Microdispensing is the technique of producing liquid media dosages in volumes of less than one microlitre. The continuing miniaturization in almost all technical areas creates constant challenges for industry, development and research facilities. Microdispensing is one of those challenges. Ever smaller amounts of adhesive, liquid, oil, grease and a multitude of other media must be dispensed reliably and accurately in dosage and placement with short cycle times. The precise positioning and quantity of fluids such as glue, reagents or any other substance has a great influence on the overall quality of a medical device. A few examples are:
Micro-dosing systems with a quantity as small as 50 picolitres Volumetric systems for use with adhesives and spraying systems for silicone coating needles and other surfaces Microdispensing is also used in non-medical applications, like on-demand soda flavoring (the Coca-Cola Freestyle and Pepsi Spire), inkjet printing, and 3-D printing.
Dispensing techniques There are two basic types of dispensing techniques: classic contact dispensing and non-contact dispensing.
Contact dispensing In contact dispensing, the drop forms at the exit of a nozzle, and is deposited by contact, while the drop is still on the nozzle. The technique is as old as the wish to divide a medium, stored in a big container, into smaller amounts. A good example for this is applying adhesive with a tube: To apply the adhesive requires contact between the tip of the tube and the part for the bead of adhesive to be transferred. This method has disadvantages:
Slow dispensing Part has to be touched Part could be damaged Adhesive forms threads Adhesive is not in the expected place Adhesive amounts are difficult to reproduce Despite all of these disadvantages, contact dispensing is still used in the majority of automated processes today, because of:
A lack of knowledge regarding non-contact dispensing systems Few manufacturers for non-contact dispensing systems No direct access to dispensing area (e.g. undercuts) Medium can not be dispensed without contact Dispensing without stress to Medium is possible More precise dispensing especially at bead dispensing Simple to clean in most cases
Typical technologies for contact dispensing Gear pump
high-frequency pulsation high performance always valves no solids possible Pressure-time systems
many components pragmatic evaluation of quantity and control add. power source: air volume flow depends on pressure, time and temperature
Non-contact dispensing (Jetting) In non-contact dispensing, the drop also forms at the end of a nozzle, but far enough away from the target area that the drop separates from the nozzle before it hits. This, too, is a very old technique, as old as squirting liquid from a tube. Because of increasing requirements in regards to cycle time and accuracy in almost all areas of production, non-contact dispensing is constantly gaining importance. A good example for this is the attachment of very small electronic parts (SMD parts) onto printed circuit boards and substrates. For this, the part carrier only needs to be positioned in one plane - after that the adhesive can be transferred without contact. The following examples show the advantages of non-contact dispensing:
Removal of a feed motion to the part Time saving through ejection of adhesive No contact with part (no damage) Even spread of adhesive topography independent of part topography and surface structure Non-contact dispensing can be divided in two different methods:
Jet-forming dispensing Dynamic drop dispensing
Jet-forming dispensing Jet-forming dispensing exists when the flow velocity of a medium at the nozzle exit is high enough that the effects of gravitation and surface tension on the separation of the fluid from the nozzle are of secondary importance. This state is characterized by the Weber number:
W e = ρ v 2 D σ {\displaystyle \mathrm {We} ={\frac {\rho v^{2}D}{\sigma }}}
where
The physical border line between drop- and jet-forming is around a Weber-number of 8. At this point the dynamic pressure of the flowing medium exceeds the pressure from the surface tension of the drop, which therefore sticks to the nozzle. This transitional stage can be demonstrated at a water tap by gradually increasing the flow, going from the dropping status until a continuous water jet has formed. The Weber-number in this case is, however, clearly above 8, because of the jet exit conditions of the nozzle. By using the Weber-number, the theoretical lower limit of the mass flow can be found for the jet-forming conditions. In actual applications, to assure a safe dispensing process, the real Weber-numbers chosen should be between 20 and 50. For a calculated estimation of the fluid flow velocity in the nozzle, for fluids with Newtonian flow behavior, the formula for capillary fluid flow according to the Hagen–Poiseuille law has been proven.
Q = Δ P π r 4 8 μ L {\displaystyle Q={\frac {\Delta P\pi r^{4}}{8\mu L}}}
To avoid atomizing of the fluid at the nozzle exit, the fluid flow in the nozzle must be laminar, which is the case as long as the Reynolds number (Re) of the nozzle is smaller than the critical Reynolds-number of the nozzle:
R e < R e c r i t {\displaystyle Re<Re_{crit}}
Reynolds-number of the nozzle:
R e = ρ v D μ {\displaystyle Re={\frac {\rho vD}{\mu }}}
Critical Reynolds-number of the nozzle:
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