In order to weld a thermoplastic part using ultrasonics, four factors must come together. First, the material must have a sufficient loss modulus. This is a very fancy way of saying that material must be able to be heated by repeated rapid application of the compression/tension cycle that the pounding motion of the horn/sonotrode creates. Some materials are too flexible to heat sufficiently, some have too much internal lubricity. Second, the material to be joined must have flow characteristics and chemical compatibility consistent with intermingling of the materials such that the result is a weld and not merely surface adhesion (though in some applications surface adhesion may be enough). Third, sufficient amplitude must be available that the thermoplastic material gets hot enough to flow before the stresses of the clamp force and amplitude destroy the joint. Fourth, point contact or line contact must exist such that the heat created by amplitude and clamp force is localized in the joint area. This last can be accomplished by the use of energy director or shear joint designs.