Effects of Pressure and Time on Dot Size

The syringes and needles are disposable and there are several sizes and materials available to customise it to the types of adhesives, the viscosity, and the quantity that should be applied (Fig. 9). Filling the syringe can be done easily with a simple equipment. 

We will see in the chapter Robot Application of Adhesives and Sealants later in this Handbook that there are also robots, with two and three axes, that may use these syringes, for instance to apply adhesives at very high speeds in electronics, and also on revolution parts. The syringes offer a very low weight and therefore can be moved at very high speed with a small robot for fast and very accurate jobs in electronics (Fig. 10). 

These syringe applicators with very fine needles may apply very small dots or drops down to one hundredth of a milligram. They are widely used in electronics for surface mounting, chips bonding and in jewellery and watches. The application of the adhesive may be controlled with a special camera or an optical fibre detector. The viscosities of the adhesives may range from 100 mPas (cyanoacrylates) to 20,000 mPas (epoxies, silicones). 

When dispensing two parts of epoxies, remember that in the epoxy curing process the viscosity will increase with time thus affecting the dot size. The control of the dispenser should be adjusted to compensate for the increase of viscosity. 

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. 

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Figure 6 Effect of pressure and time on dot size in syringe application.

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