Abstract
The specimens from an unsaturated polyester resin with a nonuniformly distributed hardener were made using a mechanically stirring vessel which could control mixing conditions. The effect of nonuniformity of hardener distribution and post-cure on mechanical properties was investigated. The flexural strength was evaluated from the viewpoint of reliability engineering based on the strength distribution. The flexural strength of specimens, made by hand mixing with a good stirring effect, showed the normal distribution and a small standard deviation. On the other hand, the specimens made using a mechanically stirring vessel didn't obey the normal distribution and had wide dispersion. Thus they showed the complicated distribution in the strength for each different molding condition. After post curing of all specimens which were made using the mechanically stirring vessel, the strengths and elastic moduli increased and no differences in these properties for any molding conditions were observed. However, the standard deviation in the flexural strength of the post cured specimens was extremely large in comparison with that of the unpostcured specimens. This is due to the increase of residual stress based on shrinkage developed during post curing. Attention should be paid to that the mechanical properties of unsaturated polyester resins could be improved by post curing; on the other hand, however, they also could disnerse widely due to the increase of residual stress.
