CASTING INNOVATIONS Casting Mold Dryers Help Increase Efficiency
To achieve the highest throughput in a sand casting shop, the refrac- tory wash could be force-dried using a convection oven. These ovens are called sand core dryers, mold dryers or core drying ovens. They operate at 200-400F and dry the cores or molds in 10-30 minutes, as opposed to the 10-20 hours required by passive dry- ing at ambient temperature. The oven temperature and drying time depend on the mold size, sand porosity, and the efficiency of the oven delivering the heat. To minimize energy use and reduce operating costs, the oven should be designed with tight temperature uniformity, high air velocity (velocities greater than 5,000 ft. per minute are common) and good end seals to prevent hot air loss. This de- sign provides good results with a lower operating temperature. Common accessories used with sand core dryers are forced air coolers to cool the sand molds for handling, humidity sensing systems to optimize the exhaust rate, belt conveyors, slat conveyors, and PLC control. Due to high production rate requirements, many casters
utilize conveyors to carry the molds through the dryer. A company in Milwaukee needed to dry a zircon- based core wash on sand molds used in the manufacture of large gears. The molds had dimensions of 70 x 50 x 40 in. (1,778 x 1,270 x 1,016 mm) and weighed 3,000 lbs. each. They needed to be dried at the rate of one every 5 minutes, for a total production rate of 36,000 lbs. per hour. Conventional conveyor designs were not up to the task. The traditional design utilized a flat wire belt conveyor riding on a friction style slide bed, a structural
steel frame to support the moving conveyor belt. At this combination of mold weight and conveyor speed (1 foot per minute), a conventional design would quickly wear out due to the heavy load on the conveyor and the resulting friction between the belt and slide bed. Wisconsin Oven (East Troy,
Wisconsin) met the challenge with a roller belt support system whereby the conveyor belt rode on top of specially designed high temperature rollers (Fig. 1). The rollers utilized a high temperature lubricant that reduced the friction by a factor of eight in comparison to a conventional friction style slide bed arrangement. In addition to significantly extending the life of the conveyor belt, this reduced design allowed the use of a smaller conveyor drive motor. The result was a long-lasting conveyor design that was also rugged and energy efficient. Te conveyor oven had an
Fig. 2. Dual conveyors provide a reduced footprint and extra heating flexibility.
operating temperature of 575F (302C), and a heated length of 21 ft. (6,400mm), which required 42 support rollers
Fig. 1. A conveyor belt rides on high temperature rollers. October 2016 MODERN CASTING | 43
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