Fig. 4. Since the installation of a sand cooler, the average of daily return sand temperature at Weil McLain decreased by 40F (4C).
Fig. 5. Muller effi ciency improved from 55 to 75%.
lbs. and processes 120 tons/hour of green sand on its “A” line. The sand is mixed and mulled in a continuous mixer, which normally operates at a 70-second retention time. The water and bond addition in the muller is controlled by an automatic tester based on feedback measurements of moisture, compactability and green compression strength. T e return sand temperature varied
considerably within the same day when measured before mulling (Fig. 1). At the beginning of the shift, the sand temperature was high because the stored sand in the silo maintained the temperature overnight, even after a weekend shutdown. After a short time, the sand temperature started drop- ping when cooler sand from molds left overnight was put back in the system. After this short period of cool sand, the temperature increased rapidly when hot sand from molds poured in the morning was recycled. From then on, the temperature increased gradually during the day, sometimes to as high as 160-180F (71-82C). Studies have shown that sand returned to the muller should be no hot- ter than 100F (37.8C), or 15F (9.4C) above ambi- ent temperature for ideal green sand preparation (Table 1). Prepared sand should exit the muller below 100F to prevent
34 | MODERN CASTING July 2011
temperature buildup as the sand is used repeatedly. When return sand approaches ele- vated temperatures, preparing it in the mullers and mixers becomes more dif- fi cult. T e moisture that must activate and plasticize the clay is used to cool the sand, driving off the heat through evaporation in the form of steam. T e time it takes to lower the temperature is subtracted from the overall mull- ing time, leading to deteriorated sand properties. T e sand becomes more fri- able, which results in sand inclusions. Even if the sand does appear to be mulled suffi ciently, the hot sand tends to dry out at molding, and loose sands persist prior to metal pour. At Weil McLain, the green com-
pression strength measured at the muller tended to decrease when the return sand temperature increased
(Fig. 1). A statistical analysis from one day’s data showed the relation- ship between the two variables was signifi cant and indicated 50% of the variation of green compression strength was explained by sand tem- perature variation. When moisture was included in the equation, the relation- ship increased to 53%. T e moisture had to be increased in the sand pro- portionally to the sand temperature to compensate for water loss due to the high sand temperature. T e prepared sand moisture
measured at Weil McLain’s molding line averaged 3.44%. T e total process variation of the moisture was 1.7%. Low moisture content resulted in low compactability and more friable mold surfaces. When Weil McLain increased the moisture content because the hot sand was being mulled ineffi ciently, it went up to levels that can cause casting defects, such as burn-on, which is a severe sand adherence that cannot be removed by shotblasting (Fig. 2). T e increase in mois-
ture is also related to the increase of bond addition in the preblend. Analy- sis of the relationship between moisture and active clay showed that additives contained in the preblend will absorb water to some degree.
Fig. 6. Regression analysis of green compression strength vs. return sand temperature showed a linear relationship.
Trial and Error As green compres-
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