Adhesive and Sealant Applications in the Assembly Shop

There are lots of adhesive applications in the assembly shop and in the manufacture of components. A complete list is not given, but adhesive use is illustrated by examples. The larger quantities of adhesives used on the assembly line are for bonding of insulation pads, interior fittings, instrument panels, and roof modules, and for direct glazing. When the applications do not require a surface coating adhesive or when smaller assembly pads are to be bonded or a droplike or continuous extrusion of the adhesive is sufficient, hot melts can be used with advantage. When higher strength is not necessary, ethylene-vinyl acetate (EVA), polyamide (PA), or thermoplastic rubber compounds can be chosen. Otherwise, reactive hot-melt adhesives would be preferred. They can be applied as common hot melts, but they are cross-linking afterward and therefore provide bond strength like that of two — part urethane adhesives and good durability at higher temperatures. In some cases, adhe­sive bonding with cyanoacrylates or methacrylates, which cure in a few seconds, is used only as a temporary fixative to assist assembly.

When joining is required over the entire surface, pressure-sensitive adhesives. adhe­sive tape systems, or hot-melt adhesive foils can be used. For health reasons, conventional rubber-based solvent cements are used rarely today. There is a trend to replace them with hot melts or water-based adhesives. With new application techniques the hot melts can be

coated on substrates like solvent-based adhesives. Water-based adhesive systems fre­quently have the disadvantages of a longer open time and insufficient tack. An additional mechanical fixing is often necessary. The adhesive bonding of interior roof linings is an example of the use of water-based instead of solvent-based adhesives.

The primary sealants in common use in the assembly shop (e. g., for sealing of assembled air-conditioning systems or air filters) are:

1. Polyisobutylene-based compounds

2. Butyls

3. Moisture-curing one-component polyurethane adhesives

The adhesive bonding of plastic assembly components such as instrument panels, spoilers, spare wheel boxes, roof parts, trim assembles, and fenders is generally with one — or two- component polyurethanes. Frequently, a primer is used as pretreatment to improve adhe­sion. Adhesive tape systems can provide good results for the bonding of dash panels, trim lines, insignia parts, and rear view mirrors, for example.

Today the use of polyurethane adhesives is a common practice for adhesive bonding of windows in a car body, called direct glazing. Compared to the former glazing technique using rubber seals or polysulfide materials, direct glazing has the following significant advantages:

1. Possibility of completely automatic application (see Fig. 10)

2. High-performance sealing, matching the safety standards

3. Higher body strength

4. Smoothly designed car bodies

5. Improved aerodynamics

In addition, with direct glazing windows can be used as design and engineering elements of a car body (flush glazing). One — or two-component adhesives are used, which can be applied either warm or at room temperature and which are moisture — or hot-curing or

Figure 10 Sketch of automatic manufacturing equipment for direct glazing.

Figure 11 Robotized extrusion of a polyurethane adhesive onto a windshield.

curing with hardener. One-component moisture-curing compounds are very common. Generally, the bonding process includes pretreatments using specific cleaner and primer for both the glass surface and the car body flange. To protect the adhesive joint against ultraviolet rays, ceramic silk-screen printing on the glass and a black glass primer are used. New developments utilize their requirement primerless direct glazing. As mentioned above, the complete process of direct glazing can be performed fully automatically. Figure 10 shows a sketch as an example of such manufacturing equipment. In Fig. 11a robotized extrusion onto a windshield is shown. The robot is holding the windshield and leads it along a stationary swiveling nozzle, and after the adhesive is applied, puts it in the body opening (see Fig. 12). The accuracy in fitting is controlled by sensors.

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