Comparison of the Izod impact resistance of thermoplastic specimens with molded and machined notches

Pendulum impact testing is commonly used to evaluate a polymer's overall toughness. Most standard pendulum testing methods utilize notches in the test specimens. The American Society for Testing and Materials has specifications associated with the procedure for machined notches. The purpose of a notch, according to ASTM D256, is to produce a stress concentration which will provide the site for initiation and promote a brittle fracture as opposed to ductile failure. There is no mention of molded-in notches in the ASTM procedure. The International Standard Organization has specifications associated with machined notches although there are also allowances for molded-in notches.;The purpose of this study was to investigate the difference of impact resistance using both molded-in and machined notches for a variety of materials. In addition, 3 to 4 different radii values were evaluated for the machined and molded notch samples in order to better understand notch sensitivity. Different melt flow rates (MFR) of the each material were also used in order to determine if MFR had any significant effect on impact resistance.;The results from this study show that in general, molded-in notch samples display a higher impact resistance when compared to machined notch samples. The effect of notch radius was similar for both molded and machined notch samples. As the notch radius increased, the impact resistance increased. In general, the notch sensitivity was greater for the molded-in notch samples compared to the machined notch samples. The majority of materials evaluated showed that the lower melt flow rate material has a higher impact resistance and higher notch sensitivity. Finally, higher melt flow materials had higher variance or coefficient of variance in impact resistance.