In the production of vinyl siding panels having the appearance of rough-sawn cedar shake shingles, for example, as disclosed in U.S. Pat. No. 3,593,479 and U.S. Pat. No. 5,455,099, it is known to injection mold the panel or vacuum-form a precut sheet and then trim and punch and form the edge portions of the sheet, to form a hook-shaped lower portion along the panel and an upper portion which defines a groove and forms a nailing flange. It is also known to extrude or form a flat sheet of plastics material and then direct the sheet onto the upper run of an endless flexible belt conveyor for vacuum-forming the sheet, for example, as disclosed in U.S. Pat. No. 3,982,868 and U.S. Pat. No. 5,314,325. The endless flexible belt is constructed of a porous material and defines cavities into which the hot plastic sheet is sucked by a vacuum source under the upper run of the endless belt. The door sheet is then cooled and cut at longitudinally spaced intervals to produced vacuum-formed garage door panels or other articles. It has been found that the endless flexible conveyor belts for producing vacuum-formed articles are expensive to construct, have a relatively short service life and do not conduct heat rapidly away from the vacuum-form sheet.