Design Characteristics

Design Advantages

Adhesives offer certain valuable design advantages. Unlike rivets or bolts, adhesives produce smooth contours that are aerodynamically and aesthetically beneficial. Adhesive bonding offers a method of joining that is not affected by the unevenness of surfaces. Greater part tolerances are possible using gap-filling adhesives. Adhesives also offer a better strength-to-weight ratio than other methods of mechanical fastening.

Adhesives can join any combination of solid materials regardless of shape, thickness, or mismatch in physical properties such as coefficient of thermal expansion or elastic modulus. Certain substrates may be too thin or too small to weld reproducibly without distortion. Thus, medical products and microelectronics are often assembled with adhesives. Non-metallic materials, such as plastics, elastomers, ceramics, and many paper products, can be joined together and to one another more economically and efficiently with adhesive bonding than with other methods.

Adhesives may also be a good way of adding options or additions to a line of modular manufactured items that share a common design. This allows the elimination or reduction of extra holes for mechanical fasteners and can eliminate expensive machining or stamping steps on the common part.

Design Disadvantages

The adhesive joint must be carefully designed for optimum performance. Design factors include the type of stress, environmental influences, and production methods to be used.

Many rigid adhesives do not work well when the service stress acts to peel or cleave the substrates from one another. Such stresses can often be reduced or eliminated by careful joint design. Seldom can a joint, which is designed for mechanical fastening, be used successfully for adhesive bonding without minor revision. Sometimes such revisions result in added expense or manufacturing steps.

There are no standards to guide the user with regard to design limits or to provide a safe design margin. These will depend on the adhesive and substrate, on the production methods, on the specific end-use environment, and on many other factors that are often not foreseen at the time of design development. Therefore, it is very difficult to predict the useful life of a bonded joint.

Simple life estimation processes used in other industries (such as Arrhenius plots to predict the aging of electrical wire insulation) are not effective with adhesives because of the numerous and sometimes competing reactions that can occur within a bonded joint. The only effective method of estimating the useful life of an adhesive bond is to do prototype testing under environmental conditions that will accelerate aging. Yet, one must be certain that these accelerated conditions do not cause reactions that are normally not experienced in the actual application.