Sewing, which joins individual panels together with another textile element (thread), provides adequate strength, elasticity and aesthetic properties. However, sewing produces discontinuous joints and perforated seams. To produce continuous and non-perforated seams, other joining methods are necessary. Thermal bonding and ultrasonic bonding (welding) involve melting and cooling of thermoplastics at the joining interface. These thermoplastic components may be present as hot-melt fibers, powders, films, or the sheath on bicomponent fibres.
Principle of Welding
Welding as a process in garment manufacture involves the sealing together of thermoplastic materials but the heat is not applied externally as is normally the case with the fusing of interlinings. The heat must be generated within the thermoplastic materials; for example, in special threads which have been woven into the back of a decorative motif during its construction.
The process used can be similar to that described under high frequency fusing, or more commonly, it is done by means of ultrasonic. If two hard materials are vibrated against each other, they become hot at the point of contact. If plies of thermoplastic materials are placed between the points of vibration, heat will be generated internally in the materials where they touch and, if this heat is sufficient, they will melt and can be pressed together so that a bond is formed.
A device which changes an input of electric current into mechanical vibrations, is known as a transducer. It is used to generate the heat. Frequencies in the ultrasonic region of around 20,000 cycles per second are used. In ultrasonic welding of clothing materials, the vibrations developed in the transducer are transferred to a "horn" and the heat is generated between this vibrating horn and stationary anvil. Fabric plies are placed between the horn and the anvil. The horn, in addition to vibrating, maintains the pressure necessary to form a weld once the surfaces to be joined have been melted. The amount of heat generated depends on the amplitude of the vibrations, the frequency, the pressure between the two surfaces, and the length of time they are allowed to continue vibrating in contact.
The amplitude is pre-set for a particular application, depending on the materials to be welded and the type of operation, that is, joining, edge finishing, etc. Pressure is adjusted by experiment and the time is usually around 2 seconds.
Friction and heat are generated at the interface areas of two fabric plies by the ultrasonic vibrations (around 20,000 cycles per second, beyond normal human hearing range) set up by the tool.
Ultrasonic welding: frictional heat created by the vibrating sewing horn fuses the fabrics in the pattern of the stitching wheel.
In addition to the stamping and plunge type operations, the welding principle can be used in rotary machines. As shown, a rotating wheel replaces the anvil and, if the wheel has a dot pattern on it, an effect similar in appearance to stitching can be created. The appearance of the machine is similar to that of a flat bed sewing machine but without the thread control systems.
The materials to be welded must be at least 65% thermoplastic, though there is scope for welding materials with a high natural fibre content if a plastic film is inserted between the plies being sealed together. The methods make use of a welding machine working on a "plunge" principle, that is, it welds a limited area or length of material at a time.
Although thermoplastics can be welded by ultrasound, different thermoplastics have different weldability because of their differences in structure, melt point, modulus and other additives, such as flame retardation and colorants, etc. Welding dissimilar materials has to meet two requirements:
Adhesives
Adhesive is a material which must be heated so that it melts and sticks as mentioned in lamination passage. It is different in principle from welding which uses the garment material to form the bond and a means other than direct heat to achieve it. Next Session will introduce the applications of adhesives as a joining method to make seams and hems in the fabric.
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