Induction Welding Heats Up Under (Low) Pressure

Ultrasonic. Hot plate. Spin. Vibration. Laser. All of these welding methods can join thermoplastic components. Yet when the joint has to be structural, hermetic, free of distortion, very long, or made between tough-to-join materials, induction welding really starts to stand out from the crowd.

In use for more than 30 years, induction welding relies on a radio frequency generator and an induction coil to deliver electromagnetic energy to the joint interface. It does so with the help of conductive thermoplastic compounds placed within the joint. Highly filled with ferromagnetic materials and formulated for compatibility with the mating part, these susceptor compounds absorb the electromagnetic energy from the work coil. That energy causes the susceptor to melt, generate heat, and ultimately fuse the mating parts. "Think of the induction coil as equivalent to the primary winding of a transformer, and the susceptor implant as equivalent to the secondary winding," explains Russell Nichols, operations manager for Norwood, NJ-based Ashland Specialty Chemicals' assembly business (www.ashchem.com/ascc/specialty/emabond.asp).

The whole process takes place within specially designed fixtures that bring the mating parts together under low pressures in close proximity to the induction coil. The process takes only seconds for a typical weld and can support a fair amount of automation; for example, the susceptor implants can be put in robotically or even molded into the mating parts using a two-component molding machine.

Induction welding may have been around for a while, but this welding method continues to evolve and now offers several advantages:

Strong joints

According to Nichols, the melted susceptor flows to fill completely any gaps within the joint and forms a molecular bond with the adjoining surfaces of the mating parts. "The failure mode is typically one of part failure," he says, noting that the process has traditionally been used to make pressure vessels. To maximize the joint strength, particularly in thin-wall parts, tongue-and-groove joints have been used to good results. "They can provide a large interfacial bond area within a thin-wall joint," Nichols notes.

Fights distortion

Induction welding doesn't significantly affect the dimensional stability of the mating components because the electromagnetic energy precisely targets the susceptor. It spares areas of the part adjacent to the bond line from the displacement of resin and distortion that can accompany welding based on conductive heating or the application of mechanical energy to the joint via the mating materials, Nichols says.

Materials line-up grows

Traditionally, induction welding has focused on polyolefins. Nichols estimates that more than 80% of induction welding jobs still involve olefinic materials. But developments in susceptor compounds and welding process control have opened up other opportunities. Recently various nylons have come on strong. Nylon assemblies, particularly those used in automotive underhood or fuel-component applications, can benefit from induction welding's ability to create vapor-proof, high-pressure joints. Nichols won't divulge any specific applications, citing confidentiality agreements, though he did say that some new induction-welded applications are on the verge of going into production. "Strong growth is occurring in automotive applications using nylon materials," he says. Beyond nylon, induction welding can also join a growing number of dissimilar and highly filled materials, thanks to a growing line-up of susceptor compounds. One patented application involves permanently attaching a Santoprene TPV automotive hose to a rigid PP part, eliminating the need for hose clamps.

Handles big parts

Induction welding handles big parts and multiple joints easily. As an example, Nichols cites one production job-an automotive load floor-whose eight individual joints created a bond line of nearly 300 inches.