Effect Of Oil Palm Fibre Addition On The Mechanical Properties of
Shell Mould Investment Casting
SOH WEN HANN
A thesis submitted in fulfilment of the
requirements for the award of the degree of
Master of Engineering (Mechanical Engineering- Advanved Manufacturing
Technology)
Faculty of Mechanical Engineering
University of Technology Malaysia
DECEMBER 2009
iii
ACKNOLEDGEMENT
I would like to take this opportunity to express my highest gratitude to my
supervisor Associate Professor Dr. Mohd. Hasbullah bin Idris for the continuing
support, advises, encouragement, patience and constructive opinions throughout this
project.
I would like to say thanks to all the staffs in Casting Laboratory
andMechanics of Material Laboratory especially Mr. Wan and Mr Amir due to their
helping hand to fulfill the requirement of this thesis.
Special thanks to Mr. Lee Chung Sing for his sincere assistance and advises
on the laboratory’s works.
Last but not least, I would like to thank my parents, sisters, brother and
friends, for their understandings, encouragement and constantly morale support on
this project.
iv
ABSTRACT
This research is aimed to investigate the effect of addition of organic fibre on
the mechanical properties of the investment casting ceramic mould based on different
length of fibres in the investment. Modulus of rupture (MOR) and permeability of
the ceramic mould are evaluated. The MOR test contains three systems: green
(undewaxed), green (dewaxed) and fired. The permeability test contains two systems:
green (dewaxed) and fired. For the variation of fibre length, 3mm, 5mm, 7mm, 9mm
are established. In each system, the MOR and permeability are compared between
the fibre addition specimens and the non fibre addition specimen. Oil palm empty
fruit bunch fibre is used as the additive in the investments. Colloidal silica and zircon
sand are used to produce the ceramic mould. Firing temperature used is 700ºC. MOR
three point bending testing machine is used to evaluate the strength of the mould.
Permeability tester is used to evaluate the permeability of the mould upon the
addition of fibres. The results obtained indicated that for the MOR test, the green
(dewaxed) system has the highest MOR values than the fired and green (undewaxed)
systems. The fired system is shown to have lowest MOR values among the three
systems. For the permeability test, as expected. the fired system fibre addition
specimens have higher permeability value than the green system.
v
ABSTRAK
Pengkajian ini bertujuan untuk mengkaji kesan tambahan fiber atas sifat-sifat
mekanikal seramik acuan dalam tuangan invesmen berdasarkan panjang fiber yang
berlainan. Modulus of Rupture (MOR) dan ketelapan seramik acuan dikaji. Ujian
MOR terdiri daripada 3 sistem: hijau (tanpa panyalilinan), hijau (degan penyalilinan)
dan penembakan. Ujian ketelapan terdiri daripada 2 sistem: hijau (dengan
penyalilinan) dan penembakan. Untuk panjang fiber yang berlainan, 3mm, 5mm,
7mm, 9mm ditubuhkan. Untuk setiap sistem, MOR dan ketelapan dibandingkan
diantara spesimen yabg ditambah fiber dan spesimen yang tiada fiber. Fiber kelapa
sawit diguna sebagai tambahan dalam invesmen. Colloidal silica dan pasir zircon
diguna untuk membuat acuan seramik. Suhu penembakan digunakan ialah 700ºC.
Mesin MOR digunakan untuk mengukur kekuatan acuan. Mesin ketelapan digunakan
untuk mengukur ketelapan acuan. Hasil yang didapati menunjukkan bahawauntuk
ujian MOR, sistem hijau (dengan penyalilinan) mempunyai MOR yang paling tinggi
dibandingkan degan sistem hijau (tanpa panyalilinan) dan penembakan. Sistem
penembakan didapati bahawa mempunyai MOR yang paling rendah diantara 3
sistem-sistem tersebut. Untuk ujian ketelapan, macam yang dijangkakan, sistem
penembakan dengan tambahan fiber mempunyai ketelapan yang lebih tinggi daripada
sistem hijau.
vi
TABLE OF CONTENTS
CHAPTER
1
2
TITLE
PAGES
DECLARATION
ii
ACKNOWLEDGEMENT
iii
ABSTRACT
iv
ABSTRAK
v
TABLE OF CONTENTS
vi
LIST OF TABLES
ix
LIST OF FIGURES
x
LIST OF SYMBOLS
xii
INTRODUCTION
1
1.1 History of Investment casting
1
1.2 Recent researches in Investment casting process
2
1.3 Problem statement
3
1.4 Objectives
4
1.5 Scope
4
LITERATURE REVIEW
5
2.1 Investment casting
5
2.2 Type of investment casting process
6
vii
2.2.1 Block molding investment casting process
6
2.2.2 Shell molding investment casting process
7
2.3 Pattern assembly
2.3.1 Die
8
2.3.2 Pattern wax
10
2.3.3 Preparation and injection of wax
11
2.3.4 Other materials as pattern
13
2.4 Investment
3
8
15
2.4.1 Slurry
16
2.4.2 Slurry parameters
26
2.4.3 Stucco
29
2.5 Dewaxing
29
2.6 Firing
31
2.7 Casting
33
METHODOLOGY
36
Overview
36
3.1 Pattern making
38
3.2 Slurry
39
3.2.1 Refractory filler
39
3.2.2 Refractory binder
39
3.2.3 Slurry Mixture
39
3.3 Organic fibre additive
41
3.4 Stucco
42
3.5 Preparation of samples for modulus of rupture (MOR) test
42
3.5.1 Samples making
42
viii
3.5.2 Dewaxing
46
3.5.3 Firing
46
3.5.4 MOR test
47
3.6 Permeability test
4
5
50
3.6.1 Samples making
50
3.6.2 Dewaxing
54
3.6.3 Firing
54
3.6.4 Permeability sample test
54
RESULTS AND DISCUSSIONS
Overview
57
4.1 MOR test
57
4.2 Permeability test
60
CONCLUSIONS AND RECOMMENDATION
68
REFERENCES
70
ix
LIST OF TABLES
FIGURE NO
TITLE
PAGE
2.1
Typical properties for the common grades of colloidal silica
18
2.2
Typical properties for ethyl silicate
20
2.3
Typical properties for the zircon sand
23
2.4
Typical properties for the fused silica
24
2.5
Typical properties for the alumina
25
2.6
Typical properties for the aluminosilicate
26
3.1
Slurry system for MOR test and Permeability test with
variation of fibre length
40
3.2
MOR test with variation of fibre length
48
3.3
Permeability test with variation of length of fibre
55
4.1
Results of MOR and permeability tests
61
x
LIST OF FIGURES
FIGURE NO
TITLE
PAGE
3.1
Process stages
37
3.2
Die dimension (in mm)
38
3.3
Slurry mixture
41
3.4
Mould formation sequence for MOR test of variation of
fibre length
44
3.5
Wax pattern coated with primary layer
45
3.6
Wax pattern coated with secondary layer, fibres and stucco
45
3.7
Wax pattern coated with seal coat layer
46
3.8
MOR specimen dimensions (in mm)
48
3.9
Universal Tester Machine(model Instron 4206, 100kN motor) 49
3.10
MOR samples
49
3.11
Mould formation sequence for permeability test of variation
of fibre length
52
3.12
Wax pattern coated with primary layer
53
3.13
Wax pattern coated with secondary layer, fibres and stucco
53
3.14
Permeability tester
56
4.1
Surface structure of the no fiber addition ceramic mould
62
4.2
Surface structure of the 3mm fiber addition ceramic mould
62
4.3
Surface structure of the 5mm fiber addition ceramic mould
62
4.4
Surface structure of the 7mm fiber addition ceramic mould
63
xi
4.5
Surface structure of the 9mm fiber addition ceramic mould
63
4.6
Graph of MOR (green undewaxed)
64
4.7
Graph of MOR (green with dewaxing)
64
4.8
Graph of MOR (fired)
65
4.9
Graph of MOR comparison of green undewaxed, green with
dewaxing and fired system
65
4.10
Graph of permeability (green with dewaxing)
66
4.11
Graph of MOR (fired)
66
4.12
Graph of MOR comparison between green and fired system
67
xii
LIST OF SYMBOLS
Si
Silicon
HCL
Hydrochloric acid
O
Oxygen
Cl
Chlorine
C
Carbon
H
Hydrogen
Hg
Mercury
Na
Sodium
Zr
Zirconate
N
Nitrogen
Pa
Pascal
MOR
Modulus of rupture
1
CHAPTER 1
INTRODUCTION
1.1 History of Investment casting
The investment casting or lost wax process has been utilized through history
for some 5000 years for the production of ornamental objects, statues and jewelry. It
is considered the most ancient of metal casting arts. Technological advances have
also made it the most modern and versatile of all metal casting processes.
Investment casting methods are regarded as precise fabrication processes for
components having intricate shape and requiring excellent surface finish and
dimensional accuracy 1. These methods, which include shell moulding, solid mould
casting and numerous similar methods, have been used for decades in jewellery, for
casting artworks, in surgery implants and in dental applications.
History of the "lost wax" process can be traced back to 4000 B.C and the use
of the process by early civilizations is recorded through the Egyptian and Romans
eras to the Bronze Age and Medieval times in Europe 2. The Aztec gold-smiths of
pre-Columbian Mexico used the lost wax process to create much of their elaborate
jewelry, and some very nice examples of these works are from the tribe of Quimbaya,
2
who lived in a little area between the Cauca River and the Micos and Gauiaya rivers.
The few castings which survived the plunder of treasure hunters show complete
mastery of a difficult and involved technique that must have taken years of trial and
error to develop.
Investment casting process emerged in the early 1940’s in USA and UK and
this process benefited from the jewelers and dentists’ work, contributing a major
breakthrough for manufacturing of complex components with high precision,
accuracy, complex geometries and sizes and good surface finish 2. Until today, many
researches have been done to improve and enhance the investment casting process,
reducing the defects and other problems associated with process.
1.2 Recent researches in Investment casting process
In this research by Qingbin Liu, Ming C.Leu, Von L.Richards and Ross
Laurent 3, the researches used ice as the pattern for the investment casting. The
advantages of the ice are cheap, readily availability, contractions during melting,
high quality and good surface finish. The ceramic moulds were done at sub-zero
temperature in favor of the ice pattern used. Experimentation was carried out to
evaluate the fracture toughness of the ice pattern-made ceramic mould. Surprisingly,
the samples tested had exceedingly low strength which led to more understanding
and analysis of the failure of the mould. However, the ice pattern method proved to
be viable method as this was demonstrated with casting of M8 bolt by the researches.
The addition of fibres into the slurry to improve the conditions of the
investment casting has steadily become a practice for the foundries and manufacturer.
The use of alcohol based binder such as ethyl silicate has been limited environmental
protection bodies due to the toxic fumes and gas emitted. The use of colloidal silica
3
binder which is water based thus increases. However, the water based colloidal silica
gels very slowly and needs of expensive polymer additions such as latex polymer.
In the research by S.Jones and C.Yuan 4, nylon fibres was added to the
colloidal based slurry and the results on the strength and permeability of the mould
were compared to the polymer addition slurry. The addition of fibres has proven a
viable method rivaling the use of expensive polymer with improved permeability and
enhanced green strength.
Another research was also done to address the above problems. In this
research by Lee C.S and Sulai E 5, rice husk had been used as the additive to the
slurry instead of fibres. Experiments were done to test the strength and the
permeability of the ceramic mould produced. Mould permeability showed
improvement with the addition of rice husk. However the strength decreased with the
addition of rice husk. Further work will be done to address this problem.
1.3 Problem statement
The use of fibre as an alternative to the expensive polymer to improve the
permeability and strength had been researched by S.Jones and C.Yuan 4. However,
the research on how the orientation of fibre and fibre length in the slurry will
influence the mould strength and permeability has never been done. In this paper, the
effect of variation in fibre length on the mould green and fired strength and
permeability will be studied
4
1.4 Objectives
1) To determine the viability of organic fibres in producing investment casting
ceramic mould
2) To study the effects of oil palm fibre addition to the mechanical properties
such as strength (green and fired) and permeability of shell mould of
investment casting based on different fibre length.
3) To compare the permeability and strength of fibre-modified ceramic mould to
the ordinary mould.
1.5 Scope
For this thesis work, couple of scopes are defined to achieve the objectives of
the project and are shown as below:
1) The fibre length is varied for 3mm, 5mm, 7mm, 9mm.
2) Refractory binder used is colloidal silica.
3) Refractory filler used is zircon sand.
4) Fibre used is organic oil palm fibre.
5
CHAPTER 2
LITERATURE REVIEW
2.1 Investment casting
Investment casting is also known as the lost wax casting, lost pattern, hot
investment and precision or cire-perdue (French – literally, wax – lost). Investment
casting process has been used in many manufacturers due to its good surface finish
of the final product produced. Investment casting also yields good dimensional
accuracies, complex geometry and design flexibility without the necessity of
extensive machining or other fabrication/finishing work required to provide a final
product. The investment casting process consists of the following main processes:
a) Wax injection process
b) Ceramic mould building process
c) Casting of molten metal
Each of these processes is vital to the quality of final product and the lead
time of the whole production process. Therefore, each process has its own
parameters that must be controlled and moderated.
6
2.2 Type of investment casting process
2.2.1 Block moulding investment casting process
Block moulding is one of the techniques used for investment casting. In this
technique, the pattern wax is made from the metal die and is coated with primary
investment slurry material. The slurry of suitable binder plus alumina, silica gypsium,
zirconium silicate or mixtures of these and other refractories are then poured into a
container surrounding the pattern. The coated pattern wax is then put inside an openended container or flask. The secondary investment is then poured inside the
container and the block mould is let to dry and cure for hours. This forms the block
mould.
The block mould is then undergoes dewaxing where the pattern wax melts
and is removed from the block mould. The block mould is then fired to get the
required strength. Molten metal is then poured into the cavity which is left by the
pattern wax in the block mould. The mould is then knocked to get the required cast
part. Any alloy that can be melted is amenable to investment casting by adapting
refractory and mould temperature to the requirements of the metal being poured 6.
The part then undergoes further machining.
7
2.2.2 Shell moulding investment casting process
The shell moulding process is the most common technique for investment
casting. Shell moulding process is also called ceramic shell process because it often
asscociates with ceramic materials. Unlike block moulding, the expendable pattern
wax is coated with primary and secondary investment slurry. The investment slurry
materials are refractory materials such as refractory binder and refractory filler.
Typical binder used for shell moulding investment casting process are colloidal silica
and ethyl silicate. Typical refractory fillers are alumina, fine zircon sand and fused
silica. The formulation of the investment is of great importance in obtaining a strong
and sturdy shell mould. The investment is also important to get the required
thickness for the shell mould. Compared to the block moulding technique, the shell
moulding technique can minimize the usage of investment materials, the refractory
filler and the expensive refractory binder. After the coating of the investment, the
pattern is then stuccoed to increase the integrity of the shell mould. Typical stucco
material used are coarser zircon sand and. The coated pattern is then let to dry and
cured for hours.
The pattern undergoes the dewaxing process by using autoclave technique.
The autoclave technique is the most common technique used in the industry to
remove the pattern from the shell mould. The dewaxing process is also one of the
most crucial part in shell moulding investment casting because the melting wax can
cause volumetric expansion and subsequently breaks the shell mould if extra
precaution is not taken to address the expansion. After undergoes dewaxing process,
the pattern wax will melt and then leave the shell mould by draining the wax.
Residual wax is burned to ensure no pores inside the shell mould. Firing or sintering
process follows up to ensure the shell mould gets the required strength and integrity,
to get the fired strength. The shell mould is put inside a high temperature furnace for
firing purpose.
8
The shell is then removed from the furnace and ready to be cast. The molten
metal of high temperature is cast into the shell through designed sprues and risers to
minimize the solidification shrinkage. The shell is then left for few hours to let the
molten solidify. The shell is then knocked to get the part out.
The shell moulding process is more preferred than the block moulding
process by the manufacturer because of the following characteristics:
a) Shell moulding process can minimize the wastage of the refractory filler and
binder.
b) It can reduce the total lead time in producing the part. Unlike the block
moulding process, where more slurry is used and the time for waiting the
slurry to cure is prolonged.
c) The shell moulding process is more suitable for casting of heavy metal.
2.3 Pattern assembly
2.3.1 Die
Die is used in investment casting process for production of the wax pattern.
The wax is injected into the die to get the required shape of the final product. The
accuracy and the dimensional aspects such as internal surface roughness and the
thermal expansion of the die are the significant factors to influence the precision of
the investment casting. Therefore die is an important aspect in producing a precision
casting and final product. It is usual for the caster to use precision-machined full
metal dies for producing wax patterns when large numbers of highly accurate
components are required 7.
9
A die is very costly to produce, as the manufacturers of die have always
wanted to produce a highly precise die as accurate as possible, with minimum
tolerances and as close as what the blue print indicates. The material of the die must
also be carefully chosen in terms of the coefficient of thermal expansion and the
strength. This will add to the considerable lead time in investment casting process
and could be much worse if the manufacturer had to rework the die if the tool fails to
produce wax or expendable pattern with high accuracy.
There are various materials that are suitable for die production, such as metals
such as alloy and polymer such as vulcanized rubber. Metals are greater heat
conductor than polymer and hence are able to produce wax patterns with high quality.
On the other hand, rubber mould such as silicone mould had been used in fabricating
ice pattern by past research
3
due to the ease of the ejection of ice patterns using
silicone mould. Steel dies are most satisfactory for long production runs and are
machined from the solid by die sinking and assembly in the tool room: this technique
gives the highest standard of accuracy 6.
There are a number of processes that can be used to manufacture die such as
forging, casting and machining. The selection of processes for die manufacturing
depends on the die shape and shape for economical reasons. Dies in fusible alloys are
formed by casting and require the preliminary production of a master pattern or metal
replica of the final casting 6. Manufacturers of die often use processes such as milling,
turning grinding to machine forged die block which is made from forging process.
Dies are often heat treated to get the required strength and integrity to withstand the
required load and high temperature of molten metal.
10
2.3.2 Pattern wax
Pattern for investment casting proves is made by injecting the pattern material
into dies of the desired shape. Wax is the most commonly used pattern material in
investment casting process. For a successful investment casting, pattern wax must
have the following characteristic 8:
a) Has the lowest possible thermal expansion
b) Its melting point is not much higher than the ambient temperature
c) Longer solidification time.
d) Resistant to breakage
e) Smooth and wettable surface
f) Low viscosity
g) Low ash content
Pattern wax Pattern waxes are commonly consisting the following components 9:
a) Petroleum waxes
b) Natural waxes
c) Natural and synthetic resins
d) Organic fillers
Petroleum waxes such as paraffin is commonly used because of its relatively
low price compared to other raw materials 9. Paraffin wax controls and enhances the
rheological properties and this will in turn affect the fluidity and injection parameters
of the pattern wax blend.
Natural waxes such as bayberry wax, candelilaa wax and carnauba wax are
derived from plant, vegetables, trees and leaves. Other natural wax such as beeswax,
shellac wax and Lanaolin wax are derived from animals. Natural waxes affect the
surface finish, hardness and properties of the pattern wax blend 9.
11
Resins such as natural resins and synthetic resin are used to improve the body
and strength of the pattern wax
10
. Some other factors that are also influenced by
resins are shrinkage and solidification characteristics of the pattern wax, tackiness
and also rigidity of the pattern wax. Natural resins are derived from natural sources
such as pine trees, crude oil, and coal tar 9. Synthetic resins are mad made resins
from esterification of organic compounds.
Organic fillers are used as an alternative to improve the various
characteristics of the pattern wax. Some research had been done on finding the
suitable material for the development in pattern wax. Senay and Akar
8
had done a
research in using soybean as an alternative organic filler in investment casting
pattern wax compositions. The research was done to find out the effects addition of
soybean to the sprue wax and compare the results to the current pattern wax available
in the market. Soybean has some characteristics such as low ash content and high
protein.
The results had proved the addition of soybean enhanced the fluidity,
reducing surface roughness and improving thermal expansion of the sprue wax.
Contact angle of the sprue wax had also been reduced while viscosity had increased.
This proved the addition of soybean can transform the sprue wax into a pattern wax.
2.3.3 Preparation and injection of wax
The handling of pattern wax can greatly affect the injection stage as well as
the quality of the wax pattern. The preparation starts with the melting of the wax as
the wax is typically stored as solid form when not in use. The common temperature
used for melting the wax are about 80ºC - 90ºC 9. Overheating of the wax can oxidize
some of the raw materials and may cause the wax to become brittle or rubbery 9.
Conditioning of the wax is also an important aspect as it will also affect the quality
of the wax pattern in terms of dimensional accuracy and surface smoothness. The
contraction percentage of the wax will increase if not carefully handled and will
cause dimensional inaccuracy. Some of the important factors that affect the amount
12
of dimensional contraction are: injection temperature and cycle time, injection flow
rate and pressure 7.
Most waxes are injected below their melting point and sometimes well below
the melting point of the components 9. Injection temperatures are ranged from 43 ºC
to 77 ºC 11. In this way, waxes are in pasty form which is in the form of solid and
liquid mixture. This is to ensure the expansion of the wax during injection and the
injection energy consumption can be minimized 8, and to reduce inaccuracies due to
thermal expansion. For pasty form wax, the injection pressure needed is somewhat
high to ensure the less dwell time and less cavitation in heavy section 9. Injection of
wax can also be done in liquid form. However, this requires the control of the speed
of wax during injection for ideal mould filling. Disadvantages of liquid form wax is
that it can have higher thermal expansion than the semi solid or pasty form wax and
liquid wax are prone to air inclusions and turbulence during injection 9. It was also
found that the increase of injection pressure can lead to decrease in contraction 7.
Injection flow rate is directly proportional to injection pressure and therefore high
injection flow rate can also decrease the contraction of the wax. Some typical
pressure used in past research 7 are up to 2 MPa.
Equipment for wax injection can be pneumatic unit with a closed, heated
reservoir tank that is equipped with a thermostat, pressure regulator, heated valve,
and nozzle and is connected to the shop air line for pressurization 11. A die or mould
is held against the nozzle with one hand while the valve is operated. The wax is then
injected into the mould with the valve operated. The Pattern can also be produced by
gravity pouring by hand operated injection gun, or by the use of low pressure
injection equipment to inject the pattern wax into the mould, most quantity is based
on automatic injection machines offering close control of temperature, pressure and
speed of injection 6.
13
Some blends of waxes commonly used in investment casting 6:
a) Paraffin wax 50% and stearine 50% - suitable for small, relatively complex
castings as the mixture has low softening temperature low strength and large
shrinkage.
b) Paraffin wax 60%, Carnauba wax 25%, Cerasin wax 10% and bees wax 5% suitable for small sized, thin-walled patterns.
c) Paraffin wax 30%, Carnauba wax 30%, Cerasin wax 10%, bee wax 10% and
dammar resin 10% - suitable for thin-walled, medium to large size casting
requiring close dimensions.
d) Cerasin wax 20%, resin 20% and polystyrene 30% - suitable for the production o
thin-walled and large sized castings requiring tight dimensions and enhanced
surface finish but has low fluidity.
2.3.4 Other materials as pattern
Past research
3
had involved the use of ice as the pattern materials. Ice is
obtained by freezing the water at sub zero temperature. Ice pattern requires different
treatment than ordinary wax patterns in terms of mould, investment, “dewaxing” and
other processes related to the investment casting. Ice pattern investment casting
requires the moulding process to be done below water freezing temperature.
Removal of ice pattern is done by letting the ice to melt at room temperature and
drain from the mould. The use of ice patterns has several significant advantages over
the conventional wax patterns:
a) Conventional waxes demonstrate stresses as they are heated during dewaxing
3
. These stresses are caused by the thermal expansion of the wax during
dewaxing as wax behaves as a partially crystalline linear polymer and
exhibits an abrupt expansion in volume. These stresses appear to be the cause
that of cracks to the ceramic mould. On the other hand, ice patterns exhibit
contraction during melting because of the nature characteristic of the water.
This means there is no or less stress applied to the ceramic mould.
14
b) Ice is much cheaper than investment casting wax and it can be obtained from
water supply.
c) Ice pattern also has demonstrated better surface finish to the ceramic mould
and thus better finish at the final product.
Plastics had also been used as pattern materials for investment casting. A
general purpose grade of polystyrene plastic is usually used. Other plastics such as
polyethylene, nylon, erhyl cellulose, and cellulose acetate had also been used.
Advantages of polystyrene and other plastics over conventional investment casting
waxes 11:
a) Plastics have the ability to be moulded at high production rates on automatic
equipment and their resistance to handling damage even in extremely thin
section.
b) Plastic pattern can be stored indefinitely without deterioration as conventional
pattern wax deteriorates with time in quality.
c) A further advantage is the lower coefficient of expansion, pattern dimensions
being consequently less sensitive to temperature variation 6.
d) Plastics are less expensive in price compared to the wax patterns.
Polystyrene appears in various foamed form with different densities as a
result of the softening and gasifying characteristics upon heating of polystyrene. This
makes the material to be easily formed into various shapes and fabricated. The
injection of polystyrene frequently associated with high injection pressure than
pressure used for conventional investment casting wax as this is a part of the
behaviour of polystyrene as a viscous liquid in the temperature range 200ºC to 350ºC.
The main weakness of plastic pattern materials is their abrupt thermal expansion
during heating that causes cracks to the ceramic moulds due to their relatively high
coefficient of thermal expansion then the ceramic materials.
15
Another version of investment casting
12
, called the Mercast process uses
liquid mercury as the pattern material. Liquid mercury was let to freeze at -57ºC or
below in a die. The slurry composition for this method was clearly different from the
common slurry composition for investment casting wax pattern. As the working
temperature for this process is at sub zero temperature, the refractory binder used
was clearly ethyl silicate instead of colloidal silica, which is alcohol based, otherwise
the colloidal silica will freeze at sub zero temperature due to its water base
characteristic. Mercury exhibits little expansion and contraction during melting or
freezing and produces highly accurate and less crack ceramic mould. This process
was less used or no longer used due to the main drawbacks:
a) The expansive equipment required for creating low temperature conditions,
b) High cost of mercury itself,
c) Health hazards associated with the handling and use of mercury
2.4 Investment
After the pattern wax has been ejected from the metal die, investment
materials are then coated onto it. The investment materials are divided into slurry and
stucco. The slurry is further consisting of refractory binder and refractory filler. Each
of the binder and filler plays an important role in obtaining the required strength of
the ceramic mould. Investment is made by mixing the refractory binder and filler
together and the required viscosity and pH are measured and obtained. The pattern is
then dipped into the container which contains the slurry for some time. This duration
is needed to allow for excessive slurry to drip for the next coatings to be applied.
16
Each coating of the slurry also requires duration to gel in order to allow for
successive coatings, from 24 hours to 72 hours, depending on the type of refractory
used. This is important to control the moisture removal and obtain excellent bonding
condition as the slurry gels. The first coat of slurry onto the pattern is termed primary
coating. This coating usually requires fine refractory filler materials to have a fine
surface finish, as this primary coating is the root of the ceramic mould.
The second and successive coatings are termed secondary or backup coatings.
The main features of investment casting shell mould quality are sufficient green
strength to withstand wax removal without failure and to withstand the weight of cast
metal 13. These coatings usually require coarser particles refractory in order to build
up the thickness which is the key of the ceramic mould green strength. Ceramic with
very low green strength is prone to cracking during wax removal.
2.4.1 Slurry
A good slurry formulation is to ensure the quality of the ceramic mould
produced. The aim of good slurry formulation is to produce stable slurry. Stable
slurry is a slurry that achieved a given set of parameters such that it is in a usable
condition that can be obtained time and time again on subsequent slurry makeup 13.
The primary coating of slurry is especially important as the primary coating
determines the quality of the surface finish of the casting. Poorly wet-in slurry will
not develop its maximum strength potential and may result in serious shell problems
such as cracking. Slurry also provides refractory protection to ensure no metal
penetration through the mould
13
. The slurry composition must also be chemically
stable and not to cause reaction with the molten alloy. There are many factors that
affect the slurry quality:
17
a) refractory type
b) flour particle size distribution
c) solid loading
d) viscosity
e) plate weight
The slurry consists of refractory binder and refractory filler as mentioned.
Other additions include wetting agent, catalyst, polymers and fibre additives.
a) Refractory binder
The use of refractory binder in the slurry is to provide bonding between the
refractory particles or fillers. The binder also acts as the base or root for the ceramic
mould produced. The role of binder is also to impart sufficient strength to the mould
to withstand the stresses that develop during its preparation 14. There are two types of
common used refractory binder, namely colloidal silica and ethyl silicate. Other
binder materials had also been produced such as sodium silicate binder, colloidal
alumina and colloidal zirconia binder.
i) Colloidal silica
Colloidal silica is the most common used binder due to its water based
characteristic which is friendly to the environment. Colloidal silica is produced by
removing sodium ions from sodium silicate or sodium hydroxide. Thus, colloidal
silica is alkaline in nature.
There are two types of common grades colloidal silica typically used in
investment casting industry 15. One of them is the one with small particle size 8nm
and the other would be large particle size at 12nm. Colloidal silica has a very low
18
viscosity, typically less than 7cps, which allows for a high solids loading of
refractory filler especially in primary slurry coating. The main weakness of colloidal
silica is that due to its water based characteristic, it suffers from long duration gelling.
On the other hand, its slow gelling time is said to be a advantage too because it
allows sufficient time for the manipulation of operation to ensure a smooth and even
coating, and total surface coverage at the stuccoing stage 13. Colloidal silica binders
also produce mould with very low green strength which are prone to cracking upon
wax removal process 4. Thus, polymers have to be added to increase the ceramic
green strength.
A special type of colloidal silica has been developed to meet high
temperature application such as directional solidification and single casting processes
and to successfully create shells for the specific geometry complexity of the trailing
edge of a blade casting 16. This type of colloidal silica, code name EHT binder, is a
water based, polymer free colloidal silica sol. One of its primary features is specially
controlled particle size and particle size distribution. The results of experiments of
EHT binder on the modulus of rupture (MOR) test, creep test and plate weight test to
those of conventional colloidal silica. The EHT binder build shell is stronger in terms
of MOR and creep resistance.
Table 2.1 Typical properties for the common grades of colloidal silica
VALUE
PROPERTIES
Percent Silica
Small (grade)
30 %
Large (grade)
30 %
pH @ 25ºC
10
10
Surface Area (m2/g)
345
240
Particle Size (nm)
8
12
% Na2O (wt %)
0.56%
0.35%
SiO2/Na2O Ratio (wt)
50
85
Viscosity @ 25ºC (cps)
5
4
Specific Gravity @ 25ºC
1.22
1.21
Particle Charge
Negative
Negative
Stabilizing Counter Ion
Sodium
Sodium
19
ii) Ethyl silicate
Ethyl silicate is an acid based refractory binder. Ethyl silicate is produced by
the reaction of silicon tetrachloride with ethyl alcohol 11.
SiCl4 + 4ROH
Si(OR)4 + 4HCl
where SiCl4 is silicone tetrachloride and Si(OR)4 is tetraalkyllorthosilicate.
The basic compound of formed is tetraethylorthosilicate (Si(OC2H5)4). The
grade used for investment applications is designated ethyl silicate 40 consisting of a
mixture of ethyl polysilicates averaging necessary for foundries to perform this
chemical operation themselves
11
. Due to its alcohol based, ethyl silicate gels much
faster than colloidal silica binder. This is because alcohol evaporates faster than
water in the presence of air. In the past research 3, ethyl silicate had been used as the
binder due to its alcohol base for ice patterns. This is because ethyl silicate alcohol
base solidifies at below water freezing temperature.
In another past research
17
, the aim of this research was to examine setting
behaviour of ethyl silicate-bonded investment mixed with silica sol accelerated by a
catalyst, such as aqueous ammonium carbonite or aqueous ammonia, and to use as an
investment mould for casting dental ceramics. This research concluded that ethyl
silicate-bonded investment mixed by hydrochloric acid HCl mixture and ammonium
carbonite solution for dental ceramics had setting behaviours, because setting time
was about 10 minutes and setting expansion was controlled by its concentration of
silica sol 17. The results showed that ethyl silicate investment can be used for dental
cast moulds material.
Ethyl silicate is, however, not without its weakness. It is much expensive than
colloidal silicate and poses environmental hazard due to its alcohol base. The
Environmental Protection Act in 19924 limited the degree of emissions allowed from
20
processes involved with alcohol substance. As a result of this, the investment casting
industries have decreased the usage of ethyl silicate and go over for colloidal silica.
This is because alcohol base ethyl silicate can cause intoxication to human bodies
after a long duration of inhalation.
Table 2.2 Typical properties for ethyl silicate
PROPERTIES
VALUE
Molecular Formula
C8H2O4Si
Flash Point
37.2 ° C
Lower Explosive Limit
1.3 %
Upper Explosive Limit
23.0 %
Melting Point
-49.9 ° C
Vapor Pressure
1.0 mm Hg
Specific Gravity
0.933
Boiling Point
168.9 ° C
Molecular Weight
208.3
iii) Sodium silicate
Sodium silicate is also known as water glass. Compared to the more common
colloidal silica and ethyl silicate, this kind of binder is not often used by investment
casting industries. Sodium silicate is obtained from the reaction of sodium carbonate
and silicon dioxide in molten form:
Na2CO3 + SiO2
Na2SiO3 + CO2
21
where Na2CO3 is sodium carbonate and SiO2 is silicon dioxide. Na2SiO3 is the product:
Sodium silicate.
Sodium silicate is a white solid and readily soluble in water. It is an alkaline
solution. Sodium silicate is stable in neutral and alkaline solutions. Silicate ion reacts
with hydrogen ions to form silicic acid in acid solution.
Sodium silicate is not so commonly used because it has poor refractoriness
which cannot be used in high temperature application as the properties will change if
subjected to high heat. Sodium silicate is not resistant to the steam atmosphere of
dewaxing autoclave 11.
b) Refractory filler
Refractory filler or refractory powder is used in investment casting process to
form the slurry needed for ceramic mould. The refractory loadings, like the binder,
also is an important factor that affects the ceramic mould strength. The filler serve as
the area in the ceramic of absorbing the impact when the mould is stressed. Therefore,
selection of filler material for shell making is very important. The selection is
dependent on a wide variety of factors, which can affect the properties of slurry, shell
strength and casting and also the economy of the process 13. Among these factors are
refractory filler type, refractory particle size and particle size distribution. Filler type
directly affects the properties of the ceramic as different filler has different
mechanical and thermal properties. Particle size affects the thickness of the mould
and directly influences the mould strength. The particle size distribution affects the
mould strength in such a way that the impact can be absorbed effectively if the filler
particles are evenly distributed and arranged. There are a few types of refractory
filler such as zircon, fused silica and alumina.
22
i) Zircon sand
Zircon sand is a form of mineral zircon. It is a mineral belonging to the group
of nesosilicates. Its chemical name is zirconium orthisilicate and chemical formula is
ZrSiO4 . Zircon sand is commonly used as refractory filler in investment casting due
to its fine particle size. Zircon sand’s high refractoriness characteristic makes it
viable for investment casting as it can withstand high temperature while retaining its
strength. At high temperature applications, it also maintains its chemical inertness
and is highly resistant to thermal shock. Zircon was also found to be a promising
candidate for refractory because of its high hardness and high modulus of elasticity.
Zircon has higher density than other refractory materials such as alumino-silicate or
silica. This makes zircon sediments faster than those refractories 13.
In the past research 18, zircon was also found to enhance the crystallization of
fused silica when they were mixed together and heated to a high casting temperature.
This slowed the rate of dilation of volume of the test specimen. This is because the
zircon sand expedite and sped the formation of cristoballite, which has a higher
density than the fused silica particles. This lower the thermal expansion of the
ceramic mould upon heating.
In another past research 13, zircon sand was also found to have positive effect
on the plate weight of the slurry and a large amount is desired for ceramic mould
making. This is because increase in plate weight with the increase of zircon sand has
been attributed to the presence of more number of fine particles in ceramic mould.
This will help to prevent shell cracking during dewaxing because as the zircon
amount increases, the higher solid loadings in the slurry will reduce shrinkage during
drying of coating and increase the green strength. This is due to zircon fine size to fill
in the gap of other refractory particles. The same research also stated that high
density of zircon facilitates the settling of the shell material into finer details of a
pattern and fine particles also prevent settling of the slurry when not in use. These
23
researches had proven zircon sand to be a viable refractory in investment casting
industry.
Table 2.3 Typical properties for the zircon sand
PROPERTIES
VALUE
Molecular Formula
ZrSiO4
Source
Natural Mineral
Composition
66 % ZrO2. 33 % SiO2
Thermal Expansion (in/in/ºF)
4.0 x 10-6
Specific Gravity
4.60
ii) Fused silica
Fused silica is a type of silica in the type of silica glass. Fused silica is a type
pf noncrystalline form if silicon dioxide with the chemical formula of SiO2. It is
made by melting natural quartz sand and then solidifying it to form glass
11
. Fused
silica is also called fused quartz or vitreous glass. Therefore fused silica is in
amorphous state. Fused silica is commonly used in investment casting process as
refractory due to its near zero thermal expansion. It also has exceptionally good
thermal shock resistance and good chemical inertness upon high temperature heating.
Its good chemical inertness prevents any reaction with between the ceramic mould
with the alloy cast. Fused silica is low in density providing about 23% more volume
per unit weight, which also makes lighter shells easier to remove after casting in the
knockout and cleanup operations 13.
Fused silica made shells have roughly half the hot crushing strength of
aluminosilicate
19
. This is due to the angular grains of fused silica that causes the
shell has considerable porosity and crushes easily. Fused silica also produces stable
24
slurry where unlike zircon, alumina and aluminosilcates which contain multi-valent
cations that can leach into the slurry and cause micro gelling of the binder as this
results in weak shells and reduces slurry life.
In the past research 3, alumino silicate was used first as the refractory for the
slurry for the ice pattern. However, mould made from this refractory exhibited low
strength and could crack easily. Thus fused silica was employed to produce crack
free mould for the research. This is because of thermal expansion coefficient of fused
silica is lower than that of aluminosilicate.
In another research
13
, fused silica had been mixed with zircon to produce
slurry for the pattern. This combination had shown good flowability on the wax
pattern. Fused silica also was shown to appear more equaixed and large agglomerates
can be observed when the surface morphology of the particles of fused silica is
magnified.
Table 2.4 Typical properties for the fused silica
PROPERTIES
VALUE
Molecular Formula
SiO2
Source
Man-Made
Composition
99.8 % SiO2
Thermal Expansion (in/in/ºF)
3.0 x 10-7
Specific Gravity
2.20
25
iii) Alumina
Alumina is a type of amphoteric oxide of aluminum with the chemical
formula of Al2O3. The chemical name of alumina is aluminum oxide. Alumina is
produced from bauxite ore by the Bayer process 11. It is more refractory than silica or
mullite and is less reactive toward many alloys than siliceous refractories. Alumina is
an excellent refractory oxide, has been widely used to make crucibles for melting
superalloys, but not for titanium and its alloy, and due to high reactivity of titanium
melts
20
. These titanium melts are extreme reactive to refractory oxides during
casting, resulting in a reaction layer or oxygen-enriched surface layer, which
degrades the properties of the cast parts. However, the interfacial reaction between
alumina mould and titanium castings could be reduced significantly by the proper
choice of binder 20.
In this research
20
, it was reported that zirconia sol was used as the binder
with the alumina refractory and this showed that through the metallographic analysis,
microhardness measurements and composition analysis, the alumina was stable and
suitable for use as shell mould when casting TiAl based alloys.
Table 2.5 Typical properties for the alumina
PROPERTIES
VALUE
Molecular Formula
Al203
Source
Man-Made
Composition
99.8 % Al2O3
Thermal Expansion (in/in/ºF)
7.5 x 10-6
Specific Gravity
3.80
26
vi) Aluminosilicate
Aluminosilicate is made by calcining fireclays or other suitable materiaos to
produce a series of products rangining in alumina content from about 42 – 72%, with
the remainder being silica and impurities
11
. Refractoriness characteristics such as
able to withstand high temperature applications without having its properties and
chemical
inertness
increase
with
alumina
content,
and
so
does
cost.
Aluminosilicate’s fired pellets are crushed or ground and carefully sized to produce a
range of powder sizes for use in slurries, and granular materials for stucco use 11.
Table 2.6 Typical properties for the aluminosilicate
PROPERTIES
VALUE
Molecular Formula
Al2O3.SiO2
Source
Natural/Man-Made
Composition
47 % Al2O3
Thermal Expansion (in/in/ºF)
5.3 X 10-6
Specific Gravity
2.62
2.4.2 Slurry parameters
There are several parameters that affect the quality of the slurry, stability of
the slurry and the life of the slurry and these in turn influence the green strength of
the ceramic mould produced, creep resistance and permeability of the mould. These
parameters are viscosity of the slurry, pH value, density and plate weight of the
slurry. Some of the parameters are interrelated with each other. These parameters are
in turn influenced by the binder and refractory filler used. The refractory particles
size also affects the viscosity and pH value of the slurry.
27
a) Viscosity
Viscosity is the measure of the resistance of a fluid flow which is deformed
by either shear stress or extensional stress. In order for a slurry to be considered
stable, it must be well mixed to a point where the viscosity of the slurry is stable 13.
Slurry with viscosity value which is too high prevents good coverage of the pattern
wax. Low value of viscosity also causes uneven coverage of slurry on the pattern.
Viscosity also is one of the factors that affect the thickness of the shell mould. This
in turn affects the strength of the mould. Viscosity of slurry is usually measured in
investment casting foundry by using Zahn viscosity cup series. Other measurement
devices also include Ford viscosity cup series and Shell cup series.
In this research
21
, it reported the effects of slurry on the thickness and
strength of the mould, and pick up weight (the plate weight) of the slurry. Viscosity
is influenced by the refractory particle size used. It is obvious that coarser particles
make higher viscosity slurry. It is also affected by total solids loading in the slurry
and to a lesser extent, the type of binder used like colloidal silica or ethyl silicate 15.
b) pH of slurry
pH of slurry is affected by the type of refractory binders used. In the case of
colloidal silica binder, the pH is kept to alkaline value. For colloidal silica based
slurry, if pH is reduced below 7 (acidic), the subunits of colloidal silica will tend to
fuse together, creating silica gel hence hardened. This is because colloidal silica
consists of amorphous particles in a water dispersion that is separated by negative
electrical charges. These negative charges is provided by sodium hydroxide. When
slurries are made, pH of the slurry drops when refractory flour is added. This causes
the electrical charges to drop and the ability of the silica particles to repel each other
decreases, causing them to gel fast 15. This is not wanted in the slurry making process
28
as this causes the binder to gel before it can provide bonding to all the refractory
filler. This can be solved by adding alkali like sodium hydroxide.
For ethyl silicate based slurry, it is most stable when the pH is equal to 2, thus
gelling time is longest when pH=2 3. On the other hand, the slurry is most unstable
and the corresponding gelling time is much shorter when the pH value is between 5
and 6. When the pH value is less than 1.0, the slurry is also unstable. Gelling can be
accelerated by raising the pH. This can be done by adding ammonia as ethyl silicate
is readily gelled when mixed with ammonia.
c) Plate weight
Plate weight is a measurement used to determine slurry pickup on a metal
plate
15
. It is useful in controlling slurry coverage and adhesion on the pattern wax,
but also for rheology or flow characteristics. The weight of slurry pickup represents
the amount of slurry retained and hence the coatability of the slurry 13. Plate weight
is one of the method to indirectly measure thixotropy of the slurry and coating
thickness. Both primary slurry and secondary slurry can undergo plate weight test for
controlling the stability and quality of the slurry. If the plate weight characteristic is
not adequate for a certain slurry, this causes uneven slurry coating on the pattern wax.
This causes porosity and also stucco penetration and in turn affects the strength of
the ceramic mould produced.
d) Density
Primary coating often has higher density than secondary or backup coating.
This is because primary coating has higher viscosity and this in turns increases the
density. The primary coating also has finer particles than secondary coating resulting
in higher density.
29
2.4.3 Stucco
Each layer of slurry is stuccoed with coarser refractory particles. The aim of
stucco is to minimize drying stresses in the coatings by presenting a number of stress
concentration centers which distribute and hence reduce the magnitude of the local
drying stress 4. The second purpose of stuccoing is to present a rough surface, thus
facilitating a mechanical bond between the primary coating and the back up or
secondary coating. The particle size of the stucco is increased as more coats are
added to maintain mould permeability and to provide bulk strength to the mould 4.
2.5 Dewaxing
After the mould has been coated with primary and secondary slurry and
stuccoed, the next step is the dewaxing process. Dewaxing process is where the
pattern wax is removed from the mould, and this is a critical process where the
ceramic mould is subjected to high stresses due to the thermal expansion of the
pattern wax. The stresses are incurred when the pattern wax expands when heated.
This is due to the thermal expansion coefficient of pattern wax is much larger than
the one of the refractory materials. Rework cost increases and this in turn increases
the lead time as the work to repair the damage mould is needed.
This stresses can be alleviated by heating the mould extremely rapidly from
the outside in 11. This causes the surface layer of wax to melt very quickly before the
rest of the pattern can heat up appreciably. Through this method, molten can drain
from the mould or drain into the mould and this provides some space to amend the
expansion caused by the rest of the pattern wax.
30
Technique for dewaxing process commonly used in investment casting
industries are autoclave dewaxing and flash dewaxing. Autoclave dewaxing is the
most widely method. The ceramic shell mould is put inside a saturated vessel, and is
subjected to high temperature heat steam with moisture content. This vessel is
generally equipped with a steam accumulator to ensure rapid pressurization due to
rapid heating is the key process 11. Operating pressure and duration of dewaxing used
in the past research was approximately from 726kPa – 750kPa and 10 minutes 22.
In the same research, it was reported that autoclave dewaxing altered the
properties of the pattern wax such as strength and hardness of the wax. This due to
the moisture content present in the autoclave process causes water appearance in the
wax and the wax needs to be stirred and heated to remove water for purification
purpose. This causes some selective removal of the components or particles of the
wax and thus alters the properties of the wax. Due to the altered properties, the wax
does not have long life. This increases the cost of purchasing new pattern wax.
For flash dewaxing process, it is carried out by inserting the shell mould into
a hot furnace at 870 to 1095°C 11. The furnace is equipped with an open bottom so
that wax can fall out of the furnace as soon as it melts. Under these conditions a steep
temperature gradient produces superficial melting of the wax before it undergoes
appreciable expansion 6. In this technique, dewaxing and firing are completed in a
single operation of about 2 hours Wax from this method can be reclaimed for future
use 11.
Another technique that is used for dewaxing is to employ microwave
equipment to dewax the pattern. The wax used in the investment casting process
includes materials with dielectric properties, which are able to be heated by
microwave absorption 22. In this method, a microwave oven is used operating with a
certain frequency, power, temperature and power. Microwave energy is applied
rather than conventional thermal energy that is obtained upon heating the mould.
Wax removed from microwave technique last longer and can be reclaimed for future
31
use. However, the duration of dewaxing using this technique increases as longer time
is needed to melt the whole pattern wax
In this research 22, the microwave dewaxing technique showed lower volume
expansion of the wax compared to the autoclave technique. This is due to the non
thermal phenomena where the microwave energy is not converted into thermal
energy but directly transferred to energy modes within the molecule of lattice.
Therefore, there is no thermal vibration which causes a more packed molecule
structure in the wax compared to the autoclave dewaxing.
2.6 Firing
After the mould has been dewaxed, it is subjected to firing process. Firing
process is also called sintering process, which is to strengthen the mould and to burn
off residues. The ceramic materials are heated below its melting point until they
adhere together. The aim of firing is also to burn of moisture content to prevent any
alteration to the properties of the shell mould and porosity due to air bubbles. Typical
firing temperature used in investment casting foundry is usually 1000°C, while other
temperatures have also been used. Heating rate is controlled to ensure no crack
occurs at the ceramic mould due to abrupt thermal expansion of the ceramic
materials.
Firing can be done in multiple stages, depending on the condition of the
mould. Multiple stages firing allows the time for amendment and repair to be made at
the mould’s crack. Cracked moulds can be repaired with ceramic slurry or special
cements 11. This will not heal cracks, but will seal them and reinforce the shell so that
it can be successfully cast.
32
Precise and controlled thermal treatment plays the crucial role in producing
quality shell mould. Temperature, time and kiln or furnace temperature influence the
properties of fired ceramic mould
23
. Different types of ceramic products require
different firing conditions. This involves the time, firing temperature and rate of
heating. Literatures had been done on developing the theoretical firing schedule
based on volume change of the ceramic body and allowing other factors that affect
firing 23. It was reported that the total firing time of the theoretical curve is dependent
on the type of body, density of the setting, thickness of the ceramic body and the kiln
cross section. Thermal analysis such as differential thermal analysis (DTA),
thermogravimetric analysis (TGA), differential scanning calorimetry (DSC) is the
best tool to detect firing defects.
Some defects are encountered during firing process. One of the main
characteristics of ceramic firing is the drastic reduction in volume
23
. This leads to
reduction in the size of the shell mould and creates inaccuracies compared to the blue
print product. Therefore a controlled heating rate is needed. A body containing an
excess of quartz will be at risk if the rate of firing or cooling is allowed to pass too
quickly through the quartz inversion temperature, in which instance the sudden
expansion or contraction of the quartz may be sufficient to cause cracking 24.
Defect occurs also in the condition where the firing is done with uneven
heating in the kiln. This is in the case where one part of the product is fired at a
temperature than the others. This cause uneven strain on the surface of the ceramic
shell fired 24. Slurry which has not been given enough time to dry completely during
firing suffers from steam generation within it. This cause porosity that weakens the
shell mould.
The objective of firing is also to preheat the mould to a specific temperature
to facilitate casting of molten metal. High mould temperature improves mould filling
of the molten metal and low mould temperature retards the mould filling such as fast
solidification of the molten and prevention of the rest of the molten metal from
33
entering the mould due to the chilling effect of the mould leads to fast heat transfer
rate. Many moulds are wrapped with ceramic-fibre blanket to minimize the
temperature drop that occurs between the preheat furnace and the casting operation
or to provide better feeding by insulating selected areas of the mould
furnaces are used for mould firing and preheating
11
.Gas-fired
11
. Most furnaces are equipped
with firebrick insulation to prevent heat loss to the environment. Furnace with
ceramic-fibre insulation and luminous wall furnaces are also used. Ceramic
recuperators are equipped at the furnace to reduce the smoke emission and obtain
high fuel efficiency.
2.7 Casting
After firing, molten metal is cast into the ceramic mould. The molten metal in
the ceramic mould is let for full solidification. The mould is then subjected to
knockout to obtain the final solidified metal. This is the final product desired by the
caster and manufacturer. Therefore, besides the ceramic mould, the quality of the
final product also depends on the solidification of the molten metal. Poor
solidification leads to shrinkage defects of metal cast and this leads to dimensional
inaccuracies to the final product.
These defects can be reduced by having directional solidification and having
good molten fluidity. Directional solidification occurs when the first wave front of
molten metal which reaches the mould wall and fully solidifies and this followed by
second wave front of molten metal which also fully solidifies when it reaches the
first wave. This continues to the core of the casting. In perfect directional
solidification, there are no shrinkage defects or impurities.
Fluidity is defined as the ability for a molten to flow through and fill a mould
cavity before solidification occurs 1. The fluidity controls the molten filling of
34
moulds and the sharpness of cast details. Fluidity is therefore an important property
for the successful production of castings. Cleanliness of the molten influences the
fluidity of the molten metal. Molten metal which has high impurities, inclusion
particles and dissolved gases greatly reduces the fluidity.
Parameters which influence the mould filling of the metal include:
a) Casting temperature of the alloy
b) Type of alloys
c) The mould preheat temperature
d) The thickness of the mould
e) The molten metal pressure head
The casting temperature of the alloy greatly influences the solidification.
Higher casting temperature molten has longer solidification time and this promotes
better mould filling. Lower casting temperature makes the molten metal fast
solidifies due to small temperature difference between the molten and the mould.
This is proven in this research 1, where the use of superheat to the molten leads to
better mould filling. This is because the enthalpy of the molten metal increases with
the pouring temperature. The molten metal remains liquid for a longer period of time.
However, if the pouring temperature is too high, some of the alloys elements would
be burned off 25. The amount of shrinkage also increases with pouring temperature.
Type of alloys includes long freezing range alloys and short freezing range
alloys. For long freezing range alloys, directional solidification is harder to achieve.
This is because the range between solidus point and liquidus point is long for this
type of alloys. Some part of the molten already solidifies while the rest has not. This
creates shrinkages pores and obstacles to the passage of the molten and thus
preventing directional solidification. For short freezing range alloy, the range
between the solidus point and liquidus point is short. Therefore all portion of the
molten solidifies concurrently, promoting directional solidification.
35
The mould is preheated to a certain temperature during firing process before
casting. This temperature is called the mould preheat temperature. Increasing preheat
reduces the amount of heat the mould can absorb and the temperature difference
across the mould 25. This allows the metal to remain in liquid form for a longer time
and promote fluidity. However, higher mould temperature promotes mould metal
reactivity and had a deleterious effect on the surface finish 1. Low mould preheat
temperature allows faster heat transfer between the mould and metal, resulting fast
solidification. Fast settling alloys need high mould preheat temperature and slow
settling alloys need low preheat temperature.
The thickness of the mould also affects the fluidity of the molten metal and
the effective mould filling. The heat transfer rate is inversely proportional to the
mould thickness. This is because the distance through which the heat is transferred is
short. This is important especially for thin section where the mould filling is more
difficult.
When the molten is poured from a height into the mould, there is a pressure
head associated with the height. If the height value is large, pressure head would be
large for the molten metal. This promotes the flow of molten metal into the mould.
Therefore, there is a linear relation between mould filling capacity and metal head 1.
However, large height is associated with large turbulence and gas pickup and these
factors lead to defects.
Vacuum casting has been practiced by many foundries to improve the casting
conditions. The use of vacuum casting increases the mould filling capacity and
improves the fluidity of the molten metal. In vacuum casting, the ceramic mould is
placed in a vacuum chamber. A partial vacuum is drawn in the chamber and around
the mould while the molten metal is poured into it. Less air is involved in casting,
thus reducing the gas porosity and pore sizes.
36
CHAPTER 3
METHODOLOGY
Overview
Chapter 3 explains in detail the laboratory work that has been done for this
research. The laboratory work has been done in the air conditioned room in the
casting laboratory in Universiti Teknologi Malaysia. The room temperature is set at
25ºC and the relative humidity of that room is approximately 50%.
The laboratory work is divided into 2 parts: the work for MOR test specimens
and the work for permeability test specimens. MOR test is divided into 3 systems,
namely: green (undewaxed), green (dewaxed) and fired. Workpieces are made for
these 3 systems. The laboratory work for MOR test specimens is divided into 6
stages: pattern making, investment (slurry, stucco and fibre), dewaxing, firing,
trimming of workpiece and testing. Permeability test is divided into 2 systems,
namely: green (dewaxed) and fired. The laboratory work for permeability test
specimens is divided into 5 stages: pattern making, investment (slurry, stucco and
fibre), dewaxing, firing, trimming of workpiece into required size and testing. The
investment stage for each part takes the longest time to complete due to the duration
the slurry and stucco take to dry and gel. During the dipping pattern into the slurry
and stuccoing, the challenge faced is to make sure the complete coverage of the
slurry and stucco powder all over the pattern due to occasionally, the shrinkage of the
slurry on the surface of the pattern leaves dents and holes which will lead to cracking
during dewaxing.
37
During the dewaxing process, care has to be taken due to fact that despite
after ensuring the even coverage of slurry and stucco on the surface of the pattern,
cracking is sometimes inevitable. Therefore, slower rate of fire of the dewaxing
chamber is employed to minimize the cracking. After dewaxing, firing process is
conducted. Care must also be taken where the workpiece should not be taken out
from the firing chamber immediately just after firing is finished. This is to prevent
cracks on the ceramic workpiece caused by the contracting and expanding due to
large temperature difference between the firing chamber and the surrounding.
Process stages
MOR test:
Permeability test:
Pattern making
Pattern making
Investment (slurry, stucco
Investment (slurry, stucco
and fibre)
and fibre)
Dewaxing
Dewaxing
Firing
Firing
Trimming of workpiece
Testing
Testing
Figure 3.1 Process stages
38
3.1 Pattern making
The pattern is made by injecting the pattern wax into the die. The wax used in
this research is the conventional investment casting pattern wax.
Figure 3.2 Die dimension (in mm)
The wax is heated to a temperature of 65ºC. Then the wax is then poured into
the aluminium die. The pouring is done by using a sprue The pattern is left to cool in
the die for about 1-2 hours to allow full solidification. The pattern is then removed
from the die by continuous knocking the die. The pattern is coated with interface
agent or wetting agent.
39
3.2 Slurry
3.2.1 Refractory filler
For this research, zircon sand is used as the refractory filler in the slurry
system. For the primary coating, 300 mesh zircon sand is used. For the secondary or
backup coating, mesh size of 200 is used for the coarser zircon sand.
3.2.2 Refractory binder
In this research, the binder used is the colloidal silica binder.
3.2.3 Slurry Mixture
For primary coating, 1000 ml of colloidal silica binder is measured and
poured into a container. Zircon sand of 300 mesh size is added into the container and
the mixture is being stirred by a mixer. The slurry viscosity for the primary coating
needed is approximately about 25 second, measured using a Zahn cup 5. For
secondary coating, the viscosity required is in the range of 10s for the slurry,
measured by Zahn cup 5. The procedure is the same as in the primary coating, except
that the 200 mesh size of Zircon sand is used. For the seal coat, the viscosity is about
25 second.
40
Table 3.1 Slurry system for MOR test and Permeability test with variation of fibre
length
MOR [Green (undewaxed and
dewaxed) and Fired]
No
Primary
coating
Drying
time
Permeability
Dipping
Dipping
time
time
Viscosity
Dipping
time
Drying
time
25s
30s
24H
25s
30s
24H
10s
30s
90-120m
10s
30s
45-90m
25s
30s
24H
25s
30s
24H
(1)
Secondary
coating
(2-6)
Seal coat
(7)
41
Figure 3.3 Slurry mixture
3.3 Organic fibre additive
The organic fibre that has been chosen for this research is the oil palm
fibre. The addition of organic oil palm fibre is to determine it can be used as an
alternative than polymer for the colloidal silica based slurry and also to see the effect
of it on the green and fired strength and the permeability of the mould.
The effect of various length of the fibre addition is considered. The fibre
length can be divided into 3mm, 5mm, 7mm and 9mm. Each fibre has an average
diameter of 30µm. The fibres are cut these lengths by using scissors. The fibres are
then washed with water to purify the content from any dirt or dust. The fibres are
then dried to remove moisture and water.
42
3.4 Stucco
The refractory material used to form the stucco system is Molochite and the
grade of the Molochite used is fine. The molochite flour is stuccoed by sprinkling the
sands by bare hands onto the mould. 50/80 mesh size for primary coating and 30/80
mesh size for secondary coating.
3.5 Preparation of samples for modulus of rupture (MOR) test
3.5.1 Samples making
Same primary and secondary slurry, stucco system and type of fibres are used
for all types of samples. Refer to section 3.2, 3.3, 3.4 for slurry composition, organic
fibres and stucco system. There are 5 variations of length of fibres:
a) 3mm length
Preparation of fibres is done by cleaning the fibres from any undesirable
particles and oil stain by repeated washing with water. The fibres are then let to dry
to remove any moisture content. They are chopped to 3mm length after drying. 200g
of 3mm long fibres are measured then mixed in the secondary slurry.
The pattern is dipped into the primary slurry for 30 seconds. The pattern or
mould is taken out and then stuccoed with molochite. The mould is then allowed to
dry for 24 hours. The pattern is then coated with secondary slurry with fibres for 30
seconds and let for semi-gel for 15 minutes and then stuccoed with molochite. The
43
mould is then let to fully gel for 90-120 minutes for next secondary coating. These
procedures for secondary coatings are done for 5 times. The mould is then coated
with the seal coat and let to dry for 24 hours.
b) 5mm length
The preparation of the fibres is the same as the 3mm length section except
that the fibres are chopped to 5mm length. 200g of 5mm length fibres are measured
and mixed into the secondary slurry. The coating procedures are the same as the
3mm length section.
c) 7mm length
The preparation of the fibres is the same as the 3mm length section except
that the fibres are chopped to 7mm length. 200g of 7mm length fibres are measured
and mixed into the secondary slurry. The coating procedures are the same as the
3mm length section.
d) 9mm length
The preparation of the fibres is the same as the 3mm length section except
that the fibres are chopped to 9mm length. 200g of 9mm length fibres are measured
and mixed into the secondary slurry. The coating procedures are the same as the
3mm length section.
e) No fibre
For this system, no fibre is added and the coating procedures and slurry
system remain the same.
44
For each variation of length, 3 ceramic moulds are made for the green
(undewaxed), green (dewaxed) and the fired system. For the green (undewaxed), the
ceramic mould does not undergo dewaxing and firing process.
Primary slurry
Dry for 24 hours
Stucco
Secondary coating with
fibres (semi gel 15
minutes) and then stucco.
Dry for 90-120 minutes
Seal coat
Figure 3.4 Mould formation sequence for MOR test of variation of fibre
length
45
Figure 3.5 Wax pattern coated with primary layer
Figure 3.6 Wax pattern coated with secondary layer, fibres and stucco
46
Figure 3.7 Wax pattern coated with seal coat layer
3.5.2 Dewaxing
The dewaxing temperature used is from 200°C to 300°C for approximately
10-15 minutes. The melting wax is allowed to drain from the ceramic mould through
the wax drain valve to a container for future use.
3.5.3 Firing
After the dewaxing of the pattern, the ceramic shell mould then undergoes
firing process. The firing process is done by putting the ceramic mould into a furnace.
The firing temperature used in this research is 700ºC. The firing is done for 60-75
minutes to strengthen the mould. If cracks are found at the surface of the mould, the
47
mould will be allowed to cool off and then undergo repair. The cracks can be
amended by putting slurry into the crack gap and the mould is again undergone firing
at a lower temperature to ensure perfect condition. The moulds are let to cool off in
room temperature for further testing experiments.
3.5.4 MOR test
MOR test is run for green (undewaxed), green (dewaxed) and fired state of
the ceramic mould. The green state with undewaxed is cut directly in half and
trimmed to the specimen size without dewaxing. Each specimen has a dimension of
approximately 100mm in length X 10mm in width. The mould is trimmed to the
specified size by using diamond grind wheel. These specimens then undergo the
modulus of rupture test (MOR) to determine the green and fired strength at a tester.
MOR test is carried out in accordance to BS 1902-4.5:2000. The machine used is the
Universal Tester Machine (model Instron 4206, 100 kN motor). The test is
performed on 3 specimens for each variation of fibre orientation and variation of
fibre length. Average MOR value is then taken. Rate of increase of load is 0.15MPa/s.
PMax , the fracture load is recorded. The MOR, σMax, was calculated using equation:
σMax =
3Pmax L
2WH 2
where PMax is the fracture load, W and H are the width and thickness of sample
fracture area, L is the span length. The average of σMax is taken for the 3 specimens
for each test.
48
Table 3.2 MOR test with variation of fibre length
Length
3mm
5mm
7mm
Test, σMax (MPa)
Green
Green
(undewaxed)
(dewaxed)
1)
2)
3)
1)
2)
3)
1)
2)
3)
9mm
1)
2)
3)
No fibre
1)
2)
3)
Fired
Average
Green
Fired
Figure 3.8 MOR specimen dimensions (in mm)
49
Figure 3.9 Universal Tester Machine (model Instron 4206, 100 kN motor)
Figure 3.10 MOR samples
50
3.6 Preparation of samples for permeability test
3.6.1 Samples making
The primary and secondary slurry, stucco system and type of fibres used for
all types permeability test samples are the same as MOR test. Refer to section 3.2,
3.3, 3.4 for slurry composition, organic fibres and stucco system. There are 5
variations of length of fibres:
a) 3mm length
Preparation of fibres is done by cleaning the fibres from any undesirable
particles and oil stain by repeated washing with water. The fibres are then let to dry
to remove any moisture content. They are chopped to 3mm length after drying. 200g
of 3mm long fibres are measured then mixed in the secondary slurry.
Galvanized pipe with threads is poked into the table tennis wax ball. The wax
ball is dipped into the primary slurry for 30 seconds. The ball is taken out and then
stuccoed with molochite. The ball is then allowed to dry for 24 hours. The ball is
then coated with secondary slurry with fibres for 30 seconds and let for semi-gel for
15 minutes and then stuccoed with molochite. The ball is then let to fully gel for 4590 minutes for next secondary coating. These procedures for secondary coatings are
done for 5 times. Seal coat is applied at the end and let for drying for 24 hours.
51
b) 5mm length
The preparation of the fibres is the same as the 3mm length section except
that the fibres are chopped to 5mm length. 200g of 5mm length fibres are measured
and mixed into the secondary slurry. The coating procedures are the same as the
3mm length section.
c) 7mm length
The preparation of the fibres is the same as the 3mm length section except
that the fibres are chopped to 7mm length. 200g of 7mm length fibres are measured
and mixed into the secondary slurry. The coating procedures are the same as the
3mm length section.
d) 9mm length
The preparation of the fibres is the same as the 3mm length section except
that the fibres are chopped to 9mm length. 200g of 9mm length fibres are measured
and mixed into the secondary slurry. The coating procedures are the same as the
3mm length section.
e) No fibre
For this system, no fibre is added and the coating procedures and slurry
system remain the same.
52
For each variation of length, 2 ceramic moulds are made for the green
(dewaxed) and the fired system.
Primary slurry
Stucco
Dry for 24 hours
Secondary coating
with fibres (semi
gel 15 minutes)
and then stucco.
Dry for 45-90
minutes
Seal coat
Figure 3.11 Mould formation sequence for permeability test of variation of fibre
length
53
Figure 3.12 Wax pattern coated with primary layer
Figure 3.13 Wax pattern coated with secondary layer, fibres and stucco
54
3.6.2 Dewaxing
Dewaxing for permeability test samples is the same as MOR test samples.
Refer to section 3.5.3 for dewaxing procedures.
3.6.3 Firing
Firing for permeability test samples is the same as MOR test samples. Refer
to section 3.5.4 for firing procedures.
3.6.4 Permeability sample test
For the permeability test, a permeability tester is used. The permeability test
is performed on the green (dewaxed) and fired ceramic moulds. Rubber tube of
permeability tester is connected to the threads of the galvanized pipe. The room
temperature is recorded. Testing is made in accordance to BS 1902: Section
10.2:1994. The permeability of the shell is determined by recording the flow of air
through a shell mould of known dimensions under a known pressure difference and
at a given temperature 26. Samples are heated at a rate of 20ºC/min up to 100ºC and
the tester rubber tube is inserted to the samples through the hole. The pressure
difference and flow are recorded at every 5 minutes from the 0 minute to 20 minutes.
3 specimens are tested for each variation of length and each system (green and fired).
Average permeability value is then taken. The permeability is then calculated from
the following equation:
55
μ=
Vc l
ap
with Vc =
V1T
T1
where:
η is the dynamic viscosity of air at the temperature of the test (in Ns/m2)
Vc is the volumetric flowrate of air corrected for expansion at elevated temperatures
(in m3)
V1 is the volumetric flowrate of air at room temperatures (in m3)
l is the thickness of the shell mould (in m)
a is the inner surface of the hollow shell mould (in m)
P is the pressure difference across the test piece (in N/m2)
T is the elevated temperature (in Kelvin)
T1 is the room temperature (in Kelvin).
Table 3.3 Permeability test with variation of length of fibre
Length
3mm
5mm
7mm
Green
(dewaxed)
1)
2)
3)
1)
2)
3)
1)
2)
3)
9mm
1)
2)
3)
No fibre
1)
2)
3)
Fired
Average
Green
Fired
56
Figure 3.14 Permeability tester
57
CHAPTER 4
RESULTS AND DISCUSSIONS
Overview
Chapter 4 discusses about the tests results which have been obtained from the
laboratory work. There are 2 tests performed: MOR and permeability tests. MOR test
is divided into 3 systems, namely: green (undewaxed), green (dewaxed) and fired.
Permeability test is divided into 2 systems, namely: green (dewaxed) and fired.
Results are compared among these systems and reasonings have been made to
explain each comparison. Surface structure magnification pictures and comparison
graphs are added to clarify the explanation.
4.1 MOR test
From the results of the MOR test from table 4.1, it can be seen that the green
(dewaxed) specimens have the highest MOR values among the 3 test systems. This
can be explained that for the green (dewaxed) system, some of the fibres added are
not burnt away in the shell after dewaxing. In addition, the dewaxing adds some
sintering effect to the shell mould. These 2 factors contribute to the increase in MOR
values and also explain why the dewaxed system has higher MOR than the
undewaxed system (since there is no sintering due to no dewaxing in the undewaxed
system).
58
For the green (dewaxed) specimens, the 3mm fibre addition test specimen has
the highest MOR value, 3.51 MPa, and the fibre addition test specimens have lower
values: 5mm (3.3MPa), 7mm (3.17 MPa), 9mm (2.31 MPa) and no fibre addition
(2.26 MPa), This can be explained that the when the fibres are short while the same
weight is maintained, the amount of fibres increases in number and this leads to the
evenly orientation and spacing of the fibres in the ceramic mould. Since the tensile
strength of all the fibres is the same, therefore the tensile strength is higher in
systems with shorter fibre length. The ceramic mould strength then increases with the
addition of fibres of shorter length.
The fired test specimens have the lowest MOR values among the 3 systems
except for the no fibre (MOR = 4.17 MPa) and 7mm fibre addition (MOR = 2.58
MPa) specimens. This is because after firing, all the fibres added are completely
burnt away, leaving porosities in the shell mould. Burning of fibres leaves passage
and holes leads to less combination of the grains therefore causes lower strength.
This causes the test specimens to have lower strength when compared to the green
systems. This is proven in the past research 4. Passage left by the burnt out fibres
leave porosities and this has greater effect than the sintering effect by firing. This
happens because the fibres prevent the grain in the slurry to combine with each other
during sintering. The no fibre addition specimens for the fired system has the highest
MOR (4.17 MPa) compared to the fibre addition specimens in the same system
suggesting that there are no voids left in the shell mould since no fibre is added.
The 3mm fibre addition specimens have the lowest strength (1.57 MPa) in the
fired system than the 5mm and 9mm fibre addition specimens. This is explained by
the same reasoning from the dewaxed system: when the fibres are short while the
same weight is maintained, the amount of fibres increases in number. Therefore the
3mm has the highest amount of fibres in the ceramic mould. After firing, all the
fibres are burnt out for all types of specimens and as the 3mm specimen has the
highest amount of fibres burnt out, thus it has the highest amount of voids left and
therefore lowest strength compared to the 5mm and 9mm specimens. Supposedly the
7mm specimen has the second highest MOR in the fibre addition specimen of the
59
fired system. However, the 7mm specimen has a slightly higher MOR (2.58 MPa)
than the 9mm specimen (1.66 MPa). This maybe caused by the uneven placing of the
fibres during the experiment in the 7mm fibre addition specimens mould which leads
to the increase of strength.
For the green (no dewaxing) specimens, this system generally has higher
MOR values than the fired system except for the no fibre addition (MOR = 1.56 MPa)
and the 7mm fibre addition (MOR = 1.94 MPa) specimens. The shells are cut into
specimens size without dewaxing and firing. Therefore the fibres added are fully
intact. The fibres tensile strength contributes to the MOR of the ceramic shell mould,
making them harder. This can be seen that for the no fibre addition specimen, the
MOR value is less than that of the fibres addition specimens, suggesting that fibres
are influencing the ceramic mould strength greatly in the green (no dewaxing)
condition. The 3mm fibre addition specimen has the highest MOR (2.14 MPa) in the
green (no dewaxing) system than the 5mm, 7mm and 9mm fibre addition specimens.
This also has the same reasoning: when the fibres are short while the same weight is
maintained, the amount of fibres increases in number. Therefore, the ceramic mould
strength then increases with the addition of fibres of shorter length.
60
4.2 Permeability test
From the results of the permeability test from table 4.1, for the green system,
the 3mm, 5mm and 9mm fibre addition specimens have identical permeability value
-16
(5.00655 x10
m2). This is because of the fibres are still left intact and no additional
voids and porosities are present in the mould. The 7mm fibre addition specimen has
-16
slightly higher permeability value (5.0488 x10
m2) due to of some cracks obtained
in this specimen during the dewaxing process. For the fired system, the fibre addition
specimens (3mm, 5mm, 7mm, 9mm) have higher permeability value than their
counterpart in the green system. This is because of the same reasoning as the MOR
test: the fibres are burnt away during firing process and many voids are left behind.
This adds to the additional porosities in the ceramic mould, therefore increases the
air penetration inside the ceramic mould.
For the fired system, the 3mm fibre addition specimen has the highest
-16
permeability value (5.36575 x10
m2) than the 5mm, 7mm, 9mm and no fibre
addition specimens. This can be explained that when the fibres are short (3mm)
while the same weight is maintained, the amount of fibres increases in number and
this leads to the evenly spacing of the fibres on the each layers of the slurry.
Therefore when the fibres are burnt out during firing process, there is large amount
of voids left and thus maximizes the air penetration. The 9mm fibre addition
specimen has the lowest permeability (5.17553 x10
-16
m2) value among the fibre
addition specimens in the fired system. This is because the 9mm specimen has the
lowest amount of fibres in the ceramic mould, leaving fewer voids after firing
process.
61
Table 4.1 Results of MOR and permeability tests
Green (no
dewaxing)
MOR
Green (dewaxed)
Fired
MOR
Permeability
m2 (x10-16)
MOR
No
Fibre
1.56
2.26
4.75309
4.17
3 mm
2.14
3.51
5.00655
1.57
5.36575
5 mm
2.07
3.3
5.00655
1.645
5.26002
7 mm
1.94
3.17
5.0488
2.58
5.21777
9 mm
1.81
2.31
5.00655
1.66
5.17553
Permeability
m2 (x10-16)
4.92206
62
a
b
c
Porosity
Porosity
Porosity
Figure 4.1 Surface structure of the no fiber addition ceramic mould.
(a) Green undewaxed. (b) Green dewaxed. (c) Fired
(10X magnification)
a
b
Fibre unburn
c
Fibre unburn
Voids left
Figure 4.2 Surface structure of the 3mm fiber addition ceramic mould.
(a) Green undewaxed. (b) Green dewaxed. (c) Fired
(10X magnification)
a
b
Fibre unburn
c
Fibre unburn
Voids left
Figure 4.3 Surface structure of the 5mm fiber addition ceramic mould.
(a) Green undewaxed. (b) Green dewaxed. (c) Fired
(10X magnification)
63
a
b
Fibre unburn
c
Fibre unburn
Voids left
Figure 4.4 Surface structure of the 7mm fiber addition ceramic mould.
(a) Green undewaxed. (b) Green dewaxed. (c) Fired
(10X magnification)
a
b
Fibre unburn
c
Fibre unburn
Voids left
Figure 4.5 Surface structure of the 9mm fiber addition ceramic mould.
(a) Green undewaxed. (b) Green dewaxed. (c) Fired
(10X magnification)
64
MOR (green undewaxed)
2.5
MOR (MPa)
2
3mm
5mm
1.5
7mm
9mm
No fibre
1
0.5
3mm
0
Figure 4.6 Graph of MOR (green undewaxed)
MOR (green dewaxed)
4
3.5
MOR (MPa)
3
2.5
3mm
5mm
7mm
2
1.5
9mm
1
0.5
0
Figure 4.7 Graph of MOR (green with dewaxing)
No fibre
65
MOR (Fired)
4.5
4
No fibre
MOR (MPa)
3.5
7mm
3
2.5
2
3mm
5mm
1.5
9mm
1
0.5
0
Figure 4.8 Graph of MOR (fired)
MOR comparison between 3 systems
No fibre
4.5
4
3.5
MOR (MPa)
3
2.5
3mm
5mm
7mm
9mm
Green
undewaxed
Green
dewaxed
Fired
2
1.5
1
0.5
0
Figure 4.9 Graph of MOR comparison of green undewaxed, green with dewaxing
and fired system
66
Permeability m2 (x10e-16)
Permeability (green)
5.1
5.05
9mm
5
4.95
3mm
5mm
7mm
4.9
4.85
No fibre
4.8
4.75
4.7
Figure 4.10 Graph of permeability (green with dewaxing)
Permeability m2 (x10e-16)
Permeability (fired)
5.4
5.3
5.2
5.1
3mm
9mm
5mm
7mm
5
No fibre
4.9
4.8
Figure 4.11 Graph of MOR (fired)
67
Permeability m2 (x10e-16)
Comparison of permeability (green and fired)
5.4
5.3
5.2
3mm
5mm
5.1
7mm
9mm
5
4.9
No fibre
4.8
4.7
Fired
Green undewaxed
Figure 4.12 Graph of MOR comparison between green and fired system
68
CHAPTER 5
CONCLUSION AND RECOMMENDATION
In the past decades, ethyl silicate has been the primary binder used in the
investment casting industries. However, its toxicity nature due to the fact that it is
alcohol based caused many countries to forbid its usage. Therefore the use of the
binder colloidal silica increases due to its water based nature. However, ceramic
mould produced by using colloidal silica has a longer time to gel than the ethyl
silicate based. This leads to the increase in production time and eventually the lead
time. In the past research
26
, the colloidal silica ceramic mould was proven to have
low green strength than the ethyl silicate based ceramic mould. This causes the
mould to have cracks during dewaxing and the pouring of molten metal. Polymers
such as latex based or PVA have been used to increase the green strength. However,
the polymer is very expensive.
Therefore, few researches had been done to find alternatives to replace the
polymer to increase the strength and the permeability of the ceramic shell mould.
This research 4 had been done to analyse the effects of synthetic fibre addition on the
ceramic shell strength and permeability. The research found that with the addition of
synthetic fibres, the fired strength did not have much difference with the green
strength, indicating the synthetic fibre addition did not influence the strength greatly.
Furthermore, the green and fired strength of fibre addition mould are lower compared
to the polymer addition mould. However, the permeability was proven to increase.
In this research 5, the rice husk was added to the ceramic shell mould and the strength
and permeability had been tested. Results indicated that generally, the ceramic shell
69
mould with rice husk addition has lower strength than the one of no rice husk
addition. However, the permeability was proven to increase.
In this current research, organic fibre- the oil palm fibre has been used as the
alternative to evaluate the strength and the permeability of the ceramic shell mould.
For the strength test (MOR), 3 systems had been used, namely the green
(undewaxed), green (dewaxed) and fired. Results showed that for generally, fired
strength of fibre addition moulds has lower strength than that of the no fibre addition
one. The green (undewaxed) and green (dewaxed) strengths were proven higher than
the fired strength. In the green (dewaxed), the strength of the fibre addition mould is
much lower than the no fibre addition mould.
The permeability, as expected is higher in the fired mould than the green
mould due to the same reasoning, burning of fibre in the mould. The permeability of
the non fibre addition mould in the fired condition is lower than the fibre added one.
This is similar in the green condition too.
The results obtained indicate that the fibre can enhance the strength of the
mould in green condition, and this can be utilized in the production process where no
firing is required. The fibre addition mould in fired condition with lower strength is
recommended to be used in small production application due to the fact that the low
strength would not be exceeded and the shell would not break in those conditions. It
is not recommended for large production application such as production of big shaft,
large machine parts and etc because the low strength would not sustain the large
amount of dewaxing and heavy weight of pouring.
Future work should be done on the method of placement and orientation of
fibres in the ceramic mould to see the effects of different orientation of fibres in the
different section of mould on the strength and permeability. Different types of fibres
can also be considered for future work.
70
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2) E.Green-Spikesley. Investment casting. Materials in Engineering Applications.
1979. Vol 1.
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