Converting from mechanical fasteners to adhesives and tapes can offer many benefits. These include flexibility in design, light weighting, increased product strength, prevention of water intrusion, processing ease, increased manufacturing throughput and improved product aesthetics.
A broad class of 3M adhesives and bonding tapes can be used for fastening in diverse applications in industries such as appliances, electronics, transportation, construction and signage. They can easily be used on multiple substrates like metal, wood, glass, plastics and composites.
To take full advantage of these benefits, paying attention to joint design is important. Table 1 illustrates a variety of joint types.
A variety of factors can influence joint design for a particular application: These can be grouped into physical, manufacturing and end-user factors. Evaluating all of them is critical for effective joint design.
The orientation of a joint influences the loading or stresses on the bond. These stresses include tensile, shear, peel and cleavage stress.
An example of tensile stress would be two small, rigid, bonded blocks being pulled away from each other. If pulled in a purely tensile mode, the stress would be balanced across the entire adhesive surface area. However, because bonded materials usually bend and flex, and bonded surfaces are often not perfectly parallel, there are very few joints that exhibit purely tensile stress.
In shear stress, forces pull parallel to or along the adhesive bond and, where the bond is in pure shear mode, the stress is spread over the entire bond area. However, because bonded parts generally flex, the part may deform, thereby creating peel stress in addition to shear stress. The amount of peel stress that occurs depends on the stiffness of both the adherends and the adhesive.
Peel stress and cleavage stress largely concentrate at one edge of a joint, rather than being spread throughout the joint. When at least one substrate is flexible, it can "peel" away from the other substrate, such as peeling masking tape off of a wall. If both substrates are relatively rigid, the joint may be in cleavage, for instance, bonding a shelf to a wall and then placing a heavy object on the shelf. In both peel and cleavage modes, stress tends to be much higher at the separating edge of the joint and propagates from that point back through the bond. Again, the response of the joint will depend on the stiffness of the adherends and the adhesive.
In creating adhesive designs, there are several objectives: 1) favoring tensile and shear stress rather than peel stress and cleavage stress; 2) moving to a design in which more of the adhesive area works against the stresses acting on the bond and 3) reducing or eliminating regions where stresses concentrate.
Manufacturing considerations should be evaluated when converting from a mechanical joint to an adhesive-based joint. Some of these factors include preferences in adhesive set time and requirements for parts handling post-assembly.
Aesthetic appeal is another important aspect of product design and the desired appearance for a final product will influence joint design. Also, gap-filling by adhesives can affect joint dimensions and should be carefully considered.
Often structural adhesives or bonding tapes can simply be substituted for mechanical fasteners without redesigning a joint. For example, the typical design for a multi-panel traffic sign uses batten strips (or splice plates) to hold the panels together at the edges and uses wind beams (or stiffeners) for strength and stiffness. Traditionally these would be attached with countersunk screws or by welding.
Adhesives offer an advantage because signs can be assembled significantly faster, without expensive equipment and with less skilled labor, resulting in a sign that continues to meet strength and durability requirements. In this case, the assembly joints are already compatible with the requirements for creating joints using bonding tape or adhesive. Stresses are primarily in tensile and shear modes or a balanced load situation across the bonded surfaces.
In another signage application, when using fasteners, stud bolts would be placed, holes would be drilled in the plastic sign and then fastened with retaining nuts. In contrast, by using bonding tape in face to face or overlap joints, there is no need to drill holes in the face of the sign.
Also, lighter, thinner materials can be used because there are no longer concerns about stud welds showing through the sign.
When spot welding a hat channel stiffener to a panel, an overlap panel is welded in place. In contrast, an overlay bond (of either a structural adhesive or a bonding tape) can simply be substituted for welding.
In other situations, joints can easily be redesigned to take advantage of the benefits adhesives can provide. If the original design creates stress on the adhesive in a concentrated or unbalanced mode (i.e., in peel or cleavage mode), the objective is redesigning toward the balanced modes obtained with shear and tensile stress.
For instance, in designing a frame, instead of welding a butt joint to bond two pieces of the frame, adhesives and a corner key can be used to easily stiffen the corner, thereby making it rigid and holding it in place.
A welded butt joint can be converted to a cylindrical (or slip) joint in which one part is slipped into another. For example, in a bicycle frame, a casing is formed that wraps around one cylindrical piece of the bicycle frame while the other cylindrical piece is inserted into the casing.
The joint is then in shear stress rather than in cleavage mode so the joint will provide much greater performance.
In another case, a bracket is mounted on a wall then a panel, such as a mirror, is hung on the bracket. Here an L-shaped hanging bracket can be replaced with a T-shaped bracket.
Alternatively, a slit can be cut in the backing and a bracket threaded through and bonded to the back of the product. Therefore, the stress at the top edge, which was in cleavage, is counterbalanced by a compressive force at the bottom edge of the bracket. Because the bracket will not push through the wall, the weight of the object being hung on the bracket will be distributed in shear stress rather than in cleavage mode.
3M offers a wide range of innovative adhesives that can be selected for optimal bond area and anticipated loadings in joint designs.
Through the 3M IATD TSR Program and the 3M IATD Design Solutions Program, 3M also helps customers by conducting design reviews and identifying the best design solutions.