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The spool valve (Fig. 3c) is a variation of time-pressure technology that offers higher precision and speed than the basic syringe or diaphragm valves, and can be used with higher pressure. The spool valve meters material flow with a spool that is seated in a circular, fluid outlet seal. The spool is attached to a piston and when it is cycled on, the spool pulls out of the valve seal and the pressurized material flows to the dispensing tip. When shut off, the spool moves back into the seal, and the motion of the spool pulling back through the material causes a "suck back" action. The spool controls the flow of material to the tip, and the longer the spool is in the open position and the higher the pressure on the material, the greater the quantity dispensed.
The spool valve is best used for dispensing lines of material and fills. It is not accurate in dispensing material dots. It excels in dispensing homogeneous materials of medium to high viscosity One area where it is quite effective is dispensing room temperature vulcanizing fluids. It is also used for gasketing and sealing applications, or wherever bulk dispensing is required.
Needle valve technology adds a valve that has a sealing element consisting of a long, tapered needle and cone-shaped seat (Fig. 3d). In most cases, the needle sits inside the tip. Material is dispensed by lifting the needle out of the tip, or valve seat. The farther the needle pulls back from the tip, the longer the needle is held out of the tip, and the higher the pressure applied to the material, the greater the quantity of material dispensed.
The needle valve offers more volumetric accuracy and speed than the other variations of time-pressure technology because the action of the needle provides better metering of the fluid flow, particularly when used to dispense low-viscosity materials. It handles the dispensing of lubricants and oils far better than the previously mentioned technologies, and often is used for medium-precision patterns and fills in applications such as gasketing and sealing.