Paper presented at Wearable Futures Conference, University of Wales, Newport, Wales, 14-16 September 2005.
Abstract
Manufacture of garments with added electronic functionality requires incorporation of viable joining methods for both the textiles and any wire interconnections. There is potential conflict between the precision microjoining techniques required for the electronic components and the larger scale seaming methods of the garments as a whole. Novel approaches are now being considered for garment manufacture. Fabric is most commonly joined by stitching, a highly labour-intensive process which renders production cost-prohibitive in many parts of the world. It also makes holes in the fabric, which can disrupt wire conductors and impair the strength and sealing performance of the resulting seam.In searching for automated processes for seaming of fabrics, companies have looked to welding and adhesive bonding, particularly for synthetic fabrics. Recent studies now show that laser welding offers the opportunity to increase the automation of the seaming process whilst also providing good performance, appearance and seam sealing in a single step. The application of laser welding to textiles is presented with reference to demonstration studies in air-bags,furniture and garments. The associated methods for making reliable interconnections in conductors for increased functionality in garments are also reviewed.
Introduction
Technical developments in textiles and electronics are providing the opportunity for the garments with improved comfort and functionality. With this potential in mind, many application areas have been identified from leisure clothing to medical monitoring areas. [9] These applications, together with the financial implications are driving the development of manufacturing methods for a new range of advanced garments. The financial drivers relate to recent developments in the worldwide textile industry. The production of commodity textiles and textile products in Europe is declining rapidly. This decline is mainly due to the disparity in labour costs between Europe and the Far East, a factor enhanced by the removal of all EU textile and clothing import quotas allowing free import to the EU from January 2005. The potential for securing employment in what are currently commodity textiles companies by diversifying into technical textiles and 'smart' garments with added functionality and value is therefore clearly apparent.
There are two major ways of countering the challenge of low labour production in China and other Far East countries:
- Diversification into products with high added value.
- Automation to reduce local labour costs.
Impact can be made in both these areas by providing garments with increased functionality and with new manufacturing methods. In this paper we consider the potential for laser welding methods to provide sealed seams for textiles and review the methods available for making interconnects between electrical conductors.
In particular, the case of manufacturing outdoor garments is considered. The requirements of the joining related manufacturing processes are as follows:
- Sealed seams for rain protection
- Reliability of the seams
- Comfort and appearance
- Moisture resistance of the electronic items and interconnects
- Reliability of the electronic function in wearing and washing
- Ease of manufacture and automation, cost
Potentially it is possible to carry out the textile seaming simultaneously with the procedures for incorporating the electronics. Textile based conductors have been developed and with suitable design the required circuit of conductors around the garment to connect power supplies, sensors, switches and actuators can be built as the garment is constructed. At this stage, however, it is necessary to consider the processes for the textile seaming and the electrical interconnects separately and then to consider those that could be carried out at the same time given suitable materials and garment designs.
Welding and adhesive methods for textiles
Welding and adhesive bonding are most commonly used where a sealed seam is required, but they offer the opportunity to develop increasingly automated procedures. Ultrasonic welding is used to cut and weld non-woven materials for face masks, filters and clean room capes. HF welding and heat sealing is used for welding coated fabrics for garments such as life vests as well as wider industrial applications such as inflatable boats and air ships. Laser welding is used to attach fabrics to rigid components such as microphones and work is ongoing to develop procedures and equipment for automated welding of garments. Adhesive bonding is used widely to attach seam sealing tape to stitched seams in outdoor clothing and is starting to be used in place of stitching by some manufacturers.
Welding is a thermal process [10] requiring melting of material at the fabric surfaces that are being joined. It is applicable to fully or partially synthetic fabrics with thermoplastic components (e.g. nylon, polyester or polypropylene yarns and PVC or polyurethane coatings), which are compatible when melted together. As a general rule the fabrics to be welded must be of the same thermoplastic. The material to be melted may be the fibres of the fabric, a thermoplastic coating or a film added at the joint in combination with the fabric fibres. Heating of the joint interface is achieved using a range of different methods.
Heat sealing - This technique uses an external heat source, often a heated bar or roller. As the bar is pressed against the fabric layers they are softened and a joint can be made. The heating is by conduction through one of the fabric layers. The process is rapid, taking 1-3 seconds when a bar is used or at a few m/min when using rollers. The outer surface of the fabric is melted in this process.
Dielectric or HF welding - This technique [4] generates heat in a polymer as a result of the application of a high frequency electric field. The oscillating field generates motion in the molecules of certain polymer types (e.g. PVC, polyurethanes), which is converted to heat. This is often described as bulk heating in contrast to heat sealing which is purely conductive heating. Typically the electrodes are shaped to the components to be welded and the process generates melting throughout the thickness of the materials. The process takes approximately 1-3secs for most materials.
Ultrasonic welding - In this process [1] heat is generated as a result of high frequency (20-40kHz), low amplitude mechanical vibrations applied under compression to the joint line. In plunge mode welding, the fabric is held under pressure onto a patterned anvil by a vibrating horn, the process typically taking around 1 second to complete. Applications include strapping, belt loops, filters and vertical blinds. In rotary/continuous mode, the fabric is fed between a fixed vibrating horn and a rotating wheel to provide a continuous seaming process. The wheels are normally patterned to provide local intensification of the ultrasonic energy and to provide a patterned seam. They may also incorporate a cutting edge to enable' cut and seal' processing. Seaming rates of at least 30m/min have been described. In both cases, the fabrics are melted throughout their thickness. The ability to ultrasonically weld textiles depends on their thermoplastic content and the desired end result. As a minimum, the material must have uniform thickness and thermoplastic content of 65%. Yarn density, tightness of weave, elasticity of material and style of knit are all factors which can influence the weld ability.
Laser welding [3,5] - Heating is carried out by passing an infrared laser beam through one of the fabrics to be absorbed by the lower fabric or a coating on the lower fabric. The fabrics are held under pressure at the time or just after the application of the laser by a sliding clamp or a roller. Melting is generated at the joint interface only, so the outer surfaces of the fabric are unaffected. Welding rates of at least 30m/min are possible depending on the fabrics being processed. The equipment is typically easily programmable for design or size changes. Variations exist using laser sources which heat the fabrics directly at a nip point as they are being rolled together. More detail is provided below.
Adhesive bonding utilizes a third material at the interface between the fabrics to be joined. This binds either chemically or mechanically with the fabric surface and typically infiltrates the fibrous materials to generate strength.Adhesives can be selected for application to most synthetic or natural fabrics and may be used to make joints between dissimilar materials and between fabrics and rigid forms. The thermally remeltable forms generate strength almost instantly after heating. The heat may be used to re-melt powder, film or solidified liquid forms at the interface using thermal sources that are often similar to those used for welding (see above) and include ovens (laminating), heat sealing, HF, ultrasonic and laser. The chemically reactive adhesives may take from seconds to a few minutes curing for the bond to be completed. Hot-melt adhesives are selected such that they melt at a lower temperature than the fabrics to be bonded. The fabric outer surfaces are therefore unaffected by the process. Production rates up to a few m/min are applicable.
Electronic interconnection methods
The electronic interconnection method is selected in parallel with the conductor material system. For use with outdoor clothing the conductive paths must be insulated from shorting as a result moisture from internal or external sources. The conductors therefore need to be either locally insulated or sealed within waterproof sections of the garment. These two options provide two distinct design cases. Interconnection methods are therefore required that will provide reliable conducting joints between insulated conductors and retain or reinstate the insulation for use in the first design case. In the second case the interconnection of bare conductors is required. In either of these cases the joints will often be made in close proximity to textiles or the conductor may be part of the textile. The interconnection method will preferably make a connection without affecting the appearance of performance of the surrounding textile.
It is also possible to use a higher density of conductors in a textile if they are insulated. Textiles, being inherently flexible, allow some movement of fibres within them. Uninsulated conductors need to be kept a few millimetres apart for reliable operation.
Conductors for use in textiles are typically of the following types: [11]
- Metal fibres (individual or in yarn)
- Silver coated nylon fibres
- Woven ribbons of above
- Insulated conductors
- Elastic/metal wound fibres
- Conductive polymer fibres
- Composite fibres with conductive particles
A wide range of methods has been considered for conductor interconnection. [7,6] Some arise directly from techniques developed for mass production in microelectronics. Others are being developed specifically for conductors in textiles. Typically joining is carried out through welding or soldering of the metal conductors directly, however with the advent of composite conductors or indeed the use conducting fibres intimately mixed with other fibres in a textile to form a composite structure, other opportunities are arising. It may be possible to weld the thermoplastic matrix of the composite or the thermoplastic fibres of the textile in such a way that the conducting elements are brought into close contact. Therefore new methods can be considered for textile interconnects that approach the problem by welding the polymer rather than the metal. The following methods may be considered for making electronic interconnects in textiles:
Mechanical fastening - Metal pop-studs, stapling, crimping and conducting Velcro [8] are examples of fastening methods not involving material melting.
Heat sealing or hot bar welding - This involves the use of a heated bar to melt and provide pressure to either the metal conductor, a solder deposit or the thermoplastic of the textile or composite.
Ultrasonic welding - A joint between two parts is created by delivering pulsed high-power ultrasound energy (>20kHz) to melt the point contact at the joint area. Heat is generated by friction as the metal conductors are vibrated against each other. This type of welding can be used for joining plastic, metal or dissimilar materials and is particularly useful in building small, complex assemblies.
Laser welding - The laser may either be directed at melting the metal, the polymer component or both. Selection of the laser wavelength and power intensity allows control over what is heated. Precise positioning of the laser beam is possible to provide small-scale joints.
Laser soldering - Pre-positioned solder is remelted using a laser heat source.
Conductive adhesive - Adhesive may be applied in tape or paste forms, and thermally or chemically cured.
Resistance welding [2] - The conducting parts of the interconnect are heated by making electrical contact of them between two electrodes and passing a current through the joint.
Soldering - Solder paste or rod is remelted using a heated tool and allowed to flow over the conductors to be joined.
Many of these processes are discussed below where the results of an initial assessment into connecting textiles incorporating conductors is described.
Laser welding for textiles
Process
The laser welding process consists of three stages which can be merged into one process if the absorber material is incorporated in advance of welding:
- Application of the laser absorber material system to the textile.
- Assembly of the seam and application of clamping pressure.
- Irradiation of the seam with a near infrared laser to melt the material where the absorber has been applied and create a permanent weld.
Application of absorber material system - The material system (given the name Clearweld ® ) may be applied in the form of a low-viscosity liquid, which dries rapidly to leave a very thin deposit on the surface of the textile. It may also be deposited on the surface of an interleaving film that is compatible with the textiles to be joined. A third option is to incorporate the absorber in a compatible interleaving film or into the textile fibres. In all cases the fabric can be prepared in advance of welding.
Assembly of the seam - The seam, including any interleaving film, is assembled and held in place such that the interface between the textiles can be irradiated through one of the textiles. The assembly also applies a clamping pressure to the joint during welding, without hindering access of the laser.
Irradiation - A near infrared laser, such as a diode laser providing radiation at a wavelength of 940nm, is used to irradiate the seam. The absorber material system absorbs the laser radiation, concentrating the heat at the interface between the textiles.
A thin film of polymer is melted in each textile and the application of clamping pressure brings these films into contact. The pressure is maintained as the films cool and solidify to produce a permanent weld.
Equipment
The laser equipment types suitable for application to textiles are the flat-bed system with a laser mounted on a gantry, a robotically manipulated laser source working over a shaped support and a 'sewing machine' type system where the textile is manipulated through the equipment manually.
Flat bed - The laser is mounted on a moving gantry above a fixed flat bed. The textiles are positioned on the flat bed, with absorber in place. The laser is manipulated around the joint line, and pressure is applied at the same time using either a slider or a roller. The textiles may be covered with a transparent cover sheet for this operation. This equipment is suitable for seams with no 3-dimensional shape and may be used for welding very large items. To date this has been used with manual placement of the textiles followed by automated welding, rather like laser cutting systems. More complete automation requires the manipulation of the textiles to be developed.
Robot mounted - The laser is mounted on a robot providing manipulation over a 2 or 3-dimensionally shaped support. The textiles are mounted on the support and a roller system is applied as the laser beam is manipulated round the seam. This is suited to more complex garments. The support and automated manipulation of the textiles still need to be developed to provide a fully automated version of this equipment.
Laser 'sewing machine' - The textiles are manipulated by hand through a roller system that applies pressure. The laser beam is directed to a point just in advance of the rollers. The equipment is less amenable to automation than the flat-bed or robotic systems.
Benefits and limitations
Benefits
Automation potential - especially with flat bed and robotic systems.
External appearance retained - the melting occurs at the interface only.
Good seam strength - 40-100% of the parent material strength in a tensile test.
Leak tight seams feasible.
Clean.
Fast - welding speeds of over 20m/min have been demonstrated.
Wide range of synthetic materials processable - nylon, polyester and polypropylene fabrics and coated or laminated fabrics with foam or membrane layers.
Multiple layers can be welded selectively using a flat construction - internal structures can be generated by welding only where absorbing material is placed.
Limitations
One part must be transparent to the infrared laser - some textiles cannot be processed due to the presence of certain colorants or additives.
Variable heating in some patterned fabrics - also due to differential absorption in certain colorants.
Investment in new technique/equipment - training, capital equipment and infrastructure.
Natural materials to be studied - these cannot be processed by melting but laser initiated bonding may be possible with interlayer films.
Examples
To date the laser welding process has been applied experimentally to pure synthetic fabrics, blends of synthetic and natural fibres, woven and non-woven textiles, polymer coated laminates and to waterproof laminates.
General garments
Demonstration garments have been prepared in polyester fabrics of two types; fleece and plain woven material. The welding was carried out on a flat bed system with manual positioning of the pieces. The shirt in woven polyester is shown in Fig.1. Another garment in black fleece also include welded zips.
Outdoor garments
Seams have been welded in nylon laminated with polytetrafluoroethylene (PTFE) membrane and in polyester laminated with bi-component polyurethane/PTFE membrane. In the latter material welding was carried out using a laser power of500W at a welding speed of 8m/min. Sealed seams were produced which exceeded standard requirements in hydrostatic testing of 3psi.
Air bags
Airbags are an important application for the nylon 6,6 fibre produced by INVISTA of Gloucester, U.K. Recent developments in automotive safety have led to the introduction of airbags mounted in the sides of the seats and in the roof above the doors (curtain airbags). These provide protection from side impacts and during multiple rollovers, which require the curtain bags to remain inflated for at least 10 seconds to offer effective protection. This requirement adds extra steps to the manufacturing process. Laser welding can provide a rapid, automated method for sealing a seam against gas or fluid leakage, and airbag samples were prepared and tested. The results from these trials were promising with the welded seams showing leak versus pressure performance within the range achieved using conventionally sealed seams. Moreover, the highly automated welding procedure offers the potential for cost savings by reducing the time and number of steps involved in airbag manufacture.
Inflatable buildings/air ships
Seams in PVC and polyurethane coated textiles have been prepared. In these cases the laser is used to melt only the fabric coating, so the ultimate strength is dependent on the strength of the coating lamination to the fabric. In the higher strength materials lap joints were produced with 87% of the parent material strength for the PVC coated fabric. The seams pass room temperature and elevated temperature tensile test standards to BS6F100.
Upholstery (Automotive and household)
Demonstrations have included welding of foam backed fabrics for automotive seating studies in bed manufacturing with Silent night Beds, Lancashire, U.K. These included attaching the mattress information label, producing a welded joint in the mattress side panel border in quilted fabric, and welding the fabric onto the PVC-coated wooden drawer front of a divan. All of the bed applications were successfully welded and subjected to industry standard testing with no visible deterioration of the seams. In all cases significant time savings could be realised through automation compared with current methods.
Processes for wire interconnects
Preliminary results are presented assessing a number of different joining techniques for fabric to fabric, yarn to yarn and component lead to fabric connections.
Materials
Two woven fabrics were assessed for connection to themselves and to LED leads. Two conducting strands were also used:
- Silver coated nylon woven with either nylon or cotton and nylon
- Lacquered copper twisted into a yarn with viscose
- LED leads
Procedures and results
Laser welding
The trials were made with a Nd:YAG laser at the infrared wavelength (1064 nm). The parameters changed from one test to another were the energy or the pulse width or the number of shots. During the first part of the tests, the object to weld was at the focal distance from the focusing lens. The following tests were made:
- Copper + viscose wire: the copper was melted and welded creating a conducting connection, and the viscose was burnt in the region of the weld.
- Silver coated nylon. The best result was obtained with pulse energy E=1.5 J, pulse width = 16 ms, repetition rate = 4 Hz, PMMA on top, 5 pulses. The two pieces of textile were bonded together with a little damage to the textile and some conduction. (See Fig.2).
- Silver coated textile with cotton: parameters as above but with 20 pulses. The joint was not as strong as with the 100%nylon fabric.
During the second part of the tests, the laser was used out of focus with a spot size of approximately 2mm diameter. With E=1.8 J, pulse width=16ms, repetition rate=4Hz, PMMA on top and one pulse, the two pieces can be joined. There was a little damage to the textile. This damage can perhaps be avoided by further increasing the spot size. (See Fig.3). The conductivity of the joint was good. Laser welding therefore offers the opportunity of welding either the metal or the polymer parts of the fabric, and potentially to weld the metal parts though a layer of fabric ora layer of insulation.
Resistance welding
Conventional resistance welding relies on the passage of current through a conductive material to generate heat. When two components are placed in contact with one another, and current and force are applied via opposed electrodes,heat is generated at the component interface. This process was used in an attempt to join thin Ag coated nylon fabric together.
An AC power supply was used together with an opposed electrode weld head. Welded joints were achieved using a welding current of 650A, a weld time of 0.2sec and an electrode load of 6kg.
A very weak but conductive joint was achieved, however the passage of current through the very small conductive elements in the fabric was sufficient to melt the Ag and the nylon. The remaining interface between the fibres was very small and weak.
This process seems less appropriate than the Hot Bar approach.
Conclusions
In a review and trials to assess the joining related requirements of advanced garments the following conclusions have arisen:
- Laser welding can be used to provide waterproof seams in laminated fabrics, with a potential for automation of the process in future.
- Ultrasonic welding and laser welding can be used to make conductive joints in woven fabrics of nylon and silver coated nylon. The same laser may be used for welding conductive tracks as for the rest of the textiles welding.
- Conductive adhesive can be used to connect large LED leads to a silver coated nylon conductive fabric.
- Laser welding can be used to join two individual lacquered copper fibres twisted in viscose yarn with damage to the viscose.
- Resistance hot bar welding can be used to connect large LED leads to a silver coated nylon conductive fabric.
Further research
Further work on laser welding for garments will address the development of equipment to support the fabrics and ultimately towards automated processing.
A wider range of interconnection methods will be assessed with the aim of making a connection between insulated wires embedded in textiles without damage to the insulation or the textile. Laser welding will be addressed further to assess the potential of making conducting connections whilst also making the rest of the welded seam.
References
- Devine, J (1998). Ultrasonic bonding of plastics and textiles for medical and other devices. International conference on Advances in Welding Technology, Columbus OH, pp261-270
- Dhawan, A, Seyam, A, Ghosh, T, Muth, J (2004). Woven fabric-based electrical circuits. Part 1 evaluating interconnect methods. Textile Res J 74(10), 2004, pp913-919
- Hilton, P, Jones I, Sallavanti R, (2000). Laser welding of fabrics using infrared absorbing dye. Conference, Joining of Advanced and Speciality Materials III, St Luois MO, pp136-141.
- Hollande, S, Laurent, J, Lebey T, (1998). High frequency welding of an industrial thermoplastic polyurethane elastomer coated fabric. Polymer vol.39, no.22, pp5343-5349
- Jones, I (2005). Improving productivity and quality with laser seaming of fabrics. Tech Text Int, May 2005, pp35-38
- Linz, T, Kallmeyer, C (2005). Integration of microelectronics into textiles. Proc. 3rd International Avantex Symposium, Frankfurt am Main, 6-8 June 2005
- Locher, I, Kirstein T, Troster G, (2004). Routing methods adapted to e-textiles. Proc 37th International Symposium on Microelectronics, Nov 2004
- New Scientist article (2004). New Scientist, 184, No.2470, 23 Oct 2004, p23
- Paradiso R, Wolter, K (2005). Wealthy - A wearable health care system: New Frontier on E-textile. Int Newsletter on Micro-Nano Integration, April 2005, p10-11
- Wise, R (1999). Thermal welding of polymers. TWI members report 667/1999, pp5-16
- Xue, P, Xiaoming, T, Leung M, Zhang, H (2005). Wearable electronics and Photonics. Woodhead Publishing, Cambridge, pp82-84