Radio Frequency Welding of Plastics

By: Brian Reuter

RF welding is a basic technology, and the basic devices necessary to affect such a weld have not changed since the inception of the process. Today, as in 1942, we need a generator to provide RF, a transmission line to transfer power, a press to apply force and an electrode in the desired geometric pattern to be welded.

The terms "Radio Frequency (RF) Welding or Sealing" are often used interchangeably with "High Frequency (HF) or dielectric welding or sealing." When matter is brought into contact with an electromagnetic field, some portion of the electromagnetic field will go through a change of energy state. As a result, it will be converted to heat and dissipated within the contacted matter. The degree to which this con-version will occur, or the efficiency of this conversion of energy state is dependent on the atomic and molecular structure of the matter, the frequency of the electromagnetic field, and the field potential (Volt-age/cm). The term dielectric heating correctly describes this phenomenon at any frequency while RF or HF heating describes the process over the lim-ited frequency range from 1 to 200 megacycles/sec (megahertz/sec).

The area where most of the technological changes have taken place is in the components from which the individual devices are constructed. Solid state components have replaced mercury vapor rectifier tubes. Digital timers have replaced industrial timers. Programmable Logic Controllers (PLC) have replaced relay logic.

When a PLC is used with linear and optical encodes, precise control can be achieved over the various functions that determine the specific characteristics of the weld. Using these types of devices it is possible to monitor and control functions of time, pressure, current and voltage and their profiles.

When modern material handling systems are used in conjunction with these devices, high speed automatic production systems can be built. Many hundreds of such systems are in use throughout the U.S. These systems manufacture a wide variety of products for the automotive, stationary products, and medical industries.

The continuing stream of new RF responsive materials being brought to the market further impact the industry. In addition, additives and RF responsive adhesives are continually being developed for specialized applications. It is now possible to bond materials that in the past were considered unsuitable for the RF process. These changes are opening up a new range of products that can now be manufactured by this time proven technology. This will have a great effect in the medical industry, as it tries to eliminate the use of vinyl.

Both electron beam and laser welding, when initially discovered, were thought to be possible replacement technologies. However, these technologies have been found to be more applicable for spot or seam welding of metals or other rigid materials where welding times are measured in minutes and hours. In RF welded products, welding times are measured in seconds or fractions thereof. Guideline believes the likelihood of these becoming competing technologies is very low. In Guideline's opinion there is nothing on the horizon that will replace RF welding in the next 5 to 10 years. Its place will be as secure as it is today, not only as the economically preferred way to weld certain materials, but in many cases the only feasible method.

The U.S. companies most likely to be the major players 10-20 years in the future are Hall Dielectric Machine Co. and Thermex Therma-tron Inc. Both of these companies have depth in their engineering and technical staffs; a core of older, seasoned technical professionals, with a good mix of younger talent behind them being groomed to take the lead.

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