fashioned from a sheet into a sleeve by overlapping a pair of its edges 46 and 47 and by joining these edges into a longitudinal seam by the application of heat adhesive or the like as shown in fig4 the sleeve 33 is positioned over the cylindrical portion of the rubber bag 32 and may be held in this position by taping it along its ends to the rubber bag the sleeve 33 is fabricated so that its outside diameter corresponds to the desired finished inside diameter of the sidewall 11 of the tank the length of the sleeve is generally equal to the length of the tank sidewall 11 the rubber bag 32 and sleeve 33 may be encased in a protective sheath of polyvinyl acetate film or nylon film or similar material which will protect the molding resin against the chemical action of vulcanizing agents or residues thereof associated with the bag or sleeve the inflatable core is normally inflated with air to a final pressure of about 25 30 p s i this pressure is adequate to properly compact the fiber body to form an article having a predetermined suitable fiber to resin ratio under these pressures the non expandable sleeve 33 does not appreciably stretch the tank sidewalls.