Ultrasonic Welding of Plastics
When a rubber ball is dropped, it never bounces back to its original height because some of the potential energy is converted to heat and sound. This is an illustration of the process on which ultrasonic welding is based. In ultrasonic plastics welding, a metal tool (a horn) delivers energy to one of the parts to be welded. The part on the other side of the joint area rests on a rigid anvil, ensuring the energy applied is dissipated in the weld zone. A small projection in the weld zone called the energy director is flexed by the oscillating force of the horn at rates of 10,000 to 70,000 times per second (10-70 kHz). Viscoelastic losses occurring in this projection cause the polymer to melt and flow across the bond line creating a weld. The most common frequencies for ultrasonic plastics welding are in the range from 20 to 40 kHz and power levels between 1 and 4 kilowatts. Oscillation amplitudes at the end of the horn range from 20 to 80 microns.
Ultrasonic welding can be accomplished at various distances from the end of the horn ranging from only a fraction of a millimeter (near field) up to several centimeters (far field). Most welds are made in the near field. Ultrasonic welding joins amorphous thermoplastics like polystyrene more efficiently than semicrystalline ones like PET. ABS and high impact polystyrene (HIPS) are among the easiest polymers to weld ultrasonically because in addition to being amorphous, they also have a high modulus. Polyolefins and elastomers are more difficult candidates for ultrasonic welding because in addition to being crystalline, they are also relatively soft.
Joint design and location are very important in ultrasonic welding. Molding tolerances must be kept within narrow limits to produce successful welds. Substrates for butt joints must be designed with an energy director (usually triangular in shape and 0.2 to 0.3 mm high). Interference or shear joints involve welding two parts that have an interference contact of about of between 0.2 and 0.5 mm. Shear joint designs are commonly used for semicrystalline polymers, or when watertight seals or high joint strengths are needed.
Ultrasonic welding is probably the most commonly used thermoplastic welding process because it is very fast (fractions to a few seconds) and usually produces welds that are relatively free of flash. This cleanliness makes it attractive to the medical products industry for clean room assembly operations. In addition, ultrasonic welding is relatively easy to automate and it is particularly suitable for high volume production. Basic functions of the welder such as weld energy, collapse, trigger force, and pressure can now be microprocessor-controlled. This permits real-time feedback and control of welding conditions during the weld cycle. The ability to vary machine parameters during the weld cycle increases productivity and yield.
EWI Lab equipment ranges from 15 kHz to 60 kHz and 100 to 8 kilowatts power and our staff has more than 20 years experience in ultrasonic plastics welding.
Contact EWI now to learn how we can help you develop part designs and welding process parameters for your assembly challenges. |
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