Ultrasonic welders are available in several different frequencies, such as 15, 20 and 40 kilohertz, and a variety of power supplies, such as 1,000, 1,500, 2,000, 3,000 and 4,000 watts. It’s important to select the optimal frequency and power for your application. Most applications can be processed by 20 kilohertz, which can be achieved by a variety of power supplies. "The 20-kilohertz frequency has become the industry standard because it is above the normal range of human hearing--approximately 18 kilohertz--and creates the necessary power and amplitude to enable melting in most thermoplastics," says Brian Gourley, manager of the technical services group at Sonics & Materials Inc. (Newtown, CT).

"Ultrasonic welding at 40 kilohertz is particularly suited for smaller, precision plastics assembly applications requiring gentler action," adds Gourley. "Horns are typically about 50 percent smaller than at 20 kilohertz and have lower amplitude, which causes less stress on the parts being welded.

"With 15 kilohertz, most thermoplastics can be welded, especially those fabricated from high-performance engineering resins. This lower frequency also enables larger horn design, thus facilitating the welding of larger assemblies. Uniform amplitude across the horn increases weld integrity, ensuring superior results. There is also significantly less attenuation, allowing many softer plastics to be welded--at greater far field distances than with other frequencies."

Robert Bishop, president of Sonitek Corp. (Milford, CT), believes power or wattage is the most important thing to look for when choosing an ultrasonic welder. "The more power a machine has, the more amplitude it can maintain while under load," he points out. "All the bells and whistles available are useless if your welder is stalling or overloading during the welding cycle.

"It doesn’t cost much more for someone considering buying a new welder to purchase a 2,000-watt welder over a 1,000-watt welder. That 1,000 extra watts could be the difference in success when welding semi or highly crystalline materials, such as nylons or polyesters with glass fillers and far field welds."

Other experts agree that ultrasonic welders need to provide enough power output. "The biggest mistake is when end users try to cut corners buying mechanically weak welders or welders that do not provide enough power," warns Dr. Dominic Friederich, executive vice president and general manager of Herrmann Ultrasonics Inc. (Schaumburg, IL). "The result is that they will have to run their application with lower forces than required, which results in longer weld times, thus degrading the plastic resin in the melt zone, causing porosity, leakers and lesser strength."

According to Bishop, the biggest mistake end users make when selecting ultrasonic welders is forgetting to talk specifics about their application first. Material, joint design, seal requirements, aesthetics, cycle duty rate, delivery requirements and process reporting needs should be considered.

Bishop says it’s important to watch out for applications that are loud, such as insertion; create plastic particulate where not tolerated, such as flash or plastic dust in a medical part chamber; cause stress damage or failure of internal components, such as circuit boards populated with fragile components; and cause cracking of thin-walled applications or other stress risers.

The key factors that determine which process you should choose are:

  • Type of thermoplastic material (crystalline vs. amorphous).
  • Compatibility to one another (for example, ABS to PC is good, but ABS to nylon is not).
  • Weld strength (tack weld or hermetic seal).
  • Field of weld. How close is the joint interface to the face of the horn (1/4 inch or less is almost always ideal)?
  • Cycle rate duty requirement. How many parts do you need to make a second?
"It’s hard to beat the cycle rate a sonic welder provides," says Bishop. "Each application is engineered to meet the customer’s specific needs and it’s important that the customer clearly defines those needs to the vendor. Ultrasonic welding is not a black art, but only experienced engineers and technicians can say that."