Mp Casting 3i5f66

ME010 404

Manufacturing Process

1 Dept of Mechanical Engineering, ICET Mulavoor

Module I Patterns: - types, allowances, color code – Molding sand:- constituents, types, properties, testing, types of mould, molding machines – Cores:- sands, types prints, machines, chaplets, forces acting on molding flasks - Gating system:- fluid flow and heat transfer in metal casting, elements and design of gating system, sprue, gating ratio, slag trap system – Risering:- risering design, chills, feeding devices - Cupola operation -pouring and cleaning of castings - defects in castings - inspection and quality control - Casting:- continuous, strip, shell mold, vacuum, investment, slush, pressure, die, centrifugal, precision investment, squeeze casting and semi solid metal forming, economics and surface finish obtainable - casting machines - comparison of casting with other production processes. (Include necessary figures and equations).

Manufacturing Process Manufacturing Process can be defined as making products from raw materials by various processes, machinery & operations following a well organized plan for each activity required. Manufacturing is a complex activity which involves materials, capital, energy, and people. A variety of machinery, equipment tooling with various levels of automation (computers etc), and material handling are involved.

Classification of manufacturing Process CASTING: In metalworking, casting involves pouring liquid metal into a mold, which contains a hollow cavity of the desired shape, and then allowing it to cool and solidify. The solidified part is also known as a casting. Eg:- sand casting, die casting, shell casting, investment casting etc. ING: Fabrication process which involves ing of two or more materials to form a single mass. eg:- Welding, Soldering, Brazing, Adhesive bonding etc. FORMING: Metal Forming is a process in which shape of the materials are changed to desired shapes by subjecting them to stresses greater than yield stress of the material. Forming is a Plastic Deformation Process where volume and mass of the metals are unchanged. eg. Rolling, forging, extrusion etc. MACHINING:Machining is a collection of processes wherein material is brought to a specified geometry by removing excess material using various kinds of tooling to leave a finished part that meets specifications. Eg:-Turning, milling, shaping etc.

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CASTING In metalworking, casting involves pouring liquid metal into a mold, which contains a hollow cavity of the desired shape, and then allowing it to cool and solidify. The solidified part is also known as a casting, which is ejected or broken out of the mold to complete the process. Casting is most often used for making complex shapes that would be difficult or uneconomical to make by other methods. Casting processes have been known for thousands of years, and widely used for sculpture, especially in bronze, jewellery in precious metals, and weapons and tools. Traditional techniques include lost-wax casting, plaster mold casting and sand casting. The modern casting process is subdivided into two main categories: expendable and nonexpendable casting.

1). Expendable mold castings Expendable mold casting is a generic classification that includes sand, plastic, shell, plaster, and investment (lost-wax technique) moldings. This method of mold casting involves the use of temporary, non-reusable molds. 2). Non-Expendable mold castings Non-expendable mold casting differs from expendable processes in that the mold need not be reformed after each production cycle. This technique includes at least four different methods: permanent, die, centrifugal, and continuous casting. This form of casting results in improved repeatability in parts produced Advantages of Castings The metal casting process is extensively used in manufacturing because of its many advantages. 1. Molten material can flow into very small sections so that intricate shapes can be made by this process. 2. It is possible to cast practically any material that is ferrous or non-ferrous. 3. As the metal can be placed exactly where it is required, large saving in weight can be achieved. 4. The necessary tools required for casting molds are very simple and inexpensive. As a result, for production of a small lot, it is the ideal process. 3 Dept of Mechanical Engineering, ICET Mulavoor

5. There are certain parts made from metals and alloys that can only be processed this way. 6. Size and weight of the product is not a limitation for the casting process. Limitations 1.

Dimensional accuracy and surface finish of the castings made by sand casting processes are a limitation to this technique.

2.

The metal casting process is a labor intensive process.

SAND CASTINGS Sand casting, also known as sand molded casting, is a metal casting process characterized by using sand as the mold material. Over 70% of all metal castings are produced via a sand casting process. Sand casting is relatively cheap and sufficiently refractory even for steel foundry use. In addition to the sand, a suitable bonding agent (usually clay) is mixed or occurs with the sand. The mixture is moistened, typically with water, but sometimes with other substances, to develop strength and plasticity of the clay and to make the aggregate suitable for molding. The sand is typically contained in a system of frames or mold boxes known as a flask. The mold cavities and gate system are created by compacting the sand around models, or patterns, or carved directly into the sand.

PATTERNS Pattern is the replica of the object to be made by the casting process, used to prepare the cavity into which molten metal will be poured during casting process. The quality of the casting produced depends upon the material of the pattern, its design, and construction. The costs of the 4 Dept of Mechanical Engineering, ICET Mulavoor

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. Typically, materials used for pattern making are wood, metal or plastics. Wax and Plaster of Paris are also used, but only for specialized applications. Mahogany is the most commonly used material for patterns, primarily because it is soft, light, and easy to work. Pattern Material Selection Patterns may be constructed from the following materials. Each material has its own advantages, limitations, and field of application. Some materials used for making patterns are: wood, metals and alloys, plastic, plaster of Paris, plastic and rubbers, wax, and resins. To be suitable for use, the pattern material should be: 1.

Easily worked, shaped and ed

2.

Light in weight

3.

Strong, hard and durable

4.

Resistant to wear and abrasion

5.

Resistant to corrosion, and to chemical reactions

6.

Dimensionally stable and unaffected by variations in temperature and humidity

7.

Available at low cost

The usual pattern materials are wood, metal, and plastics. The most commonly used pattern material is wood, since it is readily available and of low weight. Also, it can be easily shaped and is relatively cheap. The main disadvantage of wood is its absorption of moisture, which can cause distortion and dimensional changes. Hence, proper seasoning and upkeep of wood is almost a pre-requisite for large-scale use of wood as a pattern material.

Different types of patterns: The common types of patterns are: 1. Single piece pattern:

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

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2. Split pattern: These patterns are split along the parting plane (which may be flat or irregular surface) to facilitate the extraction of the pattern out of the mould before the pouring operation. For a more complex casting, the pattern may be split in more than two parts. 3. Loose piece pattern: When a one piece solid pattern has projections or back drafts which lie above or below the parting plane, it is impossible to with draw it from the mould. With such patterns, the projections are made with the help of loose pieces. One drawback of loose feces is that their shifting is possible during ramming.

4. Gated pattern: A gated pattern is simply one or more loose patterns having attached gates and runners. Because of their higher cost, these patterns are used for producing small castings in mass production systems and on molding machines. 5. Match plate pattern: A match plate pattern is a split pattern having the cope and drags portions mounted on opposite sides of a plate (usually metallic), called the "match plate" that conforms to the contour of the parting surface. The gates and runners are also mounted on the match plate, so that very little hand work is required. This results in higher productivity. This type of pattern is used for a large number of castings. Piston rings of I.C. engines are produced by this process. 7 Dept of Mechanical Engineering, ICET Mulavoor

6. Cope and drag pattern: A cope and drag pattern is a split pattern having the cope and drag portions each mounted on separate match plates. These patterns are used when in the production of large castings; the complete moulds are too heavy and unwieldy to be handled by a single worker. 7. Sweep pattern: A sweep is a section or board (wooden) of proper contour that is rotated about one edge to shape mould cavities having shapes of rotational symmetry. This type of pattern is used when a casting of large size is to be produced in a short time. Large kettles of C.I. are made by sweep patterns. 8. Skeleton pattern: For large castings having simple geometrical shapes, skeleton patterns are used. Just like sweep patterns, these are simple wooden frames that outline the shape of the part to be cast and are also used as guides by the molder in the hand shaping of the mould. This type of pattern is also used in pit or floor molding process.

Skeleton pattern Follow board pattern 9. Follow board pattern: It is used for ing a pattern which is very thin and fragile and may collapse under pressure when sand above the pattern is being rammed.

Colour Codes in Pattern 8 Dept of Mechanical Engineering, ICET Mulavoor

Patterns have some identifiers such as colors on them, each of which has different meaning that represent different treatments and requirements for the patterns. The color-coding for patterns in sand casting is as follows 1) BLACK: Surfaces of casting are to be left un machined 2) RED: Surfaces of casting to be machined 3) RED strips on Yellow Background: Seats for loose pieces 4) Yellow: Core Prints PATTERN ALLOWANCES Pattern allowance is a vital feature as it affects the dimensional characteristics of the casting. Thus, when the pattern is produced, certain allowances must be given on the sizes specified in the finished component drawing so that a casting with the particular specification can be made. The selection of correct allowances greatly helps to reduce machining costs and avoid rejections. The allowances usually considered on patterns and core boxes are as follows: 1.

Shrinkage or contraction allowance

2.

Draft or taper allowance

3.

Machining or finish allowance

4.

Distortion or camber allowance

5.

Rapping allowance

Shrinkage or Contraction Allowance All most all cast metals shrink or contract volumetrically on cooling. The metal shrinkage is of two types: i. Liquid Shrinkage: it refers to the reduction in volume when the metal changes from liquid state to solid state. To for this shrinkage; riser, which feed the liquid metal to the casting, are provided in the mold. ii. Solid Shrinkage: it refers to the reduction in volume caused when metal loses temperature in solid state. To for this, shrinkage allowance is provided on the patterns. The rate of contraction with temperature is dependent on the material. For example steel contracts to a higher degree compared to aluminum. To compensate the solid shrinkage, a shrink rule must be used in laying out the measurements for the pattern. 9 Dept of Mechanical Engineering, ICET Mulavoor

Shrink rule for various materials are Steel – 20.8 mm / m Aluminum – 17 mm / m Brass – 15.3 mm / m

Draft or Taper Allowance By draft is meant the taper provided by the pattern maker on all vertical surfaces of the pattern so that it can be removed from the sand without tearing away the sides of the sand mold and without excessive rapping by the molder.

Draft allowance varies with the complexity of the sand job. But in general inner details of the pattern require higher draft than outer surfaces. The amount of draft depends upon the length of the vertical side of the pattern to be extracted; the intricacy of the pattern; the method of molding; and pattern material. Table 2 provides a general guide lines for the draft allowance.

Table 2 : Draft Allowances of Various Metals

Pattern material

Wood

Height of the Draft angle given surface (External surface) (inch)

1

3.00

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Draft angle (Internal surface)

3.00

1.50 1 to 2

2.50 1.00

2 to 4

1.50 0.75

4 to 8

1.00 0.50

8 to 32

Metal and plastic

1.00

1

1.50

3.00

1 to 2

1.00

2.00

2 to 4

0.75

1.00

4 to 8

0.50

1.00

8 to 32

0.50

0.75

Machining or Finish 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. Machining or finish allowances are therefore added in the pattern dimension. The amount of machining allowance to be provided for is affected by the method of molding and casting used viz. hand molding or machine molding, sand casting or metal mold casting. The amount of machining allowance is also affected by the size and shape of the casting; the casting orientation; the metal; and the degree of accuracy and finish required. The machining allowances recommended for different metal is given in Table 3. Table 3 : Machining Allowances of Various Metals

Metal

Dimension (inch)

Allowance (inch)

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Cast iron

Cast steel

Non ferrous

Up to 12

0.12

12 to 20

0.20

20 to 40

0.25

Up to 6

0.12

6 to 20

0.25

20 to 40

0.30

Up to 8

0.09

8 to 12

0.12

12 to 40

0.16

Distortion or Camber Allowance Sometimes castings get distorted, during solidification, due to their typical shape. For example, if the casting has the form of the letter U, V, T, or L etc. it will tend to contract at the closed end causing the vertical legs to look slightly inclined. This can be prevented by making the legs of the U, V, T, or L shaped pattern converge slightly (inward) so that the casting after distortion will have its sides vertical ( (Figure 2). The distortion in casting may occur due to internal stresses. These internal stresses are caused on of unequal cooling of different section of the casting and hindered contraction. Measure taken to prevent the distortion in casting includes: i.

Modification of casting design

ii.

Providing sufficient machining allowance to cover the distortion affect

iii. Providing suitable allowance on the pattern, called camber or distortion allowance (inverse reflection)

12 Dept of Mechanical Engineering, ICET Mulavoor

Figure 2: Distortions in Casting Rapping Allowance Before the withdrawal from the sand mold, the pattern is rapped all around the vertical faces to enlarge the mold cavity slightly, which facilitate its removal. Since it enlarges the final casting made, it is desirable that the original pattern dimension should be reduced to for this increase. There is no sure way of quantifying this allowance, since it is highly dependent on the foundry personnel practice involved. It is a negative allowance and is to be applied only to those dimensions that are parallel to the parting plane.

MOLDING SAND Sand casting is the most widely used metal casting process in manufacturing. Almost all casting metals can be sand cast. Sand castings can range in size from very small to extremely large. Some examples of items manufactured in modern industry by sand casting processes are engine blocks, machine tool bases, cylinder heads, pump housings, and valves etc. Most sand casting operations use silica sand (SiO2). A great advantage of sand in manufacturing applications is that sand is inexpensive and very resistant to elevated temperatures. In fact, sand casting is one of the few processes that can be used for metals with high melting temperatures such as steels, nickel, and titanium. Usually sand used to manufacture a mold for the casting process is held together by a mixture of water and clay. A typical mixture by volume could be 89% sand, 4% water, 7% clay. Control of all aspects of the properties of sand is crucial when 13 Dept of Mechanical Engineering, ICET Mulavoor

manufacturing parts by sand casting; therefore a sand laboratory is usually attached to the foundry. Use of Binder In Sand Casting: A mold must have the physical integrity to keep its shape throughout the casting operation. For this reason, in sand casting, the sand must contain some type of binder that acts to hold the sand particles together. Clay serves an essential purpose in the sand casting manufacturing process, as a binding agent to adhere the molding sand together. In manufacturing industry other agents may be used to bond the molding sand together in place of clay. Organic resins, (such as phenolic resins), and inorganic bonding agents, (such as phosphate and sodium silicate), may also be used to hold the sand together. In addition to sand and bonding agents, the sand mixture to create the metal casting mold will sometimes have other constituents added to it in order to improve mold properties. Types of Sand Used In Sand Casting: There are two general types of sand used in the manufacturing process of sand casting. Naturally Bonded- Naturally bonded sand is less expensive but it includes organic impurities that reduce the fusion temperature of the sand mixture for the casting, lower the binding strength, and require a higher moisture content. Synthetic Sand- Synthetic sand is mixed in a manufacturing lab starting with a pure (SiO2) sand base. In this case, the composition can be controlled more accurately, which imparts the casting sand mixture with higher green strength, more permeability, and greater refractory strength. For these reasons, synthetic sand is mostly preferred in sand casting manufacture. Properties of A Sand Casting Mixture: Type And Content Of Binder And Other Additives: As mentioned, controlling the type and content of the sand binder and other additives is the key to controlling the properties of the casting's mold sand mixture. Moisture Content: Moisture content affects the other properties of the mixture such as strength and permeability. Too much moisture can cause steam bubbles to be entrapped in the metal casting. Grain Size: This property represents the size of the individual particles of sand. Shape of Grains: This property evaluates the shape of the individual grains of sand based on how round they are. Less round grains are said to be more irregular.

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Strength: The explanation of strength is, the ability of the sand casting mixture to hold its geometric shape under the conditions of mechanical stress imposed during the sand casting process. Permeability: The ability of the sand mold to permit the escape of air, gases, and steam during the sand casting process. Collapsibility: The ability of the sand mixture to collapse under force. Collapsibility is a very important property in this type of casting manufacture. Collapsibility of the mold will allow the metal casting to shrink freely during the solidification phase of the process. If the molding sand cannot collapse adequately for the casting's shrinkage, hot tearing or cracking will develop in the casting. Flowability: The ability of the sand mixture to flow over and fill the sand casting pattern during the impression making phase of the manufacturing process, more flowability is useful for a more detailed casting. Refractory Strength: During the pouring of the molten metal in sand casting manufacture, the sand mixture in the mold must not melt, burn, crack, or sinter. The refractory strength is the ability of the mold sand mixture to withstand levels of extreme temperature. Reusability: The ability of the mold sand mixture to be reused to produce other sand castings in subsequent manufacturing operations. When planning the manufacture of a particular casting, some properties of a sand casting mold mixture are contradictory to each other. Tradeoffs in different properties are often needed to achieve a compromise that provides a sand casting mold mixture with adequate properties for the specific part and casting application. There are some things to consider when selecting a sand mixture for a manufacturing process. Small grain size enhances mold strength, but large grain size is more permeable. Sand casting molds made from grains of irregular shape tend to be stronger because of grain interlocking, but rounder grains provide a better surface finish. A sand casting mold mixture with more collapsibility has less strength, and a sand casting mixture with more strength has less collapsibility.

Sand Conditioning For a Metal Casting Operation: If the sand is being reused from a previous sand casting manufacturing process, lumps should be crushed and then all particles and metal granules removed, (a magnetic field may be used to assist in this). All sand and constituents should be screened. In industrial practice shakers, rotary screens, or vibrating screens, are used in this process. Then continuous screw-mixers or mulling machines are used to mix the sand uniformly.

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Types of Molds Used In Sand Casting: Green Sand Molds: A green sand mold is very typical in sand casting manufacture, it is simple and easy to make, a mixture of sand, clay and water. The term green refers to the fact that the mold will contain moisture during the pouring of the casting. Manufacturing Considerations And Properties Of Green Sand Molds: 

Has sufficient strength for most sand casting applications



Good collapsibility



Good permeability



Good reusability



Least expensive of the molds used in sand casting manufacturing processes



Moisture in sand can cause defects in some castings, dependent upon the type of metal used in the sand casting and the geometry of the part to be cast.

Dry Sand Molds: Dry sand molds are baked in an oven, (at 300F - 650F for 8-48 hours), prior to the sand casting operation, in order to dry the mold. This drying strengthens the mold, and hardens its internal surfaces. Dry sand molds are manufactured using organic binders rather than clay. Manufacturing Considerations And Properties Of Dry Sand Molds:  Better dimensional accuracy of sand cast part than green sand molds 

Better surface finish of sand cast part than green sand molds



More expensive manufacturing process than green sand production



Manufacturing production rate of castings are reduced due to drying time



Distortion of the mold is greater, (during mold manufacture)



The metal casting is more susceptible to hot tearing because of the lower collapsibility of the mold 16 Dept of Mechanical Engineering, ICET Mulavoor



Dry sand casting is generally limited to the manufacture of medium and large castings

Skin Dried Molds: When sand casting a part by the skin dried mold process a green sand mold is employed, and its mold cavity surface is dried to a depth of .5 - 1 inch. Drying is a part of the manufacturing process and is accomplished by use of torches, heating lamps or some other means, such as drying it in air. Manufacturing Considerations And Properties Of Skin Dried Molds: 

The cast part dimensional and surface finish advantages of dry sand molds are partially achieved



No large oven is needed



Special bonding materials must be added to the sand mixture to strengthen the mold cavity surface

Cold Setting Processes: In industrial sand casting manufacture, sometimes non-traditional binders other than those used in the above classifications of sand molds may be used. These binders may be made of a variety of things, such as synthetic liquid resins. Conventional casting binders require heat to cure while these when mixed with the sand, bond chemically at room temperature. Hence the term cold setting processes. Technically advanced, these relatively recent sand casting processes are growing in manufacturing. While more expensive than green sand molds, cold setting processes provide good dimensional accuracy of the casting, and have high production applications. CORES: For metal castings with internal geometry cores are used. A core is a replica, (actually an inverse), of the internal features of the part to be cast. Like a pattern, the size of the core is designed to accommodate for shrinkage during the metal casting operation. Unlike a pattern, a core remains in the mold while the metal is being poured. Hence, a core is usually made of a similar material as the mold. Once the metal casting has hardened, the core is broken up and removed much like the mold. Depending upon the location and geometry of the core within the casting, it may require that it is ed during the operation to prevent it from moving or shifting. Structural s that hold the core in place are called chaplets. The chaplets are made of a material with a higher melting temperature than the casting's material, and become assimilated into the part when it hardens. Note that when manufacturing a metal casting with a permanent mold process, the core will be a part of the mold itself. 17 Dept of Mechanical Engineering, ICET Mulavoor

The core print is an added projection on the pattern and it forms a seat in the mold on which the sand core rests during pouring of the mold. The core print must be of adequate size and shape so that it can the weight of the core during the casting operation.

FORCES ACTING ON THE MOULD As the mould fills it becomes exposed to high metallostatic pressures which tend to displace or distort the mould sections and cores. These forces can be accurately predicted and contained by foundry measures. The first need is for a dense, rigid mould, since the pressure tends to expand the mould cavity, especially in greensand practice. Rigidity of the mould parts can be increased by using box bars or cover plates to reinforce the sand mass; these measures become increasingly necessary with moulds of large area. Assuming a rigid mould, the next concern is with the force tending to separate the mould parts. The upward force acting on a flat mould surface is Where  = density of the metal [kg/m ] h = head of metal, m A = superficial area, m

F= 9.81  h A [N],

A completely flat mould surface gives rise to the maximum lifting force: calculations for other shapes can thus be safely based upon their projected areas. The force is resisted by using box clamps and arrangements of plates and tie-bars to hold the mould parts together and by 18 Dept of Mechanical Engineering, ICET Mulavoor

weighting the top part is  h A [kg] including the weightthe latter case the minimum weight required is of the cope itself.

GATING SYSTEM The assembly of channels which facilitates the molten metal to enter into the mold cavity is called the gating system. Alternatively, the gating system refers to all age ways through which molten metal es to enter into the mold cavity. The nomenclature of gating system depends upon the function of different channels which they perform. 

Down gates or sprue



Cross gates or runners



Ingates or gates

The metal flows down from the pouring basin or pouring cup into the down gate or sprue and es through the cross gate or channels and ingates or gates before entering into the mold cavity.

Goals of Gating System The goals for the gating system are 

To minimize turbulence to avoid trapping gasses into the mold



To get enough metal into the mold cavity before the metal starts to solidify 19 Dept of Mechanical Engineering, ICET Mulavoor



To avoid shrinkage



Establish the best possible temperature gradient in the solidifying casting so that the shrinkage if occurs must be in the gating system not in the required cast part.



Incorporates a system for trapping the non-metallic inclusions

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Hydraulic Principles used in the Gating System Reynold's Number Nature of flow in the gating system can be established by calculating Reynold's number

Where RN

=

Reynold's number

V

=

Mean Velocity of flow

D

=

diameter of tubular flow

µ

=

Kinematic Viscosity



=

Fluid density

= Dynamic viscosity / Density

When the Reynold's number is less than 2000 stream line flow results and when the number is more than 2000 turbulent flow prevails. As far as possible the turbulent flow must be avoided in the sand mold as because of the turbulence sand particles gets dislodged from the mold or the gating system and may enter into the mould cavity leading to the production of defective casting. Excess turbulence causes 

Inclusion of dross or slag



Air aspiration into the mold



Erosion of the mold walls

Bernoulli's Equation Used to calculate flow velocities,

Where

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h

=

height of liquid

P

=

Static Pressure

v

=

metal velocity

g

=

Acceleration due to gravity



=

Fluid density

Turbulence can be avoided by incorporating small changes in the design of gating system. The sharp changes in the flow should be avoided to smooth changes. The gating system must be designed in such a way that the system always runs full with the liquid metal. The most important things to in deg runners and gates are to avoid sharp corners. Any changes in direction or cross sectional area should make use of rounded corners. To avoid the aspiration the tapered sprues are designed in the gating systems. A sprue tapered to a smaller size at its bottom will create a choke which will help keep the sprue full of molten metal. Types of Gating Systems The gating systems are of two types: 

Pressurized gating system



Un-pressurized gating system

Pressurized Gating System 

The total cross sectional area decreases towards the mold cavity



Back pressure is maintained by the restrictions in the metal flow



Flow of liquid (volume) is almost equal from all gates



Back pressure helps in reducing the aspiration as the sprue always runs full



Because of the restrictions the metal flows at high velocity leading to more turbulence and chances of mold erosion

Un-Pressurized Gating System 22 Dept of Mechanical Engineering, ICET Mulavoor



The total cross sectional area increases towards the mold cavity



Restriction only at the bottom of sprue



Flow of liquid (volume) is different from all gates



aspiration in the gating system as the system never runs full



Less turbulence

GATING RATIO The term "gating ratio" is used to describe the relative cross-sectional areas of the components of a gating system. It is defined as the ratio of sprue area to the total runner area to the total gate area. A gating system having a sprue of 1 sq cm cross-section, a runner of 3 sq cm cross-section, and three gates, each of 1 sq cm cross-section, will have a gating ratio of 1:3:3. Gating ratios are grouped in two classes, viz., pressurised and unpressurised systems. In the pressurised system, the proportions of sprue, runner, and gate areas are so arranged that back pressure is maintained on the gating system by a fluid film restriction at the gates. This requires that the total gate area is not greater than the area of the sprue. Gating ratios such as 1:0.75:0.5, 1:2:1 and 2:1:1 will therefore produce a pressurised system. A pressurised system keeps itself full of metal. This system is generally suitable for ferrous metals and brass. The unpressurised system of gating produces lower metal velocities and permits greater flow rates. It reduces turbulence in the gating system and spurting in the mould cavity. On the other hand, this system requires careful design to ensure complete filling, and large-sized runners and gates, which reduce the yield and increase the wastage of metal. Further, equal flow in the case of multiple gating is difficult to achieve. The unpressurised system is generally adopted for metals such as aluminium and magnesium. The ratios used are 1:2:2, 1:3:3, etc. Riser Riser is a source of extra metal which flows from riser to mold cavity to compensate for shrinkage which takes place in the casting when it starts solidifying. Without a riser heavier parts of the casting will have shrinkage defects, either on the surface or internally. Risers are known by different names as metal reservoir, feeders, or headers. Shrinkage in a mold, from the time of pouring to final casting, occurs in three stages. 1. during the liquid state 23 Dept of Mechanical Engineering, ICET Mulavoor

2. during the transformation from liquid to solid 3. during the solid state First type of shrinkage is being compensated by the feeders or the gating system. For the second type of shrinkage risers are required. Risers are normally placed at that portion of the casting which is last to freeze. A riser must stay in liquid state at least as long as the casting and must be able to feed the casting during this time. Functions of Risers 

Provide extra metal to compensate for the volumetric shrinkage



Allow mold gases to escape



Provide extra metal pressure on the solidifying mold to reproduce mold details more exact

Design Requirements of Risers 1. Riser size: For a sound casting riser must be last to freeze. The ratio of (volume / surface area)2 of the riser must be greater than that of the casting. However, when this condition does not meet the metal in the riser can be kept in liquid state by heating it externally or using exothermic materials in the risers. 2. Riser placement: the spacing of risers in the casting must be considered by effectively calculating the feeding distance of the risers. 3. Riser shape: cylindrical risers are recommended for most of the castings as spherical risers, although considers as best, are difficult to cast. To increase volume/surface area ratio the bottom of the riser can be shaped as hemisphere.

Chills A chill is an object used to promote solidification in a specific portion of a metal casting mold. Normally the metal in the mold cools at a certain rate relative to thickness of the casting. When the geometry of the molding cavity prevents directional solidification from occurring naturally, a chill can be strategically placed to help promote it. There are two types of chills: internal and external chills.

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Internal chills are pieces of metal that are placed inside the molding cavity. When the cavity is filled, part of the chill will melt and ultimately become part of the casting, thus the chill must be the same material as the casting. External chills are masses of material that have a high heat capacity and thermal conductivity. They are placed on the edge of the molding cavity, and effectively become part of the wall of the molding cavity. This type of chill can be used to increase the feeding distance of a riser or reduce the number of risers required.

CUPOLA FURNACE The molten iron is usually produced in a cupola furnace. This is a vertical cylindrical steel shell with a “well” at the bottom to collect the molten metal. The inside can be made of fire bricks, but is normally constructed of steel, with a water jacket for cooling and lined with clay. The well at the bottom is lined with sand and the furnace is charged through a door at the top with pig iron or scrap iron, coke and limestone. Air is necessary for combustion and this is blown into the furnace through several tuyeres (blast holes) near the base. There are cold blast cupolas where the air enters at atmospheric pressure and hot blast cupolas where the air is preheated by the heat from the exhaust gases. This type of cupola has an increased melting rate, produces hotter metal, and uses less coke. The coke burns fiercely thanks to the air blast and the metal soon melts. The limestone (CaCO3) acts as a flux, which decomposes in the cupola to form lime (CaO) which in turn combines with oxides to form a liquid (slag), which floats on top of the molten metal. This is removed through a slag hole below the tuyeres. The cupola extends above the charging door to produce a chimney where the exhaust gases are removed and cleaned before entering the atmosphere. The coke can be fired directly to start the combustion process, but electricity, oil or gas are often used with carbon being added to the charge.

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Various Zones of Cupola Furnace Various numbers of chemical reactions take place in different zones of cupola. The construction and different zones of cupola are : 1. Well The space between the bottom of the tuyeres and the sand bed inside the cylindrical shell of the cupola is called as well of the cupola. As the melting occurs, the molten metal is get collected in this portion before tapping out. 2. Combustion zone 26 Dept of Mechanical Engineering, ICET Mulavoor

The combustion zone of Cupola is also called as oxidizing zone. It is located between the upper of the tuyeres and a theoretical level above it. The total height of this zone is normally from 15 cm. to 30 cm. The combustion actually takes place in this zone by consuming the free oxygen completely from the air blast and generating tremendous heat. The heat generated in this zone is sufficient enough to meet the requirements of other zones of cupola. The heat is further evolved also due to oxidation of silicon and manganese. A temperature of about 1540°C to 1870°C is achieved in this zone. Few exothermic reactions takes place in this zone these are represented as: C + O2 → CO2 + Heat Si + O2 → SiO2 + Heat 2Mn + O2 → 2MnO + Heat 3. Reducing zone Reducing zone of Cupola is also known as the protective zone which is located between the upper level of the combustion zone and the upper level of the coke bed. In this zone, CO2 is changed to CO through an endothermic reaction, as a result of which the temperature falls from combustion zone temperature to about 1200°C at the top of this zone. The important chemical reaction takes place in this zone which is given as under. CO2 + C (coke) → 2CO + Heat Nitrogen does not participate in the chemical reaction occurring in his zone as it is also the other main constituent of the upward moving hot gases. Because of the reducing atmosphere in this zone, the charge is protected against oxidation. 4. Melting zone The lower layer of metal charge above the lower layer of coke bed is termed as melting zone of Cupola. The metal charge starts melting in this zone and trickles down through coke bed and gets collected in the well. Sufficient carbon content picked by the molten metal in this zone is represented by the chemical reaction given as under. 3Fe + 2CO → Fe3C + CO2 5. Preheating zone Preheating zone starts from the upper end of the melting zone and continues up to the bottom level of the charging door. This zone contains a number of alternate layers of coke bed, flux and metal charge. The main objective of this zone is to preheat the charges from room temperature to about 1090°C before entering the metal charge to the melting zone. The preheating takes place in this zone due to the upward movement of hot gases. During the preheating process, the metal charge in solid form picks up some sulphur content in this zone. 6. Stack 27 Dept of Mechanical Engineering, ICET Mulavoor

The empty portion of cupola above the preheating zone is called as stack. It provides the age to hot gases to go to atmosphere from the cupola furnace. CASTING DEFECTS The following are the major defects, which are likely to occur in sand castings 

Gas defects



Shrinkage cavities

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Molding material defects



Pouring metal defects



Mold shift

Gas Defects A condition existing in a casting caused by the trapping of gas in the molten metal or by mold gases evolved during the pouring of the casting. The defects in this category can be classified into blowholes and pinhole porosity. Blowholes are spherical or elongated cavities present in the casting on the surface or inside the casting. Pinhole porosity occurs due to the dissolution of hydrogen gas, which gets entrapped during heating of molten metal. Causes The lower gas-ing tendency of the mold, which may be due to lower venting, lower permeability of the mold or improper design of the casting. The lower permeability is caused by finer grain size of the sand, high percentage of clay in mold mixture, and excessive moisture present in the mold. 

Metal contains gas



Mold is too hot



Poor mold burnout

Shrinkage Cavities These are caused by liquid shrinkage occurring during the solidification of the casting. To compensate for this, proper feeding of liquid metal is required. For this reason risers are placed at 28 Dept of Mechanical Engineering, ICET Mulavoor

the appropriate places in the mold. Sprues may be too thin, too long or not attached in the proper location, causing shrinkage cavities. It is recommended to use thick sprues to avoid shrinkage cavities. Molding Material Defects The defects in this category are cuts and washes, metal penetration, fusion, and swell. Cut and washes These appear as rough spots and areas of excess metal, and are caused by erosion of molding sand by the flowing metal. This is caused by the molding sand not having enough strength and the molten metal flowing at high velocity. The former can be taken care of by the proper choice of molding sand and the latter can be overcome by the proper design of the gating system. Metal penetration When molten metal enters into the gaps between sand grains, the result is a rough casting surface. This occurs because the sand is coarse or no mold wash was applied on the surface of the mold. The coarser the sand grains more the metal penetration. Fusion This is caused by the fusion of the sand grains with the molten metal, giving a brittle, glassy appearance on the casting surface. The main reason for this is that the clay or the sand particles are of lower refractoriness or that the pouring temperature is too high. Swell Under the influence of metallostatic forces, the mold wall may move back causing a swell in the dimension of the casting. A proper ramming of the mold will correct this defect. Inclusions Particles of slag, refractory materials, sand or deoxidation products are trapped in the casting during pouring solidification. The provision of choke in the gating system and the pouring basin at the top of the mold can prevent this defect. Pouring Metal Defects The likely defects in this category are

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

Mis-runs and



Cold shuts.

A mis-run is caused when the metal is unable to fill the mold cavity completely and thus leaves unfilled cavities. A mis-run results when the metal is too cold to flow to the extremities of the mold cavity before freezing. Long, thin sections are subject to this defect and should be avoided in casting design.

A cold shut is caused when two streams while meeting in the mold cavity, do not fuse together properly thus forming a discontinuity in the casting. When the molten metal is poured into the mold cavity through more-than-one gate, multiple liquid fronts will have to flow together and become one solid. If the flowing metal fronts are too cool, they may not flow together, but will leave a seam in the part. Such a seam is called a cold shut, and can be prevented by assuring sufficient superheat in the poured metal and thick enough walls in the casting design. The mis-run and cold shut defects are caused either by a lower fluidity of the mold or when the section thickness of the casting is very small. Fluidity can be improved by changing the composition of the metal and by increasing the pouring temperature of the metal. Mold Shift The mold shift defect occurs when cope and drag or molding boxes have not been properly aligned.

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INSPECTION OF CASTINGS: A large number of methods have been developed to inspect castings for defects that may occur during their production. Such inspections may be in process inspections or finished product inspections. In process inspections are carried out before a lot of castings have been completed to detect any flaws that may have occurred in the process so that corrective measures can be taken to remove the defect in the remaining units. Finished product inspections are carried out after the castings have all been completed to make sure that the product meets the requirements specified by the customer. Defective castings may be salvaged or completely rejected to be re-melted for their material content depending upon the nature and extent of defect. The inspection methods may also be divided into destructive or non-destructive categories depending upon the magnitude of damage done to the casting during inspection. Destructive methods generally relate to sawing or breaking off of parts of the castings at places where voids or internal defects are suspected. Castings may also be damaged during strength tests. Destructive tests suffer from the disadvantage that the saw cuts may miss the flaw or the sample may not represent the behavior of the entire lot. Because of these reasons non-destructive tests are generally more commonly relied upon than destructive tests. Some of the prominent non destructive methods are described below: Visual Inspection: It consists of inspecting the surface of the casting with naked eye or sometimes with a magnifying glass or microscope. It can only indicate surface defects such as blow holes, fusion, swells, external cracks, and mismatch. Almost all castings are subjected to certain degree of visual inspection. Dimensional Inspection: Dimensional inspection is carried out to make sure that the castings produced have the required overall dimensions including allowances for machining. It may sometimes be necessary to break a part of the casting to take measurements of inside dimensions. Sound Test: 31 Dept of Mechanical Engineering, ICET Mulavoor

This is a rough test to indicate a flaw or discontinuity in a casting. The casting is suspended from a suitable free of all obstructions and tapped at various places on its surface with a small hammer. Any change in the tone produced indicates the existence of a flaw. The method cannot indicate the exact location and extent of the discontinuity.

Impact Test: Used as a standard method of determining the impact resistance of materials. In this test the casting is subjected to a blow from a hammer of known weight striking or falling on the casting. Defective castings fail under the impact of the blow but the method is very crude and unreliable.

Pressure Test: This test is carried out on castings required to be leak proof. All openings of the castings are closed and a gas or fluid under pressure is introduced in it. Castings having porosity leak under this pressure. The leakage may be detected by submerging the casting in a water tank or using a soap film if the pressure is applied by compressed air. If a liquid is used for applying pressure the leakage can be found by visual inspection. Radiography:

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Radiography uses X-rays or gamma rays penetrating through the castings and giving a shadow picture on a photographic film placed behind the work piece. These rays have very short wave length of the order of 0.001 Angstrom (10-10m) units to 40 Angstrom units for x-rays and 0.005 to about 3 Angstrom units for gamma rays compared to about 5500 Angstrom units for the centre of the visible spectrum. The ability of these waves to penetrate through metal depends also on the density of the metal and as such they can penetrate more easily in places where there is less metal that those where more metal is present leading to a shadow picture formation on the film. Any defects in the casting can easily be identified from this picture. Because of their shorter wave length gamma rays have a better penetration through the metal and are more commonly used.

Magnetic Particle Testing: This test is used for detecting cracks in metals like cast iron and steel which can be magnetized. For carrying out the test the casting is magnetized and then fine particles of iron or steel are spread on its surface. Presence of a crack or void in the casting results in interruption of the magnetic field and leakage of magnetic flux at the place of the crack. The particles of iron or steel spread on the casting surface are held by this leaking flux giving a visual indication of the nature and extent of crack. Very small cracks or voids at or near the surface which may not even be detected by radiography are easily revealed by this method.

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Penetrant Testing: This method also is used for detecting very small surface cracks and has the advantage over the magnetic particle method that it can be used for any material. The parts to be tested are either dipped into or covered with a penetrant testing liquid ( fluorescent or nonfluorescent dyes) which has very good wetting and penetrating ability. The liquid is drawn into the cracks or voids by capillary action.

After the penetrant has been applied to the surface to be tested extra penetrant is wiped off the surface is dried and a developer (acetone, isopropyl alcohol) applied to it. This developer helps in drawing out the penetrant so that it becomes visible on the surface. The penetrant liquids often contain materials which fluoresce under ultraviolet light or a die to indicate their presence.

Ultrasonic Testing: 34 Dept of Mechanical Engineering, ICET Mulavoor

Ultrasonic testing is used to detect defects like cracks, voids or porosity within the interior of the casting. The method uses reflection and transmission of high frequency sound waves. Ultrasonic sound waves much higher than the audible range are produced and made to through the casting.

The time interval between the transmitted ray and reflected ray is recorded by a cathode ray oscilloscope. Any crack or void in the casting results in reflection or some of the sound from the crack which appears as a pip between the two pips representing the thickness of the casting. The depth of the crack from the surface of the casting can be easily calculated from the distance between these pips.

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CASTING PROCESS Continuous casting Continuous casting is a process which turns liquid steel into slab or bloom. Liquid steel is filled in ladle and transferred to turret from upstream plant by crane, charged to a tundish, then distributed and flew into several molds which circulated by cooling water. Liquid steel starts cooling down, solidifying and forming a shell outside in, pulled into arc-shape strands, through secondary cooling sprays to a complete solidification, then straightened, and cut into pieces according to each order. This semi product called slab (rectangular type) or bloom (square type) is conditioned if necessary then shipped to downstream for further treatment. Relative to other casting processes, continuous casting generally has a higher capital cost, but lower operating cost. It is the most cost- and energy- efficient method to mass-produce semifinished metal products with consistent quality in a variety of sizes and shapes. Cross-sections can be rectangular, for subsequent rolling into plate or sheet, square or circular for long products, and even “dog-bone” shapes, for rolling into I or H beams.

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Shell Mold Casting The first step in the shell mold casting process is to manufacture the shell mold. The sand we use for the shell molding process is of a much smaller grain size than the typical green sand mold. This fine grained sand is mixed with a thermosetting resin binder. A special metal pattern is coated with a parting agent, (typically silicone), which will latter facilitate in the removal of the shell. The metal pattern is then heated to a temperature of 350F-700F degrees, (175C-370C). The sand mixture is then poured or blown over the hot casting pattern. Due to the reaction of the thermosetting resin with the hot metal pattern, a thin shell forms on the surface of the pattern. The desired thickness of the shell is dependent upon the strength requirements of the mold for the particular metal casting application. A typical industrial manufacturing mold for a shell molding casting process could be .3in (7.5mm) thick. The thickness of the mold can be controlled by the length of time the sand mixture is in with the metal casting pattern. The excess "loose" sand is then removed, leaving the shell and pattern. The shell and pattern are then placed in an oven for a short period of time, (minutes), which causes the shell to harden onto the casting pattern. Once the baking phase of the manufacturing process is complete, the hardened shell is separated from the casting pattern by way of ejector pins built into the pattern. It is of note that this manufacturing technique used to create the mold in the shell molding process can also be employed to produced highly accurate fine grained mold cores for other metal casting processes.

Two of these hardened shells, each representing half the mold for the casting, are assembled together either by gluing or clamping.

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The manufacture of the shell mold is now complete and ready for the pouring of the metal casting. In many shell molding processes, the shell mold is ed by sand or metal shot during the casting process.

Vacuum mold casting A special pattern is used for the vacuum mold casting process. It is either a match-plate or a cope and drag pattern with tiny holes to enable a vacuum suction. A thin plastic sheet is placed over the casting pattern and the vacuum pressure is turned on, causing the sheet to adhere to the surface of the pattern.

A special flask is used for this manufacturing process. The flask has holes to utilize vacuum pressure. This flask is placed over the casting pattern and filled with sand.

A pouring cup and sprue are cut into the mold for the pouring of the metal casting.

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Next, another thin plastic sheet is placed over the top of the mold. The vacuum pressure acting through the flask is turned on, and the plastic film adheres to the top of the mold.

In the next stage of vacuum mold casting manufacture, the vacuum on the special casting pattern is turned off and the pattern is removed. The vacuum pressure from the flask is still on. This causes the plastic film on the top to adhere to the top and the plastic film formerly on the pattern to adhere to the bottom. The film on the bottom is now holding the impression of the casting in the sand with the force of the vacuum suction.

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The drag portion of the mold is manufactured in the same fashion. The two halves are then assembled for the pouring of the casting.

During the pouring of the casting, the molten metal easily burns away the plastic.

Investment casting

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Investment casting is a manufacturing process in which a wax pattern is coated with a refractory ceramic material. Once the ceramic material is hardened its internal geometry takes the shape of the casting. The wax is melted out and molten metal is poured into the cavity where the wax pattern was. The metal solidifies within the ceramic mold and then the metal casting is broken out. This manufacturing technique is also known as the lost wax process. The first step in investment casting is to manufacture the wax pattern for the process. The pattern for this process may also be made from plastic; however it is often made of wax since it will melt out easily and wax can be reused. Since the pattern is destroyed in the process, one will be needed for each casting to be made. When producing parts in any quantity, a mold from which to manufacture patterns will be desired.

Since the mold does not need to be opened, castings of very complex geometry can be manufactured. Several wax patterns may be combined for a single casting. Or as often the case, many wax patterns may be connected and poured together producing many castings in a single process. This is done by attaching the wax patterns to a wax bar, the bar serves as a central sprue. A ceramic pouring cup is attached to the end of the bar. This arrangement is called a tree, denoting the similarity of casting patterns on the central runner beam to branches on a tree. The casting pattern is then dipped in a refractory slurry whose composition includes extremely fine grained silica, water and binders. A ceramic layer is obtained over the surface of the pattern. The pattern is then repeatedly dipped into the slurry to increase the thickness of the ceramic coat. In some cases the pattern may be placed in a flask and the ceramic slurry poured over it. Once the refractory coat over the pattern is thick enough, it is allowed to dry in air in order to harden. The next step in this manufacturing process is the key to investment casting. The hardened ceramic mold is turned upside down and heated to a temperature of around 200F-375F (90C175C). This causes the wax to flow out of the mold, leaving the cavity for the metal casting. The ceramic mold is then heated to around 1000F-2000F (550C-1100C). This will further strengthen the mold, eliminate any leftover wax or contaminants and drive out water from the mold material. 41 Dept of Mechanical Engineering, ICET Mulavoor

The metal casting is then poured while the mold is still hot. Pouring the casting while the mold is hot allows the liquid metal to flow easily through the mold cavity, filling detailed and thin sections. Pouring the metal casting in a hot mold also gives better dimensional accuracy, since the mold and casting will shrink together as they cool. After pouring of the molten metal into the mold, the casting is allowed to set as the solidification process takes place.

The final step in this manufacturing process involves breaking the ceramic mold from the investment casting and cutting the parts from the tree.

Slush casting Slush casting is a variation of permanent mold casting that is used to produce hollow parts. In this method neither the strength of the part nor its internal geometry can be controlled accurately. This metal casting process is used primarily to manufacture toys and parts that are ornamental in nature, such as lamp bases and statues. When producing a cast part using the slush casting method, a permanent mold is employed and set up. The mold is clamped together and prepared for pouring. 42 Dept of Mechanical Engineering, ICET Mulavoor

After pouring the mold will set, as solidification begins to take place. The main principle of this casting process relies on the fact that when a metal casting hardens in a mold, it will solidify from the mold wall towards the inside of the casting. In other words a metal skin forms first, (as the external geometry of the part). This skin thickens as more of the metal casting's material converts to a solid state.

In slush mold casting, during the solidification of the material, when the solid-liquid boundary has reached a certain point, the mold is turned over and the remaining liquid metal from the casting is poured out. This will leave only the solidified skin with the exterior geometry of the metal cast part and a hollow interior. The longer the metal casting was allowed to solidify before pouring out the excess metal, the greater the casting's wall thickness will be.

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The cast part is then removed from the die and allowed to cool.

Pressure casting Pressure casting, also known in manufacturing industry as low pressure casting or pressure pouring, is another variation of permanent mold casting. Instead of pouring the molten metal into the casting and allowing gravity to be the force that distributes the liquid material through the mold, pressure casting uses air pressure to force the metal through the gating system and the metal casting's cavity. This process can be used to cast high quality manufactured parts. Often steel castings are cast in graphite molds using this process. For example, in industry, steel railroad car wheels are cast with this method.

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This is a permanent mold process and the manufacture of the mold in pressure casting is standard in most regards. Two blocks are machined extremely accurately, so they can open and close precisely for removal of metal parts. The casting's gating system is machined into the mold. The gating system is set up so that the molten material flows into the mold from the bottom instead of the top, (like in gravity fed processes). The mold is set up above the supply of liquid metal to be used for the casting. A refractory tube goes from the entrance of the gating system down into the molten material. During manufacture by this process, the chamber that the liquid material is in is kept air tight. When the mold is prepared and ready for the pouring of the metal casting, air pressure is applied to the chamber. This creates pressure on the surface of the liquid, that in turn forces molten material up the refractory tube and throughout the mold. Pressure used in pressure casting is usually low, 15lbs/in2 could be typical for industrial manufacture using this process. The air pressure is maintained until the metal casting has hardened within the mold. Once the cast part has solidified, the mold is opened and the part is removed.

Die casting Die casting manufacture is characteristic in that it uses large amounts of pressure to force molten metal through the mold. Since so much pressure is used to ensure the flow of metal through the mold, metal castings with great surface detail, dimensional accuracy, and extremely thin walls can be produced. Wall thickness within castings can be manufactured as small as .02in (.5mm). Like in all permanent mold manufacturing processes, the first step in die casting is the production of the mold. The mold must be accurately created as two halves that can be opened and closed for removal of the metal casting. The mold for die casting is commonly machined from steel and contains all the components of the gating system. In a die casting production setup, the mold, (or die), is designed so that its mass is far greater than that of the casting. Typically the mold will have 1000 times the mass of the metal casting. So a 2 pound part will require a mold weighing a ton! Due to the extreme pressures and the continuous exposure to thermal gradients from the molten metal, wearing of the die can be a problem. However in a well maintained manufacturing process, a die can last hundreds of thousands of cycles before needing to be replaced. Die Casting Machines In addition to the opening and closing of the mold to prepare for and remove castings, it is very important that there is enough force that can be applied to hold the two halves of the mold together during the injection of the molten metal. Flow of molten metal under such pressures will 45 Dept of Mechanical Engineering, ICET Mulavoor

create a tremendous force acting to separate the die halves during the process. Die casting machines are large and strong, designed to hold the mold together against such forces. In manufacturing industry, die casting machines are rated on the force with which they can hold the mold closed. Clamping forces for these machines vary from around 25 to 3000 tons.

In industrial manufacture the process of die casting falls into two basic categories, hot chamber die casting and cold chamber die casting. Although these processes vary from each other, both employ a piston or plunger to force molten metal to travel in the desired direction. The pressure at which the metal is forced to flow into the mold in die casting manufacture is on the order of 1000psi to 50000psi (7MPa to 350MPa). This pressure is able for the tremendously intricate surface detail and thin walls that are often observed in metal castings manufactured by this technique. Once the mold has been filled with molten metal, the pressure is maintained until the casting has hardened. The mold is then opened and the casting is removed. Ejector pins built into the mold assist in the removal of the metal casting. In most manufacturing operations, the internal surfaces of the mold are sprayed with a lubricant before every cycle. The lubricant will assist in cooling down the dies as well as preventing the metal casting from sticking to the mold. After the casting has been removed and the lubricant applied to the mold surfaces, the die are clamped together again then the cycle will repeat itself. Cycle times will differ depending upon the details of each specific die casting manufacturing technique. In some instances, very high rates of production have been achieved using this metal casting process.

Hot Chamber Diecasting Machines In hot chamber die casting manufacture, the supply of molten metal is attached to the die casting machine and is an integral part of the casting apparatus for this manufacturing operation. 46 Dept of Mechanical Engineering, ICET Mulavoor

Submerged Plunger type

It is also called a gooseneck machine because of the shape of the metal age way. In this machine the melting pot, usually made of cast iron, is a part of the machine. The gooseneck containing a cylinder and metal age way is kept immersed in the metal pot. The plunger in the gooseneck cylinder is actuated either hydraulically or pneumatically. In operation the plunger is withdrawn letting the liquid metal into the gooseneck cylinder through the port provided. When the die halves are closed and ready for casting the plunger forces the liquid metal entrapped in the cylinder into the die through the gooseneck age and a nozzle. After a predetermined time interval the plunger is retracted allowing the liquid metal in the gooseneck channel and nozzle to fall back into cylinder. The die halves are opened and the solidified casting is ejected from the die. Hot chamber machines are designed to operate almost automatically and fast. Cold Chamber Diecasting Machines The metal in this case is melted in a separate furnace and the required quantity of metal is ladled to the machine. A plunger operated hydraulically forces the metal into the die. Injection pressures of 28 kPa to 250 kPa are possible in cold chamber machines. The machine is semiautomatic in that after the metal is ladled into the cold chamber the rest of the operation is automatic. Hot chamber machines are made in capacities varying from 0.25 to 7.5 MN and cold chamber ones from 1 to 10 MN. 47 Dept of Mechanical Engineering, ICET Mulavoor

True Centrifugal casting The centrifugal casting is a metal casting technique that uses the forces generated by centripetal acceleration to distribute the molten material in the mold. Cast parts manufactured in industry include various pipes and tubes, such as sewage pipes, and gas pipes, also bushings, rings, the liner for engine cylinders, and brake drums. The molds used in true centrifugal casting manufacture are round, and are typically made of iron, steel, or graphite. Some sort of refractory lining or sand may be used for the inner surface of the mold.

In this process the mold is rotated about its axis at a predetermined speed. Molds for smaller parts may be rotated about a vertical axis. However, most times in true centrifugal casting manufacture the mold will be rotated about a horizontal axis. The effects of gravity on the material during the metal casting process make it particularly necessary to cast longer parts with forces generated from horizontal rather than vertical rotation.

The molten material for the cast part is introduced to the mold from an external source, usually by means of some spout. The liquid metal flows down into the mold. Once inside the cavity, the centripetal forces from the spinning mold force the molten material to the outer wall. Molten

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material for the casting may be poured into a spinning mold or the rotation of the mold may begin after pouring has occurred. The metal casting will harden as the mold continues to rotate. It can be seen that this casting process is very well suited for the manufacture of hollow cylindrical tubes. The forces used in this technique guarantee good adhesion of the casting material to the surface of the mold. Thickness of the cast part can be determined by the amount of material poured. The outer surface does not need to be round. Polygonal geometries such as squares and other shapes can be cast. However, due to the nature of the process, the inner surface of a part manufactured by true centrifugal casting must always be round. Semi-Centrifugal Casting Semicentrifugal casting manufacture is a variation of true centrifugal casting. The main difference is that in semicentrifugal casting the mold is filled completely with molten metal, which is supplied to the casting through a central sprue. Castings manufactured by this process will possess rotational symmetry. In semi-centrifugal casting manufacture a permanent mold may be employed. However, often industrial manufacturing processes will utilize an expendable sand mold. This enables the casting of parts from high temperature materials.

The molten material for the metal casting is poured into a pouring basin and is distributed through a central sprue to the areas of the mold. The forces generated by the rotation of the mold ensure the distribution of molten material to all regions of the casting. As the metal casting solidifies in a rotating mold, the centripetal forces constantly push material out from the central sprue/riser. This material acts to fill vacancies as they form, thus avoiding shrinkage areas. It can 49 Dept of Mechanical Engineering, ICET Mulavoor

be seen that during the semicentrifugal manufacturing process, the material in the outer regions of the casting, (further from the center of the axis of rotation), is subject to greater forces than the material in the inner regions. When the metal casting solidifies, the outer region of the cast part forms of dense material. The greater the forces under which the molten metal solidified, the denser the material in that region. So the density of a cast part manufactured by semicentrifugal casting will increase as you travel radially outward from the center. Centrifuge Casting Centrifuge casting is the third main branch of centrifugal casting processes used for industrial manufacture of cast parts. Centrifuge casting is different in that castings manufactured by the centrifuge casting process need not have rotational symmetry. With centrifuge casting, metal castings of desired shapes can be manufactured with all the distinct benefits of castings produced by a centrifugal casting process.

In centrifuge casting manufacture, molds employed to produce the desired castings are arranged around a central sprue. These molds contain all the necessary geometry for the cast part, as well as the gating system. Runners travel from the central sprue to the mold entrances. During the pouring phase of centrifuge casting manufacture, molten material is introduced into the central sprue. The entire system is rotated about an axis with the central sprue at the center of rotation. When an object is rotated, forces are produced that act directly away from the center of the axis of rotation and centripetal forces from the rotating apparatus push this material outward from the center, through the runners and into the molds. When the correct amount of molten metal to manufacture the casting is poured and distributed completely into the molds, the apparatus will continue to rotate as solidification is occurring. 50 Dept of Mechanical Engineering, ICET Mulavoor

Squeeze casting Squeeze casting is a method combining casting and forging technologies. In contrast to other casting techniques (sand casting, die casting), in which a molten metal is poured (injected) into the mold cavity after the two parts of the mold are assembled, squeeze casting mold is closed after a portion of molten metal has been poured into the preheated bottom die. The upper die lowers towards the bottom die causing the melt to fill the mold cavity. The squeezing pressure is applied until full solidification of the casting. A scheme of the process is shown in the picture:

Squeeze castings are characterized by:  low shrinkage and gas porosity;  enhanced mechanical properties because of fine grain structure caused by rapid solidification;  good surface quality. Squeeze casting is commonly used for processing aluminum and magnesium alloys. This process is also used for fabrication of reinforced metal matrix composites where molten aluminum infiltrates a fiber reinforcing structure.

51 Dept of Mechanical Engineering, ICET Mulavoor

Semi-solid Metal Forming Semi-solid metal casting (SSM) is a near net shape variant of die casting. The process is used with non-ferrous metals, such as aluminium, copper, and magnesium. The process combines the advantages of casting and forging. SSM is done at a temperature that puts the metal between its liquidus and solidus temperature. Ideally, the metal should be 30 to 65% solid. The metal must have a low viscosity to be usable, and to reach this low viscosity the material needs a globular primary surrounded by the liquid phase. For aluminum alloys rheocasting and thixocasting are the two common processes to produce semi-solid castings. 1.Thixocasting-- Thixocasting utilizes a pre-cast billet with a non-dendritic microstructure that is normally produced by vigorously stirring the melt as the bar is being cast. Induction heating is normally used to re-heat the billets to the semi-solid temperature range, and die casting machines are used to inject the semi-solid material into hardened steels dies. Thixocasting has the ability to produce extremely high quality components due to the product consistency that results from using pre-cast billet that is manufactured under the same ideal continuous processing conditions that are employed to make forging or rolling stock. The main disadvantage is that it is expensive due to the special billets that must be used. Other disadvantages include a limited number of alloys, and scrap cannot be directly reused.

2. Rheocasting--Unlike thixocasting, which re-heats a billet, rheocasting develops the semi-solid slurry from the ordinary molten metal by a rapid undercooling in combination with the shearing of the melt. This is a big advantage over thixocasting because it results in less expensive feedstock, in the form of typical die casting alloys, and allows for direct recycling.

52 Dept of Mechanical Engineering, ICET Mulavoor