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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.

Basic Principle of Ultrasonic Welding

  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.

Advantages of Ultrasonic Welding

Fast Cycle Times

Very Accurate

Highly Repeatable, Excellent for High Volumes and Automation

Environmentally Safe

  Versatile well suited for various applications

   No Consumables

Material Consideration

  • 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

Other Welding Process

ULTRASONIC PLASTIC WELDING

VIBRATION WELDING

 

SPIN
WELDING

INFRARED
WELDING

HOT PLATE WELDING

 

ULTRASONIC FILM & FABRIC PROCESSING

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630-797-4900

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ussales@dukane.com