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Dukane / Resources / Our Processes / Ultrasonic Plastic Welding It's the connection that counts

What is Ultrasonic Welding?

Ultrasonic plastic welding (USW) is the joining or reforming of thermoplastics through the use of heat generated from high-frequency mechanical motion. It is accomplished by converting high-frequency electrical energy into high-frequency mechanical motion. That mechanical motion, along with applied force, creates frictional heat at the plastic components’ mating surfaces (joint area) so the plastic material melts and forms a molecular bond between the parts.

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Ultrasonic Welding Process

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Basic Principles of Ultrasonic Welding

Parts in Fixture
1. Parts in Fixture
Horn Descends & Contacts
2. Horn Descends & Contacts
Force Applied
3. Force Applied
Weld Time
4. Weld Time
Hold Time
5. Hold Time
Horn Retracts
6. Horn Retracts
  1. Parts in Fixture: The two thermoplastic parts to be assembled are placed together, one on top of the other, in a supportive nest called a fixture.
  2. Ultrasonic Horn Contact: A titanium or aluminum component called a horn is brought into contact with the upper plastic part.
  3. Force Applied: A controlled force or pressure is applied to the parts, clamping them together against the fixture.
  4. Weld Time: The ultrasonic horn is vibrated vertically 20,000 (20 kHz) or 40,000 (40 kHz) times per second, at distances measured in thousandths of an inch (microns), for a predetermined amount of time called weld time. Through careful part design, this vibratory mechanical energy is directed to limited points of contact between the two parts. The mechanical vibrations are transmitted through the thermoplastic materials to the joint interface to create frictional heat. When the temperature at the joint interface reaches the melting point, plastic melts and flows, and the vibration is stopped. This allows the melted plastic to begin cooling.
  5. Hold Time: The clamping force is maintained for a predetermined amount of time to allow the parts to fuse as the melted plastic cools and solidifies. This is known as hold time. (Note: Improved joint strength and hermeticity may be achieved by applying a higher force during the hold time. This is accomplished using dual pressure).
  6. Horn Retracts: Once the melted plastic has solidified, the clamping force is removed and the ultrasonic horn is retracted. The two plastic parts are now joined as if molded together and are removed from the fixture as one part.

Key Advantages:

  • Fast Cycle Times: Ultrasonic welding delivers remarkably fast bonding. Most parts are bonded in less than three seconds, with the vast majority bonding in less than one second. With clever machine and part design, it is possible to process more than one part per second with a single welder.   
  • Cost Effective: Ultrasonic welding is often one of the most cost-effective methods of plastic welding. The equipment is comparatively inexpensive and versatile and can often be used to weld multiple parts with a simple tooling change-over. Ultrasonic welding typically consumes less energy than other common plastic bonding techniques. By eliminating the need for adhesives or fasteners, this method not only reduces manufacturing time and complexity but also eliminates the ongoing cost associated with those extra materials.   
  • Versatile Application: Ultrasonic welding is adaptable to a wide range of materials, including molded thermoplastic components, woven and nonwoven thermoplastic fabrics and films, and even some soft metals. It can also be used to cut or form other materials such as food products, packaging materials, thermoset resins like rubber or urethanes, and natural fibers like cloth. Additionally, ultrasonic welding can attach various thermoplastic materials to non-plastic parts through methods such as staking, swaging, and inserting.   
  • Precision: Advanced ultrasonic welding equipment, such as the Dukane Infinity series of welders, can control an ultrasonic weld within a few microns of dimensional precision. Additional controls over energy and force provide multiple ways to achieve specific numerical targets in the bonding process.   
  • Highly Repeatable: Ultrasonic welding machines offer extensive process controls, enabling precise management of the bonding process. They are ideally suited for modern manufacturing environments where repeatability and consistency across multiple machines and multiple facilities are crucial.   
  • Elimination of Consumables: Ultrasonic welding is an excellent alternative to processes that rely on adhesives, mechanical design features, or fasteners to join parts. This method requires only the thermoplastic within one of the parts, eliminating the cost and complications associated with glues and fasteners.   
  • Versatile Equipment: In ultrasonic welding, only the tooling is specific to a particular assembly. The horn (sonotrode) and holding fixture are designed for a specific task, but the rest of the equipment can be easily repurposed for different parts or processes. This flexibility allows you to weld multiple parts on a single system by simply changing the tooling. Additionally, you can often repurpose welding equipment when a project reaches the end of its life cycle.    
  • Environmentally Friendly: Ultrasonic welding requires only the welder, tooling, a small amount of electricity, and the parts themselves. This eliminates the need for harsh glues and chemical epoxies typically used for bonding. With proper machine design and safety measures, there are no harmful effects from being around ultrasonic welding equipment, making it a safe choice for any manufacturing environment. 

Material Considerations

  • To bond two thermoplastic parts, it is necessary that the materials be chemically compatible. Otherwise, even though both materials may melt together, there will be no molecular bond.
  • A good example would be trying to weld polyethylene to polypropylene. Both of these semi-crystalline materials have a similar appearance and many common physical properties. However, they are not chemically compatible and are therefore unable to be welded to each other.
  • Like thermoplastics (i.e., materials with the same chemical properties) will weld to themselves. For example, one ABS part will weld to another ABS part.
  • Dissimilar thermoplastics may be compatible only if their melt temperatures are within 40ºF (6ºC) and they are of like molecular structure. For example, it is likely that an ABS part could be welded to an acrylic part because their chemical properties are compatible.
  • Generally speaking, only similar amorphous polymers have an excellent likelihood of being welded to each other. The chemical properties of any semi-crystalline material make each one only compatible with itself. When the materials to be welded are compatible, several other factors may affect the adhesive bonding of the parts. These factors include hygroscopicity, mold release agents, lubricants, plasticizers, fillers, flame retardants, regrind, pigments, and resin grades.

Joint Design Considerations

The joint design of the mating pieces is critical in achieving optimum assembly results. The design depends on factors such as the type of plastic, the part’s shape, and the requirements of the weld.

There are many different joint designs, each with its own advantages. Some of these designs are discussed later in this section. There are three basic requirements in joint design:

  • A uniform contact area
  • A small initial contact area
  • A means of alignment
Ultrasonic Welding Tongue and Groove Energy Director Joint

Ultrasonic Welding Tongue and Groove Energy Director Joint

Ultrasonic Welding Shear Joint

Ultrasonic Welding Shear Joint

Ultrasonic Welding Step Energy Director

Ultrasonic Welding Step Energy Director

Ultrasonic Welding Tongue and Groove Single Shear Joint

Ultrasonic Welding Tongue and Groove Single Shear Joint

Ultrasonic Welding Tongue and Groove Double Shear

Ultrasonic Welding Tongue and Groove Double Shear

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