Abstract: Two types of high impact polystyrene(HIPS) resins were injection molded several times to determine the changes in mechanical properties and chemical structure. After multiple injections, all the bending strength, elongation at break, fracture strength, and impact strength of the material decreased. The molecular structure of HIPS was detected with 13C-NMR. The relationship between mechanical properties and molecular structure was discussed. A new conception of multiple-mold-factor was proposed to evaluate the plastic process stability.

Key words: repetitous injection molding, chemical structure, mechanical property, high impact polystyrene, nuclear magnetic resonance, relevance