Injection molding of thermosetting materials such as low profile SMC/BMC composites found increasing application in the transportation industry in the eighties. Such automotive parts as front end panels and rear/hatchback doors have grown in usage. The rear doors have reached exceptional production levels of 2500/day in a single plant.
The injection process offers the advantages of greater automation for the mass production of body panel parts compared to compression molding. However, the injection molding of fiber reinforced low profile composites suffers from a severe reduction in physical properties. This is particularly true for impact strength which can be one-third that of similar compression molding materials. A primary reason for this is due to the degradation of the reinforcement during the processing/molding. Efforts at increasing the physical properties through processing changes have many times caused problems with the surface smoothness of the moldings. Should major improvements in impact strength be coupled with excellent surface appearance the injection process could be applicable to automotive hoods, doors, deck lids, etc. The resultant application of automation could provide a more favorable cost structure for these composites versus metal even at high volumes of production.
This paper reports on a concerted study of this problem involving examination of the organic matrix system, shrinkage control materials, interfacial agents, glass sizing changes, chemical thickening and various processing parameters and injection molding machines. This work has resulted in truly super Class A surface molding at impact strengths 80% above those of the best standard injection materials. Progress of this nature signals the beginning of a new day in low profile thermoset injection molding.