Pressure-time dispensing system

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. 

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

A wide variety and numerous suppliers of automated dispensing equipment are available as listed in Table 4.22. An important consideration in selecting an automated, contact-type dispensing system is the pump used in the system, of which there are three types time-pressure, auger, and linear piston or positive displacement. " ... 

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. 

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. 

In the class of flow boundary condition-based devices predominantly the flow is generated by a fixed volume displacement within a certain period of time which leads to a preset volume flow. The pressure inside the system follows from the Navier-Stokes equation and depends on the geometry and the liquid properties. Feedback from the pressure gradient that builds up to the volume flow is neglected in this case. Such a volume displacement can be achieved by displacing a membrane or a piston or by squeezing a tube or a capillary by a very powerful actuator that is not affected by the pressure that is built up internally in the dispensing system. 

A heated dispensing system is used here for illustration, however, many of the system variables are equally relevant for cold dispense systems. In a typical heated sealer application (Fig. 1), bulk unloader A would be the on-line unloader. The on-line unloader is the unit currently heating the material to application temperature and pumping the material to the changeover manifold at the required temperature, pressure, and flow. Bulk unloader B is the standby unloader and will usually be at a setback (reduced) temperature to minimize the amount of time the sealer is at application temperature while ensuring that minimum time is allotted to come up to full application temperature when commanded from the system controller. 

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 ... 

The syringe dispense system is a very popular system and ideal for adhesives and sealants packaged in standard syringes. The principle is that a timed pulse of air is applied to the back of the syringe and this then forces the adhesive through the nozzle tip. The quantity dispensed is varied by the pressure applied, the nozzle diameter and the time of the air pulse [2]. Figure 8.8 shows a typical syringe dispenser. 

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

The dispensing system may also be the cause of bubbles any liquid will absorb air in solution when it is pressurised and this air is released in the form of bubbles when the liquid adhesive returns to atmospheric pressure. The best example of this is a bottle of lemonade. The lemonade appears perfectly clear until such time when the top is unscrewed and the pressure is released and immediately bubbles will appear... 

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. 

The chemical systems for RIM all have one characteristic in common they require a RIM machine to convert liquid raw materials into quality plastic products. Assuming a properly formulated chemical system, the quality of the end product results from the ability to measure, control (Chapter 3), and adjust temperature, ratio, pressure, and other essential process parameters of the RIM dispensing machine. Such exacting control leads to a reduction in start-up time, minimal rejects and touch-up work, reproducible product quality, and the ability to pinpoint changes in product properties. 

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