Dispensing methods

The method used to apply the adhesive or sealant can have as much to do with the success of the joining operation as the kind of material applied. The selection of an application or dispensing method depends primarily on the form of the adhesive liquid, paste, powder,... 

Brushes, simple rollers, syringes, squeeze bottles, and pressurized glue guns are manual methods that provide simplicity, low cost, and versatility. These methods are probably the most widely used because of their simplicity. Manual dispensing methods allow application of adhesive to only a small segment of a surface and are particularly effective for small or irregular parts or low-volume production. 

One of the most attractive precision dispensing methods for low-viscosity adhesives is the use of microjet printing technology.10 This technology is based on piezoelectric demand mode ink-jet printing, which can produce droplets of polymeric resins 25 to 125 flin in diameter, at rates up to 1000 drops per second. 

High Pressure Ion Chromatography is a relatively recent (1975) and immensely significant development of conventional ion exchange chromatography by Small and co-workers of The Dow Chemical Company. This technique has revolutionized instrumental wet chemical analysis in that micro-amounts of an ion or mixtures of ions may be separated and assessed quantitatively in a matter of minutes compared with hours using traditional techniques. Accurate volumetric dispensing methods means that a quantitative analysis of ions may be achieved whatever their initial concentration. 

It is a simple technology compared with some other dispensing methods. 

Of the mass-transfer dispensing methods, screen printing and stencil printing are the oldest and most widely used. Screen printing has been used for over 40 years in the electronics industry to apply thick-film conductors, resistors, and dielectrics in fabricating circuits on ceramic and plastic-laminate substrates. Screen printing is also used as a batch process for depositing electrically conductive and insulative adhesives to interconnect devices on thin-film and thick-film hybrid microcircuits. 

Table 4.13 Advantages and limitations of adhesive dispensing methods... 

Table 4.14 Cross-reference of dispensing methods and materials ...

Compatible rheology with the dispensing method (clog-free, no stringing) and with assembly requirements (dot profiles, green strengths)... 

In the study by Cheng et al. there was no apparent difference between the freeze-thaw results and results from added dissolution, such as the agitation and the repeated aspiration/dispensing methods. This is understandable as the compound loss overall from freeze-thaw cycling is minimal as is compound precipitation. Visual inspection of the glass vial s bottom indi-... 

Table 4.13. Advantages and Limitations of Adhesive Dispensing Methods... 

Table 4.14. Cross-reference of Dispensing Methods and Materials ...

Dispensing System/ Manufacturer Work Area Dimensions (X,Y, in) Z-Axis Travel (in) Dispensing Method/Speed (dph) Applications Examples of Compatible Adhesives... 

An important consideration in the choice of a dispensing method is the shape of the parts to be bonded. Within the constraints imposed by the performance of the selected adhesive, early considerations of manufacturing problems caused by the shape of the bond area are wise. The engineer should keep in mind that modifications to the adhesive are sometimes possible and these changes may allow the use of a more suitable means of application. For instance, in some cases, solvents may be used to allow the use of spraying methods. Mild application of heat may reduce viscosities so products will flow satisfactorily through rather small nozzles. 

It is probable that the ambient temperature maintained in the factory will require a relatively close control. The viscosity of most liquids and pastes will vary with temperature. Since many dispensing methods will require constant adhesive viscosity in order to insure uniform application of the product, it may be necessary to control the temperature of the adhesive. This is usually accomplished by providing some way of controlling the temperature of at least the immediate supply of adhesive. 

Automation of these dispensing methods might be undertaken for two reasons economy of scale or uniformity of application. As volume increases, there will come a point where substantial capital expense for machines will be justified by the decreasing cost of the labor involved. For some high-volume items, the automated system can be quite sophisticated. 

The suggestions given above have, of necessity, been quite general. The success of any one of these techniques depends to some degree, on the physical properties of the materials being dispensed. Therefore, experimental work will usually be required to confirm the suitability of a particular adhesive for use in a particular dispensing method. 

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