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Engineering School
Mechanical Engineering Department
Fabrication Processes Laboratory
Practice 10: Sand Casting
THEORETICAL FRAMEWORK
Casting is one of the oldest manufacturing process; it consists in pouring molten metal or another material (such as plastic) into a mold to produce a part with a specific pattern or shape. This process is capable of producing complex shapes in a single piece, varying its size from very large to very small, including internal cavities. Some examples of produced parts are engine blocks, cylinder heads, transmission housings, pistons, and more. There are a lot of casting processes, but we can group them in 4 types, which are the following:
- Ingot Casting and Continuous Casting ,
- Expendable Mold, Permanent-Pattern Casting Processes
- Expendable Mold, Expendable-Pattern Casting Processes
- Permanent-Mold Casting Processes For any type of casting operation, there are some factors that must be taken into consideration:
- Solidification of the metal from its molten state, and accompanying shrinkage.
- Flow of the molten metal into the mold cavity.
- Heat transfer during solidification and cooling of the metal in the mold.
- Mold material and its influence on the casting operation.
Solidification of Metals
When talking about solidification, pure metals have clearly defined melting and freezing points,
and solidification takes place at a constant temperature. When the temperature of the molten metal decreases down to the freezing point, the latent heat of fusion is given while the temperature remains constant. After this phase change, solidification is complete and the metal cools to room temperature. Later, the casting will contract as it cools, due to the contraction from a superheated state to the solidification temperature, and cooling from the solidification temperature to the room temperature. A significant change in density may occur due to the phase change. When talking about alloys, the freezing and melting points are not clearly defined, instead they solidify over a range of temperatures. Solidification begins when the temperature of the metal drops below the liquidus phase and is completed when the temperature reaches its solidus phase. The composition and state of the alloy is described by its own phase diagram. Because all castings must possess certain properties to meet design and service requirements, the relationship between the properties and the structure developed during solidification are important. Figure 1. (a) Temperature as a function of time for the solidification of pure metals. Note that freezing takes place at a constant temperature. (b) Density as a function of time^1_._
Fluid Flow
Fluid flow is extremely important when talking about casting operations. The structure and the gating system of a casting mold need to be properly designed to ensure an adequate fluid flow so that common problems such as cooling turbulence and gas entrapment can be avoided. Before pouring it, the molten metal needs to be carefully handled to avoid forming oxides and the introduction of impurities into the molten metal. Oxides and impurities may affect fluid flow leading to common problems such as gas entrapment, cooling turbulence, incomplete fulfillment of cavities, etc. Two basic principles of fluid flow are relevant in gating design: Bernoulli’s theorem and the law of mass continuity Bernoulli’s theorem is based on the principle of the conservation of energy and relates pressure, velocity, elevation of the fluid at any location in the system, and the frictional loses in a fluid system. (^1) Obtained from: Kalpakjian, S., & Schmid, S. R. (2009). Manufacturing Engineering and Technology. Pearson.
Heat Transfer of Castings
Heat transfer is a very important aspect of a casting process during the complete cycle from pouring to solidification and cooling of the casting to room temperature. Heat flux at different locations in the system depends on many factors relating casting materials, the mold and process parameters. For example, when casting thin components, the flow rate must be high to avoid premature cooling and solidification. On the other hand, the flow rate must not be so high as to cause excessive turbulence with detrimental effects on the properties of the casting. Solidification time The solidification time is a function of the volume of a casting and its surface area and is given by 𝑆𝑜𝑙𝑖𝑑𝑖𝑓𝑖𝑐𝑎𝑡𝑖𝑜𝑛 𝑡𝑖𝑚𝑒 = 𝐶 ( 𝑉𝑜𝑙𝑢𝑚𝑒 𝑆𝑢𝑟𝑓𝑎𝑐𝑒 𝐴𝑟𝑒𝑎) 𝑛 Equation 4. Chvorinov's rule where 𝐶 is a constant that reflects (a) the mold material, (b) the metal properties (including latent heat), and (c) the temperature. The parameter 𝑛 has a value between 1.5 and 2, but usually is taken as 2. Shrinkage Metals shrink or, in special cases, expand during the solidification and cooling process. Shrinkage causes dimensional changes and sometimes cracking and is a result of the following phenomenon
- Contraction of the molten metal as it cools before it begins to solidify
- Contraction of the metal during phase change from liquid to solid (latent heat of fusion)
- Contraction of the solidified metal as its temperature drops to ambient temperature The largest amount of shrinkage or expansion occurs during cooling of the casting Figure 2. Volumetric expansion or contraction of various cast metals^2 (^2) Obtained from: Kalpakjian, S., & Schmid, S. R. (2009). Manufacturing Engineering and Technology. Pearson.
Sand Casting
Sand casting is an expendable-mold and permanent-pattern casting process. It consists of placing a pattern, having the shape of the desired casting, in sand to leave an imprint, after that a gating system is the next step. Once the gating system is defined, the molten metal is then poured into the mold to fill the resulting cavity, leaving the molten metal to solidify and cool down. After the casting has solidify and cool down, the sand mold is torn apart to take out the casting and give some finishing operations. Sands Most sand-casting operations use silica sands. There are two general types of sands: naturally bonded and synthetic. Synthetic sands are preferred because the composition can be controlled more accurately. Several factors are important in the selection of sand for molds, and certain tradeoffs with respect to properties are involved. Sand having fine, round grains can be packed closely and, thus, forms a smooth mold surface. Although fine-grained sand enhances mold strength, the fine grains also lower mold permeability (where fluids and gases penetrate through pores). Good permeability of molds and cores allows gases and steam evolved during the casting to escape easily. The mold also should have good collapsibility to allow the casting to shrink while cooling and avoid defects in the casting, such as hot tearing and cracking. Types of sand molds There are three basic types of sand molds: green-sand , cold-box , and no-bake molds. Green sand is the most common type and the “green” refers to the fact that the sand is moist while the metal is being poured. In the skin-dried method, the mold surfaces are dried. These molds generally are used for large castings because of its higher strength, better dimensional accuracy and surface finish of the casting, on the other hand, distortion of the mold is greater, they’re susceptible to hot tearing because of their low collapsibility and the production rate is slower. In no-bake mold process, a synthetic liquid resin is mixed with the sand and the mixture hardens at room temperature. No-bake mold processes are also known as cold-setting processes. The cold-box mold process uses organic and inorganic binders that are blended into the sand to chemically bond the grains for greater mold strength. Patterns Patterns for sand casting are commonly made of wood, plastic, or metal. Sometimes a combination of two types of materials may happen to give extra strength or wear resistant to critical regions depending on the characteristics of the casting process. Patterns are usually coated with a parting agent, so they can be easily removed from the sand mold. There are different types of patterns depending on the applications.
- One-piece patterns are generally used for simpler shapes and low-quantity production. They are typically made of wood.
- Split patterns are two-piece patterns, made so that each part forms a portion of the cavity for the casting, allowing the casting of complex shapes.
- Match-plate patterns are a common type of mounted pattern, in which two-piece patterns are constructed by securing each half of one or more split patters to the opposite sides of a single plate. In such constructions, the gating system can be mount on the drag side of the pattern. Cores For castings with internal cavities or passages, such as those found in an automotive engine block or a valve body, cores are utilized. Cores are placed in the mold cavity to form the interior surfaces of the casting and are removed from the finished part during shakeout and further processing. Like molds, cores must possess strength, permeability, the ability to withstand heat, and collapsibility; hence, cores are made of sand aggregates. The core is anchored by core prints, which are recesses added to the pattern to locate and support the core and to provide vents for the escape of gases. A common problem with cores is that they may lack enough structural support in the cavity. To keep the core from shifting, metal supports (chaplets) may be used to anchor the core in place. Cores generally are made in a manner like that used in sand mold making; the majority are made with shell, no-
OBJECTIVES
- The student will understand the specific security norms that applies to the practice.
- The student will learn the principal characteristics of the sand casting process.
- The student will learn the advantages and limitations of a sand casting process.
- The student will learn the structure of a common sand mold and the importance of its features.
- The student will learn the steps to make a sand mold.
- The student will make different sand molds to analyze the impact of its features
- The student will analyze the defects of each casting and its possible cause
SECURITY
To use the laboratory’s induction furnace, it is necessary to take the following precautions: To use the basic measuring instruments during this practice is necessary to consider the following precautions:
ATENTION! REASON
Do not apply excessive force on the measuring instrument. This may cause a permanent deformation on the instrument or the part. Clean the part and the contact surface area between the instrument and the part. Achieve an accurate measurement Keep measuring instrument clean all the time To maintain calibration and to not lose accuracy in the measuring instrument The measuring instruments should not be left on surfaces where there are chips, grease or any other type of pollutant. This may cause permanent damage to the measuring instrument asdfasdfasdf hxjfoasdfjioas jijiojiojo
ATENTION! REASON
Always use safety glasses when using any machine. The risk that a chip of the material or an extremely hot grain of sand jumps to an eye and makes a permanent damage is too real. Make sure that all connections are rightfully connected and isolated. The conditions of the cables, the machine and its connections the base for a secure process. Don’t get too close to the induction furnace without the proper safety equipment The induction furnace works at very high temperatures, sometimes producing hot gases when melting a metal and without the safety equipment burns and accidents may happen Never grab the crucible without the use of gloves and its handles The crucible is at very high temperatures so grabbing it without the proper safety equipment may lead to 3rd^ degree burns if it enters in contact with the skin. Do not grab the casting when it is recently released from the sand mold The casting and the sand are extremely hot, so do not grab anything without the use of handlers Use gloves when grabbing the casting once it has cool down The casting may have some regions with extremely sharp edges which may lead to accidental cuts. Make sure that the workpieces or blanks are disconnected and at room temperature for their manipulation. Do not have instant contact with the parts after the process is done. Is imperative to avoid contact with energized or extremely hot parts to prevent accidents.
MATERIAL, TOOLS AND EQUIPMENT
1) Material
- Aluminum ingots and scrap
- Silica sand
- Wooden frames
- Match-plate patterns
- Water
2) Tools
- Vernier caliper
- Wooden rammer
- Handlers
3) Equipment and Machinery
4) Safety Equipment
- Gloves
- Safety glasses
- Leather Apron
- Leather Sleeves
PROCEDURE
- Print and read all the practice before going to the laboratory. Bring the pre-report answered by hand and stapled.
- You should arrive on time to the practice, following the dress code (cotton t-shirt, jeans, closed and strong shoes), no jewelry. Bring the pre-report answered and the report, both printed and stapled.
- The instructor will apply a quiz at the beginning of the session to evaluate the student’s comprehension of the theoretical framework; the quiz will also be used to take assistance.
- The instructor will explain the principal characteristics of the sand casting process and the induction furnace.
- The instructor will explain the importance of the features of a sand mold as well as the pattern.
- The instructor will mention the steps to create the sand mold, while giving all the tools and necessary materials.
- While the students are making the sand molds, the lab personnel will start to melt aluminum using the induction furnace.
- With the help of the lab personnel or instructor, the molten metal will be poured into the different sand molds
- Everyone will wait until the metal has solidified and cool down before manipulation.
- At the end of the session, the students will analyze the parts obtained and will compare the results obtained and will concluded the causes for such results.
2) Research and describe the different types of patterns for sand casting, the materials for the
pattern and their impact on the casting.
Loose pattern
Normalmente producido en madera. Algunos patrones sueltos se pueden dividir en dos mitades para
facilitar el moldeado.
Gated patterns
Más complejos que los patrones sueltos, con compuerta o "montados", los patrones generalmente se
montan para incorporar un sistema de funcionamiento y compuerta junto con el patrón para facilitar la
productividad, se elimina el corte manual y otros pasos de moldeo para mejorar la reproducibilidad de
los moldes.
Match-plate pattern
Producidos comúnmente en madera, suelen moldearse con las porciones de la “cope” (superior) y de
“drag” (inferior) del patrón montadas en lados opuestos de una placa para acelerar el proceso de
moldeo. Los sistemas de compuerta normalmente también están incrustados en la placa de
coincidencia, junto con los accesorios utilizados para montar estos patrones en máquinas de moldeo.
Investment casting die Los troqueles son a menudo herramientas de metal complejas con al menos dos partes, donde la cavidad interior se mecaniza para tomar la forma del patrón de cera deseado. Se inyecta cera en estos troqueles, después de enfriar, se separa el troquel y se extrae el patrón de cera de una pieza. Los núcleos de metal y otras piezas se pueden ensamblar en estos troqueles de fundición de inversión para formar cavidades interiores y otras partes complejas de la fundición deseada. Bibliographic reference that was consulted: Reliance Foundry Co. Ltd. (2021). Casting Patterns and Patternmaking. Retrieved 1 October 2021, from https://www.reliance-foundry.com/blog/what-is-a- casting-pattern
3) Describe the main features of a sand mold. (Runners, risers, gating system, etc.)
Copa de vertido (Pouring cup) El metal líquido se vierte directamente en la copa de vertido. Este ayuda a separar los deshechos del metal, reduce la turbulencia y ayuda a mantener la cantidad de líquido correcto. Bebedero y corredor (Sprue and runner) El metal fluye desde la copa de vertido hacia el bebedero cónico, luego a por el canal, que se introduce en las compuertas. Compuertas (gates) El metal fluye a través de las compuertas para llenar el molde. Las compuertas pequeñas se usan para piezas fundidas que se solidifican lentamente, mientras que las puertas más grandes se utilizan para piezas fundidas que se solidifican rápidamente. Las compuertas deben colocarse con cuidado para promover una solidificación direccionada. Elevador(riser) El tubo ascendente es un depósito que evita la contracción de las cavidades. Para que un elevador funcione correctamente, este debe enfriarse más lentamente que la fundición. Los patrones con compuerta Se incluye el sistema de compuerta en el cuerpo del patrón principal. Bibliographic reference that was consulted: Reliance Foundry Co. Ltd. (2021). Casting Patterns and Patternmaking. Retrieved 1 October 2021, from https://www.reliance-foundry.com/blog/what-is-a-
casting-pattern
4) Describe the most common defects in a casting and their probable causes.
Shrinkage (encogimiento) Como todos los materiales, el metal se contrae/encoge cuando se enfría. Los patrones se hacen más grandes que las dimensiones exactas especificadas para el producto final, debido a que, si fueran del mismo tamaño, la fundición sería más pequeña de lo requerido. Distorsión La fundición puede distorsionarse, ya sea por enfriamiento o por algún orificio en la arena. Los patrones pueden distorsionarse intencionalmente para compensar la distorsión de enfriamiento esperada. Bibliographic reference that was consulted:
5) Mention the advantages and limitations of a sand casting process.
advantages limitations Los moldes de arena se pueden producir fácilmente en casi cualquier aleación ferrosa o no ferrosa. El costo de la arena es relativamente bajo. Tamaño, peso y forma: Se pueden producir en pesos desde onzas hasta más de 200 toneladas. Debido a que el costo de las herramientas puede ser mínimo, la fundición en arena puede ser apropiada para una sola pieza. El Sand Casting puede ofrecer una de las mejores exactitudes en dimensión y repetibilidad, pero este proceso, en promedio, proporcionará menos precisión que otros procesos de fundición comunes. Aunque el costo de las herramientas es más barato a comparación de otros procesos de fundición, todavía existe un arte en la creación de patrones. Los patrones pueden ser voluminosos, por lo que el almacenamiento debe gestionarse Las piezas de fundición en arena tienden a tener un acabado más rugoso que otros procesos de fundición.
- Was there any defect present in the different casted parts? If there were any defects, mention possible ways to avoid them.
- From the technical perspective, explain the experience of using the sand casting process, the common defects, the types of features needed for a high-quality casting.
PRACTICE PRE-REPORT AND REPORT DELIVERY
Make sure that your pre-report & report have all the data in the identification table and that you have answered all the elements required before delivering it to the instructor. After that, go to the storage to deliver the materials, tools and accessories and make sure to clean the work area before leaving. Si, en el resultado había ciertas imperfecciones en el objeto, “misruns” que serian como pequeñas laminas en la zona donde se unía el molde superior con el molde inferior, probablemente por el hecho de que la arena no estaba suficientemente compactada en esas zonas. Se podría evitar aplastando con más uniformemente compactada. Se necesita paciencia para el experimento ya que el proceso es tardado y necesita mano de obra en el trabajo, se pueden ver los encogimientos y distorsiones por el mal manejo del aplastamiento de la arena o el no utilizar las dimensiones contra el encogimiento.
