MEE 205 Fundamentals of Manufacturing Process Unit I Metal Casting Proce Process ss
Table of Contents
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Fundamentals of metal casting
Fluidity of molten metal
Sand casting
Shell mold casting
Investment casting
Plaster mold casting
Die casting
Defects in casting
Testing and inspection of casting MEE 205 Fundamentals of Manufacturing Process
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Table of Contents
2
Fundamentals of metal casting
Fluidity of molten metal
Sand casting
Shell mold casting
Investment casting
Plaster mold casting
Die casting
Defects in casting
Testing and inspection of casting MEE 205 Fundamentals of Manufacturing Process
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Fundamentals Fundamenta ls of Metal Casting
The casting process basically involves
Pouring of molten metal into a mold pattern after the part to be manufactured
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Allowing the molten metal to cool
Removing the metal from the mold
The important considerations in casting operations are
The flow of molten metal into the mold cavity
The solidification and and cooling of the metal in the mold
The influence of the type of mold material
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Fundamentals of Metal Casting – Gating system
[www.hackaday.io/project/2434/logs]
[http://nptel.ac.in/courses] 4
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Fundamentals of Metal Casting – Gating system
A good gating design should ensure proper distribution of molten metal without excessive temperature loss ,turbulence,
gas entrapping and slags
Very
slow
pouring,
require
longer
filling
time
and
solidification will start even before filling of mould
This can be restricted by using super heated metal, but in this case gas solubility will be a problem
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Faster pouring can erode the mould cavity MEE 205 Fundamentals of Manufacturing Process
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Fundamentals of Metal Casting – Gating system
So designing the gating system is important and it depends on the metal and molten metal composition. For example, aluminium can get oxidized easily
Gating systems refer to all those elements which are connected with the flow of molten metal from the ladle to the mould cavity
The purpose of gating system is to deliver the molten metal to mold
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Fundamentals of Metal Casting – Gating system
The elements of a gating system are
Pouring Basin
Sprue
Sprue Base Well
Runner
Runner Extension
Gate or Ingate
Riser [http://nptel.ac.in/courses]
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Characteristics of a gating system A gating system should be able to do the following
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Permit complete filling of the mold cavity
Requires minimum time to fill the mold cavity
Minimum turbulence so as to minimize gas pickup
Regulate rate at which molten metal enters the mold cavity
Prevent unwanted material from entering mould cavity MEE 205 Fundamentals of Manufacturing Process
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Characteristics of a gating system (contd..)
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Establish suitable temperature gradients
No mould erosion
Simple and economical design
Easy to implement and remove after solidification
Maximum casting yield
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Fluidity of molten metal
The capability of molten metal to fill mold cavity is called
fluidity . It consists of two basic factors
Characteristics of the molten metal ‒ Viscosity, Surface tension, Inclusions ‒ Solidification pattern of the metal / alloy
Casting parameters ‒ Mold design, Mold materials and surface characteristics, ‒ Rate of pouring ‒ Degree of superheat, Heat transfer
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Fluidity of molten metal (contd..) Characteristics of molten metal
Viscosity – resistance to flow – its sensitive to temperature (viscosity index) increases, fluidity decreases
Surface tension - the elastic tendency of liquids which makes them acquire the least surface area possible – high surface tension of the liquid metal reduces fluidity. Oxide films on the surface of molten metal have a significant adverse effect on fluidity Example: an oxide film on the surface of pure molten aluminium triples the surface tension
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Fluidity of molten metal (contd..)
Inclusions – can have adverse significant effect on fluidity. This effect can be verified by observing the viscosity of a liquid such as oil with and without sand particles in it
Solidification pattern of alloy – the manner in which solidification tales place, can influence fluidity. Fluidity is inversely proportional to freezing range. •
Shorter the range, higher the fluidity . Conversely lower fluidity for longer freezing range
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Fluidity of molten metal (contd..)
Mold design – the design and dimensions of the sprue, runners and risers all influence fluidity
Mold material and its surface characteristics – the higher the thermal conductivity of the mold and the rougher the surfaces, the lower the fluidity of the molten metal.
Degree of superheat – superheat improves fluidity by delaying solidification
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Fluidity of molten metal (contd..)
Rate of pouring – slower the rate of pouring molten metal into the mold, the lower the fluidity because of the higher rate of cooling
Heat transfer – factor affects directly the viscosity of the liquid metal
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Types of Casting process Sand casting Basic Permanent Mold casting g n i t s a c d l o m e l b a d n a p x E
Plaster Mold Casting Ceramic Mold Casting Shell Mold Casting Vacuum casting Expanded Polystyrene casting
Slush Casting g n i t s a c d l o m t n e n a m r e P
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Pressure Casting Vacuum Permanent Mold Casting Die casting Centrifugal Casting Centrifuge Casting Ingot Manufacture Continuous Casting
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Sand Casting
Sand casting typically relies on silica-based materials, such as synthetic or naturally-bonded sand
Casting sand generally consists of finely ground, spherical grains that can be tightly packed together into a smooth molding surface
The casting is designed to reduce the potential for tearing, cracking, or other flaws by allowing a moderate degree of flexibility and shrinkage during the cooling phase of the process
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Sand Casting Process (contd..)
The sand can also be strengthened with the addition of clay, which helps the particles bond more closely
Many automotive products, such as engine blocks and housings, are manufactured through sand casting
Its main advantages as a casting process include •
Relatively inexpensive production costs, especially in low-volume runs
•
The ability to fabricate large components
•
A capacity for casting both ferrous and non-ferrous materials, low cost for post-casting tooling
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Disadvantages of Sand Casting Process
Despite its benefits, sand casting yields a lower degree of accuracy than do alternate methods and it can be difficult to sand cast components with predetermined size and weight specifications
Furthermore, this process has a tendency to yield products with a comparatively rough surface finish
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Plaster Mold Casting Process
Plaster casting is similar to the sand casting process, using a mixture of gypsum, strengthening compound, and water in place of the sand
The plaster pattern is typically coated with an anti-adhesive compound to prevent it from becoming stuck against the mold, and the plaster is capable of filling in any gaps around the mold
Once the plaster material has been used to cast a part, it usually cracks or forms defects, requiring it to be replaced with fresh material
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Plaster Mold Casting Process (contd..)
The advantages offered by plaster casting include •
A very smooth surface finish
•
The ability to cast complex shapes with thin walls
•
The capacity for forming large parts with less expense than other processes, such as investment casting
•
A higher degree of dimensional accuracy than that of sand casting
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Disadvantages of Plaster Mold casting Process
This process tends to be more expensive than most sand casting operations, and may require frequent replacements of plaster molding material
It is usually more effective and cost-efficient when the quality of the surface finish is an important requirement
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Investment Casting Process
Investment casting uses a disposable wax pattern for each cast part.
The wax is either injected directly into the mold or precoated with a liquid refractory material before injection.
Molten forming material is then poured into the mold and allowed to harden and set into the shape of the wax pattern.
The component is then ejected, while the wax pattern is melted out of the casting and made available for reuse.
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Investment Casting Process (contd..)
Investment casting is often used to manufacture parts for the aerospace and automotive industries, as well the military
Some of the central advantages of using an investment casting process include A
high degree of accuracy and precise dimensional results
The
ability to create thin-walled parts with complex geometries
The capacity Relatively
for casting both ferrous and non-ferrous materials
high-quality surface finish
and detail in final
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Disadvantages of Investment Casting Process
Although it is highly precise, investment casting is usually more expensive than other comparable casting techniques, and is typically only cost-efficient when sand or plaster castings cannot be used
However, the expense can sometimes be compensated for with reduced machining and tooling costs due to investment castings’ quality surface results
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Shell Mold Casting Process
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Shell Mold Casting Process (contd..) 5
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Die Casting Process
Die casting is a method of molding materials under high pressure and usually involves non-ferrous metals and alloys, such as zinc, tin, copper, and aluminium
The mold is coated with lubricant to help regulate the die’s temperature and to assist with component ejection
Molten metal is then injected into the die under high pressure, which remains continuous until the work piece solidifies
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Die Casting Process (contd..)
This pressurized insertion is rapid, preventing any segment of the material from hardening before being cast
After the process is completed, the component is taken out of the die and any scrap material is removed
A few of the major advantages provided by die casting include •
Close size and shape tolerances
•
High component dimensional consistency and uniform design
•
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A reduced need for post-casting machining
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Disadvantages of Die casting Process
Despite its advantages, die casting has relatively high tool costs, making it more cost-efficient in high-volume product runs.
It can also be difficult to ensure the mechanical properties of a die cast component, meaning these products usually do not function as structural parts
Topics for self study: Ceramic Mold Casting and Centrifugal casting process
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Castings Inspection Methods Non-Destructive testing methods Non-destructive testing gives the metal casting facility the capability of assuring the quality of a casting without destroying it. 1. Visual Inspection Method - sand holes, excessively rough surface, surface shrinkage, blowholes, misruns, cold shuts, and surface dross or slag 2. Dimensional Inspection - To ensure a part meets dimensional requirements, such as tolerances, a metal casting facility can check the dimensional accuracy of a part manually or with a coordinate measuring machine (CMM). 30
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Castings Inspection Methods (contd..) 3. Dye Penetrant and Fluorescent Powder Testing
For tiny cracks, pores or other surface glitches that are hard to detect by the human eye, dye penetrant testing is used for both ferrous and nonferrous materials.
In this method, a colour dye solution is applied to the surface of the casting.
The dye, which is suspended in penetrating oil, will find its way into the surface defects.
When a special developer is applied, the defects are clearly indicated.
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Castings Inspection Methods (contd..) 4. Magnetic particle inspection is quick, inexpensive and sensitive to defects, particularly shallow (0.003 in.) surface cracks and other lineal indications.
It detects small cracks on or near the surface of ferrous alloys that can be magnetized (basically any ferrous alloy except austenitic material). A high-amperage, low-voltage current is passed through the casting, which establishes a magnetic field.
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Castings Inspection Methods (contd..) 5. Ultrasonic Testing - Internal defects that are detected by radiography may also be detected by sound.
In casting inspection, ultrasonic testing uses high frequency acoustic energy that is transmitted into a casting. Because ultrasonic testing allows investigation of the cross-sectional area of a casting, it is considered to be a volumetric inspection method.
The high frequency acoustic energy travels through the casting until it hits the opposite surface or an interface or defect.
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Castings Inspection Methods (contd..) 6. Eddy Current Inspection - The eddy current inspection method is applied to the detection of cracks at or near the surface.
An electrically charged coil carrying an alternate current causes an eddy current to flow in any nearby metal.
The eddy current may react on the coil to produce substantial changes in its reactivity and resistance , and that reaction is used to pinpoint small cracks or defects
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Castings Inspection Methods (contd..) 7. Pressure Leak Testing - When a casting is specified to be pressure tight or leak-proof, it is often tested by sealing openings in the casting and pressurizing it with air, inert gas or water.
When water, or hydrostatic, pressure is used, water seeping through the casting wall indicates leaks.
If air or gas pressure is used, the pressurized casting is put into a tank of clear water.
The appearance of bubbles indicates the air has penetrated through the casting wall and a leak is present.
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