Low density structural RIM is a well established process that is used to make polyurethane foam matrix composites for automotive interior trim substrates. This technology has gained this position because these substrates are high in quality, durable, and most importantly, light weight. A vehicle equipped with low density structural RIM door panel substrates will save about 40% of the substrate weight, over one equipped with substrates made from thermoplastics or other traditional materials. Low density reinforced RIM is a similar process that is used to make polyurethane foam matrix composites for interior trim substrates. These composites will save about 15% of a door panel substrate weight. Since saving weight is a goal for the automotive engineer, low density structural RIM systems have been developed to produce even lighter weight composites. This development yields composites with a specific gravity of 0.30 compared with composites currently employed which have a specific gravity of 0.60. These composites can save over 55% of the weight of a door panel substrate and reduce material cost. This paper describes the physical properties of these composites and demonstrates their utility as interior trim substrates, as well as comparing them to low density reinforced RIM materials.