Automation 101: Automated Ultrasonic Welding

Where a strong bond is needed between separate thermoplastic parts in a manufactured product, automated ultrasonic welding is almost always the best joining method to use. With no waste or high energy demands, ultrasonic welding fuses parts together by generating enough friction that the parts essentially melt together.  

Products in nearly any market – from automotive to medical, electronic to household, and beyond – utilize ultrasonic welding in order to provide high strength joints at low cost. As part of our Automation 101 series, here we look at the ultrasonic welding process, its features and benefits, and ways to automate ultrasonic welding within a manufacturing environment.

Automated Ultrasonic Welding in Industrial Manufacturing 

When we think of welding, we most likely imagine a gas or electric welding process such as arc welding or wire feed welding. In these welding styles, high amperage electrical current is used to liquify metallic or plastic materials. These molten materials are mixed together by maneuvering the weld tool, creating a fused joint where the two materials have melted and cooled into each other. The melting process blends the two materials together at the molecular level, reorganizing their composite structure, and creating a commingled interface that is very strong and relatively large in engagement area.  

Not all applications require such high-intensity energy to create a bond, and in fact, some applications cannot tolerate the high temperatures or stresses imposed by electric welding.Here, alternative welding processes can be quite beneficial.Since our blog focuses on industrial manufacturing automation topics, let’s hone in on the most relevant and preferred welding method for non-structural plastic and metallic joining: ultrasonic welding.  

What Is Ultrasonic Welding?

To start, what is ultrasonic welding? You might recognize the term “sonic” in the name, and you’d be correct to guess that it involves sound to create a bond. More accurately, we may say that ultrasonic welding utilizes frequency generation occurring at a range largely outside of human perception (the ultrasonic range occurs at and above 18 kHz, whereas the average adult human can perceive sounds up to about 17 kHz). The Handbook of Plastics Joining by Michael Troughton explains that “ultrasonic welding uses ultrasonic energy at high frequencies (20–40 kHz) to produce low amplitude (1–25 μm) mechanical vibrations.”  

In other words, an ultrasonic weld uses high frequency soundwaves to create a resonant vibration in a welding apparatus, which is then physically pressed into two materials such that plastic deformation occurs, and the molecules of the separate materials collapse and blend into each other, creating a fused weld. The sound frequency, amplitude and physical force then are the variables adjusted to tailor ultrasonic welding to a wide variety of plastic and metallic materials.  

Ultrasonic Production Welding Methodology  

Let’s next discuss what parts make up an ultrasonic welding device. An ultrasonic welding kit consists of three main components:

1. The transducer, an electrical device containing either piezoelectric crystals or capacitive (ceramic) diaphragms, which when exposed to precise electrical frequencies, cause harmonic oscillation occurring at ultrasonic frequencies. These ultrasonic frequencies are then transferred to the sonotrode in the form of mechanical vibration, either directly or via a booster that amplifies the sonic signal.    
2. The sonotrode is the mechanical implement that  physically contacts the material to be welded, transferring the ultrasonic motion from the transducer into the weld joint. The sonotrode is critical in that it must transmit the welding frequency without distortion into the weld joint, and it must also firmly press the two materials together during the welding process.    
3. The anvil sits below the materials being welded.It is the solid base support that the sonotrode presses against, holding the lower material firmly in place while the upper material is ultrasonically vibrated into plastic deformation. If the two material pieces were to resonate equally, no welding would occur. The friction between the stationary base material and the oscillating upper material is what creates the weld.  

These three components are often referred to as the “welding stack.” The stack is essentially everything you need (other than a power supply) in order to complete a weld joint. Beyond the stack, you can have a wide range of ancillary components that support positioning, error-checking, operator safety and other goals of the welding process; more on that in a minute.  

Using the Welding Stack

In practice, we find that the welding stack is utilized in one of two ways: either by hand or by fixture. Handheld ultrasonic welders are inexpensive and easy to use, but they leave weld integrity and quality largely to chance. Material fusing in any form is a particularly precise science of melting point, molecular covalence and weld fuse purity. Ultrasonic welding adds additional critical parameters by requiring a precise oscillating frequency and sonotrode pressure to assure a reliable bond, all of which vary between materials and weld sizes.  

Handheld welders offer many convenient features for low-risk applications, such as dial-operated frequency and amplitude adjustment, portable components, practically no set-up time needed and relatively low cost of ownership. However, these conveniences may directly impact weld reliability because they introduce variation in weld integrity, and so must be used only where the risk of potential weld failure is permissible. For example, using ultrasonic welding to fuse two clothing textiles together to form a pocket is very low-risk to perform with a handheld welder. In contrast, the use of a handheld welder is not recommended for fusing similar cloth materials together for a vehicle seat belt where human safety is involved.  

For most applications in professional manufacturing, a fixture-mounted ultrasonic welder is the preferred solution. To envision a fixture-mounted welding setup, imagine a structural frame holding the welding stack and the part to be welded all firmly in fixed positions, and mechanized implements performing the welding stroke motion in lieu of human hands. Using a fixed mounting bracket for the welding stack, a secured anvil and base, and with firmly mounted control buttons and knobs, nearly all of the variability of hand welding is removed from the equation. This fixture assures that components are oriented and operated in a consistent fashion across multiple parts, and that errors resulting from human handling are decreased to a minimum.    

How to Automate Ultrasonic Production Welding      

Applying an automated approach to ultrasonic welding follows the same considerations as automating most any manufacturing task: We examine ways to mechanize human motion, to digitize and encode human technique and “skill,” and to use logic controls to replace and speed up human judgement. In short, we build a computer-controlled machine to weld for us!

The design of an automated ultrasonic welding machine can be started by considering a few key objectives for your application:

• Scale: Is your part small or large? A small-scale machine will offer a compact, cost-effective solution for small parts (less than 12 inches in maximum length or width) and with a small quantity of welds to be made concurrently (one to four welds). Anything physically larger or with a higher quantity of welds would be best suited to a large-scale machine instead.  
• Cycle time: If you only have a handful of parts to weld each day, a machine without advanced sensors and self-checking algorithms would work well. Otherwise, if you have a large volume of parts to process as fast as possible, adding poka-yoke self-checking and error-avoiding capabilities to your machine will greatly reduce cycle times. 
• Single- or multi-part compatibility: A small-scale or fixed-setup machine works well when you only have a single part to weld per station, allowing it to be built without reconfiguration in mind. If you have multiple part shapes and sizes to run through the ultrasonic weld machine, a large-scale reconfigurable machine is needed. Reconfigurable machines have fixtures, slide bases, motion actuators and multiple position sensors that can all be adjusted to work between different or evolving part profiles.
• Material compatibility: In either a large- or small-scale machine, adding robust PLC and HMI controls that can store multiple preset programs is the way to go if you have many materials to switch between. These preset programs, especially when coupled with self-checking sensors on the machine, eliminate the need to manually re-setup the weld station when switching parts types.  
• Quality control: If you’d like to offload some tasks performed by your QC department directly to the machine, you can include advanced PLC functionality that captures, records, archives and reports data while the weld process occurs.  Built-in quality reporting reduces human inspection time, error halts and rejects or failures, while benefiting cost margin, regulatory compliance and customer satisfaction.  

What Are Your Automated Ultrasonic Welding Needs?

Here at AMS, we offer two semi-standard plastic joining systems that can tackle your ultrasonic welding, staking and forming needs:  

• Our PJ-301 model is a small-scale, cost-effective, PLC-controlled station particularly useful for low part variations at high volumes.  
• Our PJ-401 model is a large-scale, multi-material, multi-part configuration, highly reconfigurable platform with outstanding productivity.  

Both systems utilize the same high-quality welding components and precision positioning actuators, modern PLC and HMI controls, and advanced safety features. More so, both machines are highly customizable around your specific requirements. Our proprietary ultrasonic welding process uses no consumables and no additional hardware to complete a reliable weld every time: no glue, backing strips, fasteners, stake bosses or other materials needed.  

As an added value, we can help evaluate your application in our onsite material test lab.  Using samples of your material, we can test a variety of welding and joining processes in order to find the absolute best recommendation for your project. Once the desired weld process and specifications are approved by you, those results are guaranteed in the system provided.  

With successful projects in medical device, automotive, plastic and general fabrication, AMS’s automated ultrasonic welding solutions are the ideal choice for your next plastic joining project. Contact us today for a free application review.