Microwave Curing Of Core Binders And Coatings M. J. Skubon 183

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M i c r o w a v e Binders a n d Curing of people and inability to generate sufficient growth capital for economic survival. C o r e Coatings Advantages of Microwave Heating For Foundrymen M. J. Skubon Advantages of microwave heating in processing of cores and molds are as follows: C o b e r Electronics Inc Stamford, Connecticut Introduction While enormous growth has occurred in fast-reacting sand binder chemistry and core and mold producing equipment, some fundamental problems have been created at the same time. The speed at which these rapidly produced cores and molds are m a d e is not matched downline in the foundry operation so that final molding can be accomplished in the least amount of time from the production of cores and molds. A major portion of cores and molds need to be coated, postbaked and often glued together. The advantage of microwave processing after initial core production is substantial. Understanding of microwave equipment capabilities and functional design engineering is necessary for successful applications. While energy savings are significant with microwave heating, its speed, controllability and other unique capabilities must be effectively used to derive effective economical benefits. The end result can be a very timely tool to answer foundries' growing problems with energy procurement and cost, escalating costs of manufacturing space, shortage of trained technical A F S Transactions 78-09 • speed of processing - increased throughput • processes cores and molds at considerably reduced surface temperatures which greatly facilitates handling and promotes stronger labor relations • low core surface temperature after microwave processing eliminates sand binder thermoplasticity problems • complete removal of water from cores and molds with associated casting gas defect reduction • compatibility with low-cost hot-melt adhesives • potential to eliminate separate core glue curing cycle from coating drying • ability to use new microwave sand binders n o w coming on the market which can be water-base and nonpolluting and require lower cost coreblowers and coreboxes • complete control of processing due to uniform heating and accurate instrumentation which is only possible with microwave electronics • not dependent on natural gas. History The development of microwave technology goes back to World W a r II and the advent of radar. Initially radar and other military and defense systems were the sole application of microwaves Today, while these military systems in addition to a gamut of 183 communication applications are developing with a great deal of technical sophistication, the industrial and commercial markets for microwaves are coming of age. Total U S industrial sales of h o m e microwave ovens was approximately 2.7 million units in 1977 for a total value of about $980 million. These figures represent only 7 % of the total h o m e market. In 1970, by way of contrast, there were roughly 40,000 ovens sold into homes. In industry microwave heating is used in food processing, continuous and batch curing of rubber compounds at fast throughput rates, investment shell dewaxing, curing of plastic foams and, of course, curing of cores and drying of coatings. Microwave heating technology is not simply an interesting new approach to heating and curing - it is a proven tool which the foundry industry needs and is using today. H o w Microwave W o r k s When an electromagnetic wave is propagated in a heatable dielectric material, its energy is converted to heat. T o understand this more fully, the molecular properties of dielectric materials must be examined. Water is the major dielectric material in the foundry industry which is heated by microwave energy. The water molecules consist of hydrogen and oxygen ions arranged so that the molecule is electrically neutral. Because of this arrangement, however, the electrical charges within the molecule have a dipole m o m e n t and are said to be polar. Different molecules have varying degrees of polarity. A n electric field exerts a twisting force on a polar molecule which attempts to align the molecule with the field. W h e n the direction of the electric force is reversed, the molecule attempts to reverse its orientation. However, in so doing, frictional forces created by the molecules rubbing together have to be overcome and energy is thereby dissipated as heat. Friction generates heat and the dielectric becomes hot. Electrical energy which should be stored in the dielectric material is in part lost as heat, a phenomenon called dielectric loss. Other materials used in foundries which are good heatable dielectrics are phenolic resins of all types, furan hotbox resins, urethane nobakes and all the inorganic systems — sodium silicate and phosphate type. Green sand is also very heatable due to its water content and to carbon and impurities in the reclaimed sand. Reactivity of these binders increases as the temperature from heating increases. This heating rate increase can be rapid; however, with microwave electronics it is controllable. Microwave heating can raise the temperature of the binder to the point at which its exothermic nature carries curing reaction through completion. All the organic binders listed have been cured by microwave to temperatures in excess of 400F (204C). O v e n Selection and Design Selection of a microwave oven is predicated primarily on throughput of cores or molds to be processed, type of sand binder, size of the cores or molds and amount of water in the binder or coating. Once the quantity, weight and dimensions of the cores or molds required to be processed is k n o w n and finally the weight of water borne by the coating and adhesive, then the microwave system can be roughly gaged. The next step is to calculate the power and time required to process the bonded sand and water. The power is calculated using a value of between 2.0 and 3.0 of water removed per microwave k w h and a binder cure rate 184 between 30 and 300 lb of mixed sand per k w h depending on the materials being used. With water the theoretical m a x i m u m rate is 3.35 lb removed per microwave kwh. Actual power and time can only accurately be determined by laboratory testing of coated core pieces or better yet entire cores or molds in an appropriate microwave oven, preferably set up in the coreroom. A s all sand binders evolve some quantity of solvent vapors when heated, it is important to have sufficient air velocity and flow to carry these vapors out of the oven cavity. This air must be heated so as to maintain air temperature above the dewpoint of water — approximately 130F (54C) — otherwise water and other solvent vapors m a y recondense in the oven cavity. These vapors can be made to evolve so quickly that cores or molds crack due to high vapor pressure behind the sand surface. A. m i n i m u m power-time curing cycle must be achieved so as to maximize the speed capability of microwave and allow for necessary pressure relief within the core or mold. The oven will be either batch or conveyorized type. Oven cavity length is a function of the residence time for satisfactory heating found in testing. A typical system would have a residence or heating time of 2-8 min. For a conveyorized oven this time is controllable by varying the conveyor speed. This gives full process control capabilities and versatility for processing a wide range of core sizes and materials. Batch ovens require turntables and microwave m o d e mixers to assure uniform microwave field distribution for even heating. The volume of air and velocity are important in achieving optimum system performance. Air volume and exhaust pressure requirements are calculated from the amount of sand binder in addition to the water contained in coatings and adhesives. The air flow pattern in a conveyor oven is typically countercurrent to the conveyor flow. Air enters on the exit side of the oven and is exhausted on the input side. This helps for complete water and solvent removal as the last part of the drying process is the most difficult. The air is heated by either flowing it by separate resistance heaters, past the magnetron or microwave power tube or a combination of both. Given the volume of air the size of the heaters can be calculated. This air temperature should be controllable to about 180F (82C). Electronics involved with the oven selection are the magnetron type and size, the system used to transmit microwave power from the tube to the oven and finally power control and process regulation. Because microwave heating and curing is so rapid, accurate power control is imperative. There are two concepts of microwave power generation. O n e involves the use of a commercial microwave tube while the other uses a microwave tube of industrial design. Commercial tubes are aircooled, while industrial tubes generally are water-cooled. Key equipment features necessary for a modern reliable microwave processing system can be summarized as follows: 1) Reliable continuous operation in the adverse foundry environment via • control of electrical environment through cooling • steel conveyor meeting most demanding foundry standards • use of fully heavy industrial N E M A , JIC and O S H A standards for industrial tubes in sealed environments rather than commercial standards. 2) Ability to operate system without failing, at high microwave power levels into marginal core and coating loads by using the best available microwave tube protection. 3) Versatility to process a wide variety of cores, molds and coatings by selecting adequate microwave power and use of waveguide tuning sections to maximize absorption of lb power by core or coating. 4) Complete electronic control including full A F S Transactions instrumentation. This is necessary to optimize performance in drying and curing quality and economics and can only be accomplished with a modern electronic system. Maintenance As with all foundry equipment microwave ovens require both routine preventive maintenance and repairs. Routine maintenance includes cleaning and monitoring cooling systems, cleaning and lubricating the oven cavity and its conveyor or turntable. Dirt buildup on the conveyor or turntable and on the walls of the oven cavity will absorb some microwave power and rob the system of efficiency. The latest generation conveyorized systems have shown virtually no dirt buildup in oven cavity walls due to the reliable operation of the support air system. For this reason it is essential that any air filters present be kept clean and blower and fan motors be lubricated. Magnetron tube life varies between 2000 and 10,000 hr depending on the tube size and type and method of its protection. Commercial-type microwave power tubes have lives of about 2000 hr, while the higher powered industrial tubes are between 6000 and 10,000 hr. Performance Factors a n d Operating Costs Use of microwave to date in the foundry industry has been justified largely on the basis of processing speed. Only recently have accurate operating cost comparisons been made. These comparisons are against conventional resistance-heated and gas-fired ovens, both with supporting high-velocity air. The industry today is beginning to recognize the total-system advantages of microwave. Whether batch or continuous, microwave ovens are initially more expensive than conventional ovens. They are economically justified through proper use of their speed, smaller space requirement, energy savings and improved core and mold quality. With microwave heating, water-base coatings can be completely dried with considerably less input of Btu's than is required with a conventional oven. The microwave dried coating is significantly drier also, which means less gasassociated casting scrap such as hydrogen porosity and blows. With all oven systems there are energy losses involved which are associated with their design and efficiency. In microwave there is roughly a 1 0 % loss in converting from A C to D C power. There is a further 30-40% loss associated with the conversion to microwave energy. The total system operating efficiency of a microwave oven varies between 35 and 4 5 % from A C power into microwave energy coupled into dehydrating water. A conventional oven will operate at less than 1 0 % total system efficiency. The drying rate of water-base coatings is an excellent example of the speed and efficiency of microwave heating. With conventional ovens about 5 0 % of the total drying time is consumed drying the first 8 0 % of the water. The last 2 0 % of water then consumes about 5 0 % or the balance of the drying time. Because microwave energy penetrates immediately, the A F S Transactions entire core or mold mass, all the water or sand binder is energized uniformly and heating starts at the same time everywhere through the mass. N o time is wasted while heat convects from the core surface to the interior as occurs in conventional systems. The drying or curing time cycle required by microwave is only limited by the ability of the core or coating to vent itself without bursting, cracking or blistering from rapid volume expansion due to the binder or coating solvent-forming vapors. A solvent cannot be dried without going from the liquid to the vapor state. This drying time has been as fast as 60-75 sec in actual coated production cores. With a given microwave oven cavity the amount of microwave power can be increased and drying time can literally be only a few seconds to cure or dry. Experience to date in drying water-base coatings shows a definite relationship as regards the coating's composition and physical properties. The objective in drying coatings is to couple or absorb the m a x i m u m amount of microwave energy into the coating's water. The starting water concentration in the fluid coating is very important. This water concentration can vary from about 3 0 % to 7 5 % by weight. Foundrymen should choose a coating with as little water content as possible. This is easier said than done as the coating is required first to coat the sand surface evenly and penetrate to a critical depth. It also must produce smooth and defect-free casting surfaces. Coating and microwave oven suppliers can help with this selection. The coating's refractory should preferably be transparent; for example, talcs, micas, pyrophyllite, clays, ground silica, alumina or various alumina silicates are all good refractories that absorb little microwave energy. Carbonaceous refractories on the other hand absorb much microwave power. This can be either through direct absorption or through localized resistance heating between two adjacent particles. Experience has shown coatings based on carbonaceous refractories cure out very quickly initially; however, before all the coating's water is dehydrated the carbon begins to preferentially absorb power and makes complete drying difficult. The surface temperature of fully dried cores coated with carbonaceous refractory coatings will be considerably higher than if an insulating refractory is used. The coating's depth of penetration is also a factor. This depth should not be excessive; otherwise high water concentrations will be absorbed into the coating and drying time will be increased to remove this deep-seated water. This extra time is required due to high water-vapor pressure deep underneath the core's surface posing a vapor relief problem. The last factor of consequence is the coating's permeability. High permeable coatings definitely can be dried faster in a microwave oven. This assumes that the foundryman is trying to dry the coating in the least amount of time. Rapid heating rates with low permeability coatings result in core and coating cracking and coating blistering. Because of the considerably faster drying rates for coatings in microwave ovens, there is a significant reduction in runs, sags and tear drops after drying. Further, the general dried coating film thickness is more uniform. This is due entirely to the water evaporating more quickly and leaving behind a higher solids coating with a higher viscosity which inhibits flow. Because the sand binder and coating refractories are always somewhat microwave-absorbent, they compete for energy with water when drying coated precured cores or molds. Effective preferential coupling to the water load is achieved by reducing the core's residence time to a m i n i m u m in combination with high coupling efficiency to the water load. The surface temperature of microwave-dried cores or molds 185 will be a function of the oven's microwave power, residence time, type of sand binder and type of coating refractory. Long residence times — approaching 8 min — combined with carbonaceous coating refractories, would yield coated core surface temperatures of 130-180F (54-82C). This is 100°F (56°C) cooler than is conventional. Conclusions The foundry industry's wide expansion into the modern fast- 186 curing chemical sand binder systems, including reclamation, is complemented by microwave heating. While initial microwave equipment costs are high, their wide acceptance appears inevitable. If the fast-reacting core and mold system's processing speeds are to be fully exploited, the speed inherent in microwave is essential. Current microwave installations and most new interest is for drying water-base coatings. The greatest potential, however, for microwave could well be for rapid curing, pollution-free, novel, sand binder systems. A F S Transactions