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Apparent Hardness vs. Thickness in Overmold Applications

One of the key properties of thermoplastic elastomers supplied by PolyOne GLS is hardness. Durometer is the international standard for measuring the hardness of rubber, sponge rubber, plastic, and other nonmetallic materials. Very soft materials, such as gels and micro-cellular foam and sponge, are rated on the “Shore OO”. Many rubbers, soft plastics, urethanes and TPE’s are rated on the “Shore A” scale. The hardness of most thermoplastic resins is rated on the Shore D scale.

Hardness is defined as a materials’ resistance to indenture. The materials’ hardness is a function of both thickness and product grade. For over-mold applications, the thinner the elastomer is on the substrate, the harder it will feel. Table 1 illustrates some common GLS Materials, their supplied hardness, and the relationship between hardness and thickness. The degree of measured hardness change is dependent upon the hardness of the base material. It is important to understand that the material does not really change the hardness. The measured materials’ hardness is a function of the test method and the thickness of the part. This is the reason that the ASTM Method specifies a material thickness of 0.250 inches. This is the range where hardness is independent on thickness. 

 

GLS Grade Supplied hardness Thickness & The Apparent Hardness (Shore a)
0.027in 0.033in 0.063in 0.133in 0.265in 0.398in
Versaflex™ CL2000X 3 77 71 45 4 3 2
Dynaflex™ G 6713 13 76 69 42 17 13 14
Dynaflex™ G 7930 33 77 70 45 35 33 32
Dynaflex™ G 7960 60 78 74 64 65 60 63
Dynaflex™ G 7980 80 86 85 82 85 80 84

 

One of the controlling factors for relative hardness is a function of both the substrate and the TPE thickness. When hardness is measured, it is typically reported as an instantaneous hardness or a 10-second delay hardness. GLS typically reports hardness using the 10 second delay method

There are a number of important factors that affect the “feel” of the part, including modules and creep of the TPE, or texturing of the part surface. Your GLS Representative can assist with the product grade selection for your specific application.

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