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The principles of ultrasonic bonding and web handling.

The Principles Of Ultrasonic Bonding And Web Handling

Ultrasonics have been used for more than 25 years in various manufacturing industries as an effective means of bonding or "welding" rigid thermoplastic and other materials. The nonwovens industry, however, did not begin employing ultrasonic bonding, to any large extent, until only a few short years ago.

One of the earliest applications was in the processing of fabrics for bedspreads and mattress pads, using the "Pinsonic" process, developed by Branson Ultrasonics and Crompton & Knowles. Today, manufacturers of nonwovens are discovering the wide-ranging potential for using ultrasonics to bond, laminate, emboss and slit multiple layers of similar or dissimilar materials. To understand how it can be used, and how it differs from other bonding technologies, manufacturers of nonwoven products should first understand what ultrasonic bonding is and how it works.

The Essence of Ultrasonic Bonding

Ultrasonic energy is simply mechanical vibratory energy which, by definition, is operated at frequencies exceeding 18,000 Hz, beyond the range of normal human hearing. Pressure and vibration are applied (between the ultrasonic horn and anvil) to the area to be bonded, causing intermolecular mechanical stress within the material. Thermal energy is released, causing softening to occur at points of limited contact (Figure 1).

Major components of ultrasonic equipment are the horn, the power supply and the converter. The horn focuses the sound wave to a single plane. The power supply generates the power and provides automatic frequency control, constant amplitude and controls for CIM (computer-integrated-manufacturing).

Today we have two central ways to apply ultrasonics: through the Plunge mode, for single strike bonding, such as spot welding (used primarily for rigid plastics), and through a rotary drum, for continuous bonding of narrow or wide webs. This article concentrates exclusively on rotary drum applications for nonwoven materials. Rotary drum technology uses a custom made steel cylinder to localize the energy from the ultrasonic horn, matching a specific pattern designed by the customer. As the materials pass over the drum, virtually any pattern can be achieved, including pictures and words.

Multiple rolls of material, ready to be processed, are mounted on the equipment. As winders and unwinders draw the materials between the rotary drum and the ultrasonic horn, the various layers are simultaneously exposed to vibration and pressure, concentrating the bonds to the raised drum pattern.

Thermal bonding melts material from the outside, causing a wider area than sometimes wanted to be bonded. The resulting effect is to stiffen the material, reducing its loft. Thermal bonding, by nature, also seals more material, reducing its breathability and absorption.

Ultrasonic bonding, on the other hand, melts material from the inside out. As a result, bonding takes place only at the point of contact (between the horn and the drum pattern). An area as small as a pin head can be effectively bonded without impacting the surrounding material in any way. This offers benefits for many applications that require high loft, softness, breathability and/or high absorption, such as gloves, clean room wipes, hospital gowns, diapers, filters and face masks. Ultrasonic laminating and slitting also produces an edge with no loose fibers. This is important in many applications, such as medical wipes and filters.

Chemical bonding, stitching and mechanical bonding all use consumables, adding to the manufacturer's material and handling costs. Each of these bonding techniques also create more variables that could cause downtime during equipment failure. Needles can break. Threads can snag. Chemicals need to be stored, handled and disposed. Ultrasonics, conversely, consume no material. It is simpler by design, and relatively maintenance free. Also, compared with some processes and fabric weights, the ultrasonic process is faster, with reported speeds approaching 150 meters a minute.

Another benefit with ultrasonics is the significant conservation of energy. Because ultrasonics do not generate heat from the outside to achieve effective bonding, ultrasonics uses 300--1000% fewer watts per bond site than thermal bonding (considering the heat loss). Ultrasonics create heat only at the concentrated point of bonding.

Applications for Nonwoven Materials

Polyester, nylon, polypropylene, polyethylene, PVC, urethane, saran, EVA and surlyn can all be bonded and laminated ultrasonically (Table 1). Natural fabrics (cotton, paper) can also be laminated by inter-layering with a thermoplastic. The plastic is melted from the inside out, while the rotary drum forces it into the natural fibers.

Materials can be bonded throughout (maximum 60%) or only at the edges, depending on the intended application. For example, sanitary napkins with end seals can be produced ultrasonically on a rotary drum. Production speeds are 350-400 napkins a minute. Another interesting application is high density quilt, formed from three plies, including knit fabric, a fiber batt and a film. The combination of high loft and material would make thermal bonding difficult and sewing would be too slow.

Web Handling Equipment

Besides a few technical parameters, applications are limited only by the imagination. An important consideration, however, is the web handling equipment upon which the ultrasonics will be mounted. Combining the experience of web handling capability with knowledge of ultrasonic bonding plays a key role in the success of automating for rotary drum ultrasonic bonding.

The desired outcome for each end product must be looked at individually. How many web layers will need to be laminated? What type of material? What thicknesses? Is the material flexible, stretchable or taut? These factors will influence the tensions that must be applied in the unwind and rewind mechanisms. Will mechanical tensioning be sufficient or are electronic load sensors necessary?

What type of bond patterns will be needed? Will slitting be required? How wide will the material be? These factors will determine the design of the rotary drum(s) and the ultrasonic horn(s) for the machine.

If there is slitting, there will be selvage material, so the equipment design has to take this into consideration. How wide a web is it starting with and what width is the end product? Will the finished webs immediately go into a secondary processing step or will they be finished products ready to be shipped?

A manufacturer of equipment for continuous (rotary drum) ultrasonic bonding, with experience in both web handling and ultrasonics can produce a machine for maximum efficiency. End users attempting to build rotary drum machines on their own tend to over-build, often spending more money than necessary.

Summary

Ultrasonic bonding, laminating, embossing and slitting offer many exciting possibilities for manufacturers of nonwovens products. The principles of ultrasonics have been explained and the advantages of ultrasonics over other bonding technologies have been outlined, including far less consumption of materials, equipment, time and electrical energy. A range of thermoplastics can be bonded ultrasonically, including blends with non-weldable or natural fibers, such as cotton and paper. The result is a bond of two or several web layers that retain their loft, softness, breathability and absorption.

The future of ultrasonic bonding for the nonwovens industry is limited only by the imagination. Every day new applications are being developed and ultrasonics are increasingly becoming a part of the manufacturing process. [Tabular Data Omitted]

PHOTO : Figure 1: Rotary anvil (drum) concept for continuous bonding. Simultaneous vibration (from the ultrasonic horn) and pressure (from the rotary drum) bonds two or more layers of web material.

PHOTO : Close-up of material passing between a Branson ultrasonic horn and rotary drum. Layers of material to the right before bonding; to the left after bonding, showing preciseness of bond concentration, retaining loft, softness, breathability and absorption.

GEORGE G. GIL President, Chase Machine, West Warwick, RI and JAMES MENGASON Marketing Director, Branson Ultrasonics, Danbury, CT
COPYRIGHT 1991 Rodman Publications, Inc.
No portion of this article can be reproduced without the express written permission from the copyright holder.
Copyright 1991 Gale, Cengage Learning. All rights reserved.

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Author:Gil, George G.; Mengason, James
Publication:Nonwovens Industry
Date:Oct 1, 1991
Words:1253
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