Positive Displacement Valves Information
Positive displacement valves are precision dispensing devices designed to provide precise fluid dispensation depending on applications. They function by opening quickly, releasing product, and closing again. This process may take place numerous times per second. Positive displacement valves use physical moving force to drive material out of the valve. They are typically used to dispense materials with high viscosities.
Positive displacement valves use one of two mechanical means to distribute their product, either by auger (screw) or a rod or piston. Auger pumps are driven by a feed screw. As the feed screw turns, it forces the media out of the valve. Rod or piston pumps use a moving shaft to drive media towards its destination. In most cases, both of these valve styles use pneumatic power as the driving force, although hydraulic and mechanical drives may also used.
The primary value of positive displacement valves is that they can drive viscous fluids and meter them in very precise amounts. The dispensed output may be in the form of a bead, dot, or fill (also called potting). Bead dispensed product is at a consistent flow rate to allow a bead (or line) of material to be made. The dimension of each bead is uniform, usually metered in mils, or thousandths of an inch. Dot dispensation is a pinpoint type application that is highly precise. Dots are also generally measured in mils. Fill, also known as "potting,” is a dispensation technique that forces a precise volume of media out for accurate filling applications. Glue and other cements are often dispensed in this form in industrial assembly applications. The amount of fill released is measured in terms of shots. Each shot is generally measured in terms of cubic centimeters (cc).
Single component positive displacement valves offer a number of specific advantages when working with viscous fluids. First, their single part construction lacks specific weak points that multiple part devices have. Therefore, they can accommodate the greater pressures and stresses that accompany high viscosity products. Additionally, these positive displacement valves have fewer seam locations that could allow external air ingress that would cause aerobically curable products to harden. The result of this would be product backing up in the system and eventual seizure, leading to production downtime and extensive cleaning.