Ch 5 Casting processes - kau 5 Casting processes.pdfMachining allowance Shrinkage allowance Metal 3.0mm % 2.0 Steel 3.0mm % 1.0 cast iron 1.5mm % 1.5 copper 1.5mm % 1.25 Aluminum 1.5mm

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Dr. Abdel-Wahab El-Morsy Faculty of Engineering - Rabigh

Basic WorkshopBasic Workshop

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Plastic Deformation Process

� Casting (Liquid state)

� Forging (solid state)

� Rolling (solid state)

� Deep Drawing (solid state)

� Extrusion (solid state)

� Wire Drawing (solid state)

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Casting is a manufacturing process by which a liquid material is usually

poured into a mold, which contains a hollow cavity of the desired shape, and

then allowed to solidify. The solidified part is also known as a casting, which

is ejected or broken out of the mold to complete the process.

Casting Process

Casting processes are divided according to the specific

type of molding method used in casting, as follows:

1. Sand casting2. Centrifugal casting3. Permanent casting4. Die casting5. Investment casting


Sand CastingSand casting consists basically of pouring molten metal into appropriate

cavities formed in a sand mold. The sand may be natural, synthetic, or an

artificially blended material.

Casting Process

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Mold Making

Melting and pouring the metals

Removal and Trimming

Casting Inspection

Sand Casting

Pattern makingPrepare the sand

Casting Process


Sand Casting

Casting Process

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Sand Casting

Casting Process


The main tooling for sand casting is the pattern that is used to create the mold

cavity. The pattern is a full size model of the part that makes an impression in

the sand mold. However, some internal surfaces may not be included in the

pattern, as they will be created by separate cores.

Pattern

The pattern is actually made to be slightly larger than the part because the casting

will shrink inside the mold cavity. Also, several identical patterns may be used to

create multiple impressions in the sand mold, thus creating multiple cavities that

will produce as many parts in one casting

Several different materials can be used to fabricate a pattern, including:

• Wood,

• Plastic, and

• Metal.

Casting Process

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A pattern can be classified into four types:Solid pattern

A solid pattern is a model of the part as a single piece. It is the easiest

to fabricate, but can cause some difficulties in making the mold.

Split pattern

A split pattern models the part as two separate pieces that meet along

the parting line. Using two separate pieces allows the mold cavities in

the cope and drag to be made separately and the parting line is already

determined

Match-plate pattern

A match-plate pattern is similar to a split pattern, except that each half

of the pattern is attached to opposite sides of a single plate.

Cope and drag pattern

A cope and drag pattern is similar to a match plate pattern, except that

each half of the pattern is attached to a separate plate and the mold

halves are made independently.

Pattern

Casting Process


Shrinkage

Allowances must be made on patterns to counteract the contraction in size as the

metal cools. The amount of shrinkage is dependent on the design of the coating,

type of metal used, solidification temperature, and mold resistance.

Machining

Allowances are required in many cases because of unavoidable surface impurities,

warpage, and surface variations.

Length of pattern required

L = L0 (1 + ∆∆∆∆)

L = pattern length

L0 = part length

∆ = shrinkage allowance

Casting Process

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Machining allowance Shrinkage allowance Metal

3.0 mm 2.0 % Steel

3.0 mm 1.0 % cast iron

1.5 mm 1.5 % copper

1.5 mm 1.25 % Aluminum

1.5 mm 1.5 % Tin

1.5 mm 1.5 % Zinc

Pattern Making

Casting Process


The Process Cycle for Sand Casting

� The first step in the sand casting is to create the mold for the casting and this step must

be performed for each casting.

� A sand mold is formed by packing sand into each half of the mold around the pattern.

� When the pattern is removed, the cavity that will form the casting remains.

� Any internal features of the casting that cannot be formed by the pattern are formed by

separate cores which are made of sand.

� The use of a lubricant also improves the flow the metal and can improve the surface

finish of the casting.

� The cores are positioned and the mold halves are closed and securely clamped

together. It is essential that the mold halves remain securely closed to prevent the loss

of any material

Mold-Making

Casting Process

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Sand

The sand that is used to create the molds is typically silica sand that is mixed with a type of

binder to help maintain the shape of the mold cavity. Sand is inexpensive and is resistant to

high temperatures, allowing many metals to be cast that have high melting temperatures

• Greensand mold - Greensand molds use a mixture of sand (90%), water (3%), and a clay

or binder (7%) and are the least expensive and widely used.

• Skin-dried mold - A skin-dried mold begins like a greensand mold, but additional bonding

materials are added and the cavity surface is dried. Dry skin molds are more expensive

and require more time.

• Dry sand mold - In a dry sand mold (or cold box mold), the sand is mixed only with an

organic binder. The mold is strengthened by baking it in an oven. The resulting mold has

high dimensional accuracy, but is expensive.

• No-bake mold - The sand in a no-bake mold is mixed with a liquid resin and hardens at

room temperature.


Casting Process


The quality of the sand that is used also greatly affects the quality of the casting and is

usually described by the following five measures

� Strength - Ability of the sand to maintain its shape.

� Permeability - Ability to allow venting of trapped gases through the sand. A higher

permeability can reduce the porosity of the mold, but a lower permeability can result in a

better surface finish.

� Thermal stability - Ability to resist damage, such as cracking, from the heat of the molten

metal.

� Collapsibility - Ability of the sand to collapse or compress, during solidification of the

casting. If the sand can not compress, then the casting will not be able to shrink freely in

the mold and can result in cracking.


Sand

Casting Process

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Sand casting is able to make use of almost any alloy. An advantage of sand casting is

the ability to cast materials with high melting temperatures, including steel, nickel, and

titanium. The four most common materials that are used in sand casting are shown

below, along with their melting temperatures.

Materials Melting temperature

Aluminum alloys 660°C

Brass alloys 1082°C

Cast iron 1088-1260°C

Cast steel 1371°C

Materials


Casting Process


� After the mold has been clamped, the molten metal can be ladled from its holding

container in the furnace and poured into the mold.

� Enough molten metal must be poured to fill entire cavity and all channels in the mold.

� The filling time is very short in order to prevent early solidification of any one part of

the metal.

Pouring and Cooling

� When the entire cavity is filled and the molten metal solidifies, the final shape of the

casting is formed.

� The mold can not be opened until the cooling time has elapsed. The desired cooling

time can be estimated based upon the wall thickness of the casting and the

temperature of the metal.

� If some of the molten metal cools too quickly, the part may exhibit shrinkage, cracks,

or incomplete sections.


Casting Process

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� After the solidification, the mold can simply be broken, and the casting removed.

� This step (shakeout) is typically performed by a vibrating machine that shakes the sand

and casting out of the flask.

� Shot blasting is sometimes used to remove any remaining sand, especially from internal

surfaces, and reduce the surface roughness.

� During cooling, the material from the channels in the mold solidifies attached to the part.

This excess material must be trimmed from casting either manually via cutting or sawing,

or using a trimming press.

� The scrap material that results from this trimming is either discarded or reused in the sand

casting process.

� The scrap may need to be reconditioned to proper chemical composition before it can be

combined with non-recycled metal and reused.

Removal and Trimming


Casting Process


Pattern Making

Casting Process

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Pattern Making

Casting Process


� A pattern must have a draft if it is to be pulled from the sand without

damaging the mold.

� The draft : 0.25 – 1.0°.

Pattern Making

� Sharp inside corners are to be avoided whenever possible.

� When they do occur they may be rounded off with fillets

made of wax or wood.

Casting Process

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Runners & Channels

Casting Process


ChannelsChannelsRunners & Runners &

Casting Process

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ChannelsChannelsRunners & Runners &

Casting Process


Explain using clean schematic diagrams the steps of mold preparation for

casting a hollow cylinder with an external diameter of 120mm, an internal

diameter of 70mm and a length of 120 mm.

Example

Casting Process

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100

75

25

10

0

50

Example

Casting Process


Example

100+2

50+

125

+0.

5

100+

2

25+

0.5

Casting Process

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Die Casting

� Die casting is a manufacturing process that can produce geometrically complex metal

parts through the use of reusable molds, called dies.

� The die casting involves the use of a furnace, metal, die casting machine, and die.

� The metal, typically a non-ferrous alloy such as Al or zinc, is melted in the furnace

and then injected into the dies in the die casting machine.

� There are two main types of die casting machines

� Hot chamber (used for alloys with low melting temperatures, such as Zn),

� Cold chamber (used for alloys with high melting temperatures, such as Al)

In both machines, after the molten metal is injected into the dies, it rapidly cools and solidifies into

the final part, called the casting.

Casting Process


� The first step is preparation and clamping of the two halves of the die.

� After lubrication, two die halves are closed and securely clamped.

� Sufficient force must be applied to the die to keep it securely closed .

Clamping

� The metal is transferred into a chamber where it can be injected into die.

� Once transferred, molten metal is injected at high pressures into the die.

� This pressure holds the molten metal in the dies during solidification.

Injection

� Molten metal will begin to cool and solidify once it enters the die cavity.

� When the cavity is filled and the metal solidifies, final shape is formed.

Cooling

Die Casting

Casting Process

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Die Casting

� The die halves can be opened and an ejection can push the part out.

� The ejection must apply some force to eject the part because during

cooling the part shrinks and adheres to the die.

Ejection

� During cooling, the material in channels will solidify attached to casting.

� Excess material, along with any flash, must be trimmed from the casting.

Trimming

Casting Process


The steps in Die Casting process

Die Casting

Casting Process

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Die Casting

Casting Process


Defect Causes

Flash� Injection pressure too high

� Clamp force too low

Unfilledsections

� Insufficient shot volume

� Slow injection

� Low pouring temperature

Bubbles� Injection temperature too high

� Non-uniform cooling rate

Hot tearing � Non-uniform cooling rate

Ejector marks� Cooling time too short

� Ejection force too high

Die Casting

Casting Process

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The permanent mold casting process consists of the following steps:

� Mold preparation - First, the mold is pre-heated (150-250°C) to allow better metal flow and

reduce defects. Then, a coating material is applied to the cavity surfaces to facilitate part

removal and increase mold lifetime.

� Mold assembly - The mold consists of at least two parts. In this step, the mold halves are

clamped together.

� Pouring - The molten metal is poured at a slow rate into the mold through a sprue at the top

of mold. The metal flows through a runner system and enters the mold cavity.

� Cooling - The molten metal is allowed to cool and solidify in the mold.

� Mold opening - After the metal has solidified, the two mold halves are opened and the casting

is removed.

� Trimming - During cooling, the metal in the runner system and sprue solidify attached to the

casting. This excess material is now cut away.

Permanent Mold Casting

Casting Process



� Permanent mold casting is a metal casting process that shares similarities to both sand and

die casting.

� Sand casting uses an expendable mold which is destroyed after each cycle.

� Permanent mold casting, like die casting, uses a metal die that is typically made from steel

or cast iron and can be reused for several thousand cycles.

� Because the molten metal is poured into the die, permanent mold casting is often referred to

as gravity die casting

� Permanent mold casting is typically used for high-volume production of small, simple metal

parts with uniform wall thickness.

� Non-ferrous are typically used in this process, such as Al-alloys, Mg-alloys, and Cu-alloys.

� Irons and steels can also be cast using graphite molds.

� Common permanent mold parts include gears and gear housings, and other automotive and

aircraft components such as pistons and wheels

Casting Process

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Casting Process



Advantages:

� Can form complex shapes

� Good mechanical properties

� Many material options

� Low porosity

� Scrap can be recycled

Disadvantages:

� High tooling cost

� Long lead time possible

Casting Process

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Centrifugal Casting

� Centrifugal casting is a metal casting process that uses centrifugal force to form

cylindrical parts. This differs from most metal casting processes, which use gravity

or pressure to fill the mold.

� In centrifugal casting, a permanent mold made from steel, cast iron, or graphite is

typically used. However, the use of expendable sand molds is also possible.

� Casting process is usually performed on a horizontal centrifugal casting machine

(vertical machines are also available). and includes the following steps:

� Centrifugal casting is used to produce axisymmetric parts, such as cylinders or disks,

which are typically hollow.

� Due to the high centrifugal forces, these parts have a very fine grain on the outer

surface and possess mechanical properties approximately 30% greater than parts

formed with static casting methods.

Casting Process


• Mold preparation - The walls of a cylindrical mold are first coated.

• Pouring - The metal is poured into rotating mold, without the use of runners.

The centrifugal force drives the material towards the mold walls.

• Cooling - The mold remains spinning as the metal cools.

• Casting removal - After solidified the casting, the rotation is stopped and the

casting can be removed.

• Finishing – While centrifugal force drives

the dense metal to the mold walls, any less

dense impurities or bubbles flow to the

inner surface of the casting.

Centrifugal Casting

Casting Process

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� Investment casting is often referred to as "lost-wax casting" because the wax pattern

is melted out of the mold after it has been formed.

� Lox-wax processes are one-to-one (one pattern creates one part), which increases

production time and costs relative to other casting processes.

� Investment casting can make use of most metals, most commonly using Al-alloys,

Mg-alloys, cast iron, stainless steel, and tool steel.

� This process is beneficial for casting with high melting temperatures that can not be

molded in plaster or metal.

� Parts that are typically made by investment casting include those with complex

geometry such as turbine blades.

� High temperature applications are common, which includes parts for the automotive,

aircraft, and military industries

Investment Casting

Casting Process


• Pattern creation - The wax patterns are typically injection molded into a metal die and are

formed as one piece. Several of the patterns are attached to a central wax gating system, to

form a tree-like assembly.

• Mold creation - The "pattern tree" is dipped into a slurry of ceramic and then dried to

form a ceramic shell around patterns. The shell is then placed into an oven and the wax is

melted out leaving a hollow ceramic shell.

• Pouring - The mold is preheated in a furnace and the molten is poured into the gating

system of the mold, filling the mold cavity.

• Cooling - After the mold has been filled, the molten metal is allowed to cool and solidify

into the shape of the final casting.

• Casting removal - After the metal has cooled, the mold can be broken and the casting

removed.

• Finishing - Finishing operations such as grinding or sandblasting are used to smooth the

part at the gates.

Investment Casting

Casting Process

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Investment Casting

Casting Process

Documents

Ch 5 Casting processes - kau 5 Casting processes.pdfMachining allowance Shrinkage allowance Metal 3.0mm % 2.0 Steel 3.0mm % 1.0 cast iron 1.5mm % 1.5 copper 1.5mm % 1.25 Aluminum 1.5mm