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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 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.
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.
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
Cover Story Material selection for plastic design.
Major Focus
Automotive applications.
Electrical and electronic applications.
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