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Bonding technologies: ultrasonic bonding.

Ultrasonic energy is now a well established tool of any industry using thermoplastic materials. Ultrasonic energy's usefulness actually precedes thermoplastic applications and originates in World War Il sonar technology, followed by industrial applications for non-destructive testing, industrial ultrasonic cleaning and ultrasonic plastics joining.

The use of this technology in fabric and fiber bonding goes back to 1970 and is probably best known in the textile industry for ultrasonically formed mattress pads and bedspreads via the "Pinsonic" process. It is the Pinsonic process that has provided the foundation for using ultrasonic energy to laminate and/or form nonwoven fabrics and products. The process was so successful because of the inherent advantages that generally apply to all welding of thermoplastics by ultrasound, including speed, efficiency, good bond integrity, elimination of consumables and ability to be automated. Table 1 describes the bondability of various materials.


Ultrasonic energy is simply mechanical vibratory energy that, by definition, operates at frequencies beyond audible sound. This means frequencies that are beyond 18,000 Hz, the generally accepted upper threshold of the normal human hearing range. A wide range of frequencies are used depending on the application. When the application involves melting and reforming thermoplastic, as in the bonding of nonwoven fibers, the frequency most commonly used is 20,000 Hz. Reasons for the selection are based upon power levels required and the size of the ultrasonic tools, both of which are affected by frequency. It is easier to generate and control high power levels at the lower ultrasonic frequency because the ultrasonic tool is a resonant member whose size is

inversely proportional to its resonant frequency.

Process Variables And The Bonding Process

When examining the process variables from the ultrasonic point of view, one finds that there are only three: amplitude, pressure and time. Since the desired result is ultimately energy to melt and bond fibers, it can be simply stated that power is a function of horn vibration amplitude and pressure and that energy is a function of power used and time. These process variables are roughly established by prior experience and then finally adjusted to meet the needs of the specific application.

Amplitude is determined by the selection of the booster and the horn design and then automatically and accurately controlled or regulated by the electronic power supply. Pressure is usually generated by a pneumatic press and is easily adjusted and regulated. Time is the function of throughput speed that determines the dwell time of the fiber under the ultrasonically vibrating horn.

Other variables that become fixed are weld area, fiber type and amount of fiber Within certain limits, the ultrasonic variables can be changed in relation to one another to get the same result, but changes in non-ultrasonic variables - such as fiber type, blend or weight - will require one or more new changes in the ultrasonic variables to ensure adequate energy into the bond area.

The process for welding or bonding thermoplastic materials is essentially the same whether the parts to be joined are rigid injection molded parts or fibers of thermoplastic materials. Pressure and vibration are applied to the area to be bonded, which results in intermolecular mechanical stress. This causes melting to occur at points of limited contact where stress is maximized. Exposure time is controlled to ensure adequate energy into the material to achieve a bond.

Process Advantages And Bonding Applications

What is the difference between ultrasonic bonding and other techniques such as stitchbonding, needlepunching, chemical bonding or thermal bonding? Ultrasonic bonding can compete with or complement some of the more common processes because, first of all, no consumables such as chemical binders or needles and thread are required. Secondly, energy is needed or expended only at the precise location and area of the bond site.

Additionally - and this is a major difference between ultrasonic bonding and thermal bonding - heat energy is not conducted through the fiber to be bonded but instead, generated within the fiber itself, minimizing degradation of material through excessive heat. Finally, compared to some processes and fabric weights, the ultrasonic process is faster, with reported speeds in excess of 100 FPM.

Earlier it was stated that ultrasonic bonding could compete with or complement some of these processes. Consider the possibility of increasing a needle loom or stitch bonding machine productivity by pre-bonding ultrasonically, allowing reduction of needlepunching or stitch density and increasing productivity without sacrificing quality or, even better, producing fabric with new characteristics.

It should help increase understanding if some actual applications are identified. Without a doubt, the oldest application in ultrasonic bonding of this type is the now familiar Pinsonic process for laminating webs of nonwoven fabric, fiberfill and woven shell fabric in the formation of mattress pads and bedspreads. It is estimated that half of the pads and bedspreads used in the U.S. are produced by this process. Ultrasonic bonding is particularly attractive for this application because it eliminates the mechanisms and costs associated with needles and thread and at the same time allows virtually any pattern design without compromising productivity and quality.

This process has been in production for more than 20 years. Compared to the manufacture of mattress pads and bedspreads all other similar applications are small in scope; however, it is in these applications that the future lies. Among them are window coverings and the laminating of webs to produce replacements for cotton duck canvas used in tents, auto, boat and furniture coverings as well as combining coated and uncoated fabrics for use as moisture and thermal barriers.

Another application is the stabilizing of staple fiber into a nonwoven. It is in this area that we find most of the promise and most of the work to be done. Prototype and production machines have been built and run, while research by industry leaders continues. Application work and production is also in progress to make fabrics for other applications including apparel, wall coverings, drapes and curtains. Finally, the rotary drum principle is being considered to manufacture end products such as small filters and sponges made of various layers of materials.

In summary, ultrasonic bonding of thermoplastic containing fibers in woven and nonwoven fabrics is an established process with a significant successful history. This success is supported by the earlier and greater success of ultrasonic bonding of rigid injection molded parts from just about any industry that can be mentioned. The real work has just begun to make this process a significant contributor to the future success of the nonwoven industry. Investigations must continue to optimize fiber design, blends, bond patterns and web preparation techniques. The real challenge is the cooperative combination of these disciplines with the ultrasonic process and the market needs.

About the author:

When he authored this paper, Gary Flood was textile market manager at Branson Ultrasonics Corporation, Danbury, CT, a manufacturer of ultrasonic bonding equipment. Mr. Flood has since been promoted to regional manager, responsible for the mid-Atlantic U.S. region.
COPYRIGHT 1992 Rodman Publications, Inc.
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Copyright 1992 Gale, Cengage Learning. All rights reserved.

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Title Annotation:Nonwoven Bonding Technologies: There's More Than One way to Bond a Web; ultrasonic energy uses in nonwoven fabric production
Author:Flood, Gary
Publication:Nonwovens Industry
Date:Oct 1, 1992
Previous Article:Bonding technologies: chemical bonding.
Next Article:Bonding technologies: needlepunching.

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