Over-molding is the injection molding process where one material (usually a TPE) is molded onto a second material (typically a rigid plastic). If properly selected, the over-molded TPE will form a strong bond with the plastic that is maintained in the end-use environment. The use of primers or adhesives is no longer required to achieve an optimum bond between the two materials.

Basically, the "feel" of a soft touch over-mold is dependent upon a combination of material properties (hardness, modulus, and coefficient of friction), texture and the TPE wall thickness.

Thickness Effects
When choosing a soft touch TPE, designers usually ask for the softest material available. What they do not know is that the soft durometer of a TPE adds little value to the concept of "cushion" when thickness of the TPE is below a certain point (typically > 0.040"). This means that the thinner the TPE over-mold, the harder it will feel - the actual hardness effect is dependent on the thickness of the TPE over-mold. One way of getting around this issue is to incorporate multiple ribs that are placed closely together to create the perception of thickness without using a large amount of material.

Hardness vs. Modulus
One common myth in the TPE industry is that the durometer (or hardness) of a material is directly related to its flexibility. This is not always true; for example, a 65 Shore A SEBS material is much more flexible than a 65 Shore A TPU. Instead of using Shore Hardness, a more suitable measure of flexibility is the flexural modulus, which measures a material's resistance to bending.
A higher flexural modulus typically means that a material will feel more stiff and unyielding.