Around the casting methods especially in metal casting methods, sand casting is the most prominent and used method to cast metals in industry. We call the place where they make sand casting operations a foundry. Because of the greatness of the market, there are lots of kinds of foundries around, nearly every county. So, the sand casting market is very big. Here, we explain the general characteristics and technical details of sand casting methods.
What is Sand Casting?
There is a general terminology that we use in sand casting techniques.
In sand casting operations or processes, we pour molten metal from a mouth, then this poured molten material fills the cavity inside the sand casting mold. This is the most basic explanation of sand casting operations.
As you see in the above illustration, there are a bunch of terms related to sand casting methods. General sand casting equipment has to cope and drag sections. In most basic aspects, the cope is the upper part of the sand casting mechanism. Drag is the lower part of the sand casting mechanism. And these two halves of the sand casting system are divided by a parting line.
The parting line is very important because they place the cavity of the mold symmetrically generally inside the mold. These copes, we place to drag and parting line system inside a flask that holds the sand together. We can divide this flask also into two parts from the parting line.
Pouring Cup, Downsprue, and Runner
The pouring cup is where we pour the molten metal into the cavity of the sand mold. We must design the pouring cup to prevent turbulent flux inside the gate system. Tribulation can lead to porosities because of trapped gases.
Downsprue is where molten metal travels vertical direction to reach the mold cavity. Also, we must design a downsprue to obtain the required flux characteristics from molten material.
The runner is the horizontal gate through that molten materials pass from it to reach the mold cavity. This is another part of the gating system of the sand casting system. We must design it also to obtain enough flux characteristics from molten metal. These runners, downsprue and pouring cups constitute the gating system that molten metal travels into the mold cavity.
The riser is a very important part of the sand casting system. Riser provides molten metal into the cavity in the solidification phase. So, we must design the riser to solidify after the metal inside the mold cavity solidifies. When the system solidifies, we cut the riser.
In a part that is produced with the sand casting method, there can be internal cavities that we can not produce only with a mold cavity. To obtain these internal shapes and cavities, we use cores. We produce cores also from sand, and also we can produce from other materials such as ceramics, etc. The placement of cores inside the mold cavity is also a very important thing to obtain a decent part.
Mold Cavities in Sand Casting
We produce patterns of produced parts also to obtain a sand cavity that molten metal will pour inside it. We generally produce these patterns from wood, polymer foams, etc.
These designed patterns are slightly bigger than the original sand-cast metal part. This is because in the phase of transition from liquid to solid, metals shrink. We call this phenomenon ‘shrinkage’ in casting. We must consider shrinkage when molding and they design the mold cavities. Engineers are making shrinkage calculations by engineers, and they must make very detailed designs to obtain the required shapes in the required dimensions. After these shrinkage calculations, they produce the patterns.
They place the produced patterns into the flasks, and they spill sand with binders to fill all flasks. And we place the model symmetrically around the parting line. When the binders inside the sand, packs the sand, they remove the pattern by unmantling the cope and dragging the sides of the flasks.
With the addition of this pattern, they design proper gating and riser systems to obtain successful sand casting operations.
When we pour the molten metal inside the mold, air traps inside the sand. This trapped air can lead to a problem of porosities inside the produced cast part. Because of these porosities, the mechanical properties of produced part can be affected adversely. Self-ventilating system of the sand mold, lets the trapped air go out from the mold cavity.
In permanent metal molds, they drill special venting holes to take the trapped air out.
Sand Casting Mold Sands
As it implies with its name, we use sand in mold making. These sands are very special and they need to have some properties to obtain good casting operations. Here, we will explain;
- General ingredients of mold sands that we use in the sand casting market.
- General properties of mold sands to achieve successful sand casting operations.
- Classification of mold sands that are available in the sand casting market.
What are the Constituents in Mold Sands?
As you know, we obtain the sand from silica, and mold sands are generally mixed with other minerals. So, in general, what makes mold sand special is the ingredients.
In general, mold sand includes around 90% of typical sand, 7% of clay which binds the sand to obtain mold shape, and 3% of water for shaping purposes. Clay is a very important ingredient to obtain sand mold. After the shaping of mold sand, the solidification of clay binds the sand grains to obtain a solid mold structure.
Sometimes, we can use other phenolic resins instead of clay. Thermosets solidify with the application of light, from which we can obtain solid mold structures of this nature.
Shaping the Mold Sands
After mixing up the constituents of mold sands, the shaping of this mixture before solidification is very important. Before shaping mold sand, we must build a general system to obtain mold. In this system, there are parts of the system;
- Flasks constitute the external boundary to fill inside it with mold sand. After the sand mold solidifies, we obtain the mold in the shape of the flask.
- To obtain a mold cavity with mold sand, we use patterns. They place patterns and gating systems inside the flask, and they spill the sand mold inside it. After solidification, we obtain mold cavity and gating systems.
‘Packing’ of sand inside the flask is also a very important process. We must pack the sand inside the flask to obtain a solid mold structure. There are some mechanisms to pack the sand inside the flask.
- Sand pack inside the flask by slinging the sand at high speed.
- We can use pneumatic machines to pack the sand inside the flask by squeezing it.
- Jolting of sand from another flask into the new flask to obtain mold shape.
What are the Required Features of Mold Sands?
They create the mold sands and molds from sands must have some properties to meet the situations originating from metal casting.
- Thermal Stability: These are the most important characteristics of mold sands. We must prepare the constituents of mold sands to withstand the high temperatures that originate from metal casting processes. The surface of the internal cavity of molds must withstand the thermal effects of liquid metal inside them.
- Permeability Of Sand Mold: Gases are produced in the pouring process of liquid metal. Also, there is an atmospheric gas inside the mold cavity. When they pour the liquid metal inside the mold, these gases must evacuate from the mold cavity. Otherwise, we obtain porosities and unintended shapes from sand casting operations. So, the required permeability of sand mold is very important, and the grainy structure of sand provides this property. The permeability of sand mold also changes with the changing grain shape of sand. When the sand grains are round in shape, their permeabilities will be much higher. But, when the sand grains have very different shapes, we obtain very good packing and good surface characteristics.
- Collapsibility: Molds that are produced from sand and other constituents must be collapsed after the casting operation is done. We will use this sand in other casting operations.
- Strength Of Mold: We must establish a good packing to obtain the required strength to withstand different conditions that originate from casting operations. As we stated above, the strength of mold depends on the sand grain structure, quality of the used binders, etc.
- Reusability: Once we collapse the mold, we must use the extracted mold sand in another sand casting operation. Reusability is very important in terms of costs.
Classification of Mold Sands
In general, there are three types of conventional sand molds that we use in sand casting operations.
- Dry-Sand Molds: We use organic binders instead of clay products. In baking ovens, they bake prepared sand molds at 200-300C degrees of temperature. With this kind of sand mold, we can obtain better surface characteristics and dimensional properties. But, this method is much more expensive rather than other methods. Baking time is an also obstacle in the serial production manner. Thus, we use this method in the production of medium to big-sized metal products, in small quantities.
- Skin Dried Molds: With the addition of special binders into the mixture of the sand mold, only the surfaces of mold cavities dry with the application of light, heat, etc. Around 10-20mm depth of cavity surface generally dries. It is much more economical if we compare it with dry-sand molds.
- Green Sand Molds: Green sand mold method is the most used and most conventional method to obtain sand molds. They have a percentage of moisture in pouring time. We obtain the required permeability, strength, and collapsibility with the smart design of mold structures. But the moisture inside the sand mold structure can cause some defects in casting. And surface characteristics are lower than other methods.
Buoyancy Effect of Cores in Sand Casting
To obtain a neat sand casting operation, we must evaluate all the critical points in terms of engineering. After we obtain the cope, drag, mold cavity, and gating systems, they clamp the two halves of the sand casting system to each other. In this situation, the system is ready for the pouring of liquid metal. We place the cores also into the molding sand to obtain the internal shapes of metal parts.
To this extent, we must consider the term in the design of cores and placement of cores. When we place the core inside the sand and we pour the liquid metal, we must consider the buoyancy effect. Here, we will explain this buoyancy effect and how to eliminate it in casting operations.
In sand casting, buoyancy is applied to the cores from the liquid metal that is poured inside the mold cavity. As you know from Archimedes’ buoyancy law, the lifting force applied to an object equals, the weight of the liquid is replaced by the volume of this object minus the weight of the object. So, if the density of molten metal is bigger than the core’s density, the lifting force that originates from this situation can damage the mold structure.
So, we must consider this buoyancy force in the design of core and core placement. The placement places must withstand these buoyancy forces to prevent sand damage.
Densities of Different Metals in Liquid Form
These are the densities of some metals in liquid form. You can easily calculate the buoyancy forces to eliminate the damaging inside core placement zones;
- Cast Iron(Gray): 7.16 g/cm^3.
- Pure Copper: 8.73 g/cm^3.
- Steel: 7.82 g/cm^3.
- Pure Aluminum: 2.70 g/cm^3.
- Aluminum-Copper(92%Al): 2.81 g/cm^3.
- Aluminum-Silicon: 2.65 g/cm^3.
- Pure Lead: 11.30 g/cm^3.
- Brass: 8.62 g/cm^3.
If you know the density and the volume of the core that you used in the mold cavity, you can easily calculate the buoyancy forces that the placement of the core must withstand. And you can do required reinforcements for these areas to prevent damage.
Metal alloy systems have a very profound effect on our society and most metal alloy systems are very important as an engineering material. First shapes of metal alloys are generally available with casting methods, especially sand casting methods. But, alloy systems have very specific solidification characteristics. Because of these solidification characteristics in sand molds, there are some issues and parameters that we must consider in the phase of mold design of alloy systems. Here, we explain the solidification characteristics of metal alloys in sand molds.
How Metal Alloys Solidify in Sand Molds?
As you see, there are ‘liquidus’ and ‘solidus’ points. The temperatures of these liquidus and solidus points are different. And also these liquidus and solidus temperatures change with the changing composition of metal alloy. You can see this change on the curve sets of the copper-nickel alloy on the right side. Solidus and liquidus temperatures decrease with the increasing copper percentage in the copper-nickel alloy. If you pick a place between these liquidus and solidus curves, you will obtain a phase between liquid and solid.
Above the liquidus curve, the all-alloy system will be liquid. And below the solidus curve, all the alloy systems will be solid. If you take a ‘tie line’ in a specific percentage of copper, you can obtain an exact temperature-time curve of this composition of the alloy system. For example, above, we take 50% of the copper alloy tie line.
As you see in the left curve, the pouring temperature is the highest. When the alloy system is poured into the gating system of the sand casting system, the temperature begins to decrease. When the temperature reaches the ‘freezing point’, sectional solidification of alloy systems starts. We call this freezing point is liquidus point. But, unlike the pure metal systems, the temperature is not constant during the solidification. Up to the completion of freezing, the temperature decreases gradually.
When the solidification completes, the temperature is at the ‘solidus’ point. These solidus and liquidus temperatures, correspond to the edges of the tie line that we take from the temperature-composition curve.
How is The Dendritic Growth in Metal Alloys?
The dendritic growth in metal alloys is also quite different from pure metals. The dendritic growth starts with the formation of finer grains near the mold walls, upon rapid cooling. And these grains turn into the spikes of grains and these spikes turn into the grain branches oriented inside the mold.
But these grain branches are rich with metal of which melting temperature is lower. These dendrites surround the liquid and another alloy compound that has a higher melting temperature. We call this section of solidification a mushy zone. With the advancing of solid dendrites, this mushy zone turns into a solid phase at last. But on the microscale, alloy compounds do not spread regularly through the mold. We call this phenomenon as segregation of different compounds on the microscale.
Dendritic growth finishes with the depletion of the compound that has a lower melting temperature. So, the remaining other phase forms finer grains at the innermost of the sand mold. We call this situation also bulk segregation.
As you understand, this is a very different mechanism of solidification. We must design the mold designs and gating systems, and the part that is made with alloy systems by considering this issue.
Sand Casting Patterns
To produce these complex geometries in sand casting from metals, we must open a proper mold cavity inside the cast sand. We obtain these cavities with patterns.
How to Design a Pattern in Sand Casting Operations?
To obtain an internal void inside sand to fill it with cast metal, we must design and produce a pattern of a produced part. These patterns must have the same shapes as the original part. But we must consider some parameters in the phase of pattern design;
- A thickness of after-machining operations: After we obtain the cast part from its mold, surface characteristics may not be good as well that the customer desires. So, we can apply additional machining operations to obtain the required surface characteristics. A bit of thickness ratio must be left in the phase of pattern design.
- Ration for shrinkage: Shrinkage is another parameter. All the materials shrink after solidification generally. So, the pattern must design a bit oversized according to the original shape of the part to compensate for this shrinking.
In general, three types of materials are used in the design of patterns; Wood, metal, and plastics. Metals are very costly to produce. Various machining operations must be applied. But for a high quantity of productions, metal are better.
Wood patterns are also used, and they are very cheap and shapable. But, during the packing of sand upon wood patterns, they can abrade and deform. So, they are suitable for low-quantity of production.
In terms of both cost and production quantity, plastics take place between woods and metals. They can be preferred for middle-quantity production of cast parts.
What are the Types of Sand Casting Patterns?
For san casting operations, casting patterns are produced according to the complexity of produced part and the quantity.
- Solid patterns: Solid patterns are a one-piece pattern that has completely the same shape as the original part. They are suitable for low-quantity productions with complex parts. Specifying of parting line for these patterns can be a problem. This is the type that has the easiest production.
- Split patterns: This is a type of pattern that has two halves of solid patterns. The difference is, there is no requirement for the judgment of the parting line. Sliced geometry gives the parting line.
- Plate patterns: This is also the type of pattern in that two halves of a part are divided by a plane, and these two halves are not detachable. We can determine the Cope and drag sections easily and building the sand casting system with these patterns is much easier. So, it is suitable for higher production rates. Production of these plate patterns is quite hard if we compare it with others.
- Cope-Drag patterns: Cope-drag patterns are sliced types of plate patterns. We add gating systems, and risers also into this type of pattern. Production of sand casting systems is easier than other with cope-drag patterns, but production of these patterns is quite hard.
Pouring Molten Metal in Sand Casting
Because of its differences, pouring molten metal into the sand casting mold cavity is not an easy thing. You need to design the pouring and fluxing of molten metal inside the gatings. So in that way, you need to design a proper gating system to optimize the pouring parameters;
- Pouring temperature,
- Pouring rate,
How to Adjust Pouring in Sand Metal Casting?
As we stated above, there are three parameters to design pouring and gating to obtain the required pouring form. The first parameter is the pouring temperature.
Pouring temperature is the temperature when a metal enters the mold cavity in sand casting. We call this temperature also ‘superheat’. The pouring temperature is generally adjusted above the melting temperature of molten metal.
The molten metal must not start to solidify up to the end of the pouring process in sand casting. If the molten metal starts to solidify in the pouring process, uneven solidification and casting can occur. Uneven distribution of metal can also occur. But if the difference between the melting temperature and pouring temperature of molten metal is too high, it means that excessive energy is consumed to heat the material. This can be too expensive on a commercial basis.
So, a designer, or engineer must design the pouring temperature according to these parameters. Optimization of pouring temperature is a very important thing in sand casting.
The second parameter in the pouring process of sand casting is the pouring rate. Other parameters that are affecting the casting performance, depend on this parameter. If the pouring rate is too high, excessive turbulence can occur. Excessive turbulence is not desired thing because of some reasons that we explain below.
If the pouring rate is low, uneven solidifications of molten metal inside gates can occur, and this is a very serious problem for the healthy metal casting process. So, the pouring rate must be optimized according to the own parameters of different sand casting scenes. This is also a very important and professional thing.
The third parameter that affects the casting pouring performance is turbulence. In fluid dynamics, turbulence is one of the most important parameters. So, in metal pouring, incompressible fluid flux is a prominent phenomenon. If the turbulence is very high, it can damage the sand mold and sand gates. Unlike water, the density of molten metal is very high. Because of that, the destructive effect of turbulence of molten metal is also high compared with water. If the surface of the mold cavity is damaged, the part that will be produced will be defective.
To prevent mold erosion caused by turbulence of molten metal in gates, the turbulence of fluxing molten metal must be calculated professionally. The pouring rate and shape of the pouring cup and gates are designed to prevent turbulences. Streamlined, laminar flow is desired.
As you understand, these three parameters are interrelated to each other. You can not design or define a pouring parameter of molten metal inside the mold cavity individually. All of these three parameters must be considered same time. So, this requires an engineering profession.
What is ‘Shrinkage’ in Casting Operations?
In the most basic approach, shrinkage is the phenomenon that occurs because of the contraction of material with the decreasing temperature. It has a very specific mechanism in terms of metal casting and it needed to be understood for successful casting processes.
The solidification of metals in casting molds takes place by starting to solidify from mold walls. But before that, the liquid must give some heat energy to the surroundings to lower the pouring temperature to the level of melting temperature. During this temperature decrement in the liquid phase, the decrement of the volume of the liquid metal takes place.
When the solidification starts, the temperature is not decreased for pure metals and decreased for metal alloys. But the liquid phase, changes into the solid phase. So, solidification shrinkage takes place at this time. There is another contraction because of the phase change. In general, the solid state of the material is denser than the liquid phase.
After the completion of solidification of metal, temperature decrement goes on lowering up to room temperature. Another decrement of volume takes place for cast metal. This is the shrinkage phase for the solid state.
Because of these shrinkage steps in casting molds, ‘shrinkage cavities’ occur inside the cast metal. These shrinkage cavities are major problems in terms of the mechanical properties of produced parts from cast metal.
How to Compensate Shrinkage in Metal Casting Operations?
During the design process of all mold and casting systems of a metal part, shrinkage is considered in different means. The first solution to volumetric change due to the shrinkage phenomenon in casting operations; designing the mold cavity patterns higher in volume compared with the original part. This shrinkage ratio is calculated by engineers, and patterns are designed bigger than the original parts, to obtain exact dimensions after solidification.
As we stated above, shrinkage cavities are also important problems resulting from shrinkage thing. Risers are added to mold cavities and different places of mold gates to compensate for the shrinkage cavities. Risers must solidify after the part itself to compensate to fill shrinkage cavities with metal fluids.
What are the Shrinkage Ratios of Different Metals?
Here is the list of some shrinkage values of metals and metal alloys;
- Carbon Steel: 1.6-2.1%,
- Nickel: 2.1%
- Gray Cast Iron: 0.8-1.3%
- White Cast Iron: 2.1%
- Zinc: 2.6%
- Chrome Steel: 2.1%
- Magnesium: 2.1%
- Aluminum Alloys: 1.3% on average.
- Tin: 2.1%
- Magnesium Alloys: 2.1% on average.
- Yellow Brass: 1.3-1.6%
The volumetric calculations for shrinkage are done according to these values, to design patterns.
As you see above, sand casting is a very important topic that we need to consider for the production of metal parts. So, it is one of the most common manufacturing methods available in the production industry.
There are various kinds of parameters that we need to consider for sand casting operations. Around these parameters, the most important ones are patterns, the flow of the metals, and casting defects. We need to have exact solutions for these problems.
These are the general and most important topics about sand casting operations. Finally, do not forget to leave your comments and questions below about the sand casting operations.
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FAQs About Sand Casting Operations
It is a complete process that we obtain a cavity to pour the molten metal inside it. We create this cavity in special sand for casting operations. And we use patterns to create these cavities. Once the molten metal solidifies in the cavity, we destroy the sand to take the metal from it.
The main purpose of the sand casting operations is, we create complex shapes from metals which is very hard to produce with conventional methods.
The general requirements for sand casting operations are sand, flask, pattern, and molten metal. We pour the molten metal into the sand cavity to obtain the metal parts successfully.
Yes, there are lots of common applications of sand casting operations. These operations are very common in different kinds of industries. Also, sand casting operations have a v