Over-Molding

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.

Coefficient of Friction
When two surfaces are dragged flat against each other, the resulting resistance is characterized as friction. The coefficient of friction (COF) characterizes the degree of force required to move one surface across another - either from a complete stop (static friction) or when the surface is already moving (kinetic friction). Typically, TPE's are described as rubbery or "grippy".  One area that product designers often misunderstand is the relationship between durometer and COF. Most believe that the softer the TPE, the greater the COF - this is a very general statement and is not true in all cases.