Athletic shoe assembly: After hot melt film has been applied to synthetic suede, the film is pressed.

Photo courtesy Kenneth Straka, New Balance

Ultrasonic Welding in Action

Look at a pair of athletic shoes. While traditional shoes may be made of a single material such as canvas or suede leather, many athletic shoes have several materials such as lightweight plastic polymers, suede or synthetic suede, and mesh combined. These composite materials make the shoes light, flexible, durable and breathable. For example, one style of New Balance athletic shoe has an upper portion which consists of three parts.

  • A pattern of synthetic suede called a vamp - The vamp makes up the majority of the upper shoe including the tip, tongue and eye rows for laces.
  • A pattern of synthetic suede called a saddle eye row - The saddle eye row contains the uppermost two eyelets to strengthen the lacing and reduce wear.
  • A layer of mesh - The mesh surrounds the heel portion of the vamp and the upper part of the opening around the ankle.

But how do you put these materials together? Most commonly, shoe companies sew the materials together. About two to three years ago, however, New Balance set out to make the upper part of a shoe without stitching. After experimenting with polymer adhesive films and irons, they came up with a way of making this portion of the shoe using ultrasonic welding.

To assemble the upper part of the shoe, workers start with a piece of synthetic suede material. They use an iron press to bond a thin sheet of hot melt film to the back of the material. Next, an ultrasonic welding assembly presses a pattern vamp into a suede material. Likewise, a similar ultrasonic welding machine presses the saddle eye row from another piece of synthetic suede. The vamp shape gets cut out of the suede. The saddle eye row and mesh material get welded ultrasonically to the vamp. In the processes, the frictional heat from the ultrasonic welder melts the hot melt film, which bonds the saddle eye row and the mesh material to the vamp. The finished vamp then gets shaped and bonded to the sole and heel pieces using water-based solvents.

According to Kenneth Straka, Senior Product Developer for New Balance, the ultrasonic welding methods have increased productivity by saving time. Not only do the ultrasonic welders distribute heat more evenly than iron presses, they also heat up and cool down faster. So, the assembly process requires fewer steps and is faster than traditional sewing methods.

Now that we have seen how ultrasonic welding is used to bond various materials, let's look at the advantages and disadvantages of this technique.