Materials Sciences and Applicatio n, 2011, 2, 1143-1160
doi:10.4236/msa.2011.28155 Published Online August 2011 (
Copyright © 2011 SciRes. MSA
Foundry Coating Technology: A Review
U. C. Nwaogu*, N. S. Tiedje
Technical University of Denmark, Department of Mechanical Engineering, Institute of Production and Process Technology, Kgs.
Lyngby, Denmark.
Email: *
Received April 27th, 2011; revised May 16th, 2011; accepted May 31st, 2011.
The importance of foundry coating in improving the surface quality of castings cannot be over emphasized. The appli-
cation of mould and core washes creates a high thermal integrity barrier between the metal and the mould resulting in
the reduction of the thermal shock experienced by the sand system. These thermal shock leads to series of surface de-
fects such as veining/finning, metal penetration, burn-on/in, scab, rat tail, erosion etc. The use of coatings reduces the
tendency of occurrence of these defects. However, the understanding of the coating, its components, characteristics and
mechanism of action is important. In this review, a detailed description of these topics and examples are provided
where necessary. A potential area of research in foundry coating development, using sol-gel process is suggested. The
application of sol-gel technology in the development of foundry coatings is a novel approach.
Keywords: Coating, Refractory Materials, Application Methods, Characterization, Sol-Gel Technology
1. Introduction
Research in coatings for various applications such as
aesthetics, corrosion protection, wear resistance, thermal
barrier, self-cleaning, antifouling etc. have been very
wide spread but not much is going on in the area of
foundry coatings in recent times. The use of foundry
coatings for moulds and cores during casting is very
necessary as a means of achieving high quality surface
finish of castings more especially in complex internal
channels created by use of cores. This is despite the con-
siderable advances that have taken place over the recent
years in binder and sand technology giving the foundries
greater opportunity to choose and control these basic
foundry raw materials. Since casting surface finish de-
pends largely on sand particle grading, it might be sup-
posed that a proper selection of a particular grade of sand
would be the only requirement to achieve the desired
casting surface quality. However, there are other factors
to be considered, such as the ability to vent off the gases
produced during casting, economic use of a binder, non
availability of sand with required grading, etc., these
make the use of coatings the more practicable approach
In filling a mould with liquid metal its surface is sub-
jected to thermal, mechanical and physicochemical ac-
tions. The oxidation products of the metal, reacting with
the mould material, form low-melting materials such as
silicates, which lubricate the grains of the quartz sand
well. This promotes penetration of the metal into the in-
ter-granular spaces and the formation of mechanical
pick-up which is difficult to remove from the casting
surface. Considering that the sand moulds and cores are
highly porous, the production of castings in these materi-
als without pick-up and other surface defects is possible
only with protection of the surfaces of moulds and cores
with refractory coatings. The fundamental requirements
for the refractory coatings are minimum porosity, high
refractoriness and reduction of the physicochemical reac-
tion at the metal-coating interface (lubrication, solution,
penetration) [2]. These refractory coatings are used to
make better castings and to reduce costs. Castings sur-
face quality is improved because the coating produces
smoother metal surfaces, either by filling the spaces be-
tween the sand grains or by providing, to the metal, a
surface smoother than the mould surface itself. Further
improvement from the coatings is due to the cleaner and
better peel of sand at shakeout and elimination of certain
defects such as metal penetration, veining, erosion, sand
burn-in etc [3,4].
Controlling casting quality and increasing productivity
are top priorities for foundries to become more competi-
tive in a global casting market and coatings can help to
provide the required remedy. Addressing the issue of
mould/core moisture can lead to improvements in pro-
ductivity and help keep foundries competitive. Identify-
ing problems like poor mould/core density and moisture
in the both core and mould is challenging, but advance-
Foundry Coating Technology: A Review
ment in coating technology enhances the engineering of
refractory coatings as a quality-control tool to help iden-
tify these issues. The presence of moisture can lead to a
scrapped casting, but coatings that indicate when drying
is complete can address this issue. Coating technologies
that change colour offer visual confirmation that the
coating is dry. This confirmation may also indicate poor
sand compaction in a core or mould, as these areas will
absorb more moisture from the coating and take longer
time to dry. Therefore, a visually obvious colour change
based on moisture content permits these new refractory
coatings to act not only as a barrier between the metal
and the mould or core but also as a quality diagnostic [5].
The objective of this paper is to collate as much as
possible the significant works and results on foundry
coatings in the past and to give insight to a novel tech-
nology for the production of foundry coatings with
greater potential towards improving the surface quality of
castings from readily available raw materials at a cheaper
cost. This paper provides a detailed understanding of the
constitution of foundry coatings while providing alterna-
tives to the foundry coating components depending on
the metal to be cast and their properties and compatibility
with sand properties such as grain size and grain size
distribution and binder properties.
2. Groups of Foundry Coatings
Foundry coatings may be divided into two groups, those
applied dry and those applied wet.
2.1. Coatings for Dry Application
For dry application, the most widely used is Plumbago.
Other dry coatings used to a lesser extent include mica,
white talc and wheat flour. These materials are either
shaken or blown onto mould or core surfaces from
open-mesh cloth bags. Plumbago is a finely ground blend
of graphite containing 80% to 90% of particles that will
pass through a 200-mesh (75 micron). The graphite may
be amorphous (no definite crystal structure) or crystalline
(having definite particle shape or flaky). Graphite will
not melt at the highest foundry temperatures but its car-
bon is driven off by oxidation at these temperatures de-
pending on the air (containing oxygen) available at the
metal-mould interface. Amorphous graphite oxidizes easier
than does crystalline graphite. Plumbago is applied dry
only on green sand moulds [3].
2.2. Coatings for Wet Application
Mould and core coatings for wet application are of two
types, carbon-base and carbon-free coatings. Both types
are sold in either powder or paste form. The adherence of
the coating on the mould or core surface depends on the
moisture in the sand. Carbon-based coatings may contain
several types of graphite, coke, anthracite or any of the
numerous combinations that can be made from these
materials. Carbon-free coatings may contain silica, mica,
zircon flour, magnesite, olivine, clays, talc or a combina-
tion of these materials. Many coatings formulations con-
tain both carbonaceous and non-carbonaceous raw mate-
rials to take the advantage of the synergistic characteris-
tics of both types [3]. Foundry coatings for wet applica-
tion are also classified into two, based on their carrier
systems. Those employing an aqueous carrier and those
in which organic solvent carriers are used. The former
must be dried after application while the later are
self-drying or can be ignited and dried by their own
combustion. Both classes of coating make use of the re-
fractory materials [6].
3. Components of a Coating (Coating
A refractory coating on the mould or core should have
the following characteristics:
Sufficient refractory properties to cope with the metal
being poured
Good adhesion to the substrate to prevent spalling
Be permeable to minimize air entrapment
Be fast in drying
No tendency to blistering, cracking or scaling on dry-
Good suspension and remixing properties
Minimize core strength degradation
Provide adequate protection against metal penetration
Good stability in storage
Good covering power
Good application properties by the method chosen
Leveling well and minimizing runs and tear drops
For a coating to achieve these characteristics, the
coating will consists of
Refractory filler
Liquid carrier
Suspension agents (Rheology control system)
Binder agents
Additives as shown in Figure 1
3.1. Refractory Filler (Filler Materials)
Refractory materials are substances or minerals that have
high melting points and are difficult to fuse except at
very high temperatures. They are processed at high tem-
perature and/or intended for high temperature applica-
tions [7,8]. Refractoriness has been defined by Commit-
tee C-8 of the American Society for Testing and Materi-
als (ASTM) as “...the capability of maintaining the de-
sired degree of chemical and physical identity at high
temperatures and in the environment and conditions of
use.” The melting temperature of refractory materials is
Copyright © 2011 SciRes. MSA
Foundry Coating Technology: A Review
Copyright © 2011 SciRes. MSA
Figure 1. Coating components.
an important characteristic showing the maximum tem-
perature of use and represents fundamental point in phase
diagrams used in high temperature chemistry, metallurgy,
ceramics etc [9].
In coatings, refractory materials are dispersed in the
binder and constitute the skeleton of the coating film.
They increase the density, viscosity and hardness of the
coating film and reduce the permeability.
There are characteristics other than resistance to high
temperatures that refractory materials should exhibit.
These include:
Suitable particle shape, particle size (PS) and particle
size distribution (PSD),
Chemically inert with molten metal,
Not be readily wetted by molten metal,
Not contain volatile elements that produce gas on
Have consistent cleanliness and pH
Be compatible with new chemical binders as they are
The significance of particle shape, PS and PSD are
elaborated below in the following paragraphs. The other
factors are readily understood.
At a given refractory material loading, particle shape
determines the mechanical properties of the coating ma-
trix. The particle shape is usually described by a dimen-
sionless parameter, the aspect ratio–this is the ratio be-
tween the average diameter and average thickness of the
particle. The higher the aspect ratio of the particle of re-
fractory material, the higher the reinforcing effect on the
coating matrix will be.
The particle size distribution (PSD) of a refractory
material is usually given as a cumulative curve, indicat-
ing the amount per volume or weight of particles (%),
which are smaller than a given size. PSD can be adjusted
by grinding and classification. The coarsest particles act
as points of highest stress concentration, where crack or
fractures occur under loading. Impact strength is signifi-
cantly improved by using finer particles [10].
It is generally assumed that a sieve analysis sharply
defines between the different sizes of particles compris-
ing aggregate materials. According to [3], such is not the
case. On any particular sieve one finds particles ranging
from those just able to pass through the preceding sieve
to those just unable to pass through to the following sieve.
As a result of this lack of sharp differentiation between
the particle sizes on adjacent sieves, it is difficult to sim-
ply screen aggregate material and secure particles of
uniform size on each of two successive sieves (Figure
From a practical aspect, the refractory material should
be available in large quantities at reasonable prices [8].
In foundry coatings, refractory materials determine the
efficiency of the coating. The refractory filler may be
either a single material or a blend of materials selected
for specific applications. They make up 50% to 70% of
the coating. Fillers are chosen for their particle size and
shape, density, sintering point, melting point, thermal
conductivity, thermal expansion and reactivity towards
the metal being cast and the mould or core material on
which it is applied [11]. These refractory materials in-
clude Plumbago, silica, graphite, coke, anthracite, zircon
flour, magnesite, Chalmette, olivine, clays, talc, chromite,
alumina, mica [3,12,13]. The material of which sand
moulds and cores are made generally exert influence
upon the surface quality of the castings formed from
these moulds and cores. This is because they have a high
degree of porosity to the extent that the pores tend to be
filled with molten metal causing high surface roughness
on the castings.
Therefore, with the application of refractory coatings
on the surface of the moulds and cores that will be in
contact with the molten metal, the refractory particles
tend to fill these pores on drying, thereby creating a
smooth inert surface on the moulds and cores. These re-
fractory materials have different properties and are se-
lected depending on the metal to be cast. The more
common refractory materials are discussed below.
Silica flour is a commonly used refractory filler, par-
ticularly in steel foundries. The silica flour should con-
tain minimum 98% silica and not more than 1% moisture
[11]. The fusion point of silica flour is 1734˚ [9]. At ap-
proximately 650˚C (1100˚F), silica refractory filler has
an expansion of 1.6%. Silica fillers are well known for
use as pigments, reinforcing agents and the like.
Commercially available silica and other metals oxides
are often derived from burning volatile metal halides
with various fuels and oxidants. Silica filler has been
Foundry Coating Technology: A Review
Figure 2. Schematic portrayal of range in sizes of sand aggregate retained on No 70 sieve to demonstrate non-uniformity on
any sieve [3].
produced by direct combustion of silicon powder as re-
ported in [14]. Silica flour does not excessively increase
viscosity [15]. However, it is reported in [16] that as the
content of silica filler increased the coefficient of thermal
expansion of the composite decreased while the viscosity
Zircon flour is a highly refractory material and is
primarily used for coatings in steel foundries. Good qual-
ity zircon flour suitable for foundry work should contain
minimum 64% zircon oxide (ZrO2), 30 to 35.5% silica
and maximum of 0.5% TiO2 + Fe2O3. Refractory uses of
zircon require low interstitial water content. This trans-
lates into low loss on ignition. Excessive internal radia-
tion damage to zircon crystals (metamict zircon) can
cause an increase in the loss on ignition of a zircon
product. The desire of the refractory market for a low
loss on ignition implies that a low picocurie/gram re-
quirement is placed on zircon products [8].
It has a specific gravity of about 4.5 and a pH value of
the water-based coating of not more than 9 [11].The
melting temperature is 2727 ± 10˚C [9]. The high heat
conductivity, about double that of silica, promotes quick
formation of a solidified metal layer and helps in pro-
ducing castings with a fine grained structure. Its higher
density than that of silica prevents metal penetration [17].
Graphite refractory materials are most commonly
used for coatings in iron foundries and for non-ferrous
castings. Molten metal does not wet graphite and sand
grains coated with graphite coatings resist metal penetra-
tion. This is the reason why graphite, Plumbago and car-
bon are usually used in mould and core coatings except
those for steel [6]. Mould and core coatings containing
carbonaceous ingredients are not used for steel, particu-
larly low-carbon steels. The reason for this is because
steel is sensitive to the carbon content and if there is
carbon pick-up, the properties of the steel will change [3].
The graphite used is naturally flaky type, silvery white in
appearance, of a fine powder form and free from gritty
particles. Good quality graphite for foundry use should
have ash content of about 12 to 15% maximum; volatile
matter 3% maximum; and moisture content 1% maxi-
mum [11]. Graphite inclusion in mould and core coatings
also improves stripping during shakeout. A highly useful,
desirable, substantially non-porous, smooth, non-spalling
mould surface can be produced on porous moulds by
subjecting them to a treatment with a controlled amount
of colloidal graphite suspended in a volatile carrier or
vehicle followed by a drying after the treatment and fi-
nally baking at a relatively high temperature. The mould
surfaces prepared in this manner possess substantially no
pores, at least those of a size which can be penetrated by
molten metal. With the presence of pores which cannot
be penetrated by molten metal it is considered to be sub-
stantially non-porous. The treatment, it is believed, in-
troduces colloidal graphite particles into the mould pores
and the subsequent baking fixes or anchors them in such
a way as to prevent removal unless the mould surface
itself is worn or cut away [18].
Olivine is orthosilicate of magnesium and iron
(MgFe)O·SiO2 and it occurs as forsterite and fayalite. Its
density, conductivity and refractoriness are higher than
those of silica. Its fusion point is high—about 1800˚C-
and as such it is favoured for heavy sections of alloy steel
casting. Its resistance to slag reaction makes it suitable
for the casting of high manganese steels. Olivine refrac-
tory material can also be used for the casting of non fer-
rous castings of intricate nature [17]. Olivine is used in
preference to silica sand to overcome the silicosis hazard
[19] (Silicosis is a form of respiratory disease caused by
inhalation of silica dust, and is marked by inflammation
in the upper lobes of the lungs).
Talc is a hydrous magnesium silicate mineral with the
chemical formula (Mg3Si4O10·(OH)2) and the softest
mineral on Mohr’s scale of hardness. Talc is widely used
as a filler material [20]. Talc Mohr’s hardness is 1 and
density of 2.6 - 2.8 g/cm3. It is used in many industries
because of its characteristics—low hardness, adhesion
capability (surface coating), high melting temperature,
chemical inertness, hydrophobic, organophilic, platy, low
electrical and high thermal conductivity [20,21]. The
inert and lamellar platy natures of talc improve its crack-
ing resistance, adhesion and barrier properties. Talc is
Copyright © 2011 SciRes. MSA
Foundry Coating Technology: A Review1147
practically insoluble in water and weak acids and alkalis.
Above 900˚C, talc progressively loses its hydroxyl groups
and above 1050˚C, it recrystallizes into different forms of
enstatite (anhydrous magnesium silicate). Talc’s melting
point is at 1500˚C [22].
Mica is a plate-like crystalline aluminosilicate and has
been widely used as reinforcing filler in polymer matrix
due to its excellent mechanical, electrical and thermal
properties as well as lower cost than carbon or glass fi-
bres [23]. Chemically they contain complex silicate of
aluminium and alkalis with hydroxyl. They crystallize in
monoclinic system. Some varieties may contain iron,
magnesium, lithium. There are seven important mica
minerals: Muscovite or potassium mica, H2KAl3(SiO4)3;
Paragonite or sodium mica, H2NaAl3(SiO4)3; Lepidolite
or lithium mica, K·Li·Al(OH, F)2Al(SiO4)3; Phlogopite or
magnesium mica, H2KMg3Al(SiO4)3; Biotite or magne-
sium iron mica, (H2K)(Mg, Fe)3Al(SiO4)3; Zinnwaldite
or lithium iron mica, Li2K2Fe2Al4Si7O24; and Lepidome-
lane or iron mica, (H, K)2(Fe, Al)4(SiO4)5. Muscovite is
the commonest of all and whenever the word mica is
used it is understood to mean muscovite. No other natu-
ral substance has been found to possess the properties
equal to those of mica. Of all the known varieties of mica
only muscovite and phlogopite are of commercial im-
portance. Muscovite finds the largest use while phlo-
gopite has a limited application. On the other hand phlo-
gopite is superior to muscovite in heat resistance. Mus-
covite can withstand temperatures up to 700˚C, and
phlogopite up to about 1000˚C. Phlogopite is, therefore,
preferred where a high temperature is required [24]. Mica
can be used as refractory filler in foundry core and mould
coatings to eliminate or reduce finning defect in castings
because of its lamellar plate-like nature [8].
Clays used for the manufacture of refractory fillers are
the kaolinites. In the kaolinites there are equal numbers
of silica and alumina sheets and equal numbers of silicon
and aluminium atoms. The basic composition is Al2O3-
2SiO2·2H2O [25]. Kaolinite crystals are normally hex-
agonal disks which are built up by laying double sheets
of alumina octahedral and silica tetrahedral on top of one
another [26]. Kaolin clay is the most extensively used
particulate mineral in the filling of coating of paper
[27,28]. Since kaolin clay is fine and refractory and has
found application in coating for papers, it also has poten-
tial application in foundry coating technology.
Other filler materials and their properties are provided
in Table 1.
Different filler materials and their functions in the ma-
trix depending on their various particles sizes are pre-
sented in Figure 3.
3.2. Liquid Carrier
The liquid career is the medium containing the coating
constituents and also serves as the vehicle to transport the
filler materials onto the sand substrate [11]. Therefore,
the coatings are typically suspensions of high melting
point refractories in a liquid carrier. Liquid carrier con-
stitutes about 20 to 40% of the coating. After application,
it is necessary to dry the coating to prevent gas formation
when the hot metal is poured into the mould. The forma-
tion of gases may cause casting defects. After the liquid
carrier is removed by evaporation or combustion, a pro-
tective refractory layer is deposited on the surface of the
mould or core [29,30]. This layer prevents or minimizes
the penetration of molten metal into the sand, reduces or
prevents "burn-on" and erosion of the sand, and generally
improves the quality of a casting surface. However, there
are many factors to consider with carrier selection, in-
cluding: compatibility of carrier with sand binder and/or
refractory, method of drying, flammability and “burning”
characteristics; toxicity and odour; application; labour
and floor space. The most commonly used carrier are
water-based (aqueous) and spirit-based (organic solvent)
3.2.1. Aqueous-Based Carrier
In this class, water is used as the carrier. Water is cheap
and readily available but drying in an oven is usually
necessary to remove it before casting [11,12,29]. Water
Table 1. Other filler materials and their respective proper-
ties [17].
Data ChamotteChromite MagnesiteChrome-
Availability abundantGood Good Good
(approx.) 1780 1850 1850 1850
Thermal expansion
(×1000 mm/m) 0.0052 0.007 0.014 0.012
Thermal conductivity
(WK-1m-1) 6-9.5 9-15 20-30 13-20
Wettability with molten
metal No wettingNo wetting No wettingNo wetting
Figure 3. Classification of fillers according to average parti-
cle size [31].
Copyright © 2011 SciRes. MSA
Foundry Coating Technology: A Review
is non flammable and non toxic. It has no flash point.
Water is the safest of the carriers. From environmental
view point, the use of water-based coatings is highly
recommended. However, apart from requiring heat to dry
water-coatings, complete drying of deep pockets in a
reasonable time can be difficult. It has greater tendency
for tears or runs compared to organic-based coatings. It
reduces the tensile strength of urethane no-bakes, cold
box and silicate sands. It increases the potential for core
breakage. There is also possible degradation during core
storage. Moreover, aqueous-based coatings can freeze
3.2.2. Organic Solvent-Based Carrier
Organic solvent-based or spirit-based coating usually
contains isopropanol (isopropyl alcohol) as liquid carrier
for coating constituents, and the coating is dried by ig-
niting and burning off the isopropanol [12]. This is typi-
cal of organic solvent-based carriers which also include
methanol, ethanol, hydrocarbons and chlorinated hydro-
carbons. They dry very fast. Isopropanol is recommended
for use on large moulds and cores [11]. Isopropanol has
good combustion characteristics with slow burning front
and a moderate hot flame. This reduces the chance of
over-heating the sand surface and subsequent problems
of sand friability. Isopropanol is also technically accept-
able because it is compatible with a wide range of sus-
pension agents and resin binders also used in the formu-
lation of these coatings. Most of the organic sol-
vent-based carriers are referred to as air-drying carrier.
These include carbon tetrachloride, methylenechloride,
chloroethene and chloroform. They rely for efficiency on
a rapid rate of evaporation which places them in a more
hazardous category than isopropanol. They are also not
versatile as isopropanol in the formation of foundry
coatings. In many cases, they call for specialized forms
of gelling media and resin binders [6]. The use of organic
solvent-based coatings is threatened by environmental
issues because they are toxic and flammable [30].
3.3. Suspension Agents (The Rheology Control
There is no difficulty in keeping solid particles in per-
manent suspension in a liquid if both have the same spe-
cific gravity. This is not the case with foundry sand
coatings. The maintenance of solid particles in suspen-
sion is achieved by addition of suspension agents. These
agents provide the suspension system that prevents the
filler particles from agglomerating and separating out
during storage of the coating over extended periods. It
ensures that the coating is homogeneous and ready for
application with the minimum of agitation. It also con-
trols the flow properties of the coating and is designed to
suit the application method that is used [11,12]. The sus-
pension agent makes up 1 to 5% of the coating.
When water is used as the carrier liquid, bentonite clay
is used as a suspension agent. Bentonite swells and forms
a gel when mixed with water. Time must be allowed for
gelling to proceed to completion. Two kinds of bentonite
are in common use, one linked with calcium and the other
with sodium ions. As a suspension agent bentonites ini-
tially of the sodium type are preferred. Calcium bentonite
is converted to sodium bentonite by treatment with so-
dium carbonate. This treatment affects the swelling power
of the clay and makes control of the viscosity of the coat-
ing unpredictable. Apart from the difficulties with quality
control of the bentonite, it has the disadvantage of tending
to induce shrinkage cracks in the coating when dried. In
view of the drawbacks associated with bentonite, substi-
tutes are found in polysaccharide and certain forms of
carboxymethyl cellulose. Polysaccharides require special
mixers, which few foundries possess, to obtain optimum
suspension [6]. The cellulose type does not require this
special mixers and do not induce shrinkage cracks as does
dried bentonite as shown in Figure 4 below.
With organic solvent-based carrier systems, different
suspension agents are used. Modified bentonite known
also as organic bentonite or bentone will gel and increase
the viscosity of organic liquids such as alcohols and sol-
Figure 4. Surfaces of coated cores (a) cracking of coating
induced by bentonite and (b) cracking eliminated by using
carboxymethyl cellulose [6]
Copyright © 2011 SciRes. MSA
Foundry Coating Technology: A Review1149
vents. Bentones result from a base exchange of the inor-
ganic Ca and Na cation for an organic one which is qua-
ternary ammonium. Examples of suitable suspension
agent for organic solvent-based carrier are hydrogenated
castor oil and quaternary alkyl ammonium montmorillo-
nite gels [12].
3.4. Binding Agents
Binding agents are various materials which act to hold
the particles of refractories together and attach them to
the sand surface. The quantity of binder required for this
purpose increases a little as the particle size of the re-
fractory decreases, thereby increasing the surface area for
a given ratio in the coating. However, it makes up to 1 to
5% of the coating. It is important to determine the mini-
mum quantity of the binding agent, because too little
results in poor adhesion but, excess produces brittle
coating which may crack on drying and spall off during
casting. Furthermore, resins and similar organic binders
evolve gas on heating. Thus, any undispersed binder col-
lected in partially dried areas of moulds or cores will
cause local concentration of gas generation. In this way,
defects such as porosity and lapping can result. It is also
worthy to note that most organic binders and many sus-
pension agents used in water suspensions are subject to
biological degradation. For longer storage of the coating,
precautions must be taken to suppress these reactions.
Such reactions do not occur with spirit-based coatings.
Binders used for water suspensions include sulphite
lye, various clays (bentonite and kaolin), dextrin, molas-
ses, sugars, silica ester and resins (furan and phenol)
soluble or miscible with water. For spirit-based suspen-
sions, natural or synthetic resins are required. These in-
clude furan, phenol, urea formaldehyde, phenol formal-
dehyde, novolac and natural wood resins.
4. Coating Application Methods
Several variables dictate the choice of application method.
Part geometry and size, appearance of the coating finish,
and production rate, allinfluence the type of application
method. Facility constraints will also determine the choice
of application method. The configuration of the applica-
tion equipment is dependent on space or climate. Systems
can be manually or automatically controlled. Other sys-
tems may require extra equipment, such as holding tanks
or outside air supply to operate properly.
Similar application systems may operate at widely
varying parameters. The viscosity of the coating material,
the desired thickness of the final coating, and the com-
plexity of the part will determine the best operating-
parameters for the application method. Thus, part tem-
peratures, dip times or number of coats are put into con-
One factor that is important to all application methods
is the transfer efficiency of coating material onto the part.
Transfer efficiency is the percentage of solid coating
material used that actually deposited on the surface of the
part. The amount of solvent in the coating material
is irrelevant. The higher the transfer efficiency, the better,
as more coating material adheres to the part and less is
wasted. Transfer efficiency ranges from 25% to 40% for
conventional spray systems to almost 100% for dip and
powder coating methods. Much of the pollution and waste
created from organic finishing operations can be mini-
mized or eliminated by improving the transfer efficiency
of the application system. If the transfer efficiency cannot
be improved, pollution control technology and waste
handling measures must be employed [32]. The following
are different methods of applying foundry coating on
cores or moulds.
1) Brushing and swabbing
2) Spraying
3) Dip coating
4) Flow coating
4.1. Brushing and Swabbing
Brushing and swabbing methods of applying coatings are
used in many foundries. The effort imparted by brushing
helps to force the refractory particles into the pores of the
sand surface, which is a desirable feature. The swab is a
most useful aid in coating interior of difficult pockets and
re-entrant angles. Both methods give uneven thickness
and strives from brush motion is visible on casting. They
also depend on the skills of the operator. There is also the
risk of sand-coating mixture due to frothing and this ini-
tiates metal penetration [6].
4.2. Spraying
Spraying is a much faster means of application widely
used in foundries of all types. It is important to pay greater
attention to the coating composition because less me-
chanical effort is available to force the particles into the
pores between the sand grains. Selection of the solid
constituents and the overall viscosity is more critical for
sprayed coating than for brushing and swabbing. Spray
methods use specially designed guns to atomize the
coating into a fine spray. This method along with brushing
suffers the disability of not being able to coat deep re-
cesses thoroughly. One reason for this is the back pressure
of air which prevents refractory deposition in the cavity.
The system of airless spraying provides a means of
overcoming this disadvantage. Airless Spray has higher
transfer efficiency and lower chance of blowback. Again,
it is more efficient when a flat surface is involved which is
also placed vertically during spraying [6,33].
The above discussion refers to liquid coating mixtures;
Copyright © 2011 SciRes. MSA
Foundry Coating Technology: A Review
however, a group of researchers from Austria developed a
new method of spraying dry coating on substrates over-
coming the inherent disadvantages of the use of wet
coating. The process is called electrostatic or tribostatic
powder spraying method, also designated as EPS method.
In this process, the surfaces of the substrate is first made
conductive (if it is not a conducting material) by spraying
electrically conducting polymer solutions on them. Then
the powder coatings can be applied. According to the
developers, this novel coating process has been tested on
all popular binder systemsfrom cold box, through hot
box and furan to inorganic types [34].
4.3. Dip Coating
Dip coating techniques can be described as a process
where the substrate to be coated is immersed in the liquid
or coating and then withdrawn with a controlled speed
under controlled temperature and atmospheric conditions.
Coating thickness increases with a faster withdrawal
speed. The deposited thickness is determined by the bal-
ance of forces at the stagnation point on the coating sus-
pension surface as shown in Figure 5. The faster the
withdrawal speed the more coating suspension is pulled
up onto the substrate surface because there is no time for
the suspension to flow back down to the coating pool.
During sol-gel dip coating, the coating suspension is
rapidly concentrated on the surface of the substrate by
gravitational draining with associated evaporation and
condensation reactions. Dip coating is usually used for
cores and is well suited for automatic applications. Dip
coating enhances a high production rate and high transfer
efficiency (almost 100%) and relatively little labour is
required. The effectiveness of dip coating depends greatly
on the viscosity of the coating, which thickens with ex-
posure to air unless it is carefully managed. The viscosity
of the coating must remain practically constant if the
deposited film quality is to remain high and the same. To
maintain viscosity, solvent must be routinely added as
makeup. This results in high volatile organic compounds
(VOC). Dip coating is not suitable for objects with hol-
lows or cavities [33]. Other factors that determine the
effectiveness of dip coating include coating density and
surface tension. Better surface penetration is obtained than
with spraying because of the head pressure of the coating
in the dip tank. Even thickness of surface is necessary so
as to maintain dimensional accuracy and true reproduction
of contour. Uneven coating is at its worst when it runs
down as tears. This defect can be encouraged by the nature
of the surface to be coated but is mainly due to the kind of
the suspension agent used in the coating. Tears and similar
coating faults are sources of high gas evolution and cast-
ing defects may result [6]. The coating can be cured by a
number of methods such as conventional thermal, UV, or
Figure 5. A schematic diagram of dip-costing process [35].
IR techniques depending on the coating formulation [35].
4.4. Flow Coating
Flow coating is a method of applying a refractory coating
that can be described as wetting the moulds or heavy cores
with a garden hose at low pressure. With flow coating the
mould or core is maneuvered so it is at an angle (20 to 40˚
to the vertical) in front of the operator [35] and coating
applied through a hose as seen in Figure 6, starting at the
top and in lateral movements progressively working down
to the bottom. Flow coating is usually used for large or
oddly shaped parts that are difficult or impossible to dip
coat. Coatings applied by flow coating have only a poor to
fair appearance unless the parts are rotated during drip-
page. Flow coating is fast and easy, requires little space,
involves relatively low installation cost, requires low
maintenance, and has a low labour requirement. Required
operator skill is also low. Flow coating achieves a high
coating transfer efficiency, often 90% and higher. Prin-
cipal control of dry-film thickness depends on the coating
viscosity [33]. Flow coating can eliminate all the various
problems associated with the other coating techniques
such as spraying, dipping or brushing. For flow coating to
be effective, it must create a surface and sub-surface
coating. Surface coating provides a barrier to the metal
and improves surface finish. The sub-surface coating
penetrates the surface of a mould or core to fill the voids
Figure 6. Flow coating method, it is seen that the mould is
inclined at an angle.
Copyright © 2011 SciRes. MSA
Foundry Coating Technology: A Review1151
between the sand grains. This reduces the possibility of
metal penetration and veining [36].
5. Drying of Coating
After coating application, each coating must be ‘dried’,
which means that the suspension agent (water, alcohol or
volatile agents) must be completely removed. These sub-
stances do penetrate the mould or core material and do
not have any protective effect for the mould or core. On
the contrary, it can cause severe problems of gas forma-
tion, blows, slag entrapment, porosity, blistering, and
penetration and drastically reduce the strength of the
mould or core. The methods of removal are different
depending on the type of coating [30,37-39].
5.1. Drying Organic Solvent-Based Coatings
In the past, foundries typically used solvent-based carri-
ers because they dry quickly without external heating (air
drying). They are also referred to as self-drying coatings.
This takes a lot of time [30,37,38]. Consequently, flame
torching became the accepted means of drying coated
cores and moulds. However, workplace environmental,
health and safety concerns, as well as economic consid-
erations emanating from the rapidly increasing cost of
petrochemicals based solvent, continue to enhance the
development and use of water-based coating technolo-
gies [39].
5.2. Drying Water-Based Coatings
The trend today is towards water-based coatings. But
they require longer drying times using air drying and
conventional ovens compared to organic solvent-based
coatings. The drying temperature must exceed 100˚C, but
lower than the temperature at which the binder system is
destroyed (mostly 250˚C) [37]. Different drying tech-
niques such as high intensity lights, microwave, drying
tunnels and infrared ovens can be applied to water-based
coatings. It was reported in [40] that the high intensity
lights and drying tunnels did not dry fast enough as ex-
pected to prevent coatings from dripping and losing
thickness uniformity. Microwave drying used non-selec-
tive heat that penetrated the sand cores and caused them
to disintegrate. Infrared ovens, however, dry the coated
cores or moulds quickly without damaging the sand bod-
ies. Application of infrared heating for mould and core
coating can reduce drying time by 85%. The energy sav-
ing comes from the controllability of the infrared unit,
which brings the mould surface to the desired tempera-
ture and then cycles off in a predetermined time sequence.
Less heat is dissipated to the surroundings. The infrared
elements direct the heat more effectively at the mould
and can dry deep cavities and mould pockets – thus con-
tributing to better casting quality. The sub-surface of the
mould is not affected. An additional advantage of using
infrared heating is that only 25% of the floor space occu-
pied by the resistance oven was required [41]. A signifi-
cant development in water-based coatings is the feature
in which there is a distinctive colour change as the coat-
ing dries and transitions from the wet to the dry state as
shown in Figure 7. This change in colour offers visual
confirmation that the coating is dry. Not only that this
shows when drying is complete, it can also serve as a
quality control tool. When drying takes longer time than
necessary it will mean that the moisture content is high
and can be adjusted. This feature saves energy used in
drying thereby saving cost [39].
6. Characterization of Coatings
In order to understand the behaviour of coatings contain-
ing refractory materials, there is need for characterization
of the coatings. The parameters that characterize foundry
coatings are discussed below.
6.1. Specific Gravity
Specific gravity is the unit weight per unit volume. Spe-
cific gravity is a quick test that allows inferences to be
drawn about the total solids and refractory components
present in the coating [42]. The knowledge of the spe-
cific gravity of the suspension agent and that of the re-
fractory material is critical. There would be no difficulty
in keeping the refractory material in permanent suspen-
sion in the suspension agent if they have similar specific
gravity [6]. The specific gravity also gives a fair idea of
the refractory material content of the coating. Water has
a lower specific gravity of 1. When it used to dilute a
coating with relatively higher specific gravity component;
the specific gravity of the coating is reduced.
6.2. Viscosity
Viscosity, a measurement of material flow properties, is
Figure 7. Colour changing Zircon Foundry Coatings
changes colour from Yellow to Pink, Pink to Yellow on
drying or ignition. Available both in Water and Solvent
based [43].
Copyright © 2011 SciRes. MSA
Foundry Coating Technology: A Review
Copyright © 2011 SciRes. MSA
coating property that must be controlled during applica-
tion. They suggested that it must therefore be used with
one or more additional tests, such as, Hercules surface
tension and % solid content. Measurement of Baume´ is
shown in Figure 9.
the best test for evaluating coatings because of its high
correlation with the dried deposit on the core. There are
several different methods of measuring viscosity. The
most commonly applied in foundries is the flow cup
method as shown in Figure 8. The flow cup measure of
viscosity requires the use of a cup with a specific size of
hole in the bottom to match the material being used. A
stopwatch is used as the cup is lowered into the coating
and then taken from the surface of the coating after it has
filled. The time it takes the coating to drain through the
hole is the viscosity in number of seconds [44].
6.4. Solid content
The solids in the coating must be measured because they
are the refractory materials that provide protection to the
core or mould. The higher the percent solids, the more
protection the coating offers. The solid content of a coat-
ing determines some other important parameters of the
coating such as the density, viscosity, thickness, cover-
age etc [48]. Therefore, the knowledge of the amount of
solid in the coating is very important for reproducibility
of these properties. The percent solid content can be de-
termined by dividing the weight of the dried coating by
the original weight and multiplying by 100.
6.3. Baume´ Parameter
The Baume´ test is the most common test used in foun-
dries to control coating because it is quick and easy. The
test is performed with a hydrometer. It usually consists of
a thin glass tube closed at both ends, with one end
enlarged into a bulb that contains fine lead shot or mer-
cury. The glass tubular end contains a calibrated scale in
degrees Baume. The Baume scale of numbers relates to
the specific gravity and body of a coating. After mixing
the coating sample thoroughly, the hydrometer is imme-
diately floated in the coating slurry. When it stops sink-
ing, the degrees Baume is read directly from the hy-
drometer scale [44]. Baume is a simple test to help
measure dilution consistency. However, there is a poten-
tial for operator variability, and test parameters must be
carefully controlled. Operator consistency in placing the
hydrometer into the coating and length of test time are
critical. When Baume test is used in combination with
the specific Gravity measured by Gravimetric method,
the combined results can be a more useful diagnostic tool.
Many metal casting facilities also include viscosity test
in their refractory coating control test procedures [42]. L.
6.5. Colloidal Stability
Colloidal stability is describing the formation of uniform
suspension of the particles in the coating matrix. The
stability of particles is determined by their resistance to
The formation of uniform suspensions of particles can
be understood by calculation of the sedimentation rates
assuming that the particles are spherical so that Stokes’s
Law may be applied. Equating gravitational and fric-
tional forces:
dx 4π
Sedimentation rate,/6π
dt 3
 
η = viscosity of coating
Winardi et al. [46], reported that coating viscosity is
typically reported in degree Baume. Higher Baume´
number indicates higher viscosity.
ρ = density of coating
ρ´ = density of refractory particle material
r = radius of the refractory particle (assuming a
spherical particle)
It was also reported in [47] that Baume when per-
formed in a controlled laboratory environment tracks
well certain coating properties, but fails to identify the g = acceleration due to gravity
Figure 8. Measurement of viscosity with a flow cup [45].
Foundry Coating Technology: A Review1153
Figure 9. Measurement of oBaume´ [45].
Figure 10. Measuring the wet thick layer of a coating [45].
The stability of small particles is surprising, since sur-
face tension leads to very high pressure differences
across surfaces with small radii of curvature. For a parti-
cle of radius r, density ρ, and relative molar mass M,
with surface tension γ, the pressure difference across the
curved surface, pr, compared to that across a flat surface,
po, is given by the Kelvin equation.
RTln M
 (2)
Thus small particles should tend to dissolve while lar-
ger particles should grow as observed in Oswald ripening
of precipitates [49].
6.6. Coating Thickness
Coating thickness is usually measured using a destructive
test. To date no reliable non-destructive test is being ap-
plied by the foundry industry to measure the consistency
of the coating layer thickness applied on the cores or
moulds. In some tests, the cores are sectioned and the
measurements were taken using a microscope [47,48].
In some other methods, the coating is removed from a
flat surface on a core and the difference in the cored sur-
face and the coated surface is measured.
The amount of surface deposit can be used as a refer-
ence for future comparisons and making decisions about
coating allowance in casting design. There is a strong
correlation between the viscosity of the coating and the
coating thickness [44,48]. However, coating dry thick-
ness has proved difficult to measure, so what is generally
done is to measure the wet coating layer thickness using
the elcometer wet film “comb” as shown in Figure 10.
The elcometer wet film combs can be used in accordance
with following standards; ISO 2808-7B, ASTM D
4414-A, BS 3900-C5-7B and NF T30-125. The film
combs have various lengths on their sides. These stan-
dards specify that wet film comb be perpendicular to the
substrate and the thickness of the coating lies between
the biggest value wet tooth and the smallest value dry
tooth values [50]. The wet coating layer thickness will be
correlated to the dry coating thickness, if the volume to
solids ratio of the coating is known [37,50]. As a rule of
thumb dry coating thickness is 50% of the wet coating
thickness [50].
In dip coating, the coating thickness is mainly defined
by the withdrawal speed, the solid content (density), the
surface tension and the viscosity of the liquid. The coat-
ing thickness can be calculated from landau-Levich
equation [35]. This equation gives the wet coating layer
thickness on a vertically withdrawn flat plate.
where hw = Wet coating thickness
υ = withdrawal speed
Copyright © 2011 SciRes. MSA
Foundry Coating Technology: A Review
ρ = density
γLV = Liquid-vapour surface tension
g = acceleration due to gravity
To calculate the dry film thickness these equations
need to be modified. It was reported in [51], that Yan et
al. derived Eq. (4) for dry film thickness, hd.
0.84 0.5
hQ g
 
here Q is called a dimensionless flux and is given b
T is absolute temperature. ξ is defined as:
where ρs is the solvent density and ρp is the particle den-
sity. ηo is obtained from the viscosity of the solution as a
function of the matrix concentration Cm, and according to
Eq. (7)
 (7)
In Eq. (7), η is the viscosity of the particle solution
with concentration Cp, and the viscosity of the solvent is
6.7. Coating Penetration Depth
The distance the coating penetrates the core is an impor-
tant feature to a coating’s success. A coating that lies
entirely on the surface of the cores is not anchored well
and will most likely spall away. A coating that penetrates
too much will over degrade the core. Coating penetration
is also a function of core density. A core that is blown
too tightly resists coating penetration, while one blown
softly acts like a sponge and absorbs much water. There-
fore, any evaluation of coating penetration should be
done on a core that is of normal production quality. It is
also note worthy that core release agents may waterproof
the core and affect coating penetration. Coating penetra-
tion is evaluated by cutting a coated dried core and ob-
serving how far the coating penetrates the core. The
usual reference is sand grain penetration. A normal level
of penetration is 2 – 4 sand grains [44]. It was reported in
[47], that this is not the most precise methodology be-
cause sand grain sizes differ from one foundry to the
other. Moreover, a batch of foundry sand has a known
distribution of a variety of grain sizes within it, which
also makes using sand grain count as a measuring system
inadequate. Lower surface tension increases the depth of
coating penetration. As coating penetration increases, the
thickness of the proud layer decreases while the reverse
is the case if the proud layer increases [48]. Thermal ex-
pansion increases with the thickness of the proud coating
layer (the layer on the surface of the substrate) [47].
Therefore, an optimum proud layer thickness is needed to
reduce the expansion defects on the casting made with
these cores. This requires that the coating penetration
depth is controlled.
6.8. Coating Permeability
Coating permeability is the amount of gas that can pass
through the coating. The level of permeability is detected
by both the type and amount refractory materials that are
used in the coating formulation and the dry film thick-
ness deposit on the core. The permeability of the coating
on the core is measured using a laboratory permmeter. A
coating with low permeability is desirable when directing
evolved gases to vent through specific areas of the core.
A high permeability coating is best when the goal is the
evacuation of core gases through the coating. The per-
meability of the coating at the coating-metal interface
may be different than that measured on the core. Some
constituents of the coating may quickly thermally de-
compose leaving voids that result in higher permeability.
Some may soften and flux resulting in lower permeabil-
ity [44]. High permeability coating will reduce the time
required for removing the degradation products and will
increase the metal fill velocity, often leading to blister
and fold defects. Low permeability coating will slow
down the metal velocity, which causes the molten metal
to lose the adequate thermal energy required for com-
plete pyrolysis, traps the degradation products and leads
to misrun or partial fill. It has been reported in [51] that
mould filling times decreased with permeability of the
coatings. A standard approach to characterize the per-
meability of porous materials is to use Darcy’s law (Eq.
8), which relates volumetric flow and pressure gradients
with the properties of the fluid and porous materials.
K = permeability value in units of Darcys;
µ = viscosity of the fluid in centipoises;
Q = volumetric flow rate measured in cm3/sec;
L = length of specimen in cm in the flow direction;
A = cross-sectional area of the specimen perpendicular
the direction of gas flow in cm2,
ΔP = (P2P1) = pressure drop over the specimen
P2 = pressure at outlet side of the specimen in atmos-
P1 = pressure at inlet side of specimen in atmospheres
Eq. (8) is valid when KA/µL is a constant in the lami-
nar flow region (slow viscous flow) [52,53][ i.e. for very
Copyright © 2011 SciRes. MSA
Foundry Coating Technology: A Review1155
small Reynolds number (Re) [54]. The upper limit is at a
value of Re between 1 and 10. At a high Reynolds num-
ber, the deviation from Darcy’s law will be observed.
The Darcian permeability coefficient K indicates the ca-
pability of the porous medium to transmit fluids. Theo-
retically, the permeability coefficient only depends on
the porous medium’s properties. At high pressures, the
turbulent and inertia flow become more dominant so that
Darcy’s law is no longer valid. The transition from the
linear (Darcy’s law) to the nonlinear regime˚Ccurs
gradually as the Reynolds number increases. Therefore,
the classical approach to macroscopically characterize
the effect of inertia and turbulence on flow through real
porous media is to use Forchheimer’s equation (Eq. 9),
which includes parabolic parts in the equation consider-
ing the influence of inertia and turbulence [51,54].
 (9)
where V = fluid velocity averaged over the total
cross-section of the porous specimen (Q/A)
β = inertial parameter
ρ = density of the fluid
This equation macroscopically quantifies the
non-linear effect [55]. Research [51] has shown that the
deviation from Darcy’s law (which occurs at Re = 1 – 10)
cannot be attributed to turbulence, and inertia forces are
more appropriate to explain the deviation. The role of
inertial effects over such a transition at high Re from
linear to nonlinear flow in the pore space was success-
fully simulated in the laminar regime without including
turbulence effects [55]. However, the random aspect of
the pore distribution induces a highly heterogeneous lo-
cal flow which becomes turbulent at high Reynolds’ re-
gimes [56].
6.9. Core Degradation
Core degradation varies from coating to coating. The
longer a core stays wet, the more core degradation will
take place. So, it is the best practice to put cores into an
oven heat zone as quickly as possible after the core is
coated. Most coatings use surfactants as wetting agents to
allow the coating to penetrate the proper depth. These
surfactants change the surface tension of the water, mak-
ing it worse for core degradation. To evaluate the effect
refractory coating on core strength, dip one set of cores
and leave the other set undipped. Place both sets in the
drying oven until dry and allow them to cool to ambient
temperature approximately one hour. Then, when cool,
evaluate both sets of cores for strength. The comparative
loss in strength of coated cores will most likely be sub-
stantial [44]. It was reported in [37] that the strength of
core and mould material will decrease about 30% with
alcohol based coatings and 50% for water based coatings.
This is in agreement with the authors’ findings in the
investigation of the strength of core materials. The pub-
lication of these results is on the way.
6.10. Wettability and Surface Tension
The deposition of a coating on a solid substrate generates
new interface between dissimilar materials and involves
considerations of wettability, spreading, interface evolu-
tion and adhesion. The wettability of a solid by a liquid is
characterized in terms of the angle of contact that the
liquid makes on the solid [57]. The basic law governing
the equilibrium of a liquid drop on a surface was formu-
lated by Thomas Young σ.
The drop is shaped by the resultant forces pulling at
the three-phase contact line of the drop, where the
solid/liquid, liquid/gas and solid/gas interfaces meet, in
the plane of the solid as shown in Figure 11. The forces
(per unit length) acting at this line are the surface ten-
sions and their balance yields the famous Young’s equa-
 
where σSG , σSL and σLG are solid/gas, solid/liquid and
liquid/gas surface tensions, respectively [57,58].
According to Taylor’s depiction of liquid droplet
shape on solid surface, the droplet height, h = 2asin
(θ*/2), where a is the capillary length (a = (σ/ρg)1/2, σ, the
liquid surface tension and ρ, its density, a = 2.7 mm for
water). It shows that gravity g can affect drop shape be-
sides the three phase forces. Only if the drop is small
enough that the effect of gravity is negligible, which
typically is the case for drops of millimetre size down to
micrometres, the drop will have the shape of a spherical
cap and the liquid/gas interface meets the solid surface at
an angle θc, which is called the contact angle of a flat
surface [58]. The condition θ < 90˚ indicates that the
solid is wetted by the liquid, such a surface is referred to
as a hydrophilic surface and θ > 90˚ indicates nonwetting,
and the surface is called a hydrophobic surface. Wet-
tability of a solid surface is governed by the chemical
properties and the microstructure of the surface. Wet-
tability is mainly determined by its interfacial free energy
Figure 11. Shape of droplet on a smooth surface[57].
Copyright © 2011 SciRes. MSA
Foundry Coating Technology: A Review
Copyright © 2011 SciRes. MSA
7. Potential Area of Exploitation in Foundry
Coating Development: the Sol-Gel
(σSG). The greater, the free surface energy, the easier, the
liquid can spread upon and vice versa.
Young’s equation applies to ideal surfaces that are
perfectly smooth and devoid of all chemical and struc-
tural inhomogeneities. The contact angle measured on a
rough surface (called the Wenzel angle, θw) does not
obey Young’s equation; it is related to the equilibrium
(Young’s) angle θy [59], by Equation (11)
7.1. Introduction
Sol-gel technology is discovered in the late 1800s and
extensively studied since the early 1930s. Various steps
in the sol-gel process to control the morphology of the
final product for particular properties meant for specific
applications are shown in Figure 12. Coating production
is one of the techniques for controlling the morphology
of the product. Sol-gel coating technology has been ap-
plied to various areas of coating production for corrosion
protection [60,61], wear resistance [62], thermal barrier
[63][, anti-soiling [64], anti reflective [65,66] etc but has
not been applied to the foundry industry. A literature
survey reveals that surprisingly little research [67-69] is
carried out on a topic that is of great importance for the
surface quality of castings. The application of sol-gel
technology in the production of foundry coatings is a
novel research area undertaken by our research group.
Cos cos
where r is the ratio of true wetted area to the apparent
area. Equation (11) is called the Wenzel equation.
Wenzel’s equation applies to equilibrium angles on
rough surfaces and not to advancing and receding angles
of a droplet on a rough solid surface that give rise to
contact-angle hysteresis. Hysteresis, H, is defined as the
difference of the advancing and receding angles (i.e., H =
θa - θr) and arises because the liquid-vapour interface does
not retrace its original path when it recedes on the solid, so
that spreading is thermodynamically irreversible. Because
roughness hinders the contact line motion by creating
energy barriers, the system can reside in any of the po-
tential wells accessible to it that are commensurate with
the vibrational (or thermal) energy of the droplet [58].
The technology gained much of its popularity in the
glass and ceramics production. Sol-gel technology is an
area of materials science. It denotes a process by which
largely inorganic polymers are synthesized through the
formation of a colloidal suspension (sol) and gelation of
the sol to form a network in a continuous liquid phase
(gel). A “sol” is a dispersion of colloidal particles. A
“gel” is an interconnected polymeric network formed by
assembly of the sol. The gelation proceeds through stages
by which the product’s rigidity is increased. The final
material produced in a room temperature synthesis is a
porous glasslike solid, which is termed a xerogel [70].
This xerogel is the sol-gel component of the coating be-
ing produced and tested for foundry application. The
precursors for synthesizing the colloids consist of a metal
or metalloid element surrounded by various reactive
ligands. Metal alkoxides are the most popular because
they react readily with water.
In many industrial processes like that found in foun-
dries, the substrate (core in foundries) is immersed in a
liquid coating material, and then withdrawn to leave a
liquid film on the substrate. The film (coating) thickness
depends upon the surface tension, withdrawal speed,
substrate geometry, roughness, and viscosity. The dis-
persion of fine, granular solids in a liquid vehicle is a basic
step in preparing paints and other coating materials and
involves particle transfer across a gas-liquid interface. The
transfer of non-wettable solids into liquids requires the
solid to overcome a surface energy barrier at the liq-
uid-gas interface, and energy must be expended to assist
the transfer of non-wettable solids. Once the solid enters
the liquid, the capillary (attractive) forces and gas bridges
between solids control such phenomena as agglomeration,
dispersion, and air entrapment. The inter-particle forces
between dispersed solids are due to liquid surface tension
and pressure difference across the curved liquid-vapour
boundary between contacting solids. The maximum in-
ter-particle force, F, due to capillary forces between two
touching spheres is
The most widely used metal alkoxides are the alkox-
ysilanes, such as tetramethoxysilane (TMOS) and tetra-
ethoxysilane (TEOS). The sol-gel process offers many
advantages over other methods of producing coatings and
films. For example, the low processing temperature; the
possibility of changing the sol composition, thereby pro-
ducing a change in film and coating microstructure and
2(2) LG
low processing cost compared to some other competitive
process such as ceramic (powder) method, chemical
deposition process etc [72].
where R is the radius of the sphere. The force increases
with increasing liquid surface tension and decreasing
contact angle and particle radius. These forces affect the
viscosity, density, and sedimentation behaviour of the
suspension and the properties of the coating deposited
using the suspension [58].
7.2. Sol-Gel Reactions
The characteristics and properties of a particular sol-gel
inorganic network are related to a number of factors that
Foundry Coating Technology: A Review 1157
Figure 12. Various steps in the sol-gel process to control the final morphology of the product [71].
affect the rate of hydrolysis and condensation reactions,
such as, pH, temperature and time of reaction, reagent
concentrations, catalyst nature and concentration, H2O/Si
molar ratio (R), aging (structure modifications with time
depending on time and temperature, solvent and pH con-
ditions) temperature and time, and drying. Of the factors
listed above, pH, nature and concentration of catalyst,
H2O/Si molar ratio (R), and temperature have been iden-
tified as the most important. Thus controlling these fac-
tors, it is possible to vary the structure and properties of
the sol-gel derived inorganic network over wide ranges.
At the functional group level, three reactions are used to
describe the sol-gel process: hydrolysis, water condensa-
tion and alcohol condensation.
Generally speaking, the hydrolysis reaction, through
the addition of water, replaces alkoxides groups (-OR)
with hydroxyl groups (OH). Subsequent condensation
reactions involving the silanol groups (Si-OH), which are
hydroxylated species, produce siloxane bonds (Si-O-Si)
under release of water (oxolation), whereas the reaction
between a hydroxide and an alkoxide leads to siloxane
bonds (Si-O-Si) under release of an alcohol (alkoxola-
Si OR + H2O Si OH + ROH This
process is called hydrolysis
Si OH + HO Si Si O Si + H2O
This condensation process is called oxolation
Si OR + HO Si Si O Si +
ROH This condensation process is called alkoxolation
Under most conditions, condensation starts before hy-
drolysis is complete. However, conditions such as, pH,
H2O/Si molar ratio (R), and catalyst can force completion
of hydrolysis before condensation begins. Additionally,
because water and alkoxides are immiscible, a mutual
solvent such as an alcohol is utilized. With the presence
of this homogenizing agent, alcohol, hydrolysis is facili-
tated due to the miscibility of the alkoxides and water. As
the number of the siloxane bonds increases, the individ-
ual molecules are bridged and jointly aggregate in the sol.
When the sol particles aggregate, or inter-knit into a
network, a gel is formed. Upon drying, trapped volatiles
(water, alcohol, etc.) are driven off and the network
shrinks as further condensation can occur.
The first step of the hydrolysis of the Silicon alkoxides
can occur by acid-catalysed or base-catalysed processes.
Mineral acids (HCl) and ammonia are most generally
used; however, other catalysts are acetic acid, KOH,
amines, KF and HF. It can generally be said that sol-gel
derived silicon oxide networks, under acid catalysed
conditions, yield primarily linear or randomly branched
polymers which entangle and form additional branches
resulting in gelation. On the other hand, silicon oxides
networks derived under base-catalysed conditions yield
more highly branched clusters which do not interpene-
trate prior to gelation and thus behave as discrete clusters
having larger sol particles and large pores between the
interconnected particles. Hence the choice of acid or base
catalysis has a substantial influence on the nature of the
gel which is formed [49,73].
7.3. Application of Sol-Gel Process in Foundry
Coating Formulation
The application of sol-gel coating process in foundry
coating production is a novel area of research undertaken
by the authors at Technical University of Denmark, DTU
and the coating producers at Danish Technological Insti-
tute, DTI along with the expertise of the industrial part-
ners [48,74]. Chemically bonded sand cores are dip
coated with the sol-gel coatings containing different filler
materials. The core-coating interactions are investigated
Copyright © 2011 SciRes. MSA
Foundry Coating Technology: A Review
using advanced microscopy and spectroscopy. During
casting, the thermal behaviour of the coated cores is
monitored using a data acquisition soft ware. The mod-
eled results for commercial casting modeling and simu-
lating soft ware are correlated to experimental results
accordingly. After casting, the surface and subsurface
quality of the castings are examined with a 3 D optical
surface roughness measuring microscope and a scanning
electron microscope respectively. The results so far ob-
tained show a significant potential in the sol-gel process
towards, improvement of the surface quality of casting.
8. Conclusions
This is a review about foundry coating technology. It
includes but not limited to the coating components,
methods of application and characterization parameters.
New area of innovation for further development and im-
provement of foundry coating technology was also in-
troduced. Following the ongoing discussions, this review
has thrown more light in the foundry coating technology.
The information in this report will help foundries to
identify the right parameters to enhance the performance
of their coatings to produce castings with excellent sur-
face finish. On the other hand foundry coating manufac-
turers will find this report highly resourceful for the im-
provement of their various existing products.
9. Acknowledgements
The authors wish to acknowledge the Danish Agency for
Science, Technology and Innovation for their financial
support for the ongoing project.
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