Synthetic resin binder:
The sand used for shell moulding consists of a mixture of the following ingredients are following:
(a) Dry sand grains, AFS fineness 60 to 140 distributed over 4 to 5 screens.
(b) Synthetic resin binder, 3 to 10 % by weight. Resins that may be utilized are the phenolformaldehydes, alkyds, urea formaldehydes, and polyesters. The resin should be a thermosetting plastic, and is utilized as a powder in dry mixtures. It might also be applied as a liquid and dried on the sand grains then. For moulding, the mixture should be dry and free flowing.
The shell is cured in two stages. While the sand mixture drops onto a pattern heated up to approximate 350 to 700oF, the plastic partially thermosets and builds coherent sand shell next to the pattern. The thickness of this shell is approximate ¼ to ¾ inch and is based on the pattern temperature, dwell time on the pattern, and the sand mixture. The shell, yet on the pattern, may then be cured by heating up it to 450 to 650oF for 3 to 1 min. stripping the shell from the mould presents a difficulty as the shell is very strong and tightly grips the mould. A mould release agent, or parting agent, is utilized to get clean stripping while the ejector pins push the shell off the pattern. Silicone parting solutions, sprayed on the pattern, have been found out satisfactory.
Shell moulding is possibly utilized more for making cores than moulds. A variant of the procedure, called as the hot-box procedure, employs a heated core box. The moulding mixture again have 1.5 to 4.0 % resin of the furane or furfuraldehyde kind. Heat from the core box causes the catalysts to begin an exothermic polymerization procedure. As the sand temperature grow up, the resin polymerizes and the mass hardness. Moulds are made up by assembling the hot-box cores.