PURPOSE:To produce the preform for a composite material having a good molding size without having a spring back by applying vibrations to a mold while maintaining a prescribed outside dimension after molding the preform under pressurization by molding whiskers or fibers. CONSTITUTION:A prescribed amt. of an inorg. binder is compounded with the ceramic whiskers or short fibers and the mixture is uniformly agitated and mixed by a mixing machine. A material which is cured by heating and does not react with a combined base material is preferable as the above-mentioned inorg. binder. The powder mixture 2 obtd. in such a manner is housed into the preform forming mold 1 and is pressurized to a prescribed thickness by a piston 3. While the piston 3 is held as it is, the forming mold 1 is disposed on a vibrating table 4 and the forming mold 1 is vibrated. The residual stresses of the whiskers, etc., generated by the compression pressure of the pressurization are released in this way. The powder mixture 2 is thereafter heated to cure the compounded binder mentioned above, by which the preform is obtd.
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