abstract advantages and limitations of the process

Mohammad Amir, Lecturer, Department of Mechanical Engineering., BHCET 1

Manufacturing Technology, ME-202-E Unit- 1 (Metal Casting Processes) 1 February 2010

Syllabus:

Metal Casting Processes: Advantages and limitations, sand mold making procedure.

Patterns and Cores: Pattern materials, pattern allowances, types of pattern, color coding.

Molding materials: Molding sand composition, sand preparation, sand properties and testing, Sand molding

processes

Abstract: Casting is one of the oldest metal shaping method known to human beings. It generally means pouring of

molten metal into a refractory mould with a cavity of the shape exactly to be made, and allowing that to solidify. When

solidified the desired metallic object is taken out from the refractory mould either by breaking the mould or taking the

mold apart. The solidified object is called as molding and the process as founding.

Advantages and Limitations of the process: The various advantages of the process are as follows:

Molten material can flow into in any small section in the mold cavity and as such any intricate shapes internal or

external can be made with the casting process.

It is possible to cast practically any material be it ferrous or non ferrous.

The tooling cost is almost negligible as very simple tools are used here as compared to the other manufacturing processes.

This process is best suitable for the small batch production as well as trial production.

I t is possible in casting process, to place the amount of material where exactly required.

Castings are generally cooled down at constant rate hence directional properties like stress concentration remains same in all the directions.

Can be made of large sizes and weights up to 200 tonnes. Limitations of the process are as follows:

Dimensional accuracy and surface finish achieved by casting process is not good in many cases.

Labour required in the sand casting process is relatively high.

Hard metals casted are difficult to be machined later after the process for further surface finishing.

Applications of the process: Typical applications of sand casting process are cylinder blocks, liners, machine tool beds, pistons, piston rings, mill rolls, wheels, housings, water supply pipes and special sand bells.

Terms used in Casting Process: Flask: A molding flask is one which holds the sand mold intact, depending upon the position of the flask in the

mold structure it is referred to by various names such as drag-lower molding flask, cope-upper molding flask and cheek-intermediate molding flask used in three-piece molding. It is made up of wood for temporary applications and more generally of metal for long-term use.

Pattern: Pattern is a replica of the final object to be made with some modifications. The mould cavity is made with the help of the pattern.

Parting line: This is the dividing line between the two moulding flasks that makes up the sand mould. Split pattern it is also the dividing line between the two halves of the pattern.

Bottom board: This is a board normally made of wood which is used at the start of the mould making. The pattern is first kept on the bottom board, sand is sprinkled on it and then the ramming is done.

Facing sand: The small amount of carbonaceous material sprinkled on the inner surface of the moulding to give a better surface finish to the castings.



Parting line: This is the dividing line between the two molding flasks that makes up the sand mould split pattern it is also the dividing line between the two halves of the pattern.

Parting line: This is the dividing line between the two moulding flasks that makes up the sand mould, in split pattern it is also the dividing line between the two halves.

Facing sand: The small amount of carbonaceous material sprinkled on the inner surface of the moulding cavity to give a better surface finish to the castings.

Moulding sand: It is the freshly prepared refractory material used for making the mould cavity, lt is the mixture of silica, clay and moisture in appropriate proportions to get the desirable results and it surrounds the pattern while making the mould.

Backing sand: It is what constitutes most of the refractory material found in the mould. This is made of used and burnt sand.

Core: It is used for making hollow cavities in castings.

Pouring basin: A small funnel shaped cavity at the top of the mould into which the molten metal is used.

Spruce: The passage through which the molten metal from the pouring basin reaches the mould cavity. ln many cases it controls the flow of metal into the mould.

Runner: The passageway is n the parting plane through which molten metal flow is regulated before they reach the mould cavity.

Gate: The actual entry point through which molten metal enters the mould cavity.

Chaplet: Chaplets are used to support cores inside the mould cavity to take care of its own weight and overcome the metal-o-static forces.

Chill: Chills are metallic objects which are placed in the mould to increase the cooling rate of castings to provide uniform or desired cooling rate.

Riser: It is a reservoir of molten metal provided in the casting so that hot metal can flow back into the mould cavity when there is a reduction in volume of metal due to solidification.

Sand Mould making procedure: The procedure for making a typical sand mould is described in the following steps: First a bottom board is placed either on the moulding platform or on the floor, making the surface even. The drag (moulding flask) is kept upside down on the bottom board along with the drag part of the pattern at the centre of the flask on the board. There should be enough clearance between the pattern and the walls of the flask which should be of the order of 50 to 100 mm. Dry facing sand is sprinkled over the board and pattern to provide a non-sticky layer. Freshly prepared moulding sand of requisite quality is now poured into the drag and on the pattern to a thickness of 30 to 50 mm. Rest of the drag flask is completely filled with the backup sand and uniformly rammed to compact the sand. The ramming of sand should be done properly so as not to compact it too hard, which makes the escape of gases difficult, nor too looses o that mould would not have enough strength. After the ramming is over, the excess sand in the flask is completely scraped using a flat bar to the level of the flask edges. Now, with a vent wire which is a wire of 1 to 2 mm diameter with a pointed end vent holes are made in the drag to the full depth of the flask as well as to the pattern to facilitate the removal of gases during casting solidification. This completes the preparation of the drag.



The finished drag flask is now rolled over to the bottom board exposing the pattern. Using a slick, the edges of sand around the pattern is repaired and cope half of the pattern is placed over the drag pattern aligning it with the help of dowel pins. The cope flask on top of the drag is located aligning again with the help of the pins. The dry parting sand is sprinkled all over the drag and on the pattern. A spruce pin for making the spruce passage is located at a small distance of about 50 mm from the pattern. Also a riser pin if required is kept at an appropriate place and freshly prepared moldings and similar to that of the drag along with the backing sand is sprinkled. The sand is thoroughly rammed, excess and scraped and vent holes are made all over in the cope as in the drag. The spruce pin and the riser pin are carefully withdrawn from the flask. Later the pouring basin is cut near the top of the spruce. The cope is separated from the drag and any looses and on the cope and drag interface of the drag is blown off with the help of bellows. Now the cope and the drag pattern halves are withdrawn by using the draw spikes and rapping the pattern all around to slightly enlarge the mould cavities that the mould walls are not spoiled by the withdrawing pattern The runners and the gates are cut in the mould carefully without spoiling the mould. Any excess or loose sand found in the runners and mould cavity is blown away using the bellows. Now the facing sand in the form of a paste is applied all over the mould cavity and the runners which would give the finished casting a good surface finish.

A dry sand core is prepared using a core box, After suitable baking, it is placed in the mould cavity as shown in Fig. below. The cope is replaced on the drag taking care of the alignment of the two by means of the pins. The mould, as shown in Fig. is ready for pouring.

Pattern Materials: The usual pattern materials are wood, metal and plastics.

Wood: The most commonly used pattern material is wood, the main reason being the easy availability and the low weight. Also it can be easily shaped and is relatively cheap. But the main disadvantage of wood is its absorption of moisture as a result of which distortions and dimensional changes occur. A good construction may be able to reduce the warpage to some extent. Hence, proper seasoning and upkeep of wood is almost a pre-requisite for large scale use of wood as a pattern material. The usual varieties of wood commonly used for making patterns are pine, mahogany, teak, walnut and deodar. Besides the wood, the plywood boards of the veneer type as well as the particle boards are also used for making patterns. Because of their availability in various thicknesses, their higher strength and no need for seasoning are the reasons for their usage. However, they can be used only in patterns which are of flat type (pattern plates) and no three dimensional contours.



Choice of the pattern material depends essentially on the size of the casting, the number of castings to be made from the pattern, and the dimensional accuracy required. For very large castings, wood may be the only practical pattern material. Moulding sand being highly abrasive for large scale production, wood may not be suitable as a pattern material and one may have to opt for metal patterns. Because of their durability and smooth surface finish, metal patterns are extensively used for large scale casting production and for closer dimensional tolerances. Though many materials such as cast iron, brass, etc. can be used as pattern materials, aluminum and white metal are most commonly used. These Are light and can be easily worked, and are corrosion resistant. Since white metal has very small shrinkage, the white metal pattern can be made use of for making additional patterns without worrying about the double shrinkage allowances. Most metal patterns are cast in sand moulds from a master wood pattern provided with the double shrinkage allowance.

Plastics: Plastics are also used as pattern materials because of their low weight, easier formability, smooth surfaces and durability. They do not absorb moisture and are therefore, dimensionally stable and can be cleaned easily. The making of a plastic pattern can be done in sand clay moulds or moulds made of Plaster of Paris. The most generally used plastics are cold setting epoxy resins with suitable fillers. With a proper combination it is possible to obtain a no shrink plastic material and as such double shrinkage allowances may not be required. Polyurethane foam is also used as pattern material. It is very light and can be easily formed into any shape required. It can be used for light duty work for small number of castings for the conventional castings and for single casting in the case of full mould process, where the pattern is burned inside the mould without withdrawing. This plastic has a very low ash content and hence can be burned inside the mould. The pattern material is to be chosen based on the expected life of the pattern.

Metals: Metals are also used as the material for the preparation of the patterns. Metallic patterns are reuseable and can be used for relatively longer period of time. These patterns does not get distort or damages quickly with respect to the time. But also there are some disadvantages with these types of patters as they are heavy and also the cost incurred in the production of metallic patterns is more.

Pattern Allowances: The dimensions of the pattern are different from the final dimensions of the casting required. This is required because of various reasons. Various types of pattern allowances are as follows:

Shrinkage: All the metals shrink on cooling except Bismuth. This is because of the inter-atomic vibrations which are amplified by an increase in temperature. However, there is a distinction to be made between the liquid shrinkage and solid shrinkage. Liquid shrinkage refers to the reduction in volume when the metal changes from liquid to solid state at the

solidus temperature. To account for this risers are provided in the moulds. Solid shrinkage is the reduction in volume caused, when metal looses temperature in solid state. The snriakage

allowance is provided to take care of this reduction.

Draft: At the time of withdrawing the pattern from the sand mould, the vertical faces of the pattern are in continual contact with the sand, which may damage the mould cavity, as shown in Fig. To reduce the chances of this happening, the vertical faces of the pattern are always tapered from the parting line. This provision is called draft allowance. Draft allowance varies with the complexity of the job. But in general, inner details of the pattern require higher draft than outer surfaces. Table 7.2 is a general guide to the provision of drafts. The draft

Fig: Effect of draft on pattern withdrawing.



allowance given varies for hand moulding and machine moulding. More draft needed to be provided for hand moulding compared to machine moulding. In machine moulding the actual draft given varies with the condition of the machine (new, rigid, properly aligned, etc. require less draft). One thing to be noted here is that draft is always provided as an extra metal over and above the original casting dimensions as shown in the following

Finish or machining allowance: The finish and accuracy achieved in sand casting are generally poor and therefore when the casting is functionally required to be of good surface finish or dimensionally accurate, it is generally achieved by subsequent machining. Also, ferrous materials would have scales on the skin which are to be removed by cleaning. Hence extra material is to be provided which is to be subsequently removed by machining or cleaning process. This depends on dimensions, the type of casting material and the finish required. This may range from 2 to 20 mm. The machining allowance provided would ultimately have to be removed by machining. Hence the cost of providing additional machining allowance should be carefully examined before finalizing. The type of machining allowance provided would depend on the metal cast, the type of moulding

used, the class of accuracy required on the surface and the complexity of surface details. One way of reducing the machining allowance is to keep entire casting in the drag flask such that dimensional variation and other defects due to the parting plane are reduced to a minimum.

Shake allowance: Before withdrawal from the sand mould, the pattern is rapped all around the vertical faces to enlarge the mould cavity slightly which facilitates its removal. Since it enlarges the final casting made, it is desirable that the original pattern dimensions should be reduced to account for this increase. There is no sure way of quantifying this allowance, since it is highly dependent on the foundry personnel and practices involved. It is a negative allowance and is to be applied only to those dimensions which are parallel to the parting plane. One way of reducing this allowance is to increase the draft which can be removed during the subsequent machining.

Distortion allowance: A metal when it has just solidified, is very weak and therefore is likely to be distortion prone. This is particularly so for weaker sections such as long flat portions, V, U sections or in a complicated casting which may have thin and long sections which are connected to thick sections. The foundry practice should be to make extra material provision for reducing the distortion. Alternatively, the shape of pattern itself should be given a distortion of equal amount in the opposite direction of the likely distortion direction.

Types of Patterns: There are various types of patterns depending upon the complexity of the job, the number of castings required and the moulding procedure adopted.

Single piece pattern: These are in expensive and the simplest type of patterns. As the name indicates they are made of a single piece as shown in Fig. This type of pattern is used only in cases where the job is very simple and doesn’t create any withdrawal problems. It is also used for applications in very small scale production or in prototype development. This pattern is expected to be entirely in the drag. One of the surfaces is expected to be flat which is used as the parting plane.



Split pattern or low piece pattern: This is them most widely used type of pattern for intricate casting. When the contour of the casting makes it withdrawal from the mould difficult or when the depth of the casting is too high then the pattern is split into two parts so that one part is in the drag and the other in the cope. The split surface of the pattern is same as the parting plane of the mould. The two halves of the pattern should be aligned properly by making use of the dowel pins which are fitted to the cope half. These dowel pins match with the precisely made holes in the drag half of the pattern and thus align the two halves properly as seen in Figure.

Fig: Split Pattern

Gated pattern: This is an improvement over the simple pattern where the gating and runner system are integral with the pattern. This would eliminate the hand cutting of the runners and gates and help in improving the productivity of a moulder.

Cope and drag pattern: These are similar to split patterns. In addition to splitting the pattern, the cope and drag halves of the pattern along with the gating and risering systems are attached separately to the metal or wooden plates along with the alignment pins. They are called the cope and drag patterns. The cope and drag moulds may be produced using these patterns separated by two molders but they can be assembled to form a complete mould. These types of patterns are used for castings which are heavy and inconvenient for handling as also for continuous production.

Match plate pattern:

These are extensions of the previous type. Here the cope and drag patterns along with the gating and the risering are mounted on a single matching metal or wooden plate on either side as shown in Fig. On one side of the match plate the cope flask is prepared and on the other, the drag flask. After moulding when the match plate is removed, a complete mould with gating is obtained by joining the cope and the drag together. The complete pattern with match plate is entirely made of metal, usually aluminum for its light weight and machinability. But when dimensions are critical, the match plate may be made of steel with necessary case hardening of the critical wear points. The pattern and gating are either screwed to the match plate in the case of a flat parting plane or are made integral in case of an irregular parting plane. The casting of a match plate pattern is done usually in plastic moulds, but sometimes sand moulds are also used When the cope and the drag patterns are similar the pattern may be kept on only one side of the plate and is used for making both the drag as well as the cope. These are generally used for small castings with high dimensional accuracy and large production. The gating system is already made and attached to the match plate. Several patterns can be fixed to a single match plate, if they are sufficiently small in size. These patterns are used for machine moulding. These are expensive but increased production can be a favourable aspect to choose these types of patterns.



Fig: Match Plate Pattern

Loose piece Pattern: This type of pattern is also used when the contour of the part is such that withdrawing the pattern from the mould is not possible. Hence during moulding the obstructing part of the contour is held as a loose piece by a wire. After moulding is over, first the main pattern is removed and then the loose pieces are recovered through the gap generated by the main pattern. Moulding by the loose piece pattern is a highly skilled job and is generally expensive and therefore should be avoided till possible.

Fig: Loose Piece Pattern

Follow board Pattern: This type of pattern is adopted for those castings where there are some portions which are structurally weak and if not supported properly are likely to break under the force of ramming. Hence the bottom board is modified as a follow board to closely fit the contour of the weak pattern and thus support it during the ramming of the drag. During the preparation of the cope, no follow board is necessary because the sand which is compacted in the drag will support the fragile pattern.

Fig: Follow Board Pattern



Sweep pattern: It is used to sweep the complete casting by means of a plane sweep. These are used for generating large shapes which are axi-symmetrical or prismatic in nature such as bell shaped or cylindrical as shown in figure. This greatly reduces the cost of a three dimensional pattern. This type of pattern is particularly suitable for very large castings such as the bells for ornamental purposes used which are generally cast in pit moulds.

Fig.: Sweep Pattern (Procedure of Sweep)

Skeleton pattern: A skeleton of the pattern made of strips of wood is used for building the final pattern by packing sand around the skeleton. After packing the sand, the desired form is obtained with the help of a strickle as shown in Fig. The type of skeleton to be made is dependent upon the geometry of the workpiece. This type of pattern is useful generally for very large castings, required in small quantities where large expense on complete wooden pattern is not justified.

Fig.: Skeleton Pattern

Pattern Colour Code: The patterns are normally painted with contrasting colours such that the mould maker would be able to understand the functions clearly. The colour codes used are:

Red or orange on surfaces not to be finished and left as casted.

Yellow on surfaces to be machined.

Black on core prints for unmachined openings.

Yellow stripes on black on core prints for machined openings.

Green on seats of and for loose pieces and loose core prints.

Diagonal black stripes with clear varnish on to strengthen the weak patterns or to shorten a casting.

Moulding sand composition: The main ingredients of any moulding sand are:

The silica grains (SiO2),

The clay as binder, and

Moisture to activate the clay and provide plasticity. Silica Sand: The sand which forms the major portion of the moulding sand (up to 96%) is essentially silica grains, the rest being the other oxides such as alumina, sodium (Na2O + K2O) and Magnesium oxide (MgO + CaO). These impurities should be minimised to about 2% since they effect the fusion point of the silica sands. The main source is the river sand which is



used with or without washing. Ideally the fusion point of sands should be about 1450 °C for cast irons and about 1550 °C for steels. In the river sand, all sizes and shapes of grains are mixed. The sand grains may vary in size from a few micrometers to a few millimeters. Shape of the grains may be round, sub-angular, angular and very angular. The size and shapes of these sand grains greatly affect the properties of the moulding sands. Zircon sand is basically a zirconium silicate (ZrSiO4). The typical composition is ZrO2- 66.25%, SiO2- 30.96%, Al2O3 - 1.92%, Fe2O3. - 0.74%, and traces of other oxides. It is very expensive. In India it is available in the Quilon beach of Kerala. It has a fusion point of about 2400°C and also a low coefficient of thermal expansion. The other advantages are high thermal conductivity, high chilling power and high density. It requires a very small amount of binder (about 3%). It is generally used to manufacture precision steel castings requiring better surface finish and for precision investment castings. Chromite sand: It is crushed from the chrome ore whose typical composition is Cr2O3, - 44%, Fe2O3 - 28%, SiO2 -2.5%, CaO-0.5%, and (Al2O3.+MgO)-25%. The fusion point is about 1800°C. It also requires a very small amount of binder (about 37%). It is also used to manufacture heavy steel castings requiring better surface finish. It is best suited to austenitic manganese steel castings. Olivine sand: It contains the minerals fosterite (Mg2SiO4) and fayalite (Fe2SiO4). It is very versatile sand and the same mixture can be used for a range of steels.

Table: Comparison of various sand properties

Clay: Clays are the most generally used binding agents mixed with the moulding sands to provide the strength, because of their low cost and wider utility. The most popular clay types used are

Kaolinite or fire clay (Al2O3.2SiO2.2H2O), and Bentonite (Al2O3.4SiO2.2H2O n H2O).

Kaolinite has a melting point of 1750 to 1787 °C and Bentonite has a melting temperature range of 1250 to 1300 °C. Of the two, Bentonite can absorb more water which increases its bonding power. The clays besides these basic constituents may also contain some mixtures of lime, alkalies and other oxides which tend to reduce their refractoriness. There are basically two types of Bentonites, one with sodium as adsorbed ion which is often called western Bentonite and the other with calcium ion called southern Bentonite. Sodium Bentonite produces better swelling properties- volume increases some 10 to 20 times, high dry strength which lowers the risk of erosion, better tolerance of variations in water content, low green strength and high resistance to burnout, which reduces clay consumption, in contrast the



calcium Bentonite have low dry strength but higher green strength. It is possible to improve the properties of calcium Bentonite by treating it chemically with soda ash (sodium carbonate). The clay chosen for moulding sand should give it the requisite strength for the given application taking into account the metal being cast and thickness of the casting. Normally, the river sand contains a large amount of clay and therefore can be directly used. Water: Clay is activated by water so that it develops the necessary plasticity and strength. The amount of water used should be properly controlled. This is because a part of the water absorbed by clay help in bonding while the remainder up to a limit helps in improving the plasticity but more than that would increase the strength and formability. The normal percentages o f water used are from 2 to 8. Besides these three main ingredients, many other materials also may be added to enhance the specific properties. For example, cereal binder up to 2% increases the strength; pitch obtained as a by-product in coke making if used in percentages up to 3 would improve the hot strength; and saw dust up to 2% may improve the collapsibility by slowly burning, and increase the permeability. There are other materials such as sea coal, asphalt, fuel oil, graphite, molasses, iron oxide, etc. which are also used for obtaining specific properties. Comparative properties of moulding sand with various clays are shown in Table below.

Sand Preparation:

One of the most important requirements for the preparation of sand is thorough mixing of its various ingredients. This is essential to ensure uniform distribution of the various components in the entire bulk of sand. During the mixing process any lump present in sand is broken up and clay is uniformly enveloped around the sand grains and moisture is uniformly distributed. Besides manual mixing, equipments called Mullers are normally used in foundries to mix the sands. These are essentially of two types, batch type and continuous. A batch Muller consists of one or two Muller wheels and equal number of plough blades all of them connected to a single driving source. The Muller wheels are large and heavy, and continuously rolls inside the Muller bowl. The plough blades ensure that the sand is continuously agitated. A combined action makes the sand mixed thoroughly.

Fig: Batch Muller



Moulding sand properties: The properties of moulding sand are dependent of a great extent on a number of variables. The important among them are:

Sand grain shape and size

Clay type and amount

Moisture content

Method of preparing sand mould Sand grains: The shape and size of the sand grain would greatly affect the various mouldings and properties. The sand grain size could be coarse or fine. Similarly the grain shape could be round or angular. The coarse grains would have more void space between the grains which increases the permeability. Similarly, the finer grains would have lower permeability however they provide better surface finish to the casting produced. The distribution of the grain size also plays an important role. For example, a widely distributed sand would have higher permeability than the one with the same fineness number but where all the grains have the same size. Angular sand grains require higher amounts of binder. The round grains would have lower permeability compared to angular grains because of the irregular shape of the latter. The same has been depicted in Fig.

Figure: Variation of permeability with grain size

Clay and Water: Besides the grain size clay and water have also a large influence on the properties of the moulding sand. As the percentage of water increases in the sand its strength also I,proves but after reaching a threshold level the strength decreases.

Types of sands:

Various types of moulding sands are as follows:

Facing sand: This sand is used next to the pattern to obtain cleaner and smoother casting surfaces. Generally, sea coal or coal dust (finely divided bituminous coal of 2 to 8%) is mixed with the system sand to improve the mouldability and surface finish. The sea coal being carbonaceous, will slowly burn due to the heat from the molten



metal and give off small amounts of reducing gases. This creates a small gas pressure in the surroundings of the cavity such that molten metal is prevented from entering into the silica grains or fuse with them. This helps in generating good casting surface and also lets the moulding sand peel off from the casting during shake out.

Mould wash: Purely carbonaceous materials such as sea coal, finely powdered graphite or proprietary compounds are also applied on to the mould cavity after the pattern is withdrawn. This is called the mould wash and is done by spraying, swabbing or painting in the form of a wet paste. These are used essentially for the following reasons: to prevent metal penetration into the sand grains and thus ensure a good casting finish, and to avoid mould-metal interaction and prevent sand fusion. For depositing the mould wash, either water or alcohol can be used as a carrier. But because of the problem of getting the water out of the mould, alcohol is preferred as a carrier. The proprietary washes are available in powder, paste or liquid form. The powder needs to be first prepared and applied where as the paste and liquid can be applied straightaway.

Backing sand: This is normally the reconditioned foundry sand and is used for ramming the bulk of the moulding flask. The moulding flask is completely filled with backing sand after the pattern is covered with a thin layer of facing sand. Since the casting is not affected to any great extent by the backing sand, it usually contains the burnt facing sand, moulding sand and clay.

Parting sand: This is the material which is sprinkled on the pattern and to the parting surfaces of the mould halves before they are prepared, to prevent the adherence of the moulding sand. This helps in easy withdrawal of the pattern and easier separation of the cope and drag flasks at parting surface. It is essentially a nonsticky material such as washed silica grains.

Properties and types of sand moulds: In order to produce sound castings, moulds are required to have some specific properties. Some of them are:

It must be strong enough to withstand the temperature and weight of the molten metal.

It must resist the erosive action of the flowing hot metal.

It should generate minimum amount of gases as a result of the temperature of the molten metal.

It should have good venting capacity to allow the generated gases to completely escape from it Moulds that are used for sand casting may broadly be classified as

Green sand moulds.

Dry sand moulds.

Skin dried moulds.

Green sand moulds: Green sand is the moulding sand which has been freshly prepared from silica grains, clay and moisture. In a green sand mould, metal is poured immediately and the castings taken out. These are most commonly used and are adapted for rapid production, where the moulding flasks are released quickly. They require less floor space as no storage is involved. As the mould is produced, the casting is prepared. Thus it is the least expensive of all. Also, the tendency for hot tearing of the castings is less in green sand moulds. But mould erosion is common in these type of moulds. The permeability of these moulds should be properly controlled, otherwise blow holes and gas inclusions are likely to form.

Dry sand moulds: These are the green sand moulds which are completely dried by keeping in an oven between 150- 350°C for 8 to 48 hours depending on the binders in the mouldings and. These moulds generally has higher strengths than the green



sand mould and are preferred because they are less likely to be damaged during handling. These are generally used for medium to large castings. Better surface finish and dimensional accuracy can be achieved by dry sand mould. The main disadvantages are the likely distortion of the mould caused during the baking process, susceptibility to hot tearing of castings and longer production cycles. Also, this is more expensive than the green sand mould.

Skin dried mould: Though the dry sand mould is preferable for large moulds because of the expense involved, a compromise is achieved by drying only the skin of the mould cavity with which the molten metal comes in contact, instead of the full mould. The skin is normally dried to a depth of 15 to 25 mm. using either torches or by simply allowing them to dry in atmosphere. This can also be done in pit moulding. However, pouring of metal should be completed immediately after the drying process such that moisture from the undried portion would not penetrate the dried skin.

Melting Practice: After moulding, melting is the major factor which controls the quality of the casting. There are a number of methods available for melting foundry alloys such as pit furnace, open hearth furnace, rotary furnace, cupola furnace, etc. The choice-of the furnace depends, on the amount and the type of alloy being melted. For melting cast iron, cupola in its various forms is extensively used basically because of its lower initial and melting cost.

CUPULA: For melting of cast iron in foundry the Cupola Furnace is used. A diagramatic sketch of this furnace is given in Fig. A

Fig. A. COPULA FURNACE

Fig. B below illustrates a cross-sectional view of Cupola. It has a construction in the form of a hollow vertical cylinder made of strong mild steel plates and riveted or welded at the seams. Welded joints are more common in modern practice. In large Cupolas the lower portion is made of comparatively thicker plates so as to make it strong enough to hold the upper structure and fire brick lining. Thus, the stress in the whole structure is distributed uniformly. Also, such Cupolas are further strengthened by providing the Brick Retaining Rings at suitable heights. The Bottom Door of the shell can be in one piece, hinged to a supporting leg, or in two pieces: each piece hinged separately to the two opposite legs. When the cupola is in operation, the Bottom door is supported by a Prop so that it may not collapse due to the



large weight of the charge and coke, etc., it carries. When we do not need the COPULA for further operation, the charge feeding is stopped, air supply cut off and the Prop is removed. As soon as the Prop is removed the Door drops down providing a clear space for the coke fire, residue of the molten metal with slag and the sand bed to fall down and thus the fire inside ceases gradually.

Fig. B Cross-sectional view of COPULA Furnace

A Wind Chamber or Wind Belt, as it is more commonly known, encircles the cupola shell at a place little above the bottom of the shell. This belt is connected to the furnace blower by means of a Blast Pipe. The amount of air required is forced into the chamber by the blower, which enters the furnace through openings called Tuyeres. These Tuyeres are provided all around the shell and have a definite number and size depending upon the amount of air required. Charging Door is located at a suitable height above the charging platform. This platform is of robust mild steel construction, supported on four strong steel legs, and is provided with a Ladder. Weighed quantities of Metal, Coke, Scrap and Flux are collected on this platform, which is charged into the COPULA as and when required. The top of the COPULA is provided with a Mesh Screen and a Spark Arrester. It is a cone shaped construction, as shown in the diagram. This attachment facilitates a free escape of the waste gases at the same time deflects the spark and the dust back into the furnace. In some COPULAS the upper portion is made tapered with the top diameter as about half of the inside diameter of the cupola at the smelting zone. Small Cupola say from 500 kg to 1000 kg capacity, are better known as Cupolettes. They are quite self-sufficient in operation and have almost all the accessories which a large cupola possesses except the Spark Arrester and Charging Door. Since the height of these Cupolettes is very small, say 2.5 meters to 4



meters, charging is done from the top of the Cupolette. They are fixed on two reunions inside the bearings, mounted on the supporting legs, so that they can be tilted to become horizontal for providing the fire Brick lining. This lining is provided in all cupolas, irrespective of the size to withstand the high temperature produced inside the furnace.

Other Furnaces: Reverberatory Furnace:

In these furnaces the fuel burners fire within a refractory hood above the metal bath. These are generally used to melt large amounts of metal for example, aluminium to supply to holding furnaces such as those used with pressure die casting machines. These use gas fired burners located generally high in the furnace transferring the heat by radiation to the walls and roof. As the walls and roof become incandescent they radiate the heat to the metal bath. These furnaces are simple and have relatively low capital cost. Thus these are generally used for melting large volumes of metal.

Crucible Furnace: Smaller foundries generally prefer the crucible furnace. The crucible is generally heated by electric resistance or gas flame. In these the metal is placed in a crucible of refractory metal and the heating is done to the crucible thus there is no direct contact between the flame and the metal charge. This type of melting is very flexible since it suits a variety of casting alloys. Degassing and any metal treatment can be completed in the crucible before it is removed for pouring. Melt quality and temperature can also be controlled reasonably well.

Induction Furnace: The induction furnaces are used for all types of materials, the chief advantage being the heat source is isolated from the charge and the slag and flux would be getting the necessary heat directly from the charge instead of the heat source. The stirring effect of the electric current would cause fluxes to be entrained in the melt if they are mixed along with the charge. So flux is generally added after switching off the current to the furnace. Then sufficient time must be allowed for the oxides to be removed by the flux as slag before transferring the metal for pouring. High frequencies help in stirring the molten metal and thus help in using the metal dwarf (chips). Low cost raw materials could, therefore, be used and at the same time better control of temperature and composition can be achieved.

Ladles: The molten metal from the furnace is tapped into the ladles at requisite intervals and then poured into the moulds. Depending on the amount of metal to be handled, there are different sizes of ladles. They may range between 50 kg to 30 tones depending upon the casting size. For grey cast iron, since the slag can be easily separated, top pouring ladles would be enough. But for steels; to separate the slag effectively, the metal is to be poured from the bottom with the help of the bottom pour ladle. The bottom pour ladle has an opening in the bottom that is fitted with a refractory nozzle. A stopper rod, suspended inside the ladle, pulls the stopper head up from its position thus allowing the molten alloy to flow from the ladle. As the metal in the ladle loses a large amount of heat to the surrounding atmosphere by radiation it is necessary to account for this drop in the temperature of the casting metal.

abstract advantages and limitations of the process

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