result, the casting is subjected to a concentrated source of stress, which causes it to distort, shrink, and break throughout the subsequent process, lowering the final product's quality. This is something that should be avoided while designing a sand casting mold.
The shape of the part might be highly intricate due to the unique characteristics of sand casting mold design. In most cases, there are numerous connections. Junction is how we refer to them. L, X, V, Y, and X-T are the five different types of junctions. Local mass concentrations are also produced by these junctions, as previously stated. Cracks, shrinkage, straining, and other issues will occur. Reducing the excessive local mass concentration induced by the junction is the optimal sand casting design solution. Finally, it must blend seamlessly into the final output.
To begin, we must recognize that most metals, such as ordinary steel, copper, aluminum, magnesium, and zinc, undergo a shrinkage reaction during solidification. When it comes to sand casting design management, it's important to remember that there should be a machining allowance at the interface between two sand castings.
In other words, curved edges at the sand casting interface should be avoided. This is because their shrinkage is proportional to the freezing point of the material and the volume ratio of the casting's surface area.
Recycling Molding Sand
The lumps are cooled and crushed once the sand has been shaken off a complete casting. A magnetic field is often used to help remove all particles and metal granules. Shakers, rotary screens, and vibrating screens are used to screen all sand and components. The cleaned sand can then be returned to the molding sand production cycle from the start.
Mullers are used to combine the sand, bonding agent, and water for molding sands. Aerators are used in conjunction with aerators to loosen the sand and make it more moldable. Scoop trucks or belt conveyors carry prepared sand to the molding floor, where it is shaped into molds; the molds may be placed on the floor or transferred to a pouring station via conveyors. At a shakeout station, the castings are separated from the adhering sand after pouring. By belt conveyor or other ways, the utilized sand is returned to the storage bins.
Casting sand is frequently recovered and reused during the manufacturing process. Approximately 100 million tons of sand are utilized in production each year, according to industry estimates. Only four to seven million tons of that total are discarded, and other businesses frequently repurpose even that sand.
Chapter 4: Applications and Advantages of Sand Casting
This chapter will discuss the applications and advantages of sand casting.
Applications of Sand casting
The applications of sand casting include:
Air compressor pistons
Engine oil pans
Gas and oil tanks
Advantages of Sand Casting
As much as sand casting can have disadvantages such as:
Low material strength - When compared to a machined item, the material strength is low because of the excessive porosity.
Low dimensional precision - Dimensional accuracy is particularly poor when shrinking and finishing the surface.
Flaws are unavoidable - Defects or quality differences, such as shrinkage, porosity, pouring metal defects, and surface defects, are inherent in any metallurgical process. When compared to other casting methods like die casting and investment casting, sand casts have a high level of porosity.
Post-processing — if a tighter tolerance is necessary to interface with other mating parts, a further machining operation is frequently required. In comparison to tooling and material costs, processing costs are significant.
The advantages outweigh these sand casting disadvantages. The advantages of sand casting include:
It is used to form complex shapes
It can produce very large parts
Very low cost for tooling
It is versatile and applicable to all metals, including high melting point ones.
Easy to scale
Complex geometries with thin-wall sections
Low production cost
Complex geometries with thin-wall sections
Sand casting is a manufacturing process in which liquid metal is poured into a sand mold, which contains a hollow cavity of the desired shape, and then allowed to solidify. When casting, a liquid material is poured into a mold, which then solidifies to give the desired shape. Casting materials include metal, concrete, epoxy, plaster, and clay. It is essential to select the right method/technique in sand casting, cognizant of the type of sand intended for use.
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