PURPOSE: To mold a front cover material neatly at a high efficiency by applying high frequency heating thereto white the whole peripheral rim including not only the side rims of a sheet material but also corner ends is inverted inwardly over the base material at one time.

CONSTITUTION: A base material 4 such as cardboard paper is wrapped along the peripheral rim thereof with a sheet material 2 made of vinyl or the like to mold a front cover material. The surface plate 17 is provided with a concave section smaller in the dimensions than the sheet material 2 while the mold 3 is composed of four L- shaped sectional frames 31 fastened on the inner surface thereof in such a manner as to slide radially. A rotary disc 35 is loosely fitted into the center of a press support plate 5 with a wheel handle 36 fastened thereon. An arm 34 connects the periphery of the disc 35 and a horizontal doubling plate 32 of the sectional frames 31. With the turning of the wheel handle, the frames 31 are closed to form one whole mold. The cooperation between the drawing and relowering of the press during the process makes the extruded peripheral portion of the front cover inversely folded. The folded parts are paressed together and welded by high frequency heating.