Traditional and modern moulding methods

1. MOULDING METHODS

2. MOULDING PROCESSES
Casting processes can be classified into following FOUR categories:
1.Conventional Moulding Processes
a. Green Sand Moulding
b. Dry Sand Moulding
c. Flask less Moulding
2.Chemical Sand Moulding Processes
a. Shell Moulding
b. Sodium Silicate Moulding
c. No-Bake Moulding
3.Permanent Mould Processes
a. Gravity Die casting
b. Low and High Pressure Die Casting
4.Special Casting Processes
a. Lost Wax or Investment Casting
b. Ceramics Shell Moulding
c. Evaporative Pattern Casting
d. Centrifugal Casting

4. Green Sand Moulding
• Green sand is the most diversified moulding method used in metal casting
operations. The process utilizes a mould made of compressed or
compacted moist sand. The term "green" denotes the presence
of moisture in the moulding sand. The mould material consists of silica
sand mixed with a suitable bonding agent (usually clay) and moisture.
• This Method also Known as Expandable Moulding Process

5. • In this process, granular refractory sand is coated with a mixture of bentonite clay,
water and, in some cases, other additives The additives help to harden and hold the
mold shape to withstand the pressures of the molten metal.
• The green sand mixture is compacted by hand or through mechanical force around a
pattern to create a mold.
• The mechanical force can be induced by slinging, jolting, squeezing or by
impact/impulse.
• The following points should be taken into account when considering the green sand
molding process:
– For many metal applications, green sand processes are the most cost-effective of all metal forming
operations
– These processes readily lend themselves to automated systems for high-volume work as well as short runs
and prototype work
– In the case of slinging, manual jolt or squeeze molding to form the mold, wood or plastic pattern materials
can be used. High-pressure, high-density molding methods almost always require metal pattern
equipment;
– High-pressure, high-density molding normally produces a well-compacted mold, which yields better
surface finishes, casting dimensions and tolerances;
– The properties of green sand are adjustable within a wide range, making it possible to use this process
with all types of green sand molding equipment and for a majority of alloys poured.

6. Green Sand Moulding
Advantages
• Most metals can be cast by this method. Pattern costs and material costs
are relatively low. No Limitation with respect to size of casting and type of
metal or alloy used.
Disadvantages
• Surface finish of the castings obtained by this process is not good and
machining is often required to achieve the finished product.

7. Dry Sand Moulding
• Dry sand casting is a sophisticated form of green sand process, in which the
sand mold is baked at a given temperature (110 -260 0 c)to make it stronger.
• This process in mostly used in large foundries to produce big ferrous and non-
ferrous castings like engine blocks, construction parts, etc.
• Dry sand casting ensures precise size and perfect dimensions of the metal
casting products.
• These are Basically Two types of drying of moulds are often required.
1. Skin drying and 2. Complete mould drying
• In skin drying mould is partially dried and a firm mould face is produced.
Shakeout of the mould is almost as good as that obtained with green sand
moulding. The most common method of drying the refractory mould coating
uses hot air, gas or oil flame. Skin drying of the mould can be accomplished
with the aid of torches, directed at the mould surface.
• In Complete mould drying entire mould is dried out.

8. Shell Moulding

9. Shell moulding
• The shell moulding is a casting process in which the mould is a thin shell of
9 mm thick.
• This is made of sand held together by thermosetting resin binder.
• A metal pattern is heated and placed over a box containing sand mixed
with thermosetting resin The dump box is inverted so that sand and resin
mixture fall on the hot pattern, causing a layer of the mixture to partially
cure on the pattern surface to form a hard shell The box is positioned to
the previous stage, so that loose, uncured particles drop away
• Sand shell is heated in oven for several minutes to complete curing.
• The shell mold is removed from the pattern and two halves of the shell
mold are assembled, supported by sand or metal shot in a box, and
pouring is completed

10. Advantages
• The surface of the shell mould is smoother than conventional green sand
mould. This permits easier flow of molten metal during pouring and better
surface finish on the final casting.
• Surface finish of the order of 2.5 μm can be obtained. Good dimensional
tolerances of the order of ± 0.25 mm can be reached in a small to medium
sized parts.
• Machining operations are reduced because of good surface finish.
• Can be mechanized for mass production and will be economical too.
Disadvantages
• Expensive metal pattern is required, and hence not suitable for small
quantities.
Examples of parts made using shell molding include gears, valve bodies,
bushings, and camshafts.

11. Nobake or Airset Systems
• In order to improve productivity and eliminate the need for heat or gassing to cure mold and
core binders, a series of resin systems referred to as nobake or airset binders was developed.
• In these systems, sand is mixed with one or two liquid resin components and a liquid catalyst
component. As soon as the resin(s) and catalyst combine, a chemical reaction begins to take
place that hardens (cures) the binder.
• The curing time can be lengthened or shortened based on the amount of catalyst used and
the temperature of the refractory sand.
• The mixed sand is placed against the pattern or into the core box. Although the sand mixtures
have good flowability, some form of compaction (usually vibration) is used to provide
densification of the sand in the mold/core.
• After a period of time, the core/mold has cured sufficiently to allow stripping from the core
box or pattern without distortion.
• The cores/ molds are then allowed to sit and thoroughly cure. After curing, they can accept a
refractory wash or coating that provides a better surface finish on the casting and protects the
sand in the mold from the heat and erosive action of the molten metal as it enters the mold
cavity.

12. The NoBake process Advantages
• The capability to use wood, and in some cases, plastic patterns and core
boxes.
• Good casting dimensional tolerances due to the rigidity of the mold
• Good casting finishes
• Typically easy shakeout (the separation of the casting from the mold after
solidification is complete)
• The ability to store cores and molds indefinitely.

13. Investment Casting

14. Investment casting
• In this casting process, a pattern made of wax is coated with a refractory
material to make the mold surface, after which the wax is melted away
while pouring the molten metal. “Investment” means “to cover
completely” which refers to the coating of the refractory material around
the wax pattern. This is a precision casting process. Using this we can
make castings of high accuracy with intricate details.
• Wax patterns are first made
• Several patterns can be attached to a sprue to form a pattern tree, if
required
• The pattern tree is coated with a thin layer of refractory material and later
covered with thick coating to make the rigid full mold

15. • Heating of mold in inverted position to melt the wax and permit it to drip
out of the cavity
• The mold is preheated to a high temperature so that contaminants are
eliminated from the mold
• The molten metal is poured and it solidifies
• The mold is removed from the finished casting
Refractory coating
• Slurry of very fine grained silica or other refractory, in powder form, mixed
with plaster to bond the mold into shape. The small grain size of the
refractory material delivers smooth surface and captures the intricate
depths of the wax pattern.
• Mold is allowed to dry in air for about 8 hours to harden the binder.

16. Advantages
• Complex and intricate parts can be cast
• Tolerances of 0.075 mm are possible
• Good surface finish is possible
• In general, additional machining is not required – near net shaped part
Applications
Steels, stainless steels, high temperature alloys can be cast - Examples of parts:
machine parts, blades, components for turbine engines, jewellery, dental fixtures

17. Permanent mold process
Disadvantage of expendable molding processes is that for every casting a new
mold is required.
Here in Permanent mold processes it uses:
• Only metal mold for casting
• Molds are generally made of steel, CI
• Materials that can be cast: Al, Mg, Cu based alloys, CI (affect the mold life,
hence not used)
• Cores are also made of metal, but if sand is used then called semi
permanent-mold casting
Advantages
Good surface finish, dimension tolerance, rapid solidification causes fine
grains to form giving stronger products
Limitations
Restricted to simple part geometries, low melting point metals, mold cost is
high. Best suitable for small, large number of parts

19. Gravity Die Casting
• A form of permanent mold casting is when the molten metal is poured into the mold,
either directly or by tilting the mold into a vertical position.
• In this process, the mold is made in two halves from cast iron or steel. If cores are to be
used, they can be metal inserts, which operate mechanically in the mold, or sand cores,
which are placed in the molds before closing (semi-permanent molding).
• The mold halves are preheated and the internal surfaces are coated with a refractory.
• If static pouring is to be used, the molds are closed and set into the vertical position for
pouring; thus, the parting line is in the vertical position.
• In tilt pouring, the mold is closed and placed in the horizontal position at which point
molten metal is poured into a cup(s) attached to the mold. The mold then is tilted to
the vertical position, allowing the molten metal to flow out of the cup(s) into the mold
cavity.

20. Advantages
• Superior mechanical properties because the metal mold acts as a chill
• Uniform casting shape and excellent dimensional tolerances because
molds are made of metal
• Excellent surface finishes
• High-production runs
• The ability to selectively insulate or cool sections of the mold, which helps
control the solidification and improves overall casting properties.

21. Die Casting
In this process, high pressure of app. 7 to 350 MPa is used to pressurize the
molten metal into die cavity. The pressure is maintained during solidification.
Category
• Hot chamber machines Die casting
• Cold chamber machines Die casting
Hot chamber machines
• Molten metal is melted in a container attached to the machine, and a piston
is used to pressurize metal under high pressure into the die.
• Typical injection pressures are between 7 and 35 MPa.
• Production rate of 500 parts/hour are common.
• Injection system is submerged into the molten metal and hence pose problem
of chemical attack on the machine components. Suitable for zinc, tin, lead,
Mg.

23. Cold chamber machines
Molten metal is poured from an external unheated container into the mold cavity
and piston is used to inject the molten metal into the die cavity.
• Injection pressure: 14 to 140 MPa.
• Though it is a high production operation, it is not as fast as hot chamber
machines.
• Die casting molds are made of tool steel, mold steel, maraging steels. Tungsten
and molybdenum with good refractory qualities are also used for die cast
steel, CI.
Advantages of Die Casting
• High production rates and economical
• Close tolerances possible of the order of ±0.076 mm
• Thin section with 0.5 mm can be made
• Small grain size and good strength casting can be made because of rapid
cooling
Disadvantage
• Power requirement is high

25. Low pressure casting
• In the earlier casting process, metal flow in mold cavity is by gravity pull or
by applying pressure, but in low pressure casting, liquid metal is forced
into the cavity under low pressure, app. 0.1 MPa, from beneath the
surface so that metal flow is upward.
Advantage
• molten metal is not exposed to air, gas porosity and oxidation defects are
minimized Vacuum permanent mold casting: variation of low pressure
casting, but in this vacuum is used to draw the molten metal into the
mold cavity

26. Centrifugal casting
In this method, the mold is rotated at high speed so that the molten metal is distributed by
the centrifugal force to the outer regions of the die cavity
Types : True Centrifugal Casting, Semi-centrifugal Casting
Orientation of the mold can be Horizontal or Vertical
True centrifugal casting
• Molten metal is poured into a rotating mold to produce a tubular part (pipes, tubes,
bushings, and rings)
• Molten metal is poured into a horizontal rotating mold at one end. The high-speed
rotation results in centrifugal forces that cause the metal to take the shape of the mold
cavity. The outside shape of the casting can be non-round, but inside shape of the
casting is perfectly round, due to the radial symmetry w.r.t. forces

28. If the G-factor is very less, because of the reduced centrifugal
force, the liquid metal will not remain forced against the mold
wall during the upper half of the circular path but will go into the
cavity. This means that slipping occurs between the molten
metal and the mold wall, which indicates that rotational speed of
the metal is less than that of the mold.

29. Vertical centrifugal casting
• In this because of the effect of gravity acting on the liquid metal, casting
wall will be thicker at the base than at the top. The difference in inner and
outer radius can be related to speed of rotation as,
• It is observed from the eqn. that for Rit = Rib, the speed of rotation N will
be infinite, which is practically impossible.
• Solidification shrinkage at the exterior of the cast tube will not be an issue,
because the centrifugal force continually moves molten metal toward the
mold wall during freezing. Impurities in the casting will be on the inner
wall and can be removed by machining after solidification.

30. Squeeze casting
• Also known as liquid-metal forging
• Near net shape process - the initial production of the item is very close to the final
(net) shape, reducing the need for surfacefinishing
• Combination of casting and forging
• Squeeze casting is simple and economical, efficient in its use of rawmaterial
• The process generates the highest mechanical properties attainable in acast product
• The process was introduced in the United States in 1960 and has since gained
widespread acceptance

31. • The squeeze casting process uses an accurately measured or metered
quantity of molten metal which is poured into a heated mould via alaunder
• The mould is closed to produce an internal cavity in the shape of the
required component
• The mould is coated with a suitable release agent and for squeeze casting it
is usually in the form of a graphite coating
• Pressure continues to be applied to the molten metal until it has solidified
and forms the required component
• The press is then withdrawn and the component isejected

32. • Non ferrous alloys like aluminum, magnesium, and copper alloy components are
readily manufactured using this process
• The squeeze casting process, combining the advantages of the casting and
forging processes, has been widely used to produce qualitycastings
• Because of the high pressure applied during solidification, porosities caused by
both gas and shrinkage can be prevented oreliminated
• The cooling rate of the casting can be increased by applying high pressure during
solidification, since that contact between the casting and the die is improved by
pressurization, which results in the foundation of fine-grainedstructures

34. Advantages
• Offers a broader range of shapes and components than other manufacturing
methods
• Little or no machining required post casting process
• Low levels of porosity
• Good surface texture
• Fine micro-structures with higher strength components
• No waste material, 100% utilization

35. Disadvantages
• Costs are very high due to complextooling
• No flexibility as tooling is dedicated to specific components
• Process needs to be accurately controlled which slows the cycle time down
and increases process costs.
• High costs mean high production volumes are necessary to justify
equipment investment

36. Thixocasting
• Thixo is a general term used to describe the near-net shape forming
process from a partially melted non dendrite alloy within a metal die.
• Thixotropy- viscosity decreases with time due to constant agitation or
strain.
• If component shaping is performed in a close die it is called thixocasting.
• If component shaping is performed in an open die it is called
thixoforming.

38. Process
Thixocasting consists of three separate stages:
• The production of a pre-cast billet having special globular microstructure.
• The re-heating of these billets to the semi solid casting temperature.
• Casting of component by pressing these billets to die cavity.

39. Process
• In Thixocasting, a solid billet with a globular grain structure is prepared by
stirring the melt first while the billet is being cast, and then supplied to
the die caster.
• This billet is then reheated to the semi-solid temperature (between the
solidus and liquidus temperatures of the alloy) by the die caster, using an
induction furnace.
• It is then placed in a shot sleeve and injected into the die cast machine.
• Since the melt is partly solidified, proper thermal control of the die is
essential to ensure proper filling of the die cavity.

40. Advantages
• Energy efficiency: metal is not being held in the liquid state over long periods of
time.
• Production rates are similar to pressure die casting or better.
• Smooth filling of the die with no air entrapment and low shrinkage porosity gives
parts of high integrity (including thin-walled sections) and allows application of the
process to higher-strength heat-treatable alloys.
• Lower processing temperatures reduce the thermal shock on the die, promoting die
life and allowing the use of non-traditional die materials.

41. • Fine, uniform microstructures give enhanced properties.
• Reduced solidification shrinkage gives dimensions closer to near net shape and justifies
the elimination of machining steps.
• Surface quality is suitable for plating.
• Thixocasting has quite a low yield strength, high fluidity, low forming load and low surface
roughness.
• Especially in the thixocasting process, a complex geometry product can be obtained by
only one step forming.
• This technology has been widely applied in nonferrous metal forming and satisfactory
results were derived, but not with ferrous metal.

42. Disadvantages
• The cost of raw material for thixoforming can be high and the number of
suppliers small.
• Process knowledge and experience has to be continuously built up in
order to facilitate application of the process to new components.
• Personnel requires a higher level of training and skill than with more
traditional processes.
• Temperature control: the solid fraction and viscosity in the semi-solid state
are very dependent on temperature.

43. Applications
• Semi-solid processing is ideally suited for the production of large volume
die casting components, including light weight, high strength components
for automobiles.
• For aluminum alloys typical parts include engine suspension mounts, air
manifold sensor harness, engine blocks and oil pump filter housing.
• For magnesium alloys, semi-solid casting is typically used to produce
extremely thin walled castings, such as computer and camera bodies.
Because of this the process can be applied to rapid prototyping needs
and mass production.

45. Rheo-Casting Process
• The Rheo Casting process involves using slurry in a semi solid state with the amount of
benefits directly linked to the fraction solid at the time of casting.
• Rheo-casting involves preparation of SSM slurry directly from the liquid alloy, followed by
a forming process such as High Pressure Die Casting (HPDC). With “Rheo” processes the
alloy is cooled into a semi-solid state and then is introduced into a die without the
presence of an intermediate solidification step; semi-solid slurry with non-dendritic solid
particles is produced from a fully liquid regular alloy. It is cooled to obtain the desired
fraction solid and then it is cast into a part. Component shaping directly from SSM slurries
is inherently attractive due to its characteristics, such as overall efficiency in production
and energy management.
• A critical advantage of rheo-casting is the ability to cast the metal at a wide range of
fraction solids. The majority of the process advantages of using non-dendritic, semi-solid
alloys are dependent on the amount of solid at the time of casting. Reduction of
shrinkage, a decreased amount of latent heat, and the magnitude of viscosity are
dependent upon and increase of the percentage of solid in the alloy.