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Mold and core coating constituents
- 1. Mold and Corecoating Constituents Prepared by Asfand shahid
- 3. The need fora coating When liquid metal is cast into a sand mold or against a core, there may be a physical effect and a chemical reaction at the sand/metal interface. Either may result in surface defects on the finished casting
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- 5. Physical effect Metal penetration liquidmetal penetrates the pores of the sand mould giving a rough surface to the casting Dependence Metallostatic pressure Dynamic pressure penetration in lower part of casting more severe where the metal stream impact on the mold surface in high density metals iron or steel Sand expansion Stresses can form in bonded sand due to the thermal expansion this can lead to cracks
- 6. Chemical effect Burn-on A chemicalreaction occurs between sand and metal. It occurs with impure sand some impurities, particularly alkalis, reduce the refractoriness of the sand some binders mostly those based on sodium silicate which have poor refractoriness so that liquid phases are produced at temperatures as low as 900°C. Carbonaceous defects Organic mold and core binders degrade at high metal pouring temperatures, forming carbon bearing gases which can lead to carbon pick-up or surface pock marks due to lustrous carbon.
- 7. Refractory coating A refractorycoating on the mold or core can reduce or eliminate metal penetration and sand burn-on. For many years, foundries made their own coatings, usually ‘blackings’, based on coke dust or graphite. Ferrous and aluminum alloys show metal penetration, burn-on due to the high casting temperature and high density.
- 8. Coating selection An effectivecoating should have the following characteristics: Sufficiently refractory properties to resist the metal being poured. Good adhesion to the substrate. No blistering, cracking or scaling tendency on drying. Good suspension and remixing properties. Good stability in storage. Good covering power. Good application properties.
- 9. Choice of coatingand form of supply Applica tion method Drying method Equipme nt required Carrier Mode of supply Performa nce required
- 10. Types of coating Powdercoating The attraction of a powder coating is that it costs less to transport than a liquid, it is easy to store. The characteristics of a coating prepared from a powder pre-mix are highly dependent upon the mixing equipment used and the procedure adopted. If simple paddle mixers are used, long mixing times followed by ageing up to 24 hours may be required to develop the correct suspension and rheology. It is preferable to use special high shear mixers to develop the liquid coating and avoid the need for ageing Cream and ready-for-use coating A ‘ready-for-use’ paint is the easiest to use. So-called ready-for-use paints may also be supplied as light creams or slurries which require a small water addition to allow the user to control the final consistency of the coating to suit his particular application.
- 11. Components of a coating Foundrycoatings consist of: Refractory filler Liquid carrier Binder Rheology control system
- 12. The refractory filler This is the most important constituent since its properties decide the efficiency of the coating. The filler may be either a single material or a blend selected for specific applications. Fillers are chosen for their particle size and shape, sintering point, melting point, thermal conductivity, thermal expansion and reactivity towards the metal being cast and the molding material on which it is being used. Zircon, being highly refractory, is used for the most demanding applications such as steel or iron castings where section thickness is great. Graphite and coke are commonly used for lighter iron castings, often in combination with zircon, silica and tar.
- 13. Liquid carrier Theliquid carrier is the vehicle for the total coating composition and serves to transport the filler onto the sand substrate. It must be removed before casting takes place. The most commonly used carriers are water and isopropanol, although other solvents such as methanol, ethanol, hydrocarbons and chlorinated hydrocarbons have also been used. The use of water based coatings on cores and molds made with chemically bonded sand can effect the strength of the bond, and may give rise to casting defects due to surface friability of the core or mold.
- 14. • Fast airdrying • Obsolete due to environmental concern • For large mold • Dries faster than water • Removed by burning • RDT(Rapid drying technology) • Shortening drying time • Cheap • Drying in oven • Readily available Water Foseco Chlorinated hydrocarbons Isopropanol
- 15. Binder The functionof the binder is to bond together the filler particles and to provide adhesion to the mold or core. The binder often interacts with the whole of the coating composition and therefore cannot be considered in isolation. Silicate binders are particularly badly affected by water based coatings and spirit based coatings should be used instead. Coldbox-with this method, sand is coated with one of several binders, such as liquid sodium silicate or phenolic urethane, and catalyzed by a gas (such as carbon dioxide or sulfur dioxide) passing through the sand.
- 16. The rheology controlsystem This provides the suspension system that prevents the filler from 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 controls the flow properties of the coating and is designed to suit the application method that is used.
- 17. Application methods forcoatings Brushing Dipping Swabbing Spraying Overpouring
- 18. Application methods Dipping Submergingthe core in the liquid coating It is the fastest method of application The core is dipped manually or mechanically Dependence Consistency of the coating Rheological properties Uniform thickness of coating Decrease in viscosity upon the application of shear and regained by the removal of core Brushing Simple technique One or more layered apply Allows brush marks to flow out after application The coating can be of almost jelly-like consistency in the can but will flow readily when brushed The viscosity decreases slowly upon the action of shear and regains quickly but not immediately Extremely good suspension of the filler
- 19. Application methods Swabbing Implementused has long Easier to penetrate with long blisters and apply a thick smooth coating with minimum of dripping less tendency to mold damage or residual brush marks in the finished coating Slow process and require skilled operator to achieve best result Saprying Much faster technique than brushing but penetration and coverage is not as good as with brushing Sprayed coatings are of much lower viscosity than brushing or dipping varieties Skill is required to ensure that deep pockets and re-entrant angles are fully coated They are applied under pressure from a spray gun delivering a spray of finely divided droplets on to the surface to be coated
- 20. Application methods Overpouring/Flow-coating Itis particularly suitable for larger cores and molds it consists of a pumping system and a specially designed nozzle. Process The coating is continuously pumped through the nozzle which directs it over the core, the excess runs back into a reservoir to be recirculated through the nozzle once more. The gel system should allow an even deposition of coating without the risk of residual runs and tear marks.
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- 22. Equipment for mixingand applying coatings Dip tanks should be fitted with a paddle stirrer to maintain the coating in suspension without excessive movement, which could affect the rheology of the coating overpouring or flowcoating system consists of a method of pumping a continuous flow of coating over the core, allowing the surplus to run off the core to a catchment tray from which it is returned to the storage tank. Larger cores and molds are usually coated at a fixed station having a coating storage tank of about 100 litre capacity allowing about 4 minutes uninterrupted coating time. Small portable systems may be used, in which a portable catchment tray is placed under the work and the coating pumped over the core through a coating gun by displacement with low pressure compressed air.
- 23. Drying oven Corescoated with water based coatings must be thoroughly dried before casting. Warm air, infra-red and de-humidifying ovens may be used Good air circulation is needed to carry away the moisture Air temperatures up to 100°C may be used Large mold dried using portable warm air blowers, or a gas torch