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Casting and its types
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CASTING AND WORKING OF
METALS
Materials Processing
• Materials processing is the science and technology by which a material is converted into a useful shape with a structure and properties that are optimized for the service environment.
• Materials processing by hand is as old as civilization; mechanization began with the Industrial Revolution of the 18th century, and in the early 19th century the basic machines for forming, shaping, and cutting were developed, principally in England. Since then, materials-processing methods, techniques, and machinery have grown in variety and number.
• Numerous processes and operations can be involved in the manufacture of products and components and many of them are associated with the production of a desired shape.
Categories of manufacturing processes:
• The processes used to convert raw materials into finished products perform one or both of two major functions: first, they form the material into the desired shape; second, they alter or improve the properties of the material. Manufacturing processes can be categorized on the basis of liquid and solid states.
Casting Process
• The processing of materials in liquid state is commonly known as casting, in which the molten material (mostly metals) is converted into a desired shape by pouring the material into the mold. Cast products can have extremely complex shapes but also posses structures that are produced by solidification and are therefore, subject to such defects as shrinkage and porosity.
Mechanical manufacturing processes
• Forging, rolling, extrusion, wire drawing, swaging, roll forming, deep drawing are the mechanical manufacturing processes, are performed in solid state. These are performed on the basis of tendency to deform which can be referred as deformation processes, using temperature dependence. Deformation processes exploit the ductility of certain materials mostly metals and produce the desired shape by mechanically moving or rearranging the solid though a phenomenon known as plasticity. These processes can have high rates of production, but generally require powerful equipments and dedicated tools or dies. Their brief description is as follow:
•
– Forging• Forging is a manufacturing process involving
the shaping of metal using localized compressive forces. Forging is often classified according to the temperature at which it is performed: "cold," "warm," or "hot" forging or at room temperature. Warm or hot forgings are performed for the less ductile materials. Highly ductile can be forged at room temperature. It is usually hot working process.
– Rolling• Rolling is a metal forming process in which
metal piece is passed through a pair of rolls to lessen the crossectional area or thickness to get the required shape. Rolling is classified according to the temperature of the metal rolled. If the temperature of the metal is above its recrystallization temperature, then the process is termed as hot rolling. If the temperature of the metal is below its recrystallization temperature, the process is termed as cold rolling. It is usually hot working process.
– Extrusion• Extrusion is a process used to
manufacture objects of a fixed cross-sectional profile. A material is forced through a die of the desired cross-section. It is also classified as warm, hot and cold extrusions depending upon the material nature. It is usually hot working process.
– Wire drawing• Wire drawing is a manufacturing
process in which a metal piece is pulled through a die orifice by means of a tensile force is applied from the outside. It is usually cold working process.
Machining Processes
• Machining is a term used to describe a variety of material removal processes in which a cutting tool removes unwanted material from a work piece to produce the desired shape. It can be done at room temperature. The removal processes are capable of outstanding dimensional precision, but produce scrap when material is cut away to produce the desired shapes.
• Basically it is used to finish the component, to make the accurate size. Punching, blanking and piercing, stamping are the well known machining processes. Their brief description is as follow:
• •
– Punching• Punching is a forming process that uses
a punch press to force a tool, called a punch, through the work piece to create a hole via shearing. The punch often passes through the work into a die. Punching is often the cheapest method for creating holes in sheet metal in medium to high production volumes.
– Stamping
• It is a type of forging. Stamping includes a variety of sheet-metal forming manufacturing processes, such using a machine press as stamping press, This could be a single stage operation where every stroke of the press produce the desired form on the sheet metal part, or could occur through a series of stages. The process is usually carried out on sheet metal.
– Blanking and piercing
• Blanking and piercing are shearing processes in which a punch and die are used to modify webs. The tooling and processes are the same between the two, only the terminology is different: in blanking the punched out piece is used and called a blank; in piercing the punched out piece is scrap.
Consolidation or joining processes
• It is a technique of manufacturing. This process is used to join the different components to join together to get a single article according to the requirement. Complex products can often be assembled from simple shapes but the joint areas are often affected by the joining process and may possess characteristics different from the original material. Welding, Brazing, soldering and adhesive bonding and mechanical joining are the well known techniques.
•
– Welding• It is a fabrication that joins materials,
usually metals or thermoplastics, by causing coalescence (to join into a single mass). It is mostly used for the assembling purposes.
– Brazing• Brazing is a metal-joining process
whereby a filler metal is heated above and distributed between two or more close-fitting parts by capillary action (the ability of a liquid to flow against gravity where liquid spontaneously rises in a narrow space such as a thin tube).
– Soldering• Soldering is a process in which two
or more metal items are joined together by melting and flowing a filler metal into the joint, the filler metal having a lower melting point than the work piece.
•
– Adhesive bonding• Adhesive bonding is used to fasten
two surfaces together, usually producing a smooth bond. This joining technique involves glues, epoxies, or various plastic agents that bond by evaporation of a solvent or by curing a bonding agent with heat, pressure, or time.
– Mechanical joining• Mechanical Joining is a process for
joining parts through mechanical methods, which often involve threaded holes. Joining parts using screws or nuts and bolts are common examples of mechanical joining.
•
Powder Metallurgy
• It is a technique of manufacturing components in which we perform working on the powder of metals. That powder metal is converted into the required component. High accuracy is achieved in this process. It is used only for small component manufacturing. In this process, no finishing and machining is required.– Steps:
• Powdering of metal• Selection of binder• Mixing or blending with other powders to make homogeneous mixture.
• Compacting (the process of shaping metal powder in a die through
the application of high pressures.)• Sintering (it is an agglomeration process in which recrystallization of the
mineral is achieved. The process of sintering is used to remove the porosity and increase the strength of materials. In this, the material does not melt.)
•
Casting Technology
Casting is a manufacturing process where a solid is melted, heated to proper temperature (sometimes treated to modify its chemical composition), and is then poured into a cavity or mold, which contains it in the proper shape during solidification. Thus, in a single step, simple or complex shapes can be made from any metal that can be melted. The resulting product can have virtually any configuration the designer desires.
Types of Casting processes
• The casting process is subdivided into two main categories: expendable and non-expendable casting. It is further broken down by the mold material, such as sand or metal, and pouring method, such as gravity, vacuum, or low pressure.
• Expendable mold casting:• Expendable mold casting is a generic classification that
includes sand, plastic, shell, plaster, and investment (lost-wax technique) moldings. This method of mold casting involves the use of temporary, non-reusable molds. Sand casting, Plaste mold casting, Shell molding, Investment casting, Evaporative-pattern casting are the types of expandable mold casting.
Non-expendable mold casting
Non-expendable mold casting differs from expendable processes, in that the mold need not be reformed after each production cycle. This technique includes at least four different methods: permanent, die, centrifugal, and continuous casting.
Advantages of Casting process:There is no limitation of producing
castings of any size or weight. This advantage is not in other manufacturing processes.
Complicated shape castings can be produced. Complicated patterns are key to complex shapes.
Hollow castings can be produced.Any composition of materials can be
used for producing required casting. Any composition can be achieved. Mechanical manufacturing processes can do this job.
Some casting processes are net shape; other is near net shape.
Dimensional modification is achieved.Dimensional accuracy is achieved. Machining is
not required but surface finishing is required. In sand casting the finishing is required and machining also but for investment casting, machining is not required due to high accuracy but finishing can be required.
Metal casting is a process highly adaptable to the requirements of mass production. Large numbers of a given casting may be produced very rapidly.
• Weight saving is possible. Component made with casting process is lighter than the component made with other machining processes.
• Casting can be made with hair like precision or accuracy provided proper molding and casting technique is employed.
• Only castings have the advantage of fibrous structure. Casting leaves component with its solid fibrous structure which inherit great compressive strength. So, component subjected to compressive strength are made with casting.
• In soft material casting, melting is not always necessary, in some cases only pressing deformation is enough for soft material that is entered into the cavity.
•
Metallurgical Advantages of Casting of Metals:
• This process can give any required micro structure to obtain the required mechanical properties. We can change the microstructure by changing the grain boundaries.
• Grain size control is possible in casting process by controlling the rate of solidification. High rate of solidification gives fine size grain which has high strength as compared to coarse size that is achieved at low rates of solidification. During the solidification, the uniformity of crystallization gives strength. So mechanical properties are controlled by the solidification rate.
• If we perform sintering of the casting, high dense structure can be achieved.• Strength and lightness in certain light metal alloys, which can be produced
only as castings.• Casting provides versatility. Wide range of properties can be attained by
adjusting percentage of alloying elements.• Casting is one of cheapest method for mass production.•
Limitations of Casting process:
• Though casting is cheapest for Mass Production, it becomes non economical for JOB production.
• Sand casting leaves rough surface which needs machining in most of cases. It adds up the cost in production.
• Cast products are superior for compressive loads but they are very poor in tensile or shock loads. They are brittle.
• In sand casting, porosity is achieved.
• To manufacture small sized castings with high accuracy, good machining and finishing; the type of casting chosen raise to go expensive.
• It is almost impossible to design a part that cannot be cast by one or more of the commercial casting processes. But it will be the skill of a manufacturer to adopt such a casting process to get
a) good results and b) lowest cost expenditures. • The various casting process are distinguished primarily by the mold material (whether
sand, metal or other material) and pouring method (gravity, vacuum, low pressure or high pressure). All share the requirement tht the material should solidify in a manner that will maximize the properties and avoid the formation of defects.
• Basic Requirements of Casting processes• (For both expendable and non-expendable) 1- Mold Production• The material for making mold is sand, wax, ceramic materials and metallic
materials. Metallic materials are used for specifically for non-expendable use mold.
• The mold cavity having the desired shape and size must be produced with due allowance for shrinkage of solidifying material. Grey cast iron is used that doesn’t shrink due to the presence of the atoms of carbon atoms fibers at their place and has the ability to resist high pressure and to resist the vibration hazard effects while white cast iron shrinks.
• Strength of casting is very important. Engine blocks are made of steel alloys to resist vibration. If simply steel is used, then it will be deformed earlier. The alignment is disturbed.
• Either attempt single-use molds or double-use molds, but keep in front the economical factor. The more economical single-use molds are usually preferred for the production of smaller quantities.
• A sand mold is shown in the figure below:
• Production of mold must have following parts:• The flask is the rigid metal or wood frame that holds the molding aggregate.• Cope is the name given to the top half of the flak or mold.• Drag refers to the bottom half of the flask or mold.• Core is a sand shaped that is used to produce the hollow castings.• Core print is added to the pattern, core or mold and is used to locate and support
the core within the mold.• The molten metal and core combine to produce the Mold Cavity, the shaped hole
into which the molten metal is poured and solidified to produse desried casting.• Riser is made for the information of comlete pouring of metal in the mold.• The Runner gives a flow to molten metal to mold cavity to compensate the
shrinkage.• Gating System (pouring cup+sprue+runner)) is the network of connected channels
used to deliver the molten metal to the mold cavity.• Metal travels down a sprue is the vertical portion of the gating system.• Pouring cup is the portion of gating system that initially recieves the molten metal
and control its delievery to the rest of the mold.• Vents for the escape of gases.• The parting line is the interface that separate cope and drag in the flask.• Draft is the on the pattern or casting that permits to be withdrawn from the mold.• Casting is used to describe both the process and the product.
2- Melting Process• A melting process must be capable of providing
molten material at the proper temperature and in the desired quantity of acceptable quality and at reasonable cost.
• To get the good quality, we have to decrease the tendency of metal to react with oxygen, to avoid the formation of oxide (corrosion). Because if metal converts into oxide, then the melting of pure metal will not be achieved. In this case we will get the molten oxidezed metal that will make the casting not fair. Non ferrous metals have high tendency of forming oxides than the ferrous metals. So avoid oxidation. For this purpose flux is added in metal during the metal process. The covering flux is added to avoid oxidation. This flux flows on the molten metal to provide the barrier to the contact of air with metal. For Al, chloride flux is used mostly. Since iron has low tendency to react with air, so in this case, the covering flux is not urgently required but the use of flux is better.
• Cleaning fluxes are added in the molten metal to separate out the slag to avoid the porosity and weakness in the strength of mold for casting.
• To remove the air bubbles from the molten metal, it is known as de-oxidation and for this purpose agents used are known as de-oxidizers. For alloying elements, Ferro alloys are used as de-oxidizers that give the de-oxidation, as well as alloying addition. Ferro alloys (M.P ranges 1300-14000C) are the alloys of ferrous and non ferrous e.g. Fe-W, Fe-Si, Fe-Mo. If we don’t require the addition of alloying elements, then Al is added as de oxidizer.
3- Pouring Tech It is a technique by which the moulds are filled in with molten metal.There must be provisions for air or gases inside the mould to come out when the molten metal is poured in. When the hot metal enters the mould cavity, it may generate various gases due to chemical reactions. The mould design should allow these gases to escape, so that the molten metal can spread and fill the mould cavity completely. It helps in producing defect free, fully dense and quality casting components.Prior to the pouring, the mold produced must be heated to increase the strength of mold by the escape of gases, for no crack, to avoid porosity and thermal instability. The mold can burst due to the pressure of gases.For the escape of gases, besides provisions, some ingredients can also be applied. For example wood charcoal.Whenever the mold is used for pouring of metal, pour gate is organized.
• 4- solidification process, better design and control at this stage helps in getting quality output. Required mechanical properties of the casting can be achieved by controlling the rate of solidification of the molten metal.
5- After proper solidification, the casting should be removed from the mould i.e. casting removal. Generally, expendable moulds are broken apart and destroyed after each casting is produced without any difficulty. But, using re-usable moulds may cause major challenges from designers' point of view on the removal of casting from permanent moulds.
6- Various cleaning, finishing and inspection operations are performed after the casting removal from the mould.
Pattern
• A pattern is simply the duplicate or model or replica of the component which has to be manufactured by the casting process.
• Important property of a Pattern• It is slightly larger in the size than the original size casting to be produced because a
pattern has:• Shrinkage allowance:• Almost all the metals shrink or contract volumetrically after solidification and
therefore the pattern to obtain a particular sized casting is made over sized by an amount equal to shrinkage or contraction.
• It depends upon mainly on the cast metal or alloy used for the casting and pouring temperature of the metal or alloy. The shrinkage allowance given to pattern for Aluminum casting is not suitable for the steel casting.
• Machining allowance• It is given to the pattern for surface finishing of the casting produced, to remove
surface imperfections, to require exact casting dimensions (actual size achievement by grinding).
• How much machining allowance is applied to the pattern, depends upon:• Nature of metal: i.e. Ferrous and non-ferrous. Ferrous metals get scaled whereas non-
Ferrous ones don’t. • Size and shape of casting: Longer castings tend to warp and need more allowance to be
added to ensure that after machining the casting will be alright.
Selection of material for Pattern
• The following factors assist in selecting proper pattern material:
a) The number of castings to be produced. Metal pattern are preferred when the production quantity is large.
b) The desired dimensional accuracy and surface finish.c) Nature of molding process. E.g. sand casting frequently
use wood pattern while investment casting require wax pattern.
d) Shape, complexity and size of casting.e) The chances of repeat order.
Materials used for making Patterns
• Wood • The most common material for making pattern for sand casting is the wood.• Advantages:
1. in expensive and easyly available.• 2. for mass scale production.• 3. Easy machining and shaping.• 4. Easy to obtain good surface finish.• 5. Can be used for complex shapes and large castings.• 6. Light in weight.• 7. Can be repaired easily.• Limitations:• Absorbs moisture and hence swelling is there.• Short life patterns.• Poor wear and abrasion resistance.• Cannot withstand rough handling.• Weak as compared to metallic patterns.• Applications:• Wooden pattern are used where the number of castings to be produced is small and
the pattern size is large.• Recently, ply wood (synthetic material like wood) is used for the heavy castings (40-50
tons). Natural wooden pattern used for the normal sized castings.
• Metals• Metal patterns are cast from wooden patterns.• Advantages:• They do not absorb moisture.• They possess life much longer than wooden patterns.• They do not warp.• Give good surface finish.• Excellent wear and abrasion resistant.• Good machine ability.• Limitations:• Expensive• Heavy in weight than wooden patterns.• A chance of rusting/oxidation.• Not easily repaired (Al patterns).• Machining is not easy as wooden patterns.• Applications:• Wooden and metallic patterns are used for mass scale production.• Patterns are also used, made by plastic, rubber, plaster and wax
(investment casting) and poly styrene.
• Wax • Wax patterns are used in investment casting.• Advantages:• Gives good surface finish• Has ceramic coating• High dimensional accuracy• Not need machining• After being molded, the wax pattern is not taken out of the mold like
other patterns as wooden, rubber and metal patterns; rather the mold is inverted and heated; the molten wax comes out.
• Wax can be recycled.• Important notes: • Only both the wax and poly styrene patterns are consumable
patterns. Both evaporate but wax has ability to only melt also. So wax (molten obtained after molding) can be recycled but polystyrene cannot be recycled (due to its fuming on heating).
• Wood, rubber, metal patterns etc. are not consumable because they are taken out of the mold after molding.
Four Important Qualities of Molding/Core Sands
• The sands used for making molds must have following four important qualities:• Refractoriness• It is the ability to withstand high temperatures without melting, fracture or
deterioration. Refractoriness is provided by the basic nature of the sand. This property is very important for very high M.P metals/alloys such as steel whose high M.P depends upon the ingredients. Zircon sand has highest refractoriness and can be used to make sand mold. For low melting M.P metals, silica sand or chromite sand can be used.
• Cohesiveness• Cohesiveness is the ability of molding sand to retain a given shape when packed
into a flask. It is obtained by using binders such as clay (bentonite, kaolinite or illite) that becomes cohesive when moistened. Molasses (not for cores, only for molding sand), resins, oils and sodium silicate are also used. (Volcanic ash)
• Permeability• It is the ability of mold or core to escape gases through the sand. It is a function
of the size and shape of sand particles, the amount and type of clay used or any binder, the moisture content and the compacting pressure. Gases are produced due to sand ingredients also. If permeability is not there, there is a chance for cracking. Due to retention of air or gases in the molten metal, casting defects may be produced.
•
To increase permeability, saw dust or wood dust is added (in large amount for fine particle sands). For the facilitation, vents are produced through the mold.
• Collapsibility• It is the ability to accommodate metal shrinkage after solidification and
provides easy removal of the casting through the mold on its disintegration (shakeout).
• This property is sometimes enhanced by adding cereals or other organic materials, such as cellulose that burn out when they come into contact with the hot metal. This property can also be enhanced by adding talcum powder, graphite powder. Graphite powder plays role to avoid stickness of casting to the molding sand cavity.
• Types of Molding Sands• Natural Molding Sands (2) Synthetic Molding Sands • Natural Molding Sands:• Natural sands are simply the base sands and natural sands have 5-10%
water and 10-15% clay. They have some good properties required for the molding but are not perfect.
• Types of Base Sands and their Properties:• It is the type of sand which is used to make the mold with binder. They have
some applications without binders.• Silica Sand• On beach, coastal areas, in river beds mostly in India, Pakistan and Brazil of
good quality. Sub-continent is rich in good quality sand. • Composition depends upon geological history, but main contents are not
changed. Main component is silica (SiO2). It has 94 to 98 % silica.• Fusion point is 17600C (pure) but usually less melting point due to the presence
of impurities. For high melting temperature metals, it is not used For Aluminum (low M.P), brass, bronze it is good to use but for steel (high M.P) it is not good.
• It has poor refractoriness. It is mixed with other sands to get the good refractoriness.
• Bad surface finish. Machining is required in the sand used casting process.• It is chemically reactive with certain basic metals.• It has high thermal expansion which can cause casting defects with high melting
point metals.• It has low thermal conductivity which can lead to unsound casting.• Because of common and abundance, it is of low cost.
• Olivine Sand((Mg,Fe)2SiO4.• Olivine is a mixture of orthosilicates of iron and magnesium from
the mineral dunite.(Dunite granular, green igneous rock composed of coarse grains of olivine, is the source of the world's supply of chromium. It weathers to a dun brown color)
• It is free from silica. • It has good fusion point of 17600C. • It has good thermal stability so has good refractoriness. • Because it is free from silica, therefore it can be used with basic
metals, such as manganese steels. • Olivine sand has high thermal conductivity. • Olivine has low thermal expansion. • This sand gives good surface finish. • It is relatively high costly than the silica sand.
• Chromite Sand• It is the solid solution of spinels.(MgAl2O4 )
• Chromite (Cr2O4) is the main content. Silica is in very small amount.
• It has high fusion point of 18500C.• It has good refractoriness. • It has good chemical inertness. • It has high thermal conductivity.• It has low thermal expansion.• It gives good surface finish.• This sand is rare, so it is expensive. Therefore it’s only
used with expensive alloy steel casting and to make cores.•
• Zircon Sand• Zircon sand is a compound of approximately two-
thirds zirconium oxide (Zr2O) and one-third silica.• Zircon persists in sedimentary deposits and is a
common constituent of most sands. • It has the highest fusion point of all the base sands
at 2,600 °C.• It has very low thermal expansion. • It has a high thermal conductivity.• Because of these good properties it is commonly used
when casting alloy steels and other expensive alloys.• This sand gives good surface finish.• It is expensive and not readily available.
• Chamotte• Chamotte is made by calcining fire clay (Al2O3-SiO2) above 1,100 °C. • Its fusion point is 1,750 °C i.e. gives relatively low refractoriness.• It has low thermal expansion. • This is mixed with other sands to improve refractoriness.• Its disadvantages are, coarse grains, which result in a poor surface finish. • It is the second cheapest sand but it is still twice as expensive as silica. • Silica and chamotte sands have bad surface finish and chromite, olivine and
zircon sands have good refractoriness but they all have not sufficient cohesiveness. So these natural sands are not perfect in properties. So we prepare synthetic molding sands using moisture, binders, additives etc.
• Important Note:• Corrosion is the reverse of extraction.• Synthetic Molding Sand:• The main ingredients of any synthetic molding sand are:• Base sand,• Binder, and• Moisture• Molding sands are prepared synthetically to produce the good molding
properties.
Binders and their types
Binders are used to adhere the particles of sand to each other for the strength of molds. Or it is also defined as it is the glue that holds the mold together. Binders improve cohesiveness.
• Types of Binders:• Clay and water• Oil• Resin• Sodium silicate• • Clay and water:• A mixture of clay and water is used as binder.• It is used where the clay content is less than 15% in the molding sand. • Oils:• Oil use as a binder has restricted conditions.• Long time use or overheating makes the mold hard and brittle. • However due to their increasing cost, they have been mostly phased out. • The oil also required careful baking at 100 to 200 °C to cure, otherwise
wasting of mold.
• Resins:• Resin binders are natural or synthetic high melting
point gums. • These are sticky and in liquid form. • The two common types used are urea
formaldehyde (CH2O)(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 casting.
• Sodium Silicate:• Sodium silicate [Na2SiO3] is a high
strength binder used with silica molding sand.
• To cure the binder, carbon dioxide gas is used, after making mold, which creates the following reaction:
• The mold produced using this binder is hard but has excellent surface quality.
• Na2O(SiO2) + CO2----------Na2CO3 + SiO2 + Heat
Shell Molding
• Introduction:• Shell molding is newest of casting processes. Shell molding replaces
conventional sand molds by shell molds made up of relatively thin, rigid shells of approximately uniform wall thickness. The molten metal is filled in the shell mold and then allowed to solidify.
• Steps:• Metal pattern production• First of all, a metal or steel pattern having the profile of the required
casting. Let say a flask is to be produced. Cop and drag part of the steel pattern are produced.
• Preparation of molding sand• The mixture of silica sand and the thermosetting phenolic resin is used
to produce the molding sand to produce the mold shell. The phenolic resin gives good strength and epoxide resin gives better strength. There use is depending upon the metal used to be cast.
•
• Preparation of the shell• There are two methods to produce the shell. The
patterns are sprayed with a solution of lubricating agent or release agent containing silicone to prevent the shell sticking to the metal pattern in both the cases.
• (1) The steel pattern is preheated in oven 175-270oC. The pattern is placed over the dump box and then inverted. The molding sand is dumped over the steel pattern. Heat from the pattern partially cures the molding sand and the shell is produced of desired thickness drying and makes the resin sticky present in it. The pattern and sand mixture are then inverted, allowing the excess (uncured) sand to drop free. The pattern with adhering shell is then placed in an oven, where additional heating completes the curing process to give rigidity.
• (2) The sand mixture is spread over the steel pattern and then it is heated in the oven up to 600-700oC. The resin hardens and hence the shell is produced.
• The thickness of the shell depends on the pattern temperature and time of contact but typically ranges between 10 and 20 mm. High temperatures produces high thickness of the shell.
• Stripping of the shell• The hardened shell, with tensile strength between 350 and 450 psi, is then stripped from
the steel pattern.• Clamping of two or more shells in a pouring jacket• Two or more cooled shells are then clamped or glued together with a thermosetting
adhesive to produce a mold. Before this, we use carbon powder or talcum powder as parting agent for easy disintegration or to release easily. To provide extra support during the pour of molten metal, shell molds are often placed in a pouring jacket surrounded by the sand or steel shots.
• Pouring of molten metal• The molten metal is poured into the shell. The metal should not come out of parting line.
Otherwise, we will require machining. The heat of molten metal starts burning resin binder of the mold and the gases evolved escape through the permeable shell walls.
• • To get the casting• By the time the casting has solidified, the binder has completely burn out and the shell
mold disintegrates easily. So, casting is extracted. Since shell is no re-useable so it is an expendable mold casting process.
• Disadvantages:• Not for heavy
castings for ferrous or non-ferrous (only up to 10 kg).
• Steel pattern cost is high.
• Resins costs are comparatively high.
• Uneconomical on small scale production.
Advantages:For complex shape castings.For both ferrous and non-ferrous.No separate runner, risers and sprue are not required.Excellent surface finish.High dimensional accuracy.Use of resin produces smooth surface of casting.Cleaning, machining and other finishing cost can be significantly reduced.Low labor cost.Thin shells provide easy permeability.The burn out resin gives good collapsibility and shakeout characteristics.Pouring jacket sand is re-useable.
INVESTMENT CASTING
• Introduction:• It is a very old process-used in ancient china and Egypt and most recently
performed by dentists and jewelers for a number of years. Products such rocket components and jet engine turbine blades required the fabrication of high precision complex shapes from high melting point metals that are not easily machined. It offers unlimited freedom to complexity of shapes and types of materials to be cast.
• After being molded, the wax pattern is not taken out of the mold like other patterns like wooden, rubber and metal patterns.
• This process is also known as lost wax casting process because wax pattern during process melts on heating and comes out of the mold, in which the molten metal is poured.
• Steps:• Production of master pattern• A modified replica of the desired product made from metal, steel, plastic or wood
is made for the production of master die.• Production of master die from the master pattern• A die is produced from the master pattern usually of metal or steel. It is made of
usually of Al because Al has tendency to extract heat very rapidly and material cools down rapidly after melting.
• Production of wax patterns• Wax patterns are made by pouring molten wax into the master
die and allowing it to harden. Release agents, such as silicone sprays or talcum powder are used to assist in pattern removal from the master die.
• Assembling of wax pattern on common wax sprue• Using heated tools and melted wax, a number of patterns can be
attached to a central wax sprue and runner system to create a pattern cluster or a tree. If the product is sufficiently complex that is pattern could not be withdrawn from a single master die, the pattern may be made in pieces and assembled prior to attachment.
• Coating of the cluster or tree with a thin layer of investment material
• The wax pattern and wax sprue assembly is dipped into water slurry of finely ground refractory material. A thin but very smooth layer of investment material is onto the wax pattern, ensuring a smooth surface and good details in final product.
• Investment of the wax pattern assembly for the production of mold• After the initial layer of ceramic slurry, wax pattern assembly is then invested
to produce mold. The initially invested wax pattern assembly is re-dipped in the ceramic slurry but this time, refractory sand is showered on the wet ceramic and then dried for high temperature uses. Repeat the procedure to get the required thickness of the shell. Allow the investment to fully harden.
• Removal of wax pattern from the mold• The wax pattern is removed from the investment mold by inverting the mold,
melting the wax pattern in the oven at 300-400oC. The melted wax pattern comes out. Due to this step this process is called lost-wax process.
• Heating of mold prior to pouring of molten metal• After removing the wax pattern, investment mold is heated at 800-900oC.
This baking ensures complete removal of wax from the mold, cures the mold to give the aided strength, and allows the molten metal to retain its heat and flow more readily into all of thin details and sections and good dimensional accuracy.
• Filling of unbounded sand in the flask around the investment shell mold:• That investment shell mold is kept in flask and filled with the unbounded
sand giving vibration to ensure the compaction, to provide extra support during the pour of molten metal.
• Pouring of molten metal• Molten metal is poured into the investment mold by
simply under gravity to ensure the complete filling of mold. When complex thin sections are involved, the molten metal is poured assisted by positive air pressure.
• Removal of solidified casting from the mold• After solidification, techniques such as mechanical
vibration or sand blasting are used to break the mold and remove the mold material from the metal casting.
• Separation of casting from sprue• For this purpose cutting or machining is applied, to get
the castings in their useable form.• Inspection and testing• Inspection and testing is done on the sample of casting to
investigate about its quality and standard achievement.
• Advantages:• Complex shapes can be cast as single piece.• Mass scale and high rate of production.• Thin sections can be produced.• Excellent dimensional accuracy.• Excellent details and smoother surface.• Machining can be completely eliminated or greatly reduced.• Castings do not contain any disfiguring parting line.• Sounder and denser castings free from defects.• Wax melted is re-usable.• The economic value of this process lies in its ability to produce intricate
shapes in various alloys that could probably not be produced at all by another casting process.
• Disadvantages:• A complex process and expensive.• High cost of dies to make the wax pattern.• For small casting 2-2.5 kg.• Pattern is expendable i.e. one wax pattern is to make one investment mold.• Slow process.
