There are two main factors to consider in the selection process for dispensing valves: how different valves work and the fluid characteristics of the material to be dispensed, and how it will be applied. Although this may sound simple, these considerations are complicated by at least seven basic pneumatic valves, fluid characteristics that vary and applications that call for sprays, dots or thick lines. So how exactly do you pinpoint the valve most appropriate for your dispense application?
Pneumatic valves are frequently selected for dispensing jobs, because they cycle fast and work reliably. These valves are powered by timed air pulses that open seals or gates which let a material flow. Faster cycling calls for a 4-way valve, one that needs an initial air pulse to open the seal and another pulse to snap the seal shut after the timed interval.
Spray valves work best with low-viscosity material, typically less than 1,000 cps. To ensure a proper selection, consider the angle of spray permitted by the valve and the control module.
Needle valves are ideal for single micro shot deposits for many fluids, providing the valve is suitably configured to handle the fluid. These valves are often adjustable when rated for the high pressure that lets them handle high viscosity unfilled material.
Diaphragm valves are chosen for adjustable high-cycle dispensing of many low-to-medium viscosity fluids, including glues, solvents and corrosive agents. They are light-weight and easily maintained.
Spooler valves are suitable for high-viscosity pastes, treacle, and gels. These valves operate at high material feed pressures necessary to move high-viscosity fluids. They include a suck-back or snuff-back feature. Spooler valves also work well applying beads.
Poppet valves are similar to spooler valves, with features such as suck-back, but work with low-to-mid-high viscosities. Poppets can apply a smaller shot size than a spooler valve.
Pinch-tube valves work well dispensing aggressive solvent base fluids, because they have disposable wetted parts. These designs work with moderate precision and repeatability within the confines of a pinch-tube design. The pinch-tube valve is often used for two-part, meter-and-mixed applications where the material will cure.
Positive-displacement valves are used in tasks that require highly accurate micro deposits. The choice, either a pneumatic or electrically controlled auger or screw valve, is determined by viscosity and shot size. Pneumatic valves can positively displace lower viscosities than auger valves.
VISCOSITY|
To get a better idea of where each valve works best, consider several industrial fluids and their applications: for instance, a thin material (100 cps) that cures fast, such as cyanoacrylates. The fluid will likely need purging or a valve feature that parks the tip in a material that prevents clogging between dispense cycles. The valve design must separate air-actuated moving parts from the wetted parts.
THIXOTROPY
After viscosity, consider the thixotropic nature of some fluids. These are difficult to pour, but agitate them and they take on a creamy consistency, and move easily against themselves. Be sure to consider how the material is to be fed to the valve, so the fluid is air-free.
AGGRESSIVE FLUIDS
Additional considerations include the harshness of the fluid. For example: Corrosive fluids can contain acids, alkaline, methanol and solvents. Each reacts upon the fluid wet-chamber, seals and O-rings in the wet-chamber. Therefore, a diaphragm or needle valve must be tolerant of the corrosive substances.
Abrasive fillers, often found in pastes, increase conductivity or improve a material’s dielectric properties. For this material type, select a valve with a high-hardness stainless steel wet chamber and tougher seals that stand up to constant abrasion with minimum maintenance.
Anaerobic fluids cure in the absence of air, so the material feed and the valve’s wet chamber must be porous. Valves that best handle anaerobic fluids are likely to be diaphragm or pinch-tube designs, with plastic wetted materials.
