5 Reasons Structural Adhesives Replace Mechanical Fasteners

If you’ve found yourself wondering when it’s a good time to make the switch from traditional joining methods to a structural adhesive, you’re in the right place. Traditional joining methods like riveting come with higher costs and longer assembly times. With those concerns in mind, here are five reasons why structural adhesives have already replaced mechanical fasteners in many applications and industries. As an added bonus, our Plexus® methacrylate structural adhesives expert, Curbell Plastics Fabricated Parts Sales Manager Tim McPherson, shares his experience with the products and when it might make sense to try a structural adhesive for your application.

1. Design Flexibility

In a world that appreciates a smooth, seamless aesthetic, mechanical fasteners limit design flexibility. To join substrates with mechanical fasteners, a tradesperson needs to place the substrates, drill holes, and place and pop rivets, a process that requires the joining areas to be easily accessible by tools. This causes visible distortion, not to mention rivets and bolts leave a surface noticeably irregular making it difficult to apply graphics or paint. If the substrate is thin, concentrated stress at rivet joining sites can tear or crack, limiting material choices and design flexibility.

Structural adhesives help designers achieve that sleek, modern appearance that many consumers and customers desire. “You get a much cleaner look from bonding than mechanical fasteners,” Tim McPherson said, and mentioned specialty vehicle manufacturers as a prime example. “These days semi-trucks, trailers, and emergency vehicles are so much sleeker because they’re bonded.” Class 8 truck manufacturers, for example, now offer modern, luxury-like cab interiors with clean lines and smooth surfaces without unsightly screws or rivets. Structural adhesives provide even load distribution across joined areas which reduces potential substrate failures and allows for more attractive and flexible design options. Structural adhesives are also great for awkward joining sites that may be difficult to reach for mechanical fastening or welding, but could be secured with a clamp.

2. Bonding Dissimilar Materials

The top recurring issue with dissimilar materials Tim encounters are different coefficients of thermal expansion (CTE). Each material has its own and the differing rates of thermal expansion or contraction put stress on the joining sites of mechanical fasteners. The stress on the joint can often result in substrate failures such as warping, tearing, and cracking. Many assemblers have tried industrial strength glue to bond dissimilar materials, but CTE hasn’t been kind to glue either.

It’s in situations like these where Tim likes Plexus® adhesives. They have excellent peel and shear strengths which allow for the flexibility needed between dissimilar materials with different rates of thermal expansion and contraction. “It allows for some creep,” Tim explained and used industrial glue as a traditional joining method comparison product when speaking about dissimilar materials. “Previously, if you wanted to bond wood to aluminum, the different CTEs of the materials would crack glue. Plexus® methacrylate adhesives have polymers and impact modifiers that allow for CTE where other industrial glues don’t. It’s an excellent choice for dissimilar materials.” The design flexibility that bonding dissimilar materials has allowed has enabled design engineers to address another common need: weight reduction.

3. Lightweighting

Heavy mechanical fasteners are a familiar choice and often used out of convenience rather than necessity. They provide structural strength, but the level of holding power rivets offer can be overkill for many common applications. To reduce the likelihood of thin sheet material tearing at joining sites, manufacturers use thicker and heavier substrates. They may also try to reduce gap space between rivets to help with load distribution, but that increases the weight of the final product, too.

Replacing rivets and bolts with structural adhesives immediately returns weight savings by the joining material differences alone. As far as substrates go, lighter materials that display good impact resistance are replacing thicker, heavier materials in industries ranging from sign manufacturing to automotive assembly and military vehicles. Since the challenge of joining dissimilar materials is often overcome with structural adhesives, manufacturers can diversify their products by looking beyond traditional metal materials into alternative thinner and lighter-weight plastics, composites, and metals. All that lightweighting lends itself to every business’ dream: lowering costs.

4. Reduces Costs

Throughout his 25 years at Curbell Plastics, Tim’s been impressed with the total cost savings he’s seen customers reclaim by bonding with structural adhesives. “Previously, you used to drill a thousand holes and follow with a thousand rivets,” he recalled. “You save money by eliminating time and material costs from holes and rivets, and weight related fuel costs.” In fact, time trials between rivet and Plexus® adhesive assembly processes determined Plexus® adhesives significantly decreased the time it took to assemble start to finish, even accounting for cure time. Many customers have benefited with up to 50% assembly-costs savings when switching from rivets to structural adhesives.

There are plenty of applications where mechanical fasteners or welding will be required, but finding opportunities to use bonding can assign that costly skilled labor to where it’s needed most. Generally speaking, bonding with methacrylate adhesives is an easy task that doesn’t require skilled labor. Instead of using those tradespeople on, say, joining exterior panels to steel frames, adhesives enable general laborers to assumethose responsibilities. Plexus® Guide to Bonding Plastics, Composites, and Metals provides helpful bonding information and techniques for common thermoplastics like the amount of surface preparation involved for specific plastics. The guide identifies some plastics that are difficult to bond with adhesives due to their chemical structure (like fluoropolymers and acetal) and how to approach that.

5. Extended Service Life

Structural adhesives have a documented history of a longer service life for some applications than traditional mechanical fasteners. Several automotive, heavy equipment, and emergency and specialty vehicle manufacturers have replaced mechanical fasteners with Plexus® adhesives thanks to the extended maintenance schedules they enable. Since the bond provides a continuous line of adhesive, the material load is distributed more evenly across the joint resulting in a much higher lap shear strength. Additionally, the continuous line of adhesives creates a stronger bond because of the increased surface area for adhesion. Trailer side panels have replaced mechanical fasteners with structural adhesives to eliminate cracking at joining sites that had been caused by differing material CTEs and stress from road vibration. It’s not uncommon for applications that typically require routine maintenance or repair to extend their service life by years or even decades with adhesives.

When to Use Plexus® Structural Adhesives

Tim often sees Plexus® used in automotive assembly, specialty trailer manufacturing, sign manufacturing, fabrication shops, and many other market sectors, and has seen more adoption in military applications and medical devices in recent years. Some of the most common applications for structural adhesives are panel bonding, reinforcement bonding, and bracket bonding. Applications that see repetitive dynamic stress tend to last significantly longer with Plexus® adhesives than traditional fasteners. In fatigue cycle studies, 1 square inch bonded with Plexus® MA8110 can be used to lift up to 1000 pounds every minute without failure, even after 19 years. Other benefits specific to Plexus® adhesives include low shrinkage, low odor, and UV stable product options.

So why should a design engineer consider a structural adhesive in place of mechanical fasteners? Tim immediately pointed to the strength and elongation (flexibility) of Plexus® adhesives as the number one reason they’d want to test it, especially when bonding dissimilar materials. The design flexibility and cost savings it enables have made incredible differences in his customer’s operations and assembly lines.

The opportunities for methacrylate structural adhesives seem endless. Every application that puts two or more parts together could potentially be a fit.

Here are a few questions to help identify if your riveted application is right for adhesives:

1. How much strength is required?
2. How much time does my current riveting process take including drilling and sealing?
3. Are there areas more prone to leaks?
4. Am I forced to use more expensive rivets for higher strength requirements?
5. How often does misalignment happen with riveting?
6. Are there corrosion concerns (especially joining painted steel to aluminum with a rivet)?

With these questions to guide you, Tim or one of our other experts can assist you with selecting an adhesive to start your testing process.

The Finishing Touches

There are several reasons to try structural adhesives: they allow for a wider variety of substrates to choose from, shorten assembly times, and help lower associated costs through a reduced need for skilled labor and lightweighting opportunities. Manufacturers can explore different design avenues to create unique products to diversify their product offerings, which can include switching to lighter-weight, dissimilar, or otherwise hard-to-bond substrates. When used in place of mechanical fasters, adhesives reduce the potential of repairs and replacement thanks to even load distribution and excellent strength. So why not give them a try?

This article provides general guidelines and is intended for informational purposes only. Because every situation is unique, many factors must be considered when selecting a material. It is the reader’s responsibility to conduct his or her own research and make his or her own determination regarding the suitability of specific products for any given application.