Santoprene is not usually used for these toys because they do not offer a compound which is usually considered soft enough for that useage. Most are either Silicone, plasitisol or SEBS materials. SEBS molds well over and sticks PP and is cheaper than silcone, so it is the most common material. Santoprene is better suited to muscle massage vibrators. His trouble with noise is likely that his motor does not fit well in the PP housing and the off set weight is likely striking the PP. Sounds like he has a engineering design problem and has not copied his competitors closely enough.
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