Rectangular plates of the size of 1800 x 600 x 12 mm(3) and 1200 x 600 x 12 mm(3) were selected for injection molding and injection compression molding, respectively, in order to investigate warpage characteristics of the large-sized polymer plates with orthogonal stiffener. To determine the mold system and to reduce warpage of the specimen, numerical analyses for injection molding and injection compression molding were performed by using a commercial simulation code. Experiments were performed to verify the suggested mold system and warpage of the specimen. Relatively large warpage of the injection molded product was observed and small warpage of the injection compression molded product was generated. Compression force of the injection compression molding was only 6% of the clamp force of the injection molding. Warpage of the product was reduced significantly by using the injection compression molding. The injection compression molding will be used to substitute expensive and disused wood forms with inexpensive and recyclable polymer plates for concrete casting. (C) 2010 Wiley Periodicals, Inc. J Appl Polym Sci 116: 3460-3467, 2010