Automating adhesive application

In the automated lay-up system, each layer of veneer (with exception of the top surface veneer) passes under an automatic adhesive application system. This may be a spray application, a curtain coater, or an extruder, each of which is designed to apply a uniform adhesive spread on the upper face of each veneer. After all except the top veneer have been spread with adhesive and laid together, the top veneer is added. The me of the veneer—adhesive assembly at this point should be about 8%. 

When large, flat surfaces and webs of materials are to be coated and when production reaches rates of 200 to 300 pieces per day, machine methods of application should be considered over the manual methods described above. Mechanical-roller methods are commonly used to apply a uniform layer of adhesive via a continuous roll. Such automated systems are used with adhesives that have a long working life and low viscosity. Various machine methods of adhesive application are illustrated in Fig. 17.8, and several are described below. 

As to the first point mentioned above, cost-cutting steps have to be taken seriously. Increasing automation of the adhesive application is imperative. Adhesive bonding processes without extra pretreatment of joint surfaces and without using a primer but with... 

As with many mechanical systems, this process improved with each new machine that was built, and it is a good example of the benefit of close cooperation between the adhesive manufacturer, the equipment supplier, and the applicator. Another good example of automation is the application of paste epoxies to bond the lens to the reflector of halogen auto headlamps. The degree of automation is not quite as complete since most of these machines still use human operators to load and unload lenses on the adhesive application system. The primary objective in this case is to apply a very uniform bead of adhesive around the perimeter of the lens. In this process, a two-dimensional curve-following robot carries multiple extrusion nozzles around several lenses placed on a turntable. Lenses are then manually placed on the lamp reflector. The assembly is then carried on a chain conveyor into the curing oven. 

Selection of the robot and the dispensing equipment are of primary importance when automating sealant and adhesive application processes. Those installations that have taken into account all aspects of the application requirements have been successful. 

Automated methods for preparation, adhesive application, and bond assembly give the best reproducibility and best control of durability however, such automation is frequently not possible. Manufacturers recommendations in data sheets and on containers should always be followed and particular attention paid to storage conditions, mixing procedures, adhesive or sealant application, and joint assembly. 

Although a fully automatic bonding process including adhesive application and joining of adherends is possible, it is hard to check visually the mixture condition, the applied amount of adhesive, and its application patterns. Imaging technologies are, of course, possible to use recently. However, there are several problems in terms of cost and accuracy so far. A combined process of automation and manual work is very efficient for such situations. For instance, adherend joining can remain a manual operation to check the adhesive condition mentioned above, which is helpful to avoid defects. 

An advantage of manual operation is the availability of a sequential process from adhesive mixture to pressurization. This whole process can be carried out continuously by one person. On the other hand, a parallel process is dangerous because the pot life of the adhesive applied to many queuing parts can be attained if the production line stops before the joining process. Since automation needs a parallel process, a waiting system for the adhesive application, which is activated when the following processes consume too much time, has to be installed. 

There are two principal types of automation available to the user these are the dedicated systems designed specifically for one panel or assembly (Fig. 4.6) and the now familiar robot based systems (Fig. 4.7). Both types are in use in both Europe and Japan. The choice between them is influenced by the quantities of component to be processed during the model life, the expected frequency of styling changes, complexity of the component and the cycle times required of the process. Typical uses for dedicated automation adhesive/sealant application include liquid gaskets applied to two-dimensional or simple three-dimensional shapes. 

For many bonding applications a variety of adhesives can perform adequately. Hot melt adhesives are normally chosen where process speed is critical. Since hot melts have no carrier vehicle (solvent or water), and thicken rapidly as they cool, they are limited in their ability to (1) penetrate low porosity substrates or wet out very rough surfaces (2) cut through or imbibe surface contaminants and (3) wet out high thermal conductivity substrates (e.g. metals). Nonetheless, hot melts are increasingly the adhesive of choice in automated production environments because of their fast set speed. 

In many cases, there is no clear delimitation between application and mixing of the adhesives. In particular, in the case of adhesive systems with very short pot lives, mixing, dosing and application devices are often one single unit. Investing in such equipment is not only sensible for automation purposes, but also results in savings in adhesives since wrong batches or exceeded pot lives can be avoided. 

In this paper, we have presented test results of two methods of adhesion improvement on a specific aluminum alloy and a stainless steel. The methods are based on flame pyrolysis of silane precursors and a laser pretreatment in connection with a primer. While the first method seems to be most favorable for steel surfaces, the latter gives good results on both aluminum and steel. Both methods are well suited for application in the railroad industry since they are effective under atmospheric conditions and are therefore promising for integration in an automated production line. 

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