Volume 26, Issue 2 (March 1998)
Impact Force Comparison of Polymers: Molded-Notch Versus Cut-Notch Using the ASTM D 256 Izod Impact Test Method
This study was conducted to compare the Izod impact values of injection molded polymer specimens with molded- versus machined-notch specimens. The ASTM D 256 notched Izod impact test was used to measure the impact values of both specimen types. The impact values were compared to determine if differences exist between specimens that had the molded notch and specimens that had the cut notch. Post-molding machining of the notch is the procedure required by ASTM D 256-93a, Standard Test Methods for Determining the Pendulum Impact Resistance of Notched Specimens of Plastics.
Specimens from six different polymer families were produced using the reciprocating-screw injection molding process and a two-cavity mold. The polymers tested were poly(acrylonitrile-butadiene-styrene) (ABS), high-impact polystyrene (HIPS), poly(methyl methacrylate) (PMMA), polyamide (PA), polypropylene (PP), and polycarbonate (PC).
It was found that five of the six polymers had significantly higher impact values in the molded-notch condition. Impact values for polypropylene were as much as 300% greater in the molded-notch condition. Testing of polycarbonate resulted in a bimodal distribution among cut-notch specimens; however, the molded-notch specimens fractured as significantly higher values. Although the mean impact value for molded-notch specimens of ABS was slightly higher than that of the cut-notch specimens, it was found that there was no statistically significant difference for ABS.
It was found that the geometries of the molded notches were not uniform among the different materials, even though they were produced from the same mold, Molding of the materials resulted in slightly different notch geometries when inspected at × 50 magnification.