A mold assembly for molding thermoplastic material into a solid thin shell includes a metal mold with an outer surface exposed to a heat source and an inner surface on which thermoplastic material is cast and fused to form a thin layer thermoplastic shell. A plurality of elongated generally planar fins are connected to the outer surface at predetermined spaced areas for transferring heat from the heat source to predetermined spaced areas of the inner surface. The fins are metal sheets, preferably copper, each having a discontinuous flange at one end secured to the outer surface at spaced points along the discontinuous flange. The discontinuous flange is formed by slitting or cutting one end to form spaced projections and then bending adjacent projections in opposite directions generally transversely from the elongated metal sheet. The projections arc secured to the outer surface by TIG welding.