Composite binders for precision casting shell molds

The convert mold process has been investigated, focusing on preparation of shell molds using different composite binder systems. The glassification efficiency of binder systems with different SiO2 precursor structures were investigated in terms of the Si content and dipping time, and the subsequent effects on fracture strength. Two types of the SiO2 precursor were used: tetraethyl orthosilicate of the silicate type; and hexamethyl disiloxane (HMDS), octamethyl trisiloxane, and polydimethyl siloxane of the siloxane type. Use of the siloxane type precursor does not produce a hydrolysis reaction, whereas the silicate type precursor is converted into SiO2 by a hydrolysis and condensation reactions during the drying process. The samples based on the siloxane type (except HMDS) have a much higher fracture strength, nominal value of 8 MPa, than that of samples based on the silicate type. This is a result of the enhancement of glassification. When the siloxane is added to the silicate the fracture strength is as high as about 6 MPa, independent of the Si content and dipping time. Results indicate the importance of the reaction mechanism in achieving fracture strength with related to glassification efficiency.