Pumps & Valves

IMPROVING INDUSTRIAL DISPENSING WITH PNEUMATIC VALVES

Dispensing ValveThe simple pneumatic dispensing machine is generally low 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 size 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 hand-held 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.

Dispensing Valve
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 the following benefits:
1) Increased accuracy – critical applications in areas such as microelectronics or medical devices requiring higher levels of accuracy and repeatability, validation and fine tolerances.
2) Challenging materials can be handled, e.g. Cyanoacrylate adhesives (very low viscosity) and thermally conductive RTV silicone adhesives (very high viscosity).
3) Volume – the largest dispensing barrels are typically 55ml in volume. For higher daily volumes, some form of larger reservoir is needed.
4) Automation – a valve can be fixtured to a robot and provide better stability than a dispensing barrel.

Material Feed
The material to be dispensed is presented in a reservoir, generally pressurised to provide flow. Very low viscosity materials (e.g. solvent) can be gravity fed but very high viscosity materials (e.g. grease) will require an extrusion pump to achieve flow. Dispensing barrels and cartridges can be used to feed dispensing valves – e.g. RTV silicones sold in a 310ml cartridge can be fed direct from their packaging. Other reservoirs take the form of pressure pots of 500ml to 10 litres or more.

Valve Operation
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 cut off of material flow, either by spring or reciprocal air pulse. Some designs inherently “suck-back” the material, reducing the “string” effect.

Valve Types and Selection
The main material characteristics influencing valve selection are:
1) Viscosity – very low viscosity liquids might be dispensed with a diaphragm valve or a needle valve, very high viscosity ones, like temporary solder mask, may need a spool or poppet valve.

2) Cure – with materials which cure quickly or are easily catalysed, consider using a valve with a disposable fluid path (pinch tube valve) one which is both chemically inert and very low wetting (diaphragm valve). Light sensitive materials (e.g. UV curable adhesives) will require light-tight valves, fittings and hoses.
3)  Reactivity – with high solvency or corrosive materials, metal parts might need to be stainless steel. Polymer parts like seals, valve seats and O-rings might need to be an ultra high molecular weight (UHMW) plastic, synthetic rubber or fluoropolymer elastomer like Viton.
4) Abrasiveness – e.g. fillers to increase conductivity (thermal and/or electrical), chemical resistance, physical robustness or even simple bulk, may also be abrasive, so do consider ease of component replacement.
 

Summary
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. Further information regarding Intertronics’ products can be found at www.intertronics.co.uk.

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