Metal Casting - 2 | Mechanical Engineering SSC JE (Technical) PDF Download

Metal Casting

Gating Ratio  

It is the ratio between area of sprue : area of runner: area of gate.
• For non- pressured gating system,
sprue : runner : ingate = 1 : 4 : 4
• For pressurized gating system,
Sprue : runner : ingate = 2 : 2 : 1

Top Gate

Assumption

• The molten metal is entering into mould cavity directly from the end of the sprue at atmospheric pressure.
• Top gating system is used for ferrous material of large size casting.
As
A_g is the cross section area and
V_g is the velocity of liquid metal.

h_t = h_s + h_c
A and H are the cross section of area and height of mould cavity respectively.
t_f1 is the total time to fill the mould.

 
Using Bernoulli's equation between 2 and 3


Its means that at point (2) in the sprue there will be vacuum and it will capture atmosphere into liquid metal through sand voids. This will produce blow hole in the casted part.
This phenomenon is called Aspiration effect so to avoid this, area of the sprue is changed to have uniform atmospheric pressure through out the sprue.

Bottom Gate:

Applying Bernoulli’s equation between (1) and (2),

V₁ = can be neglected because basin area >> gating system
P₁ is atmospheric pressure:

Bernoulli's between (2) and (3)

410hP₃ is atmospheric pressure cross-section area of casting is same

Putting this in equation (4).

we know

Let us consider that in time d_tf, the liquid metal comes out of gate increasing the level of metal in the cavity by dh.

Solidification Time Calculation:

• Chvorinov's Rule is a mathematical relationship first expressed by Czech engineer Nicolas Chvorinov in 1940, that relates the solidification time for a simple casting to the volume and surface area of the casting. 
• In simple terms the rule establishes that under otherwise identical conditions, the casting with large surface area and small volume will cool more rapidly than a casting with small surface area and a large volume. 

The relationship can be written as:

                    t        =        B                              (                                          V                A                                      )                                n                          ,              {\displaystyle t=B\left({\frac {V}{A}}\right)^{n},}   

V = Volume - It represents amount of heat available.
A = Surface area – It represents amount of heat disputation.

SOLIDIFICATION TIME FOR CERTAIN SHAPES

Purpose of Riser

• Riser is provided to compensate liquid and solidification shrinkage during Casting.
It is also used to show whether casting is full or not.
• So riser design should be such that it solidify after casting, so that liquid metal is available to compensate shrinkage.
• Riser should be design in such a way that it has minimum surface area. Riser volume can be taken three time the liquid shrinkage but this design has to be checked whether riser will solidify before of after casting.

Classification of Casting

1. Expendable moulds [Temporary moulds]

• Sand casting • Shall moulding
• Full moulding • CO₂ moulding
• Investment Casting

Expendable-Pattern Casting(Lost Foam)

• The expendable pattern casting process uses a polystyrene pattern, which evaporates upon contact with molten metal to form a cavity for the casting. 
• The process is also known as evaporative pattern or lost pattern and under the trade name full mold process. 
• It was formerly known as the “expanded polystyrene process” and has become one of the most important casting process for ferrous and non-ferrous metals particularly for the automotive industry.
• In this process, raw expendable polystyrene (EPS) beads, containing 5% to 8% pentane are placed in a preheated die which is usually made of aluminum. 
• The polystyrene expands and takes the shape of the die cavity; additional heat is applied to fuse and bonds the beads together. 
• The die is then cooled and opened and the polystyrene pattern is removed.

• In the expendable pattern casting process, also known as lost foam or evaporative casting, the patterns is coated with a water-based refractory slurry, derived and placed in a flask. 
• The flask is then filled with loose fine and, which surrounds and supports the patterns and may be dried or mixed with bounding agents to give its additional strength and may be derived or mixed with bonding agents to give it additional strength.

2. Permanent Moulds [Metallic Moulds]:

(i) Centrifugal Casting

(ii) Die Casting

(iii) Gravity Die casting

(iv) Pressure Die casting: (a) Hot die casting (b) Cold die casting

• In permanent mould casting process, also called hard mold casting, two halves of a mold are made from material such as cast iron, steel bronze, graphite or refractory metal alloys. 
• The mould cavity and gating system are machined into the mold and thus become an integral part. 
• To produce castings with internal cavities, cores made of metal or sand aggregate are placed in the mold prior to casting.
• This process is used mostly for aluminium, magnesium copper alloys and grey cast iron because of their generally lower melting points steels can also be cast using graphite or heat resistance metal molds.
• This process produces at high production rates casting with good surface finish, close dimensional tolerance and uniform and good mechanical properties. 
• Typical parts made are automobile pistons, cylinder heads, connecting rods, gear blanks, appliance and kitchen wares.
• Permanent mould casting is not economical for small production run and because of the difficulty in “removing the casting from the mould, intricate shape cannot be cast by this process. However, easily collapsed sand core can be used and removed from casting to leave intricate internal cavities. 
• The process is then called Semi-permanent mould casting.

3. Continous Moulding:

• Shell moulding:
  
  
  
  

A common method of making shell molds

• It is process in which the sand mixed with a thermo setting resin is allowed to come into contact with a heated metallic pattern plate so that a thin and strong shell of mould is formed around the pattern.
• It will produce better surface finish and close tolerance and object having small projection.
• The pattern is produce by metallic material and it will be heated to 200°- 230° C.
• Moulding sand and material consists of fine grain, silica phenolic resin in pressure of alcohol.
• Moulding sand will be made in contact with heated metallic pattern. Due to heat of pattern resins will be activated and the moulding sand will stick to the surface of pattern material and it will form a shell.
• The thickness of the shell will depends on the contact time between pattern material and moulding sand and this time period is known as DWELL TIME.
• After getting sufficient thickness of shell, pattern will be heated along with shell upto 300°-330°C.
• Due to this strength of shell will be increased.

Advantage

• Shell mould casting are generally more dimensionally accurate than sand casting.
• A smooth surface can be obtained
• Draft angles, which are lower than the sand casting, are required in shell moulds.
• Permeability of the shell is high and there fore no gas inclusions occurs.
• Automation is readily possible because of the simple processing involved in shell moulding.

Limitations

• The patterns are very expansive.
• The size of the casting obtained by shell moulding is limited.
• Highly complicated shapes cannot be obtained.

Application

• Small size mechanical parts those require better surface finish and accuracy.
• Cylinder block of IC engine, rockers arm, small size gate etc.

Investment Casting

• The investment casting process, also called the lost wax process. 
• The pattern is made of wax or plastic (such as polystyrene) by molding or rapid prototyping techniques. 
• The sequences involved in investment casting are shown in Figure below. The pattern is made by injecting molten wax or plastic into a metal die in the shape of the pattern. 
• The pattern is then dipped into a slurry of refractory material such as very fine silica and
  binders including water, ethyl silicate and acids. 
• After this initial coating has dried, the pattern is coated repeatedly to increase its thickness.
• The turm investment derives from the fact that the pattern is inverted with the refractory material wax pattern requires careful handling because they are not strong enough to withstand the forces involved during mold making. 
• However, unlike plastic pattern, wax can be recovered and reused.
• Although the labour and material involved make the lost wax process costly, it is  suitable for casting high melting point alloys with good surface finish and close dimensional tolerances. 
• Therefore, few or no finishing operation, which would otherwise add significantly to the total cost of the casting are required.
• Typical parts made are components for office equipment as well as mechanical component such as gears, cams, valves and ratchets.
• In this, mould is prepared around an expendable pattern. It is used for making intricate shapes which are not symmetrical.
• Investment casting produces better tolerances. Compared to shell moulding.
• It is used for making jewelry surgical equipment and blade for turbine bolt and triggers for fire arm.
• The pattern is made of either wax or mercury

Applications

• Complex shapes which are difficult to produce by any other method are possible since the pattern is withdrawn by melting it.
• Very fine details and thin section can be produced by this process.
• Very close tolerances and surface finish can be produced.
• It can produce jet engine parts, turbine blades, dentures (surgical instrument,)gold ornaments etc

Die Casting

Hot chamber die casting

• The hot chamber process involves the use of a piston, which traps a certain volume of molten metal and force it into the die gravity through a gooseneck and nozzle.
• The pressure range up to 35 MPa, with an average of about 15 MPa.
• The metal is held under pressure until it solidifies in the die. 
• To improve die life and to aid in rapid cooling die are usually cooled by circulating water or oil through various passage way in the die block.
• Low melting point alloys such as zinc, magnesium tin and lead are commonly  cast using this process
  Die casting involves the preparation of components by injecting molten metal at high pressure into a metallic die.
• In this process the mould is made up of some permanent material like cast iron, die steels copper and aluminium.
• Two halves can either be placed horizontally or vertically and when liquid metal is pound under gravity it is called gravity die casting.
• When liquid metal is injected into this permanent mould it is called pressure die casting.

Advantages

• Same mould can be used again and again this increases the production rate
• Dimensional tolerance as of the order of 0.01 to 0.03 inch.
• Rapid cooling produces high strength.
• Better section sensitivity but after some amount of uses.
• It is very economical for large scale production.

Limitations

• The maximum size of casting is limited upto certain size
• This is not suitable for all material because of the limitation on the die material.
• Normally zinc, aluminium, magnesium and copper alloys are die cast.
• The air in the die cavity gets trapped inside the casting and is therefore a problem often with die castings.
• It is used for producing crank casting, fuel injection pump, value bodies, small size connecting rod and carburetors.

Cold chamber die casting

• In cold chamber process, molten metal is processed into the injection cylinder (short chamber). The shot chamber is not heated, hence the term cold chamber.
• The metal is forced into the die cavity at pressure usually range from 20 MPa to 70 MPa.
• The machines may be horizontal or vertical in which case the shot chamber is vertical and the machine is similar to a vertical press.
• High melting point alloys of magnesium and copper are normally cast using  this method, although other metals can also be casted in this manner. 
• The main difference between these two is that in the hot chamber die casting, the heating furnace for the liquid metal is integral with the die casting machine,

Where as the cold chamber machine, the metal is melted in a separate furnace and than poured into the die casting machine with a ladle for each casting cycle which is also called shot

Slush Casting

• Hollow casting with thin wall can be made by a process called slush casting. 
• The molten metal is poured into the metal mold; after the desired thickness of solidified skin is obtained, the mould is inverted or swing and the remaining liquid metal flows out. 
• The mould halves are then opened and the casting is removed.
• Such casting is important for small production run and. is generally used for making ornamental and decorative objects such as camp bases and stems and toys from lower melting point metal such as zinc, tin and lead alloys.

NOTE:

• Very thin sections, gold jewelry, lamp bases, statue and other brass items are made up by this process.
• Toys, decorative items and camp shades are made by this process.

Blow Moulding

• Glass and plastic bottles, bulb etc. are moulded by this process.
• Glass or plastic in the semi viscous form (called gob) is placed in the die and air is blown into the die.
• As a result of that material takes the shape of die.

Centrifugal Casting

• This is a process where the mould is rotated rapidly about its axis (central axis) as the metal poured into it.
• Because of the centrifugal force, a continuous pressure will be acting on the metal as it solidifies.
• The slag oxides and other inclusions being lighter, gets separated from the metal and segregates towards centres.

Types of Centrifugal Casting 

True Centrifugal Casting

• In this process a metallic mould (in two parts with flange) is rotated at 3000
rpm using a rotating devices.
• Liquid metal is than poured into it .

• The mould is slightly inclined from the horizontal position so that the liquid metal covers the entire work length of moulds.
• Percentage yield in casting is defined as the ratio of useful material to that the total liquid material poured into the cavity it is nearly about 95%-98%.
• Cause grains settles down at the outside surface due to higher centrifugal force. The gains towards centre will be finer and finer. Such surface are called “Jagged Surfaces”
Limitation of true centrifugal casting
• Only certain shapes which are axis symmetric and having concentric hours are suitable for true centrifugal casting.
• The equipments is expensive and thus is suitable only for large shell production.

Applications

• The mechanical properties of centrifugally cast jobs are better compared to other process, because the inclusions such as slag and oxides segregates towards the centre and can be easily removed by machining.
• After the pressure acting on the metal throughout the solidification, causes the porosity get eliminated giving rise to dense metal.
• Upto a certain dimensions and thickness of object, proper directional solidification can be obtained.
• No core is required for making concentric holes in case of true centrifugal casting.
• There is no need for gates and runners, which increases the casting yield, reaching almost 100%.
• The axis of rotation can be either horizontal vertical or any angle in between.
• Components having fine grain size and high density can also be produced by true centrifugal casting.

Semicentrifugal Casting  

• In this process mould is placed on the horizontal plane and it is rotated along vertical axis.
• The outer portion of the mould will be filled by purely centrifugal action and as the liquid metal approaches towards centre the centrifugal component decreases and gravity component increases.
• The central portion is purely filled by gravity.
• The speed of rotation and percentage yield is lower than true centrifugal casting.
• It is used for making wheel, pulley, spoke wheel, alloyed wheels.

Centrifugal Casting

• As shown in Figure that a number of casting are placed on the periphery of a drum and are connected to the central sprue through individual gates.
• After solidification, gating system is disconnected to get the casted part.
• The percentage yield in this case is only 5%-10% and speed of rotation is much lower.
• The casting need not to be axis symmetric and the process is primarily used in making patterns for investment casting.
• The centrifugal process is used in order to obtain higher metal pressure during solidification.
• When casting shapes are not axis symmetrical this is suitable only for small jobs of any shape.
• It is used for making pattern used in investment casting.

Casting Defects 

Drop

Irregular projection on the top of casting caused by dropping of sand from cope.

Buckle

V-shaded depression occurring on flat casting due to expansions of ^d at the mould face before liquid metal solidifies.

Scab

Protruding surface of casting at roof. (Figure ) .

Swell

Liquid metal displaces the sand at the wall regions due to hydrostatic pressure.

Mould Shift

Due to misalignment between the two halves.

Remedies

• Heat the molten liquid metal in the furnace upto pouring temperature only.
• Convert green sand mould into dry sand mould before allowing the liquid metal into mould cavity.
• Select the moulding sand such that it has better permeability.

Discontinuties

Such as cracks, cold hot tearing and cold shut. If the solidifying metal is constrained from shrinking freely, cracking and tearing can occur. Although many factors are involved as tearing, coarse grain size and the presence of low melting point segregates along the grain boundaries, increase the tendency for hot tearing. Cold sheet is an interface in a casting that lacks complete fusion because of the meeting of two streams of liquids metal from different gates.

Incomplete Casting 

Such as misrun (due to premature solidification), insufficient volume of the metal poured and runout (due to loss of metal from mold after processing). Incomplete casting can result from the molten metal being at| too low temperature or from pourning the metal too slowly.

Inclusions

Which form during melting, solidification and molding. Generally nonmetallic, they are regarded as harmful because they act as stress raisers and reduce the strength of casting. Inclusion may form during melting when molten metal reacts with the environment or with the crucible or mold material. Chemical reaction among components in the molten metal may produce inclusion slag and other foreign material entrapped in the
molten metal also become inclusion.

Cupola

This is the commonly used melting furnace used in foundries. Cupolas are refractory lined vertical steel vessels  changed with attenuating layers of metal, like and flux, Althoughthey require major investments and are being replaced by induction furnaces, cupolas operate continuously, have high melting rates and produce large amounts of molten metal.
• steel is basically an alloy of iron and carbon.
• In addition to carbon, which imparts basic properties to steel, the other elements that are normally present in wrought steel are also found in cast steels.

Melting of Cast Iron

Wide spread use of the cupola for gray-iron melting rests upon its unique advantage.
1. Continuous melting: Foundry production is initiated since a cable of molten iron may be tapped from the furnace at regular intervals. The flow of molten iron metal and moulds for pouring may synchronized for quality production as required by the automotive, agricultural equipment.
2. Low Cost of Melting: Raw material and operating costs are lower than any other type of melting furnace producing equivalent product.
3. Chemical composition control is possible by proper furnace operation with continuous melting.
4. Adequate temperature control for fluidity in pouring casting can be obtained.
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