Investigation On Ingredients Choice & Preparation Of Water-Based Coating In Expandable Pattern Casting For Cast Iron

Coating is one of the key technologies in Expandable Pattern Casting (EPC). The properties of coating, especially for high temperature permeability and high temperature surface strength, have significant impacts on elimination of carbon defects in cast iron of EPC. Refractory powders, suspending agent and binder are three major components of EPC coating. In this investigation, the effect of collocation of refractory powders, varieties and contents of suspending agent and binder on the properties of EPC coating, including suspension, apparent viscosity, brushing, rheology, thixotropy, permeability and surface strength, were analyzed by using a cypriot graduated cylinder, coating-4 viscometer, NDJ-1 rotary viscometer, scanning electron microscopy(SEM) and ZTY intelligent permeability detector. A orthogonal experiment was utilized in compound suspending agent and compound binder to develop a cast iron EPC coating. The major results can be drawn as follows:(1) As the size of kaolin powder decreases, kaolin coating suspension and the apparent viscosity are all increased, little change is occurred on the high-temperature surface strength, and the surface strength is not higher than 25g/mm, whereas the high-temperature permeability decreased 45%. As the powder size of alumina decreases, alumina coating suspension and the apparent viscosity is increased, but is not higher than kaolin coating. The surface strength and permeability of alumina coating are higher than that of kaolin coating.(2) The addition of talcum powder with the size of 0.025mm can improve the properties of kaolin coating including apparent viscosity and thixotropy, but deteriorate the high temperature permeability and high-temperature surface strength.(3) With the content of sodium lignosulphonate increased, coating suspension rate is increased. suspension rate (72h) of paint is reached 100% after the addition of sodium lignosulphonate is 0.7%. The fluid type of sodium lignosulphonate coating is yielding pseudoplastic fluid. By the orthogonal test, the compounding suspending agent is 2% attapulgite+0.7% sodium lignosulphonate.(4) The high temperature permeability of EPC coating can be improved by PVA addition with content between 1wt% and 4wt%. The high-temperature strength can be improved with the addition of aluminium sulphate. The effect of aluminium sulphatet on the thixotropy of coating is better than that of PVA with the same content. By the orthogonal test, the compounding binder is 5% aluminium sulphate+3%PVA+5% silica sol.(5) The formula of the EPC coating for cast iron is 75% alumina,20% kaolin,5%talcum powder,2% attapulgite,0.7% sodium lignosulphonate,5% aluminium sulphate,3%PVA,5% silica sol, with proper water and octanolt, and the suspension (72h), high-temperature permeability, high-temperature surface strength, thixotropy, density and high-temperature anti-crack grade level of the coating are 100%,347,363g/mm,22.12%,1.49g/cm3 and I grade respectively. The pouring experiment results show that the cast iron EPC coating can meet the requirements on processing e and working performanc.