UNIT I - CASTING
CASTING:
Casting is a manufacturing process by which a liquid material is usually poured into a mold, which contains a hollow cavity of the desired shape, and then allowed to solidify. The solidified part is also known as a casting, which is ejected or broken out of the mold to complete the process. Casting materials are usually metals or various cold setting materials that cure after mixing two or more components together; examples are epoxy, concrete, plaster and clay. Casting is most often used for making complex shapes that would be otherwise difficult or uneconomical to make by other methods
Casting, one of the oldest manufacturing processes, consists of pouring a molten metal into a mold cavity, where it solidifies in the shape of the cavity. Casting can produce complex shapes (including internal cavities) and large parts with small material wastage. A disadvantage of casting is that during solidification, coring occurs, and as a result, solute elements are concentrated at the grain boundaries. If these elements form brittle particles, the cast alloy will have a low ductility. A post-casting normalizing heat treatment may reduce the solute segregation.
In this lab, you will make a sand mold and assist in the melting and casting processes. You will get exposure to the process parameters in casting: fluid flow, heat transfer, metal solidification rates, and design of the metal feeding system. Thermocouples embedded in thick and thin sections of each mold will be used to generate direct cooling curves. Inflection temperatures will be compared to the equilibrium liquidus and solidus of the alloy system.
PROCEDURE FOR MAKING THE CASTING
1. Make a mold from the oil bonded sand. Place one thermocouple in the thin section and one in the thick section of the mold.
2. Begin melting the metal in the induction furnace as mold nears completion. Be sure to wear safety gear around the furnace. Set-up the PC data acquisition system and the Notebook program.
3. Place the mold in the sandbox. Connect the thermocouples to the leads from the PC data acquisition system.
4. Skim the oxide from the surface of the melt. Remove furnace from the crucible, start the data collection program, and pour the metal into the sand mold.
5. CAUTION! Students not assisting in the pouring operation should stand back as splattering of the molten metal may occur. Remember, the aluminum is above 600°C. Also, the oil in the molding sand will begin to burn, giving off noxious fumes.
6. After solidification and cooling (>10 min.) remove the casting from the mold and inspect it.
Casting is the process of forming objects by pouring liquid or viscous material into a prepared mold or form.
Examples: Carburetors, frying pans, engine blocks, crankshafts, railroad-car wheels, plumbing fixture, power tools, gun barrels, machine tool is bases etc.
Properly designed and properly produced castings do not have directional properties. Casting can produce complex shapes. Cast iron has also very good dampening characteristics.
Various Casting processes
There are two major categories of casting:
Expendable molds:
Made of sand, plaster, ceramics and similar materials which are generally mixed with various binders or bonding agents, the molds are broken up to remove the casting.
Permanent molds:
Used repeatedly and are designed in such a way that casting can be easily removed and the mold used for the next casting.
Composite Molds:
Made of two or more different materials (such as sand, graphite, and metal) combining the advantages of each material. Used to improve mold strength, cooling rates and overall economics of the process.
Steps consists of:
Most sand casting operations use silica sand (SiO2).
Inexpensive and suitable as mold material because of its resistance to high temperatures
Two general types of sands:
Naturally bonded (bank sands) and
Synthetic (lake sands).
Synthetic sand is preferred (because its composition can be controlled more accurately).
Important factors in selecting sand for molds:
Sand having fine, round grains can be closely packed and forms a smooth mold surface. Good permeability of molds and cores allows gases and steam evolved escape easily. The mold should have good collapsibility to avoid defects in the casting such as (hot tearing and cracking).
So, selection of sand involves certain tradeoffs with respect to properties
Sand is usually conditional before use.
Mulling machines are used to uniformly mull (mix thoroughly) sand with additives.
Clay (bentonite) is used as a cohesive agent to bond sand particles (giving the sand strength).
Zircon (ZrSiO4), Olivine (Mg2SiO4) and Iron silicate (Fe2SiO4) sands are often used in steel foundries for their low thermal expansion.
Chromite (Fe2Cr2O4) is used for its high heat transfer properly.
Molds
A mold is a container that has the cavity or cavities of the shape to be casted.
Flask:A flask is a wood or metal frame in which a mold is made. A flask is made of two principal parts, the cope (top section) and the drag (bottom section). To increase the depth of the cope and/or the drag, intermediate sections, known as cheeks, are used
Pouring basin or pouring cap: into which the molten metal is poured.
Sprue: through which the molten metal flows downward.
Runner system:channels to carry the molten metal from the sprue to the mold cavity. Gates are inlets into the mold cavity.
Risers: supply additional metal to the casting as it shrinks during solidification.
Cores: inserts made from sand. They are placed in the mold to form hollow regions or otherwise define the interior surface of the casting.
Cores are also used on the outside of the casting to form features such as telling on the side of a casting or deep external pockets.
Vents: carry off gases produced when the molten metal comes in contact with the sand in the molds and core. Also exhaust air from the mold cavity comes out through vents as the molten metal flows into the mold.
Depending on the materials used, Molds are classified as follows:
Green-sand molds: Molds made with damp molding sand.
Skin-dried molds:
Two methods.
First-The sand around the pattern to a depth of about ½ inch is mixed with a binder so that when it is dried it will leave a hard surface on the mold. The remainder of the mold is made up of ordinary green sand.
Second-The entire mold is made with green sand and then coat its surface with a spray or wash, which hardens when it is applied.
Spray used are: linseed oil, molasses water, gelatinized starch etc.
In both of them mold is dried either by air or by a torch to harden the surface and drive cut excess moisture.
Dry sand molds: Fairly coarse molding sand mixed with a binding material is used. Flasks are of metal, since molds must be oven baked before being used. It is free from gas troubles due to moisture. Skin-dried and dry-sand molds are widely used in steel foundries.
Loam molds:It is first built up with bricks or large iron parts; these parts are then plastered over with a thick Loam mortar, the shape of the cavity being obtained with sweeps or skeleton patterns. The mold is then allowed to dry thoroughly. It needs long time to make and is not used extensively.
Furan molds:Dry, sharp sand is thoroughly mulled with phosphoric acid which acts as an accelerator==> furan resin is added and mulling is continued==>the sand materials begins to air harden almost immediately.
CO2 molds:Clean sands is mixed with sodium silicate and the mixture is rammed about a pattern. When CO2 gas is pressure-fed into the mold, the sand mixture hardens. Very smooth and intricate castings are obtained. Used for core making.
Metal molds:these are used mainly in the die-casting of low-molting-temperature alloys. Accurate with smooth finish. Eliminate much machine work.
Special molds: Plastics, cement, plaster, paper, wood and rubber are all mold materials used to fit particular applications.
Bench molding: is for small work, done on a bench of a height convenient to the molder.
Floor molding: When castings increase in size, with resultant difficulty in handling, the work is done on the foundry floor. This type of molding is used for practically all medium and large size castings.
Pit molding: Extremely large castings are frequently molded in a pit instead of a flask. The pit acts as the drag part of the flask and a separate cope is used above it. They sides of the pit are brick kind, and on the bottom there
Machine molding: Machines have been developed to do a number of operations that the molder ordinarily does by hand. Ramming the sand, rolling the mold, forming the gate and drawing the pattern can be done by these machines.
Mold preparation - For removable pattern:
For disposable pattern:
Gating System
The passage way for bringing the molten metal into the mold cavity. It includes: pouring basin, downgate or vertical passage known as a sprue, gate through which the metal flows from the sprue base to the mold cavity, a runner in large castings, which takes the metal from the sprue base and distributes it to several gate passage ways around the cavity.
Metal should enter the cavity with as little turbulence as possible at or near the bottom of the mold cavity.
Erosion of the passageway or cavity surfaces should be avoided by properly regulating the flow of metal.
Metal should enter the cavity so as to provide, directional solidification if possible. The solidification should progress from the mold surfaces to the hottest metal so that there is always hot metal available to compensate for shrinkage.
Clay or other foreign particles should be prevented from entering the mold cavity.
Skimming gates may be used to trap slag or other light particles into the second sprue hole. The gate to the mold is restricted somewhat to allow time for the floating particles to rise into the skimmer.
Three types of gate are used in mold:
Parting gates
Top gates
Bottom gates
Top gate: Conductive to a favorable temperature gradient but erosion may be high
Bottom gate: Offers smooth flow with a minimum of erosion but unfavorable temperature gradient.
Riser: Risers are often provided in molds to feed molten metal into the main cavity to compensate for the shrinkage.
There are two types of riser
Open Riser: Top of the open riser in open, it is cylinder shape
Advantages: An open riser is easy to mold,
Air can be removed from it.
Disadvantages:It is not placed in the drag.
More difficult to remove from the Casting.
Close/blind riser: Blind risers are domelike risers, found in the cope half of the flask, which are not the complete height of the cope.
Advantages: Can be placed at any position of the mold
Can be easily removed from the casting.
Disadvantages: Difficult to mold
May draw liquid metal from solidifying casting.
Chills are metal inserts used to control solidification by carrying heat away from the solidifying metal at a rapid rate.
Chills are the metal shapes inserted in molds to speed up the solidification of a particular portion of the casting.
Chills equalize the cooling rate of thin and thick sections and thus prevents hot tears.
Chills promote progressive and directional solidification.
Types chills (I) External (II) Internal.
External Chills: It is rammed up in the mold walls. An external Chill is excellent for controlling cooling rates in critical region of castings.
Internal Chills: These are of same material as the molten metal. Thus are placed in the mold cavity before casting when molten metal enters into mold cavity, melts the block, which is used as internal chills, and prevents shrinkage void.
Used to mold the sand mixture into the shape of the casting.
Wood – for small production (white pine, mahogany, cherry etc.)
Metal – for high quality production
Brass
Cast iron
Aluminum
Plastics
Advantage of metal or plastic pattern:
Pattern material selection depends on :
Strength and durability of the material selected for patterns must reflect the number of castings that the mold will produce.
Sometimes combination of materials is used to reduce wear in critical regions.
Patterns are usually coated with a partings agent to facilitate their removal from the molds.
Types of pattern
Solid or single piece pattern: generally used for simpler shapes and low quantity production. They are generally made of wood and are inexpensive.
Split pattern: many patterns cannot be made of a single piece because of the difficulty in molding. To eliminate the difficulty the patterns are made split, half rests in lower part and half in upper part.
Gated patterns: in production work where many castings are required, patterns are made of metal to give them strengths and to eliminate any warping tendency. The gates or runners for the molten metal are formed by connecting parts between the individual patterns.
Loose piece pattern: consists of loose pieces, which are necessary to facilitate withdrawing it from the mold.
Match plates: provide a substantial mounting for patterns. It consists of a flat metal or wooden plate to which the patterns and gate are permanently fastened.
Sweep pattern: they are used where the shape to be molded can be formed by the rotation of a curved line element about an axis:
Rapid prototyping:
A recent development to mold and pattern making
For example, in investment casting wax patterns can now be replaced with accurate resin patterns by rapid prototyping.
In this case CAD data are used directly (without the need for dies) to make the pattern at a fraction of the time and cost of dies for making wax patterns.
Pattern allowances:
Shrinkage: metals shrink when they cool.
Draft:
When a pattern is drawn from a mold, the tendency to tear away the edges of the mold in contact with the pattern is greatly decreased if the surfaces of the pattern are slightly tapered known as draft.
1/8 to ¼ in/foot (exterior)
¾ in/foot (interior)
Finish: positive allowance is provided for machining. For small and average-sized casting finish allowance is 1/8 inch.
Distortion: distortion allowance applies only to those castings of irregular shape which are distorted in the process of cooling because of metal shrinkage.
Shake: when a removable pattern is rapped in the mold before it is withdrawn, the cavity in the mold increases slightly. A shake allowance should be considered by making the pattern slightly smaller to compensate for the rapping of the mold.
A core is a body, usually made of sand, used to produce a cavity in or on acasting.
Examples: forming the water jacket in a water cooled engine block and forming the air space between the cooling fins of an air cooled engine.
Cores are placed in the mold cavity before casting to form the interior surfaces of the casting.
Desirable properties:
Strength (green and dry)
Permeability
Ability to withstand heat or refractoriness
Collapsibility
Friability
Core making
Core sand is placed in a core box. It can be blown into the box, rammed or packed by hand, or jolted into the box. The excess sand is struck off, and a drier plate is placed over the box. The core box is then inverted, vibrated or rapped, and drawn off the core. The core is then put in a core oven and backed.
Recesses that are added to the pattern to support the core and to provide vents for the escape of gases.
Core shifting: shifting of cores from its proper place is a major cause of defective castings.
Anchor: a core is subjected to an appreciable buoyant force when immersed in the liquid metal poured into the mold cavity.
Chaplets: serve to support cores that tend to sag or sink in inadequate core print seats. Chaplets also serve as anchor to keep the core in place during the casting process.
A chaplet is usually made of the same metal as, and becomes part of the casting.
Types of cores
Green sand core:
A green sand core is made of the same sand from which the mold has been made i.e. the molding sand.
Relatively cheap and popular.
Dry sand core:
Dry sand core unlike green sand cores are not produced as a part of the mold.
Dry sand core is made separately and independent of the mold.
Backed sand or dry sand core has a binder that must be cured with heat.
Core making machines:
Cores of regular shapes and sections may be extruded and cut to length. A central vent hole is left by a wire extending from the center of the screw.
Large cores are made by jolt-rollover, sand slinger and other machines.
Small and medium size irregular shape cores are usually made by hand. But if quantity is high, they are produced on a core blowing machine. This machine blows sand by compressed air through a core plate with holes arranged to pack the sand evenly and firming in the core box.
Core backing:
The cores that are bonded by oils must be baked for ultimate hardness and strength. The purpose of baking is to drive off moisture, oxidize the oil, and polymerize the binder.
A uniform temperature and controlled heating are necessary for baking an oil-bonded sand core. With linseed oil the temperature is raised at a moderate rate, and is held at about 200°C for about 1 hr and then is allowed to fall slowly to room conditions.
Molding machines:
Serve: To pack sand firmly and uniformly into the mold.
To manipulate the flasks, mold, and pattern.
Three types of molding machines are:
Jolt-squeeze Molding Machines:
A jolt-squeezer consists basically of a table actuated by two pistons in air cylinders, one inside the other. The mold on the table is jolted by the action of the inner piston that raises the table repeatedly and drops it down sharply on a bumper pad. Jilting packs the sand in the lower parts of the flask but not at the top. The larger cylinder pushes the table upward to squeeze the sand in the mold against the squeeze head at the top. A vibrator may be attached to the machine to loosen the pattern to remove it easily without damaging the mold.
The sand slinger:
The sand slinger achieves a consistent packing and ramming effect by hurling sand into the mold at a high velocity. Sand from a hopper is fed by a belt to a high-speed impeller in the head. A common arrangement is to suspend the slinger with counter weights and move it about to direct the stream of sand advantageously into a mold. Sand slinger can be deliver large quantities of sand rapidly and are specially beneficial for ramming big molds.
Casting defects:
1.Blow holes:
Small holes visible on the surface of the casting are called open blows where as occurring below the surface of the casting.
Causes>> High moisture in sand resulting in low permeability, very hard ramming of sand and improper venting of mold.
2.Gas holes:
These are the holes appearing on the surface when it is machined or cut into sections.
Causes>>using faulty or poor quality metal, use excessive moist sand.
3.Seam and plate:
Seam is an impressed line on casting surface and plate is in the form of a layer of metal, partially separated from the main body of the casting section by scale (plate of hard material).
Causes>>Interrupted metal flow due to abrupt changes in casting section adn sharp section.
4.Misrun:
It is a casting that is incomplete in its outermost sections, either long the to thickness is too large or because the metal was poured with insufficient superheat.
Causes>> Too cold molten metal
Too thin casting section
Too small gates.
5.Cold shut:
It is an interface within a casting that lacks complete fusion and is formed when two streams of liquid from two different directions come together after the leading surfaces are solidified.
Causes>> Metal lacking in fluidity.
Too small gates
Too cold molten metal.
6.Hot tear:
Intergrannular (along grain boundaries) failure at a high temperature the larger sections for intensive strain induced by solid contraction of adjacent thinner section.
Causes>> Excessive mold hardness.
High drag and hot strength of sand mold.
Too much shrinkage of metal while solidifying.
Too low pouring temp.
7.Shrinkage Cavities:
An internal void in a casting from the volume contraction that occurs during solidification. It causes for any casting.
(a) Design for minimum casting stresses
(b) Design for solidification
(c) Design for metal flow
(d) Cast mold design.
(e) Design for minimum casting.
(f) Functional design
Design rules:
External corner should be rounded with raddi that are 10% to 20% to section thickness. By rounding corners, the resistance of ductile metal to fatigue or static stress is increased.
In Joining section of unequal sizes the raddi plays an important role, A raddi of (a) 0.1 t the resistance to fatigue stress is united (b) 1t, there is 40 to 50% more stress endurance. (c) 4t, 120% more stress endurance than that with 0/1t radius.
CENTRIFUGAL CASTING :
Since its inception at the beginning of nineteenth century several applications developed have survived commercial exploitation. The main feature of centrifugal casting that differentiates it from all other static casting processes is pouring of molten metal into a mould that is rotated during solidification. The castings produced by this process are completely free from porosity defect and are strong (at par with similar forgings). This is due to whirling out of metal towards the periphery because of centrifugal force. Lighter impurities are also removed as being lighter these remain at the center.
Following are the main features of centrifugal casting process:
Process is suitable only for products, which have rotational symmetry.
General process is economical for ring shaped objects, tabular shaped objects and hollow cylinders, e.g. compressor cases, winding spools, furnace rollers etc.
No core is needed to form the bore as in static casting.
Temperature gradients during cooling can be controlled to some extent by controlling speed of rotation. Centrifugal pressures can be applied to advantage in checking premature freezing and imparting strength to the casting.
Main advantage of centrifugal casting is that the porosity free castings are obtained.
There are several variations of centrifugal casting process. These are :
Centrifugal casting process:
In this the mould rotates about its axis. This axis of rotation can be vertical, horizontal or inclined depending upon the shape of final product. If the axis of rotation is horizontal it is called as horizontal centrifugal casting as shown in Figure 3.1 and if the axis is vertical or inclined it is called as vertical or inclined centrifugal casting as shown in Figures 3.2 and 3.3 respectively. In this the need of center core is completely eliminated. Castings produced by this method have true directional solidification. Because of directional solidification the casting thus produced is defect free without any shrinkage, which is prevalent in sand castings.
The rotation speed selection is very important, particularly in the case of horizontal axis rotational speed plays a finite role. A speed lower than the required causes slipping and raining of the metal, which will not adhere to the mould surface. A speed higher than necessary may cause hot tears on its walls.
Semi-centrifugal Casting
In the semi-centrifugal casting process the mould is not rotated as fast as in the case of true centrifugal casting process. This is because only enough force is needed to cause the molten metal to flow first to the outer rims. In this process, mould is filled from rim to hub not from bottom to top.
This method is used for meeting large sized castings, which are symmetrical about their axis, e.g. gears, pulleys, spoke wheels etc. In this process, the metal is poured into central sprue, which in turn is forced outwards to the rim through hubs by centrifugal force. For hollow sections dry sand or CO2 core is used.
Centrifuge Centrifugal Casting
This process has the widest field of application. In this similar mould cavities are arranged symmetrically about the center axis of rotation like spokes of the wheel. Therefore multiple castings can be produced in one go. Sometimes for a large number of castings steel moulding is used. It is not a purely centrifugal process as castings produced are not rotated about their own axes and pouring pressure is different for all the castings.
DIE CASTING:
Die casting involves the preparation of components by injecting molten metal at high pressures into a metallic die. It is similar to permanent mold casting in the sense that both the processes use reusable metallic dies. The pressure is generally obtained by compressed air or hydraulically and varies from 70-5000 kg/cm2. Because of high pressures involved in the process, any narrow sections, complex shapes and fine surface details can be easily produced. Combination of high pressures and velocity of the injected liquid metal give a unique capacity for the production of intricate components at relatively low cost.
The die consists of two parts. One is called the stationary die or the cover die and is fixed to the die casting machine (as shown in figure). The second part called the ejector die is moved for the extraction of casting. The casting cycle starts when the two parts of the die are apart. The lubricant is sprayed on the die-cavity manually or by the auto lubrication system. The two die halves are closed and clamped. The required amount of metal is injected into the die. After the casting is solidified under pressure, the die is opened and the casting is ejected.
A die casting machine consists of four basic elements namely Frame
Source of molten metal and molten metal transfer
Dies
Metal Injection Mechanism.
These machines are classified on the basis of injection mechanisms and are of two types:
The main difference between these two types is that in hot chamber, the holding furnace for the liquid metal is integral with the diecasting machine, whereas in the cold chamber machine, the metal is melted in a separate furnace and then poured into the diecasting machine with a laddle for each casting cycle which is also called ‘shot’.
In this process, a gooseneck is used for pumping the liquid metal into the die cavity. The gooseneck is submerged into the holding furnace containing the molten metal. The gooseneck is made of grey, alloy or ductile iron or of cast steel. A plunger made of alloy cast iron, which is hydraulically operated moves up in the gooseneck to uncover the entry port for the entry of liquid metal into the gooseneck. The plunger can then develop the necessary pressure for forcing the metal into the die cavity. A nozzle at the end of the gooseneck is kept in close contact with the sprue located in the cover die.
The cycle starts with the closing of the die when the plunger is in the highest position in the gooseneck, thus facilitating the filling of the gooseneck by the liquid metal. The plunger then starts moving down to force the metal in the gooseneck to be injected into the die cavity. The metal is then held at the same pressure till it is solidified. The die is opened, and any cores if present, are also retracted. The plunger then moves back returning the unused liquid metal to the gooseneck. The casting, which is in the ejector die, is now ejected and at the same time the plunger uncovers the filling hole, letting the liquid metal from the furnace to enter the gooseneck.
Air pressure required for injecting the metal into the die is that of the order of 30-45 kg/cm2. Depending upon its size, this hot chamber die casting machine can produce about 60 or more castings upto 20 kg each per hour and several hundred castings per hour for single impression castings weighing a few grams.
The hot chamber process is used for most of the low melting temperature alloys such as zinc, lead and tin. For materials such as aluminum and brass, their high melting temperatures make it difficult to cast them by hot chamber process, because gooseneck of the hot chamber machine is continuously in contact with the molten metal. Also liquid aluminum would attack the gooseneck material and thus hot chamber process is not used with aluminum alloys. In the cold chamber process, the molten metal is poured with a ladle into the hot chamber for every shot. This process reduces the contact time between the liquid metal and the hot chamber.
The operation starts with the spraying of die lubricants throughout the die cavity and closing the die when molten metal is ladled into the hot chamber of the machine either manually or by means of an auto ladle. An auto ladle is a form of robotic device, which automatically scoops molten aluminum from the holding furnace and pours it into the die at the exact instance when required in the casting cycle. The pouring temperature can be precisely controlled with the help of auto ladle and hence the desired casting quality can be had. Then the plunger forces the metal into the die cavity and maintains the pressure till it solidifies. In the next step, the die opens. The casting is ejected. At the same time, plunger returns to its position completing the operation.
Cold chamber and hot chamber die casting differs from each other in the following respects :
Melting unit is not an integral part of the cold chamber die casting machine. Molten metal is brought and poured into the die casting machine with the help of ladles.
In case of cold chamber process high pressures tend to increase the fluidity of molten metal possessing relatively lower temperature and hence castings produced are denser, dimensionally accurate and free from blowholes.
In case of cold chamber process die components experience less thermal stresses due to lower temperature of the molten metal. However, dies are required to be made stronger in order to bear high pressures.
Cold chamber process has a longer cycle time compared to hot chamber process.
In case of cold chamber process as metal is ladled from a furnace, it may loose superheat and may cause defects such as cold shuts.
Advantages of Die Casting Process:
Applications
The typical products made by die casting are carburetors, crank cases, magnetos, handle bar housings, parts of scooters and motor cycles, zip fasteners, head lamp bezels, and other decorative automobile items.
SHELL MOULDING :
Shell moulding is a process in which the sand mixed with a thermosetting resin is allowed to come in contact with a heated metallic plate, so that a thin and strong shell of mould is formed around the pattern. Then the shell is removed from the pattern and the cope and the drag are removed together and kept in a flask with the necessary backup material and molten metal is poured into the mould.
Generally, dry and fine sand (90 to 140 GFN) which is completely free of the clay is used for preparing the shell moulding sand. The grain size to be chosen depends on the surface finish desired on the casting. Too fine a grain size requires large amount of resin which makes the mould expensive.
The synthetic resins used in shell moulding are essentially thermosetting resins, which get hardened irreversibly by heat. The resins, most widely used, are the phenyl formaldehyde resins. Combined with sand, they give very high strength and resistance to heat. The phenolic resins used in shell moulding usually are of the two stage type, that is, the resin has excess phenol and acts like a thermoplastic material. During coating with the sand, the resin is combined with a catalyst hexa-methylene tetramine in a proportion of about 14 to 16% so as to develop the thermosetting characteristics. The curing temperature for these would be around 150oC and the time required would be 50 to 60 sec.
The first step in preparing the shell mould is the preparation of the sand mixture in such a way that each of the sand grain is thoroughly coated with resin. To achieve this, first the sand, hexa and additives, which are all dry, are mixed inside a Muller for a period of 1 min. Then the liquid resin is added and mixing is continued for another 3 minutes. To this cold or warm air is introduced into the Muller and the mixing is continued till all the liquid is removed from the mixture and the coating of the grains is achieved to the desired degree.
Since the sand resin mixture is to be cured at about 150oC temperature, only metal patterns with associated gating are used. The metal used for preparing patterns is grey cast iron, mainly because of its easy availability and excellent stability at temperatures involved in the process. But sometimes-additional risering provision is required as the cooling in shell mouldings is slow.
The metallic pattern plate is heated to a temperature of 200 to 350 degrees depending on the type of pattern. It is very essential that the pattern plate is uniformly heated so that the temperature variation across the whole pattern is within 25 to 40 degrees depending on the size of the pattern. A silicone agent is sprayed on the pattern and the metal plate. The heated pattern is securely fixed to a dump box, wherein the coated sand in an amount larger than required to form the shell of the necessary thickness is already filled in.
Then the dump box is rotated so that the coated sand falls on the heated pattern. The heat from the pattern melts the resin adjacent to it thus causing the sand mixture to adhere to the pattern When a desired thickness of shell is achieved, the dump box is rotated backwards by 180 degrees so that the excess sand falls back into the box, leaving the formed shell intact with the pattern. The average shell thickness achieved depends on the temperature of the pattern and the time the coated sand remains in contact with the heated pattern.
The shell along with the pattern plate is kept in an electric or gas fired oven for curing the shell. The curing of the shell should be done as per requirements only because over curing may cause the mould to break down as the resin would burn out. The under curing may result in blow holes in the casting or the shell may break during handling because of the lack of strength.
The shells thus prepared are joined together by either mechanical clamping or adhesive bonding. The resin used as an adhesive may be applied to the parting plane before mechanical clamping and then allowed for 20 to 40 seconds for achieving the necessary bonding.
Since the shells are thin, they may require some outside support so that they can withstand the pressure of the molten metal. A metallic enclosure to closely fit the exterior of the shell is ideal, but it is too expensive and therefore impractical. Alternately, a cast iron shot is generally preferred as it occupies any contour without unduely applying any pressure on the shell. With such a backup material, it is possible to reduce the shell thickness to an economical level.
Shell moulding castings are generally more dimensionally accurate than sand castings. It is possible to obtain a tolerance of ± 0.25 mm for steel castings and ±0.35 mm for grey cast iron castings under normal working conditions.
A smoother surface finish can be obtained in shell castings. This is primarily achieved by the finer size grain used. The typical order of roughness is of the order of 3 to 6 microns.
Draft angles are lower than required in sand castings. The reduction in draft angles may be between 50 to 75% which considerably saves the material costs and the subsequent machining costs.
Sometimes, special cores may be eliminated in shell moulding. Since the sand has a high strength the mould could be designed in such a manner that the internal cavities can be formed directly with the shell mould itself without the need of the shell cores.
Also, very thin sections of the type of air cooled cylinder heads can be readily made by the shell moulding because of the higher strength of the sand used for shell moulding.
Permeability of the shell is high and therefore no gas inclusions occur.
Very small amount of sand needs to be used.
Mechanization is readily possible because of the simple processing involved in shell moulding.
Cylinders and cylinder heads for air cooled I. C. engines, automobile transmission parts, cast tooth bevel gears, brake beam, track rollers for crawler tractors, transmission planet carrier, steel eyes, gear blanks, chain seat bracket, refrigerator valve plate, small crank shafts are some of the common applications of shell mould castings.
UNIT II
WELDING
Welding is a materials joining process which produces coalescence of materials by heating them to suitable temperatures with or without the application of pressure or by the application of pressure alone, and with or without the use of filler material.
Welding is used for making permanent joints. It is used in the manufacture of automobile bodies, aircraft frames, railway wagons, machine frames, structural works, tanks, furniture, boilers, general repair work and ship building.
TYPES
The piece of metal to be joined are heated to a plastic state and forced together by external pressure
(Ex) Resistance welding
The material at the joint is heated to a molten state and allowed to solidify
(Ex) Gas welding, Arc welding
Classification of welding processes:
(i). Arc welding
(ii). Gas Welding
(iii). Resistance Welding
Arc welding methods
1. Metal arc welding
It is a process of joining two metal pieces by melting the edges by an electric arc. The electric arc is produced between two conductors. The electrode is one conductor and the work piece is another conductor. The electrode and the work piece are brought nearer with small air gap. (3mm app.)
When current is passed an electric arc is produced between the electrode and the work piece. The work piece and the electrode are melted by the arc. Both molten piece of metal become one. Temperature of arc is about 4000°c Electrodes used in arc welding are coated with a flux. This flux produces a gaseous shield around the molten metal. It prevents the reaction of the molten metal with oxygen and nitrogen in the atmosphere. The flux removes the impurities from the molten metal and form a slag. This slag gets deposited over the weld metal. This protects the weld seam from rapid cooling. Fig.1 shows arc welding process.
Equipments:(Refer Fig 2)
Advantages
Limitations
2. Carbon arc welding
In carbon arc welding, the intense of heat of an electric arc between a carbon electrode and work piece metal is used for welding. DC power supply is used. The carbon electrode is connected to negative terminal and work piece is connected to positive terminal, because positive terminal is hotter (4000°c) than the negative terminal (3000°c) when an arc is produced. So carbon from the electrode will not fuse and mix up with the metal weld. If carbon mixes with the weld, the weld will become weak and brittle. To protect the molten metal from the atmosphere the welding is done with a long arc. In this case, a carbon monoxide gas is produced, which surrounds the molten metal and protects it.
Carbon arc welding is used to weld both ferrous and non ferrous metals. Sheets of steel, copper alloys, brass and aluminium can be welded in this method.( Refer Fig 3)
Fig 3 Carbon Arc Welding
Comparison of A.C. and D.C. arc welding
|
Alternating Current (from Transformer) |
Direct Current (from Generator) |
1 |
More efficiency |
Less efficiency |
2 |
Power consumption less |
Power consumption more |
3 |
Cost of equipment is less |
Cost of equipment is more |
4 |
Higher voltage – hence not safe |
Low voltage – safer operation |
5 |
Not suitable for welding non ferrous metals |
suitable for both ferrous non ferrous metals |
6 |
Not preferred for welding thin sections |
preferred for welding thin sections |
7 |
Any terminal can be connected to the work or electrode |
Positive terminal connected to the work |
GAS WELDING
Oxy-Acetylene welding
In gas welding, a gas flame is used to melt the edges of metals to be joined. The flame is produced at the tip of welding torch. Oxygen and Acetylene are the gases used to produce the welding flame. The flame will only melt the metal. A flux is used during welting to prevent oxidations and to remove impurities. Metals 2mm to 50mm thick are welded by gas welding. The temperature of oxyacetylene flame is about 3200°c. Fig 3 shows Gas welding equipments.
Gas Welding Equipment
1. Gas Cylinders
Pressure
Oxygen – 125 kg/cm2
Acetylene – 16 kg/cm2
2. Regulators
Working pressure of oxygen 1 kg/cm2
Working pressure of acetylene 0.15 kg/cm2
Working pressure varies depends upon the thickness of the work pieces welded.
3. Pressure Gauges
4. Hoses
5. Welding torch
6. Check valve
7. Non return valve
Fig- 4 Gas Welding Equipment
TYPES OF FLAMES
Fig 4 shows the types of flames.
Fig 5 Types of Gas Flames
Advantages
Disadvantages
GAS CUTTING
Fig 6 Automatic Gas Cutting
Fig 7 Manual Gas Cutting
Weld joint
There are 5 basic joint types in welding
Types of weld
Fig 8 Types of Weld Joints
Weldability is the ease of a material or a combination of materials to be welded under fabrication conditions into a specific, suitably designed structure, and to perform satisfactorily in the intended service
Brazing and Soldering
Brazing
It is a low temperature joining process. It is performed at temperatures above 840º F and it generally affords strengths comparable to those of the metal which it joins. It is low temperature in that it is done below the melting point of the base metal. It is achieved by diffusion without fusion (melting) of the base
Depending upon the method of heating, brazing can be classified as
Fig 9 Brazing
Advantages
Disadvantages
Soldering
It is a low temperature joining process. It is performed at temperatures below 840ºF for joining.
Soldering is used for,
Fig 9 Soldering
Questions:
PART A – Short Questions
PART B - Essay Type Questions
1.Classify the welding process.
2.Explain the principle of arc welding process.
3.Compare the use of A.C and D.C. in welding.
4.What are the equipments used in gas welding? State their functions.
5.What is brazing? Describe briefly two methods of brazing.
6.Write short notes on the following:
7.With a neat sketch, explain metal arc welding process.
8. With a neat sketch, explain different types of flames used in gas welding
process. Also list out their uses.
Source: https://svce.ac.in/departments/auto/Lesson%20plan/II%20YEAR%20CLASS%20NOTES/ME6352/PT.doc
Web site to visit: https://svce.ac.in
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