Clearweld^TM - TWI's innovative joining process for protective clothing

By Ian Jones

To create a sealed joint at fabric seams, for use in dangerous environments or foul weather conditions, stitching followed by sealing with tape or welding is used. The welding operation, while often being a more convenient and cheaper process than the double operation of stitching and taping, usually gives a less pleasing result in terms of the appearance and comfort of the garment.

TWI's Advanced Materials and Processes department has recently developed a technique for welding thermoplastic fabrics, which melts only at the interface between materials, rather than through the full thickness. This results in a joint that has a greater flexibility and softer feel than one that is made with other welding methods.

Fabrics are most commonly joined using stitching. There are, however, a number of situations where a welding process would be more suitable. Consider the requirements for leak-proof seams such as in personal protective clothing, containment bags or waterproof jackets and tents. Seam sealing compound or tape is often added to stitched seams to give a seal, which adds cost to the joining operation. The presence of holes also has the potential of weakening the fabric in the region of the seam.

Most welding processes for fabrics apply energy to the outer surface of the material (e.g. heat sealing) or to the bulk material (e.g. dielectric welding). These result in melting of the majority of the thickness of the material to effect a seal, and hence stiffen the seam.

A more flexible welding solution uses transmission laser welding (Clearweld) to apply a well-controlled amount of heat, just to the contacting surface of the fabrics. The Clearweld process was invented by TWI, and is being commercialised by Gentex Corporation. The process uses lasers that are commercially available, and infrared absorbing materials that are currently under development, but the intention is to create a product line of consumables compatible with commercially available delivery systems (e.g. inkjet inks, inks for liquid dispensing systems, thin films etc). Resulting beneficial features of the process include:

• Control of melt volume and hence seam flexibility
• Sealed seams in one operation, avoiding use of tapes - curved seams become possible
• Potential for high speed seaming and automation
• A novel appearance to the seam - new design opportunities

To date the Clearweld process has been applied to pure synthetic fabrics (see Fig.1), blends of synthetic and natural fibres and to waterproof laminates (see Fig.2).

The work has been carried out under TWI's Core Research Programme and TWI has applied for a patent for the use of these absorbers in laser welding of plastics.

	Fig.1. Weld between blue and yellow nylon fabrics showing region of fused material in the centre and unmelted fibres on the outer surfaces
	Fig.2. Waterproof laminated fabrics welded with a Nd:YAG laser using the Clearweld technique. Note the lack of any surface melting - only the interface between the two fabric layers has been melted

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