Dimensional stability of a mark made in a photoplastic wafer as a function of time

This study investigates in vitro the dimensional stability of the Vishay's mark as a function of time. The Vishay was placed between a hardened, 1/2 inch diameter steel ball and a flat tungsten carbide cutting tool blank in an Instron testing machine. A compressive force of 10 pounds applied, and three separate marks made within the photoplastic wafer. The mark was displayed as a 10x image on the Vishay analyzer's monitor. The image was photographed, along with a standard, using a 35 mm camera tripod setup. To evaluate stability of the marks, photographs were taken at 1 hour, 1 day, 3 weeks, and 2.25 years after indentation. Each mark's image was measured three times with a Sigma-Scan digitizing software and reduced to real dimensions. The three marks were treated as a block and time as four repeated measures. An ANOVA was used to compare the mark sizes at these times. The four means across time were not significantly different (P = 0.34). Both Greenhouse-Geisser, and Huynh-Feldt epsilon correction factors for repeated measures were 0.34 which resulted in an adjusted significance level of 0.36. This indicates unequal correlations from one time to another, but does not affect the insignificant differences of these means. The Vishay photoplastic wafer can store mark size for at least 2.25 years in vitro.