Pneumatic valves dispense with accuracy

Edited by the Engineeringtalk editorial team Jan 25, 2008

For industrial liquid dispensing, pneumatically controlled valves are robust, accurate, can be cycled quickly and in many cases can be implemented at low cost with several benefits.

The simple pneumatic dispensing machine is generally low in cost and almost ubiquitous.

They improve the application of materials like single-part adhesives, coatings, sealants and inks, by enhancing a manual process to one which is more controlled and repeatable, while health and safety benefits include less mess, less exposure to potentially hazardous chemicals and fewer RSI issues.

These systems work by filling an industrial syringe or barrel with the liquid material.

Pulses of air then push the liquid out of the other end, usually through a dispensing needle or tip.

The quantity dispensed can be varied by the sise of the needle orifice (from as small as 0.15mm), the pressure applied and the time of the air pulse triggered by footswitch, finger switch or automation.

They may be handheld or mounted on positioning robots or rotary tables.

Less than 1% variance is often easily achievable, and 0.1% not uncommon.

Digital timers further improve accuracy.

For industrial liquid dispensing, pneumatically controlled valves are robust, accurate, can be cycled quickly and in many cases can be implemented at low cost with several benefits.

First, for example, critical applications in areas such as microelectronics or medical devices requiring higher levels of accuracy and repeatability, validation and fine tolerances.

Secondly, challenging materials can be handled, such as cyanoacrylate adhesives (very low viscosity) and thermally conductive RTV silicone adhesives (very high viscosity).

Thirdly, whereas the largest dispensing barrels are typically 55ml in volume, for higher daily volumes, some form of larger reservoir is needed.

Fourthly, a valve can be fixtured to a robot and provide better stability than a dispensing barrel.

Liquid is fed through a hose, to the valve actuated by a pulse of air pressure.

The amount of material dispensed can be varied by the size of the needle orifice, the pressure applied to the material reservoir and the time of the air pulse determined by a pneumatic dispensing controller.

The exception to this type of operation would be a positive displacement valve, such as a screw valve, where precise micro-deposits can be controlled by the rotation of an auger screw.

One of the main advantages of dispensing valves is that they snap close, with a positive cutoff of material flow, either by spring or reciprocal air pulse.

Some designs inherently "suck-back" the material, reducing the "string" effect.

There are quite a few options when it comes to choosing a dispensing valve, and a plethora of materials which might be dispensed through them.

Finding the optimal match is quite complicated - material rheology and reactivity needs to be understood, valve capability identified and proven, and the final outcome defined.