When cooking up their next great product, designers and manufacturers are constantly looking for ways to produce the highest-performing products while keeping assembly and materials costs low.
Often this means streamlining what materials are used. It’s unusual for bulk and excess materials to be baked into the design of a product unless there’s some aesthetic or functional, or utilitarian reason. So it’s interesting that mechanical fasteners–essentially hundreds or thousands of “extra” metal components–are used in many assemblies.
Adhesive bonding, on the other hand, is by definition, a simpler proposition. This innovation has revolutionized how products are designed by using only three components to create a bond (e.g., two substrates, the materials being bonded, and the adhesive). In addition, fewer pieces require fabrication to create the whole product because adhesively bonded joints are incredibly resilient with a strong bond or seal. Therefore, these structures are more reliable and cost-effective than products built using massive amounts of extraneous, heavy metal fasteners.
In this article, we’ll look at why mechanical fastening is still the dominant assembly method, its drawbacks, and how adhesive bonding is the true future of manufacturing. We’ll also discuss ways to ensure adhesive bonds are as strong as possible, some major considerations when developing new products to be assembled using adhesives, and how to design manufacturing processes with all these things in mind.
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Why are Mechanical Fasteners Used?
It’s important to say that mechanical fasteners aren’t inherently bad, which is why they have become the standard in manufacturing.
For example:
- It is difficult to manufacture a product as a single unit. Fabricating large, fully formed pieces has only recently become more common. As a result, subassemblies are created and fastened together mechanically.
- Historically, designing with mechanical fasteners has been the easy and most straightforward method of assembling components into a product. There were no other affordable or understood possibilities.
- Process requirements for mechanical fasteners are relatively simple and defined by specifying torque with widely available tools for measuring these specifications.
- Mechanical fasteners are usually durable and don’t typically go bad unless they’re exposed to extreme environmental conditions.
- Mechanical fasteners have been the standard forever. If it isn’t broken, why fix it?
Even with all these benefits, mechanical fasteners have some pretty unignorable drawbacks.
Firstly, it takes a lot of mechanical fasteners to assemble large products. Often this means using several different types of fasteners for the same assembly, making it difficult to keep track of which kinds need to be used, where and how many. Sometimes, it’s even necessary to over-purchase fasteners to be certain there will be enough to complete the job. As a result, these fasteners become built-in scrap and waste, costing you ROI and revenue in the long run
Furthermore, you must create holes through the parts you’re joining in assembling with fasteners. Essentially, you are purposefully creating defects and weaknesses in those parts. There is a concentration of stress at the location of the hole. So, to compensate for this stress, you have to use thicker, stronger, and heavier materials. The design of the product is directly constrained by these intentional defects so that the finished assembly can handle the stress concentration caused by the mechanical fasteners.
Using Adhesive Bonding in Place of Mechanical Fasteners
The truth is, if we do away with fasteners and swap in adhesive bonding, we’ll make better products and dramatically simplify manufacturing.
With adhesive bonding, product designers can disguise how a product is assembled because the joints are extremely low profile (as opposed to bulky, obvious fasteners) and products can look like a unitary structure as opposed to parts that were stuck together. This means products will look cleaner and sleeker which appeals to modern consumers.
Designers are also freed up to let their creativity flourish. Adhesive bonding is possible to use with a wide variety of materials that mechanical fasteners make difficult due to the additional weight and structural concerns we discussed previously. The more materials at the disposal of designers, the more innovative the products can be. Removing constraints on design is what pushes industries ahead.
Similarly, using more polymers and advanced materials allows for more organic, flowing designs with fewer visible joints. When joints can be super thin and small, it enables products to be thinner, smaller, and lighter, which often means higher performance. For example, as vehicles get lighter due to the use of adhesive bonding, they become more fuel efficient and have fewer parts that wear down and require replacements.
When you use a good adhesive bond, the bond stresses are distributed over a large area as opposed to the highly concentrated stress of a mechanical fastener. This offers the entire assembly a higher degree of strength and reliability.
Less stress overall means less scrap and lower costs.
Additionally, when you use a well-implemented adhesive bond, it requires less labor to build the product. Instead of creating holes and adding fasteners, you add a bead of adhesive where the bond is to occur. This requires far fewer touches of the assembly by technicians or other machinery. This streamlines manufacturing processes to the essential elements, reducing production costs.
As with anything, there are disadvantages to using adhesive bonding.
For example, there is the potential for degradation of the adhesive bond strength during exposure to the environment. The strength of a mechanical structure stays strong throughout its life. Adhesives are polymers, and the properties of polymers are affected by heat and humidity. So, it is important to take the intended use of a part into consideration when selecting an adhesive and choose the most appropriate one for the temperatures it will be exposed to.
Furthermore, to protect the durability of a bond, surface treatment can be used to overcome the limitations of adhesives. To do adhesive bonding well, it requires a certain amount of surface intelligence. A strong bond relies not only on a powerful adhesive and a sturdy material, but the surface of that material has to be properly treated and prepared to bond with the adhesive. The two have to stick together.
What is a Well-Implemented Adhesive Bond?
To take full advantage of the benefits of adhesive bonding that we outlined earlier, designers and industrial engineers need to understand the requirements for creating a good bond.
A good bond requires the following:
- Surface cleaning and surface measurement techniques. Just like with the tools and methods of measuring torque to secure a mechanical fastener, there are ways to ensure a surface is absolutely ready to be bonded to.
- A highly precise cleaning or treatment process that includes measuring cleanliness as it relates to bond performance is essential to creating reliable adhesive bonds.
This requires a different manufacturing mindset when compared to using only mechanical fasteners. It requires different process control and quality control.
It means product and process designers need to think early in the product development phase about the factors which make reliable bonds. They need to think about production as a single process where each step either gets you closer to a strong bond or takes you a step further away from one. Any time a surface is exposed or interacted with, it has the possibility of degrading its cleanliness and, therefore, its ability to create a strong bond with the adhesive.
Different risks are inherent to using adhesive bonding or fasteners. It’s critical that product development teams define these risks upfront and build in ways to mitigate those risks throughout the process by way of quality controls and measurements.
Using Surface Intelligence to Create Effective Adhesive Bonds
Once again, the most effective use of adhesive bonding in the design and manufacturing of products requires a different mindset than mechanical fasteners; we call that mindset surface intelligence.
Adhesive bonding is a chemical synthesis process. Using fasteners is a purely mechanical operation. When we add an adhesive to a substrate and then cure it, this often involves chemical changes to the adhesive. The top few molecular layers of the substrate’s surface immediately interact with the top few molecular layers of the adhesives. They chemically bond through the attraction of the molecules present there. These sorts of changes and reactions don’t factor into using a mechanical fastener. Surface intelligence allows us to understand this chemical process and the effect the entire production process has on our ability to create predictably resilient adhesive bonds.
It is important to be aware of seemingly simple things like surface cleanliness when using adhesives. Manufacturers who use mechanical fasteners may not be sensitive to the subtleties and requirements of surface preparations. But, with adhesive bonding, it’s crucial to measure surface cleanliness and apply surface intelligence from the earliest stages of product development all the way through to the final assembly.
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