Structural adhesives: A viable alternative to mechanical fasteners

As times change and products become more sophisticated, manufacturing techniques must be refined to accommodate new needs in structural applications. Mechanical fasteners and welding are traditional joining methods that many manufacturers are comfortable with, but these methods are not always the most practical solution for modern assembly. Mechanical fasteners, in particular, can drive up costs; limit options for materials; and cause fatigue, metal distortion, or tearing.

Strong adhesives or tapes can outperform mechanical fasteners in these structural applications, while also providing for a clean, durable design. That’s why more manufacturers are looking at industrial adhesives as a viable alternative to traditional joining methods.

Problems With Mechanical Fasteners

Traditional joining methods can present difficulties in the assembly process. Metal distortion and tearing under heavy loads, or fatigue, reduce reliability and longevity of assembled parts. To address this issue, manufacturers may try and decrease the gap size between rivets or bolts, inflating the number of needed fasteners and associated purchasing and labor costs.

Even welding can damage metals by causing heat distortion or burn-through, especially with lighter-weight substrates. Also, energy and labor often are needed to return the welded parts to a condition suitable for painting.

Sustainability in production is a hot-button issue that is only going to escalate. A lot of pressure is on manufacturers to decrease fuel use, energy consumption, and contaminant emissions. The best way to do that is to reduce the weight of materials being produced and use the lighter-weight materials in lieu of traditional heavy metals.

Unfortunately, traditional joining methods usually aren’t as effective as adhesives when joining lighter-weight materials, so reducing weight for sustainability’s sake becomes a difficult task. Lighter composite or plastic materials cannot always be welded easily, and thin sheet metal parts are prone to distortion and tearing at the concentrated points where through-part fasteners like rivets and bolts are placed.

While mechanical fasteners often can provide structural strength, the level of their holding power can be overkill for many common applications. In this way, mechanical fasteners are used out of convenience rather than necessity, when there are more simple and effective joining options available.

Choosing Adhesives

Selecting industrial adhesives for structural applications opens the door to opportunities that would not be possible when relying on mechanical fasteners or welding options. Adhesives are suitable for thin, lightweight materials. Where mechanical fasteners may rip through and damage a thin substrate, an adhesive bond provides even load distribution across the surface area of the joint. This reduces the potential for fatigue damage, lengthens the life of the bond and product, and drastically reduces replacement or repair costs.

Thin and light materials can be cheaper to manufacture than heavy metals, so using adhesives with these types of materials in assembly potentially can drive down costs as well.

Adhesives allow manufacturers to diversify the materials used in their assemblies in other ways, besides simply using different kinds of metal. Using adhesives opens up options for more composite and plastic use, which can help manufacturers decrease material costs and improve the aesthetics of the end product.

Where mechanical fasteners can limit design options for structural applications, adhesives expand them. Adhesives can be applied in areas that are inaccessible to mechanical fastening during final assembly, allowing new designs that further reduce weight, costs, and labor. From an aesthetics standpoint, adhesives can reduce and eliminate unsightly screw and rivet points and weld lines, leaving a smooth, uninterrupted surface without grinding after welding.

Adhesives can be used to bond dissimilar materials, such as metal to composites or plastics instead of metal to metal. Differentiating materials in assembly opens up a multitude of design options, and manufacturers can diversify their product offerings. Mechanical fasteners would not be a good option for joining metal to many composites, because drilling holes for fasteners would likely crack the material. When plastics are joined to metal, thermal cycling can lead to fractures within the plastic.

What type of adhesives are we talking about? Adhesive sealants provide high elongation, flexibility, and gap filling. Toughened epoxies work for applications that need a more rigid, solid combination of impact resistance and strength. Industrial tapes are not only easy to apply and convenient to handle, but also are designed to provide excellent impact load resistance on thin substrates.

Adhesive Solutions to Bonding Challenges

With more manufacturers choosing substrates over traditional metals, the right adhesives can be the answer for hard-to-bond materials.

Low surface energy (LSE) plastics like polyethylene, TPO, HDPE, PP, and some powder topcoats can be hard to bond because of their chemical composition. Typically, these materials require extensive surface preparation before the joining process even begins.

Thermal methods like friction and ultrasonic welding are considered to be suitable for LSE plastics. However, these methods typically are expensive and require ongoing tooling, which is not ideal for products that have short runs or need frequent design changes.

Even adhesives don’t always solve the surface preparation issue. But some specialty adhesives have been created to address this situation. These plastic adhesives can bond LSE materials and even reduce or eliminate the surface preparation steps.

Another challenge is trying to bond oily metal. Oily metals often require extensive preparation and cleaning, which creates extra work for fabricators. Unfortunately, these steps are necessary because the oil can interfere with robust bond formation.

That said, two-part structural acrylic adhesives are able to absorb most oils from metal surfaces, allowing the metals to bond without extensive cleaning first. This eliminates those tedious steps, saving the manufacturer time and money.

Adhesives: Going Beyond Strong

Though many manufacturers are comfortable with traditional fastening methods, modern design and sustainability factors render mechanical fasteners and welding to be less effective than alternative joining methods for some applications.

Adhesives are an effective remedy for the issues mechanical fasteners present. They also allow manufacturers to choose from a wider variety of substrates, bond dissimilar or hard-to-bond substrates, and explore different design avenues to create unique and superior products.

Shari Loushin is senior technical service specialist, structural adhesives, 3M Company, Industrial Adhesives and Tapes Division, 3M Center, St. Paul, MN 55144-1000, 888-364-3577, 3m.com/iatd.