Unit 4 Casting

Casting process Casting is 6000 year young process. The plant where casting is made is called ‘Foundry’. Foundry engineering deals with process of making castings in moulds prepared by patterns.

Foundry…

The whole process of making castings is classified in 5 stages Pattern making Moulding and core making Melting Metal handling and pouring/casting Fettling (cleaning and finishing of casting) Testing and inspection Except pattern making, all other stages to make/produce castings are done in foundry shop

Pattern making Definition : A pattern may be defined as an element(similar to the part being cast, replica) used for making cavity or impression in the mould into which molten metal is poured to produce a casting. It is not exact replica of part to be cast. It is slightly larger than the part due to various allowances like shrinkage allowance, machining allowance etc.

-It is the physical model of the casting used to make the mould -It is the physical model of the casting used to make the mould. Made of either wood or metal. -The mould is made by packing moulding sand, surrounding the pattern. -When the pattern is withdrawn, its imprint provides the mould cavity. This cavity is filled with metal to become the casting. - If the casting is to be hollow, additional patterns called ‘cores’, are used to form these cavities.

Pattern materials I) Wood : II) Metals: White pine, Mahogany, Teak, Deodar Shisam , Kail, Cherry, Maple, beach,etc. II) Metals: Steel, C.I., Brass, Al & its alloys White metal (tin base alloys)

IV) Gypsum (POP) V) Wax VI) Rubber III) Plastics Compositions based on epoxy, phenol formaldehyde & polyester resins Polyacrylates Polyethylene Polyvinyl chloride most commonly used cold curing plastics based on epoxy resins & acrylates IV) Gypsum (POP) V) Wax VI) Rubber

Wood Characteristics: Light in weight Easily available & inexpensive Good workability Suitable for gluing & joining Easy to repair

Limitations Wooden pattern is first required to be made to serve as master pattern which forms the mould into which plastic resin is poured. Moulds may be made from rubber plastics, metals or plaster of paris(mostly used)

Metals Advantages : More durable and accurate in size than wooden patterns Resistant to wear, abrasion, corrosion & swelling Not affected by moisture so retain their shape Resistant to rough handling & hence do not warp Good machinability, dimensional accuracy and stability, good surface finish

Plastics Advantages Economy in material & labour cost Lighter, stronger & highly corrosion resistant than wooden patterns No moisture absorption Smooth surface of patterns Less sticking of sand to patterns as compared to wood Strong & dimensionally stable

Wax Exclusively for investment castings Normally blends of several types of waxes are used like paraffin wax, shellac wax, bees wax Normally wax patterns are formed by injecting semi liquid or liquid wax into split die. The die is kept cool by circulating water around it. As the wax sets on cooling the die parts are separated & wax pattern is taken out

Plaster of paris (gypsum) It can be easily cast into intricate shapes and can be easily worked also It has high compressive strengths

Factors to be considered for selection of pattern materials Number of castings to be made Less- cheaper material , More- costlier material Desired dimensional accuracy & surface finish Nature of moulding process: sand casting-wood, investment casting- wax Method of moulding i.e. hand or machine Shape, size & complexity of casting Probability of design change, chances of future orders etc.

Pattern allowance Shrinkage allowance: Definition: The difference in dimensions of casting & pattern are due to the various allowances considered while designing pattern for casting. They are: Shrinkage allowance: Most of the metals used in casting work contract during cooling from pouring temperature to room temperature. This contraction takes place in 3 forms:

Liquidus contraction- for drop of temperature from pouring temperature to Liquidus temperature Solidifying contraction- for drop of temperature from Liquidus to solidus Solid contraction- for drop of temperature from solidus to room temperature

Machine allowance: (finish allowance ) This is a positive allowance provided on the pattern to compensate the material lost in machining or finishing of certain surfaces marked on drawing The amount of machining allowance depends on Kind of material(metal)- [ferrous/non ferrous] Ferrous metal- more allowance due to rusting tendency

2) Size & shape of casting – large & slender casting need more 3) Type of machining operation 4) Moulding method (large for hand moulding compared to machine moulding) 5) Casting method/condition 6) Degree of finish required

Machining allowances of various metals Metal Dimension (inch) Allowance (inch) Cast iron Up to 12 0.12 12 to 20 0.20 20 to 40 0.25 Cast steel Up to 6 0.12 6 to 20 0.25 20 to 40 0.30 Non ferrous Up to 8 0.09 8 to 12 0.12 12 to 40 0.16

Draft allowance/ Taper allowance While taking the pattern out of the mould there is possibility of damaging of edges and vertical surfaces of mould To reduce the chances of damaging of edges & surfaces the vertical surfaces of pattern are inclined or tapered, called as draft allowance This draft allowance is expressed in terms of mm/m or in degrees/side

The amount of draft depends upon Length of pattern in vertical direction(depth) Moulding method Mould material Type of surface- more on internal surface than external

Rapping/Shake allowance For ease of removal of pattern from the mould it is rapped or shacked so that surface is free from sand This slightly increases size of mould & hence casting. To compensate this a small negative allowance is provided in pattern This allowance is insignificant in small & medium sized castings. But small correction of pattern size is done for large castings

Types of patterns Factors affecting choice of pattern are Number of castings to be produced Size and complexity of shape of casting Type of moulding method to be used Following types of patterns are commonly used Solid or single piece Two piece or split pattern Multipiece pattern Match plate pattern

Gated pattern Skeleton pattern Sweep pattern Pattern with loose piece Core & drag pattern Follow board pattern Segmental pattern

Solid pattern Made out of one piece and has no joints Cheapest but requires lot of manual operations like gate cutting providing runners & risers etc Depending on shape can be moulded in one or two boxes

Two piece or split pattern Due to certain design solid pattern offers difficulty in moulding & withdrawal of pattern. In such cases the split pattern is recommended The 2 parts of split pattern are joined at parting line by means of dowels

Multipiece pattern For more complicated designs the pattern is made in more than 2 parts to facilitate easy moulding & withdrawal of pattern

A skeleton pattern For very large, easy to shape castings. When few numbers are to be made, solid patterns are uneconomical In such cases a pattern consisting of wooden frame & strip is made (called skeleton), is filled with loam sand & rammed. The surplus sand is removed by means of stickle. For symmetrical pipe like parts, only half pattern will serve the purpose of moulding both halves Ex. Boiler shells, chimney, huge pipes etc.

Pattern with loose piece Some patterns usually single piece, have loose pieces for ease of withdrawal from mould These pieces form integral part of pattern during moulding After the mould is complete the pattern is removed, leaving the pieces in sand which are later withdrawn through the cavity of the mould

Sweep pattern Advantageously used for preparing moulds of large symmetrical casting particularly of circular cross section like, Boiler shells, bells etc. Results in large saving of time, labour & material Shape of cavity obtained by rotating contoured sweep about a spindle mounted in sand in a base After removal of sweep & spindle the hole is manually filled, separately prepared core is placed in mould, gates are cut to make mould ready for pouring

Segmental pattern Used for preparing moulds of large circular castings avoiding use of solid pattern of exact size In principle similar to sweep pattern but the difference is that a sweep pattern is given a continuous revolving motion to generate the desired shape whereas the segmental pattern is a portion of solid pattern itself & mould is prepared in parts by it It is mounted on pivot & is moved to next position till part is ready Ex. Rings, wheels, rims, gears etc.

Match plate pattern It is a split pattern with cope & drag portions mounted on opposite sides of a plate called match plate The gates & runners are mounted on match plate so that very little hand work is required The match plate can have many patterns of same or different castings Aluminium is preferred to make the pattern due to its lightness and cheapness. More than one piece can be cast at a time.

Gated pattern It is one or more loose patterns having attached gates and runners Since gates and runners are not to be hand cut, time required is less Suitable for small castings in mass production

Defects in casting

Core making: Cores are placed into a mould cavity to form the interior surfaces of castings. Thus the empty space is filled with molten metal and eventually becomes the casting. Moulding: Moulding is nothing but the mould preparation activities for receiving molten metal. Moulding usually involves: (i) preparing the sand mould around a pattern held within a supporting metal frame, (ii) removing the pattern to leave the mould cavity with cores. Mould cavity is the primary cavity.

Melting and Pouring: The preparation of molten metal for casting is referred to simply as melting. The molten metal is transferred to the pouring area where the moulds are filled. Cleaning: Cleaning involves removal of sand, scale, and excess metal from the casting. Burned-on sand and scale are removed to improved the surface appearance of the casting. Excess metal, in the form of fins, wires, parting line fins, and gates, is removed. Inspection of the casting for defects and general quality is performed.

Making a simple sand mould 1) The drag flask is placed on the board 2) Dry facing sand is sprinkled over the board 3) Drag half of the pattern is located on the mould board. Dry facing sand will provide a non-sticky layer. 4) Moulding sand is then poured in, to fill the drag completely.

5) Sand is then tightly packed in the drag by means of hand rammers 5) Sand is then tightly packed in the drag by means of hand rammers. Peen hammers (used first close to drag pattern) and butt hammers (used for surface ramming) are used. 6) The ramming must be proper i.e. it must neither be too hard or soft. Too soft ramming will generate weak mould and imprint of the pattern will not be good. Too hard ramming will not allow gases/air to escape and hence bubbles are created in casting resulting in defects called ‘blows’. Moreover, the making of runners and gates will be difficult.

7) After the ramming is finished, the excess sand is levelled/removed with a straight bar known as strike rod. 8) 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 pouring and solidification. Done by vent rod. 9) The finished drag flask is now made upside down exposing the pattern.

10) Cope half of the pattern is then placed on the drag pattern using locating pins. The cope flask is also located with the help of pins. The dry parting sand is sprinkled all over the drag surface and on the pattern. 11) A sprue pin for making the sprue passage is located at some distance from the pattern edge. Riser pin is placed at an appropriate place.

12) Filling, ramming and venting of the cope is done in the same manner. 13) The sprue and riser are removed and a pouring basin is made at the top to pour the liquid metal. 14) Pattern from the cope and drag is removed. 15) Runners and gates are made by cutting the parting surface with a gate cutter. A gate cutter is a piece of sheet metal bent to the desired radius.

16) The core for making a central hole is now placed into the mould cavity in the drag. Rests in core prints. 17) Mould is now assembled and ready for pouring.

Function/characteristics Prepare cavity of desired shape & size (to produce mould) It may carry additional projections called core prints, to produce seats for cores(if casting requires cores) Runners, gates and risers may form a part of pattern The pattern establishes the parting line of parting surfaces in the mould

Properly finished patterns with smooth surfaces can avoid or reduce casting defects A pattern is used to position a core before ramming of moulding sand

CORE MAKING :- A core is a body of refractory material (sand or metal) which is set into prepared mould before closing and pouring it, for forming holes , recesses , projections , undercuts and internal cavities.

Core characteristics Good dry sand cores should have the following characteristics: Good dry strength and hardness after baking 2. Sufficient green strength to retain the shape before baking 3. Refractoriness(Material that absorb more hear-3000 C) 4. Surface smoothness 5. Permeability(porosity) 6. Lowest possible amount of gas created during the pouring of casting 7. Good collapsibility

Essential qualities of core :- The cores are subjected to more thermal & mechanical effects than molding sand because they are surrounded from all sides (except at the ends ) by molten metals. Hence, core sand should have high dry strength , surface strength, refractoriness, permeability and sufficient collapsibility. Strength can be improved by proper selection of kind of sand and good binder. Collapsibility is essential to avoid formation of cracks in casting after cooling . It can be improved by using oil as binding material. Metal cores are less frequently used.

Core baking • After cores are made they are taken to ovens for baking • Baking removes moisture and hardens core binders • generally core sands are poor conductors of heat and hence heat penetrates slowly into the interior sections of the cores • In a core having thin and thick sections, the thin sections will be over baked, while thick sections will be optimally baked • Over baking of cores will result in destroying the binders and hence core will be just a heap of sand • cores that are not baked fully will create an excess of gas and cause blows in castings

Types of cores :- The cores are usually classified according to position in mould as under. 1.Horizontal core :- - most commonly used - usually placed at parting line

2. Vertical core :- - fitted with vertical axis. - two ends of core seats in cope and drag - maximum portion of core is supported in drag.

3. Balanced core :- - used when blind holes are to be produced along horizontal axis - Length of core seat is more to prevent falling of core in mould.

4. Hanging / Cover core :- - no support at bottom . Entire core is contained in drag only. - dry sand cores can be suspended to cope by fastening wires or rods.

Core Boxes: Def..: A core box is essentially a type of pattern made of wood or metal into which sand is rammed or packed to form core. Types of core boxes: 1. Half core boxes: To prepare two halves which are cemented later to form complete core.

2. Dump core box: used to prepare complete core in it . Generally rectangular cores are prepared in these boxes . In construction it is similar to a half core box.

3. Split core box: It is made in two parts which are joined by means of dowels to form the complete cavity for making the core.

4. Stickle type core box: Stickle is often used when a core with an irregular shape is required. Core is produced by striking off the core sand from top of the core box with a piece called stickle board.

Gating System: It refers to all the passage ways through which metal enters a mould cavity. Requirements of gating system: i) Minimum turbulence of aspiration of mould gases during flow of metal to prevent sand erosion of gas pickup. ii) Facilitate directional solidification towards the riser during metal flow by proper establishment of temperature gradients on mould surfaces. iii) Well regulated flow of metal in cavity ( complete filling of mould within shortest possible time) iv) minimum excess of metal should be left in gates and risers.

v) Prevent entry of loose sand, oxide and slag in mould cavity. vi) Avoid erosion of mould walls. These requisites can be achieved by: 1. Controlled pouring 2. Use of proper pouring equipment. 3. Pouring metal at proper(specific) temperature 4. Correct design of spruce, runner and gates

Parts of gating systems:

1. Pouring basin: The main purpose of the pouring basin is to establish proper flow system as rapidly as possible . It may made of core sand or metal and may be cut in the cope of sand mould. Functions: To make it easier for ladle operator to direct the flow of metal from crucible to spruce. To maintain flow rate of metal. To reduce turbulence at spruce entrance To help in separating slag and dirt from metal before entering the spruce.

2. Sprue: The vertical passage for the cope and connecting the pouring basin through the runner or gate is called the sprue. Requirements for sprue size: a. The spruce must be small enough for the pourer to keep it full during the entire pouring operation and the metal to enter the mould cavity at a velocity that avoids turbulence. b. The sprue must be large enough for the mould cavity to fill completely without lapse or misrun

Sprue sizes vary from 10 mm2. For work below 12kg Sprue sizes vary from 10 mm2. For work below 12kg. poured weight to 50 mm2 for heavy castings. The base of the sprue is usually enlarged and made deeper than the runner called as sprue well. It serves as a cushion to falling weight of a metal and absorbs its kinetic energy. The width and depth of sprue well are about 1 ½ times those of runner. Runners: In the large castings, molten metal is usually carried from the sprue base to several gates around the cavity through a passage way called the runner.

Gates: The gate is passage that finally leads molten metal from runner into mould cavity. The location and size of gates are so arranged that the mould can be filled in quickly with a minimum amount of cutting of mould surfaces by the flowing of metal. The gates should be placed so as to avoid the development of cracks after cooling. The gate connection should be located so that they can be removed readily without damaging casting. Types of Gates: The gates are classified according to position in mould cavity. 1) Top gates 2) Bottom gates 3) Parting gates

Top gates Bottom gates Parting gates

1) Top gates: - Molten metals enters the cavity from top. - since the molten metal falls directly in mould cavity, the mould should be hard and strong enough to resist erosion by dropping metal. - The proper temperature gradients are formed (hot metal at top ) to achieve proper solidification. Advantages of top gate: 1) simplicity of molding. 2) low consumption of additional metal. 3) Achievement of proper temperature gradient.

Limitations: Erosion of mould surface due to dropping liquid . More chances of oxidation due to splashing of metal. More possibility of turbulence and air entrapment . Not suitable for non ferrous casting (Al & Mg) because of tendency of dross formation Application: For gray C.I. For castings with heavy bosses (like railway driving wheels), centers and hollow cylinders. For moulds made of refractory materials.

2) Bottom Gates : - made in bottom side of cavity in drag portion. - used to minimize turbulence & prevent mould erosion. - directional solidification is not achievable. Advantages: 1. Minimum splashing ,reducing chances of oxidation. 2. Minimum turbulence & metal erosion as compared to top gate. 3. Quality of cast surface is good.

Limitations: 1. May result in incomplete mould filling due to metal flow choke off ( as freezing take place at bottom) 2. Involves greater complexity of molding. 3. Creates unfavorable temp. gradient making it difficult to achieve directional solidification. Applications: 1. employed for steel castings in order to reduce erosion & gas entrapment . 2. to prevent splashing.

3. side gate or Parting line gate: - metal enters the cavity at the parting line. Advantages: 1. Simple to construct (even by pattern itself) 2. Less time taking and produce good results when drag is not deep. 3. Best compromise between bottom & top gate. 4. Promotes directional solidification when it is used as riser. Limitations: 1. when P.L is not near bottom of mould cavity or drag is deep. Results in turbulence. 2. May result in erosion or washing of mould.

RISERING OF CASTINGS: A riser is hole cut or molded in cope to permit the molten metal above the highest point in castings. The functions of riser are : To enable pourer to see the metal as it falls in mould cavity. To facilitate ejection of steam, gas and air from mould cavity. To serve as feeder to feed the molten metal in to the main castings to compensate for its shrinkage. Requirements of an effective riser it should have sufficient volume as it should be the last part of casting to freeze. it should facilitate directional solidification towards riser. it must be completely cover the sectional thickness that requires feeding.

A. A thin layer cools and solidifies where the metal touches the mold surface. B. Heat is sucked out of the metal anywhere it is touching the mold, and crystal dendrites form along the surface layer C. As heat continues to leave via the metal/mold interface, the dendrites grow. D. Metal in the center finally cools enough to start to crystallize. Small seed crystals form. E. These seed crystals multiply and grow until the casting is solid.

Solidification Time Solidification time = C(volume/surface area)2 Where C is a constant that depends on mold material and thickness, metal characteristics and temperature.