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techniques can be described as a process where the substrate to be coated is immersed in the liquid or coating and then withdrawn with a controlled speed under controlled temperature and atmospheric conditions.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.