Adhesives for Assembly
by Edward A.Y. Fisher, engineering manager
Question: When choosing an assembly process for a manufactured part, why should adhesives be considered over other processes, such as welding?
Answer: Adhesives offer several major benefits over other assembly methods. They distribute stress load evenly over a broad area, eliminating stress concentration that occurs when mechanical fasteners are utilized and increasing the overall strength of the assembly. Since adhesives are applied inside the joint, they are invisible within the assembly. Adhesives resist flex and vibration stresses and form a seal, as well as a bond, protecting the internal components from harsh environments.
They join irregularly shaped surfaces more easily than mechanical or thermal fastening, negligibly increase the weight of an assembly, create virtually no change in part dimensions or geometry, and quickly and easily bond dissimilar substrates. Adhesives are one-size-fits-all and assembly can be easily automated.
Question: When selecting adhesives during the design process, what should manufacturers take into consideration to avoid production issues?
Answer: To select an appropriate adhesive for an application, a designer should consider how the component will be assembled and what substrates will be bonded. Also, it is critical that the adhesive specified during the design phase is appropriate for the production process. At a minimum, the following questions should be asked when specifying an adhesive:
- What kind of joint stress will the assembly see? Tensile? Compressive? Peel?
- Does the design include difficult-to-bond substrates like polypropylene or nylon?
- Are there dissimilar metals that may cause thermal expansion problems when heated/cooled?
- Are any of the parts UV absorbing, making a UV curable adhesive inappropriate?
- Are there shadowed areas that will not see UV light?
- Will surface treatments (plasma, corona treatment) enhance bonding?
- Will the substrates and adhesive perform properly in the end use environment?
- Are there temperature-sensitive substrates that canít tolerate heat, making the selection of a heat cure or even a UV cure adhesive inappropriate?
Question: How can I be sure Iím using the correct adhesive for the manufacturing situation?
Answer: Currently there are a multitude of adhesives available that fall into six families that are most commonly used in manufacturing environments. Each of these families offer a unique combination of performance and processing benefits. Manufacturers that dedicate significant up-front time to research and select the proper adhesive for an application will save significant time and expense later in manufacturing and reliability. The benefits and disadvantages of each family are shown in the image above.