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METAL CASTING
Dr.M. Bala Theja,M.Tech,Ph.D
Associate Professor
Department of Mechanical Engineering
CONTENTS
• Introduction
• Selection of casting processes
• General design considerations for casting
• Casting tolerances
• Simulation in casting design
• Product design rules for sand casting
INTRODUCTION
• The solidified piece of metal, which is taken out of the
mould is called “Casting”.
• A plant where the castings are made is called a
“Foundry”.
• The casting process is also called as “Founding”.
• The word “Foundry” is derived from Latin word
“Fundere” meaning “melting and pouring”.
Steps in casting process
1. Melting the metal.
2. Pouring it into a previously made mould or cavity
which conforms to the shape of the desired
component.
3. Allowing the molten metal to cool and solidify in the
mould.
4. Removing the solidified component from the mold,
cleaning it and subjecting it to further treatment, if
necessary
Advantages of casting process
1. Parts (both small and large) of intricate shapes can be
produced.
2. Almost all the metals and alloys and some plastics can
be cast.
3. A part can be made almost to the finished shape
before any machining is done.
4. Good mechanical and service properties
5. Mechanical and automated casting processes help
decrease the cost of castings.
6. The number of castings can vary from very few to
several thousands.
CLASSIFICATION OF CASTING PROCESS
SELECTION CASTING PROCESS
Application of casting materials
SELECTION OF CASTING PROCESS
The following are the important factors that
influence the selection of a casting process
• The number of castings to be processed
• The size and/or weight of the casting
• The shape and intricacy of the product
• The amount and quality of finish machining needed
SELECTION OF CASTING PROCESS
• The required surface finish
• The prescribed level of internal soundness (pressure
tightness) and/or the type and level of inspection to be
performed
• The permissible variation in dimensional accuracy for a
single part and part to part consistency through the
production run
_____________________________________________________
The casting characteristics of the copper alloy –
EXAMPLE given below
GENERAL DESIGN
CONSIDERATIONS OF CASTINGS
1. The shape of the castings should be as simple as
possible. That helps to reduce the cost of patterns,
cores and moulds
2. Casting should be made as compact as possible. Large
steel castings of complex shape are divided into two or
more castings, they can be joined by welding.
3. To facilitate removal, provision should be made of draft
(1/2 to 3 degrees) on the castings vertical surfaces.
The draft is greater for the inside surfaces than for
exterior surfaces.
GENERAL DESIGN
CONSIDERATIONS OF CASTINGS
4. Projecting details (bosses, lugs,etc) or undercuts
should be avoided or the pattern elements for them
should be made so that they do not hinder the removal
of pattern from the mold.
GENERAL DESIGN
CONSIDERATIONS OF CASTINGS
5. Wherever possible, avoid complex parting lines on the
pattern, because these increase the cost of moulding
operations. Parting lines should be in a single plane, if
practicable
GENERAL DESIGN
CONSIDERATIONS OF CASTINGS
6. Avoid concentration of metals so that no shrinkage
cavities are formed. For this reason, bosses, lugs, pads
should be avoided unless absolutely necessary. Metal
section is too heavy at bosses which is difficult to feed
solid
GENERAL DESIGN
CONSIDERATIONS OF CASTINGS
7. The position of the castings surfaces during metal
pouring must be taken into account, since gas blow
holes may form on the castings upper horizontal
surfaces. Critical surfaces of castings should lie at the
bottom part of the mold.
8. The thickness of the casting walls is determined
depending on the size and mass of the casting, its
material and the casting method. Inner sections of the
castings, resulting from complex cores, cool much
slower than outer sections and variation in strength
properties
9. Whenever feasible, the castings should be designed
with uniform section thickness, because shrinkage
defects( porosities, cracks) may arise in the thickened
portions. An abrupt change in section and sharp
corners act as stress risers in the finished casting,
create turbulence during pouring and hinder proper
feeding of the casting
GENERAL DESIGN
CONSIDERATIONS OF CASTINGS
10.The sharp corners are eliminated with a radius at the
corner from one-half to one-third of the section
thickness
GENERAL DESIGN
CONSIDERATIONS OF CASTINGS
GENERAL DESIGN
CONSIDERATIONS OF CASTINGS
11. Ribs are used for two purposes:
i) to increase stiffness ii) to reduce weight
• Thickness of the rib should be 0.8 times the casting
thickness.
• Ribs should be rounded at edges and correctly filleted.
• Avoid complex ribs. Wide and low ribs are safer than
thin and high ones.
• When two ribs cross each other, localized heavy cross-
sections result. This indicate hot spot where melt
solidifies only after adjacent zones have solidified,
resulting shrinkage cavities. They can be avoided by
offsetting the ribs.
GENERAL DESIGN
CONSIDERATIONS OF CASTINGS
• The staggered ribs cause less distortion than the
regularly spaced ribs.
• Ribs should be used chiefly for static loads. These
should be avoided where impact loads are expected
since these increase the rigidity of the parts.
GENERAL DESIGN
CONSIDERATIONS OF CASTINGS
12.Due to the same reasons as for cross ribs, bring or join
minimum number of sections together
13.Inside diameter of a cylinder or bushing should be
greater than the wall thickness of the casting. If it is
less, it is better to cast solid. Holes can be produced by
cheaper and safer methods than by coring.
14.Don’t use iron castings for impact and shock loading
15.Don’t use cast iron at temperature above 3000C, since
its strength decreases after 3000C.
GENERAL DESIGN
CONSIDERATIONS OF CASTINGS
16. A material that has a large solidification shrinkage will
result in hot-shortness(hot tears). The straight arms
can result in hot tears, but S-shaped arms can
straighten a little to accommodate the required
shortening on and after solidification
GENERAL DESIGN
CONSIDERATIONS OF CASTINGS
17.Provide places where holes are to be drilled, to
reinforce the walls of the casting, design (a) is not good,
because the drilled holes should be normal to the
surfaces, top and bottom to eliminate drill breakage.
Design(b) is much more satisfactory because it
eliminates drill breakage, reduces drilling time and
saves lot of material.
GENERAL DESIGN
CONSIDERATIONS OF CASTINGS
18.Load the iron castings in compression, as afar as
possible. Tensile and bending stresses can be
eliminated or minimized by proper design.
19.The casting shape should allow easy cut-off the gating
system elements and removal of cores.
20.It is preferable to dispose the entire casting in the drag
if the casting construction permits doing so. This helps
rule out mismatch.
21.The mould should have a minimum number of cores or
no cores at all, if possible. It is advisable to use
projection cods.
GENERAL DESIGN
CONSIDERATIONS OF CASTINGS
22. The cavities in castings should have extensions roomy
enough to receive the core prints of cores. It is
undesirable to support cores with chaplets since they
sometimes do not weld enough with the metal being
cast.
23. The casting design should provide for easy removal of
core materials and reinforcements and should make
for ease of cleaning and fettling after the shake out
operation. In order to remove core material from
internal cavities, special bosses with holes should be
provided on the casting. After the cleaning, the holes
are stopped with the plugs. The outer contour of the
casting should be free of deep blind pockets and
recesses. The cavities should have openings of
sufficient size of facilitate stripping.
GENERAL DESIGN
CONSIDERATIONS OF CASTINGS
24. Casting drawings should indicate locating surfaces
which are to be used in the machining of the castings
and also in the checking of the castings. The locating
surfaces should be formed by the pattern and should
lie in the same mould half, so that relative
displacements of mould parts and the cores do not
affect accuracy of these surfaces. These locating
surfaces are not needed for part functioning and can
be removed after machining when necessary
Examples of locating surfaces are:
a) Centre holes on shafts.
b) Centring recess 1 and end face 2 on the skirt of an
automotive engine piston. Fig (a)
GENERAL DESIGN
CONSIDERATIONS OF CASTINGS
c) Flats 1 of bosses 2 on cast blanks, provided for their
proper loading and clamping., Fig (b)
d) Bosses 1 on turbine blade blanks, Fig(c )
e) Two locating holes 1 on housing-type castings. Fig (d)
GENERAL DESIGN
CONSIDERATIONS OF CASTINGS
25.Cooling and solidification of molten metal start at the
surface of the mould cavity and crystals form there
first. Then the crystal growth takes place inwards,
normal to all surfaces. At the central plane of the
casting, the formations from different sides intersect.
The metal in this plane is weak because the crystals do
not join perfectly or even porosity may result there. In
a casting with sharp corners, fig (a), a plane of
weakness extends from corner to corner, exactly where
the stresses are apt to be maximum. In castings with
corners rounded off, Fig(b)
GENERAL DESIGN
CONSIDERATIONS OF CASTINGS
• the plane of weakness runs uniformly through out the
centre, where the stresses are extremely low. Also, the
heat flows propagating normally to the corner walls
intersect and develop a ‘hot zone’ within the corner
inner area. The corner walls here are made 20 to 25%
thinner than those in the areas farther from the corner.
26.The design of a cast part must take into account the
casting properties of the alloy being cast.
CASTING
TOLERANCES
CASTING TOLERANCES
• The lowest tolerances of all the casting processes are
maintained in die casting
• This process is limited to non ferrous metals and high
volume production
• Investment castings can also be produced with low
tolerances and applies to both ferrous as well as non
ferrous metals
• Of all the processes, sand casting requires the highest
tolerance
CASTING TOLERANCES
• And castings that use coarse sand molds generally
develop high surface roughness
• Die castings and investment castings have low
roughness since these do not use sand molds
SIMULATION IN CASTING DESIGN
• In this day and age, customers, especially in the
automotive industry, would be more likely to request
castings with high quality (Q), quick delivery(D) and at
a low cost(C).
• A tool that foundries may use to achieve the three goals
is to apply Computer Aided Engineering into their
process, in this case is by using computer simulation
software for casting
PROCEDURE OF USING CASTING
SIMULATION SOFTWARE
1. Build a model of the casting design including the gating
system and all other material used with the casting, such
as chills, cores, sleeves etc. This step may be done by
using a CAD system.
PROCEDURE OF USING CASTING
SIMULATION SOFTWARE
2. Input required data needed for computation, such
as the physical, mechanical and heat properties of the
metal, properties of the mold or die, pouring
temperature, pouring time, pressure, etc.
PROCEDURE OF USING CASTING
SIMULATION SOFTWARE
3. Computation of the simulation , which different
casting simulation programs may have different
approaches in simulation the results. Some well
known approaches for example are the numerical
simulation approaches(FEM, FDM), the geometrical
approach, the meshless method, etc
PROCEDURE OF USING CASTING
SIMULATION SOFTWARE
4. Simulated results and interpretation of results
The results from the simulation program may be shown
in the form of graphs or colored figures with numerical
results depending on what criterion is used, such as
the temperatures in each section of the casting at a
given time, solidification times, hot spots, material
density, etc. These results must be translated into
useful information to evaluate if a casting is sound or
not, or what must be done to improve the casting
design and start from step 1 once again
PROCEDURE OF USING CASTING
SIMULATION SOFTWARE
CASTING SIMULATION METHODS
1. Numerical approach
• Finite Element Method (FEM)
• Finite Difference Method (FDM)
2. Geometric approach
• K-Contour Method
3. Computer Wave Front Analysis
• Pour-out Method
• Cubic Spline Functions
4. Mesh less Method
5. Grid-based simulation system
LIMITATIONS OF CASTING SIMULATION
SOFTWARE
• The most accurate simulation results require that all input
parameters be set carefully to match the real casting,
including material, chemical composition, mold material,
poring temperature, pouring time, heat transfer coefficient
values, cooling curves, expansion and shrinkage rates etc.
• Casting simulation software cannot predict all types of
defects that may occur in a casting, such as processing
defects, human error, additional chemical elements added to
the molten metal etc.
• A foundry engineer must be able to notice what kinds of
defects occur and what causes them to occur so the problem
can be treated as casting simulation software can only
simulate the solidification process.
PRODUCT DESIGN RULES
FOR SAND CASTING
1. AVOID SHARP ANGLES AND MULTIPLE SECTION
JOINTS
• Solidification of the molten metal begins at the mold
face, from which crystals grow into the casting at right
angles.
• A straight section of constant thickness results in
uniform cooling which will in turn produce uniform
material properties.
• Sharp angles can cause large temperature variations in
the casting, which often lead to casting defects
PRODUCT DESIGN RULES
FOR SAND CASTING
PRODUCT DESIGN RULES
FOR SAND CASTING
• A well designed casting brings the minimum number of
sections together at intersections and avoids acute
angles.
• Wherever a number of sections converge, the
appropriate solution is to create a large hole like the
center of a web.
PRODUCT DESIGN RULES
FOR SAND CASTING
2. DESIGN SECTIONS OF UNIFORM THICKNESS
• Designing for uniform thickness also reduces the
amount of material in a casting, saving weight and
reducing machining, and results in a stronger casting.
• If larger masses of the metal are unavoidable, the
designer should make them accessible for feeding either
directly or with a riser.
• If section thickness are too small, then feeding
problems may occur.
• The increased cost of scrap caused by incomplete
feeding will normally be higher than the material
savings in a lighter casting.
PRODUCT DESIGN RULES
FOR SAND CASTING
PRODUCT DESIGN RULES
FOR SAND CASTING
3. PROPORTION INNER WALL THICKNESS
• Inner sections in a casting cool more slowly than a
section exposed to the mold face.
• If a complex geometry is necessary, the designer should
reduce the inner section thickness to 80% of the outer
wall thickness.
• Also, core section thicknesses should always be greater
than the section thickness of the surrounding metal.
• If the core is too small, it will become overheated and
slow down the solidification rate of the surrounding
metal, leading to the possibility of defects.
PRODUCT DESIGN RULES
FOR SAND CASTING
4. CONSIDER METAL SHRINKAGES IN DESIGN
• Almost all alloys shrink as they solidify
• While the patternmaker is the one affected by the
shrinkage, the designer must still compensate for it in
the design.
• Ina good design, the section thicknesses decrease as
the distance from the feed system or riser increases
• In order to accomplish this, the designer must be
familiar enough with the casting process to be able to
visualize how the casting will be fed and adjust the
castings dimensions to assist the metal flow.
• The greater the shrinkage of the metal, the more the
designer must consider it when designing the casting.
PRODUCT DESIGN RULES
FOR SAND CASTING
PRODUCT DESIGN RULES
FOR SAND CASTING
5. USE A SIMPLE PARTING LINE
• A flat plane, known as a straight parting line,
separating the two mold halves, results in more
economical casting than a tiered or contoured
separating surface.
• More complex parting lines often result in fewer parts
per mold, more costly patterns, less accuracy, and
increased scrap.
• The parting line should be positioned so that it has
minimal effect on the functional characteristics of the
part.
PRODUCT DESIGN RULES
FOR SAND CASTING
• Locating the parting line in less critical parts of the
casting is desirable for two main reasons
• First, dimensions around the parting line are the
hardest to control.
• Additionally flash occurs at the parting line
• If the surface around the parting line is not critical,
then flash removal costs will be lower.
PRODUCT DESIGN RULES
FOR SAND CASTING
6. DEFINE APPROPRIATE MACHINING ALLOWANCES
• The machining allowances is material added to the
casting to compensate for dimensional and surface
variations in the as-cast part.
• The amount of stock added is a function of the size of
the surface to the machined and to a lesser degree the
machining method and the final accuracy required
• Minimal additional material is needed if only flatness,
possibly with some un machined surface areas, is
desired.
• A large allowance is required if the full surface is to be
machined without any imperfections
• Normal machining allowances vary from 0.25cm fro
small castings(<15cm) to as much as 2.5 cm for larger
castings(>250cm).
PRODUCT DESIGN RULES
FOR SAND CASTING
7. USE ECONOMICAL TOLERANCES
• The tolerances achievable by a foundry vary depending
on the types of processes employed at the facility.
• Tighter tolerances may be obtained by machining,
which significantly increases the cost of the casting
• The most basic tolerance is the linear tolerance. It
refers to how precisely the distance between two points
can be produced
• Linear tolerances of +/- 1.0mm are readily achievable
for small castings.
• An additional factor of +/- 0.03mm should be added for
every centimeter over 15cm for larger parts

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Metal Casting.pptx

  • 1. METAL CASTING Dr.M. Bala Theja,M.Tech,Ph.D Associate Professor Department of Mechanical Engineering
  • 2. CONTENTS • Introduction • Selection of casting processes • General design considerations for casting • Casting tolerances • Simulation in casting design • Product design rules for sand casting
  • 3. INTRODUCTION • The solidified piece of metal, which is taken out of the mould is called “Casting”. • A plant where the castings are made is called a “Foundry”. • The casting process is also called as “Founding”. • The word “Foundry” is derived from Latin word “Fundere” meaning “melting and pouring”.
  • 4. Steps in casting process 1. Melting the metal. 2. Pouring it into a previously made mould or cavity which conforms to the shape of the desired component. 3. Allowing the molten metal to cool and solidify in the mould. 4. Removing the solidified component from the mold, cleaning it and subjecting it to further treatment, if necessary
  • 5. Advantages of casting process 1. Parts (both small and large) of intricate shapes can be produced. 2. Almost all the metals and alloys and some plastics can be cast. 3. A part can be made almost to the finished shape before any machining is done. 4. Good mechanical and service properties 5. Mechanical and automated casting processes help decrease the cost of castings. 6. The number of castings can vary from very few to several thousands.
  • 9. SELECTION OF CASTING PROCESS The following are the important factors that influence the selection of a casting process • The number of castings to be processed • The size and/or weight of the casting • The shape and intricacy of the product • The amount and quality of finish machining needed
  • 10. SELECTION OF CASTING PROCESS • The required surface finish • The prescribed level of internal soundness (pressure tightness) and/or the type and level of inspection to be performed • The permissible variation in dimensional accuracy for a single part and part to part consistency through the production run _____________________________________________________ The casting characteristics of the copper alloy – EXAMPLE given below
  • 11.
  • 12.
  • 13. GENERAL DESIGN CONSIDERATIONS OF CASTINGS 1. The shape of the castings should be as simple as possible. That helps to reduce the cost of patterns, cores and moulds 2. Casting should be made as compact as possible. Large steel castings of complex shape are divided into two or more castings, they can be joined by welding. 3. To facilitate removal, provision should be made of draft (1/2 to 3 degrees) on the castings vertical surfaces. The draft is greater for the inside surfaces than for exterior surfaces.
  • 14. GENERAL DESIGN CONSIDERATIONS OF CASTINGS 4. Projecting details (bosses, lugs,etc) or undercuts should be avoided or the pattern elements for them should be made so that they do not hinder the removal of pattern from the mold.
  • 15. GENERAL DESIGN CONSIDERATIONS OF CASTINGS 5. Wherever possible, avoid complex parting lines on the pattern, because these increase the cost of moulding operations. Parting lines should be in a single plane, if practicable
  • 16. GENERAL DESIGN CONSIDERATIONS OF CASTINGS 6. Avoid concentration of metals so that no shrinkage cavities are formed. For this reason, bosses, lugs, pads should be avoided unless absolutely necessary. Metal section is too heavy at bosses which is difficult to feed solid
  • 17. GENERAL DESIGN CONSIDERATIONS OF CASTINGS 7. The position of the castings surfaces during metal pouring must be taken into account, since gas blow holes may form on the castings upper horizontal surfaces. Critical surfaces of castings should lie at the bottom part of the mold. 8. The thickness of the casting walls is determined depending on the size and mass of the casting, its material and the casting method. Inner sections of the castings, resulting from complex cores, cool much slower than outer sections and variation in strength properties
  • 18. 9. Whenever feasible, the castings should be designed with uniform section thickness, because shrinkage defects( porosities, cracks) may arise in the thickened portions. An abrupt change in section and sharp corners act as stress risers in the finished casting, create turbulence during pouring and hinder proper feeding of the casting GENERAL DESIGN CONSIDERATIONS OF CASTINGS
  • 19. 10.The sharp corners are eliminated with a radius at the corner from one-half to one-third of the section thickness GENERAL DESIGN CONSIDERATIONS OF CASTINGS
  • 20. GENERAL DESIGN CONSIDERATIONS OF CASTINGS 11. Ribs are used for two purposes: i) to increase stiffness ii) to reduce weight • Thickness of the rib should be 0.8 times the casting thickness. • Ribs should be rounded at edges and correctly filleted. • Avoid complex ribs. Wide and low ribs are safer than thin and high ones. • When two ribs cross each other, localized heavy cross- sections result. This indicate hot spot where melt solidifies only after adjacent zones have solidified, resulting shrinkage cavities. They can be avoided by offsetting the ribs.
  • 21. GENERAL DESIGN CONSIDERATIONS OF CASTINGS • The staggered ribs cause less distortion than the regularly spaced ribs. • Ribs should be used chiefly for static loads. These should be avoided where impact loads are expected since these increase the rigidity of the parts.
  • 22. GENERAL DESIGN CONSIDERATIONS OF CASTINGS 12.Due to the same reasons as for cross ribs, bring or join minimum number of sections together 13.Inside diameter of a cylinder or bushing should be greater than the wall thickness of the casting. If it is less, it is better to cast solid. Holes can be produced by cheaper and safer methods than by coring. 14.Don’t use iron castings for impact and shock loading 15.Don’t use cast iron at temperature above 3000C, since its strength decreases after 3000C.
  • 23. GENERAL DESIGN CONSIDERATIONS OF CASTINGS 16. A material that has a large solidification shrinkage will result in hot-shortness(hot tears). The straight arms can result in hot tears, but S-shaped arms can straighten a little to accommodate the required shortening on and after solidification
  • 24. GENERAL DESIGN CONSIDERATIONS OF CASTINGS 17.Provide places where holes are to be drilled, to reinforce the walls of the casting, design (a) is not good, because the drilled holes should be normal to the surfaces, top and bottom to eliminate drill breakage. Design(b) is much more satisfactory because it eliminates drill breakage, reduces drilling time and saves lot of material.
  • 25. GENERAL DESIGN CONSIDERATIONS OF CASTINGS 18.Load the iron castings in compression, as afar as possible. Tensile and bending stresses can be eliminated or minimized by proper design. 19.The casting shape should allow easy cut-off the gating system elements and removal of cores. 20.It is preferable to dispose the entire casting in the drag if the casting construction permits doing so. This helps rule out mismatch. 21.The mould should have a minimum number of cores or no cores at all, if possible. It is advisable to use projection cods.
  • 26. GENERAL DESIGN CONSIDERATIONS OF CASTINGS 22. The cavities in castings should have extensions roomy enough to receive the core prints of cores. It is undesirable to support cores with chaplets since they sometimes do not weld enough with the metal being cast. 23. The casting design should provide for easy removal of core materials and reinforcements and should make for ease of cleaning and fettling after the shake out operation. In order to remove core material from internal cavities, special bosses with holes should be provided on the casting. After the cleaning, the holes are stopped with the plugs. The outer contour of the casting should be free of deep blind pockets and recesses. The cavities should have openings of sufficient size of facilitate stripping.
  • 27. GENERAL DESIGN CONSIDERATIONS OF CASTINGS 24. Casting drawings should indicate locating surfaces which are to be used in the machining of the castings and also in the checking of the castings. The locating surfaces should be formed by the pattern and should lie in the same mould half, so that relative displacements of mould parts and the cores do not affect accuracy of these surfaces. These locating surfaces are not needed for part functioning and can be removed after machining when necessary Examples of locating surfaces are: a) Centre holes on shafts. b) Centring recess 1 and end face 2 on the skirt of an automotive engine piston. Fig (a)
  • 28. GENERAL DESIGN CONSIDERATIONS OF CASTINGS c) Flats 1 of bosses 2 on cast blanks, provided for their proper loading and clamping., Fig (b) d) Bosses 1 on turbine blade blanks, Fig(c ) e) Two locating holes 1 on housing-type castings. Fig (d)
  • 29. GENERAL DESIGN CONSIDERATIONS OF CASTINGS 25.Cooling and solidification of molten metal start at the surface of the mould cavity and crystals form there first. Then the crystal growth takes place inwards, normal to all surfaces. At the central plane of the casting, the formations from different sides intersect. The metal in this plane is weak because the crystals do not join perfectly or even porosity may result there. In a casting with sharp corners, fig (a), a plane of weakness extends from corner to corner, exactly where the stresses are apt to be maximum. In castings with corners rounded off, Fig(b)
  • 30. GENERAL DESIGN CONSIDERATIONS OF CASTINGS • the plane of weakness runs uniformly through out the centre, where the stresses are extremely low. Also, the heat flows propagating normally to the corner walls intersect and develop a ‘hot zone’ within the corner inner area. The corner walls here are made 20 to 25% thinner than those in the areas farther from the corner. 26.The design of a cast part must take into account the casting properties of the alloy being cast.
  • 32. CASTING TOLERANCES • The lowest tolerances of all the casting processes are maintained in die casting • This process is limited to non ferrous metals and high volume production • Investment castings can also be produced with low tolerances and applies to both ferrous as well as non ferrous metals • Of all the processes, sand casting requires the highest tolerance
  • 33. CASTING TOLERANCES • And castings that use coarse sand molds generally develop high surface roughness • Die castings and investment castings have low roughness since these do not use sand molds
  • 34. SIMULATION IN CASTING DESIGN • In this day and age, customers, especially in the automotive industry, would be more likely to request castings with high quality (Q), quick delivery(D) and at a low cost(C). • A tool that foundries may use to achieve the three goals is to apply Computer Aided Engineering into their process, in this case is by using computer simulation software for casting
  • 35. PROCEDURE OF USING CASTING SIMULATION SOFTWARE 1. Build a model of the casting design including the gating system and all other material used with the casting, such as chills, cores, sleeves etc. This step may be done by using a CAD system.
  • 36. PROCEDURE OF USING CASTING SIMULATION SOFTWARE 2. Input required data needed for computation, such as the physical, mechanical and heat properties of the metal, properties of the mold or die, pouring temperature, pouring time, pressure, etc.
  • 37. PROCEDURE OF USING CASTING SIMULATION SOFTWARE 3. Computation of the simulation , which different casting simulation programs may have different approaches in simulation the results. Some well known approaches for example are the numerical simulation approaches(FEM, FDM), the geometrical approach, the meshless method, etc
  • 38. PROCEDURE OF USING CASTING SIMULATION SOFTWARE 4. Simulated results and interpretation of results The results from the simulation program may be shown in the form of graphs or colored figures with numerical results depending on what criterion is used, such as the temperatures in each section of the casting at a given time, solidification times, hot spots, material density, etc. These results must be translated into useful information to evaluate if a casting is sound or not, or what must be done to improve the casting design and start from step 1 once again
  • 39. PROCEDURE OF USING CASTING SIMULATION SOFTWARE
  • 40. CASTING SIMULATION METHODS 1. Numerical approach • Finite Element Method (FEM) • Finite Difference Method (FDM) 2. Geometric approach • K-Contour Method 3. Computer Wave Front Analysis • Pour-out Method • Cubic Spline Functions 4. Mesh less Method 5. Grid-based simulation system
  • 41. LIMITATIONS OF CASTING SIMULATION SOFTWARE • The most accurate simulation results require that all input parameters be set carefully to match the real casting, including material, chemical composition, mold material, poring temperature, pouring time, heat transfer coefficient values, cooling curves, expansion and shrinkage rates etc. • Casting simulation software cannot predict all types of defects that may occur in a casting, such as processing defects, human error, additional chemical elements added to the molten metal etc. • A foundry engineer must be able to notice what kinds of defects occur and what causes them to occur so the problem can be treated as casting simulation software can only simulate the solidification process.
  • 42. PRODUCT DESIGN RULES FOR SAND CASTING 1. AVOID SHARP ANGLES AND MULTIPLE SECTION JOINTS • Solidification of the molten metal begins at the mold face, from which crystals grow into the casting at right angles. • A straight section of constant thickness results in uniform cooling which will in turn produce uniform material properties. • Sharp angles can cause large temperature variations in the casting, which often lead to casting defects
  • 43. PRODUCT DESIGN RULES FOR SAND CASTING
  • 44. PRODUCT DESIGN RULES FOR SAND CASTING • A well designed casting brings the minimum number of sections together at intersections and avoids acute angles. • Wherever a number of sections converge, the appropriate solution is to create a large hole like the center of a web.
  • 45. PRODUCT DESIGN RULES FOR SAND CASTING 2. DESIGN SECTIONS OF UNIFORM THICKNESS • Designing for uniform thickness also reduces the amount of material in a casting, saving weight and reducing machining, and results in a stronger casting. • If larger masses of the metal are unavoidable, the designer should make them accessible for feeding either directly or with a riser. • If section thickness are too small, then feeding problems may occur. • The increased cost of scrap caused by incomplete feeding will normally be higher than the material savings in a lighter casting.
  • 46. PRODUCT DESIGN RULES FOR SAND CASTING
  • 47. PRODUCT DESIGN RULES FOR SAND CASTING 3. PROPORTION INNER WALL THICKNESS • Inner sections in a casting cool more slowly than a section exposed to the mold face. • If a complex geometry is necessary, the designer should reduce the inner section thickness to 80% of the outer wall thickness. • Also, core section thicknesses should always be greater than the section thickness of the surrounding metal. • If the core is too small, it will become overheated and slow down the solidification rate of the surrounding metal, leading to the possibility of defects.
  • 48. PRODUCT DESIGN RULES FOR SAND CASTING 4. CONSIDER METAL SHRINKAGES IN DESIGN • Almost all alloys shrink as they solidify • While the patternmaker is the one affected by the shrinkage, the designer must still compensate for it in the design. • Ina good design, the section thicknesses decrease as the distance from the feed system or riser increases • In order to accomplish this, the designer must be familiar enough with the casting process to be able to visualize how the casting will be fed and adjust the castings dimensions to assist the metal flow. • The greater the shrinkage of the metal, the more the designer must consider it when designing the casting.
  • 49. PRODUCT DESIGN RULES FOR SAND CASTING
  • 50. PRODUCT DESIGN RULES FOR SAND CASTING 5. USE A SIMPLE PARTING LINE • A flat plane, known as a straight parting line, separating the two mold halves, results in more economical casting than a tiered or contoured separating surface. • More complex parting lines often result in fewer parts per mold, more costly patterns, less accuracy, and increased scrap. • The parting line should be positioned so that it has minimal effect on the functional characteristics of the part.
  • 51. PRODUCT DESIGN RULES FOR SAND CASTING • Locating the parting line in less critical parts of the casting is desirable for two main reasons • First, dimensions around the parting line are the hardest to control. • Additionally flash occurs at the parting line • If the surface around the parting line is not critical, then flash removal costs will be lower.
  • 52. PRODUCT DESIGN RULES FOR SAND CASTING 6. DEFINE APPROPRIATE MACHINING ALLOWANCES • The machining allowances is material added to the casting to compensate for dimensional and surface variations in the as-cast part. • The amount of stock added is a function of the size of the surface to the machined and to a lesser degree the machining method and the final accuracy required • Minimal additional material is needed if only flatness, possibly with some un machined surface areas, is desired. • A large allowance is required if the full surface is to be machined without any imperfections • Normal machining allowances vary from 0.25cm fro small castings(<15cm) to as much as 2.5 cm for larger castings(>250cm).
  • 53. PRODUCT DESIGN RULES FOR SAND CASTING 7. USE ECONOMICAL TOLERANCES • The tolerances achievable by a foundry vary depending on the types of processes employed at the facility. • Tighter tolerances may be obtained by machining, which significantly increases the cost of the casting • The most basic tolerance is the linear tolerance. It refers to how precisely the distance between two points can be produced • Linear tolerances of +/- 1.0mm are readily achievable for small castings. • An additional factor of +/- 0.03mm should be added for every centimeter over 15cm for larger parts