- by Kishore09331A0396 : - by Kishore09331A0396 Metal Casting Introduction: : Introduction: Metal Casting is one of the oldest materials shaping methods known. Casting means pouring molten metal into a mold with a cavity of the shape to be made, and allowing it to solidify. When solidified, the desired metal object is taken out from the mold either by breaking the mold or taking the mold apart. The solidified object is called the casting. By this process, intricate parts can be given strength and rigidity frequently not obtainable by any other manufacturing process. The mold, into which the metal is poured, is made of some heat resisting material. Sand is most often used as it resists the high temperature of the molten metal. Permanent molds of metal can also be used to cast products. Foundry: : Foundry: ” Foundry or casting is the process of producing metal/alloy component parts of desired shapes by pouring the molten metal/alloy into a prepared mould (of that shape) and then allowing the metal/alloy to cool and solidify. The solidified piece of metal/alloy is known as a CASTING”. Casting Terms: : Casting Terms: 1. Flask: A metal or wood frame, without fixed top or bottom, in which the mold is formed. Depending upon the position of the flask in the molding structure, it is referred to by various names such as drag - lower molding flask, cope - upper molding flask, cheek - intermediate molding flask used in three piece molding.
2. Pattern: It is the replica of the final object to be made. The mold cavity is made with the help of pattern.
3. Parting line: This is the dividing line between the two molding flasks that makes up the mold. Slide 5: 4. Core: A separate part of the mold, made of sand and generally baked, which is used to create openings and various shaped cavities in the castings.
5. Pouring basin: A small funnel shaped cavity at the top of the mold into which the molten metal is poured.
6. Sprue: The passage through which the molten metal, from the pouring basin, reaches the mold cavity. In many cases it controls the flow of metal into the mold.
7. Runner: The channel through which the molten metal is carried from the sprue to the gate. Slide 6: 8. Gate: A channel through which the molten metal enters the mold cavity.
9. Chaplets: Chaplets are used to support the cores inside the mold cavity to take care of its own weight and overcome the metallostatic force.
10. Riser: A column of molten metal placed in the mold to feed the castings as it shrinks and solidifies. Also known as feed head.
11. Vent: Small opening in the mold to facilitate escape of air and gases. Basic Features: : Basic Features: Pattern and Mould
A pattern is made of wood or metal, is a replica of the final product and is used for preparing mould cavity.
Mould material should posses refractory characteristics and with stand the pouring temperature.
When the mold is used for single casting, it made of sand and known as expendable mold.
When the mold is used repeatedly for number of castings and is made of metal or graphite are called permanent mould.
For making holes or hollow cavities inside a casting, cores made of either sand or metal are used. Slide 8: Melting and Pouring:
Several types of furnaces are available for melting metals and their selection depends on the type of metal, the maximum temperature required and the rate and the mode of molten metal delivery.
Before pouring provisions are made for the escape of dissolved gases. The gating system should be designed to minimize the turbulent flow and erosion of mould cavity.The other important factors are the pouring temperature and the pouring rate. Slide 9: Solidification and Cooling:
The properties of the casting significantly depends on the solidification time cooing rate.
Shrinkage of casting, during cooling of solidified metal should not be restrained by the mould material, otherwise internal stresses may develop and form cracks in casting.
Proper care should be taken at the design stage of casting so that shrinkage can occur without casting defects. Slide 10: Removal, Cleaning, Finishing and Inspection:
After the casting is removed from the mould it is thoroughly cleaned and the excess material usually along the parting line and the place where the molten metal was poured, is removed using a potable grinder.
White light inspection, pressure test, magnetic particle inspection, radiographic test, ultrasonic inspection etc. are used Schematic diagram of casting mould: : Schematic diagram of casting mould: Steps involved in making a casting: : Steps involved in making a casting: Make the pattern out of Wood , Metal or Plastic.
Prepare the necessary sand mixtures for mould and core making.
Prepare the Mould and necessary Cores.
Melt the metal/alloy to be cast.
Pour the molten metal/alloy into mould and remove the casting from the mould after the metal solidifies.
Clean and finish the casting.
Test and inspect the casting.
Remove the defects, if any.
Relieve the casting stresses by Heat Treatment.
Again inspect the casting.
The casting is ready for shipping. Advantages of metal casting: : Advantages of metal casting: Casting is one of the most versatile manufacturing process.
Casting provides the greatest freedom of design in terms of shape, size and the product quantity.
Casting imparts uniform directional properties and better vibration capacity to the cast parts.
Casting produces machinable parts.
Shapes difficult and uneconomic to obtain otherwise may be achieved through casting process.
( Contd..) Slide 15: A product may be cast as one piece, there by eliminating the need of metal joining processes.
Very heavy and bulky parts which are otherwise difficult to get fabricated may be cast.
Metals (like cast iron) dificult to be shaped by other manufacturing processes may be cast.
Casting can be designed for equal distribution of loads and for minimum stress concentration in order to achieve more strength and increased service life.
Casting process can be mechanised and usefully employed for mass production of components. Slide 16: Limitations of casting:
Dimensional accuracy and surface finish of the castings made by sand casting processes are a limitation to this technique.
Many new casting processes have been developed which can take into consideration the aspects of dimensional accuracy and surface finish.
Some of these processes are die casting process, investment casting process, vacuum-sealed molding process, and shell molding process.
The metal casting process is a labor intensive process Applications of Casting: : Applications of Casting: Transportation vehicles
Aircraft jet engine parts
Communication, Construction and Atomic Energy applications, etc.. Different Sections in Foundry: : Different Sections in Foundry: Pattern making
Sand mixing & preparation
Mould and core making
Mould assembly & handling
Fettling & finishing
Inspection & testing Raw Materials for Foundry: : Raw Materials for Foundry: Metals and alloys.
Fuels (for melting metals).
Fluxes. Metals and alloys commonly used in Foundries: : Metals and alloys commonly used in Foundries: 1. Ferrous
Nickel alloys Fuels for Melting Metals: : Fuels for Melting Metals: Coal
electricity Fluxes: : Fluxes: A flux is a low melting point material.
When heated up, it melts and combines with ash, viscous slags, sand, metallic oxides etc., and makes a fluid and easy flowing slag.
This slag being lighter comes on to the surface of the molten metal from where it can be easily skimmed, tapped or removed before pouring the metal into the mould.
Slag protects molten metal from furnace atmosphere. Refractories: : Refractories: Refractories are heat resistant materials.
They can withstand high temperatures without being fused.
Crucibles and furnace sides and bottoms containing molten metal are made up of refractories.
These are used as ladles for pouring metal into the mould.
Refractories constitute furnace walls and roof and thus minimize heat losses. Core and core prints: : Core and core prints: Castings are often required to have holes, recesses, etc. of various sizes and shapes.
These impressions can be obtained by using cores.
So where coring is required, provision should be made to support the core inside the mold cavity.
Core prints are used to serve this purpose.
The core print is an added projection on the pattern and it forms a seat in the mold on which the sand core rests during pouring of the mold.
The core print must be of adequate size and shape so that it can support the weight of the core during the casting operation.
Depending upon the requirement a core can be placed horizontal, vertical and can be hanged inside the mold cavity. Pattern having core prints. : Pattern having core prints. Pattern Making: : Pattern Making: A Pattern is a model or the replica of the object to be cast.
Except for the various allowances a pattern exactly resembles the casting to be made.
A pattern is required even if one object has to be cast. Functions of Patterns: : Functions of Patterns: A Pattern prepares a mould cavity for the purpose of making a casting.
A Pattern may contain projections known as core prints if the casting requires a core and need to be made hollow.
Patterns properly made and having finished and smooth surfaces reduce casting defects.
Properly constructed patterns minimize overall cost of the casting. Selection of Pattern Materials: : Selection of Pattern Materials: The following factors assist in selecting proper pattern material:
No. of castings to be produced.
Dimensional accuracy & surface finish.
Shape, complexity and size of casting.
Casting design parameters.
Type of molding materials.
The chance of repeat orders.
Nature of molding process. Slide 30: The pattern material should be:
Easily worked, shaped and joined.
Light in weight.
Strong, hard and durable.
Resistant to wear and abrasion .
Resistant to corrosion, and to chemical reactions.
Dimensionally stable and unaffected by variations in temperature and humidity.
Available at low cost. Materials for making patterns: : Materials for making patterns: Wood
Wax. 1. Wood Patterns: : 1. Wood Patterns: Advantages:
Easily available in large quantities
Easy to fabricate
Light in weight
Thay can be repaired easily
Easy to obtain good surface finish
Susceptible to shrinkage and swelling
Possess poor wear resistance
Abraded easily by sand action
Obsorb moisture, consequently get wraped
Cannot withstand rough handling
These are used where the no. of castings to be produced is small and pattern size is large. 2. Metal Patterns: : 2. Metal Patterns: Advantages:
Do not absorb moisture
Possess much longer life
Do not wrap, retain their shape
Greater resistance to abrasion
Accurate and smooth surface finish
Not easily repaired
Ferrous patterns get rusted
These are employed where large no. of castings have to be produced from same patterns. 3. Plastic Patterns: : 3. Plastic Patterns: Advantages:
Provides a smooth surface
Does not involve any appreciable change in size or shape
Wear and corrosion resistance
Easy to make
Good resistance to chemical attack
Plastic patterns are Fragile
These are may not work well when subject to conditions of severe shock as in machine molding (jolting). 4. Plaster Patterns: : 4. Plaster Patterns: Advantages:
It can be easily worked by using wood working tools.
Intricate shapes can be cast without any difficulty.
It has high compressive strength.
Plaster may be made out of Plaster of paris or Gypsum cement.
Plaster mixture is poured into a mould made by a sweep pattern or a wooden master pattern, in order to obtain a Plaster pattern. 5. Wax patterns: : 5. Wax patterns: Advantages:
Provide very good surface finish.
Impatr high accuracy to castings.
After being molded, the wax pattern is not taken out of the mould like other patterns; rather the mould is inverted and heated; the molten wax comes out and/or is evaporated. Thus there is no chance of the mould cavity getting damaged while removing the pattern.
Wax patterns find applications in Investment casting process. Types of Patterns: : Types of Patterns: Single piece pattern.
Split piece pattern.
Loose piece pattern.
Match plate pattern.
Follow board pattern.
Cope and Drag pattern. (a)Split pattern(b) Follow-board(c) Match Plate(d) Loose-piece(e) Sweep(f) Skeleton pattern : (a)Split pattern(b) Follow-board(c) Match Plate(d) Loose-piece(e) Sweep(f) Skeleton pattern 1. Single piece (solid) pattern: : 1. Single piece (solid) pattern: Made from one piece and does not contain loose pieces or joints.
Used for large size simple castings.
Pattern is accommodated either in the cope or in the drag.
Bodies of regular shapes.
stuffling box of steam engine. Fig: Single piece pattern : Fig: Single piece pattern 2. Split piece pattern: : 2. Split piece pattern: Patterns of intricate shaped castings cannot be made in one piece because of the inherent difficulties associated with the molding operations (e.g. withdrawing pattern from mould).
The upper and the lower parts of the split piece patterns are accommodated in the cope and drag portions of the mold respectively.
Parting line of the pattern forms the parting line of the mould.
Dowel pins are used for keeping the alignment between the two parts of the pattern.
Taps and water
stop cocks etc., Fig: split piece pattern : Fig: split piece pattern 3.Loose piece pattern: : 3.Loose piece pattern: Certain patterns cannot be withdrawn once they are embedded in the molding sand. Such patterns are usually made with one or more loose pieces for facilitating from the molding box and are known as loose piece patterns.
Loose parts or pieces remain attached with the main body of the pattern, with the help of dowel pins.
The main body of the pattern is drawn first from the molding box and thereafter as soon as the loose parts are removed, the result is the mold cavity. 4. Match plate pattern: : 4. Match plate pattern: It consists of a match plate, on either side of which each half of split patterns is fastened.
A no. of different sized and shaped patterns may be mounted on one match plate.
The match plate with the help of locator holes can be clamped with the drag.
After the cope and drag have been rammed with the molding sand, the match plate pattern is removed from in between the cope and drag.
Match plate patterns are normally used in machine molding. Fig: Match plate pattern : Fig: Match plate pattern 5. Sweep pattern: : 5. Sweep pattern: A sweep pattern is just a form made on a wooden board which sweeps the shape of the casting into the sand all around the circumference. The sweep pattern rotates about the post.
Once the mold is ready, Sweep pattern and the post can be removed.
Sweep pattern avoids the necessity of making a full, large circular and costly three-dimensional pattern.
Making a sweep pattern saves a lot of time and labour as compared to making a full pattern.
A sweep pattern is preferred for producing large casting of circular sections and symmetrical shapes. 6. Gated pattern: : 6. Gated pattern: The sections connecting different patterns serve as runner and gates. This facilitates filling of the mould with molten metal in a better manner and at the same time eliminates the time and labour otherwise consumed in cutting runners and gates.
A gated pattern can manufacture many casting at one time and thus it is used in mass production systems.
Gated patterns are employed for producing small castings. castings : castings Gating system 7. Skeleton pattern: : 7. Skeleton pattern: A skeleton pattern is the skeleton of a desired shape which may be S-bend pipe or a chute or something else. The skeleton frame is mounted on a metal base
The skeleton is made from wooden strips, and is thus a wooden work.
The skeleton pattern is filled with sand and is rammed.
A strickle (board) assists in giving the desired shape to the sand and removes extra sand.
Skeleton patterns are employed for producing a few large castings.
A skeleton pattern is very economical, because it involves less material costs. 8. Follow board pattern: : 8. Follow board pattern: A follow board is a wooden board and is used for supporting a pattern which is very thin and fragile and which may give way and collapse under pressure when the sand above the pattern is being rammed.
With the follow board support under the weak pattern, the drag is rammed, and then the fallow board is with drawn, The rammed drag is inverted, cope is mounted on it and rammed. During this operation pattern remains over the inverted drag and get support from the rammed sand of the drag under it.
Follow boards are also used for casting master patterns for many applications. 9. Cope and Drag patterns: : 9. Cope and Drag patterns: A cope and drag pattern is another form of split pattern.
Each half of the pattern is fixed to a separate metal/wood plate.
Each half of the pattern(along the plate) is molded separately in a separate molding box by an independent molder or moulders.
The two moulds of each half of the pattern are finally assembled and the mould is ready for pouring.
Cope and drag patterns are used for producing big castings which as a whole cannot be conveniently handled by one moulder alone. Fig: Cope and drag pattern : Fig: Cope and drag pattern Pattern Allowances: : Pattern Allowances: A pattern is larger in size as compared to the final casting, because it carries certain allowances due to metallurgical and mechanical reasons for example, shrinkage allowance is the result of metallurgical phenomenon where as machining, draft, distortion, shake and other allowances are provided on the patterns because of mechanical reasons. Types of Pattern Allowances: : Types of Pattern Allowances: The various pattern allowances are:
shrinkage or contraction allowance.
Machining or finish allowance.
Draft of tapper allowances.
Distortion or chamber allowance.
Shake or rapping allowance. 1.Shrinkage Allowance: : 1.Shrinkage Allowance: All most all cast metals shrink or contract volumetrically on cooling.
The metal shrinkage is of two types:
1. Liquid Shrinkage:
it refers to the reduction in volume when the metal changes from liquid state to solid state at the solidus temperature. To account for this shrinkage; riser, which feed the liquid metal to the casting, are provided in the mold.
2. Solid Shrinkage:
it refers to the reduction in volume caused when metal loses temperature in solid state. To account for this, shrinkage allowance is provided on the patterns. Slide 58: Almost all cast metals shrink or contract volumetrically after solidification and therefore the pattern to obtain a particular sized casting is made oversize by an amount equal to that of shrinkage or contraction.
Different metals shrink at different rates because shrinkage is the property of the cast metal/alloy.
The metal shrinkage depends upon:
The cast metal or alloy.
Pouring temp. of the metal/alloy.
Casting design aspects.
Molding conditions(i.e., mould materials and molding methods employed) Slide 59: Rate of Contraction of Various Metals : 2. Machining Allowance: : 2. Machining Allowance: A Casting is given an allowance for machining, because:
Castings get oxidized in the mold and during heat treatment; scales etc., thus formed need to be removed.
It is the intended to remove surface roughness and other imperfections from the castings.
It is required to achieve exact casting dimensions.
Surface finish is required on the casting.
How much extra metal or how much machining allowance should be provided, depends on the factors listed below:
Nature of metals.
Size and shape of casting.
The type of machining operations to be employed for cleaning the casting.
Molding process employed Machining Allowances of Various Metals: : Machining Allowances of Various Metals: 3. Draft or Taper Allowance: : 3. Draft or Taper Allowance: It is given to all surfaces perpendicular to parting line.
Draft allowance is given so that the pattern can be easily removed from the molding material tightly packed around it with out damaging the mould cavity.
The amount of taper depends upon:
Shape and size of pattern in the depth direction in contact with the mould cavity.
Draft allowance is imparted on internal as well as external surfaces; of course it is more on internal surfaces. Table 2 : Draft Allowances of Various Metals: : Table 2 : Draft Allowances of Various Metals: Fig: taper in design : Fig: taper in design 4. Distortion or cambered allowance: : 4. Distortion or cambered allowance: A casting will distort or wrap if :
It is of irregular shape,
All it parts do not shrink uniformly i.e., some parts shrinks while others are restricted from during so,
It is u or v-shape,
The arms possess unequal thickness,
It has long, rangy arms as those of propeller strut for the ship,
It is a long flat casting,
One portion of the casting cools at a faster rate as compared to the other. 5. Shake allowance: : 5. Shake allowance: A patter is shaken or rapped by striking the same with a wooden piece from side to side. This is done so that the pattern a little is loosened in the mold cavity and can be easily removed.
In turn, therefore, rapping enlarges the mould cavity which results in a bigger sized casting.
Hence, a –ve allowance is provided on the pattern i.e., the pattern dimensions are kept smaller in order to compensate the enlargement of mould cavity due to rapping.
The magnitude of shake allowance can be reduced by increasing the tapper. Pattern Layout: : Pattern Layout: Steps involved:
Get the working drawing of the part for which the pattern is to be made.
Make two views of the part drawing on a sheet, using a shrink rule. A shrink rule is modified form of an ordinary scale which has already taken care of shrinkage allowance for a particular metal to be cast.
Add machining allowances as per the requirements.
Depending upon the method of molding, provide the draft allowance. Pattern Construction: : Pattern Construction: Study the pattern layout carefully and establish,
Location of parting surface.
No. of parts in which the pattern will be made.
Using the various hand tools and pattern making machines fabricate the different parts of the pattern.
Inspect the pattern as regards the alignment of different portions of the pattern and its dimensional accuracy.
Fill wax in all the fillets in order to remove sharp corners.
Give a shellac coatings(3 coats) to pattern.
impart suitable colors to the pattern for identification purposes and for other informations. Pattern Colours: : Pattern Colours: Patterns are imparted certain colors and shades in order to:
Identify quickly the main body of pattern and different parts of the pattern.
Indicate the type of the metal to be cast.
Identify core prints, loose pieces, etc.,
Visualise the surfaces to be machined, etc. Moulding Materials : Moulding Materials Major part of Moulding material in sand casting are
70-85% silica sand (SiO2)
10-12% bonding material e.g., clay cereal etc.
Requirements of molding sand are:
The performance of mould depends on following factors:
Dry strength Molding Material and Properties: : 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: 1. Refractoriness: : 1. Refractoriness: It is the ability of the molding material to with stand high temperatures (experienced during pouring) with out
Cracking, buckling or scabbing,
Experiencing any major physical change.
Silica sand have high refractriness. 2. Permeability: : 2. 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. 3. Green Strength: : 3. 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.
Green strength helps in making and handling the moulds. 4. Dry Strength: : 4. Dry Strength: A mould may either intentionally be dried, or a green sand mould may lose its moisture and get dried while waiting for getting poured or when it comes in contact with molten metal being poured.
The sand thus dried must have dry strength to
Withstand erosive forces due to molten metal,
Withstand pressure of molten metal,
Retain its exact shape,and
Withstand the metallostatic pressure of the liquid material. 5. Hot Strength: : 5. 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.
In the absence of adequate hot strength, the mold may
break, erode or
get cracked. 6. Collapsibility: : 6. Collapsibility: Collapsibility determines the readiness with which the molding sand,
Automatically gets collapsed after the casting solidifies, and
Breaks down in knock out and cleaning operations.
If the mould or core does not collapse, it may restrict free contraction of solidifying metal and cause the same to tear or crack. 7. Flowability: : 7. Flowability: It is the ability of the molding sand to get compacted to a uniform density.
Flowability assists molding sand to flow and pack all-around the pattern and take up the required shape.
Flowability increases as clay and water contents increase. 8. Adhesiveness: : 8. Adhesiveness: It is the property of molding sand owing to which, it
Sticks with the walls of molding boxes,
Sticks with gaggers, and
Thus makes it possible to mold cope and drag. 9. Fineness: : 9. Fineness: Finer sand mould resist metal penetration and produce smooth casting surfaces.
Fineness and permeability are in conflict with each other and hence they must be balanced for optimum results. Molding Sand Composition: : Molding Sand Composition: The main ingredients of any molding sand are:
Moisture 1. Base Sand: : 1. 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. 2. Binder: : 2. Binder: Binders are of many types such as:
Organic binders and
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. 3. Moisture: : 3. 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: : A Typical Composition of Molding Sand: Effect of moisture, grain size and shape on mould quality : Effect of moisture, grain size and shape on mould quality Classification of casting Processes: : Classification of casting Processes: 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
Sodium Silicate Molding
3. Permanent Mold Processes
Gravity Die casting
Low and High Pressure Die Casting
4. Special Casting Processes
Ceramics Shell Molding
Evaporative Pattern Casting
Vacuum Sealed Molding
Centrifugal Casting 1.Green Sand Molding: : 1.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.
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
Surface Finish of the castings obtained by this process is not good and machining is often required to achieve the finished product. 2.Dry Sand Molding: : 2.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. 3.Shell Molding Process: : 3.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. 4.Sodium Silicate Molding Process : 4.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. Slide 95: 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; Cntd.. : Cntd.. Review: : Review: Foundry activities : Foundry activities 2 types of moulding flask designs : 2 types of moulding flask designs Sand mold - opened : Sand mold - opened Sand mold - closed : Sand mold - closed Cooling curve for pure metal : Cooling curve for pure metal Mixing moulding sand with binders & adhesives : Mixing moulding sand with binders & adhesives Filling sand in moulding flasks : Filling sand in moulding flasks Melting furnace : Melting furnace Pouring molten liquid : Pouring molten liquid Knock out : Knock out Heat treatment : Heat treatment Machining : Machining final products of casting : final products of casting Reference: : Reference: A text book of Production Technology Vol. I / O.P. Khanna / Dhanpat Rai Publications
A text book of Production Technology (Manufacturing Processes) / P. C. Sharma / S. Chand & Company Ltd
Manufacturing Technology (Second Edition) / P N Rao / Tata McGraw-Hill Publishing Company Ltd
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