sion strength decreased on the A line, Weil McLain had a tendency to produce more defective parts due to mold-crack. This mold-crack also may have resulted in sand inclusion defects (Fig. 3). Weil McLain tried various meth- ods of increasing the strength of its molding sand and reducing the sand temperature. One method was recy- cling the sand through the muller one time during the weekend and every night before the start of the shift. T is practice did not signifi cantly improve green strength, except on Monday mornings. During the day, the return sand temperature increased and the green compression strength decreased. During the week, in spite of recycling the sand prior to the start of the shift, the green strength continued declining. T e metalcaster also tried add-
ing 2% prepared sand into the return sand from the batch muller of its B line during the production shift. T is showed an 8% increase in green strength, but it also increased variation in moisture and compactability. Next, Weil McLain diverted its
prepared sand and put it back into the return sand of the A line system for 45 days. Initially, the green compres- sion strength increased, but it was not sustained consistently day-to-day. Further, variation in moisture and compactabiltiy throughout the day and daily did not improve. T e sand temperature increased to 150-160F (66-71C) during the day.
Success With a Sand Cooler
Weil McLain decided in 2007 a sand cooler would be necessary to
improve its green sand properties and the quality of its products. In Sep- tember 2008, the company installed a sand cooling system with the follow- ing features: • continuous sand weighting system; • complete control of mixing by moisture, weight and temperature sensors installed in the sand stream;
• capability of controlling screw feed- ers with up to three variable speeds for additive dosing, such as bond;
• automatic compensation of water additions for evaporation losses generated by temperature, mixing time, storage time and ambient conditions;
• onscreen display of useful informa- tion concerning equipment opera- tions, including graphic trends of main process variables and appro- priate messages and alarms. Since the installation of the sand
cooler, the daily average return sand temperature has decreased (Fig. 4). The summer before the cooler was installed, the average sand tempera- ture reached 160F. The next sum- mer, the maximum average tempera- ture was 120F. T e reduction of return sand
temperature resulted in an increase in green compression strength (a 40% increase as measured by the tester and 25% based on lab testing). T e maximum green strength was obtained with a partial addition of bond (50-70%) into the sand cooler, beginning in December 2008. T is bond addition raised the green com- pression strength beyond the maxi- mum desired level (26 psi), without increasing the amount of active clay. T e working bond also has
increased since the sand cooler instal- lation, while moisture in the prepared sand has been reduced by 0.6%. Muller effi ciency has increased from 55 to 75%, which is mainly driven by the increase in green compression strength (Fig. 5-6.). T e amount of active clay has been reduced from 9% to 7.5%. T e desired compactability of the sand at the point of use was main- tained at 32% +/-4% before and after the sand cooler installation. However, the compactability-to-moisture ratio increased. T e mulling-mixing system is able to develop the desired tempera- ture with less water. Green sand permeability increased
by 29%, confi rming that hot sand above 100F (38C) negatively aff ects this property. T e reduction in bond addition also may have contributed to the permeability increase. Chronic casting defects such as
sand inclusions and related defects were signifi cantly reduced after the installation of the cooler. Scrap due to sand inclusions was reduced 70% on the boiler section used for this study (Fig. 7). Similarly, the burn-on scrap for the same part was reduced by more than 90% (Fig. 8). No other process change was performed for this casting, so the scrap reduction is mainly attributed to the improvement in green sand properties. On all A line parts, scrap due to sand-related defects was reduced by 39%.
Michael Mroczek is general manager of foundry operations, T omas Wozniak is director of manufac- turing and Carlos Crespo is quality assurance man- ager for Weil-McLain, Michigan City, Ind. Stephen Neltner is technical service engineer and Vic LaFay is research and technical development manager for S&B Industrial Minerals, Cincinnati.
Fig. 7. Scrap due to molding sand-related defects were reduced 70% on the boiler section.
Fig. 8. Scrap due to burn-on defects was reduced 90% after the installation of the sand cooler.
July 2011 MODERN CASTING | 35
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