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Exploring the Ultrasonic Welding Stack
by Ken Holt, Herrmann Ultrasonics, Inc.

Assembly Update

The purpose of this article is to explain the ultrasonic welding stack: what the components are, what they do, and the specifications and technology used in terms that an engineer just getting acquainted with the process can understand easily. Some familiarity of the ultrasonic welding process is assumed, as this article is not intended to be a primer for ultrasonic welding (as space precludes my explaining all facets of this field). Many reference materials are available that delve deeply into the basic theory, practice, and advanced topics of this technology.

The term “ultrasonic welding stack” refers to the assembly consisting of the ultrasonic converter, booster, and sonotrode as shown in Figure 1. Synonymous to sonotrode is the term “horn.” The explanation begins at the source of the mechanical vibrations used in the welding process, i.e., the converter, also referred to as the transducer. This component contains the piezoelectric crystals that produce the ultrasonic vibrations used for welding. The converter receives high-frequency, high-voltage electrical signals from the welding system’s generator and its piezoelectric crystals expand and contract in response to this alternating voltage signal. These crystals deform when they are subjected to an electric signal and when this signal is alternated, the crystals are set into a vibration at the output frequency of the ultrasonic generator.

These vibrations are compression waves sent longitudinally, or down, through the stack and introduced into the plastic part that is to be welded. Welding of the plastic part occurs because one part of the assembly couples with this movement while the other part remains stationary. The high-frequency striking of these two parts against one another is the basis for the generation of frictional heat. This raises the temperature to the melting point of the plastic and, coupled with a compressive force supplied by the welder, makes a weld between the two parts.

The frequency of operation is not adjustable and is referred to in the specifications of the welding system. Various frequencies are available in the marketplace and the most common are 15, 20, 30, 35, and 40 kHz. The frequency used to weld a certain part is based upon the size of the part, or the desired weld area sizing, with smaller parts welded better with higher frequencies. The converters are non-user serviceable devices and should be serviced only by the manufacturer or authorized service companies, as the manufacture of the converters requires that exacting standards of cleanliness and compression be applied to the piezoelectric crystals.

The output of this converter component is a mechanical vibration – hence, the converting of electrical signals to mechanical vibrations – and is measured in microns of vibration from the “at rest” position with no voltage applied to fully extended expansion. (Remember, the vibration is a compression/retraction of the components.) This also can be measured in the peak to peak excursion or, in other words, as full extension to full retraction. Typical values for its output amplitude, origin to peak, are approximately 20 kHz = 10 microns, 30 kHz = 7-8 microns, 35 kHz = 6.5 microns, and 40 kHz = 3-4 microns. (For reference, a value of 20 microns = .0008".) These values can be affected by electronic regulation within the generator interface, i.e., running at 80 percent amplitude reduces the excursion by 20 percent.

The vibrations from the converter are then introduced into what is called a booster or amplitude coupler. This device either amplifies or attenuates its input amplitude as the wave travels through it, depending upon the shape of the booster, or more exactly, the ratio of mass above and below the center of the booster. The center horizontal plane is referred to as the nodal point. This mass ratio dictates the ratio of input amplitude to the output amplitude which is then introduced to the sonotrode and then into the part to be welded.

A nodal point is the plane through the component in which there is no longitudinal vibration and occurs close to the geometric center of the device. This term will be used later in regard to the sonotrode or horn, as it also has this same effect of shape on the gain. For example, if the ratio of mass above the nodal point to that below is 2:1 (twice as heavy on the top as on the bottom), the booster is said to have a gain ratio of 1:2 (twice as much output amplitude as input). This property is the result of the law of conservation of momentum. As the larger mass moves, it induces a larger and faster movement in the less massive front portion. Typical gain ratios are 1:0.6 (attenuating), 1:1, 1:1.5, 1:2, and 1:2.5 (Figure 2). These are the most common ratios found in industrial practice. Boosters can be made of either aluminum or titanium for longer life and are tuned from the factory to vibrate at the output frequency of the generator. The following is an example of the gain: converter output of 10 microns x booster gain of 2.5 = 25 microns introduced to the next component in the stack, the sonotrode.

Sonotrode or Weld Horn
The sonotrode or weld horn, the most critical and influential component in the set that contacts the plastic part, is designed to provide the optimum contact area and amplitude of vibration to the parts being welded. In concert with the booster, the amount of amplitude of vibration can be tailored to a specific plastic polymer material and, less important, to the geometry to the part. Various polymers require different amounts of amplitude to be welded based on many factors but primarily, on their glass transition temperature or melt point and “stiffness” or loss modulus.

Acrylic is the easiest to weld due to its low Tg and relatively high stiffness. On the other hand, polypropylene with its semi-crystalline structure being able to absorb vibrations (high loss modulus) and being relatively soft with a high melt point is harder to weld, particularly in thin wall structures. The overall sonotrode design is driven by the final application to a high degree. Sonotrodes are made of either titanium, aluminum, or hardened steel of various grades to suit the final use. Titanium is favored for its high rate of transmission of sound waves, durability, and relative hardness. Aluminum can be used and has a much lower cost in regard to material and machining. However, it suffers from being relatively weak and because of its oxidization, causes marks on the plastic parts and subsequent need for some type of plating. Steel is very stiff and hard and is used in applications that are abrasive to the sonotrode surface such as welding of glass-reinforced materials, i.e., glass-reinforced nylons in automotive applications. Steel must be put through a heat treating process to allow it to transmit ultrasonic vibrations so a further cost is incurred. Thus, the choice of materials is dependent on many factors, including cost, how many cycles or welds it will make, required lead time, the material to which it will contact, production cycle, etc.

The design of the sonotrode is based upon the part's material and shape. Again, the amount of amplitude is calculated in concert with the converter output, booster gain, and sonotrode gain, and the proper amount is introduced into the part. Too much amplitude and the process will be unstable, as the weld will occur too quickly and damage may be caused to the part. Too little amplitude and the weld will not occur uniformly and consistently, weld times will be too long, and the part may be damaged by the introduction of energy into the top of the mating part rather than into the joining area between the two parts.

The design of these tools is best left to those with the knowledge, experience, and the required electronic testing devices to make them correctly. Damage arising from incorrect tuning can mean catastrophic failure to either the converter and/or the generator and can get very expensive. Sonotrodes must be tuned to run correctly at the frequency of vibration that is output from the generator causing the vibration of the converter and booster. The damage alluded to can result if the generator/converter cannot run the sonotrode at the frequency for which it is designed. Large power draws can occur at the incorrect frequencies and damage these other components. Sonotrodes, like boosters, are tuned to run at that frequency. An analogy would be the tuning of a musical instrument. With proper tuning, a sonotrode runs with minimal power and 100 percent of the energy is used to vibrate longitudinally at the specific frequency +/- a small bandwidth either side of the generator output.

There are many designs of sonotrodes that are used throughout the industry and these designs have evolved over the years (Figure 3). Simple part designs will require only a slight modification to the contact face of the sonotrode in one of these known designs. When sonotrodes are configured to contact a specific part, the tuning and frequency requirement can get tricky. Experienced tooling engineers use not only past knowledge but also FEA (finite element analysis) software and electronic frequency testing equipment to ensure a singularity of frequency output (i.e., with no parasitic or alternative frequencies that sap power). The larger the plastic part, the deeper the required cavity is cut into the face, or the more curved the surfaces, the more difficult this process becomes. Throughout, the sonotrode must vibrate efficiently and with enough amplitude to melt the plastic part.

The amplitude must be consistent across the face of the sonotrode. As sonotrodes get larger, the values of the output amplitude can start to get uneven across the face. This is critical in that areas with differing amplitudes will weld differently. FEA is used on the larger sonotrodes to predict and help point out machining steps that could remediate such problems. Such steps, while expensive, must be taken to assure reliability of the welding with large process windows. To ensure the largest process window, each sonotrode should go through a quality evaluation since there are variables, which include material composition variables and manufacturing tolerances.

An excellent step when purchasing a new sonotrode is to “benchmark” the frequency and power draw of the unit when new. This is done by using the “test” function of the generator, which introduces a low power signal to the welding stack and outputs of frequency and power draw are displayed (Figure 4). These values should be recorded for future reference. If this component becomes suspect in production at a later time, these benchmarks can be used to diagnose the cause of the trouble.

As sonotrodes wear during extended production, the surface will become marked or pocked. Many times, these marks can be alleviated by the manufacturer so as to extend the life of the sonotrode. However, since the overall length dictates the frequency of vibration, changes in length caused by machining can change the tuning of the sonotrode. Typically, these can be refaced or resurfaced a few times depending on the gain of the sonotrode. Again, this is best left to those with the experience and equipment to do so.

The ultrasonic stack consisting of converter, booster, and sonotrode provides the necessary ultrasonic vibration in order to efficiently and precisely assemble the plastic parts in production. Understanding the terminology used to describe these parts, the way these components interact, and the functions of each will give the user a more thorough understanding of the process and allow for more informed decisions.

Ken Holt has been in the plastics industry since 1985. An SPE member since 1987, he started in the custom molding industry with a structural foam molder and has been involved in high-power ultrasonic welding of plastics since 1993. He has written for various publications and has conducted training and education in seminar environments. Holt currently is employed by Herrmann Ultrasonics, Inc., Bartlett, Ill., as its application lab manager, where he deals with all facets of the ultrasonic welding process from initial design reviews to final field testing of equipment. He can be reached at (630) 626-1635 or email