UNIT-1

CASTING

Syllabus:

Casting: Steps involved in making a casting – Advantage of casting and its applications;

Patterns - Pattern making, Types, Materials used for patterns, pattern allowances and their

construction; Properties of moulding sands.

Methods of Melting - Crucible melting and cupola operation – Defects in castings;

Casting processes – Types – Sand moulding, Centrifugal casting, die- casting, Investment

casting, shell moulding; Principles of Gating – Requirements – Types of gates, Design of

gating systems – Riser – Function, types of Riser and Riser design.

Casting Definition: Casting is a manufacturing process in which a liquid material is usually poured into a mould, which contains a hollow cavity of the desired shape, and then allowed to solidify. The solidified part is ejected or broken out of the mould to complete the process.

CASTING

PARTS OF CASTING

· COPE & DRAG: In foundry work, the terms cope and drag refer respectively to the top and bottom parts of a two-part casting flask, used in sand casting. The flask is a wood or metal frame, which contains the molding sand, providing support to the sand as the metal is poured into the mold.

· GATING SYSTEM: Gating system is that it leads the pure molten metal to flow through a ladle to the casting cavity, which ensures proper and smooth filling of the cavity.

· RISER: A riser or feeder head is a vertical passage made in the cope to store the liquid metal and supply the same to the casting as it solidifies. Functions: Store sufficient liquid metal and supply the same to the casting it solidifies there by avoiding volumetric shrinkage of the casting

· VENT: The main function of a vent (or vents) is to remove any air or gas that is in the mold during the casting process. 

· POURING BASIN:  Pouring basin is a reservoir in the top part of a mould into which molten metal is poured.

· SPRUE: A sprue is the passage through which liquid material is introduced into a mold. In many cases it controls the flow of material into the mold. During casting or molding, the material in the sprue will solidify and need to be removed from the finished part.

· CHOKE: The choke, which is the smallest cross-sectional area in the gating system used to control flow, can be placed near the sprue well to slow down and smooth out the flow. 

· BLIND RISER: Blind risers or closed risers are used used to feed various casting sections.

· CORE:A core is a device used in casting and molding processes to produce internal cavities. The core is normally a disposable item that is destroyed to get it out of the piece.

· MOULD CAVITY: Molten metal is poured into a mold. Once the metal cools and solidifies it takes the shape of the mold cavity.

STEPS TO MAKE CASTING

· Make the pattern. The material of the pattern can be wood metal or plastic

· With help of pattern & core prepare the mould.

· Clamp the mould properly with cores placed properly in the mould cavity.

· Melt the metal or alloy to be cast

· Pour the molten metal into the mould cavity

· Allow the molten metal to cool & solidify. Remove the casting from the mould. This operation is called “shakeout”.

· Clean & finish the casting. This operation is called fettling.

· Test & inspect the casting

· Remove the defects if any & if possible

· Stress relive the casting by heat treatment

· Again inspect the casting

· The casting is ready for the use

TYPES OF PATTERN:

· Single piece pattern: This is the simplest type of pattern, exactly like the desired casting. For making a mould, the pattern is accommodated either in cope or drag.Used for producing a few large castings, for example, stuffing box of steam engine.

· Loose piece pattern

When a one piece solid pattern has projections or back drafts which lie above or below the parting plane, it is impossible to with draw it from the mould. With such patterns, the projections are made with the help of loose pieces. One drawback of loose piece pattern is that their shifting is possible during ramming.

· Split pattern

These patterns are split along the parting plane (which may be flat or irregular surface) to facilitate the extraction of the pattern out of the mould before the pouring operation. For a more complex casting, the pattern may be split in more than two parts.

· Match plate pattern

· A match plate pattern is a split pattern having the cope and drags portions mounted on opposite sides of a plate (usually metallic), called the "match plate" that conforms to the contour of the parting surface. The gates and runners are also mounted on the match plate, so that very little hand work is required. This results in higher productivity. This type of pattern is used for a large number of castings.Piston rings of I.C. engines are produced by this process.

· Skeleton pattern

For large castings having simple geometrical shapes, skeleton patterns are used. Just like sweep patterns, these are simple wooden frames that outline the shape of the part to be cast and are also used as guides by the molder in the hand shaping of the mould.This type of pattern is also used in pit or floor molding process.

· Multi piece pattern

pattern is split into more than two parts. it facilitates in easy moulding & with drawl of pattern, pattern will be of 3 or more pieces based on the design of component.

· Sweep pattern

· A sweep is a section or board (wooden) of proper contour that is rotated about one edge to shape mould cavities having shapes of rotational symmetry. This type of pattern is used when a casting of large size is to be produced in a short time

· Follow board pattern

It is a wooden board used to support during moulding. it acts like base seat for pattern. follow bards are used for those patterns that have odd shapes. it supports week patterns & also acts like natural parting line.

Gated pattern:

A gated pattern is simply one or more loose patterns having attached gates and runners.Because of their higher cost, these patterns are used  for producing small castings in mass production systems and on molding machines.

ALLOWANCES

To compensate for any dimensional and structural changes which will happen during the casting or patterning process, allowances are usually made in the pattern

Contraction allowance / Shrinkage allowance

Shrinkage Allowance : Almost all metals shrink or contract volumetrically after solidification to obtain a particular size of casting .The metal will undergo shrinkage during solidification and contract further on cooling to room temperature. To compensate this, the pattern is made larger than the required casting. This extra size is given on the pattern for metal shrinkage is called shrinkage allowance.

The pattern needs to incorporate suitable allowances for shrinkage; these are called contraction allowances, and their exact values depend on the alloy being cast and the exact sand casting method being used. Some alloys will have overall linear shrinkage of up to 2.5%, whereas other alloys may actually experience no shrinkage or a slight "positive" shrinkage or increase in size in the casting process (type metal and certain cast irons. Shrinkage can again be classified into liquid shrinkage and solid shrinkage. Liquid shrinkage is the reduction in volume during the process of solidification, and Solid shrinkage is the reduction in volume during the cooling of the cast metal. Shrinkage allowance takes into account only the solid shrinkage. The liquid shrinkage is accounted for by risers.

Draft allowance

When the pattern is to be removed from the sand mold, there is a possibility that any leading edges may break off, or get damaged in the process. To avoid this, a taper is provided on the pattern, so as to facilitate easy removal of the pattern from the mold, and hence reduce damage to edges. The taper angle provided is called the Draft angle. The value of the draft angle depends upon the complexity of the pattern, the type of molding height of the surface, etc. Draft provided on the casting is usually 1 to 3 degrees on external surfaces (5 to 8 internal surfaces).

Finishing or Machining allowance

The surface finish obtained in sand castings is generally poor (dimensionally inaccurate), and hence in many cases, the cast product is subjected to machining processes like turning or grinding  in order to improve the surface finish. During machining processes, some metal is removed from the piece. To compensate for this, a machining allowance (additional material) should be given in the casting. the amount of finish allowance depends on the material of the casting, size of casting, volume of production, method of molding, and etc..

Shake allowance

Usually during removal of the pattern from the mold cavity, the pattern is rapped all around the faces, in order to facilitate easy removal. In this process, the final cavity is enlarged. To compensate for this, the pattern dimensions need to be reduced. There are no standard values for this allowance, as it is heavily dependent on the personnel. This allowance is a negative allowance, and a common way of going around this allowance is to increase the draft allowance. Shaking of the pattern causes an enlargement of the mould cavity and results in a bigger casting

Distortion allowance

During cooling of the mould, stresses developed in the solid metal may induce distortions in the cast. This is more evident when the mould is thinner in width as compared to its length. This can be eliminated by initially distorting the pattern in the opposite direction

DEFECTS IN CASTINGS

· It is an unwanted irregularities that appear in the casting during metal casting process. There is various reason or sources which is responsible for the defects in the cast metal.

· These defects may be the result of:(a) Improper pattern design,(b) Improper mould and core construction,(c) Improper melting practice,(d) Improper pouring practice and(e) Moulding and core making materials.(f) Improper gating system(g) Improper metal composition(h) Inadequate melting temp and rate of pouring

· Shift or Mismatch

· Shift is of two types mould shift & core shift

· Mould shift refers o side wards shift of cope with respect to drag Or improper placement of pattern in cope box.

· Core shift refers to misplacement or displacement of core from the required place.

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· Blow holes: When gases traps in side the metal during pouring it displaces the metal & forms the round or oval shape holes. These holes are called as blow holes

Pinholes

They are very small holes of about 2 mm in size which appears on the surface of the casting. This defect happens because of the dissolution of the hydrogen gases in the molten metal. When the molten metal is poured in the mould cavity and as it starts to solidify, the solubility of the hydrogen gas decreases and it starts escaping out the molten metal leaves behind small number of holes called as pinholes.

Shrinkage cavity

A shrinkage cavity is a depression or an internal void in a casting that results from the volume contraction that occurs during solidification. This occurs because of uneven or uncontrolled solidification process & when the pouring temperature of metal is too high

Swell : A swell is a slight, smooth bulge usually found on vertical faces of castings, resulting from liquid metal pressure. It may be due to low strength of mould because of too high a water content or when the mould is not rammed sufficiently

Drops or crush

Drop defect occurs when there is cracking on the upper surface of the sand and sand pieces fall into the molten metal.

Metal penetration

When the molten metal is poured into the mould cavity, at those places when the sand packing is inadequate, some metal will flow between the sand particles for a distance into the mould wall and get solidified. When the casting is removed, this lump of metal remains attached to the casting. Of course, it can be removed afterwards by chipping or grinding.

Cold shut: It is a type of surface defects and a line on the surface can be seen. When the molten metal enters into the mould from two gates and when these two streams of molten metal meet at a junction with low temperatures than they do not fuse with each other and solidifies creating a cold shut (appear as line on the casting). It looks like a crack with round edge.

· Misrun or cold sheet or short run: This defect is incomplete cavity filling. The reasons can be: - inadequate metal supply, too- low mould or melt temperature, improperly designed gates, .or length to thickness ratio of the casting is too large. When molten metal is flowing from one side in a thin section, it may loose sufficient heat resulting in loss of its fluidity, such that the leading edge of the stream may freeze before it reaches the end of the cavity.

Slag inclusion This defect is caused when the molten metal containing slag particles is poured in the mould cavity and it gets solidifies.

· Hot tears or Hot crack :They arise when the solidifying met does not have sufficient strength to resist tensile forces produced during solidification. 

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· Hot spot or hard spot

· Hot spot defects occur when an area on the casting cools more rapidly than the surrounding materials. Hot spot are areas on the casting which is harder than the surrounding area. It is also called as hard spots

Buckle:

· A buckle is a long, fairly shallow, broad, vee depression that occurs in the surface of flat castings. It extends in a fairly straight line across the entire flat surface.

· It results due to the sand expansion caused by the heat of the metal, when the sand has insufficient hot deformation. It also results from poor casting design providing too large a flat surface in the mould cavity.  

· Buckling is prevented by mixing cereal or wood flour to sand

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CENTRIFUGAL CASTING

Centrifugal casting is a method of casting parts having axial symmetry. The method involves pouring molten metal into a cylindrical mold spinning about its axis of symmetry.

The mold is kept rotating till the metal has solidified.

As the mold material steels, Cast irons, Graphite or sand may be used.

The rotation speed of centrifugal mold is commonly about 1000 RPM (may vary from 250 RPM to 3600 RPM).A centrifugal casting machine is schematically presented in the picture:

Centrifugal casting is carried out as follows:

· The mold wall is coated by a refractory ceramic coating (applying ceramic slurry, spinning, drying and baking).

· Starting rotation of the mold at a predetermined speed.

· Pouring a molten metal directly into the mold (no gating system is employed).

· The mold is stopped after the casting has solidified.

· Extraction of the casting from the mold.

The casting solidifies from the outside fed by the inner liquid metal.

Non-metallic and slag inclusions and gas bubbles being less dense than the melt are forced to the inner surface of the casting by the centrifugal forces. This impure zone is then removed by machining.

Advantages

1. Relatively very light impurities move inwards towards center. So they can be removed easily thus helping in producing sound castings.

2. Gates and risers are not needed.

3. This technique is best suited for the mass production of symmetrica lobjects and Castings yield is very high in some cases it is even equal to 100%.

4. Castings acquire high density, high mechanical strength and fine grained structure.

5. Inclusions and impurities are lighter.

6. These castings have a directional solidification starting from outside to inside.

Disadvantages Centrifugal casting process:

1. Skilled labors are to be employed for this process.

2. An inaccurate diameter of the inner surface of the casting.

3. Only some shapes can be generated by this casting process.

4. Not all alloys can be cast in this way. 

5. Centrifugal castings require very high investments.

SHELL MOULDING:

1. Phenol or urea formaldehydes (5%) are added to fine silica sand as bonding agents. No clay is added.

2. Each half of a shell mould is made on a pattern plate. This has to be made of metal, usually of steel, because of the high temperature of moulding.

3. The plate is heated to 200 - 250˚C, and is then sprayed or brushed with silicon oil, to facilitate the subsequent stripping of the shell from the pattern.

4. The pattern plate is then placed on top of the dump box containing the sand-resin mixture.

5. The dump box is inverted so that the pattern becomes covered by sand-resin mixture.

6. The resin melts, and in about 30 s the pattern becomes coated with a “shell” of resin-bonded sand. The shell is sometimes quite hard, due to the thermosettingproperties of the resin.

7. The dump box is turned back to its original position, so that the surplus of sand and resin falls back into the bottom of the dump box.

8. The pattern plate is then removed, and with the shell still adhering to it, is transferred to an oven where the shell is hardened further by curing it for about 2 min at 315˚C. The shell is then stripped from the pattern plate by means of ejector pins built into the plate. Thickness is 6 - 9 mm.

9. Two halves of the mould are then joined together by adhesives or bolts.

10. Medium productivity (approximately 30 - 220 moulds per hour).

11. Tool costs are reasonably high.

12. Higher degree of accuracy than with sand casting. Limits of ±0.25 mm can be achieved on small castings up to 100 mm in size.

13. Cores should generally be avoided wherever possible.

14. Only 1˚ of taper is required compared with 2 - 3˚ for sand castings. Faces up to 18 mm deep can be produced with virtually no taper.

15. Use of fine grained sands produces good surface finishes.

16. A shell moulding, with reduced machining and finishing, should be cheaper than an equivalent sand casting.

ADVANTAGES

· Good surface quality.

· High rough casting dimensional accuracy.

· Thin wall thickness and complex castings.

· Less manpower and molding skill requirements

LIMITATIONS:.

· High production costs and casting prices.

· High pattern costs.

· Size and weight limitation.

Die casting is a process, in which the molten metal is injected into the mold cavity at an increased pressure up to 30,000 psi (200 MPa).

The reusable steel mold used in the die casting process is called a die.

Die casting is a highly productive method of casting parts with low dimensions tolerance and high surface quality.The following parts are manufactured by die casting method: automotive connecting rods, pistons, cylinder beds, electronic enclosures, toys, plumbing fittings.The molten metal injection is carried out by a machine called die casting machine.

There are two principal die casting methods: hot chamber method and cold chamber method.

· Cold chamber die casting

· Hot chamber die casting

· Design aspects of die casting

· Advantages and disadvantages of die casting

Cold chamber die casting

In the cold chamber die casting machines hydraulically operated plunger forces a molten metal to flow in the cold cylinder (chamber).

A principal scheme of the cold chamber die casting machine is shown in the picture:

Cold chamber process:

· When the pressure chamber is filled with a molten metal the plunger starts traveling forward and builds up a pressure forcing the metal to flow through the sprue to the die cavity.

· After the metal has solidified the plunger returns to its initial position allowing a new portion of the molten metal to fill the pressure chamber.

· The die then opens and the ejector pins removes the casting from the die.

· The casting cycle now may be repeated.

Cold chamber method is mainly used for casting Aluminum alloys, Magnesium alloys, Copper alloys and zinc alloys (including zinc-aluminum alloys).

Hot chamber die casting

In the hot chamber die casting machines the pressure chamber (cylinder) and the plunger are submerged in the molten metal in the pot (crucible).

Hot chamber machines have short casting cycle (about 1 sec.). They are capable to cast thin wall casting with good filling the cavity under precise temperature control of the molten metal.

Hot chamber process may be used for casting low melting metals, which are chemically inert to the material of the plunger and other parts of the casting machine: zinc alloys (except zinc alloys containing more than 10% of aluminum), tin alloys and Magnesium alloys.

Maintenance of hot chamber machines is more expensive as compared to the cold chamber process.

Hot chamber process:

· The plunger goes up allowing the melt to fill the cylinder space. The die is closed at this stage.

· The plunger goes down forcing the melt to flow through the gooseneck into the die cavity.

· After the die has been filled with the melt the plunger is held under a pressure until the Solidification is completed.

· The die opens. The casting stays in the die part equipped with ejectors.

· The plunger goes up and the melt residuals return through the gooseneck back to the pot.

· The ejectors push the casting out of the die.

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Advantages and disadvantages of die casting

Advantages of die casting:

· High productivity.

· Good dimensional accuracy.

· Good surface finish: 2-100 µinch (0.5-2.5 µm) Ra.

· Thin wall parts may be cast.

· Very economical process at high volume production.

· Fine Grain structure and good mechanical properties are achieved.

· Intricate shapes may be cast.

· Small size parts may be produced.

Disadvantages of die casting:

· Not applicable for high melting point metals and alloys (eg. steels)

· Large parts can not be cast.

· High die cost.

· Too long lead time.

· Some gases may be entrapped in form of porosity.

Investment casting process

Investment casting is a casting process in which a mould is made around a wax pattern that is burned away when the molten material is poured in. In investment casting, a ceramic slurry is applied around a disposable pattern to form a mould. It is also called as lost wax method.

The investment casting process uses expendable patterns made of investment casting wax.

· The wax patterns are commonly prepared by injection moluding technology which involves injection of wax into a prefabricated die having the same geometry of the cavity as the desired cast part.

· The wax patterns are then attached to a gating system (a set of channels through which a molten metal flows to the mould cavity).

· The next stage is the shell building - the wax assembly is immersed into refractory ceramic slurry of hardening mixtures followed by drying. This operation is repeatedly carried out resulting in formation of a solid ceramic shell of 1/4” -3/8” (6mm – 9mm) thick.

· The next stage is dewax. At this stage the assembly is heated in an autoclave where the most of the wax is melted out. This operation is followed by burning out the residual wax in a furnace.

· The mould is then preheated to 1830°F (1000°C). Now the mould is ready for filling with a molten metal.

· Casting stage is conventional operation involving pouring a molten metal into the shell through the gating system.

· After the metal has solidified and cooled to a desired temperature, the shell is broken and the castings are cut away from the gates and sprue.

· The last stage is finishing carried out by sandblasting or machining.

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Ceramic slurry materials

· Mullite Al2O3 44-48%, SiO2 47-51%, Fe2O3 max. 1%, TiO2 max. 1%.

· Zircon flour.

· Binders: Colloidal Silica SiO2 25-45%, Na2O max.1% in distilled water; sodium silicate NaO*nSiO2*mH2O; ethyl silicate Si(OC2H5)4.

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Advantages and disadvantages of investment casting

Advantages:

· Excellent surface finish.

· Tight dimensional tolerances.

· Complex and intricate shapes may be produced.

· Capability to cast thin walls.

· Wide variety of metals and alloys (ferrous and non-ferrous) may be cast.

· Draft is not required in the molds design.

· Low material waste.

Disadvantages:

· Individual pattern is required for each casting.

· Limited casting dimensions.

· Relatively high cost (tooling cost, labour cost).

INSPECTION & TESTING OF CASTING

1. VISUAL INSPECTION

2. DIMENSIONAL INSPECTION

3. METALLURGICAL CONTROL

4. PRESSURE TESTING

5. RADIO GRAPHICAL TESTING

6. MAGNEIC TESTING

7. MAGNETIC PARTICAL TESTING

8. EDDY CURRENT INSPECTION

9. LIQUID PENETRATION INSPECTION

10. ULTRASONIC TESTING