192 Philips tech. Rev. 40,192-198, 1982, No. 7

Ink-jet printing

M. Döring

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.

Introduetion

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. Döring 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.

Droplet ejection

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.

[1] R. G. Sweet, High frequency recording with electrostaticallydeflected ink jets, Rev. sci. Instr. 36, 131-136, 1965.

[2] In the rest of this article the term 'fluid' will be used instead of'ink' .

[3] E. Stemme and S.-G. Larsson, The piezoelectric capillaryinjector - a new hydrodynamic method for dot patterngeneration, IEEE Trans. ED-20, 14-19, 1973.

[4] J. van Randeraat and R. E. Setterington (eds), Piezoelectricceramics, Philips Application-Book , 1974.

194

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.OÖRING 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 at20°C) 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.

E

t10

100 150p,m-d

Fig. S. Kinetic energy Eç 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 leaves the aperture. The drawings are based on photo-graphs made in the same way as those in fig. 4.

~~~ö~Q

Fig. 7. The modified nozzle with tubular mouth, compared with anozzle of conventional design. The figure illustrates droplet forma-tion for a low steady fluid pressure. a) With the improved design aspherical bulge forms. b) With a conventional nozzle the surround-ing surfaces are wetted and droplet formation is riot symmetrical.

Philips tech. Rev. 40, No. 7 INK-JET PRINTING 195

all and the fluid remains behind. It will now be shownthat such difficulties with droplet ejection can beprevented by careful attention to the shape of thenozzle.

Design of the nozzle

As we have seen, asymmetrical wetting of the sur-roundings of the nozzle must be avoided. One sharpedge of 90° is not sufficient to prevent the fluid fromspreading over the surface. We therefore designed anozzle in which two such edges are located closelytogether, with the aperture of the nozzle in the formof a projecting tube with sharp edges, as illustrated infig. 7a.

The superior operation of such a nozzle can be seenfrom a comparison of fig. 7a and fig. 7b. With asteady low fluid pressure, the fluid in our nozzle formsa spherical bulge. For a conventional nozzle, how-ever, the surrounding surface is easily wetted, leadingto asymmetrical droplet formation.

Nozzle technologies

If the characters on the paper are to have the desiredresolution, the diameter of the apertures of the nozzlesmust be about 50 urn. To make a nozzle as small asthis with the shape shown in fig. 7a will obviously beextremely difficult. We have however developed twotechnologies for making these nozzles economically.

The stages in the first process are shown in fig. Bato e. A brass plate has holes drilled in it of diametergreater than the final inside diameter of the nozzle.Next, a layer of nickel is applied by a chemical method;this has the same thickness as the wall of the tube (seefig. 7a). A layer of materialof at least the same thick-ness as the nickel layer is then ground away from theunderside of the brass plate. Finally, part of the brassis etched away, producing the desired tubular mouthfor the nozzle. Fig. 8f shows a scanning-electron-microscope photograph of a nozzle made in this way.

The second process is illustrated in fig. 9a and b.A spring-steel plate St with holes in it larger than thenozzles to be formed is placed underneath a nickelplate. Since the spring-steel plate is only elastically de-formed in the process, it can be used several times. Aplastic strip S is placed underneath the spring-steelplate, and the strip S is enclosed by a steel base-plateBP. The plastic strip can also be used several times bysliding it out (in a direction perpendicular to the planeof the drawing). A punch tool is located on top of thenickel plate. The punch, which has a tapering diam-eter, is driven into the nickel plate. Since the spring-steel plate supports the nickel plate and the plasticbehaves rather like a fluid, a hole of the desired special

g

f

Fig. 8. The chemical process for producing the nozzles. a) and b).Holes are drilled in a brass plate. c) A layer of nickel is then appliedby the 'electroless' method. d) The nickel layer, and some of thebrass where necessary, is removed by grinding. e) Selective etchingproduces the desired shape of nozzle. f) Scanning-electron-micro-scope (SEM) photograph of a nozzle produced by this process.Magnification about 600 x .

~~TTt~--G----ffi~4*----P---~~~~

Ni~~~~~~st~~~~~~

~~"-T';rl-- 5 --+,L,L~,__~

g b

Fig. 9. The mechanical process for producing the nozzles. A spring-steel plate St with holes in it is placed beneath a nickel plate Ni.A plastic strip S is placed beneath St, and surrounded by a base-plate BP. a) A punch tool is located on top of the plate Ni; thepunch tool consists of a guide G and a punch P. b) The force Fdrives the punch into the plate Ni. At the end of the process a partof Ni has penetrated into S. Because of the supporting action of Stand the fluid behaviour of S a hole without burrs and of the desiredshape is produced in Ni. c) SEM photograph of a nozzle producedby this process. Magnification 600 x .

196 M. DÖRING

shape and free of burrs is punched in the nickel plate.A nozzle made by this method is shown in fig. 9c. Thesharp edge inhibits the wetting of the surrounding sur-face even more than the tube-like end produced by theprevious method. Since no time-consuming drilling isrequired, the nozzles produced in this way are evencheaper.

Practical design of the print head

ln the printing process the print head with itsnozzles is moved across the paper so that it prints lineby line. The desired resolution of the characters deter-mines how many nozzles are required in the printhead [51. It is not usually possible to put all the nozzlesin a single row, for geometrical reasons. Fig. 10 showshow the printing is done when the characters have aheight of twelve dots or droplets. The nozzles arepositioned in two rows of six, offset from each otherby half the spacing in the row (see fig. lOa). Duringthe printing, the appropriate piezoelectric plates in the

o 0 0 0 0 0

o 0 0 0 0 0Q

Fig. 10. Ejection of the droplets by a print head with 12 nozzles.a) Location of the nozzles in two rows of six. b) Formation ofcharacters twelve dots high. Piezoelectric plates in the two rows ofnozzles are energized in succession, depending on the partienlarcharacter required. The droplets strike the paper at an angle equalla that of the resultant of the ejection velocity and the print-headvelocity.

pressure chambers of the two rows are energized insuccession, depending on the particular characterrequired.

Fig. 11 shows the P2131 printer developed by Philipsfrom the results of our investigations, for use with theP2000 microcomputer. Fig. 12 gives a typical print-out obtained with this machine, showing the threedifferent typefaces. The printing speed is 80 standardcharacters per second (one line per second). Since astandard character consists of a matrix of 10 X 12 dots,this corresponds to a rate of approximately 1200 drop-

Philips tech. Rev. 40, No. 7

Fig. 11. The P2131 printer developed for the Philips P2000 micro-computer. Its print head has 12 nozzles and prints characters in amatrix of LOx 12 dots.

NORMAL ABCOEFGHlJKLM @123456789

COND[NSEDA8CDEFGHIJKLM 81234567898

ENLARGED ABCOEF

graPhiCS:. -'·_"'E:3·~lmIJ· ........... .-·-.._... .· -.._ ..........r.· -..J"Ph.......r··-........~~ ..

Fig. 12. Examples of the three different typefaces that can be printedwith the P2131 machine.

lets per second. We shall see that this rate can be madehigher.

For professional purposes a higher resolution isrequired. We have therefore developed a print headwith 24 nozzles in four rows. A photograph of thishead can be seen in fig. 13. The plate with the piezo-electric elements has been removed, so that the 24conical pressure chambers are visible; the connections

'1\' \' f. '/ ,. './ n \/ ft~, •• ,->".' ~et:' '. .,. ;""~~l 0/'\ ft.l,'" ni" I.,

~,~ '.' (to " ft '/." ,/ ft , .......,;o'f, ~.ft .! '. (ft ,/ -,n ,,"ft " "0 ft .. " " '"\. i)

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Fig. 13. Photograph of the experimental print head with 24 nozzlesthat can print characters in a matrix of 24 x 20 dots. The conicalpressure chambers can be seen at the top (the plate with the piezo-electric elements has been removed) and the connections for the inkfeed are on the right. The transparent plastic body contains thechannels to the nozzles.

Philips tech. Rev. 40, No. 7 INK-JET PRINTING 197

for the ink feed can be seen on the right. The channelsthat connect the pressure chambers with the nozzlesare visible in the plastic body.For printers of this type it is important to have the

right combination of ink and paper. The ink in thenozzle must not dry out, but the paper must dryquickly enough to prevent smearing of the characters.A compromise can be found in a type of ink thatattracts moisture from the air and therefore does notdry too quickly. Smearing ofthe characters must thenbe prevented by using a type of paper that absorbs theink quickly. Paper with a high ash content is a fast inkabsorber, but its coarse fibres give large and blurreddots. Paper with a high sizing content, on the otherhand, forms well-defined dots, but does not absorbthe ink quickly. The choice of the correct type ofpaper is therefore a compromise, in which the type ofink is also a factor.

Increasing the printing speed

After a voltage pulse has been applied to the ceramicplate, the pressure in the channel at the nozzle canvary in several ways as a function oftime (seefig. 14).The degree of damping determines the time taken forthe system to return to rest. If the system is criticallydamped (curve 1), the time taken is shorter than if it isunderdamped (curve 2) or overdamped (curve 3). Thenext voltage pulse cannot be applied to the ceramicplate until the system is almost completely at rest. Fora given resonant frequency of the system the maxi-mum droplet ejection rate n is thus highest when thesystem is critically damped. Depending on the vis-cosity of the fluid, critical damping can be achieved bylocally narrowing the diameter of the channel betweenthe pressure chamber PC and the feed channel C (seefigs 2 and 3). This also has the effect of reducingmutual interference between adjacent pressure gener-ators.

p

t ' ..., ....I , ,--,

\ I_t ....,_..''...",/ _'-'-'-__ .__.,

Fig. 14. The variation of the pressure p in the channel at the aper-ture, after application of a voltage pulse to the ceramic plate, as afunction of time t. Curve 1 critically damped, curve 2 under-damped, curve 3 overdamped. For a given resonant frequency ofthe system the highest droplet-ejection rate is achieved with criticaldamping.

/y/

t

Fig. IS. Measured variation of the modulus of the admittance (thereciprocal of the impedance) of the piezoelectric plate in fig. 1, as afunction of the frequency of an applied alternating voltage. On theright are three frequency characteristics of the bilaminar platealone, for three different plate thicknesses, increasing towards theright. The quantities fBI, fB2 and!ss are the lowest natural fre-quencies of the flexure vibrations. The frequency IR is the naturalfrequency of the radial vibrations, and is independent of the platethickness. The three curves on the left were measured for the com-plete system filled with fluid, with the same variation of the platethickness. The quantities jju, fp2 andha are the lowest natural fre-quencies of the flexure vibrations when the system is filled withfluid, sincefsl :!s2 :!sa =Fn :fp2 :ha. The lowest frequency fA ofthe acoustic vibrations is independent of the plate thickness. Thewavelength associated with fA is approximately equal to twice thelength of the channel Ch in fig. 2.

At the same time, to achieve the maximum ejectionrate the lowest resonant frequency of the systemshould be as high as possible. This depends to a greatextent on the resonant frequency of the bilaminarplate in the pressure generator (see fig. I). Three fre-quency characteristics of the bilaminar plate a~onecan be seen at the right-hand side of fig. 15. For a bi-laminar plate whose thickness is increased in threesteps, fBI, fB2 and fB3 are the lowest resonant fre-quencies of the flexure vibrations of the type shown infig. 1. In these three cases the frequency f« for radialvibrations remains constant. The left-hand side offig. 15 shows, for the same variation in plate thick-ness, the three measured frequency characteristics forthe complete system filled with fluid. The lowest reso-nant frequencies [ri, fF2 and jrs originate from theflexure vibrations of the bilaminar plate. The fre-quency fA does not depend on the plate thickness andarises from the acoustic vibration with the lowest fre-quency, the wavelength being roughly equal to twicethe length of the channel Ch (see fig. 2). If we increasethe flexure frequency iF by increasing the stiffness ofthe ceramic plate, iF approaches the frequency fA ofthe acoustic vibrations. The acoustic vibrations deter-mine the dynamic 'behaviour of the system if the stiff-ness of the plate is increased further. Eventually areduction of the length of the channel Ch will give anincrease in fA.

(5) See for example J. Borne, Philips tech. Rev. 29, 205, 1968.

198 INK-JET PRINTING Philips tech. Rev. 40, No. 7

2.5mls

V 2.0

t 1.5

1.0

05

00 5 62 3 4-n

Fig. 16. Velocity v of the droplets generated by a print head as afunction of the ejection rate n. To obtain satisfactory printing, vmust not vary by more than about 50/0. Curve 1relates to the printhead in fig. 13. Curve 2 is obtained when the stiffness of the bi-laminar plate is increased by reducing its diameter. Curve 3 applieswhen the length of the channel to the nozzle is also reduced. A printhead constructed to meet these conditions gives twice the maximumdroplet-ejection rate and its volume is reduced by a factor of six.

The extent to which the maximum droplet-ejectionrate can be increased in this way is illustrated infig. 16,where the velocity v of the ejected droplets is plottedagainst the ejection rate n. When n approximates to aresonant frequency of the system the ejection rate isincreased or decreased. In practice it is found that the

ejection rate should not vary by more than about 50/0from the mean value. Curve 1 shows the variation ofthe ejection velocity ofthe print head in fig. 13. Curve2 is the characteristic of the same system, but with asmaller diameter for the bilaminar plate, which there-fore has increased stiffness. Curve 3 is the character-istic of the system, but now with the length of thechannel to the nozzle reduced as well. The graphshows that the ejection rate can now be increased to6000 droplets per second, which is twice the ratepreviously considered possible with DOD printers. Anincidental advantage of the technology is that thevolume of the print head is reduced by a factor ofabout six.

Summary. A print head of sandwich construction makes it possibleto impel ink droplets at the paper in direct response to controlsignals (the 'droplet-on-demand' (000) principle). Special nozzleshave been designed that eject the droplets with great directionalaccuracy, thus improving the quality' of the printed characters. Twotechnologies have been developed for producing these nozzleseconomically. Some practical print heads of this type are discussed;one of these is used in the P2131 printer developed for the PhilipsP2000 microcomputer. Improvement of the dynamic character-istics of the printing system allows the' droplet-ejection rate to beincreased to 6000 droplets per second, which is twice the ratepreviously considered possible with 000 printing systems.