SEDL / STP / STP907-EB / STP19978S

Influence of Mold Coverage upon the Notch Strength of R25 Sheet Molding Compounds

Shirrell, CD
Staff research engineer and senior science assistant, Polymers Department, General Motors Research Laboratories, General Motors Technical Center, Warren, MI
Polymer Systems Department of the Shell Development Co., Houston, TX

Onachuk, MG
Staff research engineer and senior science assistant, Polymers Department, General Motors Research Laboratories, General Motors Technical Center, Warren, MI

Pages: 19    Published: Jan 1986

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Abstract

The influence of a processing variable, mold coverage, upon two 25% by weight randomly oriented 2.54-cm-long glass fiber reinforced sheet molding compounds (R25 SMCs) was examined. Artificially created flaws, in the form of machined circular notches, were utilized to determine the effect of mold coverage upon the naturally occurring tensile flaw sites in these materials. The critical hole size (that size circular notch through which all specimens fail) was found to be 9.53 mm. Variations in mold coverage from 97.5 to 25% were observed to have virtually no effect upon this critical hole size. Thus, the most severe tensile critical flaw sites in R25 SMC appear to be unrelated to mold coverage. A comparison of the two R25 SMC's indicates that a rubber toughening agent reduces slightly the notch sensitivity of isophthalic polyester resin matrix sheet mold compounds. Within experimental error, the two-parameter notch strength model of Whitney-Nuismer was found to accurately describe the notch sensitivity of R25 SMC.