Abstract: Method for obtaining a plastic water-heater body, wherein(1) a plastic shell is used which is intended to form the interior of the water heater and also serves as an internal molding wall, the said shell having at least one orifice,(2) the said shell is positioned inside a mold consisting of two parts, while providing a space between the outside of the shell and the inside of the mold, the mold having an opening,(3) water is introduced into the internal shell via the said orifice,(4) a liquid mixture consisting essentially of a polyester resin composition capable of foaming and polymerizing is cast via the said opening of the mold, by cooling the outside of the mold to a temperature of approximately 10.degree. to 15.degree. C. during casting of the resin, so as to obtain a skin effect on the surface of the foam which comes into contact with the mold during polymerization of the said foam, by controlling the temperature of the expanding and polymerizing resin so that it does not exceed approximately 40.degree.