studies on silica and zircon sands bonded with silicate - ester systems

Abstract

Introduction Cold-set processes have revolutionized mould and core-making techniques and have led to a reduced need for skilled labour. Chemical binders have made steady progress in the production of moulds and cores for small, medium, and large castings. A study of published data indicates that cold-setting systems may be broadly classified under organic, inorganic, and miscellaneous binder systems. Under organic binder systems, phenolic, furan, alkyd, lignin, and free radical systems are the popular ones. In inorganic systems, sodium silicate bonded sands are the most widely used. The hardening effect in these systems is brought about by a hardener which may be a solid, liquid, or gas. Ferrosilicon, dicalcium silicate, and CO? gas are some of the commonly used hardeners with sodium silicate. Organic esters were introduced some years back as hardeners, but it is observed from the literature that the use of such esters has not yet been fully exploited, probably due to the lack of adequate information on the materials and the process. Therefore, an investigation was taken up to study the properties of sand systems bonded with sodium silicate and hardened by esters. Details of Investigation: Two grades of commercially available sodium silicates and two types of esters were made use of in the investigation. Initially, using Thin Layer Chromatography (TLC), Infrared Spectroscopy (IR), and Nuclear Magnetic Resonance Spectroscopy (NMR), the characterization of the esters was completed; ester E? was found to be ethylene glycol diacetate (EGDA) of about 99% purity. The other ester E? has been found to contain about 90% of glycerol diacetate (diacetin), the remaining 10% being accounted for by triacetin and impurities. Naturally, the properties of the sand systems hardened by these two esters will be slightly different from each other. Subsequently, experiments were carried out using silica sand and zircon sand as base moulding materials, sodium silicates B? and B? as binders, and the above esters E? and E? as hardeners (B? with E? and B? with E?). With different combinations of binder and hardener contents, investigations were carried out to study the setting characteristics, strip time, pour time, setting time, and permeability. From the findings of these trials, the compositions which gave the best possible properties for each sand system were optimized. By means of scanning electron microscopic technique and X-ray diffraction technique, the nature of bond formation on the sand particles was analyzed and justification for optimum composition made. Differential Thermal Analysis was adopted to study the thermal stability of these optimized mixes. The compositions optimized in each system were used for further investigations on the other aspects as briefed below: i) Thermal properties: The line heat source method and the melting and casting technique were adopted to assess the properties of thermal conductivity, heat diffusivity, and temperature diffusivity of the four sand systems at different temperatures. Proceeding further, the rate of heat extraction and the quantity of heat extracted from the moulds were calculated and the mould constant for each system computed. A comparison of the above thermal properties of the systems for the two base sands viz., silica and zircon was then attempted. In general, the thermal properties of the above sands were seen to be satisfactory for foundry applications. ii) Knockout property: Realizing that the casting trial method yields more realistic inferences on the knockout property than the measurement of retained strength, casting trials were conducted on all the sand-binder-hardener systems using aluminium, aluminium-12% silicon alloy, copper, and grey iron. The knockout property of each system has been quantified and compared with the other systems as well as with conventional green sand. Knockout property of the ester-hardened sands was found to be very good. iii) Friability: Experiments were planned to study the property of friability of the different systems mentioned above. The mouldability tester was made use of in the assessment of the above property. It was established through this investigation that the friability of the above sand systems is not higher than the other conventional sands in common use. iv) Surface finish and mechanical properties of castings made: In order to compare the surface finish and mechanical properties of castings produced in moulds made of the above sands, casting trials were completed with aluminium, aluminium-12% silicon alloy, copper, and grey cast iron. Zircon sand systems obviously result in better surface finish and marginally superior mechanical properties. However, all the sand systems result in acceptably good surface finish and more than the minimum of specified mechanical properties. Through the above investigations, a comparative study of the properties of sand systems comprising silica and zircon sand with sodium silicate binders and ester hardeners has been completed. The sand systems with the above ingredients develop adequate mould properties and result in acceptably good castings. The variation in properties of the systems from one another is due to the difference between the esters and the resulting difference in chemical reactions.