Adhesives form an integral part of a wide variety of fabricated products, and offer the potential to create new, challenging product designs. Structural and speciality adhesives account for about 30% of total adhesive and sealant sales, with uses in the following industries:
Synthetic adhesives have good adhesion to a variety of substrates, can be applied quickly, have many excellent properties, and can be cost effective.
It is necessary to consider the following specific design topics, in addition to all other aspects of adhesive technology.
Basic principles and rules
Product and process design; strength of materials; design principles; basics of adhesive bonding design.
Mechanical behaviour
Stresses; joint geometries; analysis; joint dimensions; criteria; hybrid joints.
Performance in-service
Creep; fatigue; impact; thermal; moisture; chemical; stress; combined effects; life prediction.
Manufacturing considerations
Pre-treatment; ease of assembly; value analysis; automation; costing and economics; quality assurance; robustness or mistake-proofing.
Why use adhesives?
Basically, to make money by offering better products.
The exact combination of reasons will vary from case to case, but advantages should be sought in the following areas:
Reduced Production Costs
These may result from:
Increased production speed
In comparison with other fabrication methods, adhesive assembly is essentially fast. Even if curing is required, this can often be accommodated 'off-line', or combined with other processing stages such as paint curing.
Material selection
In contrast to welding, adhesives allow a wide freedom of choice during material specification. It is possible to mix and match material combinations to suit product function and save production costs in ways which have been impossible in the past.
Traditional materials may be combined with new metal alloys, plastics, composites, and ceramics to give distinctive product advantages.
Absorbing the full potential of this new freedom is perhaps one of the biggest challenges in finding significant market opportunities.
Some adhesives can also function both as a bonding and sealing agent.
Joining dissimilar materials
Adhesive bonding is often the only option when joining dissimilar materials.
Design for manufacture
Adhesive assembly offers significant cost savings if material costs can be reduced and production operations can be simplified. There is ample evidence that this can be the case if the design and manufacturing functions cooperate to design or redesign the product with manufacturing in mind.
New approaches can be taken to the manufacture of subcomponents, and castings may be combined with extrusions, sheet components and parts produced in a variety of other ways.
Better production sequences
Traditional assembly methods such as welding impose fairly rigid sequences during production, and frequently demand intermediate processing to remove contamination or rectify distortion. As adhesives may be applied more flexibly within the production sequence, (after finish painting for instance), they allow new approaches to production planning, shop layout and work flow. Bottlenecks can be removed, unnecessary operations can be eliminated, and work in progress can be reduced.
Low capital costs
Many adhesive operations involve manual application and use of adhesive packages which have built-in applicators. Even when mechanised or automated application is justified for high volume work, the equipment is usually lighter and lower cost than would be used for welding.
Low production costs
This is a source of significant confusion. Costs 'per tube' of some adhesives may be high, but this should not be confused with all-in production costs or cost per product. Cost comparisons should be based on the costs of the whole joining process, including plant, preparation and other pre-assembly costs, production and rectification expenses.
All-in cost assessments of this type provide a basis for accurate comparisons, and adhesive assembly may often give significant benefits.
Assembly considerations
How adhesives are applied and cured depends on:
- whether there is a facility to change the manufacturing process;
- the curing mechanism of the adhesive;
- whether it is possible to heat cure an adhesive;
- time factors in the assembly process.
The adhesive can be applied by either an automated robotic system, a bulk dispensing system or a portable hand-held dispensing cartridge which allows the system to be mobile. Adhesives can even be applied by hand with a spatula. Which application method is chosen really depends on the volume of adhesive being used. With respect to the two part epoxies and polyurethanes, mixing and metering of exactly the right amount of component parts is vital for optimum performance of the adhesive. Equipment exists which can provide both exact metering and full mixing, as well as dispensing in exactly the right position for manufacturing the product. In heat curing, either one-part pastes or film adhesives, platens, autoclaves and vacuum presses may be required.