BONDING AND WELDING: A SEAMING ALTERNATIVE?
With sewfree or stitchless fabric joining technique gaining momentum in technical apparel as well as fashion apparel segment, the welding and bonding technology is coming under review again and again. Although the technology was primarily initiated for functional applications like waterproofing and airproofing, it is now gradually moving towards comfort and aesthetic applications. And surprisingly for a change, this technology is moving from continuous processing (faster) towards batch processing (slower). Dr. Prabir Jana, NIFT Delhi, Patrick Weissgerber, President & CEO, DAP (Duerkopp Adler PFAFF) America and Anshuman Dash, Marketing Director, H&H Asia perform a reality check on this...
Sewing, bonding and welding share the common aim of joining material plies together. Sewing is continuous joining of two or more plies of material by penetrating the material with a needle and using a third material (thread) to join plies together. The sewn seam retains the elasticity of the base material.
Welding is clocked or continuous joining of two plies of material by liquefying the material and pressing it together, material is not penetrated and no third material (thread or glue) is required, whereas bonding is clocked or continuous joining of two plies of material by applying a third material (glue) in between the plies. While the welded seam has limited elasticity in comparison to sewing, the bonded seam has similar elasticity like sewing.
Both bonding and welding can be achieved by continuous processes on especially designed machines. While sewing and welding are single stage processes, bonding is a three-stage process.
In welding, two plies of materials are fed through a set of rollers in the presence of heat and pressure. The heat can be applied through hot wedge, hot air or by ultrasonic welding.
“I strongly feel that the flatness of bonded seam will be the key driver for growth of innovative applications in coming years.” – Dr. Prabir Jana, NIFT Delhi
Hot wedge and hot air welding technology, although giving high seam strength, are only suitable for material thickness from 0.2 mm to 2.0 mm and therefore have very limited application in fashion and performance apparel segment. These are currently used in most of the technical textiles applications. Ultrasonic welding, however giving average seam strength, is suitable for 0.05 mm to 0.4 mm material thickness, and hence is fit for apparel use.
However, a comparatively common application of hot air welding is seam sealing, which is a twostage process. In seam sealing, an existing sewn seam is layered with adhesive tape to seal the needle perforations. Hot air is used to melt the adhesive in the tape and paste it over the seam line.
A quick comparison of hot wedge, hot air and ultrasonic welding is given below in Table 1:
Types of ultrasonic welding machines
The upper feeding wheel of the ultrasonic welding machine is called anvil wheel and the lower feeding wheel is called sonotrode or horn. The commercially available ultrasonic welding machines are of four types. The most popular is the vertically rotating sonotrode and anvil wheel, which works on 35kHz technology (only PFAFF machines work with 35kHz, otherwise you can also get those with 20kHz), inaudible to the human ear. The second type is the standing sonotrode and vertically rotating anvil wheel. The third type is the horizontally rotating sonotrode and vertically rotating anvil wheel. Both second and third type work on 20kHz technology, audible to the human ear, and having potentially noxious effect! The fourth type is the vertically up-down presser foot type anvil and standing sonotrode. While the vertically rotating, sonotrode and anvil wheel type is offered by almost all leading brands, the presser foot type anvil is specific to apparel use and currently offered by limited brands.
The main difference among the four types is the material feeding vertically rotating sonotrode and anvil wheel which move the material in the same direction, thus guaranteeing a smooth material movement. The material movement is not synchronized (in the same direction) in all the three types except in the second type, where the sonotrode does not transport the material at all. The curious reason behind the vertically standing and horizontally rotating sonotrode seems to be the cheaper cost of the machine.
Conditions for welding
While different types of material can be sewn or bonded together, welding requires two minimum conditions to be met. Firstly, only thermoplastic materials can be welded, and secondly, both the plies should also be having similar fibre composition to be welded together. For example, thermoplastics such as PVC (Polyvinyl chloride), PU (Polyurethane), PA (Polyamide/ Nylon), PES (Polyester), PE (Polyethylene) and PP (Polypropylene) can be welded but cotton and silk can’t be welded. Furthermore, only ply of PVC can be welded with another ply of PVC; PES cannot be welded with PVC and PU cannot be welded with PES. Ultrasonic welding can do regular welding, cut and seal or dual system of cut and seal with simultaneous welding (unique to PFAFF machines).
Logically, a non-woven would be easier to weld as it offers more surface to be joined together. If we take it to the extreme and image two plies of a woven material like a fish net, we can only join the fibres (strains) that lie on top of each other. All other areas will not be joined. This does not mean that it cannot be welded but the seam strength will be of course lower.
Bonding – A preferred choice
Bonding comparatively offers wider applications like any type of fibre compositions which can be bonded together; for example, cotton fabric can be bonded with polyester. Only disadvantage with bonding is that it is a slow process. In the first stage, the double-sided adhesive tape will be laid on one fabric layer along the seam line. In the second stage, the paper backing is removed manually followed by positioning of the second layer of fabric on the adhesive tape. The third stage is the heat pressing of the seam for final bonding.
Machines used for bonding
Although numerous brands offer continuous adhesive tape laying machines (for first stage), in reality the process is often done manually in most of the companies. There are several reasons behind this; firstly, the tape laying machines are good for laying adhesive tape continuously for long length. But, if the seam line is not a straight line but curved or if the seam line has intermittent sewn applications, then manual tape laying by small hand iron is a better option.
“Bonded seams have a predetermined life expectancy. We have a customer who is doing functional underwear for athletes. They only guarantee 20 wash cycles on some garments. A sewn or welded seam usually does not have such restrictions. I do not expect a bonded seam to fall apart during wear, but over time the glue will be dissolved, especially when talking about chemical washing cycles as seen in dry cleaners. Sewn and welded seams are resistant to washing. In a sewn seam, the thread stays in place. A welded seam consists of the material itself only. If the welded seam would open that would mean the material itself would dissolve during washing. It is not made to last forever. Huge disadvantage in my opinion.” – Patrick Weissgerber
There are also tape laying machines with cloth edge trimmer (just similar to fabric edge trimming in overlock sewing). The mechanical trimmer is a better option than ultrasonic trimmer. Although ultrasonic trimmer seals the edge (thereby producing neater edge), in reality trimming applications often require crossover seams to be trimmed and in such cases, ultrasonic trimming fails (due to variable thickness). Mechanical trimmer can trim variable thickness along seam as well as has the additional advantage of working on any fabric type whereas ultrasonic trimmer can work for thermoplastic fabric type only.
Continuous machines are also available for attaching the second layer of fabric (second stage) and heat press (third stage), however for both the stages, it is seen that manual batch process is preferred. The second stage is the most crucial stage as the quality of seam will depend on this process. As a precaution, mostly the second layer of fabric is spot welded to avoid any shifting or distortion. The heat press or final stage is 100 per cent batch process and nowadays, there are machines that offer hot press followed by cold press. Positive cooling ensures garments can be processed for next operation immediately without any fear of distortion/ permanent wrinkle/fold. In case of hot press, the glue melts and spreads as per actual shape of the seam and cold press immediately freezes it otherwise the fabric takes the original position while getting dry and we see waviness along seam. Additionally, cold press claims an increase in the bonding strength by 30 per cent due to the quenching principle.
Ultrasonic welding or bonding?
The flatness of seam is the key advantage of welding or bonding over sewing. As there is no needle penetration, no continuous movement of two plies, no chances of either puckering due to structural jamming or puckering due to ply slippage. However, there are many disadvantages of ultrasonic welding for apparel use such as poor seam strength, reverse visual look of seam appearance. The stiffness of seam is debatable among experts and needs more in-depth study. The appearance of the welded seam is determined by the choice of anvil wheel. Some experts feel it is possible to create a very elastic welded seam for lingerie (which is comfortable to wear) by using specially designed anvil. Machine manufacturers require working more with product developers to improve on seam strength and visual look, which is technically possible.
Bonding offers wider applications like any type of fibre compositions can be bonded together; for example, cotton fabric can be bonded with polyester.
Bonded seams, on the contrary, have equal seam strength, comparatively softer seam handle and absolutely neat and flat appearance. The single-most advantage of bonded seam is flat seam. It is common practice in men’s formal shirt construction to apply double-sided fused tape sandwiched between seam to create a flatter seam appearance, and bonded seam will give you that naturally! Table 2 shows the comparative advantages and disadvantages of sewing, bonding and ultrasonic welding. While sewing still enjoys overwhelming acceptability and productivity advantages, bonding is gaining niche due to appearance and comfort, and welding is yet to prove its versatility in apparel applications.
There are numerous advantages evolving out of the bonding process although the process is currently dead slow to make any productive advantage. Till the technology of applying adhesive tape changes drastically, productivity will remain an issue. While Brother briefly showed a prototype of liquid glue dispensing in continuous bonding process three years ago, industry watchers say a newer method of dispensing adhesive is the key to the commercially popularizing bonding process. Interestingly, while most of the sewing operations are faster than similarly constructed bonding operations, there may be light at the end of the tunnel. The bonding offers clever manipulation and engineering of certain seams to clock equal or faster time than sewing! One such potential operation is polo T-shirt placket making.
Vertically up-down presser foot type anvil and standing sonotrode
Horizontally rotating sonotrode and vertically rotating anvil wheel
Vertically rotating sonotrode and anvil wheel
Standing sonotrode and vertically rotating anvil wheel
Hemline of a garment with partly bonding and partly elasticated sewing
A flat zipper