CASTING PROCESS 1:Basic steps
Industrialprocess
Obtaining the Casting Geometry :
The process is referred as the study of the geometry of parts and plans, so as to improve the life and quality of casting.
The process is referred as the study of the geometry of parts and plans, so as to improve the life and quality of casting.
· Casting Patternmaking :
In pattern making, a physical model of casting, i.e. a pattern is used to make the mold. The mold is made by packing some readily formed aggregated materials, like molding sand, around the pattern. After the pattern is withdrawn, its imprint leaves the mold cavity that is ultimately filled with metal to become the casting.
n case, the castings is required to be hollow, such as in the case of pipe fittings, additional patterns, known as cores, are used to develop these cavities.
In pattern making, a physical model of casting, i.e. a pattern is used to make the mold. The mold is made by packing some readily formed aggregated materials, like molding sand, around the pattern. After the pattern is withdrawn, its imprint leaves the mold cavity that is ultimately filled with metal to become the casting.
n case, the castings is required to be hollow, such as in the case of pipe fittings, additional patterns, known as cores, are used to develop these cavities.
In core making, cores are formed, (usually of sand) that are placed into a mold cavity to form the interior surface of the casting. Thus the annul space between the mold-cavity surface and the core is what finally becomes the casting.
Molding is a process that consists of different operations essential to develop a mold for receiving molten metal.
Molding is a process that consists of different operations essential to develop a mold for receiving molten metal.
· Alloy Melting and Pouring :
Melting is a process of preparing the molten material for casting. It is generally done in a specifically designated part of foundry, and the molten metal is transported to the pouring area wherein the molds are filled.
Melting is a process of preparing the molten material for casting. It is generally done in a specifically designated part of foundry, and the molten metal is transported to the pouring area wherein the molds are filled.
· Casting Cleaning :
Cleaning is a process that refers to the different activities performed for the removal of sand, scale, and excess metal from the casting.
However, all the operations may not apply to each casting method but such processes play an important role to comply with environmental guidelines.
Cleaning is a process that refers to the different activities performed for the removal of sand, scale, and excess metal from the casting.
However, all the operations may not apply to each casting method but such processes play an important role to comply with environmental guidelines.
Obtaining the Casting Geometry
A foundry may pour a casting having little knowledge of how a casting cools down or how the metal freezes within the mold. However, if proper planning is not done the result can be gas or shrink porosity within the casting. To improve the quality of a casting the foundry engineer studies the geometry of the part and plans how the heat removal is to be controlled.
It is important to find the suitable casting geometry so as to meet the structural and solidification shrinkage needs. For some alloys, finding the right geometry can be very simple. For other alloys, obtaining that geometry is the real essence of superior casting design. In the case, that geometry is not found for difficult alloys, the foundry engineer should resort to "thermal trickery" to achieve fluid flow and heat transfer patterns, which the geometry fails to deliver.
Thermal trickery is a highly effective technique but is expensive. By eliminating thermal trickery with good design, it is possibleto achieve cost less production, processing and assembly.
The conventional method of obtaining the casting geometry is by sending blueprint drawings to the foundry. However, the development of computer hardware and software for making and analyzing solid models has enabled a quantum leap in the use of section modules to add features like increase in the stiffness of structural components and reducing the stress within them. In fact, these tools are making the power of metal casting geometry much more accessible to design engineers because they enhance significantly the ability to visualize in three dimensions. Casting Geometry plays a significant role in the casting yield.
Advantages of Good Casting Geometry
· Reduces defects, post casting operations, and rejected castings
· Significantly reduce energy and environmental impacts
· Saves energy
· Improves overall quality and life of casting
· Casting Patternmaking
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It's the science of making an original pattern or form which will be used to make a mold. The mold will allow for mass production of the original pattern by poring some sort of casting material such as metal, resin, plaster, or other material. A pattern can be both simple and complex for e.g an object that has detail on only one side like a plaque, or with detail on all sides, like a machine part. Factors that go into making a pattern includes durability, shrinkage, machine allowances, draft and undercuts.
In sand casting, the pattern is usually of wood, whereas it may be of metal or other materials in pressure or centrifugal casting. The patternmaker also decides where the sprues and risers (inlet and outlet for molten material) will be placed with respect to the pattern. Objects which have holes or depressions in some of its parts are taken care of by inserting cores. Sometimes patterns may also contain chills, solid pieces of the final material, to enable rapid cooling, resulting in quench hardening in the neighborhood of the chill.
It's the science of making an original pattern or form which will be used to make a mold. The mold will allow for mass production of the original pattern by poring some sort of casting material such as metal, resin, plaster, or other material. A pattern can be both simple and complex for e.g an object that has detail on only one side like a plaque, or with detail on all sides, like a machine part. Factors that go into making a pattern includes durability, shrinkage, machine allowances, draft and undercuts.In sand casting, the pattern is usually of wood, whereas it may be of metal or other materials in pressure or centrifugal casting. The patternmaker also decides where the sprues and risers (inlet and outlet for molten material) will be placed with respect to the pattern. Objects which have holes or depressions in some of its parts are taken care of by inserting cores. Sometimes patterns may also contain chills, solid pieces of the final material, to enable rapid cooling, resulting in quench hardening in the neighborhood of the chill.
As sand casting methods have paved the way for die casting and thin mold casting (the mold is made of a thin, workable refractory material), patterns are not much in use today, though they may still be used for small or specialized jobs.
Coremaking & Molding
Molds by themselves can only have the external shape of the pattern. That is why cores are placed inside the mold to form internal cavities. Cores are produced in a core box, which is a permanent mold, developed in tandem with the pattern. It helps in flowing the molten metal to all sides of the cores.Cores are supported either on core prints or by metal supports called chaplets. Generally the foundry molds are made of sand grains bonded together to form the desired shape of the casting. Sand is used because it is cheap, resists deformation when heated, offers a great variety of casting sizes and complexities. It also offers the added advantage of reuse of a large portion of the sand in future molds. However pattern making, melting, cleaning, and finishing operations are essentially the same whether or not sand molds are used in the casting process.
Alloy Melting and Pouring
The preparation of molten metal for casting is referred to as melting. It is usually done in a specifically designated area of the foundry, and the molten metal is transferred to the pouring area where the molds are filled. Melting may be done by gas or electricity. Various methods of pouring the molten metal are in use (e.g. gravity pouring, bottom pouring, vacuum or pressure assisted pouring).
The melting process begins with the metal specification for the casting, determining the type of scrap metal to be used to 'charge' the furnace. Once charged, the furnace uses electrodes, each supplying roughly 6,500 amps of electricity, to melt the scrap metal. Samples are taken at various points in the melt process, to ascertain the chemical composition of the molten metal. Using a spectrometer as a guide, alloys are added to the furnace to bring the molten metal to the proper specification.
At a temperature of around 3,000° F the metal reaches the desired specification. It is then poured into a preheated ladle for transfer to the pouring lines. At the pouring lines, molten metal is 'poured' into the requisite molds. Due to the lifting pressure of molten steel, molds will often be 'weighted' or 'clamped' to prevent them from separating at the Cope/Drag meeting point. Thereafter, the mold is allowed to cool for approximately 30 minutes before it is taken to the shakeout. Poured molds are then dumped into a vibrating conveyor, wherein they are broken up by the vibration, exposing the casting for removal.
The melting process begins with the metal specification for the casting, determining the type of scrap metal to be used to 'charge' the furnace. Once charged, the furnace uses electrodes, each supplying roughly 6,500 amps of electricity, to melt the scrap metal. Samples are taken at various points in the melt process, to ascertain the chemical composition of the molten metal. Using a spectrometer as a guide, alloys are added to the furnace to bring the molten metal to the proper specification.
At a temperature of around 3,000° F the metal reaches the desired specification. It is then poured into a preheated ladle for transfer to the pouring lines. At the pouring lines, molten metal is 'poured' into the requisite molds. Due to the lifting pressure of molten steel, molds will often be 'weighted' or 'clamped' to prevent them from separating at the Cope/Drag meeting point. Thereafter, the mold is allowed to cool for approximately 30 minutes before it is taken to the shakeout. Poured molds are then dumped into a vibrating conveyor, wherein they are broken up by the vibration, exposing the casting for removal.
Finally the sand from the mold is separated and processed through a reclamation system for further use.
Casting Cleaning
The fifth and the final stage of casting process is cleaning. The process refers to different activities that are performed to remove the sand, scale and excess metal from the casting. Some of the activities performed in cleaning are -· The casting is separated from the mold and transported to the cleaning department.
· Burned-on sand and scale are removed.
· Excess metal is removed (Fins, wires, parting line fins, and gates).
· Subsequently the casting can be upgraded using welding or other such as procedures.
· Final testing and inspection to check for any defects.
Advantages:
· Improves the surface appearance and finish of casting
· Improves overall quality and functionality by removing impurities, such as sand, scale and excess metal
Finally the sand from the mold is separated and processed through a reclamation system for further use.
Molds and Die Making
The change in cultural patterns and increase in outsourcing to different less developed countries has led to a drastic reformation in the industry of dies and molds. Let us see what all is required to excel in this new milieu.
The globalization is having an adverse effect on the local die and mold industry, which already is a tough and competitive business. The mold and die makers need to reorient their plans in order to meet the new standards. The process is already begun with various companies, which are either small or medium in size, entering into new supportive and collaborative efforts. Various syndications and acquisitions taking place substantiate this change.
Earlier tool making was considered as a highly skillful art. But not anymore today. With the advent of new technological tools like parametric modeling in 3D, tooling at rapid pace and machining at high speed etc, anybody can enter into tool making using right blend of these technological tools. The business structures of yore are fast becoming redundant and the need of the hour is to focus on value instead of cost, to make progress. The tool and die makers should target suitable markets. They should also invest in developing products, which are specialized materials, in proper course of action and of course on the cognizance of the customers' needs. Summarily, the whole competition should be on value rather than cost.
Earlier tool making was considered as a highly skillful art. But not anymore today. With the advent of new technological tools like parametric modeling in 3D, tooling at rapid pace and machining at high speed etc, anybody can enter into tool making using right blend of these technological tools. The business structures of yore are fast becoming redundant and the need of the hour is to focus on value instead of cost, to make progress. The tool and die makers should target suitable markets. They should also invest in developing products, which are specialized materials, in proper course of action and of course on the cognizance of the customers' needs. Summarily, the whole competition should be on value rather than cost.
Consultant Glenn Beall advised Modern Plastics that although quite a few processors have tried to aggrandize their businesses by taking care of various additional services like inventory facility, doing finishing making of packages, assembling and even decorating for their original equipment manufacturers (OEM), it is not good enough. Providing additional services would only be useful and beneficial if they are coupled with some specialty like provision of an in-house or internal painting line or perhaps the capability of electroplating. Mere offering of more services is not tantamount to adding value.
To obtain the end products in mold and die making a flow process, which is uninterrupted, is applied on a mixture, which can bind and also be used again. Such a mixture mainly consists of ventonite clay, a lubricant like oil, and sand. All of these are mixed together in such a way that on the mixture the desired impressions can be made to form back to back molds and dies. The mixture is then made to move through many zones wherein some work is performed on them. This is accomplished by using a conveyor belt, which takes the mixture through these zones. The conveyor belt is again required for spreading of mixture on it.
The process of smoothening and rolling is carried out to make the mixture uniform and consistent according to the specifications. After that a mold maker is used to impress upon the mixture in order to get mold or dies. Further, a material, which can harden the mold or die, is injected into it.
Whole of the product is then heated so that finished and cured product can be obtained from the hardenable material. The mixture is then finally separated from the final product that is cured and finished, and is transferred back to the stage from where the entire process started. Again another conveyor belt is used for transferring the mixture. The mixture again goes through the same steps to form finished products like structural material.
Offshore outsourcing of manufacturing by the automobile sector has also contributed greatly to the decline of the traditional strategies. The ones who have suffered the most because of this transportation to other economical locations are small shops, which are owned by families and employ about 5 to 100 workers. The reason being a large dependence of mold and die makers on the transportation industry, particularly automotive.
The subcontracting to other economical locations has come as a bane for the tool shops that are small. Statistics reveal that North America has lost somewhere around 150,000 jobs of tooling since 2000 just because of this outsourcing. The reason is obvious from the fact that around 60% of stamping dies and about 40% of plastic molds are engaged directly or indirectly by the automotive industry worldwide. Apart from this, over a period of five years, the market of machinery used for molding injections has gone down by about 50% in United States.
It can thus be inferred from all this data that the industry of molds and dies has ceased to be merely a skillful occupation. In fact, it has graduated into becoming a very competitive, multifaceted business. Survival and rise in this industry today would only be possible if the people involved pay attention to certain new parameters. These parameters include devoting their energies to a particular skill, strengthening their services, trying to make their products more valuable, and of course seeking markets, which could be their forte.
Hot Forming Process
Hot forming process is used very frequently for casting of industrial products and parts. In this process, heat is applied to soften the piece of metal. This metal or raw material is available in the form of sheet, bar, tube or wire. Then some form of pressure is used to alter the shape of the metal. The hot forming process can form a variety of complex parts and hold relatively tight tolerances.Most of the hot forming processes are complex in nature and involves adiabatic heating, die chill and microstructural changes. Hot forming process includes various operations in the manufacturing of high quality, critical components and parts. In most cases, the cost-effective production of near-net shape components depends on the interactions between casting, ingot breakdown, extrusion, closed-die forging, heat treatment and machining.
Advantages
· The metal "springback" effect is reduced and the part's ductility is improved.
· The desired shape is achieved with relatively low pressure and minimal residual part stress.
· Produces superior grain flow and microstructure that improves the part's mechanical properties.
· Relatively high production rates with minimal defects.
The following casting processes can be categorized under Hot Forming Process:
· Centrifugal Casting
· Ceramic Mold Casting
· Extrusion
· Forging
· Full Mold Casting
· Permanent Mold Cast
· Plaster Mold Casting
· Powder Metallurgy
· Sand Casting
· Shell Mold Casting
Centrifugal Casting
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This process can be categorized as similar to that of permanent mold method. Here as the molten metal is being poured, a permanent metal mold revolves about its axis at high speeds ranging from 300 to 3000 rpm. This can be in horizontal, vertical or inclined positions. As a result, the molten metal is centrifugally thrown towards the inside mold wall. There it solidifies after cooling. It's usually a fine grain casting with a very fine-grained outer diameter.
Molds for centrifugal castings can be divided into three types.
The Permanent Mold: Made of steel, iron or graphite. Inside surface is coated with a thin refractory wash to increase mold life. The mold is preheated before coating, so as to dry the coating and improve the adherence to the mold surface.
Rammed Mold: It consists of a steel metal flask, lined with a layer of refractory molding mix. The inside lining is coated with a refractory wash which is baked until dry and hard.
Spun or Centrifugally Cast Mold: In the metal flask a predetermined mass of refractory material in slurry form is poured.
The flask on rotation makes the refractory materials centrifuged onto the wall of the flask. The rotation is stopped and the liquid portion of the slurry drained off. It leaves the mold with a refractory coating, to be baked until dry before use.
Features of Centrifugal Casting
Rammed Mold: It consists of a steel metal flask, lined with a layer of refractory molding mix. The inside lining is coated with a refractory wash which is baked until dry and hard.
Spun or Centrifugally Cast Mold: In the metal flask a predetermined mass of refractory material in slurry form is poured.
The flask on rotation makes the refractory materials centrifuged onto the wall of the flask. The rotation is stopped and the liquid portion of the slurry drained off. It leaves the mold with a refractory coating, to be baked until dry before use.
Features of Centrifugal Casting
· Castings can be made in almost any length, thickness and diameter.
· Different wall thicknesses can be produced from the same size mold.
· Eliminates the need for cores.
· Resistant to atmospheric corrosion, a typical situation with pipes.
· Mechanical properties of centrifugal castings are excellent.
· Only cylindrical shapes can be produced with this process.
· Size limits are upto 3 m (10 feet) diameter and 15 m (50 feet) length.
· Wall thickness range from 2.5 mm to 125 mm (0.1 - 5.0 in).
· Tolerance limit: on the OD can be 2.5 mm (0.1 in) on the ID can be 3.8 mm (0.15 in).
· Surface finish ranges from 2.5 mm to 12.5 mm (0.1 - 0.5 in) rms.
Applications of Centrifugal Casting:
Typical materials that can be cast with this process are iron, steel, stainless steels and alloys of aluminum, copper and nickel. Two materials can be cast by introducing a second material during the process. Typical parts made by this process are pipes, boilers, pressure vessels, flywheels, cylinder liners and other parts that are axi-symmetric.
There are two types of Centrifugal Casting:
Ceramic Mold Casting
This process use a method very near to plaster mold casting. Plaster, plastic, wood, metal or rubber is used for making the pattern. A ceramic slurry comprising zircon, fused silica and a bonding agent is first poured over the pattern. Like rubber it hardens quickly. It is then peeled of the pattern and reassembled as a mold. The volatile materials are removed in a low temperature oven. Ceramic mold, with high temperature pours is obtained after it is baked in a furnace at about 1000 °C (1832 °F) .
Features of Ceramic Mold Casting
· Tolerances: 0.4 %,
· Surface finish: 2 - 4 µm (.075 - .15 µin)
· Wall thickness: may be as small as 1.25 mm (.050 in),
· Weights: Range from 60 g (2oz) to a 1000kg
· Draft allowance: 1° recommended.
· Patterns: Reusable and cheap.
· Casting size: generally not restricted except above 100 lb
Ceramic Mold Casting can be performed by two distinct procedures:
True Ceramic Molding: Here, the refractory grain is first bonded with calcium or ammonium phosphates. The ceramic molds are generally made by the dry pressing method. Where molds are made by pressing clay mixture with some percentage of moisture in dies under a pressure of 1-10 ton/sq inch. The mold is finally ready after they are stripped from the dies and baked in a furnace at temperatures that range between 1650-2400°F (899°C and 1316°C).
Shaw Process: Shaw process or the Ethyl silicate variation takes place in the following way. A consistent slurry is made by blending together a mixture of graded refractory filler, hydrolyzed ethyl silicate, and a liquid catalyst. It is then poured in the pattern and allowed to jell. After this, the mold is stripped and heated using a high pressure gas torch. It is then cooled, assembled and fired before pouring is done. Some times the Shaw process and the lost wax process are used in combination to gain the advantages of both the processes.
Application of Ceramic Mold Casting
Parts made from this process include impellers, complex cutting tools, plastic mold tooling etc.
Extrusion Process
Extrusion process is used for manufacturing long and straight metal parts. The shape of the cross-sections can be solid round, rectangular, to T shapes, L shapes and Tubes etc. Extrusion is done by squeezing the metal in a die by using a mechanical or hydraulic press.
Extrusion is capable of producing compressive and shear forces in the stock material. As tensile is not produced, this makes very high deformity a possibility without actually tearing the metal. A wear resistant material lines the cavity in which the raw material is contained . This helps to resist the high radial loads as the material is pushed into the die.
Features of Extrusion Process:
Features of Extrusion Process:
· Cross-section: Wide variety of cross-sections can be made.
· Minimum thickness: For steel 3 mm (0.120 in), for Aluminum and Magnesium 1mm (0.040 in).
· Minimum cross section: For steel 250 mm (0.4 in) for steel.
· Corner and fillet radii: 0.4 mm (0.015 in) for Aluminum and Magnesium, for steel the minimum corner radius is 0.8mm (0.030 in) and 4 mm (0.120 in) fillet radius.
An example of Hot Extrusion Process using Aluminium Alloy is briefly described here:
The alloyed press bars are cut into smaller pieces and heated up in an induction furnace to 450ñ500°C. The bar is then pressed with very high force using speeds between 5ñ50 m/min through a hollowed tool. As a result a profile is formed. The length of the profile ranges between 25ñ45 m. Immediately after the pressing operation, profile is cooled with air or water.
The profile is straightened and internal stresses released by stretching it in a pulling machine just after cooling. The profile is then cut into required lengths. Finally ageing gives the strength of the material. Which can be done by natural ageing at normal temperature or artificial ageing done at elevated temperature of 170ñ185°C.
Application of Extrusion Process: Trim parts as used in automobile and construction equipment, railings, window frame members, structural parts etc.
Extrusion can be of two types
Hot Extrusion
Generally done at fairly high temperatures, approximately at 50 to 75 % of the melting point of the metal. The pressures range from 35-700 MPa (5076 - 101,525 psi). To cool down the high temperatures and pressures and its adverse effect on the die life as well as other components, good lubrication is a must. Oil graphite and glass powder is preferred as lubricants.
Application of Hot Extrusion:
Aluminium, copper with their alloys are successfully used to manufacture products using hot extrusion process. Electrical wires, bars and tubes are some of the items produced.
Cold Extrusion
Cold extrusion takes place at room temperature or slightly elevated temperatures. This process is useful for withstanding the stresses created by extrusion.
The advantages of cold extrusion are:
· No oxidation process .
· Good mechanical properties provided the temperatures created are below the re-crystallization temperature. Good surface finish
Application of Cold Extrusion:
Examples of the metals that can be extruded are copper, lead, tin, aluminum alloys, titanium, molybdenum, vanadium, steel. Which are used to make parts like collapsible tubes, gear blanks, aluminum cans, cylinders etc.
In automobile sector they have found wide applications in Injection technology; Engine control; Fuel supply; Automatic transmissions Seat technology; Safety systems (restraint systems).
Forging Process
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· Swaging
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Forging technology occupies a very important place among all the manufacturing processes as it produces parts with excellent properties and with minimal wastage. Through a series of operations, metals with very simple geometry are physically deformed into products of complex configuration.In the forging process the metal is heated up by applying compressive force and given shape by plastic deformation. The compressive force is applied by hammer blows using a power hammer or a press. Powered by compressed air, hydraulics electricity or steam. The weight of the hammer can be 500 pounds to thousands of pounds. Forging has the capacity to refine the grain structure and improve the physical properties of the metal.
Forging products are consistent, without the defects of porosity, inclusion or voids, finishing operations like machining and coating operations like painting or plating can also be done very easily.
Typically a forged metal results in the following:
· Drawing Out of the Metal: Increased length decreased cross-section.
· Upsetting the Metal: Decrease in length decreased cross-section.
· Change in Length; Change in Cross-section: Resulting in favourable grain flow for strong parts.
Tips for Selecting the Right Forging Technology
· Technology to remain competitive must come out with cost effective alternatives. That is the reason computer aided techniques like CAD, CAM, CAE and Finite Element Analysis (FEA) based computer simulation, are used to selecting the right forging process.
· Understanding the forged material's flow behaviour under processing conditions.
· Knowledge of the die geometry and materials.
· Environmental considerations.
· Evaluating the mechanics of deformation process-stress and strain.
· Friction and Lubricating process.
· Nature of the Forging equipment.
Applications of Forging Process:
Wide variety of uses in different kinds of Industries:
Automobile Industry: Wheel spindles, kingpins, axle beams and shafts, torsion bars, ball studs, idler arms and steering arm.
Agro-Industries: Engine and transmission components, levers, gears, shafts and spindles to tie-rod ends, spike harrow teeth and cultivator shafts.
Aerospace: Bulkheads, hinges, wing roots, engine mounts, brackets, beams, shafts, landing gear cylinders and struts, wheels, brake carriers and discs and arresting hooks, blades, buckets couplings etc.
Hand Tools: Sledges, pliers, hammers, wrenches and garden tools, as well as wire-rope clips, sockets, hooks, turnbuckles and eye bolts are common examples.
Industrial Equipment: Connecting rods, blanks, blocks, cylinders, discs, elbows, rings, T's, shafts and sleeves.
Methods of Forging
· Swaging
Full Mold Casting
Full-mold casting technique is quite similar to investment casting, but here polystyrene foam is used as the pattern. The foam pattern is laced with a refractory material. The pattern is put in an encasing of one-piece sand mold, during the pouring of the metal. The foam vaporizes and its place is taken by the metal.Features
· Complex shaped castings: Possible without any draft or flash.
· Minimum wall thicknesses: 2.5 mm
· Tolerances: Can be held to 3% on dimensions.
· Surface finish: From 2.5µm to 25µm (0.1µin to 1.0 µin) rms.
· Size limits: From 400 g (1 lb) to several tons.
· Draft allowance: Required.
Application of Full Mold Casting:
Materials cast with this process are aluminum, iron, steels, nickel alloys, copper alloys to produce pump housings, manifolds, and auto brake components.
This process is commonly known as the lost wax casting process. It got its name because of the fact that ancient Egyptians used it to make gold jewelry hence the name Investment. Very intricate shapes with high accuracy can be made in this process.
Additionally metals which are hard to machine or fabricate can be cast with this process. Parts that cannot be produced otherwise by normal manufacturing processes like turbine blades with complex shapes, or airplane parts that needs to withstand high temperatures are examples of this process.
(A) The pattern assembly is covered with ceramic to produce a monolithic mold
(B) Melting the assembly for a precise mold cavity; firing the mold to remove residues of the pattern; developing the bond and preheating the mold ready for casting; pouring
(C) Finally knockout, cutoff and finishing processes.
The process works like this first a mold is made by making a pattern. Wax or some other materials can be used that can be melted away. The wax pattern is dipped in refractory slurry, which coats the wax pattern and a skin is subsequently formed. It is then dried. The process of dipping in the slurry and drying is continued till a firm thickness is achieved. The pattern is placed in an oven and the wax melted. This leads to a mold which can be easily filled with the molten metal.
The wax pattern can be made by a duplicating process using a stereo lithography or a similar model that has been fabricated by a computerized solid model master.
The slurry materials used are a mixture of plaster of Paris, a binder and, a refractory material. Powdered silica is used for low temperature melts. For higher temperature melts, an alumina-silicate is used as the refractory material. While silica is used as a binder. Additional coatings of sillimanite and ethyl silicate may be applied to increase the quality of the finished product.. The mold thus produced is ready for use as light castings. It may be reinforced by placing it in a bigger container and adding more slurry.
Before the pouring operation, the mold is pre-heated at about 1000ºC (1832ºF) to remove traces of wax. Pouring can be done in gravity, pressure or vacuum conditions. Mold permeability factor is to be kept in mind when using pressure, to allow the air to escape as the pouring is made.
Features of Investment Casting
The wax pattern can be made by a duplicating process using a stereo lithography or a similar model that has been fabricated by a computerized solid model master.
The slurry materials used are a mixture of plaster of Paris, a binder and, a refractory material. Powdered silica is used for low temperature melts. For higher temperature melts, an alumina-silicate is used as the refractory material. While silica is used as a binder. Additional coatings of sillimanite and ethyl silicate may be applied to increase the quality of the finished product.. The mold thus produced is ready for use as light castings. It may be reinforced by placing it in a bigger container and adding more slurry.
Before the pouring operation, the mold is pre-heated at about 1000ºC (1832ºF) to remove traces of wax. Pouring can be done in gravity, pressure or vacuum conditions. Mold permeability factor is to be kept in mind when using pressure, to allow the air to escape as the pouring is made.
Features of Investment Casting
· Allows undercuts in the pattern
· Tolerances of 0.5 % of length are possible, and for small dimensions it can be as low as 0.15%.
· Weight of the castings can range from a few grams to 35 kg (0.1 oz to 80 lb)
· Minimum wall thicknesses are about 1 mm to 0.5 mm (0.040-0.020 in)
· Parts do not require machining because of the closer tolerances.
· Smooth surface finish.
· Excellent production rates, particularly for small components
· Thorough dimensional accuracy and consistency
· High integrity castings
· Machining can be eliminated
· Minimal shot blast and grinding needed
Application of Investment Casting:
Typically materials that can be cast with this process are Aluminum alloys, Bronzes, Stainless steels, Stellite etc. Glass mold accessory castings, Valves and fittings, Gears, Levers and Splines are some of the popular usages.
The steps as depicted in the figures include: Making of heat-disposable wax or plastic patterns, and assembling them onto the gating system
Limitations
Time consuming process and costly. Exceptional surface finish possible but minute lacuna can cause rejection of castings as a result scrap rates can be high.
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Permanent Mold Cast
This process utilizes, a metal as the mold material. Ideally cast iron or Meehanite (a dense cast iron) is used. Metal or sand is used for cores. Thin layers of Clay or sodium silicate is used for coating the cavity surfaces, as they are heat resistant. The molds are pre-heated at a temperature upto 200ºC (392ºF) before the metal is poured into the cavity. Proper precaution is needed for ensuring thermal balance. This can be facilitated by using external water cooling or suitable radiation techniques. Features of Permanent Mold Cast:
· Allows use of different patterns.
· Lowers cost of production, although that depends upon the complexity of the part produced.
· Wall Thickness - typical considerations apply here such as 3mm for lengths under 75 mm), radius (inside radius = nominal wall thickness, outside radius = 3 x nominal wall thickness)
· Draft Angles - 1 to 3º on outside surfaces, 2 to 5º on inside surfaces)
· Tolerance is 2% of linear dimensions.
· Surface Finish ranges from 2.5 µm to 7.5 µm (100 µin to 250 µin).
· Part Sizes range from 50 g to 70 kg (1.5 ounces to 150 lb).
· Casting method is clean with little waste and no fumes. The process avoids the contamination problems that generally sand foundries deal with.
Application of Permanent Mold Cast:
Most common materials used are small and medium sized parts made of aluminum, magnesium and brass. Products include gears, splines, wheels, gear housings, pipe fittings, fuel injection housings, and automotive engine pistons.
Permanent Mold Castings can take various forms such as:
Slush Casting
A special type of permanent mold casting, where the molten metal is not solidified completely. After obtaining the desired wall thickness the semi- solidified molten metal is poured out.
Useful for making hollow ornamental objects such as lamps, candlesticks, statues etc.
Low Pressure Permanent Mold Casting
Yet another variation of the permanent mold casting. Instead of using gravity to help in the metal pour and flow, a low pressure of 1 atmosphere gas is applied to the molten metal. The pressure on the melt causes the mold to be completely filled up and compensates for any shrinkage on cooling. Mechanical properties are superior to permanent mold casting by about 5%.
Vacuum Permanent Mold Casting
Also a variation of the permanent mold casting. Thin wall castings can be made here. The yields are generally high since no risers are used. The mechanical properties are better than the traditional permanent mold casting by 10-15% Castings sizes can range from 200 g to 4.5 kg (6 oz to 10 lb).
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