Joining Technology of the Future

September 1, 2010
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High-performance structural adhesives deliver numerous advantages for constructors and processors alike.

Depending on the application, the adhesive can be applied manually, semi-automatically or fully automatically by robot.


Figure. Stress Distribution in Bonding and Riveting

The development of new materials with wide-ranging application potential poses major challenges for modern bonding technology. In production, adhesive technology enables the optimum use of specific material properties and the implementation of new construction methods. The use of structural adhesives has also become essential to the widely used mixed construction method, as well as the application of all kinds of metal, synthetics and fiber composite materials.

In recent years, the use of modern adhesive technologies has become increasingly prominent in the industry. There is a good reason for this, as high-performance adhesives deliver numerous advantages for constructors and processors alike. In particular, the property of bonding all kinds of materials quickly and securely is one of the reasons why adhesives are now regarded as an established part of many production processes. Almost all types of metals-both conventional standard steels and stainless steel and aluminum-as well as synthetic and fiber composite materials and glass or ceramics, can be combined adhesively with professional application. In this way, the specific material properties can be optimally utilized for a component in product manufacture.

“Structural bonding has now become an essential joining method in all industrial sectors, especially where different materials have to be bonded together,” says Gerd Dietz, Technical Customer Service manager. Industrial applications range from the manufacture of domestic appliances and building facades to filters and wind turbines. In truck construction, nearly all modern box bodies are now bonded and sealed at the same time. The sandwich design of modern wall and ceiling elements enables lightweight structures that can only be fitted by means of bonding.



The use of modern structural adhesive technologies has become increasingly prominent in industrial applications.

Advantages Over Other Joining Methods

In addition to their all-round versatility, other factors contribute to the increasingly important role of adhesives in trade and industry. On a like-for-like basis, traditional connection technologies have familiar disadvantages. Mechanical processes such as riveting or bolting only enable the transfer of force at specific points (see Figure). The flux results in peak stresses at the hole or spot edges that increase the risk of fatigue fracture and thus adversely affect the dynamic and static strength of the bond.

In contrast, when bonding connections are effected correctly, the stress or load transfer is more evenly distributed across the entire bonding surface. Compared with traditional joining methods, bonds are superior in the presence of dynamic stress. In addition, the parts to be bonded are not weakened by the drilling of holes.

Compared with welding and soldering, the use of adhesives chiefly stands out by virtue of its flexibility. Heat-sensitive materials such as aluminum or plastic, as well as very thin components, can thus be securely bonded. Adhesives cause no negative impact on the strength of the material. The low heat build-up is another key criterion when processing joining parts that already have their final surface finish (e.g., chrome-plated steel). As a non-conducting material, cured adhesives also have an insulating effect, thus preventing contact corrosion.



Adhesives cause no negative impact on the strength of the material.

Technology Overview

Depending on the field of application, various adhesive technologies with specific product properties are available for structural bonding. This primarily involves 1- or 2-component reaction adhesives that cure at room temperature or as a result of heat. Available technologies include epoxy, acrylics and polyurethane adhesives, as well as silane-modified polymers and silicone-based products. For selecting the most suitable adhesive, expert advice from the manufacturer is extremely important.

“To make the right adhesive choice, it is essential to know and take into account the subsequent everyday requirements for bonded connections,” says Dietz.

One distinguishing criterion is the difference between rigid and flexible bonds, both of which have different advantages. Conventional rigid bonds are mainly used for high mechanical transfers of force, and they replace traditional joining methods in this case. They enable a tough adhesive bond on the surfaces to be joined, and they meet all practical requirements in terms of their strength properties, as demonstrated by numerous examples in the aircraft and vehicle industry. In contrast, elastic adhesives are increasingly being used in the presence of high dynamic stress resulting from temperature differences or component movements.



Practical Applications

Adhesive technology can be practically integrated into existing production processes in all industrial sectors. Depending on the application, the coating is applied manually, semi-automatically or fully automatically by robot. For this purpose, Henkel provides a complete range of dispensing systems that ensures secure and constant production.

According to the quantity and type of adhesive used, the use of a suction device is sometimes necessary and employees are advised to wear protective clothing and equipment in each case. The principal factor in the quality of the bond is the surface quality of the joining parts. For example, inadequate cleaning of oil or grease from the fitted parts to be bonded is a common source of faults in practical application.

Another essential condition is the correct application of the adhesive by the staff. The effectiveness of the bonded structures is heavily dependent on the production conditions, including preparation of the fitted parts, correct mixing in the case of 2-component adhesives and their application, and the fixing of and adherence to curing times.

The extent to which the use of structural adhesives at metal-processing firms pays off commercially must be assessed on a case-by-case basis. In the design and manufacture of composite materials, the joining method of bonding provides significant freedom of design and the ability to integrate many functions in one component. Optimizing the materials used not only reduces the weight of the component, but also enables a more cost-effective design. In addition, the possible increase in process speed compared with many mechanical joining technologies also delivers commercial benefits. In terms of visual aspects, one advantage is the prevention of surface pitting due to spot-welded connections.

Before the industrial use of structural adhesives, tests should be conducted under realistic conditions. Henkel provides advice and services for industrial customers that want to try out the use of adhesives for their application purposes. At its locations in Garching (near Munich) and Heidelberg, Germany, the company also has state-of-the-art centers for development and application technology where tests and analyses through to customer-specific modification and processing of products can be carried out.



Outlook

In view of the prevailing trend toward lightweight design and the targets of high productivity and economic efficiency in manufacture, the use of structural adhesives opens up attractive future prospects for small and large firms alike. These adhesives will come increasingly to the fore, as resource-conserving lightweight design is hard to implement without adhesives.

In the future, the economic success of a metal-processing firm will increasingly depend on the extent to which high innovation potential is used in the manufacture of composite materials. In this context, structural bonding is an important factor, which is why it is constantly referred to as a key technology of the 21st century.



For more information, contact Henkel AG & Co. KGaA, Henkelstraße 67, 40191 Düsseldorf Germany; phone (49) 211797-9947; e-mail daniela.schenkel@henkel.com; or visit www.360bonding.com.

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