Various Manufacturing Processes Part -2 (End)

 

👉Various Manufacturing Processes Part -2 (End)

⇀CASTING PROCESS

Casting is one of the most economical methods of producing components. It is a process of forming metallic products by melting the metal, pouring it into the cavity known as mould and allowing it to solidify. in this way, the molten metal takes the shape of the mould cavity. Mould is made with the help of a pattern, which may be made of wood, metal or non-metal like polystyrene. The internal hollow shape, if any, is made with the help of a sand core of desired shape, which is made with the help of a core box. The moulds are provided with core prints where the core is properly held so that the core may not be displaced or sifted during the pouring of molten metal,. Moulds can be made of metal or non-metal, but they are generally made of sands. The product so produced is cleaned and machined to the desired dimensions.

⇀Terminology Used in Casting

Before going to the details of the casting process, defining a number of casting terminologies are important and those are given hereunder. FIGURES shows various components used in a mould structure.

ᐉDrag

Drag is the lower moulding box.            

ᐉCope

Cope is the upper moulding box.

ᐉCheck

Check is the intermediate moulding box.

ᐉPattern

Pattern is a replica of the desired final product with some modifications. The mould cavity is made with the help of the pattern.

ᐉParting Line

Parting Line is the dividing line between the two or more molding boxes. In split pattern, it is also the dividing line between the two halves of the pattern.

ᐉFacing Sand

Facing Sand is high quality sand having higher sintering point and low thermal expansion used on the face of the mould. It helps to give better surface finish to the castings, Sometimes plumbago powder, Ceyion lead and graphite are used as facing sands to prevent the metal form burning the moulding sand.

ᐉParting Sand

Parting Sand is used to prevent the joint between the halves of a mould from adhering to one another when the two parts of a moulding box are separated. It also prevents the sand from adhering to the pattern.

ᐉBacking Sand

Backing Sand is used to fill the flasks to cover the pattern.

ᐉCore

Core is used for making hollow cavity in casting.

ᐉCore Print

Core Print is provided in the pattern, core and mould to locate and support the core within the mould.

ᐉMould Cavity

The mould material and the core then combine to form the mould cavity. It is the hollow portion of mould in which the molten metal is poured to get the casting.

ᐉPermeability

Permeability is also called porosity. It refers to the ability of moulding sand to provide the passage of gaseous materials, water and steam vapor.

ᐉPouring Basin

A small tunnel-shaped cavity at the top of the mould into which the molten metal is poured.

ᐉRunner

The passage in the parting surface through which molten metal flow is regulated before the metal reaches the mould cavity.

ᐉGate

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

ᐉChaplet

Chaplet is sued to support the core inside the mould cavity to take care of its weight and overcome the metallostatic pressures.

ᐉChill

Chills are metallic objects, which are placed in the mould to increase the cooling rate of castings so as to provide uniform

or desired cooling rate.

ᐉRiser

Riser acts as a reservoir of molten metal provided in the mould so that the hot metal can flow back into the mould cavity when there is shrinkage due to solidification.

ᐉTools and Equipment

In this section, various tools generally used in the casting process are described. FIGURE depicts some of the important tools used for this purpose.

ᐉRiddle

Riddle is a wooden frame with a wire mesh bottom. It is used for removing foreign materials, such as, nails, splinters of wood, etc from the moulding sand.

ᐉRammer

Rammer is a wooden tool used for packing the sand into the mould.

ᐉShovel

Shovel is used for mixing and tempering moulding sand and for moving sand from the pile to the flask.

ᐉStrike-off Bar

Strike-off Bar is a metal bar used to remove the surplus sand from the mould after the ramming has been completed.

ᐉVent Wire

Vent Wire is a wire rod used for making opening called "vents" in the mould to allow the escape of gas and steam from the mould during the pouring operation.

ᐉSprue Pin

A Sprue Pin is either wooden or metallic pin to make an opening in the mould through which the metal is poured.

ᐉLifter

Litters are used for patching deep sections of a mould and removing loose sand from pockets of the mould.

ᐉSlick

Slicks are available in many types, which vary in shape and are used for repairing and finishing moulds.

ᐉTrowel

Trowels are available in many types and are used for making joints and finishing flat surface of the mould.

ᐉSwab

Swab is made of hemp and used for applying water to the mould around the edge of the pattern before removing the same from the mould.

ᐉGate Cutter

Gate Cutter is a piece of sheet metal, which is used to cut opening (gate) that connects the sprue with the mould cavity.

ᐉBellow

Bellows are used to blow loose sand out of the mould and are used more frequently.

ᐉMoulding Board

Moulding Board is a smooth board on which the pattern and the flask are placed during the moulding.

ᐉGagger

Gaggers are used for reinforcement of the sand in the cope.

ᐉLadle

Ladles are used to receive molten metal from the meting furnace and is used to pour the same into the mould.

ᐉCrucible

Crucibles are used as metal melting pots.

⇀Types of Foundries

Foundries are of many types according to the nature of work they undertake and the framework of their organizations.

Foundries may be categorized as follows.

👉Captive foundry

👉Jobbing foundry

👉Production foundry

👉Semi-production foundry

ᐉCaptive Foundry

The captive foundry is an integral part of some manufacturing organizations. It makes castings for the same only and all the castings made in a captive foundry are consumed mainly in the products being manufactured by that organization.

ᐉJobbing Foundry

Jobbing foundry produces normally small number of castings of a given type for different customers. Such foundries sometimes also have facilities for mass production.

ᐉProduction Foundry

The producing foundry is highly mechanized and can produce castings economically on mass production.

ᐉSemi-production Foundry

The semi-production foundry is a combination of jobbing foundry and production foundry. in other words, a semi- production foundry accepts both the production and the job work.

⇀Steps of Casting Process

Major steps of casting process are as follows.

👉Pattern making.

👉Mould and Core making.

👉Melting and Pouring.

👉Cleaning of casting, and

👉Inspection and quality control.

ᐉPattern Making

This is the first step in making a casting. Pattern is a model of a desired casting component. It can be made from any of the following

(i)Wood,

(ii)metal,

(iii) plastic,

(iv) plaster of Paris, and

(v)wax.

...

ᐉWood

Wood is the most popular material for pattern making because of a number of characteristics, namely, (i) cheapness, (ii) availability, (iii) ease of fabrication in various forms, (iv) light in weight, (v) easy to give good surtace finish.

Wood is used as a pattern material when the lot size to be produced is less.

ᐉMetal

Metallic patterns are preferred when the numbers of castings to be produced are very large in number. These patterns are not affected by the moisture of the moulding sand and they are highly wear resistant. The main disadvantages of metallic pattern are:(i) higher cost, (ii) higher weight, and (iii) tendency of rusting.

ᐉPlastics

Plastics are gaining favour day-by-day as pattern materials owing to the characteristics, namely, lightness in weight, high strength, high resistance to wear and corrosion, low solid shrinkage, fine surface finish, low cost, etc. The plastic used for pattern making is generally thermosetting resins. Phenolic resin plastic and foam plastic are widely used for the above

purpose.

ᐉPlaster of Paris

Plaster of paris is generally used for producing small patterns and core boxes containing intricate shapes which require closer dimensional control. Pattern made by plaster of paris can easily be worked. Its expansion can easily be controlled and it has very high compression strength.

ᐉWax

Wax is used only in investment casting. For this, a die or metal mould is made in two halves into which the heated wax is Poured. The die is kept cool by circulating water around it. As the wax sets on cooling, the die parts are separated and the wax pattern is taken out.

For selecting a particular material for pattern making, the following factors must be taken into consideration.

👉Number of castings to be produced

👉Mould material used

👉Type of moulding process

👉Degree of accuracy and surface finish required

👉Minimum thickness required.

⇀Types of Pattern

A host of patterns are used for the purpose of die and these are as follows:

(i) single piece pattern,

(ii) multiple-piece pattern or loose piece pattern,

(iii) split pattern,

(iv) cope and drag pattern,

(v) follow board pattern,

(vi) gated pattern,

(vii) skeleton pattern,

(viii) match plate pattern, and

(ix) sweep pattern.

ᐉSingle Piece Pattern

The Single Piece Pattern is the simplest of all the patterns. It is made in one piece and cares no joint, partition or loose pieces. FIGURE shows the view of a Single Piece Pattern.

FIGURE. Single Piece pattern

It can be moulded in one or two boxes depending upon the shape. This is the cheapest process but its use is limited since its moulding involves a large number of manual operations. FIGURE shows the casting produced with the help of single piece pattern.

FIGURE. Single Piece Casting

ᐉMultiple-piece Pattern

Castings having a more complicated design require the pattern in more than two parts in order to facilitate an easy moulding and withdrawal of pattern. This type of pattern may consist of 3, 4 or more number of parts, depending upon the design. A typical example of such a pattern is shown in FIGURE.

FIGURE. Multiple Piece Casting

ᐉSplit Pattern

Split Pattern is made in two parts which are joined at the parting line by means of dowels. At the time of moulding, the drag contains one part of pattern while cope contains the other. An example of a Split Pattern is shown in FIGURE, The Split Pattern is generally used for casting which is difficult to mould by single piece pattern.

FIGURE. Split Pattern

ᐉCope and Drag Pattern

When very large castings are to be made, the complete pattern becomes 100 heavy to be handled by a single operator. Such pattern is made in two parts which are moulded separately in different moulding boxes., After the completion of moulds, the above-mentioned two boxes are assembled to form complete cavity of which one part is contained by the drag and other in cope. Thus, in a way it is nothing but a Split Pattern of which both the pieces are moulded separately instead of being moulded in the assembled position, The Cope and Drag pattern is shown in FIGURE.

FIGURE.Cope and Drag Pattern

ᐉFollow Board Pattern

Follow Board Pattern is a wooden board which is 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 a very thin wall, require a follow board. The Follow Board Pattern is shown in FIGURE.

FIGURE. Follow Board Pattern

ᐉGated Pattern

Gated Pattern is used for mass production of small castings. Multi-cavity moulds are prepared for such castings, that is, single sand mould carries a number of cavities. The Gated Pattern is shown in FIGURE. The pattern for these castings is connected to each other by means of gate formers which provide suitable channels or gates in sand for feeding the molten metal to cavities. A single runner can be used for feeding all the cavities. This facilitates time saving and also the uniform feeding of molten metal.

FIGURE. Gated Pattern

ᐉSkeleton Pattern

Skeleton Pattern consists of wooden frame and strips. It is filled with loam sand and rammed thereafter. The Skeleton Pattern is shown in FIGURE. It is employed when the size of the casting is very large, but easy to shape, and only a few numbers are to be made.

FIGURE. Skeleton Pattern

ᐉMatch Plate Pattern

Match Plate Pattern is used where a rapid production of small and accurate castings is desired on a large scale. These patters find a great favour in machine moulding. The construction cost of Match Plate Pattern is quite high, but the same is easily compensated by a high rate of production, greater dimensional accuracy and minimum requirement for machining in the casting. These patterns are made in two pieces; one piece mounted on one side and the other on the other side of a plate, called match-plate. The plate may carry only one pattern, or a group of patterns mounted in the same way on its two sides. The plate may be of wood, steel, magnesium or aluminium. Aluminium is preferred due to its lightness and cheapness. Gates and runners are also attached to the plate along with the pattern FIGURE.

FIGURE. Match Plate Pattern

ᐉSweep Pattern

Sweep Pattern is used widely for preparing moulds of large symmetrical castings particularly of circular cross-section. This effects a large saving in time, labour and material. The Sweep Pattern is shown in FIGURE. The equipment consists of a base placed in sand mass, a vertical spindle and 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. The sweep and spindle are then removed, leaving the base in the sand. The hole made by the removal of spindle is patched up by filling the sand.

FIGURE. Sweep Pattern

The pattern and desired castings differ in several respects, like as follows.

👉The patter is slightly larger than the casting Pattern allowances like draft allowances, shrinkage allowances and machining allowances are being provided in the patter and for this reason the overall size of the pattern is bigger than the casting.

👉Each surface to be machined may be marked with a color mark on the pattern.

👉All surfaces parallel to the direction (along which the Patter is to be drawn out) are tapered at a slight angle, sloping inward to facilitate the removal of pattern. This is called draft and for external taper, the draft is normally 10° and for internal surfaces, like that of holes and pockets, the same should be made 30°.

👉Where the central hole is located, extensions, called as core prints, have been added to the pattern. These are generally used to locate and support the core.

👉The pattern may be made in two halves, which are located properly together by dowel pins.

⇀Pattern Colour Code

The patters are normally painted with contrasting colour so that the mould maker would be able to understand the functions clearly. The following colour code is used

(i) red or orange on surfaces not to be finished and left as cast,

(ii) yellow on surfaces to be machined,

(iii) black on core prints for un-machined operations,

(iv) green on seats of and for loose pieces and loose core prints, and

(v) diagonal black strips with clear varnish on stop-offs to strengthen the weak patterns or to shorten a casting.

⇀Mould and Core Making

Mould in a cavity is prepared with the help of moulding materials, The moulding materials may be metallic (like cast iron, steel, anodised aluminium, etc) or non-metallic (like sand plaster of Paris, graphite, silicon carbide, ceramics, etc). Factors for selection of moulding materials are, namely, (i) material cost, (ii) quality of the casting desired, (iii) material to be cast, (iv) number of castings to be cast, (v) clear dimensional accuracy, and (vi) size and shape of the casting.

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