1.3 fundamentals of metal casting – fluidity of molten metal – different types of casting...

Unit I

Metal Casting Process

MEE 205 Fundamentals of Manufacturing Process

Table of Contents

7/31/2015 MEE 205 Fundamentals of Manufacturing Process 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

Fundamentals of Metal Casting


The casting process basically involves

Pouring of molten metal into a mold pattern after the part

to be manufactured

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 cooling of the metal in the mold

The influence of the type of mold material


Fundamentals of Metal Casting – Gating system

[www.hackaday.io/project/2434/logs]

[http://nptel.ac.in/courses]


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

Faster pouring can erode the mould cavity



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



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]

Characteristics of a gating system


A gating system should be able to do the following

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


Establish suitable temperature gradients

No mould erosion

Simple and economical design

Easy to implement and remove after solidification

Maximum casting yield

Characteristics of a gating system (contd..)

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


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

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



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



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


Types of Casting process


Exp

and

able

mo

ld c

asti

ng

Sand casting

Plaster Mold Casting

Ceramic Mold Casting

Shell Mold Casting

Vacuum casting

Expanded Polystyrene

casting

Investment Casting

Per

man

ent

mo

ld c

asti

ng

Basic Permanent Mold casting

Slush Casting

Pressure Casting

Vacuum Permanent Mold Casting

Die casting

Centrifugal Casting

Centrifuge Casting

Ingot Manufacture

Continuous Casting

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


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

Sand Casting Process (contd..)


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

Disadvantages of Sand Casting Process

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


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

Plaster Mold Casting Process (contd..)


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

Disadvantages of Plaster Mold casting Process

Investment Casting Process


Investment casting uses a disposable wax pattern for each

cast part.

The wax is either injected directly into the mold or pre-

coated 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.


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 for casting both ferrous and non-ferrous materials

Relatively high-quality surface finish and detail in final

components

Investment Casting Process (contd..)


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

Disadvantages of Investment Casting Process

Shell Mold Casting Process


1 2

3 4


Shell Mold Casting Process (contd..)

5 6

7

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


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

• A reduced need for post-casting machining

Die Casting Process (contd..)


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

Disadvantages of Die casting Process

Topics for self study: Ceramic Mold Casting and Centrifugal casting process

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).

[www.afsinc.org]

Manufacturing Process Videos/Casting Process/Visual Inspection Method for Casting.mp4


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.

Castings Inspection Methods (contd..)

[www.afsinc.org]


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.


[www.afsinc.org]


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.

[www.afsinc.org]



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


[www.afsinc.org]


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.


[www.afsinc.org]

References


Elaheh Ghassemieh (2011). Materials in Automotive

application, State of the Art and Prospects, New Trends and

Developments in Automotive Industry, Prof. Marcello

Chiaberge (Ed.), ISBN: 978-953-307-999-8, InTech, Available

from: http://www.intechopen.com/books/new-trends-and-

developments-in-automotiveindustry/ materials-in-

automotive-application-state-of-the-art-and-prospects

P. Blain (2012) – Steel perspectives for automotive industry

Lotus Engineering, 2010

www.thomasnet.com

http://www.intechopen.com/books/new-trends-and-developments-in-automotiveindustry/


















1.3 fundamentals of metal casting – fluidity of molten metal – different types of casting...

Documents

casting process mee

flow of molten metal

molten metal viscosity

capability of molten

molten metal composition

casting testing

casting operations

super heated metal