Performance Apparel Markets - Grado Zero Espace Srl

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Performance Apparel Markets, 1st quarter 2006 Seamless knitting and stitch-free technologies ● welding using a thermoplastic film; ● welding by ultrasonic heating; and ● welding by high frequency radiation. All require energy to activate and create a bond between the two fabrics Welding using a thermoplastic film employs the melting and adhesive properties of the film It may also be used to supplement other methods if insufficient thermoplastic is present The polyurethane coating of some performance fabrics aids welding Welding with a thermoplastic film involves melting the film by applying heat and pressure so that the fabric is penetrated and a bond created The films may be called adhesive films although they are not true adhesives Seam bonding can be performed using a specially adapted sewing machine Thermoplastic tape is fed between two layers of fabric and a heated foot applies heat and pressure to melt the film Each type of film has specific properties, including— —the polymer from which it is made— Regardless of the technology used, joining technologies have one thing in common—energy is required to activate and create a bond between the two fabrics. Welding by using a thermoplastic film—a process frequently referred to as welding by adhesive or thermoplastic adhesive—utilises the melting and adhesive properties of the film to bond two fabrics together. This method may also be used in addition to welding using ultrasonic heating or welding using high frequency radiation if there is insufficient thermoplastic material present in the fabrics to be welded. Some performance apparel fabrics are made with a polyurethane coating which provides sufficient thermoplastic material for welding to be effective. WELDING USING A THERMOPLASTIC FILM Welding with thermoplastic films uses a combination of chemical and mechanical bonding. Heat and pressure are applied to a film to activate it. The film melts in response to the right temperature or pressure and the molten substance subsequently penetrates the fabric. Penetration helps to create a bond between the melted film and the fibres. Welding with thermoplastic films is also commonly referred to as welding using adhesive tapes or adhesive films—although, by definition, such films are not true adhesives. Seam bonding can be performed by using a specially adapted sewing machine. In fact it is possible to adapt almost any type of sewing machine with seam bonding equipment. Once the sewing machine has been adapted, fabric is fed into it and folded to make the seam. Thermoplastic tape is fed between the two layers of fabric to be bonded. A heated foot (platen) applies the necessary heat and pressure to melt the film. A good bond is created if the thermoplastic film has penetrated both fabrics. Each type of thermoplastic film has a specific combination of properties, and the choice of film depends on the combination required for the fabric being used. The properties are detailed below. ● One property is the polymer from which the film is made. The correct polymer must be selected to achieve a good bond. Typical © Textiles Intelligence Limited 23
Performance Apparel Markets, 1st quarter 2006 Seamless knitting and stitch-free technologies polymer types include polyurethane, polyester, nylon and polyolefin. —the thickness or gauge— —the film’s softening point, at which it begins to melt and flow— —and the melt flow index, indicating how an adhesive flows once it has melted A heat seal press may be used During production three key factors must be met precisely and monitored— —heat— —pressure— —and time Ultrasonic welding involves channelling high frequency vibratory waves to build up heat in fibres It is fast, accurate and fabrics stay soft and breathable Ultrasonic welding can be used on nylon— ● A second property is the thickness or gauge of the film, which helps to determine its weight. Heavier fabrics require a thicker film for optimum bonding while lighter weight fabrics require a thinner film. ● A third property is the film’s softening point—or the temperature at which it begins to melt and flow. As a general rule, the minimum set temperature to activate a thermoplastic film is 15°C above its softening point. Importantly, the film should have a lower melting temperature than the fabrics being bonded. ● A fourth property is the melt flow index. This is an indication of how the thermoplastic flows once it has melted. A thermoplastic with a high melt flow index will flow rapidly after melting (similar to water) whereas one with a low melt flow index will flow more slowly (more like honey). High melt flow thermoplastics penetrate more quickly into fabrics than do low melt flow thermoplastics, resulting in a better mechanical bond. In order to activate thermoplastic films, a heat seal press may be used. During the production process, three key factors must be met precisely and monitored to ensure a strong and durable bond: heat, pressure and time. ● Heat: during production the critical temperature is the glue line temperature (GLT)—which is the temperature at the interface between the two fabrics. ● Pressure: this is usually applied by a pneumatic cylinder. ● Time: for the thermoplastic to melt and flow into a fabric adequately, heat and pressure need to be applied for a precise amount of time. WELDING BY ULTRASONIC HEATING Ultrasonic welding involves channelling high frequency vibratory waves (typically 20 KHz or more) so that they cause a rapid build-up of heat in synthetic fabrics. The heat can be used to weld or cut the fabrics. Proponents claim that ultrasonic welding is fast, accurate and efficient. Also, fabrics retain their softness and breathability. Ultrasonic welding can be used to cut or weld materials such as: ● nylon; 24 © Textiles Intelligence Limited
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