Measurement of shore hardness on thin samples

Objectives:  To test a theoretical model which predicts the effect of sample thickness on  Shore Hardness.

ASTM D2240 specifies that samples should be at least 6mm thick, and measurements taken at least 12mm from any edge. However, measurements are often taken on thinner samples

Methods:

The following materials were used:

Extrude Light, Medium, and Heavy Body, MPV, and Extra Putty(Kerr USA), Evatouch Super ( GC Japan ), Oranwash VL (Zhermack, Italy) . These  were mixed according to manufacturers instructions and samples  moulded as plane sheets 1,2,3,& 6mm thick

Shore Durometer Readings were made using  a Congenix Wallace  Hardness tester.

Using the  6mm thick samples as the standard, the theoretical  prediction was based on three equations:

                            i) that, relating the Shore Hardness Reading (s) to the depth of penetration  of the indentor(d) :

                                                 d(cm) =0.254-2.54s                    1)

                               ii) that of Waters, relating the effect of sample thicknes (t) on the classical force- indentation equation for a cylindrical indenter (radius=R)

For  thinner samples, the following equation was found to apply:

                                                      d₁/d =f(t/R)                    2)          

where f(t/R) is a function only of the ratio t/R; values for   f   are tabulated in the literature.

 Here the value of s for a 6mm sample is used to calculate d

For other values of t, t/R is calculated,  f(tR) found, and d₁ calculated from 4)

This is now substituted in 1) to get the  s₁ results

 Results:

             Tests of the theory are shown in Table 1., for the hardest(1) and softest(2) materials studied.

Conclusions:

 The proposed theory fits the experimental data adequately. Note the thickness dependence is greatest with the thinnest specimens

.