Adhesive bonding is, at times, faster and less expensive than conventional fastening methods. It is well suited for high-volume production or assemblies requiring large bonded areas. As the size of the area to be joined increases, the time and labor saved by using adhesives instead of mechanical fasteners become progressively greater because the entire joint area can be bonded in one operation. Figure 1 shows the economy of large area metal-to-metal bonding compared with riveting.
The Economy of Metal-to-Metal Bonding Compared with Conventional Riveted Structures
(Source: Cagle, C. V., Adhesive Bonding Techniques and Applications, McGraw Hill, New York, 1968)
Some adhesives are especially well suited to applications requiring rapid assembly especially if the end-use requirements (i.e., strength, heat, and chemical resistance) are not too severe. The packaging industry and much of the decorative furniture industry use adhesives because they are fast and reliable. In the medical products industry, use of ultraviolet curing permits rapid assembly of syringes and other articles. Certain automotive materials are chosen for their ability to be ultrasonically welded, thereby, allowing efficient, fast, and high volume assembly operations.
At times, adhesive bonding may be more expensive than other fastening methods. However, the overall cost of the final part may be less through reduced material requirements, weight savings, elimination of other operations such as drilling, countersinking, welding, etc., and simplified assembly. Using associated production processes such as a paint-drying oven to cure the adhesive may also save costs.
Slow and critical processing requirements can be a major disadvantage of using adhesives particularly in high volume production operations. Several production concerns must be considered when bonding operations are first projected. All adhesives require clean surfaces to obtain the best results. Depending on the type and condition of the substrate and the bond strength desired, surface preparations ranging from a simple solvent wipe to chemical etching are necessary. Adhesives should never be applied over other coatings unless the characteristics of the initial coating are accurately known. The resulting bond strength will be no greater than the "weakest link in the chain".
If the adhesive has multiple components, the parts must be carefully weighed and mixed. The setting operation often requires heat and pressure. Lengthy set time makes jigs and fixtures necessary for assembly. Rigid process controls are also necessary, because the adhesive properties are dependent on the curing parameters and surface preparations.
The inspection of finished joints for quality control is very difficult. This necessitates strict control over the entire bonding process to ensure uniform quality. Non-destructive test techniques cannot quantitatively predict joint strength.
Since the true “general-purpose” adhesive has not yet been developed, the end-user should allow time to test candidate adhesives and bonding processes. Everyone involved in the design, selection, testing, and manufacture of adhesive bonded assemblies should be trained as to the critical requirements and processes.
Adhesives and sealants are sometimes composed of materials that may present personnel hazards, including flammability and dermatitis, in which case the necessary precautions must be considered. Often regional or national regulations will attempt to control the personal exposure with these materials. Workers must be trained how to handle these materials safely.
The following items contribute to a “hidden cost” of using adhesives, and they also could contribute to serious production difficulties when neglected:
- The storage life of the adhesive may be unrealistically short; some adhesives require refrigerated storage.
- The adhesive may begin to solidify before the worker is ready.
- The cost of surface preparation and primers, if necessary, must be considered.
- Ease of handling, waste, and reproducibility can be essential cost factors.
- Cleanup is a cost factor, especially where misapplied adhesive may ruin the appearance of a product.
- Once bonded, samples can't easily be disassembled; if misalignment occurs & the adhesive cures, usually the part must be scrapped.
Many of these hidden costs can be minimized by the proper choice of adhesives and processes. However, storage, cure, and waste disposal are seldom a concern in joining with mechanical fasteners, and with welding, the joining material is essentially free of charge.