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The 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, repeatable and safe. Thesedispensing 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, the pressure applied and the time of the air pulse. They may be hand-held or mountedon 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 the following benefits:
Increased accuracy for critical applications in requiring higher levels of accuracy and repeatability, validation and fine tolerances.
Handling challenging materials - e.g. very low or very highviscosity.
Volume the largest dispensing barrels are typically55ml in volume. For higher daily volumes, some form of larger reservoir is needed.
Automation a valve can be fixed to a robot and providing better stability.
The material to be dispensed is presented in a reservoir, generally pressurised to provide flow. Very low viscosity materials can be gravity fed but very high viscosity materials will require an extrusion pump to achieve flow. Dispensing barrels and cartridges can be used to feed dispensing valves, some materials can be fed direct from their packaging.
Liquid is fed through ahose, to the valve actuated by a pulse of air pressure. The amount ofmaterial 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 tothis type of operation would be a positive displacement valve, such as a screw valve, where precise micro-deposits can be controlled by therotation 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.
The main material characteristics influencing valve selection are:
Viscosity very low viscosity liquids might be dispensed with adiaphragm valve or a needle valve, very high viscosity ones may needa spool or poppet valve.
Cure with materials which cure quicklyor are easily catalysed, consider using a valve with a disposable fluid path (pinch tube valve) or one which is both chemically inert and very low wetting (diaphragm valve). Light sensitive materials require light-tight valves, fittings and hoses.
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 tobe an ultra high molecular weight (UHMW) plastic, synthetic rubber orfluoropolymer elastomer.
Abrasiveness e.g. fillers to increaseconductivity (thermal and/or electrical), chemical resistance, physical robustness or even simple bulk, may also be abrasive, so do considerease of component replacement.
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 isquite complicated - material rheology and reactivity needs to be understood, valve capability identified and proven, and the final outcomedefined.
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