Air pressurizes a syringe that feeds material to a motorized auger screw which rotates and forces the material from the dispensing tip. However, the accuracy of any specific valve model is dependent on the precision of its construction. This technology is best used for dispensing beads and lines of material, but the rotary screw valve can dispense dots with satisfactory results for a wide range of applications. It is a general workhorse in electronics assembly and PCB production, excelling in applications such as glob topping, underfill for flip chip applications and gasketing. The rotary screw valve does a far more accurate job than any of the previous valves, and works well with both adhesives and solder paste.
Positive displacement is available in two basic technologies.
The actual models available differ in design by individual manufacturer, but the key to the applications they are used for stems from which method activates the dispenser. The actual volume of material dispensed is controlled by the adjustable displacement of the piston entering the chamber. Positive Displacement Pump Motor-driven positive displacement technology is relatively new. It works by feeding material under low pressure from a holding container into a small dispensing chamber. The feed mechanism typically differs by manufacturer, but this does not affect the actual dispensing process, which uses the same piston technology.