If you use adhesives and tapes in your assembly process, you know that they are usually faster and easier to use than other fastening methods—but you’ve likely also run into this familiar challenge: you need to bond two parts, but the mating surfaces seem too small to achieve a good grip.
One common solution is to change the fastening method to something slower and more complicated, such as welding or riveting. But why should the production process be dictated by weaknesses in the design?
A much better solution is to specify an adhesive or tape specifically designed for these problem areas of corner joints and small joint assemblies. In many situations, minor joint design tweaks can improve the strength of bonded joints even more. For example, consider a use case in which composite panels are to be attached to a steel tube frame. The round profile of the tubing does not provide a large surface area for adhesive.
Following the two approaches above, the designer could specify a product like a VHB tape, which is made of a soft, conformable foam which can bring more of the rounded surface into contact. The other option is to replace the round tubing with square tubing to increase the mating surface area.
There are two general approaches to consider when designing joints for the best adhesive performance: redesigning the joint to help an adhesive bond perform better, and choosing an adhesive or tape that can perform well with an existing design.
How to Improve Joint Design for Best Bonding Results
When designing a joint, it’s critical to consider the forces acting on the joint. The key forces to consider in adhesive applications are tensile and shear stress, as well as peel or cleavage stress.
Tensile and Shear Stress
Bronze Angle Bracket
To some degree, designing against these stresses is intuitive. For example, ordinary metal brackets are often gusseted to avoid bending or breakage.
One advantage of adhesives is that due to the properties of the chemical and physical bonding at work, they can have extremely high tensile and shear strength. Adhesive coats the entire mating surfaces, which means that under tensile or shear stress, the force is distributed fully across the entire bond.
Joint design should orient the forces acting on the bond toward tensile and shear stresses as much as possible.
Peeling and Cleavage Stress
When the geometry of the joint does not allow for distribution of force along the entire bond, the adhesion can be much weaker overall. The classic analogy is the bundle of sticks: easy to break one at a time, difficult to break as a unit.
Peeling stress occurs when one of the bonded parts is flexible, such as peeling masking tape off a wall. Cleavage stress occurs when one side of the bond acts as a fulcrum and the other side is pulled apart first. A common example of adhesive bond failure due to cleavage stress happens with the type of angle bracket described below.
Designing Joints to Apply the Right Stresses
The key to designing a strong adhesive joint assembly is distributing the stress along as large an area as possible. Work to the strengths of adhesive, namely tensile and shear strength, and eliminate or reduce areas where stress concentrates.
One example of a change in joint design could be adding a flange to increase the area of the mating surface. Another example is pictured below, showing an L-bracket replaced by a T-bracket. The T-bracket repositions the load to the middle of the bonding surface, reducing cleavage stress.
Corner joints can be especially tricky, as they offer a very small contact surface and undergo many different types of stresses. For miter joints like the one pictured above, hardware can be added to the assembly that will help control the forces acting on the bond, such as a corner key. A corner key will strengthen the joint against cleavage stress, while the adhesive strengthens it against shear and tensile stress.
Choosing the Right Adhesive for Joints
While these design solutions can help make joints more suitable for bonding, many joints designed for other types of fastening are already adequate for adhesive or tape bonding. Here’s an example: the most common welding joint assemblies include corner joints, lap joints and butt joints. Depending on the thickness of the stock, these joints are ideal for certain types of 3M structural adhesives, especially lap joints.
As a designer trying to strip cost out of a design, this is the ideal situation, as you can immediately reap the benefits of adhesives and tapes. With small assemblies, it’s worthwhile to simply try out a new technique before delving into the complex engineering calculations.
The benefits of adhesives and tapes over mechanical fasteners are well known. Many manufacturers make the switch to achieve increased manufacturing throughput, easier processing, light weighting, improved ergonomics for assembly workers, sealing against water and debris, increased product strength and better aesthetics.
Critical Considerations for Choosing Adhesive for Tight Spaces
Consider your production process when selecting the adhesive for your joint. Can you accommodate set time or curing heat requirements? Different adhesives may require different bonding conditions or treatment.
What type of assembly is required? It could be as simple as attaching trim, or something more heavy-duty and structural. You should consider this question because it informs your product line selection. 3M makes adhesives that run the duty gamut from light-duty tape all the way to structural adhesive that sets stronger than the substrate it’s bonded to. If you don’t consider the duty requirements of your assembly, you risk buying the wrong product.
What are the substrate materials? Some 3M products are specifically designed for metal, plastics or for bonding dissimilar materials. This may seem arbitrary, but the chemical and mechanical properties of different substrates require different technology. For example, an adhesive designed for wood will seep into the surface features and pores of the wood and form a mechanical bond. This would not work on a material such as smooth glass.
What is the end use of the product? Consider factors including temperature and moisture exposure, vibration and aesthetics. Again, this goes back to the engineers and chemists at 3M, who create specific products for specific use cases. For example, certain epoxies become very brittle when cured. This would not be ideal for a high-vibration environment, as it would likely fail. 3M has a large selection of products, not to mention the hundreds more adhesives out there from other brands. You can save time finding the right product by contacting a 3M product expert who can help you find the right product for the job.
(Images courtesy of 3M)
3M Scotch-Weld Structural Adhesives are a line of load-bearing, tough adhesives useful for a wide variety of applications, from automotive to sporting goods. The Scotch-Weld product line includes acrylics, urethanes and instant adhesives.
While adhesive is generally easier and simpler to use than mechanical fasteners or skill-intensive welding, tape is often an ideal solution when it comes to ease of use. 3M VHB tapes are high-strength, on-contact bonding tapes and typical applications include bonding panels to a frame, bonding stiffeners to a panel or eliminating the aesthetic appearance of screw heads or rivets.
VHB tapes are designed for long-term performance and can seal against moisture and debris. If your current process involves a fastening step and a sealing step, the right VHB tape could simplify your process and improve throughput. 3M technical experts can help you select the right tape for your application.
Other Considerations for Improving Corner Joint Design
- Adhesives that fill gaps, and thicker foam tapes, will alter the dimensions of joints. This is an important consideration for sub-assemblies or applications where fit is important.
- Different substrates have different surface energies, affecting the ability of the adhesive to wet the surface and bond properly. Surface preparation is critically important in all adhesive applications, and typically requires surfaces to be clean, dry and oil-free. Surface roughness can also help some adhesives bond more strongly.
- When bonding different substrates, consider the coefficients of thermal expansion. 3M offers flexible adhesives that can accommodate these changes.
(Image courtesy of 3M)
Typically, an advantage of mechanical fasteners over adhesives and tapes is that adhesives and tapes form a strong, permanent bond, while mechanical fasteners can be loosened and removed for maintenance, repairs or access.
However, for situations such as these, you can still gain the manufacturing benefits of 3M products without turning to mechanical fasteners. Reclosable fasteners, such as hook-and-loop or 3M Dual-Lock reclosable fasteners, use an adhesive backing to bond to substrates, and can be manually detached thousands of times.
Here’s a hint: it’s easier to undo these fasteners by applying cleavage or peel force than by tensile or shear force!
3M has a wide range of adhesive products for every application, condition and substrate. The easiest, fastest way to choose the right adhesive for your task is to contact a 3M technical expert! Sign up to stay in touch!
This post is sponsored by 3M. All opinions are mine. – Isaac Maw
3M, Scotch-Weld and VHB are trademarks of 3M.