Adhesive bonded studs Vs. traditional welded studs

Industrial applications use a wide variety of techniques to fix components to metal sheets, but one popular method is by welding thread studs on to parts. However, many are finding the process has increasing limitations when modernising the design of products. Though it is a reasonably efficient process that can be automated, stud welding reaches a limit with sheets that are less than 0.5mm or when metal substrates makes welding impossible. This puts the process at odds with changes being made in many high-throughput sectors. Here, the use of aluminium, magnesium and other more exotic materials is becoming much more common, as is the use of thinner gauges of ultra high-strength steel sheets. In addition, welding is restricted to non-lacquered metal components. These factors make the more traditional approach of welding studs to sheet metal components much less viable. One alternative has come from adhesive specialist DELO, which has developed a process in conjunction with mechanical joining expert Böllhoff. The pair have been developing and testing a process of bonding studs to sheet parts using a high strength adhesive. While the process might not seem all that original at first glance, they claim it offers some unique advantages while addressing many of the shortfalls traditionally associated with the technique. The Böllhoff and DELO solution is known as the Onsert method and can be used on thin sheets, more exotic metals and even composite panels. The process uses fixing elements that are initially moulded using a transparent plastic to allow the adhesive to be cured by an LED lamp. This allows the process to be fully carried out within a short cycle time. Another advantage is that bond studs have been shown to need significantly less energy to process compared to welding. Welding requires joined substrates to be melted – an energy intensive procedure – while the Onsert method only uses an LED lamp with a wavelength of just 400nm for curing. To weld a stud with a thickness of 6mm and a circular projection to a zinc plated sheet with a wall thickness of 1mm, an effective welding current of around 18,000A is required at a resulting voltage of 2V and a welding time of 100ms, with 3.6kJ effectively implemented. For resistance welding, a typical efficiency factor of 0.53 is indicated. This equates to an energy demand of 6.8kJ per stud. By contrast, the Onsert method – including the curing lamp including the cooling system – consumed around 117W at an amplitude of 100%. The energy required for a normal irradiation time of 5s is therefore 0.6kJ per element offering a significantly more energy efficient process. The process is not without its disadvantages, however. As with most adhesive processes the time to cure is indeed longer. Taking the example above, the cure time of 5s compares to a weld time of just 100ms. While both can be considered relatively quick processes that fit high volume manufacturing processes, welding does offer a higher output capability overall. In addition, one of the most commonly cited disadvantages of using adhesive bonds is poor resistance to high temperature which can cause the adhesive to lose stability, melt and lose its bonding ability. The same is true at very low temperatures where the adhesive can become brittle and lose mechanical integrity. The Onsert has the potential to operate at temperatures of 150°C and, while this is suitable for most applications, other adhesives can theoretically be used where heat cycling or higher temperatures become a main driver over throughput. The space industry, for example, is known to use stud bonding on honeycomb panels and has proven the capability of the process in continuous and considerable heat cycling environments. Indeed, elastic adhesives can be used where change in temperatures can cause significant expansion and contraction between different substrate materials. This applies if stud bonding as a process is used on between a composite panel and a steel stud, any two substrates with significantly different rates of thermal expansion. Here, adhesives can be used to absorb any shear stress and strain to stop it localising around a stud or insert. And compared to welding processes, stud bonding avoids the issue of heat-affected zones and also eliminates metal grinding, potentially lowering unit cost and arguably improving aesthetics. Indeed, stud bonding offers another advantage in multi-material manufacture that relies on traditional bolted joints. The use of bonded studs eliminates the corrosion associated with metals with different galvanic potential, such as a steel and aluminium joint. Böllhoff and DELO are not the only ones that have been able to successfully develop and implement a bonded stud variation. Bournemouth-based BigHead has been successfully applying bonded fastenings for some time. Its focus, however, is based less around metal sheets, but instead on allowing composite panels to have studs, nuts and fastenings applied to them to allow easy assembly, and disassembly, as required – as opposed to being permanently bonded. BigHead fasteners are also surface bonded to the skin of a panel with relative ease and its wide range of 'BigHeads' are designed to suit most applications. Standard BigHead fasteners can be surface bonded to the outer skin of the sandwich panel with adhesive without any need for drilling, fully retaining the strength and structural integrity of a sandwich panel. Its fasteners can also be bonded to the inner skin of a sandwich panel by drilling through the outer skin and sandwich filling with male, female and other fixing variants available. While bonded studs offer many advantages, they are unlikely to replace welded studs completely for sheet metal applications. Welded studs continue to be used everywhere where connection elements are irremovably joined to metal carriers. Their application is versatile, widespread and can be found in many objects of everyday life from household appliances like washing machines and cooking stoves to ship and aircraft construction. The advantage of this technique include its high loading capacity thanks to full area welding, high productivity thanks to short welding times, and ease of automation.