introduction to metal casting by prof. keyur parmar g.i.t.s, prantij 1prof. keyur parmar

prof. keyur parmar 1

Introduction To Metal Casting

By Prof. Keyur parmar

G.I.T.S, prantij


Definition

• Metal casting is a process in which molten or liquid metal is poured into a mould made of sand, metal or ceramic, to form geometrically complex parts. All major metals can be cast.

• The most common materials are iron, aluminum, magnesium, zinc, steel and copper-based alloys.

• Casting is a 6000 year old process


Casting Methods


Advantages Of Casting :

• The metal casting process is extensively used in manufacturing because of its many advantages.

• Molten material can flow into very small sections so that intricate shapes can be made by this process. As a result, many other operations, such as machining, forging, and welding, can be minimized or eliminated.

• It is possible to cast practically any material that is ferrous or non-ferrous.

• As the metal can be placed exactly where it is required, large saving in weight can be achieved.

• The necessary tools required for casting moulds are very simple and inexpensive. As a result, for production of a small lot, it is the ideal process.

• There are certain parts made from metals and alloys that can only be processed this way.

• Size and weight of the product is not a limitation for the casting process.


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 moulding process, and shell moulding process.

• The metal casting process is a labor intensive process.


Casting Terms• Flask

• Drag

• Cope

• Cheek

• Pattern

• Parting Line

• Bottom Board

• Facing Sand

• Moulding Sand

• Backing Sand

• Core

• Pouring Basin

• Sprue

• Runner

• Gate

• Chaplet

• Chill

• Riser


Pattern

• A pattern may be defined as a model/replica of desired casting which when moulded in sand forms an impression called mould. The mould when filled with the molten metal forms casting after solidification of the poured metal. The quality and accuracy of casting depends upon the pattern making.

• The pattern may be made of wood, metal(cast iron, brass, aluminum and alloy steel.), plaster, plastics and wax

• The main modifications are the addition of pattern allowances, and the provision of core prints. If the casting is to be hollow, additional patterns called cores are used to create these cavities in the finished product.

• The quality of the casting produced depends upon the material of the pattern, its design, and construction. The costs of the pattern and the related equipment are reflected in the cost of the casting. The use of an expensive pattern is justified when the quantity of castings required is substantial


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.

• Runner, gates, and risers used for feeding molten metal in the mould cavity may form a part of the pattern.

• Patterns properly made and having finished and smooth surfaces reduce casting defects.

• A properly constructed pattern minimizes the overall cost of the castings.


Material Requirement Of Pattern( wood, metals and alloys, plastic, plaster of Paris, plastic and rubbers, wax, and resins)

• 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


Pattern Types

• solid or single piece patterns• split or two/multiple piece patterns• match plate pattern• cope and drag pattern• loose piece pattern• gated patterns• sweep pattern• skeleton pattern• shell pattern• segmental pattern• follow board pattern• lagged up pattern• left and right hand pattern


Loose Piece Pattern

• Some patterns usually single piece, are made to have loose pieces in order to enable their easy withdrawal from the mould. These pieces form an integral part of the pattern during moulding. After the mould it completes, the pattern is withdrawn leaving the pieces in the sand, which are later withdrawn separately through the cavity formed by the pattern.


Single Piece / Solid Pattern

• A single piece pattern is the simplest of all the patterns, is made in one piece and carries no joint, partition or loose pieces. Depending upon the shape, it can be moulded in one or two boxes. The pattern is the cheapest but its use can be done to a limited extent of production only since its moulding involves a large number of manual operations like gate cutting, providing runners and risers.


Multi-Piece Pattern

• Castings having a more complicated design than above require the pattern in more than two parts in order to facilitate an easy moulding and withdrawal of pattern. Their pattern may consist of 3, 4 or more numbers of parts, depending on their design.


Two Piece / Split Pattern

• Many times the design of casting offers difficulty in mould making and withdrawal of pattern, if a solid pattern is used. For such castings, split or two piece pattern are employed. They are made in two parts which are joined at the parting line by means of dowels. While moulding one part of the pattern is contained by the drag and the other by the cope.


Gated Pattern• They are also used in mass production of small castings. For such

castings, multi-cavity moulds are prepared i.e. a single sand mould carriers a number of cavities.

• Patterns for these castings are connected to each other by means of gate formers which provide suitable channels or gates in sand for feeding the molten metal to these cavities. A single runner can be used for feeding all the cavities.

• This enables a considerable saving in moulding time and a uniform feeding of molten metal.


Cope and Drag Pattern

• When very large castings are to be made, the complete pattern becomes too heavy to be handled by a single operator.

• Such a pattern is made in two parts which are separately moulded in different moulding boxes. After completion of the moulds, the two boxes are assembled to form the complete cavity of which one part is contained by the drag and other is cope. Thus, in a way, it is nothing but a two piece or split pattern of which both the pieces are moulded separately instead of being moulded in the assembled position.


Follow Board Pattern

A follow board is a wooden board used to support a pattern during moulding. It acts as a seat for the pattern.Such single piece patterns which have an odd shape or very thin wall require a follow board. In the former case, the hollow board is provided with a cavity corresponding to the shape of the pattern in which the pattern is seated for moulding.In the latter case, the follow- board carries a projection confirming to the inside shape of the thin walled pattern to support it during moulding. If such a support is not provided, the pattern may sag or get broken due to less wall thickness during ramming.


Sweep Pattern• Sweeps can be advantageously used for preparing moulds of large

symmetrical castings, particularly of circular cross section. • This effect a large saving in time, labour and material. The full equipment

consists of a base, suitably placed in the sand mass, a vertical spindle and a wooden template called sweep.

• The outer end of sweep carries the contour corresponding to the shape of the desired casting. The sweep is rotated about the spindle to form the cavity. Then the sweep and the spindle are removed, leaving the base in the sand. The hole made by the removal of spindle is patched up by filling the sand.


Segmental Pattern

• These patterns are used for preparing moulds of circular castings, avoiding the use of solid pattern of exact size.

• If principle they work like a sweep, but the difference is that a sweep is given a continuous revolving motion to generate the desired shape, where as segmental pattern is a portion of the solid pattern itself and the mould is prepared in parts by it. It is mounted on a central pivot and after preparing the part mould in one position, the segment is moved to the next position. The operation is repeated till the complete mould is ready.


Skeleton Pattern

• When the size of the casting is very large, but easy to shape and only a few numbers are to be made, it is uneconomical to make a large solid pattern of that size.

• In such cases, a pattern consisting of wooden frame and strips is made called skeleton pattern

• strips of wood is used for building the final pattern by packing loam sand around the skeleton with further ramming. After packing the sand desired shape or form is obtained, The surplus sand is removed by means of a stickle.

• The core can be prepared separately either with the help of a core box or another skeleton made for that, and assembled in position in the mould.


Pattern Colour Code

There is no universal method of colouring but following method is followed as a practice for colouring the patterns and core boxes.• Red for machining surface• Black for un-machined surface• Yellow for core print• Red strip on yellow base for seat for loose pieces• Without colour for parting surface


Pattern Allowances

• A pattern is always made larger than the required size of the casting considering the various allowances. These are the allowances which are usually provided in a pattern.


shrinkage or contraction allowance:

• The various metals used for casting contract after solidification in the mould. Since the contraction is different for different materials, therefore it will also differ with the form or type of metal.

• 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 mould.

• Solid Shrinkage : it refers to the reduction in volume caused when metal loses temperature in solid state

• Shrink Rule or “pattern maker’s contraction rule• A shrink rule for cast iron is 1/8 inch longer per foot than a standard rule.


Material Dimension Shrinkage allowance (inch/ft)

Grey Cast Iron

Up to 2 feet2 feet to 4 feet

over 4 feet

0.1250.1050.083

Cast Steel


over 6 feet

0.2510.1910.155

Aluminum


over 6 feet

0.1550.1430.125

MagnesiumUp to 4 feetOver 4 feet

0.1730.155


Exercise

Q1. The pattern is to be made for the above drawing from the wood material. The product material is cast iron .All dimensions are in mm. Find the dimensions of pattern.

Q2. Grey cast iron castings of dimension 80 mm are to be made in a metal mould made of aluminium alloy. The metal mould is to be made using a wooden pattern. Determine the correct dimension of the wooden pattern considering the solidification contraction only.


Exercise Solution

Q1. The shrinkage allowance for cast iron for size up to 2 feet is o.125 inch per feet For dimension 18 inch, allowance = 18 X 0.125 / 12 = 0.1875 inch » 0.2 inchFor dimension 14 inch, allowance = 14 X 0.125 / 12 = 0.146 inch » 0.15 inchFor dimension 8 inch, allowance = 8 X 0.125 / 12 = 0.0833 inch » 0. 09 inchFor dimension 6 inch, allowance = 6 X 0.125 / 12 = 0.0625 inch » 0. 07 inch

Q2. Here the wooden pattern must have a double shrinkage allowance for theshrinkage of metal mould (aluminium) and the casting (cast iron).Allowance for aluminium = (80 mm) x (1.20/100 mm/mm) = 0.96 mmAllowance for cast iron = (80 mm) x (0.80/100 mm/mm) = 0.64 mmTherefore, total shrinkage allowance = 0.96 + 0.64 = 1.60 mmHence, the dimension of the wooden pattern would be = 80 + 1.60 = 81.60 mm


Draft allowance or Taper allowance

• When a pattern is drawn from a mould, there is always a possibility of damaging the edges of the mould. Draft is taper made on the vertical faces of a pattern to make easier drawing of pattern out of the mould (Fig. 1.3). The draft is expressed in millimetres per metre on a side or in degrees..


• The amount of draft needed depends upon • The shape of casting• Depth of casting• Moulding method• Moulding material.

• Generally, the size of draft is 5 to 30 mm per metre, or average 20 mm per metre. But draft made sufficiently large, if permissible, will make moulding easier.

• For precision castings, a draft of about 3 to 6 mm per metre is required


Finish or machining allowance

• The allowance is provided on the pattern if the casting is to be machined. This allowance is given in addition to shrinkage allowance.

• In case the casting designed to be machined, they are cast over-sized in those dimensions shown in the finished working drawings. Where machining is done, the machined part is made extra thick , which is called machining allowance.

• The amount of this allowance varies from 1.6 to 12.5 mm which depends upon the type of the casting metal, size and the shape of the casting.

• The ferrous metals require more machining allowance than non ferrous metals.


• Machining Allowance is given due to following reason:• Castings get oxidised inside mould and during heat treatment. Scale thus

formed requires to be removed.• For removing surface roughness, slag, dirt and other imperfections from the

casting.• For obtaining exact dimensions on the casting.• To achieve desired surface finish on the casting.

• Machining Allowance depends up on following Factors:• Method of machining used (turning, grinding, boring, etc.). Grinding

removes lesser metal than turning.• Characteristics of metal (ferrous or non-ferrous, hard and easily machinable

or soft). Ferrous metals get oxidised, aluminium does not.• Method of casting used. Centrifugal casting requires more allowance on the

inner side. Die castings need little machining, sand castings require more.• Size and shape of the casting. For long castings, warpage is more and

greater allowance is required. Thicker sections solidify late and impurities tend to collect there. This necessitates more machining allowance.

• Degree of finish required. A higher degree of finishing requires more machining allowance.


Distortion or camber allowance

• This allowance is provided on patterns used for casting of such design in which the contraction is not uniform throughout.

• Sometimes castings, because of their size, shape and type of metal, tend to warp or distort during the cooling period depending on the cooling speed.

• This is due to the uneven shrinkage of different parts of the casting. Expecting the amount of warpage, a pattern may be made with allowance of warpage. It is called camber.


• For example, a U-shaped casting will be distorted during cooling with the legs diverging, instead of parallel . For compensating this warpage, the pattern is made with the legs converged but, as the casting cools, the legs straighten and remain parallel.

• Warpage depends on the thickness and method of casting and it is actually determined by experience. Generally 2 to 3 mm is considered appropriate for 1 metre length.

1. Casting Without Camber2. Actual casting 3. Pattern with camber allowance


Rapping or shaking allowance

• When the pattern is shaken for easy withdrawal, the mould cavity, hence the casting is slightly increased in size. In order to compensate for this increase, the pattern should be initially made slightly smaller.

• For small and medium sized castings, this allowance can be ignored. But for large sized and precision castings, however, shaking allowance is to be considered.

• The amount of this allowance is given based on previous experience.


Design Considerations in Patterns• Proper allowance should be provided.• The parting line should be carefully selected.• Proper material should always be selected• The wall thickness and sections should kept as uniform as possible. Abrupt

changes should invariably be avoided.• The use of offset pairing, instead of cores, should be encouraged to as great

extent as it is possible. Abrupt changes should invariably be avoided. • For large scale production of small castings, the use of gated or match-plate

pattern should be encouraged wherever the existing facilities permit• All sharp corners and edges should be invariably provided with suitable fillets

or otherwise rounded to enable an easy withdrawal of pattern, smooth flow of molten metal and ensure a sound casting.

• All those surfaces of the castings which are specially required to be perfectly sound and clean should be so designed that they will be moulded in the drag.

• The pattern should be given a high class surface finish as it directly effects the corresponding finish of the casting

• If gates, runners and risers are attached to the pattern, they should be properly located and their sudden contractions or enlargements should be avoided.


Classification Of Casting Process

Casting processes can be classified into following FOUR categories:

• Conventional Moulding Processes Green Sand Moulding• Dry Sand Moulding• Flask less Moulding

• Chemical Sand Moulding Processes• Shell Moulding• Sodium Silicate Moulding• No-Bake Moulding

• Permanent Mould Processes• Gravity Die casting• Low and High Pressure Die Casting

• Special Casting Processes• Lost Wax• Ceramics Shell Moulding• Evaporative Pattern Casting• Vacuum Sealed Moulding• Centrifugal Casting


Different Casting Processes


Sand Casting


Moulding Material

• Moulding Sand• Clay• Core Sand• Binders• Additives


Moulding Sand

• The general sources of receiving sands are :• Beds of sea ,River, Lakes, Granular Elements of rocks, Deserts

• Common Sources in India• Batala Sand (Punjab)• Ganges Sand (Uttar Pradesh)• Oyaria Sand (Bihar)• Damodar & Barakar Sands (Bengal-Bihar Border)• Londha Sand (Bombay)• Gigatamannu Sand ( Andhra Pradesh)• Avadi and Veeriyambakam Sand (Madras)


Sand

• Moulding Sand May of two types : • Natural & Synthetic

• Foundry sands, in general are composed of • grains of quartz or quartzite, • crystalline forms of silica • other minerals of highly siliceous

• These sands as they occur in nature, may not be suitable for foundry use.

• It may be necessary to prepare the sand by washing, grading or mixing.


Constituents Of Sand

•Silica Sand • The element silica has a density of 2.4 g/cc and a melting point of

1420°C, where as quartz, the principal constituents of silica sand has a density of 2.66 g/cc and a melting point of about 1750°C.

• The greatest drawback of silica sand for use as a mould material is the sudden expansion that occurs at 575°C and it is this expansion which governs the tendency of mould face to spall at some stare during casting causing scabs and sand inclusions.

•Binder• It can be Organic or Inorganic type• Inorganic – Clay Sodium Silicate , Portland cement • Organic – Dextrin, Molasses, Cereal binders, linseed oil, resin like phenol

formaldehyde and urea formaldehyde etc• In Foundry Shop – Kaolinite, Ball clay, Fire Clay, Limonite, Fuller’s Earth,

Bentonite are used.• Organic binder are used for core making generally.


•CLAY (Binder)• Binding agent mixed with sand to provide the sufficient

strength.• Low cost and wider utility.• Kaolinite or fire Clay and Bentonite• Melting point Kaolinite = 1780*C• Melting Point Bentonite = 1300*C• Bentonite can absorb more water which increase the

bonding strength.• It also contain small amount of Lime , Alkalies, oxides in it

to reduce their refractoriness.


• Western Bentonite• Sodium as absorbed ion• High dry strength, • better swelling, • better tolerance • low green strength, • high resistance to burnout which reduces clay

consumption.• Southern Bentonite

• Calcium as absorbed ion• Have low dry strength• Higher Green Strength• Properties can be improved by treating it with soda ash


•Moisture• Amount moisture varies between 2% to 8%• It is used for bonding clay and silica sand.

•Additives• Generally added to mould and core sand.• Coal Dust

• Producing a reducing atmosphere in casting , to take out oxygen from pores.

• Corn Fluor• Belongs to starch family of carbohydrates.• Used to increase collapsibility.

• Dextrin• It increases dry strength of mould.


• Sea Coal• Fine powdered Bituminous coal.• It Positions in the pores of silica sand.• When heated –changes to coke –fill the pores• Unaffected by water.• Sand grains are restricted to move in a dense packing

pattern• Reduces mould wall movement & Permeability• Makes mould and core surface clean and smooth.

• Pitch• Distilled form of soft coal.• It can be added 0.02% To 2%.• It Enhances hot strengths, surface finish on mould

surface, similar to sea coal.


• Wood Fluor• Fibrous material mixed with granular sand• Long thin fiber between grains of sand.• It can be added 0.05% To 2%.• It volatilizes when heated , allowing sand grains room

to expand.• It increase the mould wall movement • Decrease expansion defects• Increase Collapsibility of mould and core sand.

• Silica Fluor• Called as “ Pulverized Silica”• Added up to 3%• Increases Hot strength and Finish on Surface.• It also reduce metal penetration in wall.


Moulding Sand – Kinds / Types

•Green Sand•Dry Sand•Loam Sand•Facing Sand•Backing Sand•System Sand•Parting Sand•Core Sand


• Green Sand:• Also known as tempered or natural sand• It is prepared mixture of :

• Silica sand + 18-30% Clay. + 6-8% Moisture• FINE, SOFT, LIGHT & POROUS• Moulds prepared by this , do not require backing , hence

are known as Green Sand Moulds.• Easily Available & Low Cost.• Use – Product of Ferrous & Non-Ferrous Castings.

• Dry Sand:• Green sand that has been baked or dried in oven after

making mould and cores is DRY SAND.• It possess Strength, Rigidity, Thermal Stability.• Suitable for larger casting. (Dry Sand Moulds).


• Loam Sand:• Loam is a mixture of Sand & Clay with water to a thin

layer of plastic paste.• High Clay = 30-50%• Water = 18%• Patterns are not Used for this sand for mould making.• Mould is given a shape by sweeps.• Use – For Large Grey Iron Casting.


• Facing Sand:• Just Prepared & Forms the Face of mould.• It is directly next to the surface of pattern.• It comes in contact with molten metal.• It must possess – High strength refractoriness.• Purely made from silica sand + Clay.• Layer thickness in mould = 22-25mm• It is 10-15% of whole moulding sand.

• Backing / Floor / Black Sand:• Used to backup facing sand and fill the whole volume of

moulding flask.• Used sand is mainly employed as backing sand


• System Sand:• It is used in mechanized foundries where machine

moulding is employed.• To Fill Whole moulding flask.• Used sand re-activated with water and special additives.

• Parting Sand:• Helps the green sand not to stick to pattern.• Also allow the sand on the parting surface of cope and

drag to separate without clinging.• Clean clay –free silica sand which serves the purpose of

parting dust.


• Core Sand:• Used for making cores• Also called oil sand• Made from highly richer silica sand mixed with oil

binders such as core oil – Linseed oil, resin , light material oil etc


Properties require in moulding material or Sand

• Refractoriness• Permeability• Green Strength• Dry Strength• Hot Strength• Cohesiveness• Adhesiveness• Flowability or Plasticity• Collapsibility• General – they should be re-usable and have good

thermal conductivity so that heat from casting is quickly removed.


Refractoriness

• Ability of Moulding material to withstand high temperature of molten metal without breaking down.

• It can be only increased to limited extent• If this property is not good then the sand will burn on casting surface

and no smooth surface is obtained.• Degree of Refractoriness depends upon:

• SiO2 , Quartz Content.• Shape• Grain Size

• Higher SiO2 + Rougher grain volumetric composition –Higher is the degree of refractoriness.

• It is measured by Sinter Point of Sand instead of Melting point.• IS 1918:1996 – MTD-14


Material Melting Point *C

Silica 1710

Alumina 2020

Magnesia 2800

Thoria 3050

Zirconia 2700

Zircon 2650

Silicon Carbide 2700

Graphite 4200


Green Strength• Sand with moisture is termed as green sand.• It should have enough strength and toughness so that the

constructed mould retains its shape.• For this sand grains must be adhesive and cohesive . • By virtue of this property the pattern can be taken out of the mould

without damage or erosion.• Depends upon : Grain Shape , size and moisture content.


Dry Strength

• When moisture is completely expelled, it is called the dry sand.• When molten metal poured into a mould, the sand around the mould

cavity is quickly converted into dry sand.• It should retain the mould cavity and metallostatic forces due to

liquid metal.• It prevents the enlargement of mould cavity.


Hot Strength

• When the liquid metal in the mould cavity , the near by temperature rises and moisture becomes zero.

• At that time to hold the shape of the mould cavity is the Hot strength of the sand.


Permeability• Also termed as porosity of moulding sand in order to allow the

escape of any air, gases or moisture present/generated in mould.• During the solidification process , Large amount of gases are

expelled from the mould.• This are entrapped/absorbed Gases in furnace, steam, air and

other product gases generated by moulding sand and core sand.• This gas /air should be removed out of the mould during casting

to avoid the casting defects.• So the permeability is the nothing but the sufficient porosity of

the sand to allow this gases to escape out .• It is a function of : Grain Size, Grain Shape, Moisture, Clay

content.• Extent of ramming of sand directly affects the permeability.• It can be increased by using vent rods.


Collapsibility

• After molten metal solidification , sand mould must be collabsible, so that free contraction of metal occurs.

• So naturally - It avoids Tearing or Cracking of Metal.• This property is highly desired in cores.


Flowability or Plasticity

• It is the ability of sand to get compacted and behave like a fluid.• This property allows the sand uniform flow and distribution of sand

around the pattern against the ramming pressure .• Flowability increases with decrease in green strength an decrease in

grain size.• It also varies with moisture and Clay content.

For further go to 2nd file………..

introduction to metal casting by prof. keyur parmar g.i.t.s, prantij 1prof. keyur parmar

Documents

metal casting process

accuracy of casting

advantages of casting

investment casting process

limitations of casting

casting moulds

casting methods

definition metal casting