Metal casting process using sand as the mold material

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. In , over 60% of all metal castings were produced via sand casting.[1]

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Molds made of sand are relatively cheap, and sufficiently refractory even for steel foundry use. In addition to the sand, a suitable bonding agent (usually clay) is mixed or occurs with the sand. The mixture is moistened, typically with water, but sometimes with other substances, to develop the strength and plasticity of the clay and to make the aggregate suitable for molding. The sand is typically contained in a system of frames or mold boxes known as a flask. The mold cavities and gate system are created by compacting the sand around models called patterns, by carving directly into the sand, or via 3D printing.

Basic process

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There are five steps in this process:

1. Place a pattern in sand to create a mold.
2. Incorporate the pattern and sand in a gating system. Remove the pattern.
3. Fill the mold cavity with molten metal.
4. Allow the metal to cool.
5. Break away the sand mold and remove the casting.

Components

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Patterns

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From the design, provided by a designer, a skilled pattern maker builds a pattern of the object to be produced, using wood, metal, or a plastic such as expanded polystyrene. Sand can be ground, swept or strickled into shape. The metal to be cast will contract during solidification, and this may be non-uniform due to uneven cooling. Therefore, the pattern must be slightly larger than the finished product, a difference known as contraction allowance. Different scaled rules are used for different metals, because each metal and alloy contracts by an amount distinct from all others. Patterns also have core prints that create registers within the molds into which are placed sand cores. Such cores, sometimes reinforced by wires, are used to create under-cut profiles and cavities which cannot be molded with the cope and drag, such as the interior passages of valves or cooling passages in engine blocks.

Paths for the entrance of metal into the mold cavity constitute the runner system and include the sprue, various feeders which maintain a good metal 'feed', and in-gates which attach the runner system to the casting cavity. Gas and steam generated during casting exit through the permeable sand or via risers,[note 1] which are added either in the pattern itself, or as separate pieces.

In addition to patterns, the sand molder could also use tools to create the holes.

Molding box and materials

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A multi-part molding box (known as a casting flask, the top and bottom halves of which are known respectively as the cope and drag) is prepared to receive the pattern. Molding boxes are made in segments that may be latched to each other and to end closures. For a simple object—flat on one side—the lower portion of the box, closed at the bottom, will be filled with a molding sand. The sand is packed in through a vibratory process called ramming, and in this case, periodically screeded level. The surface of the sand may then be stabilized with a sizing compound. The pattern is placed on the sand and another molding box segment is added. Additional sand is rammed over and around the pattern. Finally a cover is placed on the box and it is turned and unlatched, so that the halves of the mold may be parted and the pattern with its sprue and vent patterns removed. Additional sizing may be added and any defects introduced by the removal of the pattern are corrected. The box is closed again. This forms a "green" mold which must be dried to receive the hot metal. If the mold is not sufficiently dried a steam explosion can occur that can throw molten metal about. In some cases, the sand may be oiled instead of moistened, which makes casting possible without waiting for the sand to dry. Sand may also be bonded by chemical binders, such as furane resins or amine-hardened resins.

Additive manufacturing (AM) can be used in the sand mold preparation, so that instead of the sand mold being formed via packing sand around a pattern, it is 3D-printed. This can reduce lead times for casting by obviating patternmaking.[3] Besides replacing older methods, additive can also complement them in hybrid models, such as making a variety of AM-printed cores for a cavity derived from a traditional pattern.[3]

Chills

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To control the solidification structure of the metal, it is possible to place metal plates, chills, in the mold. The associated rapid local cooling will form a finer-grained structure and may form a somewhat harder metal at these locations. In ferrous castings, the effect is similar to quenching metals in forge work. The inner diameter of an engine cylinder is made hard by a chilling core. In other metals, chills may be used to promote directional solidification of the casting. In controlling the way a casting freezes, it is possible to prevent internal voids or porosity inside castings.

Cores

[edit] Main article: Core (manufacturing)

Cores are apparatus used to generate hollow cavities or internal features which cannot be formed using pattern alone in molding, cores are usually made using sand, but some processes also use permanent cores made of metal.

To produce cavities within the casting—such as for liquid cooling in engine blocks and cylinder heads—negative forms are used to produce cores. Usually sand-molded, cores are inserted into the casting box after removal of the pattern. Whenever possible, designs are made that avoid the use of cores, due to the additional set-up time, mass and thus greater cost.

With a completed mold at the appropriate moisture content, the box containing the sand mold is then positioned for filling with molten metal—typically iron, steel, bronze, brass, aluminium, magnesium alloys, or various pot metal alloys, which often include lead, tin, and zinc. After being filled with liquid metal the box is set aside until the metal is sufficiently cool to be strong. The sand is then removed, revealing a rough casting that, in the case of iron or steel, may still be glowing red. In the case of metals that are significantly heavier than the casting sand, such as iron or lead, the casting flask is often covered with a heavy plate to prevent a problem known as floating the mold. Floating the mold occurs when the pressure of the metal pushes the sand above the mold cavity out of shape, causing the casting to fail.

After casting, the cores are broken up by rods or shot and removed from the casting. The metal from the sprue and risers is cut from the rough casting. Various heat treatments may be applied to relieve stresses from the initial cooling and to add hardness—in the case of steel or iron, by quenching in water or oil. The casting may be further strengthened by surface compression treatment—like shot peening—that adds resistance to tensile cracking and smooths the rough surface. And when high precision is required, various machining operations (such as milling or boring) are made to finish critical areas of the casting. Examples of this would include the boring of cylinders and milling of the deck on a cast engine block.

Design requirements

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The part to be made and its pattern must be designed to accommodate each stage of the process, as it must be possible to remove the pattern without disturbing the molding sand and to have proper locations to receive and position the cores. A slight taper, known as draft, must be used on surfaces perpendicular to the parting line, in order to be able to remove the pattern from the mold. This requirement also applies to cores, as they must be removed from the core box in which they are formed. The sprue and risers must be arranged to allow a proper flow of metal and gasses within the mold in order to avoid an incomplete casting. Should a piece of core or mold become dislodged it may be embedded in the final casting, forming a sand pit, which may render the casting unusable. Gas pockets can cause internal voids. These may be immediately visible or may only be revealed after extensive machining has been performed. For critical applications, or where the cost of wasted effort is a factor, non-destructive testing methods may be applied before further work is performed.

Processes

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In general, we can distinguish between two methods of sand casting; the first one using green sand and the second being the air set method.

Green sand

[edit] Further information: Molding sand § Green sand

These castings are made using sand molds formed from "wet" sand which contains water and organic bonding compounds, typically referred to as clay. The name "green sand" comes from the fact that the sand mold is not "set", it is still in the "green" or uncured state even when the metal is poured in the mold. Green sand is not green in color, but "green" in the sense that it is used in a wet state (akin to green wood). Contrary to what the name suggests, "green sand" is not a type of sand on its own (that is, not greensand in the geologic sense), but is rather a mixture of:

• silica sand (SiO2), chromite sand (FeCr2O4), or zircon sand (ZrSiO4), 75 to 85%, sometimes with a proportion of olivine, staurolite, or graphite.
• bentonite (clay), 5 to 11%
• water, 2 to 4%
• inert sludge 3 to 5%
• anthracite (0 to 1%)

There are many recipes for the proportion of clay, but they all strike different balances between moldability, surface finish, and ability of the hot molten metal to degas. Coal, typically referred to in foundries as sea-coal, which is present at a ratio of less than 5%, partially combusts in the presence of the molten metal, leading to offgassing of organic vapors. Green sand casting for non-ferrous metals does not use coal additives, since the CO created does not prevent oxidation. Green sand for aluminum typically uses olivine sand (a mixture of the minerals forsterite and fayalite, which is made by crushing dunite rock).

The choice of sand has a lot to do with the temperature at which the metal is poured. At the temperatures that copper and iron are poured, the clay is inactivated by the heat, in that the montmorillonite is converted to illite, which is a non-expanding clay. Most foundries do not have the very expensive equipment to remove the burned out clay and substitute new clay, so instead, those that pour iron typically work with silica sand that is inexpensive compared to the other sands. As the clay is burned out, newly mixed sand is added and some of the old sand is discarded or recycled into other uses. Silica is the least desirable of the sands, since metamorphic grains of silica sand have a tendency to explode to form sub-micron sized particles when thermally shocked during pouring of the molds. These particles enter the air of the work area and can lead to silicosis in the workers. Iron foundries expend considerable effort on aggressive dust collection to capture this fine silica. Various types of respiratory-protective equipment are also used in foundries.[4][5]

The sand also has the dimensional instability associated with the conversion of quartz from alpha quartz to beta quartz at 680 °C ( °F). Often, combustible additives such as wood flour are added to create spaces for the grains to expand without deforming the mold. Olivine, chromite, etc. are therefore used because they do not have a phase transition that causes rapid expansion of the grains. Olivine and chromite also offer greater density, which cools the metal faster, thereby producing finer grain structures in the metal. Since they are not metamorphic minerals, they do not have the polycrystals found in silica, and subsequently they do not form hazardous sub-micron sized particles.

"Air set" method

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The air set method uses dry sand bonded with materials other than clay, using a fast curing adhesive. The latter may also be referred to as no bake mold casting. When these are used, they are collectively called "air set" sand castings to distinguish them from "green sand" castings. Two types of molding sand are natural bonded (bank sand) and synthetic (lake sand); the latter is generally preferred due to its more consistent composition.

With both methods, the sand mixture is packed around a pattern, forming a mold cavity. If necessary, a temporary plug is placed in the sand and touching the pattern in order to later form a channel into which the casting fluid can be poured. Air-set molds are often formed with the help of a casting flask having a top and bottom part, termed the cope and drag. The sand mixture is tamped down as it is added around the pattern, and the final mold assembly is sometimes vibrated to compact the sand and fill any unwanted voids in the mold. Then the pattern is removed along with the channel plug, leaving the mold cavity. The casting liquid (typically molten metal) is then poured into the mold cavity. After the metal has solidified and cooled, the casting is separated from the sand mold. There is typically no mold release agent, and the mold is generally destroyed in the removal process.[6]

The accuracy of the casting is limited by the type of sand and the molding process. Sand castings made from coarse green sand impart a rough texture to the surface, and this makes them easy to identify. Castings made from fine green sand can shine as cast but are limited by the depth to width ratio of pockets in the pattern. Air-set molds can produce castings with smoother surfaces than coarse green sand but this method is primarily chosen when deep narrow pockets in the pattern are necessary, due to the expense of the plastic used in the process. Air-set castings can typically be easily identified by the burnt color on the surface. The castings are typically shot blasted to remove that burnt color. Surfaces can also be later ground and polished, for example when making a large bell. After molding, the casting is covered with a residue of oxides, silicates and other compounds. This residue can be removed by various means, such as grinding, or shot blasting.

During casting, some of the components of the sand mixture are lost in the thermal casting process. Green sand can be reused after adjusting its composition to replenish the lost moisture and additives. The pattern itself can be reused indefinitely to produce new sand molds. The sand molding process has been used for many centuries to produce castings manually. Since , partially automated casting processes have been developed for production lines.

Cold box

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Cold box uses organic and inorganic binders that strengthen the mold by chemically adhering to the sand. This type of mold gets its name from not being baked in an oven like other sand mold types. This type of mold is more accurate dimensionally than green-sand molds but is more expensive. Thus it is used only in applications that necessitate it.

No-bake molds

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No-bake molds are expendable sand molds, similar to typical sand molds, except they also contain a quick-setting liquid resin and catalyst. Rather than being rammed, the molding sand is poured into the flask and held until the resin solidifies, which occurs at room temperature. This type of molding also produces a better surface finish than other types of sand molds.[7] Because no heat is involved it is called a cold-setting process. Common flask materials that are used are wood, metal, and plastic. Common metals cast into no-bake molds are brass, iron (ferrous), and aluminum alloys.

Vacuum molding

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Vacuum molding (V-process) is a variation of the sand casting process for most ferrous and non-ferrous metals,[8] in which unbonded sand is held in the flask with a vacuum. The pattern is specially vented so that a vacuum can be pulled through it. A heat-softened thin sheet (0.003 to 0.008 in (0.076 to 0.203 mm)) of plastic film is draped over the pattern and a vacuum is drawn (200 to 400 mmHg (27 to 53 kPa)). A special vacuum forming flask is placed over the plastic pattern and is filled with a free-flowing sand. The sand is vibrated to compact the sand and a sprue and pouring cup are formed in the cope. Another sheet of plastic is placed over the top of the sand in the flask and a vacuum is drawn through the special flask; this hardens and strengthens the unbonded sand. The vacuum is then released on the pattern and the cope is removed. The drag is made in the same way (without the sprue and pouring cup). Any cores are set in place and the mold is closed. The molten metal is poured while the cope and drag are still under a vacuum, because the plastic vaporizes but the vacuum keeps the shape of the sand while the metal solidifies. When the metal has solidified, the vacuum is turned off and the sand runs out freely, releasing the casting.[9][10]

The V-process is known for not requiring a draft because the plastic film has a certain degree of lubricity and it expands slightly when the vacuum is drawn in the flask. The process has high dimensional accuracy, with a tolerance of ±0.010 in for the first inch and ±0.002 in/in thereafter. Cross-sections as small as 0.090 in (2.3 mm) are possible. The surface finish is very good, usually between 150 and 125 rms. Other advantages include no moisture related defects, no cost for binders, excellent sand permeability, and no toxic fumes from burning the binders. Finally, the pattern does not wear out because the sand does not touch it. The main disadvantage is that the process is slower than traditional sand casting so it is only suitable for low to medium production volumes; approximately 10 to 15,000 pieces a year. However, this makes it perfect for prototype work, because the pattern can be easily modified as it is made from plastic.[9][10][11]

Fast mold making processes

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With the fast development of the car and machine building industry the casting consuming areas called for steady higher productivity. The basic process stages of the mechanical molding and casting process are similar to those described under the manual sand casting process. The technical and mental development however was so rapid and profound that the character of the sand casting process changed radically.

Mechanized sand molding

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The first mechanized molding lines consisted of sand slingers and/or jolt-squeeze devices that compacted the sand in the flasks. Subsequent mold handling was mechanical using cranes, hoists and straps. After core setting the copes and drags were coupled using guide pins and clamped for closer accuracy. The molds were manually pushed off on a roller conveyor for casting and cooling.

Automatic high pressure sand molding lines

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Increasing quality requirements made it necessary to increase the mold stability by applying steadily higher squeeze pressure and modern compaction methods for the sand in the flasks. In early fifties the high pressure molding was developed and applied in mechanical and later automatic flask lines. The first lines were using jolting and vibrations to pre-compact the sand in the flasks and compressed air powered pistons to compact the molds.

Horizontal sand flask molding

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In the first automatic horizontal flask lines the sand was shot or slung down on the pattern in a flask and squeezed with hydraulic pressure of up to 140 bars. The subsequent mold handling including turn-over, assembling, pushing-out on a conveyor were accomplished either manually or automatically. In the late fifties hydraulically powered pistons or multi-piston systems were used for the sand compaction in the flasks. This method produced much more stable and accurate molds than it was possible manually or pneumatically. In the late sixties mold compaction by fast air pressure or gas pressure drop over the pre-compacted sand mold was developed (sand-impulse and gas-impact). The general working principle for most of the horizontal flask line systems is shown on the sketch below.

Today there are many manufacturers of the automatic horizontal flask molding lines. The major disadvantages of these systems is high spare parts consumption due to multitude of movable parts, need of storing, transporting and maintaining the flasks and productivity limited to approximately 90–120 molds per hour.

Vertical sand flaskless molding

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In , Dansk Industri Syndikat A/S (DISA-DISAMATIC) invented a flask-less molding process by using vertically parted and poured molds. The first line could produce up to 240 complete sand molds per hour. Today molding lines can achieve a molding rate of 550 sand molds per hour and requires only one monitoring operator. Maximum mismatch of two mold halves is 0.1 mm (0. in). Although very fast, vertically parted molds are not typically used by jobbing foundries due to the specialized tooling needed to run on these machines. Cores need to be set with a core mask as opposed to by hand and must hang in the mold as opposed to being set on parting surface.

Matchplate sand molding

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The principle of the matchplate, meaning pattern plates with two patterns on each side of the same plate, was developed and patented in , fostering the perspectives for future sand molding improvements. However, first in the early sixties the American company Hunter Automated Machinery Corporation launched its first automatic flaskless, horizontal molding line applying the matchplate technology.

The method alike to the DISA's (DISAMATIC) vertical molding is flaskless, however horizontal. The matchplate molding technology is today used widely. Its great advantage is inexpensive pattern tooling, easiness of changing the molding tooling, thus suitability for manufacturing castings in short series so typical for the jobbing foundries. Modern matchplate molding machine is capable of high molding quality, less casting shift due to machine-mold mismatch (in some cases less than 0.15 mm (0. in)), consistently stable molds for less grinding and improved parting line definition. In addition, the machines are enclosed for a cleaner, quieter working environment with reduced operator exposure to safety risks or service-related problems.

Safety standards

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With automated mold manufacturing came additional workplace safety requirements. Different voluntary technical standards apply depending on the geopolitical jurisdiction where the machinery is to be used.

Canada

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Canada does not have a machine-specific voluntary technical standard for sand-mold making machinery. This type of machinery is covered by:

Safeguarding of machinery, CSA Z432. Canadian Standards Association. .

In addition, the electrical safety requirements are covered by:

Industrial Electrical Machinery, CSA C22.2 No. 301. .

European Union

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The primary standard for sand-mold manufacturing equipment in the EU is: Safety requirements for foundry moulding and coremaking machinery and plant associated equipment, EN 710. European Committee for Standardization (CEN).

EN 710 will need to be used in conjunction with EN -1 for electrical safety, and EN ISO -1 and EN ISO -2 or EN for functional safety. Additional type C standards may also be necessary for conveyors, robotics or other equipment that may be needed to support the operation of the mold-making equipment.

United States

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There is no machine-specific standard for sand-mold manufacturing equipment. The ANSI B11 family of standards includes some generic machine-tool standards that could be applied to this type of machinery, including:

• Safety of Machinery, ANSI B11.0. American National Standards Institute (ANSI). .[12]
• Performance Requirements for Risk Reduction Measures: Safeguarding and other Means of Reducing Risk, ANSI B11.19. American National Standards Institute (ANSI). .
• Safety Requirements for the Integration of Machinery into a System, ANSI B11.20. American National Standards Institute (ANSI). .
• Safety Requirements for Transfer Machines, ANSI B11.24. American National Standards Institute (ANSI). (R).
• Functional Safety for Equipment (Electrical/Fluid Power Control Systems) General Principles for the Design of Safety Control Systems Using ISO -1, ANSI B11.26. American National Standards Institute (ANSI). .
• Sound Level Measurement Guidelines, ANSI B11.TR5. American National Standards Institute (ANSI). (R).

Mold materials

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There are four main components for making a sand casting mold: base sand, a binder, additives, and a parting compound.

Molding sands

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Molding sands, also known as foundry sands, are defined by eight characteristics: refractoriness, chemical inertness, permeability, surface finish, cohesiveness, flowability, collapsibility, and availability/cost.[13]

Refractoriness — This refers to the sand's ability to withstand the temperature of the liquid metal being cast without breaking down. For example, some sands only need to withstand 650 °C (1,202 °F) if casting aluminum alloys, whereas steel needs a sand that will withstand 1,500 °C (2,730 °F). Sand with too low refractoriness will melt and fuse to the casting.[13]

Chemical inertness — The sand must not react with the metal being cast. This is especially important with highly reactive metals, such as magnesium and titanium.[13]

Permeability — This refers to the sand's ability to exhaust gases. This is important because during the pouring process many gases are produced, such as hydrogen, nitrogen, carbon dioxide, and steam, which must leave the mold otherwise casting defects, such as blow holes and gas holes, occur in the casting. Note that for each cubic centimeter (cc) of water added to the mold cc of steam is produced.[13]

Surface finish — The size and shape of the sand particles defines the best surface finish achievable, with finer particles producing a better finish. However, as the particles become finer (and surface finish improves) the permeability becomes worse.[13]

Cohesiveness (or bond) — This is the ability of the sand to retain a given shape after the pattern is removed.[14]

Flowability – The ability for the sand to flow into intricate details and tight corners without special processes or equipment.[15]

Collapsibility — This is the ability of the sand to be easily stripped off the casting after it has solidified. Sands with poor collapsibility will adhere strongly to the casting. When casting metals that contract a lot during cooling or with long freezing temperature ranges a sand with poor collapsibility will cause cracking and hot tears in the casting. Special additives can be used to improve collapsibility.[15]

Availability/cost — The availability and cost of the sand is very important because for every ton of metal poured, three to six tons of sand is required.[15] Although sand can be screened and reused, the particles eventually become too fine and require periodic replacement with fresh sand.[16]

In large castings it is economical to use two different sands, because the majority of the sand will not be in contact with the casting, so it does not need any special properties. The sand that is in contact with the casting is called facing sand, and is designed for the casting on hand. This sand will be built up around the pattern to a thickness of 30 to 100 mm (1.2 to 3.9 in). The sand that fills in around the facing sand is called backing sand. This sand is simply silica sand with only a small amount of binder and no special additives.[17]

Types of base sands

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Base sand is the type used to make the mold or core without any binder. Because it does not have a binder it will not bond together and is not usable in this state.[15]

Silica sand

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Silica (SiO2) sand is the sand found on a beach and is also the most commonly used sand. It is either made by crushing sandstone or taken from natural occurring locations, such as beaches and river beds. The fusion point of pure silica is 1,760 °C (3,200 °F), however the sands used have a lower melting point due to impurities. For high melting point casting, such as steels, a minimum of 98% pure silica sand must be used; however for lower melting point metals, such as cast iron and non-ferrous metals, a lower purity sand can be used (between 94 and 98% pure).[15]

Silica sand is the most commonly used sand because of its great abundance, and, thus, low cost (therein being its greatest advantage). Its disadvantages are high thermal expansion, which can cause casting defects with high melting point metals, and low thermal conductivity, which can lead to unsound casting. It also cannot be used with certain basic metals because it will chemically interact with the metal, forming surface defects. Finally, it releases silica particulates during the pour, risking silicosis in foundry workers.[18]

Olivine sand

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Olivine is a mixture of orthosilicates of iron and magnesium from the mineral dunite. Its main advantage is that it is free from silica, therefore it can be used with basic metals, such as manganese steels. Other advantages include a low thermal expansion, high thermal conductivity, and high fusion point. Finally, it is safer to use than silica, therefore it is popular in Europe.[18]

Chromite sand

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Chromite sand is a solid solution of spinels. Its advantages are a low percentage of silica, a very high fusion point (1,850 °C (3,360 °F)), and a very high thermal conductivity. Its disadvantage is its costliness, therefore it is only used with expensive alloy steel casting and to make cores.[18]

Zircon sand

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Zircon sand is a compound of approximately two-thirds zirconium oxide (ZrO2) and one-third silica. It has the highest fusion point of all the base sands at 2,600 °C (4,710 °F), a very low thermal expansion, and a high thermal conductivity. Because of these good properties it is commonly used when casting alloy steels and other expensive alloys. It is also used as a mold wash (a coating applied to the molding cavity) to improve surface finish. However, it is expensive and not readily available.[18]

Chamotte sand

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Chamotte is made by calcining fire clay (Al2O3-SiO2) above 1,100 °C (2,010 °F). Its fusion point is 1,750 °C (3,180 °F) and has low thermal expansion. It is the second cheapest sand, however it is still twice as expensive as silica. Its disadvantages are very coarse grains, which result in a poor surface finish, and it is limited to dry sand molding. Mold washes are used to overcome the surface finish problems. This sand is usually used when casting large steel workpieces.[18][19]

Binders

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Binders are added to a base sand to bond the sand particles together (i.e. it is the glue that holds the mold together).

Clay and water

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A mixture of clay and water is the most commonly used binder. There are two types of clay commonly used: bentonite and kaolinite, with the former being the most common.[20]

Oil

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Oils, such as linseed oil, other vegetable oils and marine oils, used to be used as a binder, however due to their increasing cost, they have been mostly phased out. The oil also required careful baking at 100 to 200 °C (212 to 392 °F) to cure (if overheated, the oil becomes brittle, wasting the mold).[21]

Resin

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Resin binders are natural or synthetic high melting point gums. The two common types used are urea formaldehyde (UF) and phenol formaldehyde (PF) resins. PF resins have a higher heat resistance than UF resins and cost less. There are also cold-set resins, which use a catalyst instead of a heat to cure the binder. Resin binders are quite popular because different properties can be achieved by mixing with various additives. Other advantages include good collapsibility, low gassing, and they leave a good surface finish on the casting.[21]

MDI (methylene diphenyl diisocyanate) is also a commonly used binder resin in the foundry core process.

Sodium silicate

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Water glass ( sodium silicate [Na2SiO3 or (Na2O)(SiO2)] ) is a high strength binder used with silica molding sand both for cores and molds.[22]: 69–70  To cure a mixture of finely ground sand (e.g. by using a sand muller) and 3 to 4% of sodium silicate the binder, carbon dioxide (CO2) gas is used.[22]: 69–70  The mixture is exposed to the gas at ambient temperature reacting as following:[22]: 69–70 

Na 2 O ( SiO 2 ) + CO 2 ↽ − − ⇀ Na 2 CO 3 + 2 SiO 2 + Heat {\displaystyle {\ce {{Na2O(SiO2)}+ CO2 <=> {Na2CO3}+ {2SiO2}+ Heat}}}

The advantage to this binder is that it can be used at room temperature and is fast. The disadvantage is that its high strength leads to shakeout difficulties and possibly hot tears (probably due to quartz inversion[citation needed]) in the casting.[21][22]: 70  The mixed sodium silicate and sand may also be heated by a heat gun to achieve better rigideness.

Additives

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Additives are added to the molding components to improve: surface finish, dry strength, refractoriness, and "cushioning properties".

Up to 5% of reducing agents, such as coal powder, pitch, creosote, and fuel oil, may be added to the molding material to prevent wetting (prevention of liquid metal sticking to sand particles, thus leaving them on the casting surface), improve surface finish, decrease metal penetration, and burn-on defects. These additives achieve this by creating gases at the surface of the mold cavity, which prevent the liquid metal from adhering to the sand. Reducing agents are not used with steel casting, because they can carburize the metal during casting.[23]

Up to 3% of "cushioning material", such as wood flour, sawdust, powdered husks, peat, and straw, can be added to reduce scabbing, hot tear, and hot crack casting defects when casting high temperature metals. These materials are beneficial because burn-off when the metal is poured creates tiny voids in the mold, allowing the sand particles to expand. They also increase collapsibility and reduce shakeout time.[23]

Up to 2% of cereal binders, such as dextrin, starch, sulphite lye, and molasses, can be used to increase dry strength (the strength of the mold after curing) and improve surface finish. Cereal binders also improve collapsibility and reduce shakeout time because they burn off when the metal is poured. The disadvantage to cereal binders is that they are expensive.[23]

Up to 2% of iron oxide powder can be used to prevent mold cracking and metal penetration, essentially improving refractoriness. Silica flour (fine silica) and zircon flour also improve refractoriness, especially in ferrous castings. The disadvantages to these additives is that they greatly reduce permeability.[23]

Parting compounds

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To get the pattern out of the mold, prior to casting, a parting compound is applied to the pattern to ease removal. They can be a liquid or a fine powder (particle diameters between 75 and 150 micrometres (0. and 0. in)). Common powders include talc, graphite, and dry silica; common liquids include mineral oil and water-based silicon solutions. The latter are more commonly used with metal and large wooden patterns.[24]

History

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Clay molds were used in ancient China since the Shang dynasty (c.  to BC). The famous Houmuwu ding (c. BC) was made using clay molding.

The Assyrian king Sennacherib (704–681 BC) cast massive bronzes of up to 30 tonnes, and claims to have been the first to have used clay molds rather than the "lost-wax" method:[25]

Whereas in former times the kings my forefathers had created bronze statues imitating real-life forms to put on display inside their temples, but in their method of work they had exhausted all the craftsmen, for lack of skill and failure to understand the principles they needed so much oil, wax and tallow for the work that they caused a shortage in their own countries—I, Sennacherib, leader of all princes, knowledgeable in all kinds of work, took much advice and deep thought over doing that work. Great pillars of bronze, colossal striding lions, such as no previous king had ever constructed before me, with the technical skill that Ninushki brought to perfection in me, and at the prompting of my intelligence and the desire of my heart I invented a technique for bronze and made it skillfully. I created clay moulds as if by divine intelligence....twelve fierce lion-colossi together with twelve mighty bull-colossi which were perfect castings... I poured copper into them over and over again; I made the castings as skillfully as if they had only weighed half a shekel each

In , Ismail al-Jazari first described the casting of metals in closed mold boxes with sand.[26][27] Sand casting molding method was recorded by Vannoccio Biringuccio in his book published around .

In , the Ford Motor Company set a record by producing 1 million cars, in the process consuming one-third of the total casting production in the U.S. As the automobile industry grew the need for increased casting efficiency grew. The increasing demand for castings in the growing car and machine building industry during and after World War I and World War II, stimulated new inventions in mechanization and later automation of the sand casting process technology.

There was not one bottleneck to faster casting production but rather several. Improvements were made in molding speed, molding sand preparation, sand mixing, core manufacturing processes, and the slow metal melting rate in cupola furnaces. In , the sand slinger was invented by the American company Beardsley & Piper. In , the first sand mixer with individually mounted revolving plows was marketed by the Simpson Company. In , the first experiments started with bentonite clay instead of simple fire clay as the bonding additive to the molding sand. This increased tremendously the green and dry strength of the molds. In , the first fully automated foundry for fabricating hand grenades for the U.S. Army went into production. In the s the first high-frequency coreless electric furnace was installed in the U.S. In , ductile iron was invented by adding magnesium to the widely used grey iron. In , thermal sand reclamation was applied for molding and core sands. In , the "D-process" was developed for making shell molds with fine, pre-coated sand. In , the hotbox core sand process in which the cores are thermally cured was invented. In , a new core binder—water glass (sodium silicate), hardened with CO2 from ambient air, came out

In the s, additive manufacturing began to be applied to sand mold preparation in commercial production; instead of the sand mold being formed via packing sand around a pattern, it is 3D-printed.

See also

[edit]

• Casting – Manufacturing process in which a liquid is poured into a mold to solidify
• Veining (metallurgy) – Metallurgical casting defect, common sand casting defect
• Foundry sand testing
• Hand mold
• Sand rammer
• Juutila Foundry – Finnish bell foundry (Finland), est. , specialized in sand casting
• voxeljet – German manufacturer of 3D printers (Germany), 3D printing,

References

[edit]

Notes

[edit]

Bibliography

[edit]

• Degarmo, E. Paul; Black, J T.; Kohser, Ronald A. (), Materials and Processes in Manufacturing (9th ed.), Wiley, ISBN 0-471--4.
• Todd, Robert H.; Allen, Dell K.; Alting, Leo (), Manufacturing Processes Reference Guide, Industrial Press Inc., ISBN 0---0.
• Rao, T. V. (), Metal Casting: Principles and Practice, New Age International, ISBN 978-81-224--0.

Sand Casting: Types and Applications

Chapter 1: Understanding the Construction and Techniques of Sand Casting

Dating back to approximately BCE, sand casting ranks as one of the oldest methods of metal casting. Over time, developments in process management, materials, and design nuances have minimized flaws and increased its adaptability, making it the most popular and versatile metal casting technique used in contemporary industries.

Understanding Sand Casting

Sand casting involves pouring molten metal into a sand mold shaped in the desired form. Once the metal cools and takes the required shape, the sand mold is destroyed and removed. Materials traditionally used for sand casting range from metals to concrete, epoxy, plaster, and even clay.

Casting generally refers to the manufacturing process where liquid material fills a mold that contains the hollow shape of the desired product. The discussion here focuses on sand casting, which stands out for its ability in producing diverse sizes, weights, and complexity levels using a wide array of metals. Leveraging sand as the primary mold material significantly reduces overall costs compared to metal mold casting, which is often hindered by expensive mold machining.

The choice of sand largely depends on its reusability. Green sand casting enables mold reuse, in contrast to dry sand casting, where sand is often discarded post-use.

With its capacity to handle metals having high melting points, such as titanium, steels, and nickel, sand casting remains invaluable for industries like aerospace and automotive, which require cost-effective small-batch production.

Core Components of a Sand Casting Mold

Constructing a sand casting mold usually involves four primary components:

Fundamental Base Sand

The base sand creates the mold's backbone and is utilized in a pure form. A binder is added to ensure cohesion. Cores, also crafted from base sand, are indispensable in this process. Common varieties include:

• Silica sand
• Olivine sand
• Chromite sand
• Zircon sand
• Chamotte sand

Binders or Binding Agents

Binders serve as the glue holding sand particles intact. Common types include:

The company is the world’s best resin sand casting services supplier. We are your one-stop shop for all needs. Our staff are highly-specialized and will help you find the product you need.

• Clay and water
• Oil
• Resin
• Sodium silicate

Enhancements with Additives

Additives improve mold properties like surface finish, strength, heat resistance, and cushioning.

Parting Compounds

Parting compounds, powders, or liquids facilitate the removal of the pattern from the mold.

Sand Casting Methods

Beginning in China around BCE, sand casting emerged to create statues, decorations, and artifacts, later gaining prominence with airplanes and automobiles in the 20th century for producing precisely accurate parts. Recently, these antique techniques have evolved to address modern demands in the manufacturing of components.

It is assessed that 70% of metal castings worldwide are accomplished through sand casting, indispensable for fabricating engine blocks, cylinder heads, pump housings, valve bodies, and gearboxes.

Bedding-In Sand Casting

The 'bedding-in approach' is a method used for crafting a firm cylindrical mold. Sand is initially poured into the drag and rammed until the pattern is firmly pressed, with tightly compacted sand surrounding it. If any soft spots surface, further ramming ensures firmness. Bedding-in suits pit molding for larger molds as well.

False Cope Sand Casting

The false cope process creates solid cylindrical molds, paying attention to compacting sand beneath the pattern for a refined parting surface. The cope, dusted with separating sand, is gripped and rolled onto a sand bed before conducting standard ramming with a 'false cope' acting as a temporary block for forming the drag.

Flat Back Sand Casting

Flat back casting forms the mold cavity on either the drag or cope side or both. After arranging the pattern and sand, gates and sprues guide molten metal entry. Once separated, cope and drag reassembly prevents movement, ensuring consistent pouring and cooling until the part is cooled and finished.

Skin Dried Sand Casting

Skin dried sand casting incorporates a layer of dried fine-grained sand over the mold, producing a smooth finish. This refined surface is ideal for parts with rigorous finishing needs, suitable for sectors like aerospace and engine components.'

Water Glass or Sodium Silicate Sand Casting

In this method, CO2 solidifies sodium silicate-mixed sand, allowing intricate casting. Despite producing a rough finish, water glass casting is economical, demanding minimal equipment, making it an option for complex part formation.

Vacuum Sand Casting

Vacuum casting, or the V-process, employs plastic sheets and vacuum techniques for precision casting and exceptional finish. This innovation minimizes moisture flaws, binder costs, and harmful emissions, enhancing manufacturing tolerance and surface quality.

Shell Sand Casting

Shell casting, with resin-coated mold sand, achieves a high degree of accuracy and efficiency, making it suitable for mass production of detailed components. Although costlier than traditional methods, it streamlines production, reducing waste and eliminating post-production work.

The Sand Casting Process

Executing sand casting involves varying crucial stages:

Designing the Pattern – Desired Product

A reusable pattern sets the foundation by replicating the final component. Accounting for shrinkage, riser provisioning, and subsequent contraction ensures accurate dimensions post-cooling. Draft allowance simplifies removal, while machining allowance prepares for any final refinements.

Pattern Creation – Delivery System for Metal: Gates and Risers

Patterns incorporate channels for metal flow, allowing sufficient gating and riser provision. Crafted in wood, metal, EPS, or synthetics, patterns adapt based on volume and tolerance, providing structure for complex or hollow designs via additional cores.

Core Making

Cores form integral features or cavities that molds cannot, and require particular strength, hardness, and permeability to navigate molten material without distortion or gas retention.

Creating the Mold

Sourced from refractory material like sand, enhanced by binders, and supplemented with cores, molds accommodate design flexibility and cope with high pouring temperatures. Ensuring resilience while maintaining brittleness for easy removal after solidifying, molds meet diverse demands.

Pouring Metals into Molds

Pouring metal meticulously ensures proper flow, with risers counteracting shrinkage for flawless parts. Argon shielding guards reactive alloys, upholding casting quality across varying alloys with a preference for stable metal transition.

Shakeout Process

The shakeout stage disassembles the casting, salvaging mold sand for reuse. Careful reclamation reconditioned for lengthy utility ensures defense against future waste.

Concluding Casting Operations

Upon finishing, unwanted features like gates and risers undergo removal, followed by sandblasting or other interventions if necessary. Machining and heat treatment ensure specifications are met, with NDT methods verifying final quality standards before distribution.

Chapter 2: Who are the Leading Manufacturers of Sand Casting Machines?

The objective of the sand casting process is to produce metal parts and components with high precision, tight tolerance capabilities, and consistent quality. To achieve these goals, leading manufacturers have engineered advanced sand casting machines designed for versatility, efficiency, and reliable performance. Selecting the right sand casting equipment is a critical step for foundries, manufacturers, and metal casting facilities looking to optimize production, reduce defects, and ensure cost-effective operations. Below, we highlight five top-rated brands of sand casting machines widely used in the United States and Canada. Each listing includes specific molding machine models, distinctive features, and key advantages for industrial users in automotive, aerospace, machinery, and general metalworking sectors.

Hunter Foundry Machinery Corporation

Model: HLM Series Molding Machines

Features: The Hunter HLM Series sets a high standard in sand molding technology, known for its high-speed and high-pressure molding capabilities. With a horizontal parted, automatic molding machine design, these machines deliver exceptional mold integrity, minimizing the likelihood of casting defects and porosity. Integrated advanced PLC control systems ensure precise sand compaction and uniformity, critical for repeatability and dimensional accuracy. Energy-efficient operation helps reduce operating costs, while the HLM Series’ flexibility in adjustable molding sizes and quick pattern changes enhances productivity and supports a broad range of casting applications, from gray iron to non-ferrous metals.

Sinto America

Model: FBO Flaskless Molding Machine

Features: The Sinto FBO Flaskless Molding Machine delivers innovative automated sand casting by eliminating traditional flask handling, resulting in streamlined production and reduced labor costs. Its automated functions cover every step: mold making, sand compaction, stripping, and mold transfer. The user-friendly touchscreen interface combined with programmable casting parameters ensures operational efficiency and repeatable results. Designed for minimal downtime, the FBO features robust construction for longevity and is ideal for foundries seeking flexibility for jobbing and high-volume production with various sand casting alloys and pattern complexities.

DISA Group

Model: DISAMATIC C Line Molding Machine

Features: The DISAMATIC C Line is renowned globally for its high-speed, high-volume vertical molding technology, using a matchplate system for extreme dimensional precision and fast cycle times. These automatic sand casting machines incorporate advanced control systems for consistent sand compaction, accurate pattern alignment, and minimum sand core shift—crucial for complex castings in automotive and heavy machinery applications. Automatic mold handling, minimal manual intervention, and scalable configuration options make the C Line a preferred choice for large foundries targeting maximum throughput and premium casting quality.

Roberts Sinto Corporation

Model: FDNX Series Flaskless Molding Machine

Features: Roberts Sinto’s FDNX Series flaskless molding machines are engineered for diverse sand casting applications, including both ferrous and non-ferrous alloys. The series boasts high-speed operations with reliable sand compaction, precision pattern match, and robust mold handling capabilities. Customizable casting parameters, a modular design, and options for integrated sand mixers or cooling systems allow these machines to adapt to specific foundry layouts or workflows. Operators benefit from easy maintenance, rapid pattern changes, and support for environmentally friendly molding processes using reclaimed sand.

Palmer Manufacturing & Supply, Inc.

Model: No-Bake Shakeout System

Features: Palmer's No-Bake Shakeout System serves as an essential sand reclamation solution for no-bake sand casting operations. It uses advanced mechanical vibration and air separation technology to extract sand from castings efficiently, recovering reusable molding sand and minimizing waste. Adjustable shakeout intensity allows for processing different casting types and weights, while integrated sand cooling optimizes mold sand properties for reuse. With automatic sand collection and disposal, this system helps foundries lower material costs, reduce environmental impact, and consistently achieve high casting quality.

When evaluating sand casting machine suppliers and equipment, consider factors such as molding method (green sand versus no-bake), cycle time, automation level, adaptability to various casting alloys, and integration with core making or sand handling systems. Choosing established brands provides assurance of product support, parts availability, and engineering guidance for process optimization and capacity planning.

Please be aware that availability, machine specifications, and features may evolve as manufacturers innovate their product lines. For the most current sand casting machine offerings, technical documentation, and purchasing options, it’s advisable to consult directly with the respective manufacturers or authorized distributors.

Chapter 3: What are the different types of casting sand?

Sand casting is a highly versatile manufacturing technique, offering developers, designers, and engineers the flexibility to produce complex and detailed metal parts and components with precise tolerances. As one of the oldest and most widely used metal casting processes, sand casting accommodates a broad range of ferrous and non-ferrous metals—making it popular in industries such as automotive, aerospace, mining, agriculture, and heavy machinery manufacturing. Its compatibility with various alloys, cost-effectiveness for both prototyping and high-volume production, and ability to create intricate designs make sand casting the preferred choice for a wide variety of industrial applications.

Green Sand

Green sand casting is the most prevalent type of sand casting, valued for its affordability and adaptability. Despite the name, the sand isn't actually green; it's termed "green" because of its moisture content and readiness for molding without being dried or baked. This sand mixture, which includes foundry sand, bentonite clay, water, and various organic additives, is used to create expendable molds that are cost-effective and suitable for rapid, high-volume production runs. However, the sand itself is generally not recyclable and is discarded after use.

In green sand casting, the composition typically includes 75% to 85% high-silica sand, 5% to 11% bentonite clay, 2% to 4% water, and 3% to 5% other materials like powdered coal or sea coal for improving surface finish. The clay and water serve as binding agents, providing structural integrity to the mold cavity. While green sand casting is highly efficient for manufacturing large quantities of castings—such as engine blocks, cylinder heads, and automotive parts—it often does not yield parts with tight tolerances or extremely smooth surface finishes. Additional machining or finishing processes are frequently required to meet precision specifications. The flexibility and low tooling cost also make green sand a go-to choice for prototyping parts and low-to-medium production volumes.

Dry Sand

Dry sand casting involves mixing sand with clay or advanced binding agents, shaping the mixture into the mold form, and then baking or drying it in an oven to increase strength and stability. The heat treatment process ensures the mold maintains its integrity during the pouring of molten metal. This technique is commonly employed for casting large ferrous and non-ferrous metal components such as engine blocks, pump housings, gearboxes, turbine casings, and industrial machinery frames.

Dry sand casting is favored for its ability to produce detailed and dimensionally accurate castings with higher precision and improved surface finish compared to green sand casting. Similar to green sand casting, cores and sprues are used to guide and direct molten metal flow into the mold cavity. Although the process is more complex, time-consuming, and costly, it delivers superior results for parts where high strength, reliability, and consistent quality are priorities. Unlike green sand casting, the sand used in dry sand casting is usually non-reusable after the casting operation due to changes in its structural properties.

Facing Sand

Facing sand, sometimes called face sand, is used to coat or line the mold cavity surface that is in direct contact with molten metal. Typically composed of fine silica sand and high-purity clay, facing sand is freshly prepared for each casting run and is never sourced from previous projects. By being applied directly adjacent to the pattern surface, this sand ensures that the mold can withstand extreme temperatures and resist metal penetration. Facing sand must possess high refractoriness, excellent thermal stability, and good collapsibility to improve the surface finish of the casting and reduce defects such as sand inclusions or rough surfaces. This specialized molding sand ensures an exceptionally fine grain finish in the mold to support demanding quality standards.

Core Sand

Core molding sand is a tailored blend of high-purity silica sand with core oil (which may include linseed oil, resin, and mineral oil) and additional binding agents such as dextrin, cornflour, and sodium silicate. This blend is specifically engineered for manufacturing cores used to create internal cavities or complex passages in castings that cannot be achieved by two-part molds. Due to its excellent compressive strength, permeability, and ability to hold complex shapes under thermal stress, core sand is indispensable for intricate castings required in industries such as automotive and pump manufacturing. Cores made from this sand type ensure precise cooling channels, holes, and other interior details essential to product functionality.

Loam Sand

Loam sand consists of an equal mix of sand and clay, combined with sufficient water to create a malleable yet structurally sound mold material. It is commonly used for casting large, heavy components such as hoppers, turbines, and large pipes, where specially shaped molds are built up by hand. Loam sand casting is particularly suitable for on-site work or one-off production where conventional pattern-making and flask molding would be impractical or uneconomical.

Parting Sand

Parting sand, which is pure silica sand, performs the crucial function of preventing adhesion between sand mold sections during the molding process. It is sprinkled over the pattern before embedding it in the molding sand, providing a physical barrier and facilitating the easy removal of the pattern and separation of the mold halves. Additionally, parting sand is spread across the contact surfaces of the cope, drag, and cheek to ensure smooth separation and prevent sticking, reducing the risk of mold damage or casting defects.

Backing and Floor Sand

This type of casting sand, known as floor sand or backing sand, is used to fill the volume box behind the facing sand and provide critical support and rigidity to the mold cavity during pouring. Floor sand is typically reclaimed from previous castings, making it a cost-effective backing medium that adds strength, bulk, and stability to the overall mold structure. Its composition and degree of purity are less critical than facing sand, but it must still maintain sufficient strength to support heavy castings.

System Sand

System sand is specifically designed for automated or mechanized molding processes, especially in large foundries and mass-production environments. This type of molding sand is engineered for large mechanical castings due to its high refractoriness, optimal permeability, and robust strength. Used exclusively in machine molding to completely fill the flask, system sand does not require the addition of separate facing sand because the mixture is uniformly prepared and contains special additives for better performance and surface finish. Advantages include improved mold stability, better recycle rates, and consistency in cast product quality.

Molasses Sand

Molasses sand uses molasses as a natural binding agent, resulting in a strong, smooth, and collapsible core sand suitable for core making and, in some cases, intricate shape casting. This type is preferred when a high degree of surface smoothness and collapsibility is needed, such as in thin-walled or complex-shaped metal castings.

Choosing the Right Casting Sand

When selecting the ideal casting sand for your manufacturing needs, factors to consider include metal type, desired surface finish, casting size, dimensional accuracy, and production volume. Understanding these different types of sand casting materials enables better process optimization, cost management, and quality assurance. Industries seeking high-precision prototypes, durable end-use components, or large structural parts can benefit from matching the sand mixture to their specific foundry equipment, metal alloys, and project requirements.

Sand Casting Process Applications

The sand casting process is widely used for various applications, including custom automotive engine parts, pump components, machinery housings, valves, hardware, and industrial tool castings. Its versatility allows manufacturers to produce parts ranging from a few ounces to several tons, serving markets such as energy, chemical processing, marine defense, construction, and mining. Modern improvements in sand molding, pattern materials, and additive selection further enhance output quality and reduce casting defects, making sand casting a leader among foundry processes.

Leading Manufacturers and Suppliers

Chapter 4: 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

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.

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.

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.

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.

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.

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

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.

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.

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.

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.

Chapter 5: What considerations should be taken into account when performing sand casting?

When considering sand casting design, it's important to understand the intended use of the casting and any additional processes it will undergo after pouring. For visible castings, a smooth finish might be required through machining or coating. If the casting is part of a structure or assembly that demands high stiffness, heat treatment services might be necessary.

To achieve the desired final results, most castings will need to undergo some form of machining or treatment. Essential considerations include:

Detailed Prints and Drawings

The most significant feature of sand casting mold design is the provision of extremely detailed prints and drawings. A sand foundry requires a drawing for both the casting and the fully machined product.

Detailed prints are a crucial communication tool in the sand casting design process, as they convey the designer's expectations and requirements for the finished product. The following information should be included in your sand casting product design:

• Size
• Shape
• Draft
• Radii
• Tolerance level
• Defect level
• Surface finish
• Inspection requirements

When your casting design includes cast-in identification marks like a component number, foundry code, or heat lot, ensure that the size and location are clearly indicated in your detailed drawings.

Sand Casting Draft Angle

The draft angle in sand casting is a critical angle applied perpendicular to the model to facilitate easy removal of the casting from the fragile sand mold without damaging the external surfaces. Factors such as the molding process, casting design, and the depth of the pattern within the mold help determine this angle. However, many designers often overlook its importance. Choosing an appropriate draft angle can enhance the effectiveness of the tapered surfaces in the casting design.

Moreover, due to the optimized metal flow, it can contribute to reducing processing costs. Therefore, adhering to the ISO standard for draft angles in sand casting is essential and does not compromise the functionality of the design. This practice allows for more efficient mold production at lower costs while maintaining quality. Typically, the standard draft angle for sand casting is 2 degrees, with a minimum draft of approximately 1 degree for both external and internal features.

Rounds and Filet

Incorporating ample rounds and fillets is a crucial element of the sand casting design process. Properly designed rounds and fillets not only improve the visual appeal of a casting but also help in evenly distributing stress and minimizing casting-related stresses. Well-placed corner fillets further contribute to the casting process by preventing turbulent flow and ensuring that the molten metal flows smoothly to fill the mold correctly.

Parting Line

In sand casting design, accurately determining the parting line is essential. This line acts as the division point where the mold separates, and its placement is crucial as it influences both the production cost and the quality of the castings. It is important for engineering designers to clearly define and document the parting line on the casting blueprint. Relying solely on the practical experience of foundry workers for this decision may not be sufficient.

The parting line should ideally be positioned as low as possible, and the design should aim for a wide, short, and flat line. If the parting line does not align with the seam burrs, the tolerance between them should not exceed 0.020 inches.

Moreover, the maximum flash extension material for the parting line should be around 0.015 inches. If the parting line's location changes, it's important to take note, as it can affect factors such as the use of cores, the pouring position, the casting's weight, and its dimensional accuracy.

Alloy Selection

Selecting the appropriate alloy for your casting is essential, as the choice of alloy can significantly influence properties such as:

• Strength
• Durability
• Toughness
• Corrosion resistance
• Ductility
• Shrinkage
• Hardenability
• Cost
• Weldability

Undercuts and Cores

In sand casting design, the undercut feature is used to ensure that the pattern can be removed without damaging the mold during production. However, using cores can increase both the time and cost of casting. Therefore, it's important to minimize or eliminate the need for core sand sections in the design. Initially, experts provided guidelines for defining the parting line, which helped in understanding product features and avoiding undercut issues. Over time, these guidelines have evolved, making it crucial to understand the current norms and standards in sand casting design.

Cross-Sections

Maintaining a uniform cross-section or wall thickness is typically beneficial. However, in sand casting design, it might not always be feasible since many casting products cannot accommodate sudden changes in section thickness. Ideally, the thicker parts of the casting should not cool in isolation, as the cross-section is most effective when it remains uniform. This is because thicker sections take longer to cool.

Thicker sections are not influenced by the solidification of the surrounding metal and will be the last to solidify. This can lead to casting defects such as porosity or tearing if not properly managed. Therefore, it's crucial to account for the maximum allowable thickness in your sand casting design to avoid these issues.

Wall Thickness

In sand casting design, achieving uniform solidification and preventing cavity formation is essential. This involves managing the volume-to-surface area ratio of the mold. Generally, the rate at which castings solidify should be proportional to the square of this ratio.

Parts with a smaller volume-to-surface area ratio solidify more quickly compared to those with a larger ratio, and vice versa. To address this, many foundries either increase the overall thickness of the mold or add materials to support load-bearing areas. However, a more effective solution is to use stiffeners and gussets. Stiffeners enhance structural strength, while gussets help reduce local wall thickness, ensuring a more consistent solidification process.

Corners and Angles

The cooling characteristics of the material used for sand molds play a crucial role in the quality of sand castings. Therefore, this factor must be carefully considered during the design phase. Inadequate cooling of either the casting or the sand mold can lead to localized heat buildup at sharp corners and junctions of the component.

This localized heat concentration creates stress points, which can cause distortion, shrinkage, and cracking in the casting during subsequent processing, ultimately compromising the quality of the final product. It's important to address these issues in the sand casting design to avoid such problems.

Junction Design

The complexity of sand casting mold design allows for highly intricate part shapes, often involving various junctions. These junctions, such as L, X, V, Y, and X-T types, create local mass concentrations. Such concentrations can lead to issues like cracks, shrinkage, and strain. The best approach in sand casting design is to minimize these localized mass concentrations caused by junctions, ensuring they integrate smoothly into the final product.

Casting Allowance

Initially, it’s important to understand that metals like steel, copper, aluminum, magnesium, and zinc experience shrinkage as they solidify. When managing sand casting design, it’s crucial to include a machining allowance at the junctions between two sand castings.

In essence, avoid curved edges at the interfaces of sand castings. This is because shrinkage is related to the material's freezing point and the ratio of the casting's surface area to its volume.

Recycling Molding Sand

After removing the sand from a completed casting, the lumps are cooled and then crushed. A magnetic field is commonly employed to extract all metal particles and granules. Shakers, rotary screens, and vibrating screens are used to sort the sand and components. The cleaned sand is then reintroduced into the molding sand production process.

Mullers are utilized to mix sand with bonding agents and water for creating molding sands. Aerators are used alongside these mixers to loosen the sand, enhancing its moldability. Scoop trucks or belt conveyors transport the prepared sand to the molding area, where it is shaped into molds. These molds can either be placed on the floor or moved to a pouring station via conveyors. At the shakeout station, castings are separated from the residual sand after pouring. The used sand is then returned to storage bins via belt conveyor or other methods.

In the manufacturing process, casting sand is often reclaimed and reused. Industry estimates suggest that around 100 million tons of sand are used annually, with only four to seven million tons being discarded. Many companies also repurpose even the discarded sand.

Chapter 6: What are the applications and advantages of sand casting?

This chapter explores the various applications of sand casting and highlights its benefits.

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

Despite its advantages, sand casting has some drawbacks, including:

• 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.
• Internal sand mold wall surface roughness results in poor surface finishes.
• 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.

However, the benefits of sand casting often outweigh its disadvantages. Key advantages of sand casting include:

• It is used to form complex shapes
• It can produce very large parts
• Very low cost for tooling
• Recycle scrap
• 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

Conclusion

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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