Sand casting
Sand casting, also known as sand molded casting, is a
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
Basic process
There are five steps in this process:
- Place a pattern in sand to create a mold.
- Incorporate the pattern and sand in a gating system. Remove the pattern.
- Fill the mold cavity with molten metal.
- Allow the metal to cool.
- Break away the sand mold and remove the casting.
Components
Patterns
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
Tools
In addition to patterns, the sand molder could also use tools to create the holes.
Molding box and materials
A multi-part molding box (known as a
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
To control the solidification structure of the metal, it is possible to place metal plates,
Cores
Cores are apparatus used to generate hollow cavities or internal features which cannot be formed using pattern alone in moulding, 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
Design requirements
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
Processes
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
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 mould. 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
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
"Air set" method
The air set method uses dry sand bonded with materials other than clay, using a fast curing
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
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
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 1950, partially automated casting processes have been developed for production lines.
Cold box
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
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
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
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
The first mechanized molding lines consisted of
Automatic high pressure sand molding lines
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
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
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
In 1962, 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 (0.0039 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
The principle of the matchplate, meaning pattern plates with two patterns on each side of the same plate, was developed and patented in 1910, 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 (0.0059 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
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
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. 2016.
In addition, the electrical safety requirements are covered by:
Industrial Electrical Machinery, CSA C22.2 No. 301. 2016.
European Union
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 60204-1 for electrical safety, and EN ISO 13849-1 and EN ISO 13849-2 or EN 62061 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
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). 2020.[12]
- Performance Requirements for Risk Reduction Measures: Safeguarding and other Means of Reducing Risk, ANSI B11.19. American National Standards Institute (ANSI). 2019.
- Safety Requirements for the Integration of Machinery into a System, ANSI B11.20. American National Standards Institute (ANSI). 2017.
- Safety Requirements for Transfer Machines, ANSI B11.24. American National Standards Institute (ANSI). 2002 (R2020).
- Functional Safety for Equipment (Electrical/Fluid Power Control Systems) General Principles for the Design of Safety Control Systems Using ISO 13849-1, ANSI B11.26. American National Standards Institute (ANSI). 2018.
- Sound Level Measurement Guidelines, ANSI B11.TR5. American National Standards Institute (ANSI). 2006 (R2017).
Mold materials
There are four main components for making a sand casting mold: base sand, a binder, additives, and a parting compound.
Molding sands
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
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
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
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
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
Olivine sand
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
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
Chamotte sand
Other materials
Modern casting production methods can manufacture thin and accurate molds—of a material superficially resembling papier-mâché, such as is used in egg cartons, but that is refractory in nature—that are then supported by some means, such as dry sand surrounded by a box, during the casting process. Due to the higher accuracy it is possible to make thinner and hence lighter castings, because extra metal need not be present to allow for variations in the molds. These thin-mold casting methods have been used since the 1960s in the manufacture of cast-iron engine blocks and cylinder heads for automotive applications.[citation needed]
Binders
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
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
Oils, such as
Resin
Resin binders are natural or synthetic high melting point
MDI (methylene diphenyl diisocyanate) is also a commonly used binder resin in the foundry core process.
Sodium silicate
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
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
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,
Up to 3% of "cushioning material", such as wood flour,
Up to 2% of cereal binders, such as
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
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 (0.0030 and 0.0059 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
Clay molds were used in ancient China since the
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
Sand casting molding method was recorded by Vannoccio Biringuccio in his book published around 1540.
In 1924, 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
In the 2010s, 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
- 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 mould – Tool used in injection molding and printing
- Sand rammer
- Juutila Foundry – Finnish bell foundry (Finland), est. 1881, specialized in sand casting
- voxeljet (Germany), 3D printing,
References
Notes
- ^ Rao 2003, p. 15.
- ISBN 0-7506-1696-2.
- ^ a b Donaldson, Brent (2017-11-01), "Foundry Says Robotic Sand Printing a "Game Changer" for Metal Casting", Additive Manufacturing, retrieved 2017-11-14.
- PMID 31143220.
- ^ "Respirator Use and Practices in Primary Metal Operations". Foundry Management and Technology. Retrieved 2021-04-05.
- ^ Sand Casting Process Description
- ^ Todd, Allen & Alting 1994, pp. 256–257.
- ^ Metal Casting Techniques - Vacuum ("V") Process Molding, retrieved 2009-11-09.
- ^ a b Degarmo, Black & Kohser 2003, p. 310.
- ^ a b The V-Process (PDF), retrieved 2009-11-09.
- ^ Degarmo, Black & Kohser 2003, p. 311.
- ^ B11 Standards
- ^ a b c d e Rao 2003, p. 18.
- ^ Degarmo, Black & Kohser 2003, p. 300.
- ^ a b c d e Rao 2003, p. 19.
- ^ "Beneficial Reuse Of Spent Foundry Sand" (PDF). 1996.
- ^ Rao 2003, p. 22.
- ^ a b c d e Rao 2003, p. 20.
- ^ Rao 2003, p. 21.
- ^ Rao 2003, p. 23.
- ^ a b c Rao 2003, p. 24.
- ^ OCLC 85814321.
- ^ a b c d Rao 2003, p. 25.
- ^ Rao 2003, p. 26.
- ISBN 978-0-19-966226-5. Translation by the author, reproduced by permission of Oxford University Press.
Bibliography
- Degarmo, E. Paul; Black, J T.; Kohser, Ronald A. (2003), Materials and Processes in Manufacturing (9th ed.), Wiley, ISBN 0-471-65653-4.
- Todd, Robert H.; Allen, Dell K.; Alting, Leo (1994), Manufacturing Processes Reference Guide, Industrial Press Inc., ISBN 0-8311-3049-0.
- Rao, T. V. (2003), Metal Casting: Principles and Practice, New Age International, ISBN 978-81-224-0843-0.