| abstract |
An extruded hollow thermoplastic board, which includes a pair of flat and parallel sheets spaced apart and interconnected by extending ribs, generally has a plurality of depression bands in the areas where the flat sheets and extending ribs are joined. The bands, which negatively affect the surface flatness, are especially apparent for crystalline thermoplastic materials. A hollow thermoplastic board, which effectively reduces the depth of the depression bands by inclusion of locationally fixed gas pockets in the rib area during production, is disclosed in the present invention. The hollow thermoplastic board of the present invention substantially enhances the surface smoothness and is highly beneficial to applications such as printing, lamination and graphic art. The present invention also provides a method for production of the hollow thermoplastic boards of smooth surfaces. |