192 Philips tech. Rev. 40,192-198, 1982, No. 7
Printing by spraying ink directlyon to the paper is not a new idea. Indeed, Lord Kelvininvented his 'siphon recorder' in 1873. This device was capable of recording the telegraphsignals sent by cable across the Atlantic. The ink-jet principle is being applied today for print-ing out computer results. In its latest development, known as the DaD principle (Droplet OnDemand), the droplets are impelled directly at the paper on the receipt of control signals. Theauthor has improved the design of the extremely small ejector nozzles in such a way that theink droplets are always ejected in exactly the same direction. This gives much better printing.A t the same time technologies have been developed for making these special nozzles econom-ically. An improved understanding of the dynamic behaviour of the ink in the exit channel hasenabled us to double the speed of printing the characters.
For some time computer printers have been on themarket that print the characters directlyon the paperby 'shooting' ink jets or droplets at it. Since they con-tain few parts moving at high velocity, these printersare quiet in operation and are usually reliable.
Their precursor was an ink-jet oscilloscope, devel-oped in the sixties, that could record high-frequencysignals directly on paper. This was done by giving theink jet an electric charge and deflecting it in an electricfield [11. The same principle is used in a printer intro-duced in 1976 for word processors and computers.The disadvantage of the system employed in thesemachines is that the ink particles that are not chargedand deflected have to be intercepted and returned tothe system by means of a pump and filters.
The DaD principle (Droplet On Demand) does nothave this disadvantage. In this system a droplet isejected through a fine aperture and applied directly topaper, without deflection, on receipt of an electricalcontrol signal. The droplet is generated by a pressurewave in the fluid [21, produced by applying a voltagepulse to a piezoelectric ceramic [31. There are nowvarious printers that work on this principle, but they
Dipl-Phys. M. Dring is with Philips GmbH Forschungslaborator-ium Hamburg, Hamburg, West Germany.
are expensive to manufacture and therefore unsuitablefor semi-professional use. The quality of the charac-ters printed with some of them also leaves somethingto be desired.Printers operating on the DaD principle have for
some years been the subject of investigations at thePhilips laboratories in Hamburg. Wehave investigateda device that generates the pressure wave by means offlat piezoelectric plates, as discussed in this article.The development of a special nozzle has made it pos-sible to eject the droplets with precision in the samedirection. This has substantially improved the qualityof the characters produced by the print heads. Technol-ogies have also been developed for the economicalmanufacture of these nozzles, which have an insidediameter of only 50 urn. A print head based on ourinvestigations is used in the P2131 printer developedfor the Philips P2000 microcomputer. As a result ofimproving the dynamic characteristics of the printingsystem it will be possible to increase the speed offuture systems to 6000 droplets per second. This istwice the speed of current DaD printing systems.In the following we shall first consider the principle
of the print head and the mechanism of droplet for-mation, and then look at the ejector nozzle and
Philips tech. Rev. 40, No. 7 INK-JET PRINTING 193
Fig. 3. Filling the pressure chamber with fluid. Since the pressure chamber is shaped like a flatcone and the fluid feed is radial, an air bubble is trapped in the chamber, and is subsequently ex-pelled along the connecting channel Ch (see fig. 2).
methods of making it. Next some practical types ofprint heads will be discussed. The article concludeswith a description of the measures that can be takento achieve a considerable increase in printing speed.
Principle of the print head
The most important part of the print head is thefluid-pressure generator, in which the disc of piezo-electric ceramic PXE (seefig. 1) is the energizing com-
Fig. 1. The fluid-pressure generator. PXE plate of piezoelectricceramic. El and E2 electrodes consisting of evaporated metal films.M metal plate. F fluid filling the system. V direct voltage appliedacross E1 and 2, causing flexure of the bilaminar plate consistingof PX and M and setting up a pressure wave in the fluid.
Fig. 2. Part of the print head, built in a sandwich construction.B metal body in which the pressure chambers PC have beenrecessed. P plastic plate with connecting channels Ch. NP nozzleplate with nozzles N. D ejected droplet. C common fluid-feed chan-nel. See fig. I for the other symbols.
ponent. Attached to the upper and lower faces of thedisc are two electrodes El and E2, consisting of evap-orated metal films. The disc is cemented to a metalplate M, which is in contact with the fluid F. When adirect voltage V is applied between the electrodes, thedisc becomes thicker or thinner, but aradial contrac-tion or expansion also occurs [4l. The result is that the
combination of PXE and M, called the bilaminarplate, flexes as shown in an exaggerated way in thefigure. This flexing sets up a pressure wave in the f uid.
Fig. 2 shows how the pressure generator is mountedin the print head, which is a sandwich constructionconsisting of the metal plate M, the body B with pres-sure chambers PC, the nozzle plate NP with nozzles Nand the plastic plate P with communicating channelsCh. The design allows several nozzles to be placed sideby side, each with its own pressure chamber. Eachpressure chamber PC is connected to the commonfluid-feed channel C.
For sufficient pressure to be generated in the fluidthere must be no air bubbles in it. The pressure cham-ber is therefore shaped like a flat cone and has a radialconnection to the feed channel C. When the pressurechamber is filled, the capillary action of the gap at theedge of the pressure chamber causes the fluid to flowtangentially into the pressure chamber (seefig. 3). Thetwo fluid flows meet at the other side of the pressurechamber, so that a volume of air is enclosed in thecentre of the chamber. This air is subsequently ex-pelled through the channel Ch and the nozzle.
When a short rectangular voltage pulse is applied tothe electrodes of the piezoelectric plate a pressurewave is created in the fluid. The pressure wave travelsthrough the fluid into the channel Ch (see fig. 2), sothat the fluid at the nozzle N is accelerated anda column of fluid is ejected through the nozzle (seefig. 4). When the fluid has returned to its initial state,the ejected fluid column contracts and becomesseparated from the ft uid in the nozzle. The ejectedfluid then forms a droplet, whose velocity depends onthe energy contained in the voltage pulse.
 R. G. Sweet, High frequency recording with electrostaticallydeflected ink jets, Rev. sci. Instr. 36, 131-136, 1965.
 In the rest of this article the term 'fluid' will be used instead of'ink' .
 E. Stemme and S.-G. Larsson, The piezoelectric capillaryinjector - a new hydrodynamic method for dot patterngeneration, IEEE Trans. ED-20, 14-19, 1973.
 J. van Randeraat and R. E. Setterington (eds), Piezoelectricceramics, Philips Application-Book , 1974.
Two kinds of energy play a part in the formation ofa droplet. When the fluid leaves the nozzle, it containsa quantity of energy. Some of this energy - the sur-face energy - is used for generating the droplet. Theresidual energy is the kinetic energy in the droplet.
M.ORING Philips tech. Rev. 40, No. 7
the area and volume of a sphere and introducing thedensity. A plot of these relationships is given infig. 5.A value of 50 X 10-3 Nim has been taken for the sur-face tension (this value relates to ordinary inks at20C) and the droplet velocity is taken as 2 mis (a
Fig. 4. Ejection of droplets through the nozzle. The photographs were made by stroboscopicallyilluminating the nozzle at a droplet-ejection rate of about 1000 per second. Although each pictureis formed by superimposing about 100 separate images, the definition is sufficient and shows thehigh stability of the ejection process.
Fig. S. Kinetic energy E and surface energy Es per droplet, as afunction of droplet diameter d. The surface tension is SO x 10-3 Nimand the droplet velocity 2 mis. At a droplet diameter of ISO urn thetwo energies are approxirnately equal.
The surface energy Es required for forming thedroplet surface is
Es = aA,
where A is the area of the surface and a is the surfacetension. The kinetic energy Ev: of the droplet is
ti; = ~ms Vd2,
where me is the mass of the droplet and Vd its velocity.The energies Es and E; can be expressed as a functionof the droplet diameter d by using the formulae for
practical value). The figure shows that the two ener-gies are identical at a diameter of about 150 urn. How-ever, we want to use droplets with a smaller diameter,determined by the nozzle diameter. This means thateach droplet will always have a surface energy greaterthan the kinetic energy.
It can be seen from fig. 6 that droplet formation isstrongly affected by surface effects. If the emergentfluid wets the area surrounding the nozzle asymmetrie-ally, the droplet is dragged back on the side where thewetting is greatest and is deflected in that direction. Inthe extreme case the droplet does not break away at
Fig. 6. The effect of wetting the area surrounding the aperture. Ifthe surrounding area is wetted asymmetrically, the droplet is de-flected as it leav