FOUDNRY COATIN TECHNOLOGY

 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). 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- 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 . 

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.

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 . 

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. 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 . 

3. Components of a Coating (Coating

Materials)

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-ing

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. 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 desired degree of chemical and physical identity at high temperatures and in the environment and conditions of use.” The melting temperature of refractory materials is 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.

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 heating,

Have consistent cleanliness and pH

Be compatible with new chemical binders as they are developed 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 .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 2).

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 These refractory materials in-clude Plumbago, silica, graphite, coke, anthracite, zircon flour, magnesite, Chalmette, olivine, clays, talc, chromite, alumina, mica.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. The fusion point of silica flour is 1734°. 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 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 increased.

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.

It has a specific gravity of about 4.5 and a pH value of the water-based coating of not more than 9.The melting temperature is 2727 ± 10°C . 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 .

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.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. 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. 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

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 Olivine is used in preference to silica sand to overcome the silicosis hazard (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 .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 . The inert and lamellar platy natures of talc improve its cracking resistance, adhesion and barrier properties.

                                          

                                    

Talc is 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

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 fibres .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 .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

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 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 Kaolin clay is the most extensively used particulate mineral in the filling of coating of paper .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

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 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 .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

Table 1. Other filler materials and their respective proper-ties [17]. Data	Chamotte	Chromite	Magnesite	Chrome- Magnesite
Availability	abundant	Good	Good	Good
Refractoriness℃ (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 wetting	No wetting	No wetting	No wetting

Water 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 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 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 The use of organic solvent-based coatings is threatened by environmental issues because they are toxic and flammable

3.3. Suspension Agents (The Rheology Control

System)

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 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 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- 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

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-sideration.

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 . 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