Abstract: Mold insert apparatus and a method is provided for high pressure solid phase forming of relatively thin web or sheet material. The apparatus includes a die set assembly provided with a forming die set unit having a die forming cavity which receives a forming die and an opposed high pressure chamber die set unit wherein the die set units receive material to be formed therebetween when in spaced relationship and functional to clamp the material therebetween when the die set units are in closed forming positions for solid phase forming of the material against the forming die upon introduction of high pressure fluid into the high pressure chamber die set unit. A material preheating station is provided outside but generally proximal to the die forming cavity of the forming die set unit.