Backpressure Control & Purging
If the liquid/air interface is withdrawn inside the tapered area of the nozzle, the actuation does not have enough energy to move the fluid forward and eject it from the orifice. If the liquid forms a “pool” on the orifice face (left image in the figure to the left) around the orifice, the energy input might be too low to form a drop. For a drop-on-demand ink-jet system to operate, the solution/liquid to be dispensed needs to be flush with the orifice. To maintain the fluid flush at the orifice during operation, the surface tension and hydrostatic pressure forces need to balance.
Typically, maintaining the liquid interface flush at the orifice requires a negative hydrostatic pressure. While this could be achieved by arranging the fluid level in the reservoir to be below the level of the orifice, the reservoir is usually placed higher than the orifice in order to be out of the way. This is most often the situation when printing when the reservoir has to clear the substrate that is printed. When the fluid level is above the microdispenser orifice, vacuum is applied to the reservoir volume above the liquid to compensate the hydrostatic pressure from the fluid column. If the reservoir (and liquid free surface) can be adjusted under the microdispenser’s orifice, the reservoir volume above the liquid can be vented to the atmosphere; in this case, the liquid is maintained flush with the orifice by adjusting the reservoir level.
In addition to maintaining the solution flush with the orifice, it is practical to be able to apply pressure in the reservoir. The pressure either fills the tubing and the device after a reservoir fill-up, clears out contamination (smaller than the orifice) or air bubbles, or recovers dispensing in case there is a material dried out at the orifice. When pressure is applied, there is a stream of fluid that is ejected from the orifice (right image in the figure to the left).
If the fluid is maintained flush with the orifice using the height of the reservoir, the reservoir and its adjustment range should allow a position where the liquid level is 2-3” below the orifice. If the liquid is maintained flush with the orifice using vacuum, an accurate vacuum regulator should be employed. The vacuum would be several inches of water – the height of the fluid level above the orifice + 1-2” water column.
Applied pressure for purging depends on the orifice size and solution viscosity, but generally is between 10 and 25Psi and it should consist of clean and dry air or inert gases.
Some possible configurations for backpressure
The simplest configuration consists of a syringe reservoir mounted on a vertical post that can be lowered or raised. The syringe is vented during normal operation and is set at a height that makes the fluid is flush at the orifice - typically the fluid level is 1-2” under the orifice. For purging, pressure can be applied to the space above the fluid in the reservoir. This can be accomplished with a switch/valve that vents to the atmosphere in the non activated position. For any valve used for purging it is important that the pressure transients allow the transition from purging to operation without causing the liquid-air interface to pull back in the nozzle or causing the fluid to start dripping.
In another configuration, the reservoir is fixed (can be above the orifice) and vacuum is applied to the space above the liquid to maintain the liquid flush with the orifice. This configuration requires a vacuum regulator with fine control. An accurate gauge makes it easier for the user to return to previously used operation conditions. Considering that the set point for the applied vacuum is in the range of 2-3” water column, the vacuum regulator and gauge should be capable of setting / reading within 0.1” water column of the desired value. Purging requires the pneumatics circuit described at the simplest configuration described aboe; in this case the switch needs to toggle the reservoir space above the liquid from vacuum/backpressure (during operation) to pressure (during purging). Switching has similar requirements in returning to the equilibrium condition after purging.
MicroFab’s basic pressure control unit incorporates the fine vacuum & pressure regulator, the gauge (vacuum & pressure), and a valve to switch between vacuum/backpressure to pressure for purging; adjustment and switching is done manually. Some models of the pressure control unit use a computer controlled pressure regulator and manual or computer controlled switching. The capabilities are further discussed on MicroFab’s web site under Products/Pressure & Temp. Control.