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The Role of Ultrasonic Welding in Industrial Manufacturing

What do you think of when you hear the word “welding”? By common definition, welding refers to a process of joining separate materials together using heat. Fiery torches throwing off a rain of molten sparks from metal plates is what most folks envision when welding is mentioned, though this visual only really describes a small sample of welding methods used in actual practice.  

In fact, ultrasonic welding for plastics is different, but it plays a significant role in modern manufacturing.

Most welding involves an application of concentrated heat, just enough to melt and commingle the molecules of separate materials together into a single piece. By this definition, welding is often performed on metals, plastics and in some cases even fabrics. Regarding plastics, there’s a method of welding that does not rely on heat to fuse materials, known as ultrasonic welding. Read on to learn more.

Ultrasonic Welding Actuators

A Sound Approach

Ultrasonic welding can best be described by breaking apart its root words:

  • Sonic: related to sound and sound waves, typically referring to audible sounds in the human hearing range of 20 Hz to 20 kHz.
  • Ultra: Adding “ultra” to “sonic” suggests that the sound involved is outside of human hearing at elevated frequencies. For ultrasonic welding, that's certainly true, commonly reaching 20 kHz to 40 kHz (and up to 70 kHz).
  • Welding: a fabrication technique in which separate materials are fused together at the molecular level into a single piece.

Putting these root words together, one could guess that ultrasonic welding is a joining process that uses high-frequency sound waves to meld separate material pieces into one at the molecular level, and that’s spot on. Where traditional welding relies on heat to create this fuse, ultrasonic welding uses high-power mechanical vibration to nudge separate molecular structures into each other. 

Ultrasonic welding is common practice with thermoplastic materials, but can also be accomplished in certain glass and metal materials as well. And unlike traditional welding, ultrasonic welding is relatively low temperature, getting nowhere near a material’s melting point and making it suitable for heat-sensitive materials. Here’s how: 

Plastic materials come in two categories: thermoset plastics and thermoplastics. The key difference between these material types related to ultrasonic welding is that once a thermoset’s molecular structure is formed (during initial molding), it cannot be changed. Thermoplastics, on the other hand, are not limited by this permanency and are able to be modified after initial forming.  

This is the same process that can occur in suitable metal and glass materials as well, allowing ultrasonic waves to fluidize materials enough that they fuse into each other, becoming one solid piece (and without the addition of heat). 

Ultrasonic Welding Materials

The use cases for ultrasonic welding are all around us in both our consumer and professional lives. Finding signs of ultrasonic welding in everyday products can be tricky, however, as the matter is more a function of materials than applications. Said another way, ultrasonic welding is the de facto standard joining technique for particular plastic materials over all others, regardless of what application they're used for. 

Along with high-frequency vibration, ultrasonic welding is also dependent on the amplitude of those vibrations; that is, the length of the stroke that occurs at each vibration. Let's quickly divert to explain this concept. 

Ultrasonic welding equipment has three main parts: the converter, the booster and the sonotrode (also known as the “tool” or “horn”). The converter converts electrical input from a power supply (the ultrasonic generator) to physical vibrations, which are then boosted by the booster, and physically applied to the parts being welded by the horn. The horn presses down onto the parts being welded, transferring its acoustical vibrations into the materials, creating frictional heat and coercing molecules to intermingle.  

This welding action is specified by two values: amplitude and frequency. Amplitude is the physical dimension that the horn oscillates, and frequency is the number of oscillations that occur within a second. Therefore, a complete ultrasonic welding specification would read something like "20 µm amplitude @ 20 kHz frequency." 

Reading this specification in plain English, we can see that the weld horn will move a dimension of 20 µm (microns) at a rate of 20,000 strokes per second. Engineers develop this specification through calculations and physical testing for each material and joint being welded, as every weld will take a different amount of force and motion to compel separate molecular structures into one another. Some materials take very well to ultrasonic welding due to their suitability for fluidizing in this way, whereas others will refuse to commingle entirely.

Here's a quick rundown of materials that are best suited for ultrasonic welding:

  • Polystyrene
  • ABS
  • Polycarbonate
  • Acrylic
  • Acetal
  • Nylon
  • Glass
  • Some thin metallics such as aluminum, copper and nickel

Getting Things Done: Ultrasonic Welding Methods

Ultrasonic welding can be applied in many ways, including joining techniques such as:

Ultrasonic Welding Applications

Considerations and Cautions in Ultrasonic Welding

Because of ultrasonic welding's relatively straightforward path to success, it also has a low threshold for failure when key steps are rushed or overlooked. Below, we describe a few of these key considerations to keep in mind when pursuing an ultrasonic welding solution. 

Because of ultrasonic welding's relatively straightforward path to success, it also has a low threshold for failure when key steps are rushed or overlooked. Below, we describe a few of these key considerations to keep in mind when pursuing an ultrasonic welding solution. 

For ultrasonic welding to reach its potential, joints and connections must first be properly engineered. Joint geometry, dimensions, spacing, orientation, area and thickness are all details that need to be worked out to result in a reliable, repeatable bond sufficient for mass production.

Before buyers select an ultrasonic joining system to invest in, we encourage sample testing to ensure that specifications between parts and systems line up properly. Using our in-house material test lab, the AMS team can help test and recommend ideal system design criteria using samples of your actual materials, rendering results that would be guaranteed in any ultrasonic welding system purchased from AMS.

Ultrasonic welding can be performed in manual, semi-automated or fully automated fashions, scaling up or down in sophistication as needed to best suit the size and complexity of products being assembled. Automation is key to reliably achieving advanced requirements such as hermetic sealing, joining dissimilar materials and minimizing weld times. We'll use a few of our own system models to showcase different levels of solutions:

  • The PJ-301 model is a small-scale, cost-effective, PLC-controlled station particularly useful for low part variations at high volumes.
  • The larger PJ-401 model is a large-scale, multi-material, multi-part layout, highly reconfigurable platform with outstanding productivity and very fast cycle times.

Welds can fail for any number of reasons, from improper fixturing to sonotrode misalignment, material contamination to movement during welding, and so on. Robust quality control testing and documentation must be implemented ahead of full production, tested and re-tested to confirm veracity over the entire life of the ultrasonic welding operation.

Considering Ultrasonic Welding?

For a wide array of plastic parts, fabrication cost, speed and simplicity are the name of the game. For other parts, high strength, crisp aesthetics and extreme reliability are paramount. Fortunately, ultrasonic welding can apply to both of these situations and every application in between. 

While any single fabrication technique is limited by the strengths and capacities of the materials being worked with, ultrasonic welding as a joining method is a very capable solution within the realm of the plastics it’s typically used on. As ultrasonic technologies continue to develop, new engineered materials are being created as well, together suggesting a very promising future of new possibilities for manufacturers and end users of lightweight, high-strength, cost-competitive plastic products.  

Book a meeting with us today and let’s talk about your next project.

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