TYPES OF CORE COATING

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 .

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.

The above discussion refers to liquid coating mixture  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 systems—from cold box, through hot box and furan to inorganic types .

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techniques can be described as a process where the substrate to be coated is immersed in the liquid or coating and then withdrawn with a controlled speed under controlled temperature and atmospheric conditions.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. 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 . 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. The coating can be cured by a number of methods such as conventional thermal, UV, or IR techniques depending on the coating formulation

                                                  

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  and coating applied through a hose, 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. 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 between the sand grains. This reduces the possibility of metal penetration and veining

                                          

Flow coating method, it is seen that the mould is inclined at an angle.

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.

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. 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 .

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).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. 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. 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.

CHARACTERIZATION OF COATINGS

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 . 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 . 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.

2. Viscosity

Viscosity, a measurement of material flow properties, is 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. 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 second

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. 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 . L.

Winardi et al., reported that coating viscosity is typically reported in degree Baume. Higher Baume´ number indicates higher viscosity.

It was also reported in that Baume when per-formed in a controlled laboratory environment tracks well certain coating properties.

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. 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.

6.5. 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 aggregation.

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:

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.

Thus small particles should tend to dissolve while lar-ger particles should grow as observed in Oswald ripening of precipitates.

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.

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. 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 . The wet coating layer thickness will be correlated to the dry coating thickness, if the volume to solids ratio of the coating is known . As a rule of thumb dry coating thickness is 50% of the wet coating thickness.

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. This equation gives the wet coating layer thickness on a vertically withdrawn flat plate.

where hw = Wet coating thickness

υ = withdrawal speed

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

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 . It was reported in, 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. Thermal ex- pansion increases with the thickness of the proud coating layer (the layer on the surface of the substrate). 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. 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 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

     

                   

small Reynolds number (Re). 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

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. Research 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 However, the random aspect of the pore distribution induces a highly heterogeneous lo-cal flow which becomes turbulent at high Reynolds’ re-gimes

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. It was reported in that the strength of core and mould material will decrease about 30% with 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. 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-tion.

where σSG , σSL and σLG are solid/gas, solid/liquid and liquid/gas surface tensions, respectively

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 . 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 (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 , by Equation (11)

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. 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 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.