DEVELOPMENT OF SELF-HARDENING SAND SYSTEM FOR MAKING CASTINGS

Abstract

The present investigation was undertaken to develop a self--hardening.sand mixture without the use of carbon'.. dioxide gas. for the production of moulds and cores. In the self-hardening sodium silicate bonded moulding sand system, sodium bentonite additions were made in a sodium silicate banded sand mixture to develop self-hardening property at room temperature. Results of the investiga-tion indicated that the sand mix containing 6% sodium silicate and 3% sodium bentonite and 3% moisture. content by weight developed the optimum compression strength of 30 kg/cm2 as well as permeability of 321. after 24 hours of air hardening at room temperature at a relative humi-dity of 48%. The influence of heat on the hardening property. of the above sand mix was also studied and it was observed that the maximum strength developed by air hardening alone in 24 hrs at 150°C could be attained by heating in an oven in 20 minutes only. The influence of accelerators like ferro-silicon and calcium carbide was also studied for the above sand mix. It was observed that by adding 0.4% ferro silicon or 0.2% calcium carbide, the compressive strength values after 6 hrs of air hardening were comparable to the values obtained after 24 hrs of air hardening for the sand mix without any addition. Shatter toughness of the optimum ii sand mix was studied and found to be 80% which means this sand mix has good workability. A longer mixing time gave smaller compressive st. .- eng th as compared to shorter ,mixing time .which resulted in higher compressive strength. Keeping the sodium si- licate content at 6% and sodium bentonite and water con-tents being constant at 3% each, the increase of relative humidity from .48 to 75% decreased. the compressive strength by 829. However, at low relative humidity of 48%, the compressive strength can be increased by 67% if sodium slicate content is increased from 6% to 8.25% keeping so-dium bentonite and water content constant at 3% each, but permeability is reduced by 22%. However, if relative humidity is increased from 48% to 75%, and sodium silicate content is increased from 6% to 7.7% the absolute value of compressive strength, though much lower as compared to that at 48% RH, is increased from 5.36 kg/cm2 to 7.50 kg/cm2 which is a large increase of 77.2%. Using this optimum sand mixture, test gear castings of grey cast iron weighing about 30 kgs were successfully casted and it was observed that surface finish of the casting was fairly good without any casting defects like metal penetration, blow holes etc. These castings were compared with the castings made with conventional CO2 process iii and ordinary green sand casting. Ease of cleaning the adhering sands from the test casting was in the decreasing order with green sand castings self-hardening sand cas-ting and CO2 casting.