Classification of casting process:
Casting processes can be classified into following FOUR categories:
1.Conventional Molding Processes
*Green Sand Molding
*Dry Sand Molding
*Flask less Molding
2. Chemical Sand Molding Processes
*Shell Molding
*Sodium Silicate Molding
*No-Bake Molding
3. Permanent Mold Processes
*Gravity Die casting
*Low and High Pressure Die Casting
4. Special Casting Processes
*Lost Wax
*Ceramics Shell Molding
*Evaporative Pattern Casting
*Vacuum Sealed Molding
*Centrifugal Casting
Green Sand Molding:
Green sand is the most diversified molding method used in metal casting operations. The process utilizes a mold made of compressed or compacted moist sand. The term "green" denotes the presence of moisture in the molding sand. The mold material consists of silica sand mixed with a suitable bonding agent (usually clay) and moisture.
Advantages:
Most metals can be cast by this method.
Pattern costs and material costs are relatively low.
No Limitation with respect to size of casting and type of metal or alloy used
Disadvantages:
Surface Finish of the castings obtained by this process is not good and machining is often required to achieve the finished product.
Sand making mold Procedure:
1.The procedure for making mold of a cast iron wheel.
2.The first step in making mold is to place the pattern on the molding board.
3.The drag is placed on the board.
4.Dry facing sand is sprinkled over the board and pattern to provide a non sticky layer.
5.Molding sand is then riddled in to cover the pattern with the fingers; then the drag is completely filled.
6.The sand is then firmly packed in the drag by means of hand rammers. The ramming must be proper i.e. it must neither be too hard or soft.
7.After the ramming is over, the excess sand is leveled off with a straight bar known as a strike rod.
With the help of vent rod, vent holes are made in the drag to the full depth of the flask as well as to the pattern to facilitate the removal of gases during pouring and solidification.
8.The finished drag flask is now rolled over to the bottom board exposing the pattern.
Cope half of the pattern is then placed over the drag pattern with the help of locating pins. The cope flask on the drag is located aligning again with the help of pins.
9.The dry parting sand is sprinkled all over the drag and on the pattern.
10.A sprue pin for making the sprue passage is located at a small distance from the pattern. Also, riser pin, if required, is placed at an appropriate place.
11.The operation of filling, ramming and venting of the cope proceed in the same manner as performed in the drag.
12.The sprue and riser pins are removed first and a pouring basin is scooped out at the top to pour the liquid metal.
Then pattern from the cope and drag is removed and facing sand in the form of paste is applied all over the mold cavity and runners which would give the finished casting a good surface finish.
13.The mold is now assembled. The mold now is ready for pouring
Molding Material and Properties:
A large variety of molding materials is used in foundries for manufacturing molds and cores. They include molding sand, system sand or backing sand, facing sand, parting sand, and core sand. The choice of molding materials is based on their processing properties. The properties that are generally required in molding materials are:
Refractoriness:
It is the ability of the molding material to resist the temperature of the liquid metal to be poured so that it does not get fused with the metal. The refractoriness of the silica sand is highest.
Permeability:
During pouring and subsequent solidification of a casting, a large amount of gases and steam is generated. These gases are those that have been absorbed by the metal during melting, air absorbed from the atmosphere and the steam generated by the molding and core sand. If these gases are not allowed to escape from the mold, they would be entrapped inside the casting and cause casting defects. To overcome this problem the molding material must be porous. Proper venting of the mold also helps in escaping the gases that are generated inside the mold cavity.
Green Strength:
The molding sand that contains moisture is termed as green sand. The green sand particles must have the ability to cling to each other to impart sufficient strength to the mold. The green sand must have enough strength so that the constructed mold retains its shape.
Dry Strength:
When the molten metal is poured in the mold, the sand around the mold cavity is quickly converted into dry sand as the moisture in the sand evaporates due to the heat of the molten metal. At this stage the molding sand must posses the sufficient strength to retain the exact shape of the mold cavity and at the same time it must be able to withstand the metallostatic pressure of the liquid material.
Hot Strength:
As soon as the moisture is eliminated, the sand would reach at a high temperature when the metal in the mold is still in liquid state. The strength of the sand that is required to hold the shape of the cavity is called hot strength.
Collapsibility:
The molding sand should also have collapsibility so that during the contraction of the solidified casting it does not provide any resistance, which may result in cracks in the castings.Besides these specific properties the molding material should be cheap, reusable and should have good thermal conductivity.
Molding Sand Composition:
The main ingredients of any molding sand are:
*Base sand,
*Binder, and
*Moisture
Base Sand:
Silica sand is most commonly used base sand. Other base sands that are also used for making mold are zircon sand, Chromite sand, and olivine sand. Silica sand is cheapest among all types of base sand and it is easily available.
Binder:
Binders are of many types such as:
Clay binders,
Organic binders and
Inorganic binders
Clay binders are most commonly used binding agents mixed with the molding sands to provide the strength. The most popular clay types are:
Kaolinite or fire clay (Al2O3 2 SiO2 2 H2O) and Bentonite (Al2O3 4 SiO2 nH2O)
Of the two the Bentonite can absorb more water which increases its bonding power.
Moisture:
Clay acquires its bonding action only in the presence of the required amount of moisture. When water is added to clay, it penetrates the mixture and forms a microfilm, which coats the surface of each flake of the clay. The amount of water used should be properly controlled. This is because a part of the water, which coats the surface of the clay flakes, helps in bonding, while the remainder helps in improving the plasticity. A typical composition of molding sand
Dry Sand Molding:
When it is desired that the gas forming materials are lowered in the molds, air-dried molds are sometimes preferred to green sand molds. Two types of drying of molds are often required.
*Skin drying and
*Complete mold drying.
In skin drying a firm mold face is produced. Shakeout of the mold is almost as good as that obtained with green sand molding. The most common method of drying the refractory mold coating uses hot air, gas or oil flame. Skin drying of the mold can be accomplished with the aid of torches, directed at the mold surface.
Shell Molding Process:
It is a process in which, the sand mixed with a thermosetting resin is allowed to come in contact with a heated pattern plate (200 oC), this causes a skin (Shell) of about 3.5 mm of sand/plastic mixture to adhere to the pattern.. Then the shell is removed from the pattern. The cope and drag shells are kept in a flask with necessary backup material and the molten metal is poured into the mold.
This process can produce complex parts with good surface finish 1.25 µm to 3.75 µm, and dimensional tolerance of 0.5 %. A good surface finish and good size tolerance reduce the need for machining. The process overall is quite cost effective due to reduced machining and cleanup costs. The materials that can be used with this process are cast irons, and aluminum and copper alloys.
Molding Sand in Shell Molding Process:
The molding sand is a mixture of fine grained quartz sand and powdered bakelite. There are two methods of coating the sand grains with bakelite. First method is Cold coating method and another one is the hot method of coating.
In the method of cold coating, quartz sand is poured into the mixer and then the solution of powdered bakelite in acetone and ethyl aldehyde are added. The typical mixture is 92% quartz sand, 5% bakelite, 3% ethyl aldehyde. During mixing of the ingredients, the resin envelops the sand grains and the solvent evaporates, leaving a thin film that uniformly coats the surface of sand grains, thereby imparting fluidity to the sand mixtures.
In the method of hot coating, the mixture is heated to 150-180 o C prior to loading the sand. In the course of sand mixing, the soluble phenol formaldehyde resin is added. The mixer is allowed to cool up to 80 – 90 o C. This method gives better properties to the mixtures than cold method.
Sodium Silicate Molding Process:
In this process, the refractory material is coated with a sodium silicate-based binder. For molds, the sand mixture can be compacted manually, jolted or squeezed around the pattern in the flask. After compaction, CO 2 gas is passed through the core or mold. The CO 2 chemically reacts with the sodium silicate to cure, or harden, the binder. This cured binder then holds the refractory in place around the pattern. After curing, the pattern is withdrawn from the mold.
The sodium silicate process is one of the most environmentally acceptable of the chemical processes available. The major disadvantage of the process is that the binder is very hygroscopic and readily absorbs water, which causes a porosity in the castings.. Also, because the binder creates such a hard, rigid mold wall, shakeout and collapsibility characteristics can slow down production. Some of the advantages of the process are:
A hard, rigid core and mold are typical of the process, which gives the casting good dimensional tolerances;
good casting surface finishes are readily obtainable;
Permanent Mold Process:
In al the above processes, a mold need to be prepared for each of the casting produced. For large-scale production, making a mold, for every casting to be produced, may be difficult and expensive. Therefore, a permanent mold, called the die may be made from which a large number of castings can be produced. , the molds are usually made of cast iron or steel, although graphite, copper and aluminum have been used as mold materials. The process in which we use a die to make the castings is called permanent mold casting or gravity die casting, since the metal enters the mold under gravity. Some time in die-casting we inject the molten metal with a high pressure. When we apply pressure in injecting the metal it is called pressure die casting process.
Advantages
Permanent Molding produces a sound dense casting with superior mechanical properties.
The castings produced are quite uniform in shape have a higher degree of dimensional accuracy than castings produced in sand
The permanent mold process is also capable of producing a consistent quality of finish on castings
Disadvantages
The cost of tooling is usually higher than for sand castings
The process is generally limited to the production of small castings of simple exterior design, although complex castings such as aluminum engine blocks and heads are now commonplace.
Centrifugal Casting:
In this process, the mold is rotated rapidly about its central axis as the metal is poured into it. Because of the centrifugal force, a continuous pressure will be acting on the metal as it solidifies. The slag, oxides and other inclusions being lighter, get separated from the metal and segregate towards the center. This process is normally used for the making of hollow pipes, tubes, hollow bushes, etc., which are axisymmetric with a concentric hole. Since the metal is always pushed outward because of the centrifugal force, no core needs to be used for making the concentric hole. The mold can be rotated about a vertical, horizontal or an inclined axis or about its horizontal and vertical axes simultaneously. The length and outside diameter are fixed by the mold cavity dimensions while the inside diameter is determined by the amount of molten metal poured into the mold
Investment Casting Process:
The root of the investment casting process, the cire perdue or “lost wax” method dates back to at least the fourth millennium B.C. The artists and sculptors of ancient Egypt and Mesopotamia used the rudiments of the investment casting process to create intricately detailed jewelry, pectorals and idols. The investment casting process alos called lost wax process begins with the production of wax replicas or patterns of the desired shape of the castings. A pattern is needed for every casting to be produced. The patterns are prepared by injecting wax or polystyrene in a metal dies. A number of patterns are attached to a central wax sprue to form a assembly. The mold is prepared by surrounding the pattern with refractory slurry that can set at room temperature. The mold is then heated so that pattern melts and flows out, leaving a clean cavity behind. The mould is further hardened by heating and the molten metal is poured while it is still hot. When the casting is solidified, the mold is broken and the casting taken out.
The basic steps of the investment casting process are :
1.Production of heat-disposable wax, plastic, or polystyrene patterns
2.Assembly of these patterns onto a gating system
3.“Investing,” or covering the pattern assembly with refractory slurry
4.Melting the pattern assembly to remove the pattern material
5.Firing the mold to remove the last traces of the pattern material
6.Pouring
7.Knockout, cutoff and finishing.
Ceramic Shell Investment Casting Process:
The basic difference in investment casting is that in the investment casting the wax pattern is immersed in a refractory aggregate before dewaxing whereas, in ceramic shell investment casting a ceramic shell is built around a tree assembly by repeatedly dipping a pattern into a slurry (refractory material such as zircon with binder). After each dipping and stuccoing is completed, the assembly is allowed to thoroughly dry before the next coating is applied. Thus, a shell is built up around the assembly. The thickness of this shell is dependent on the size of the castings and temperature of the metal to be poured.
After the ceramic shell is completed, the entire assembly is placed into an autoclave or flash fire furnace at a high temperature. The shell is heated to about 982 o C to burn out any residual wax and to develop a high-temperature bond in the shell. The shell molds can then be stored for future use or molten metal can be poured into them immediately. If the shell molds are stored, they have to be preheated before molten metal is poured into them.
Full Mold Process / Lost Foam Process / Evaporative Pattern Casting Process:
The use of foam patterns for metal casting was patented by H.F. Shroyer on April 15, 1958. In Shroyer's patent, a pattern was machined from a block of expanded polystyrene (EPS) and supported by bonded sand during pouring. This process is known as the full mold process. With the full mold process, the pattern is usually machined from an EPS block and is used to make primarily large, one-of-a kind castings. The full mold process was originally known as the lost foam process. However, current patents have required that the generic term for the process be full mold.
In 1964, M.C. Flemmings used unbounded sand with the process. This is known today as lost foam casting (LFC). With LFC, the foam pattern is molded from polystyrene beads. LFC is differentiated from full mold by the use of unbounded sand (LFC) as opposed to bonded sand (full mold process).
Foam casting techniques have been referred to by a variety of generic and proprietary names. Among these are lost foam, evaporative pattern casting, cavity less casting, evaporative foam casting, and full mold casting.
In this method, the pattern, complete with gates and risers, is prepared from expanded polystyrene. This pattern is embedded in a no bake type of sand. While the pattern is inside the mold, molten metal is poured through the sprue. The heat of the metal is sufficient to gasify the pattern and progressive displacement of pattern material by the molten metal takes place.
The EPC process is an economical method for producing complex, close-tolerance castings using an expandable polystyrene pattern and unbonded sand. Expandable polystyrene is a thermoplastic material that can be molded into a variety of complex, rigid shapes. The EPC process involves attaching expandable polystyrene patterns to an expandable polystyrene gating system and applying a refractory coating to the entire assembly. After the coating has dried, the foam pattern assembly is positioned on loose dry sand in a vented flask. Additional sand is then added while the flask is vibrated until the pattern assembly is completely embedded in sand. Molten metal is poured into the sprue, vaporizing the foam polystyrene, perfectly reproducing the pattern.
In this process, a pattern refers to the expandable polystyrene or foamed polystyrene part that is vaporized by the molten metal. A pattern is required for each casting.
Process Description:
1.The EPC procedure starts with the pre-expansion of beads, usually polystyrene. After the pre-expanded beads are stabilized, they are blown into a mold to form pattern sections. When the beads are in the mold, a steam cycle causes them to fully expand and fuse together.
2.The pattern sections are assembled with glue, forming a cluster. The gating system is also attached in a similar manner.
3.The foam cluster is covered with a ceramic coating. The coating forms a barrier so that the molten metal does not penetrate or cause sand erosion during pouring.
4.After the coating dries, the cluster is placed into a flask and backed up with bonded sand.
5.Mold compaction is then achieved by using a vibration table to ensure uniform and proper compaction. 6.Once this procedure is complete, the cluster is packed in the flask and the mold is ready to be poured .
Vacuum Sealed Molding Process:
It is a process of making molds utilizing dry sand, plastic film and a physical means of binding using negative pressure or vacuum. V-process was developed in Japan in 1971. Since then it has gained considerable importance due to its capability to produce dimensionally accurate and smooth castings. The basic difference between the V-process and other sand molding processes is the manner in which sand is bounded to form the mold cavity. In V-process vacuum, of the order of 250 – 450 mm Hg, is imposed to bind the dry free flowing sand encapsulated in between two plastic films. The technique involves the formation of a mold cavity by vacuum forming of a plastic film over the pattern, backed by unbounded sand, which is compacted by vibration and held rigidly in place by applying vacuum. When the metal is poured into the molds, the plastic film first melts and then gets sucked just inside the sand voids due to imposed vacuum where it condenses and forms a shell-like layer. The vacuum must be maintained until the metal solidifies, after which the vacuum is released allowing the sand to drop away leaving a casting with a smooth surface. No shakeout equipment is required and the same sand can be cooled and reused without further treatment.
Sequence of Producing V-Process Molds:
*The Pattern is set on the Pattern Plate of Pattern Box. The Pattern as well as the Pattern Plate has Numerous Small Holes. These Holes Help the Plastic Film to Adhere Closely on Pattern When Vacuum is Applied.
*A Heater is used to Soften the Plastic Film
*The Softened Plastic Film Drapes over the Pattern. The Vacuum Suction Acts through the Vents (Pattern and Pattern Plate) to draw it so that it adheres closely to the Pattern.
*The Molding Box is Set on the Film Coated Pattern
*The Molding Box is filled with Dry Sand. Slight Vibration Compacts the Sand
*Level the Mold. Cover the Top of Molding Box with Plastic Film. Vacuum Suction Stiffens the Mold.
*Release the Vacuum on the Pattern Box and Mold Strips Easily.
*Cope and Drag are assembled and Metal is poured. During Pouring the Mold is Kept under Vacuum
*After Cooling, the Vacuum is released. Free Flowing Sand Drops Away, Leaving a Clean Casting
Mealting Practices:
Melting is an equally important parameter for obtaining a quality castings. A number of furnaces can be used for melting the metal, to be used, to make a metal casting. The choice of furnace depends on the type of metal to be melted. Some of the furnaces used in metal casting are as following:.
*Crucible furnaces
*Cupola
*Induction furnace
*Reverberatory furnace
Crucible Furnace:
Crucible furnaces are small capacity typically used for small melting applications. Crucible furnace is suitable for the batch type foundries where the metal requirement is intermittent. The metal is placed in a crucible which is made of clay and graphite. The energy is applied indirectly to the metal by heating the crucible by coke, oil or gas.The heating of crucible is done by coke, oil or gas. .
Coke-Fired Furnace .
1.Primarily used for non-ferrous metals
2.Furnace is of a cylindrical shape
3.Also known as pit furnace
4.Preparation involves: first to make a deep bed of coke in the furnace
5.Burn the coke till it attains the state of maximum combustion
6.Insert the crucible in the coke bed
7.Remove the crucible when the melt reaches to desired temperature
Cupola:
Cupola furnaces are tall, cylindrical furnaces used to melt iron and ferrous alloys in foundry operations. Alternating layers of metal and ferrous alloys, coke, and limestone are fed into the furnace from the top. A schematic diagram of a cupola.This diagram of a cupola illustrates the furnace's cylindrical shaft lined with refractory and the alternating layers of coke and metal scrap. The molten metal flows out of a spout at the bottom of the cupola. .
Description of Cupola:
*The cupola consists of a vertical cylindrical steel sheet and lined inside with acid refractory bricks. The lining is generally thicker in the lower portion of the cupola as the temperature are higher than in upper portion
*There is a charging door through which coke, pig iron, steel scrap and flux is charged
*The blast is blown through the tuyeres
*These tuyeres are arranged in one or more row around the periphery of cupola
*Hot gases which ascends from the bottom (combustion zone) preheats the iron in the preheating zone
*Cupolas are provided with a drop bottom door through which debris, consisting of coke, slag etc. can be discharged at the end of the melt
*A slag hole is provided to remove the slag from the melt
*Through the tap hole molten metal is poured into the ladle
*At the top conical cap called the spark arrest is provided to prevent the spark emerging to outside
Operation of Cupola:
The cupola is charged with wood at the bottom. On the top of the wood a bed of coke is built. Alternating layers of metal and ferrous alloys, coke, and limestone are fed into the furnace from the top. The purpose of adding flux is to eliminate the impurities and to protect the metal from oxidation. Air blast is opened for the complete combustion of coke. When sufficient metal has been melted that slag hole is first opened to remove the slag. Tap hole is then opened to collect the metal in the ladle.
Reverberatory furnace:
A furnace or kiln in which the material under treatment is heated indirectly by means of a flame deflected downward from the roof. Reverberatory furnaces are used in opper, tin, and nickel production, in the production of certain concretes and cements, and in aluminum. Reverberatory furnaces heat the metal to melting temperatures with direct fired wall-mounted burners. The primary mode of heat transfer is through radiation from the refractory brick walls to the metal, but convective heat transfer also provides additional heating from the burner to the metal. The advantages provided by reverberatory melters is the high volume processing rate, and low operating and maintenance costs. The disadvantages of the reverberatory melters are the high metal oxidation rates, low efficiencies, and large floor space requirements. A schematic of Reverberatory furnace.
Induction furnace:
Induction heating is a heating method. The heating by the induction method occurs when an electrically conductive material is placed in a varying magnetic field. Induction heating is a rapid form of heating in which a current is induced directly into the part being heated. Induction heating is a non-contact form of heating.
*The heating system in an induction furnace includes:
*Induction heating power supply,
*Induction heating coil,
*Water-cooling source, which cools the coil and several internal components inside the power supply.
The induction heating power supply sends alternating current through the induction coil, which generates a magnetic field. Induction furnaces work on the principle of a transformer. An alternative electromagnetic field induces eddy currents in the metal which converts the electric energy to heat without any physical contact between the induction coil and the work piece. A schematic diagram of induction furnace.The furnace contains a crucible surrounded by a water cooled copper coil. The coil is called primary coil to which a high frequency current is supplied. By induction secondary currents, called eddy currents are produced in the crucible. High temperature can be obtained by this method. Induction furnaces are of two types: cored furnace and coreless furnace. Cored furnaces are used almost exclusively as holding furnaces. In cored furnace the electromagnetic field heats the metal between two coils. Coreless furnaces heat the metal via an external primary coil.
Gating system:
The assembly of channels which facilitates the molten metal to enter into the mold cavity is called the gating system.Alternatively, the gating system refers to all passage ways through which molten metal passes to enter into the mold cavity. The nomenclature of gating system depends upon the function of different channels which they perform.
*Down gates or sprue
*Cross gates or runners
*Ingates or gates
The metal flows down from the pouring basin or pouring cup into the down gate or sprue and passes through the cross gate or channels and ingates or gates before entering into the mold cavity.
Goals of Gating System:
The goals for the gating system are
*To minimize turbulence to avoid trapping gasses into the mold
*To get enough metal into the mold cavity before the metal starts to solidify
*To avoid shrinkage
*Establish the best possible temperature gradient in the solidifying casting so that the shrinkage if occurs must be in the gating system not in the required cast part.
*Incorporates a system for trapping the non-metallic inclusions
Hydraulic Principles used in the Gating System
Reynold's Number
Nature of flow in the gating system can be established by calculating Reynold's number
RN = Reynold's number
V = Mean Velocity of flow
D = diameter of tubular flow
m = Kinematics Viscosity = Dynamic viscosity / Density
r = Fluid density
When the Reynold's number is less than 2000 stream line flow results and when the number is more than 2000 turbulent flow prevails. As far as possible the turbulent flow must be avoided in the sand mold as because of the turbulence sand particles gets dislodged from the mold or the gating system and may enter into the mould cavity leading to the production of defective casting. Excess turbulence causes
*Inclusion of dross or slag
*Air aspiration into the mold
*Erosion of the mold walls
Bernoulli's Equation:
h = height of liquid
P = Static Pressure
n = metal velocity
g = Acceleration due to gravity
r = Fluid density
Turbulence can be avoided by incorporating small changes in the design of gating system. The sharp changes in the flow should be avoided to smooth changes. The gating system must be designed in such a way that the system always runs full with the liquid metal. The most important things to remember in designing runners and gates are to avoid sharp corners. Any changes in direction or cross sectional area should make use of rounded corners.
To avoid the aspiration the tapered sprues are designed in the gating systems. A sprue tapered to a smaller size at its bottom will create a choke which will help keep the sprue full of molten metal.
Types of Gating Systems:
The gating systems are of two types:
*Pressurized gating system
*Un-pressurized gating system
Pressurized Gating System:
*The total cross sectional area decreases towards the mold cavity
*Back pressure is maintained by the restrictions in the metal flow
*Flow of liquid (volume) is almost equal from all gates
*Back pressure helps in reducing the aspiration as the sprue always runs full
*Because of the restrictions the metal flows at high velocity leading to more turbulence and chances of mold erosion
Un-Pressurized Gating System:
*The total cross sectional area increases towards the mold cavity
*Restriction only at the bottom of sprue
*Flow of liquid (volume) is different from all gates
*aspiration in the gating system as the system never runs full
*Less turbulence
Riser:
Riser is a source of extra metal which flows from riser to mold cavity to compensate for shrinkage which takes place in the casting when it starts solidifying. Without a riser heavier parts of the casting will have shrinkage defects, either on the surface or internally.
Risers are known by different names as metal reservoir, feeders, or headers.
Shrinkage in a mold, from the time of pouring to final casting, occurs in three stages.
during the liquid state
during the transformation from liquid to solid
during the solid state
First type of shrinkage is being compensated by the feeders or the gating system. For the second type of shrinkage risers are required. Risers are normally placed at that portion of the casting which is last to freeze. A riser must stay in liquid state at least as long as the casting and must be able to feed the casting during this time.
Functions of Risers:
*Provide extra metal to compensate for the volumetric shrinkage
*Allow mold gases to escape
*Provide extra metal pressure on the solidifying mold to reproduce mold details more exact
Design Requirements of Risers:
Riser size: For a sound casting riser must be last to freeze. The ratio of (volume / surface area)2 of the riser must be greater than that of the casting. However, when this condition does not meet the metal in the riser can be kept in liquid state by heating it externally or using exothermic materials in the risers.
Riser placement: the spacing of risers in the casting must be considered by effectively calculating the feeding distance of the risers.
Riser shape: cylindrical risers are recommended for most of the castings as spherical risers, although considers as best, are difficult to cast. To increase volume/surface area ratio the bottom of the riser can be shaped as hemisphere.
Casting Defects:
The following are the major defects, which are likely to occur in sand castings
*Gas defects
*Shrinkage cavities
*Molding material defects
*Pouring metal defects
*Mold shift
Gas Defects:
A condition existing in a casting caused by the trapping of gas in the molten metal or by mold gases evolved during the pouring of the casting. The defects in this category can be classified into blowholes and pinhole porosity. Blowholes are spherical or elongated cavities present in the casting on the surface or inside the casting. Pinhole porosity occurs due to the dissolution of hydrogen gas, which gets entrapped during heating of molten metal.
Causes:
The lower gas-passing tendency of the mold, which may be due to lower venting, lower permeability of the mold or improper design of the casting. The lower permeability is caused by finer grain size of the sand, high percentage of clay in mold mixture, and excessive moisture present in the mold.
*Metal contains gas
*Mold is too hot
*Poor mold burnout
Shrinkage Cavities:
These are caused by liquid shrinkage occurring during the solidification of the casting. To compensate for this, proper feeding of liquid metal is required. For this reason risers are placed at the appropriate places in the mold. Sprues may be too thin, too long or not attached in the proper location, causing shrinkage cavities. It is recommended to use thick sprues to avoid shrinkage cavities.
Molding Material Defects:
The defects in this category are cuts and washes, metal penetration, fusion, and swell.
Cut and washes:
These appear as rough spots and areas of excess metal, and are caused by erosion of molding sand by the flowing metal. This is caused by the molding sand not having enough strength and the molten metal flowing at high velocity. The former can be taken care of by the proper choice of molding sand and the latter can be overcome by the proper design of the gating system.
Metal penetration:
When molten metal enters into the gaps between sand grains, the result is a rough casting surface. This occurs because the sand is coarse or no mold wash was applied on the surface of the mold. The coarser the sand grains more the metal penetration.
Fusion:
This is caused by the fusion of the sand grains with the molten metal, giving a brittle, glassy appearance on the casting surface. The main reason for this is that the clay or the sand particles are of lower refractoriness or that the pouring temperature is too high.
Swell:
Under the influence of metallostatic forces, the mold wall may move back causing a swell in the dimension of the casting. A proper ramming of the mold will correct this defect.
Inclusions:
Particles of slag, refractory materials, sand or deoxidation products are trapped in the casting during pouring solidification. The provision of choke in the gating system and the pouring basin at the top of the mold can prevent this defect.
Pouring Metal Defects:
The likely defects in this category are
*Mis-runs and
*Cold shuts.
A mis-run is caused when the metal is unable to fill the mold cavity completely and thus leaves unfilled cavities. A mis-run results when the metal is too cold to flow to the extremities of the mold cavity before freezing. Long, thin sections are subject to this defect and should be avoided in casting design.
A cold shut is caused when two streams while meeting in the mold cavity, do not fuse together properly thus forming a discontinuity in the casting. When the molten metal is poured into the mold cavity through more-than-one gate, multiple liquid fronts will have to flow together and become one solid. If the flowing metal fronts are too cool, they may not flow together, but will leave a seam in the part. Such a seam is called a cold shut, and can be prevented by assuring sufficient superheat in the poured metal and thick enough walls in the casting design.
The mis-run and cold shut defects are caused either by a lower fluidity of the mold or when the section thickness of the casting is very small. Fluidity can be improved by changing the composition of the metal and by increasing the pouring temperature of the metal.
Mold Shift:
The mold shift defect occurs when cope and drag or molding boxes have not been properly aligned.
Casting processes can be classified into following FOUR categories:
1.Conventional Molding Processes
*Green Sand Molding
*Dry Sand Molding
*Flask less Molding
2. Chemical Sand Molding Processes
*Shell Molding
*Sodium Silicate Molding
*No-Bake Molding
3. Permanent Mold Processes
*Gravity Die casting
*Low and High Pressure Die Casting
4. Special Casting Processes
*Lost Wax
*Ceramics Shell Molding
*Evaporative Pattern Casting
*Vacuum Sealed Molding
*Centrifugal Casting
Green Sand Molding:
Green sand is the most diversified molding method used in metal casting operations. The process utilizes a mold made of compressed or compacted moist sand. The term "green" denotes the presence of moisture in the molding sand. The mold material consists of silica sand mixed with a suitable bonding agent (usually clay) and moisture.
Advantages:
Most metals can be cast by this method.
Pattern costs and material costs are relatively low.
No Limitation with respect to size of casting and type of metal or alloy used
Disadvantages:
Surface Finish of the castings obtained by this process is not good and machining is often required to achieve the finished product.
Sand making mold Procedure:
1.The procedure for making mold of a cast iron wheel.
2.The first step in making mold is to place the pattern on the molding board.
3.The drag is placed on the board.
4.Dry facing sand is sprinkled over the board and pattern to provide a non sticky layer.
5.Molding sand is then riddled in to cover the pattern with the fingers; then the drag is completely filled.
6.The sand is then firmly packed in the drag by means of hand rammers. The ramming must be proper i.e. it must neither be too hard or soft.
7.After the ramming is over, the excess sand is leveled off with a straight bar known as a strike rod.
With the help of vent rod, vent holes are made in the drag to the full depth of the flask as well as to the pattern to facilitate the removal of gases during pouring and solidification.
8.The finished drag flask is now rolled over to the bottom board exposing the pattern.
Cope half of the pattern is then placed over the drag pattern with the help of locating pins. The cope flask on the drag is located aligning again with the help of pins.
9.The dry parting sand is sprinkled all over the drag and on the pattern.
10.A sprue pin for making the sprue passage is located at a small distance from the pattern. Also, riser pin, if required, is placed at an appropriate place.
11.The operation of filling, ramming and venting of the cope proceed in the same manner as performed in the drag.
12.The sprue and riser pins are removed first and a pouring basin is scooped out at the top to pour the liquid metal.
Then pattern from the cope and drag is removed and facing sand in the form of paste is applied all over the mold cavity and runners which would give the finished casting a good surface finish.
13.The mold is now assembled. The mold now is ready for pouring
Molding Material and Properties:
A large variety of molding materials is used in foundries for manufacturing molds and cores. They include molding sand, system sand or backing sand, facing sand, parting sand, and core sand. The choice of molding materials is based on their processing properties. The properties that are generally required in molding materials are:
Refractoriness:
It is the ability of the molding material to resist the temperature of the liquid metal to be poured so that it does not get fused with the metal. The refractoriness of the silica sand is highest.
Permeability:
During pouring and subsequent solidification of a casting, a large amount of gases and steam is generated. These gases are those that have been absorbed by the metal during melting, air absorbed from the atmosphere and the steam generated by the molding and core sand. If these gases are not allowed to escape from the mold, they would be entrapped inside the casting and cause casting defects. To overcome this problem the molding material must be porous. Proper venting of the mold also helps in escaping the gases that are generated inside the mold cavity.
Green Strength:
The molding sand that contains moisture is termed as green sand. The green sand particles must have the ability to cling to each other to impart sufficient strength to the mold. The green sand must have enough strength so that the constructed mold retains its shape.
Dry Strength:
When the molten metal is poured in the mold, the sand around the mold cavity is quickly converted into dry sand as the moisture in the sand evaporates due to the heat of the molten metal. At this stage the molding sand must posses the sufficient strength to retain the exact shape of the mold cavity and at the same time it must be able to withstand the metallostatic pressure of the liquid material.
Hot Strength:
As soon as the moisture is eliminated, the sand would reach at a high temperature when the metal in the mold is still in liquid state. The strength of the sand that is required to hold the shape of the cavity is called hot strength.
Collapsibility:
The molding sand should also have collapsibility so that during the contraction of the solidified casting it does not provide any resistance, which may result in cracks in the castings.Besides these specific properties the molding material should be cheap, reusable and should have good thermal conductivity.
Molding Sand Composition:
The main ingredients of any molding sand are:
*Base sand,
*Binder, and
*Moisture
Base Sand:
Silica sand is most commonly used base sand. Other base sands that are also used for making mold are zircon sand, Chromite sand, and olivine sand. Silica sand is cheapest among all types of base sand and it is easily available.
Binder:
Binders are of many types such as:
Clay binders,
Organic binders and
Inorganic binders
Clay binders are most commonly used binding agents mixed with the molding sands to provide the strength. The most popular clay types are:
Kaolinite or fire clay (Al2O3 2 SiO2 2 H2O) and Bentonite (Al2O3 4 SiO2 nH2O)
Of the two the Bentonite can absorb more water which increases its bonding power.
Moisture:
Clay acquires its bonding action only in the presence of the required amount of moisture. When water is added to clay, it penetrates the mixture and forms a microfilm, which coats the surface of each flake of the clay. The amount of water used should be properly controlled. This is because a part of the water, which coats the surface of the clay flakes, helps in bonding, while the remainder helps in improving the plasticity. A typical composition of molding sand
Dry Sand Molding:
When it is desired that the gas forming materials are lowered in the molds, air-dried molds are sometimes preferred to green sand molds. Two types of drying of molds are often required.
*Skin drying and
*Complete mold drying.
In skin drying a firm mold face is produced. Shakeout of the mold is almost as good as that obtained with green sand molding. The most common method of drying the refractory mold coating uses hot air, gas or oil flame. Skin drying of the mold can be accomplished with the aid of torches, directed at the mold surface.
Shell Molding Process:
It is a process in which, the sand mixed with a thermosetting resin is allowed to come in contact with a heated pattern plate (200 oC), this causes a skin (Shell) of about 3.5 mm of sand/plastic mixture to adhere to the pattern.. Then the shell is removed from the pattern. The cope and drag shells are kept in a flask with necessary backup material and the molten metal is poured into the mold.
This process can produce complex parts with good surface finish 1.25 µm to 3.75 µm, and dimensional tolerance of 0.5 %. A good surface finish and good size tolerance reduce the need for machining. The process overall is quite cost effective due to reduced machining and cleanup costs. The materials that can be used with this process are cast irons, and aluminum and copper alloys.
The molding sand is a mixture of fine grained quartz sand and powdered bakelite. There are two methods of coating the sand grains with bakelite. First method is Cold coating method and another one is the hot method of coating.
In the method of cold coating, quartz sand is poured into the mixer and then the solution of powdered bakelite in acetone and ethyl aldehyde are added. The typical mixture is 92% quartz sand, 5% bakelite, 3% ethyl aldehyde. During mixing of the ingredients, the resin envelops the sand grains and the solvent evaporates, leaving a thin film that uniformly coats the surface of sand grains, thereby imparting fluidity to the sand mixtures.
In the method of hot coating, the mixture is heated to 150-180 o C prior to loading the sand. In the course of sand mixing, the soluble phenol formaldehyde resin is added. The mixer is allowed to cool up to 80 – 90 o C. This method gives better properties to the mixtures than cold method.
Sodium Silicate Molding Process:
In this process, the refractory material is coated with a sodium silicate-based binder. For molds, the sand mixture can be compacted manually, jolted or squeezed around the pattern in the flask. After compaction, CO 2 gas is passed through the core or mold. The CO 2 chemically reacts with the sodium silicate to cure, or harden, the binder. This cured binder then holds the refractory in place around the pattern. After curing, the pattern is withdrawn from the mold.
The sodium silicate process is one of the most environmentally acceptable of the chemical processes available. The major disadvantage of the process is that the binder is very hygroscopic and readily absorbs water, which causes a porosity in the castings.. Also, because the binder creates such a hard, rigid mold wall, shakeout and collapsibility characteristics can slow down production. Some of the advantages of the process are:
A hard, rigid core and mold are typical of the process, which gives the casting good dimensional tolerances;
good casting surface finishes are readily obtainable;
Permanent Mold Process:
In al the above processes, a mold need to be prepared for each of the casting produced. For large-scale production, making a mold, for every casting to be produced, may be difficult and expensive. Therefore, a permanent mold, called the die may be made from which a large number of castings can be produced. , the molds are usually made of cast iron or steel, although graphite, copper and aluminum have been used as mold materials. The process in which we use a die to make the castings is called permanent mold casting or gravity die casting, since the metal enters the mold under gravity. Some time in die-casting we inject the molten metal with a high pressure. When we apply pressure in injecting the metal it is called pressure die casting process.
Advantages
Permanent Molding produces a sound dense casting with superior mechanical properties.
The castings produced are quite uniform in shape have a higher degree of dimensional accuracy than castings produced in sand
The permanent mold process is also capable of producing a consistent quality of finish on castings
Disadvantages
The cost of tooling is usually higher than for sand castings
The process is generally limited to the production of small castings of simple exterior design, although complex castings such as aluminum engine blocks and heads are now commonplace.
Centrifugal Casting:
In this process, the mold is rotated rapidly about its central axis as the metal is poured into it. Because of the centrifugal force, a continuous pressure will be acting on the metal as it solidifies. The slag, oxides and other inclusions being lighter, get separated from the metal and segregate towards the center. This process is normally used for the making of hollow pipes, tubes, hollow bushes, etc., which are axisymmetric with a concentric hole. Since the metal is always pushed outward because of the centrifugal force, no core needs to be used for making the concentric hole. The mold can be rotated about a vertical, horizontal or an inclined axis or about its horizontal and vertical axes simultaneously. The length and outside diameter are fixed by the mold cavity dimensions while the inside diameter is determined by the amount of molten metal poured into the mold
Investment Casting Process:
The root of the investment casting process, the cire perdue or “lost wax” method dates back to at least the fourth millennium B.C. The artists and sculptors of ancient Egypt and Mesopotamia used the rudiments of the investment casting process to create intricately detailed jewelry, pectorals and idols. The investment casting process alos called lost wax process begins with the production of wax replicas or patterns of the desired shape of the castings. A pattern is needed for every casting to be produced. The patterns are prepared by injecting wax or polystyrene in a metal dies. A number of patterns are attached to a central wax sprue to form a assembly. The mold is prepared by surrounding the pattern with refractory slurry that can set at room temperature. The mold is then heated so that pattern melts and flows out, leaving a clean cavity behind. The mould is further hardened by heating and the molten metal is poured while it is still hot. When the casting is solidified, the mold is broken and the casting taken out.
The basic steps of the investment casting process are :
1.Production of heat-disposable wax, plastic, or polystyrene patterns
2.Assembly of these patterns onto a gating system
3.“Investing,” or covering the pattern assembly with refractory slurry
4.Melting the pattern assembly to remove the pattern material
5.Firing the mold to remove the last traces of the pattern material
6.Pouring
7.Knockout, cutoff and finishing.
Ceramic Shell Investment Casting Process:
The basic difference in investment casting is that in the investment casting the wax pattern is immersed in a refractory aggregate before dewaxing whereas, in ceramic shell investment casting a ceramic shell is built around a tree assembly by repeatedly dipping a pattern into a slurry (refractory material such as zircon with binder). After each dipping and stuccoing is completed, the assembly is allowed to thoroughly dry before the next coating is applied. Thus, a shell is built up around the assembly. The thickness of this shell is dependent on the size of the castings and temperature of the metal to be poured.
After the ceramic shell is completed, the entire assembly is placed into an autoclave or flash fire furnace at a high temperature. The shell is heated to about 982 o C to burn out any residual wax and to develop a high-temperature bond in the shell. The shell molds can then be stored for future use or molten metal can be poured into them immediately. If the shell molds are stored, they have to be preheated before molten metal is poured into them.
Full Mold Process / Lost Foam Process / Evaporative Pattern Casting Process:
The use of foam patterns for metal casting was patented by H.F. Shroyer on April 15, 1958. In Shroyer's patent, a pattern was machined from a block of expanded polystyrene (EPS) and supported by bonded sand during pouring. This process is known as the full mold process. With the full mold process, the pattern is usually machined from an EPS block and is used to make primarily large, one-of-a kind castings. The full mold process was originally known as the lost foam process. However, current patents have required that the generic term for the process be full mold.
In 1964, M.C. Flemmings used unbounded sand with the process. This is known today as lost foam casting (LFC). With LFC, the foam pattern is molded from polystyrene beads. LFC is differentiated from full mold by the use of unbounded sand (LFC) as opposed to bonded sand (full mold process).
Foam casting techniques have been referred to by a variety of generic and proprietary names. Among these are lost foam, evaporative pattern casting, cavity less casting, evaporative foam casting, and full mold casting.
In this method, the pattern, complete with gates and risers, is prepared from expanded polystyrene. This pattern is embedded in a no bake type of sand. While the pattern is inside the mold, molten metal is poured through the sprue. The heat of the metal is sufficient to gasify the pattern and progressive displacement of pattern material by the molten metal takes place.
The EPC process is an economical method for producing complex, close-tolerance castings using an expandable polystyrene pattern and unbonded sand. Expandable polystyrene is a thermoplastic material that can be molded into a variety of complex, rigid shapes. The EPC process involves attaching expandable polystyrene patterns to an expandable polystyrene gating system and applying a refractory coating to the entire assembly. After the coating has dried, the foam pattern assembly is positioned on loose dry sand in a vented flask. Additional sand is then added while the flask is vibrated until the pattern assembly is completely embedded in sand. Molten metal is poured into the sprue, vaporizing the foam polystyrene, perfectly reproducing the pattern.
In this process, a pattern refers to the expandable polystyrene or foamed polystyrene part that is vaporized by the molten metal. A pattern is required for each casting.
Process Description:
1.The EPC procedure starts with the pre-expansion of beads, usually polystyrene. After the pre-expanded beads are stabilized, they are blown into a mold to form pattern sections. When the beads are in the mold, a steam cycle causes them to fully expand and fuse together.
2.The pattern sections are assembled with glue, forming a cluster. The gating system is also attached in a similar manner.
3.The foam cluster is covered with a ceramic coating. The coating forms a barrier so that the molten metal does not penetrate or cause sand erosion during pouring.
4.After the coating dries, the cluster is placed into a flask and backed up with bonded sand.
5.Mold compaction is then achieved by using a vibration table to ensure uniform and proper compaction. 6.Once this procedure is complete, the cluster is packed in the flask and the mold is ready to be poured .
Vacuum Sealed Molding Process:
It is a process of making molds utilizing dry sand, plastic film and a physical means of binding using negative pressure or vacuum. V-process was developed in Japan in 1971. Since then it has gained considerable importance due to its capability to produce dimensionally accurate and smooth castings. The basic difference between the V-process and other sand molding processes is the manner in which sand is bounded to form the mold cavity. In V-process vacuum, of the order of 250 – 450 mm Hg, is imposed to bind the dry free flowing sand encapsulated in between two plastic films. The technique involves the formation of a mold cavity by vacuum forming of a plastic film over the pattern, backed by unbounded sand, which is compacted by vibration and held rigidly in place by applying vacuum. When the metal is poured into the molds, the plastic film first melts and then gets sucked just inside the sand voids due to imposed vacuum where it condenses and forms a shell-like layer. The vacuum must be maintained until the metal solidifies, after which the vacuum is released allowing the sand to drop away leaving a casting with a smooth surface. No shakeout equipment is required and the same sand can be cooled and reused without further treatment.
Sequence of Producing V-Process Molds:
*The Pattern is set on the Pattern Plate of Pattern Box. The Pattern as well as the Pattern Plate has Numerous Small Holes. These Holes Help the Plastic Film to Adhere Closely on Pattern When Vacuum is Applied.
*A Heater is used to Soften the Plastic Film
*The Softened Plastic Film Drapes over the Pattern. The Vacuum Suction Acts through the Vents (Pattern and Pattern Plate) to draw it so that it adheres closely to the Pattern.
*The Molding Box is Set on the Film Coated Pattern
*The Molding Box is filled with Dry Sand. Slight Vibration Compacts the Sand
*Level the Mold. Cover the Top of Molding Box with Plastic Film. Vacuum Suction Stiffens the Mold.
*Release the Vacuum on the Pattern Box and Mold Strips Easily.
*Cope and Drag are assembled and Metal is poured. During Pouring the Mold is Kept under Vacuum
*After Cooling, the Vacuum is released. Free Flowing Sand Drops Away, Leaving a Clean Casting
Mealting Practices:
Melting is an equally important parameter for obtaining a quality castings. A number of furnaces can be used for melting the metal, to be used, to make a metal casting. The choice of furnace depends on the type of metal to be melted. Some of the furnaces used in metal casting are as following:.
*Crucible furnaces
*Cupola
*Induction furnace
*Reverberatory furnace
Crucible Furnace:
Crucible furnaces are small capacity typically used for small melting applications. Crucible furnace is suitable for the batch type foundries where the metal requirement is intermittent. The metal is placed in a crucible which is made of clay and graphite. The energy is applied indirectly to the metal by heating the crucible by coke, oil or gas.The heating of crucible is done by coke, oil or gas. .
Coke-Fired Furnace .
1.Primarily used for non-ferrous metals
2.Furnace is of a cylindrical shape
3.Also known as pit furnace
4.Preparation involves: first to make a deep bed of coke in the furnace
5.Burn the coke till it attains the state of maximum combustion
6.Insert the crucible in the coke bed
7.Remove the crucible when the melt reaches to desired temperature
Cupola:
Cupola furnaces are tall, cylindrical furnaces used to melt iron and ferrous alloys in foundry operations. Alternating layers of metal and ferrous alloys, coke, and limestone are fed into the furnace from the top. A schematic diagram of a cupola.This diagram of a cupola illustrates the furnace's cylindrical shaft lined with refractory and the alternating layers of coke and metal scrap. The molten metal flows out of a spout at the bottom of the cupola. .
Description of Cupola:
*The cupola consists of a vertical cylindrical steel sheet and lined inside with acid refractory bricks. The lining is generally thicker in the lower portion of the cupola as the temperature are higher than in upper portion
*There is a charging door through which coke, pig iron, steel scrap and flux is charged
*The blast is blown through the tuyeres
*These tuyeres are arranged in one or more row around the periphery of cupola
*Hot gases which ascends from the bottom (combustion zone) preheats the iron in the preheating zone
*Cupolas are provided with a drop bottom door through which debris, consisting of coke, slag etc. can be discharged at the end of the melt
*A slag hole is provided to remove the slag from the melt
*Through the tap hole molten metal is poured into the ladle
*At the top conical cap called the spark arrest is provided to prevent the spark emerging to outside
Operation of Cupola:
The cupola is charged with wood at the bottom. On the top of the wood a bed of coke is built. Alternating layers of metal and ferrous alloys, coke, and limestone are fed into the furnace from the top. The purpose of adding flux is to eliminate the impurities and to protect the metal from oxidation. Air blast is opened for the complete combustion of coke. When sufficient metal has been melted that slag hole is first opened to remove the slag. Tap hole is then opened to collect the metal in the ladle.
Reverberatory furnace:
A furnace or kiln in which the material under treatment is heated indirectly by means of a flame deflected downward from the roof. Reverberatory furnaces are used in opper, tin, and nickel production, in the production of certain concretes and cements, and in aluminum. Reverberatory furnaces heat the metal to melting temperatures with direct fired wall-mounted burners. The primary mode of heat transfer is through radiation from the refractory brick walls to the metal, but convective heat transfer also provides additional heating from the burner to the metal. The advantages provided by reverberatory melters is the high volume processing rate, and low operating and maintenance costs. The disadvantages of the reverberatory melters are the high metal oxidation rates, low efficiencies, and large floor space requirements. A schematic of Reverberatory furnace.
Induction furnace:
Induction heating is a heating method. The heating by the induction method occurs when an electrically conductive material is placed in a varying magnetic field. Induction heating is a rapid form of heating in which a current is induced directly into the part being heated. Induction heating is a non-contact form of heating.
*The heating system in an induction furnace includes:
*Induction heating power supply,
*Induction heating coil,
*Water-cooling source, which cools the coil and several internal components inside the power supply.
The induction heating power supply sends alternating current through the induction coil, which generates a magnetic field. Induction furnaces work on the principle of a transformer. An alternative electromagnetic field induces eddy currents in the metal which converts the electric energy to heat without any physical contact between the induction coil and the work piece. A schematic diagram of induction furnace.The furnace contains a crucible surrounded by a water cooled copper coil. The coil is called primary coil to which a high frequency current is supplied. By induction secondary currents, called eddy currents are produced in the crucible. High temperature can be obtained by this method. Induction furnaces are of two types: cored furnace and coreless furnace. Cored furnaces are used almost exclusively as holding furnaces. In cored furnace the electromagnetic field heats the metal between two coils. Coreless furnaces heat the metal via an external primary coil.
Gating system:
The assembly of channels which facilitates the molten metal to enter into the mold cavity is called the gating system.Alternatively, the gating system refers to all passage ways through which molten metal passes to enter into the mold cavity. The nomenclature of gating system depends upon the function of different channels which they perform.
*Down gates or sprue
*Cross gates or runners
*Ingates or gates
The metal flows down from the pouring basin or pouring cup into the down gate or sprue and passes through the cross gate or channels and ingates or gates before entering into the mold cavity.
Goals of Gating System:
The goals for the gating system are
*To minimize turbulence to avoid trapping gasses into the mold
*To get enough metal into the mold cavity before the metal starts to solidify
*To avoid shrinkage
*Establish the best possible temperature gradient in the solidifying casting so that the shrinkage if occurs must be in the gating system not in the required cast part.
*Incorporates a system for trapping the non-metallic inclusions
Hydraulic Principles used in the Gating System
Reynold's Number
Nature of flow in the gating system can be established by calculating Reynold's number
RN = Reynold's number
V = Mean Velocity of flow
D = diameter of tubular flow
m = Kinematics Viscosity = Dynamic viscosity / Density
r = Fluid density
When the Reynold's number is less than 2000 stream line flow results and when the number is more than 2000 turbulent flow prevails. As far as possible the turbulent flow must be avoided in the sand mold as because of the turbulence sand particles gets dislodged from the mold or the gating system and may enter into the mould cavity leading to the production of defective casting. Excess turbulence causes
*Inclusion of dross or slag
*Air aspiration into the mold
*Erosion of the mold walls
Bernoulli's Equation:
h = height of liquid
P = Static Pressure
n = metal velocity
g = Acceleration due to gravity
r = Fluid density
Turbulence can be avoided by incorporating small changes in the design of gating system. The sharp changes in the flow should be avoided to smooth changes. The gating system must be designed in such a way that the system always runs full with the liquid metal. The most important things to remember in designing runners and gates are to avoid sharp corners. Any changes in direction or cross sectional area should make use of rounded corners.
To avoid the aspiration the tapered sprues are designed in the gating systems. A sprue tapered to a smaller size at its bottom will create a choke which will help keep the sprue full of molten metal.
Types of Gating Systems:
The gating systems are of two types:
*Pressurized gating system
*Un-pressurized gating system
Pressurized Gating System:
*The total cross sectional area decreases towards the mold cavity
*Back pressure is maintained by the restrictions in the metal flow
*Flow of liquid (volume) is almost equal from all gates
*Back pressure helps in reducing the aspiration as the sprue always runs full
*Because of the restrictions the metal flows at high velocity leading to more turbulence and chances of mold erosion
Un-Pressurized Gating System:
*The total cross sectional area increases towards the mold cavity
*Restriction only at the bottom of sprue
*Flow of liquid (volume) is different from all gates
*aspiration in the gating system as the system never runs full
*Less turbulence
Riser:
Riser is a source of extra metal which flows from riser to mold cavity to compensate for shrinkage which takes place in the casting when it starts solidifying. Without a riser heavier parts of the casting will have shrinkage defects, either on the surface or internally.
Risers are known by different names as metal reservoir, feeders, or headers.
Shrinkage in a mold, from the time of pouring to final casting, occurs in three stages.
during the liquid state
during the transformation from liquid to solid
during the solid state
First type of shrinkage is being compensated by the feeders or the gating system. For the second type of shrinkage risers are required. Risers are normally placed at that portion of the casting which is last to freeze. A riser must stay in liquid state at least as long as the casting and must be able to feed the casting during this time.
Functions of Risers:
*Provide extra metal to compensate for the volumetric shrinkage
*Allow mold gases to escape
*Provide extra metal pressure on the solidifying mold to reproduce mold details more exact
Design Requirements of Risers:
Riser size: For a sound casting riser must be last to freeze. The ratio of (volume / surface area)2 of the riser must be greater than that of the casting. However, when this condition does not meet the metal in the riser can be kept in liquid state by heating it externally or using exothermic materials in the risers.
Riser placement: the spacing of risers in the casting must be considered by effectively calculating the feeding distance of the risers.
Riser shape: cylindrical risers are recommended for most of the castings as spherical risers, although considers as best, are difficult to cast. To increase volume/surface area ratio the bottom of the riser can be shaped as hemisphere.
Casting Defects:
The following are the major defects, which are likely to occur in sand castings
*Gas defects
*Shrinkage cavities
*Molding material defects
*Pouring metal defects
*Mold shift
Gas Defects:
A condition existing in a casting caused by the trapping of gas in the molten metal or by mold gases evolved during the pouring of the casting. The defects in this category can be classified into blowholes and pinhole porosity. Blowholes are spherical or elongated cavities present in the casting on the surface or inside the casting. Pinhole porosity occurs due to the dissolution of hydrogen gas, which gets entrapped during heating of molten metal.
Causes:
The lower gas-passing tendency of the mold, which may be due to lower venting, lower permeability of the mold or improper design of the casting. The lower permeability is caused by finer grain size of the sand, high percentage of clay in mold mixture, and excessive moisture present in the mold.
*Metal contains gas
*Mold is too hot
*Poor mold burnout
Shrinkage Cavities:
These are caused by liquid shrinkage occurring during the solidification of the casting. To compensate for this, proper feeding of liquid metal is required. For this reason risers are placed at the appropriate places in the mold. Sprues may be too thin, too long or not attached in the proper location, causing shrinkage cavities. It is recommended to use thick sprues to avoid shrinkage cavities.
Molding Material Defects:
The defects in this category are cuts and washes, metal penetration, fusion, and swell.
Cut and washes:
These appear as rough spots and areas of excess metal, and are caused by erosion of molding sand by the flowing metal. This is caused by the molding sand not having enough strength and the molten metal flowing at high velocity. The former can be taken care of by the proper choice of molding sand and the latter can be overcome by the proper design of the gating system.
Metal penetration:
When molten metal enters into the gaps between sand grains, the result is a rough casting surface. This occurs because the sand is coarse or no mold wash was applied on the surface of the mold. The coarser the sand grains more the metal penetration.
Fusion:
This is caused by the fusion of the sand grains with the molten metal, giving a brittle, glassy appearance on the casting surface. The main reason for this is that the clay or the sand particles are of lower refractoriness or that the pouring temperature is too high.
Swell:
Under the influence of metallostatic forces, the mold wall may move back causing a swell in the dimension of the casting. A proper ramming of the mold will correct this defect.
Inclusions:
Particles of slag, refractory materials, sand or deoxidation products are trapped in the casting during pouring solidification. The provision of choke in the gating system and the pouring basin at the top of the mold can prevent this defect.
Pouring Metal Defects:
The likely defects in this category are
*Mis-runs and
*Cold shuts.
A mis-run is caused when the metal is unable to fill the mold cavity completely and thus leaves unfilled cavities. A mis-run results when the metal is too cold to flow to the extremities of the mold cavity before freezing. Long, thin sections are subject to this defect and should be avoided in casting design.
A cold shut is caused when two streams while meeting in the mold cavity, do not fuse together properly thus forming a discontinuity in the casting. When the molten metal is poured into the mold cavity through more-than-one gate, multiple liquid fronts will have to flow together and become one solid. If the flowing metal fronts are too cool, they may not flow together, but will leave a seam in the part. Such a seam is called a cold shut, and can be prevented by assuring sufficient superheat in the poured metal and thick enough walls in the casting design.
The mis-run and cold shut defects are caused either by a lower fluidity of the mold or when the section thickness of the casting is very small. Fluidity can be improved by changing the composition of the metal and by increasing the pouring temperature of the metal.
Mold Shift:
The mold shift defect occurs when cope and drag or molding boxes have not been properly aligned.
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