Ultimate Strength - Term used to describe the maximum unit stress a material will withstand when subjected to an applied load in a compression, tension, or shear test.

Ultrasonic Sealing - A film sealing method in which sealing is accomplished through the application of vibratory mechanical pressure at ultrasonic frequencies (20 to 40 KC.). Electrical energy is converted to ultrasonic vibrations through the use of either a magnetostrictive or piezoelectric transducer. The vibratory pressures at the film interface in the sealing area develop localized heat losses which melt the plastic surfaces effecting the seal.

Ultraviolet - Zone of invisible radiation beyond the violet end of the spectrum of visible radiation. Since UV wavelengths are shorter than the visible, their photons have more energy, enough to initiate some chemical reactions and to degrade most plastics.

Undercut - (a.) Having a protuberance or indentation that impedes withdrawal from a two-piece, rigid mold. Flexible materials can be ejected intact even with slight undercuts. (n.) Any such protuberance or indentation; depends also on design of mold.

Urea Plastics - Plastics based on resins made by the condensation of urea and aldehydes.

UV Stabilizer - Any chemical compound which, when mixed with a thermoplastic resin, selectively absorbs UV rays.