Research Of High Strength Coating For Expandable Pattern Casting

Expandable Pattern Casting(EPC) technology has been widely used as a green casting technology in the 21 st century, but casting defects such as adhering sand, hole and wrinkled skin always occurred in the production of castings, and these defects in large part is due to the coating. Refractory aggregate, binder, and deflocculating agent which are the main components for EPC coating are systematically studied by single factor test and orthogonal test in this paper. The properties of EPC coating is tested using the ZNN-D6 rotational viscometer, STZ-A direct-reading permeability tester and SZFL intelligent gas evolution tester. The raw and processed materials proportioning and preparation process of coating is obtained, that is with excellent performance, moderate price, and easy to operate for high strength of coating for EPC.In the selection and determination for refractory aggregate, two aggregate ratios, bauxite and quartz as well as bauxite and mica powder, and three kinds of aggregate ratios with bauxite, quartz and mica powder were mainly studied in this paper for the consideration of high melting point and difficult to sintering of single aggregate. The experiment results showed that the integrated performance of the coating with three aggregates was better, and the ratio was bauxite 80%, quartz 10% and mica powder 10%.In terms of binder selection and determination, the starch, silica sol, and sodium hexametaphosphate as the composite binder were used in this experiment because the composite binder has significant effect on improvement of the strength and other properties of coating. The strength, permeability, floatability and flowing property of coating were tested, and the content of composite binder that starch2%, silica sol 3%~5%, and sodium hexametaphosphate 1.5%~2.5% is finally determined.In the selection and determination for deflocculating agent, CMC and sodium bentonite were used as the composite deflocculating agent to improve the floatability of coating. The influence of the addition amount change of CMC and sodium bentonite on coating performance such as strength, floatability and gas evolution was studied. The addition amount of CMC and sodium bentonite should be CMC 1.5~2.5%,sodium bentonite 2%~3%.The optimization design for the components of coating is carried out by the orthogonal test. The ratio of various components of coating is decided using the range analysis and comprehensive balance method, and the optimized composition is CMC2.5%, sodium bentonite2%, silica sol 4%, and sodium hexametaphosphate2.5%. The performance of this proportion of coating is tested, and the 48 h floatability of the coating is 99%, the permeability is 275, the high temperature tensile strength is 6.8kg/cm2, the high temperature surface strength is 0.01 g, the searing and anti-crack grade level is I. Through “burning the shell” experiment in the laboratory and pouring examination in the factory, the strength of the coating is high, peeling performance is well, and castings are defect-free. All of these have reached the expected target. The cost of the coating is accounted and the price is 1444.5 RMB/t. The coating price is in a low cost, which can satisfy the enterprises for the requirement of the economic efficiency.