Gas injection molding of reinforced engineering plastics for structural applications is a very innovative approach, but is still in an infancy stage. The requirements for window guidance channel are: dimensional stability, high strength to weight ratio, no warpage and low wear. The composite materials with hollow tubular structure can only meet these requirements based on FEA analysis. We evaluated 30% glass filled polyester (short and long glass fibers). The part and tool designs are the key parameters for successful results which are discussed. Information on current gas injection molding technologies is discussed. The new software ‘C-Gas flow analysis’ was used to optimize gate design and process parameters. The design of experiments based on Taguchi method was used. Both available technologies, i.e., gas through nozzle and gas through runner or cavity were tested. The cross section analysis was done using CAT Scan. This is the first structural application of gas injection technology which is fully tested and approved according to GM specifications. We received Society of Plastic Industries (SPI) ‘Award of Excellence’ for design and application development in February 1990.