Improvement of local phenolic resin coated sand for shell molding

Abstract

In Thai foundries, shell molding using phenolic novolac resin coated sand is an important method for the production of near net shape components. Thai foundries use both Thai and imported resins for the shell process, hence this work has attempted to characterize these two types of resin. Thai and imported novolac resins were found to have different structural isomers giving rise to different crucial characteristics and properties including melting point, flow, and gelation time. The Thai resin had lower viscosity, shorter flow and gelation times as compared to imported resin. These overall effects are believed to be responsible for the lower bending strengths of shell molds formed with Thai resin. Scanning electron microscopy (SEM) was used to examine the fracture surfaces of test pieces. It was found that bond strength of resin coated sand test pieces increased with increasing contact area of resin bridges between sand grains. Contact areas were found to increase with the amount of resin used. Bond strengths also depended on the type of sand used in the mixes. Blends of Thai and imported novolac resins were investigated bases on the standard formula used in one foundry, 2.7% by weight of imported resin with reclaimed sand. Resin coated sand prepared using this formula generally posses bending strength in the range of 48-58 kg/cm2. Thai resin contents of 20, 40, 60, 80 and 100 by weight percent were used to produce coated sand. It was found that the resin blends of 80/20 and 70/30 imported resin to Thai resin gave equivalent bending strengths of resin coated sand to those obtained using only costly imported resin. Experimental results demonstrate a possible cost saving of up to 20-30% of imported resin in shell mold production by effective use of resin blends as the binder in resin coated sand.