Casting Processes and Defects

Casting Processes and Defects
Presented By
Mr. Vikrtant A Shimpukade

Functions of gating system
• Gate is defined as one of the channels which actually lead into the mould
cavity, and the term Gating or gating system refers to all channels by means
of which molten metal is delivered to the mould cavity
• Functions
1. To provide continuous, uniform feed of molten metal, with less errors as
possible to the mould cavity.
2. To supply the casting with liquid metal at best location to achieve proper
directional solidification and optimum feeding of shrinkage cavities.
3. To fill the mould cavity with molten metal in the shortest possible time to
avoid temperature gradient.

4. To provide with a minimum of excess metal in the gates and risers.
5. To prevent erosion of the mould walls.
6. To prevent slag, sand and other foreign particles from entering the mould.

Gating and risering system
• Gating system : casting system provides all passageways to the casting,
through which the molten metal passes to the cavity.

Components of gating system
 Pouring basin:
• This part of the gating system is made on or in the top of the mould.
Sometimes, a funnel-shaped opening which serves as pouring basin is made
at the top of the sprue in the cope.
• The main purpose of the pouring basin is to direct the flow of metal from
ladle to the sprue, to help maintaining the required rate of liquid metal flow,
and to reduce errors and at the sprue entrance.

 Sprue:
• The vertical passage that passes through the cope and connects the pouring
basin with the runner or gate is called the sprue.
• The cross-section of a sprue may be square, rectangular, or circular.
 Runner:
• In large castings, molten metal is usually carried from the sprue base to
several gates around the cavity through a passageway called the runner.
• The runner is generally preferred in the drag, but it may sometimes be
located in the cope, depending on the shape of the casting.
• It should be streamlined to avoid aspiration and turbulence.

 Gate
• A gate is a passage through which molten metal flows from the runner to
the mould cavity.
• The location and size of the gates are so arranged that they can feed liquid
metal to the casting at a rate consistent with the rate of solidification.
• A gate should not have sharp edges as they may break during passage of
the molten metal.
• However, the gates should be located where they can be easily removed
without damaging the casting.

Risering System
• A riser or a feeder head is a passage of sand made in the cope to permit the
molten metal to rise above the highest point in the casting after the mould
cavity is filled up.
• Risers serve a dual function:
1) they compensate for solidification shrinkage which is a very common
casting defect.
2) a heat source so that they freeze last and promote directional solidification.
• Risers provide thermal gradients from a remote chilled area to the riser.
• If the metal does not appear in the riser, it indicates that the mould cavity has
not been completely filled up.

Types of riser
(a) Open riser
(b) Blind riser

Casting processes
 Shell moulding process

• Shell is a special form of sand casting. This process is relatively recent and
because of its advantages it is being increasingly used
• Shell moulding process is applicable to production of castings ranging from
about 250 gram to about 25 kilogram in ferrous as well as non-ferrous
metals and alloys.
• The sand used in this method is a mixture of the following ingredients:
a) Dry fine silica sand and
b) Synthetic resin binder 3 to 10 % by weight.

• Resins used are the phenol formaldehydes, urea formaldehydes, alkyds and
polyesters in the form of fine powder.
• The resins must be thermosetting plastics because , the strength obtained
after the mould is heated & must be retained when molten metal is poured.
• The mould is formed from a mixture of fine sand (100-150 mesh) and a
thermosetting resin binder that is placed opposite to heated pattern.
• In actual practice, the metal pattern is heated to about 200 to 300 °C, the
melting point of resin
• Then after a silicon parting agent is sprayed on the surface, the resin &
sand mixture is deposited on the pattern by blowing or dumping.

• The resin starts melting and, in a few seconds, forms together with the sand
a uniform resin-soaked layer of about 4 to12 mm in thickness, depending
on the heating period.
• The pattern is then turned over to allow the unbounded sand to be removed,
leaving the shell on the pattern.
• The shell is then stripped mechanically and once more heated for 3 to 5
minutes in a special oven to cure the plastic material.

• In this way, stable shell moulds are obtained which are made in two
sections. Both sections are matched and joined by guides to obtain the
casting mould.
• Finally, they are placed in a metal case, and surrounded by about 37 mm of
steel shot, sand, and other backup material to support them during pouring.
• Applications
1. Cylinders for air-cooled engines with tapered fins
2. cams,
3. camshafts
4. air compressor crank cases
5. pistons and piston rings

Advantages
1. Floor space required per ton of castings is less compared to conventional
castings.
2. Operators can be trained easily, thus, providing more output per operator.
3. Skilled operators are not required.
4. The process can be highly mechanised.

Disadvantages
1. High pattern cost.
2. High resin cost.
3. High equipment cost.

 Pressure die casting.
• Die: shaping device
• Die casting: casting which can be made by using metal dies.
• Die casting is the art to produce accurately dimensioned parts by
forcing molten metal under pressure into split metal dies which resemble a
common type of permanent mould.
• Because of the low temperature of the die (it is water-cooled), the casting
solidifies quickly, then die parts to be separated and the casting ejected.
• If the parts are small, several parts may be cast at one time in what is known as
multiple cavity die.
• This process is particularly suitable for lead, magnesium, tin, and zinc alloys.

Types
• Two main types of machines are used to produce die castings
(1) the hot chamber die casting
(2) the cold chamber die casting machine

 working / Process
• In a hot chamber submerged plunger-type machine, the plunger operates in
one end of a gooseneck casting which is submerged in the molten metal.
• With the plunger in the upper position, metal flow by gravity into this
casting through holes, just below the plunger and the entrapped liquid metal
is forced into the die through the gooseneck channel and in-gate.
• As the plunger retracts, the channel is again filled with the right amount of
molten metal.
• The plunger made of refractory material may be operated manually or
mechanically and hydraulically, that is by means of air pressure below 150
kgf/cm2 (about 15 MN/m2).
• Heating is continued throughout the operation to keep the molten metal
sufficiently liquid.

 Working /process
• In a horizontal plunger cold-chamber machine, the plunger is driven by air
or hydraulic pressure to force the charge into the die.
• As soon as the ladle is emptied, plunger moves to the left and forces the
metal into the cavity . After the metal solidified, the core is withdrawn, and
then the die is opened.
• Ejectors are employed to remove the casting automatically from the die.

Advantages of die casting are:
1. Very high rate of production is achieved.
2. Close dimensional tolerances of the order of ± 0.025 mm is possible.
3. Surface finish of 0.8 microns can be obtained.
4. Very thin sections of the order of 0.5 mm can be cast.
Disadvantages of this process are:
1. Not economical for small runs.
2. Only economical for nonferrous alloys.
3. Heavy castings cannot be cast
4. Cost of die and die casting equipment is high.

 Centrifugal casting
• In the centrifugal casting, molten metal is poured into moulds while they are
rotating.
• The metal falling into the centre of the mould at the axis of rotation is thrown
out by the centrifugal force under sufficient pressure towards the periphery, and
the impurities present being lighter in weight are also pushed towards the
centre.
• Solidification progresses from the outer surface to inwards, thus developing an
area of weakness in the centre of the wall.
• The use of gates, feeders, and cores is eliminated, making the method less
expensive and complicated.
• Centrifugal casting can be classified into three general types:
true centrifugal, semi centrifugal, and centrifuged.

True centrifugal casting
• This employs, moulds made up of steel (with a refractory mould wash
or even a green or dry sand lining) or of graphite. Which are rotational
symmetric.
• The melt is poured while the mould rotates at its axis, which may be
horizontal, vertical or inclined at any suitable angle between 0 to 90°,
although horizontal axis of rotation is a more common practice

• While rotating, the molten metal is carried to the walls of the cavity by
centrifugal force as shown in fig.
• The metal then solidifies forms a hollow casting without the use of a
central core.
• The outside of the mould is water-cooled to accelerate solidification
• This method is ideal for hollow cylindrical castings such as bushings, cast
iron pipes, etc.

Centrifuged
• In this process, several identical or nearly similar moulds are located
radially about a vertically arranged central riser or sprue or gate.
• which feeds the metal into the cavities through a number of radially gates.

• The entire mould is rotated with the central sprue which acts as the axis of
rotation. Thus, it is not a purely centrifugal process.
• This type of casting is suitable for small, intricate parts where feeding
problems are encountered.

Defects
• Causes And Remedies
1. Shifts. This is an external defect in a casting.
Cause:
• Due to core misplaced or mismatching of top and bottom parts of the
casting usually at a parting line.
• Misalignment of flasks is another likely cause of shift.
Remedy:
• By ensuring proper alignment of the pattern or die part,
• moulding boxes, correct mounting of patterns on pattern plates, and
• checking of flasks, locating pins, etc. before use.

2. Warpage.
Warpage is unintentional and undesirable deformation in a casting that
occurs during or after solidification.
Cause:
• Due to different rates of solidification at different sections of a casting
• Large and flat sections or intersecting sections such as ribs are particularly
prone to warpage.
Remedy:
• to produce large areas with wavy, corrugated construction, or add sufficient
ribs or rib-like shapes,
• to provide equal cooling rates in all areas.
• a proper casting design can go a long way in reducing the warpage of the
casting.

3. Swell.
• A swell is an enlargement of the mould cavity by metal pressure,
Cause: This is caused by improper or defective ramming of the mould.
Remedy: To avoid swells, the sand should be rammed properly and evenly.
4. Blowholes.
• Blow holes are smooth, round holes appearing in the form of a cluster of a large number of small
holes below the surface of a casting.
• These are entrapped bubbles of gases with smooth walls.
Cause:
• Excessive moisture in the sand.
• Sand grains are too fine.
• Sand is rammed too hard.
Remedy:
• To prevent blowholes, the moisture content in sand must be well adjusted.
• Sand of proper grain size should be used.
• Ramming should not be too hard.

5. Drop.
• A drop occurs when the upper surface of the mould cracks, and pieces of
sand fail into the molten metal.
Cause:
• This is caused by low strength and soft ramming of the sand,
• insufficient fluxing of molten metal and insufficient reinforcement of sand
projections in the cope.
Remedy:
• The above factors are eliminated to avoid drop.

Casting Processes and Defects

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