Adhesives dispensing methods

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

Table 4.13. Advantages and Limitations of Adhesive 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. 

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. 

Figure 4 D5max method. Steps (1) Worker loads magnets to automatic feeder/adhesive dispenser also brings motor cans to second worker station. (2) Second worker applies activator to can housing with a semiautomatic dispenser. (3) Second worker then places can over magnets that have been previously placed into fixtures robotically. (4) Turntable cycles, engaging fixture pressure mechanism. (5) Bonded parts are automatically off-loaded to continue in production.

A needle-valve controls the amount of adhesive dispensed. Advantages of this method include lower cost and simplicity of operation compared with the auger and the linear piston types. Some disadvantages are ... 

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. 

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. 

Most adhesives dispense well and by using a small tip stringing may be eliminated. Syringe application is also the best method for filling and potting applications, with polyurethanes or silicones. 

Pin- and daub-transfer processes are also mass-transfer methods, but comprise less than 10% of the adhesive-dispensing processes used in production. In pin transfer, hard tooling consisting of an array of pins corresponding to the positions on the circuit board where components are to be attached is fabricated. The pin arrays are dipped into atray containing the... 

Dispensing Methods. There are five primary methods for dispensing adhesives, fluxes, and solder pastes ... 

Standard adhesive application methods are insufficient when small amounts of adhesive are necessary. Dispenser units, working with air or extruder feed, are often used for the application of the adhesives. Here, the handling of the COTiductive adhesive is often limited because of the particle size. Another technology utilizes shaped adhesive films. This is an attractive way to produce a complex series. Non-shaped films can be used for non-conductive bonding through the use of pressure and heat. 

Various adhesive application methods are available (dispensing, pin transfer and screen/stencil printing) and different adhesive rheologies are required in each case. 

Adhesives are classified by many methods such as dispensing method, application, and primary resin. The classification by primary resin is very useful to select a good adhesive for a given application, because the name of the primary resin involves the chemical structure of the back bone and the sketchy properties such as adhesion strength and heat resistance. In this chapter. 

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