Pressure-time dispensers

In a pressure-time dispensing system (see Section 8.3), the adhesive is forced under pressure through a valve so that the desired quantity is dispensed onto the component part. As the dispense valve opens and closes, the pressure-time response curve can be monitored just upstream of the nozzle tip. 

The major methods of dispensing anaerobic adhesives use pressure-time controlled valves, a cartridge and syringe system and screen printing. 

Pressure is applied to bulk liquid and timer-controlled valves meter the quantity dispensed. Figure 6 shows a typical pressure-time system for adhesives where the viscosity is up to approximately 100,000 mPa. Pressurization of the prodnct tank pushes the adhesive through a feed line to a dispensing valve. A timer controls the opening time of the dispenser valve to obtain a metered quantity. Regulated pressure behind the product and the valve opening time control the quantity dispensed. The adhesive can be applied in dots or in bead form. 

Fig. 6. A pressure-time typical system for dispensing adhesive...

Pressure time using advancing valve The advancing pinch valve is utilized in operator-paced or fully automatic lines where the component part itself cannot be moved to the valve nozzle. The advancing pinch valve, which can be between a stroke of 25 and 150 mm (1 and 6 in), advances the application nozzle to the component part. The dispenser valve then opens, allowing a metered quantity of product to flow through the valve nozzle on to the component part. The valve then closes and the advancing cylinder retracts from the nozzle. In this way dots or continuous beads of product may be applied. 

Pressure time using a rotospray unit This unit is used to apply anaerobic products to machined bores. The unit consists of an electric motor with an integral dispenser valve. At the end of the motor shaft (here is a plastic cup around the periphery of which are a number of holes. 

In the Automotive industry there is a trend towards complete automation of production lines. In the application of adhesives/sealants for gasketing, the principles of dispensing have normally been to use a pressure-time system, or alternatively a cartridge system with a robotic head. However, a faster system is to screen print an anaerobic on to the surface of a component. 

The most common principle of dispensing is to use the pressure/time system as described in Section 10.9.1. 

The adhesive is put under pressure and the dispense valve is opened for a certain time to provide the correct quantity of adhesive (see Figure 10.15). Since the cyanoacrylate contacts the pressurised air, the dryness of the air is important and an air filter on the mains air line is recommended. A pressure/time system is best suited for cyanoacrylates up to a viscosity of 5,000 mPa-s, i.e., self levelling products. 

A pressure/time system usually consists of a reservoir, a dispense valve and a controller. The type of dispense valve depends on whether it needs to be hand-held, stationary or advancing, and if there are standard or high precision requirements. 

Time-pressure pump dispensing is a method to deposit adhesive or solder paste by applying a pressure pulse for a specific time duration on a reservoir of material (see Rg. 40.17). A precisely controlled quantity of adhesive or solder paste emerges from the selected orifice size, which is deposited on the circuit board. Often, the material comes prepackaged in a syringe that is inserted into the machine. 

FIGURE 40.17 Time-pressure pump dispensing technique (a) The dispenser is positioned at the circuit board site (b) the dispenser is lowered to near the surface at which time, a pulse of air pushes the piston a distance Ax to dispense a set quantity of adhesive (c) the dispenser is raised from the site, leaving the adhesive deposit. 

One of the most commonly used semi-automatic dispense systems is the pressuretime method and this is extensively used for a wide range of production engineering applications. The adhesive is stored in a pressurised vessel and pressure is applied to force the adhesive to the dispensing tip. There are two distinct variations of the pressure-time method ... 

With these pressure-time systems, any variation in adhesive viscosity will lead to irregular dispensing. 

A method sometimes used is to check the pressure-time response of the adhesive dispense valve. 

Figure 10.2 shows a typical pressure-time output as the valve operates. There is a brief surge in pressure as the valve opens and then this levels out fairly quickly until the end of the elapsed dispense time. If there is a bubble in the adhesive or a blocked nozzle the pressure-time curve will be changed (Figure 10.3) and by integrating the area under the curve, the software controller [1] can quickly determine whether the dispensed quantity was within preset tolerances. 

Recently, we have developed a new version of this system, and this is shown in Figure 36. In this system, instead of the capillary being used as a blow-out pipette, the capillaries are used as wash-out pipettes. The capillaries are dispensed in a block, and the solution flows through the capillaries sequentially as they are presented to 2 press plates which apply pressure to the side of the block and maintain contact at all time. This done with 0-rings and has an effect similar to a suction cup sliding along the block. In Figure 36, it shows how 3 capillaries can be simultaneously emptied into three different channels of an autoanalyzer so that three determinations can be done simultaneously. 

A room is used for dispensing flammable liquids. The liquids are expected to have fundamental burning velocities less than 1.3 times that of propane. The room is 9 m long by 6 m wide by 6 m in height. Three of the walls are shared with an adjoining structure. The fourth and larger wall of the room is on the outer surface of the structure. The three inside walls are capable of withstanding a pressure of 0.05 bar. Estimate the vent area required for this operation. 

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