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Sand casting, the most widely used casting process, utilizes expendable sand molds to form complex metal parts that can be made of nearly any alloy. Because the sand mold must be destroyed in order to remove the part, called the casting, sand casting typically has a low production rate. The sand casting process involves the use of a furnace, metal, pattern, and sand mold. The metal is melted in the furnace and then ladled and poured into the cavity of the sand mold, which is formed by the pattern. The sand mold separates along a parting line and the solidified casting can be removed. The steps in this process are described in greater detail in the next section.
|Sand casting overview|
Sand casting is used to produce a wide variety of metal components with complex geometries. These parts can vary greatly in size and weight, ranging from a couple ounces to several tons. Some smaller sand cast parts include components as gears, pulleys, crankshafts, connecting rods, and propellers. Larger applications include housings for large equipment and heavy machine bases. Sand casting is also common in producing automobile components, such as engine blocks, engine manifolds, cylinder heads, and transmission cases.
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Disclaimer: All process specifications reflect the approximate range of a process's capabilities and should be viewed only as a guide. Actual capabilities are dependent upon the manufacturer, equipment, material, and part requirements.
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The process cycle for sand casting consists of six main stages, which are explained below.
- Mold-making - The first step in the sand casting process is to create the mold for the casting. In an expendable mold process, this step must be performed for each casting. A sand mold is formed by packing sand into each half of the mold. The sand is packed around the pattern, which is a replica of the external shape of the casting. When the pattern is removed, the cavity that will form the casting remains. Any internal features of the casting that cannot be formed by the pattern are formed by separate cores which are made of sand prior to the formation of the mold. Further details on mold-making will be described in the next section. The mold-making time includes positioning the pattern, packing the sand, and removing the pattern. The mold-making time is affected by the size of the part, the number of cores, and the type of sand mold. If the mold type requires heating or baking time, the mold-making time is substantially increased. Also, lubrication is often applied to the surfaces of the mold cavity in order to facilitate removal of the casting. The use of a lubricant also improves the flow the metal and can improve the surface finish of the casting. The lubricant that is used is chosen based upon the sand and molten metal temperature.
- Clamping - Once the mold has been made, it must be prepared for the molten metal to be poured. The surface of the mold cavity is first lubricated to facilitate the removal of the casting. Then, the cores are positioned and the mold halves are closed and securely clamped together. It is essential that the mold halves remain securely closed to prevent the loss of any material.
- Pouring - The molten metal is maintained at a set temperature in a furnace. After the mold has been clamped, the molten metal can be ladled from its holding container in the furnace and poured into the mold. The pouring can be performed manually or by an automated machine. Enough molten metal must be poured to fill the entire cavity and all channels in the mold. The filling time is very short in order to prevent early solidification of any one part of the metal.
- Cooling - The molten metal that is poured into the mold will begin to cool and solidify once it enters the cavity. When the entire cavity is filled and the molten metal solidifies, the final shape of the casting is formed. The mold can not be opened until the cooling time has elapsed. The desired cooling time can be estimated based upon the wall thickness of the casting and the temperature of the metal. Most of the possible defects that can occur are a result of the solidification process. If some of the molten metal cools too quickly, the part may exhibit shrinkage, cracks, or incomplete sections. Preventative measures can be taken in designing both the part and the mold and will be explored in later sections.
- Removal - After the predetermined solidification time has passed, the sand mold can simply be broken, and the casting removed. This step, sometimes called shakeout, is typically performed by a vibrating machine that shakes the sand and casting out of the flask. Once removed, the casting will likely have some sand and oxide layers adhered to the surface. Shot blasting is sometimes used to remove any remaining sand, especially from internal surfaces, and reduce the surface roughness.
- Trimming - During cooling, the material from the channels in the mold solidifies attached to the part. This excess material must be trimmed from the casting either manually via cutting or sawing, or using a trimming press. The time required to trim the excess material can be estimated from the size of the casting's envelope. A larger casting will require a longer trimming time. The scrap material that results from this trimming is either discarded or reused in the sand casting process. However, the scrap material may need to be reconditioned to the proper chemical composition before it can be combined with non-recycled metal and reused.
In sand casting, the primary piece of equipment is the mold, which contains several components. The mold is divided into two halves - the cope (upper half) and the drag (bottom half), which meet along a parting line. Both mold halves are contained inside a box, called a flask, which itself is divided along this parting line. The mold cavity is formed by packing sand around the pattern in each half of the flask. The sand can be packed by hand, but machines that use pressure or impact ensure even packing of the sand and require far less time, thus increasing the production rate. After the sand has been packed and the pattern is removed, a cavity will remain that forms the external shape of the casting. Some internal surfaces of the casting may be formed by cores.
Cores are additional pieces that form the internal holes and passages of the casting. Cores are typically made out of sand so that they can be shaken out of the casting, rather than require the necessary geometry to slide out. As a result, sand cores allow for the fabrication of many complex internal features. Each core is positioned in the mold before the molten metal is poured. In order to keep each core in place, the pattern has recesses called core prints where the core can be anchored in place. However, the core may still shift due to buoyancy in the molten metal. Further support is provided to the cores by chaplets. These are small metal pieces that are fastened between the core and the cavity surface. Chaplets must be made of a metal with a higher melting temperature than that of the metal being cast in order to maintain their structure. After solidification, the chaplets will have been cast inside the casting and the excess material of the chaplets that protrudes must be cut off.
In addition to the external and internal features of the casting, other features must be incorporated into the mold to accommodate the flow of molten metal. The molten metal is poured into a pouring basin, which is a large depression in the top of the sand mold. The molten metal funnels out of the bottom of this basin and down the main channel, called the sprue. The sprue then connects to a series of channels, called runners, which carries the molten metal into the cavity. At the end of each runner, the molten metal enters the cavity through a gate which controls the flow rate and minimizes turbulence. Often connected to the runner system are risers. Risers are chambers that fill with molten metal, providing an additional source of metal during solidification. When the casting cools, the molten metal will shrink and additional material is needed. A similar feature that aids in reducing shrinkage is an open riser. The first material to enter the cavity is allowed to pass completely through and enter the open riser. This strategy prevents early solidification of the molten metal and provides a source of material to compensate for shrinkage. Lastly, small channels are included that run from the cavity to the exterior of the mold. These channels act as venting holes to allow gases to escape the cavity. The porosity of the sand also allows air to escape, but additional vents are sometimes needed. The molten metal that flows through all of the channels (sprue, runners, and risers) will solidify attached to the casting and must be separated from the part after it is removed.
The sand that is used to create the molds is typically silica sand (SiO2) that is mixed with a type of binder to help maintain the shape of the mold cavity. Using sand as the mold material offers several benefits to the casting process. Sand is very inexpensive and is resistant to high temperatures, allowing many metals to be cast that have high melting temperatures. There are different preparations of the sand for the mold, which characterize the following four unique types of sand molds.
- Greensand mold - Greensand molds use a mixture of sand, water, and a clay or binder. Typical composition of the mixture is 90% sand, 3% water, and 7% clay or binder. Greensand molds are the least expensive and most widely used.
- Skin-dried mold - A skin-dried mold begins like a greensand mold, but additional bonding materials are added and the cavity surface is dried by a torch or heating lamp to increase mold strength. Doing so also improves the dimensional accuracy and surface finish, but will lower the collapsibility. Dry skin molds are more expensive and require more time, thus lowering the production rate.
- Dry sand mold - In a dry sand mold, sometimes called a cold box mold, the sand is mixed only with an organic binder. The mold is strengthened by baking it in an oven. The resulting mold has high dimensional accuracy, but is expensive and results in a lower production rate.
- No-bake mold - The sand in a no-bake mold is mixed with a liquid resin and hardens at room temperature.
The quality of the sand that is used also greatly affects the quality of the casting and is usually described by the following five measures:
- Strength - Ability of the sand to maintain its shape.
- Permeability - Ability to allow venting of trapped gases through the sand. A higher permeability can reduce the porosity of the mold, but a lower permeability can result in a better surface finish. Permeability is determined by the size and shape of the sand grains.
- Thermal stability - Ability to resist damage, such as cracking, from the heat of the molten metal.
- Collapsibility - Ability of the sand to collapse, or more accurately compress, during solidification of the casting. If the sand can not compress, then the casting will not be able to shrink freely in the mold and can result in cracking.
- Reusability - Ability of the sand to be reused for future sand molds.
There exists many ways to pack the sand into the mold. As mentioned above, the sand can be hand packed into the mold. However, there are several types of equipment that provide more effective and efficient packing of the sand. One such machine is called a sandslinger and fills the flask with sand by propelling it under high pressure. A jolt-squeeze machine is a common piece of equipment which rapidly jolts the flask to distribute the sand and then uses hydraulic pressure to compact it in the flask. Another method, called impact molding, uses a controlled explosion to drive and compact the sand into the flask. In what can be considered an opposite approach, vacuum molding packs the sand by removing the air between the flask and a thin sheet of plastic that covers the pattern.
The packing of the sand is also automated in a process known as flask-less molding. Despite the name of the process, a flask is still used. In conventional sand casting, a new flask is used for each mold. However, flask-less molding uses a single master flask in an automated process of creating sand molds. The flask moves along a conveyor and has sand blown against the pattern inside. This automated process greatly increases the production rate and also has many benefits to the castings. Flask-less molding can produce uniform, high density molds that result in excellent casting quality. Also, the automated process causes little variation between castings.
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The main tooling for sand casting
is the pattern that is used to create the mold cavity.
The pattern is a full size model of the part that makes
an impression in the sand mold. However, some internal
surfaces may not be included in the pattern, as they
will be created by separate cores. The pattern is
actually made to be slightly larger than the part
because the casting will shrink inside the mold cavity.
Also, several identical patterns may be used to create
multiple impressions in the sand mold, thus creating
multiple cavities that will produce as many parts in one
Several different materials can be used to fabricate a pattern, including wood, plastic, and metal. Wood is very common because it is easy to shape and is inexpensive, however it can warp and deform easily. Wood also will wear quicker from the sand. Metal, on the other hand, is more expensive, but will last longer and has higher tolerances. The pattern can be reused to create the cavity for many molds of the same part. Therefore, a pattern that lasts longer will reduce tooling costs. A pattern for a part can be made many different ways, which are classified into the following four types:
- Solid pattern - A solid pattern is a model of the part as a single piece. It is the easiest to fabricate, but can cause some difficulties in making the mold. The parting line and runner system must be determined separately. Solid patterns are typically used for geometrically simple parts that are produced in low quantities.
- Split pattern - A split pattern models the part as two separate pieces that meet along the parting line of the mold. Using two separate pieces allows the mold cavities in the cope and drag to be made separately and the parting line is already determined. Split patterns are typically used for parts that are geometrically complex and are produced in moderate quantities.
- Match-plate pattern - A match-plate pattern is similar to a split pattern, except that each half of the pattern is attached to opposite sides of a single plate. The plate is usually made from wood or metal. This pattern design ensures proper alignment of the mold cavities in the cope and drag and the runner system can be included on the match plate. Match-plate patterns are used for larger production quantities and are often used when the process is automated.
- Cope and drag pattern - A cope and drag pattern is similar to a match plate pattern, except that each half of the pattern is attached to a separate plate and the mold halves are made independently. Just as with a match plate pattern, the plates ensure proper alignment of the mold cavities in the cope and drag and the runner system can be included on the plates. Cope and drag patterns are often desirable for larger castings, where a match-plate pattern would be too heavy and cumbersome. They are also used for larger production quantities and are often used when the process is automated.
Another piece of tooling used in sand casting is a core-box. If the casting requires sand cores, the cores are formed in these boxes, which are similar to a die and can be made of wood, plastic, or metal just like the pattern. The core-boxes can also contain multiple cavities to produce several identical cores.
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Sand casting is able to make use of almost any alloy. An advantage of sand casting is the ability to cast materials with high melting temperatures, including steel, nickel, and titanium. The four most common materials that are used in sand casting are shown below, along with their melting temperatures.
|Aluminum alloys||1220 °F (660 °C)|
|Brass alloys||1980 °F (1082 °C)|
|Cast iron||1990-2300 °F (1088-1260 °C)|
|Cast steel||2500 °F (1371 °C)|
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- Decrease the maximum wall thickness of a part to shorten the cycle time (cooling time specifically) and reduce the part volume
- Uniform wall thickness will ensure uniform cooling and reduce defects. A thick section, often referred to as a hot spot, causes uneven cooling and can result in shrinkage, porosity, or cracking.
- Round corners to reduce stress concentrations and fracture
- Inner radius should be at least the thickness of the walls
- Apply a draft angle of 2° - 3° to all walls parallel to the parting direction to facilitate removing the part from the mold.
- Add 0.0625 - 0.25 in. (0.16 - 0.64 mm) to part dimensions to allow for machining to obtain a smooth surface.
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The material cost for sand casting includes the cost of the metal, melting the metal, the mold sand, and the core sand. The cost of the metal is determined by the weight of the part, calculated from part volume and material density, as well the unit price of the material. The melting cost will also be greater for a larger part weight and is influenced by the material, as some materials are more costly to melt. However, the melting cost in typically insignificant compared to the metal cost. The amount of mold sand that is used, and hence the cost, is also proportional to the weight of the part. Lastly, the cost of the core sand is determined by the quantity and size of the cores used to cast the part.
The production cost includes a variety of operations used to cast the part, including core-making, mold-making, pouring, and cleaning. The cost of making the cores depends on the volume of the cores and the quantity used to cast the part. The cost of the mold-making is not greatly influenced by the part geometry when automated equipment is being used. However, the inclusion of cores will slightly slow the process and therefore increase the cost. Lastly, the cost of pouring the metal and cleaning the final casting are both driven by the weight of the part. It will take longer to pour and to clean a larger and heavier casting.
The tooling cost has two main
components - the pattern and the core-boxes. The pattern
cost is primarily controlled by the size of the part
(both the envelope
and the projected area) as well as the part's complexity.
The cost of the core-boxes first depends on their size,
a result of the quantity and size of the cores that are
used to cast the part. Much like the pattern, the
complexity of the cores will affect the time to
manufacture this part of the tooling (in addition to the
core size), and hence the cost.
The quantity of parts that are cast will also impact the tooling cost. A larger production quantity will require the use of a tooling material, for both the pattern and core-boxes, that will not wear under the required number of cycles. The use or a stronger, more durable, tooling material will significantly increase the cost.
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