a mold especially adapted for forming precast , waffle - shaped concrete panels employs a one - piece mold body of plastic sheet material to provide a reusable mold structure that may be rapidly stripped and reset . each corner of the mold body is open ; thus , the sides of the mold present pliable flaps due to the inherent flexibility of the sheet material and are swung outwardly after molding is completed in order to release the molded product . external reinforcing members on the sides maintain the same rigid and true during the molding process . the corners are held closed by simple locking devices which are readily released for stripping . raised portions in the mold body form the characteristic voids in the molded waffle product , and are reinforced and rigidified by a foam filler . other features include longitudinally extending expansion joints between the mold body and reinforcing members , holdups on the pliable sides of the precasting mold , and the provision of built - in rebar supports which also form notches in the webs of the molded waffle product that serve as electrical and plumbing raceways .