Abstract
The presence of integrated ribs in sheet molding compound (SMC) panels causes some degree of deformation (rib readout) on the cosmetic side which makes the panels unacceptable for exterior applications. In this paper, experiments are described which show that: a) the coefficient of linear thermal expansion (CLTE) normal to the cos metic surface of these panels changes considerably in the area over the ribs (about 25 %) and, hence, has a considerable effect on the magnitude of rib readout; b) the ribs are filled by either "bridging and looping" or "concurrent flow," and the former mechanism causes more rib readout than the latter; c) the painting process has a detrimental effect on the ex tent of rib readout due to the differential thermal expansion of the panels in the vicinity of the ribs coupled with the thermal relaxation of the panels; d) the longer the resin maturates, the greater is the magnitude of rib readout, and since some degree of matura tion is needed for a better molding, one has to optimize the extent of resin maturation in order to achieve minimum rib readout.
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