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 2600/day
in a single plant.
The injection process offers the advantages of greater
automation for the mass production of body panel parts compared in
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 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.
K. E. Atkins, R. L. Seats, G. C. Rex, M. H. P. Montagne
Union Carbide Chemicals and Plastics Co., Inc.
Materials Innovation and Their Applications in the Transportation Industry