BALLISTIC AEROSOL MARKING

A.R. Völkel*, G.B. Anderson, F.J. Endicott, E.M. Chow, A.V. Pattekar PARC, a Xerox Company, US

ABSTRACT

Ballistic Aerosol Marking (BAM) is a radically new printing concept that combines the simplicity of a direct marking technique such as inkjet printing with the quality and color range/stability of Xerography. This is achieved by using high-speed gas jets as the transport fluid and Xerographic toner particles as the marking material.

In this paper we will provide an introduction into this novel concept, discuss design considerations from CFD and electrostatics simulations, and show some proof-of-concept results from single channel experiments. KEYWORDS Printing, direct marking, micro-fluidics, electrostatic actuation INTRODUCTION

Printing, i.e. putting marks on a substrate, can be accomplished in several different ways. In variable data printing applications, where each page is expected to have different content, two main approaches have dominated the market: direct marking concepts such as ink jet [1] and Xerography [2]. In direct marking approaches ink droplets are ejected on demand. The main advantage of this approach is its simplicity, since the whole imaging logic is reduced to the droplet ejection mechanism, which is typically thermal (bubble jet) or acoustic (piezo inkjet). Big disadvantages of these concepts are either the fact that the bulk of the ink is only used as transport medium and has to be evaporated after deposition, or that the ink is a phase change material, which needs to be heated for jetting and is less resistant to scratching once printed. Also, many water-based inks use dyes as the colorant, leading to short lived color images. Xerography on the other hand uses solid toner particles with pigments as the colorant, which obviates the need for drying or heating and provides long-term stable images. However, the process of placing these toner particles at the right spots in the correct quantity is rather complicated and involves many steps that require intricate control of charged particles in complicated electrostatic fields. TECHNOLOGY OVERVIEW

Ballistic aerosol marking combines ideas from both inkjet printing and Xerography (Figure 1): toner particles are gated into a fast flowing gas jet on demand and delivered to the marking substrate. The use of a gas jet removes the need for drying or heating the ink during the process, while the use of toner particles results in the same image quality and stability as that of Xerographic prints. Arrays of channels and the toner gating mechanism can be fabricated using standard MEMS techniques and integrated onto a single print head chip, allowing for the same compactness of other direct printing approaches. Depending on the bulk properties of the toner particles a

post fusing step (maybe integrated with a transfer roll) may be needed to complete the print.

The innovative parts of this approach are: (1) the design of the channels for the high speed gas jets (need well-controlled toner inlet section and well-collimated jets from channel exit to substrate); (2) the implementation of a gating mechanism that is robust enough to handle the occasional toner particle with the wrong electric charge; and (3) a marking system that delivers the toner particles in compact well defined pixels at high capture efficiency. In the following we will address each of these innovation areas separately.

Premixed true color

600…900  spi...

P = 1 Atm,before exit P =< 72 Atm

Focused high velocity aerosol jet (array, continuous)Fuse on impact or fuse in flight (particle V/A ratio => 0, m=>0)

Electrostatic toner gate

Venturi toner feedor

pressure forced feed

K Y

Small particle toner(on demand)

Laval type expansion pipe

C M

CO2

Fully expand before exit=> focused over many exit diams.

Figure 1: Schematic diagram of BAM device.

Locally

fluidized bedOptional PVDF strip:

Toner flow & tribo

Toner

Tribo or Moore

Plastic

moulded

part

Riston...

SU-8...

Glass

TFT drivers

addressing logic

Gas inlet, manifold

Address electrode

Electrostatic  ‘tunneling’

Venturi

Modulated

aerosol

toner inlet

Figure 2: Schematic diagram of BAM device as implemented using MEMS technology. The channels for the high speed gas jet can be either edged into silicon, or fabricated in a photo resist such as SU-8. CHANNEL DESIGN

In order to enable the delivery of toner particles with a high speed gas jet at the desired print speeds and resolution the gas jets have to be optimized for several distinct goals. (1) The gas jets have to be able to accelerate the toner particles sufficiently to enable the required printing speed, (2) the toner should be delivered in a well-focused pixel   (e.g.   ≤   41   m for a 600 dpi resolution), and (3) inlets need to be integrated that allow for controlled gating of toner particles into the gas jet on demand. Straight channels with a width constriction (a “Venturi”   nozzle)   promise   to   be   a   good   approach to achieve all three goals (Figure 3).

pape

r / su

bstr

ate

Figure 3: Schematic of micro channel. High pressure air (1-5 atm) is supplied to the channel inlet at the left. A constriction (Venturi nozzle) downstream of the channel causes a sudden pressure drop in the air jet, allowing the introduction of toner particles at close to atmospheric pressure conditions.

x [mm]

pres

sure

[Pa]

(a)

x [mm]

velo

city

[m/s

]

(b)

Figure 4: Pressure profile (a) and air speed (b) along channel (x direction) from CFD simulations for different Venturi neck width wt. The simulated channel is 4 mm long (from x-0 to x=4 mm and has width w and height h of 64 m each. At x=4.5 mm is the target (paper) plane. Simulations

The Venturi structure inside the channels causes rapid acceleration of the flowing air post-constriction, where the toner particles can be entrained with minimal pressure overhead because of a brief drop in the air pressure as the channel returns back to its original width. Figure 4a shows the pressure profile along the center of a channel as

obtained by CFD simulations [Fluent]. Through careful design of the shape of the Venturi structure the pressure at the particle inlet can be designed to be either above, at, or below atmospheric conditions. The gas jet accelerates as it passes through the Venturi structure and can reach supersonic conditions at the smallest channel cross-section at sufficient inlet pressures (Figure 4b). Depending on the cross-sectional area of the channels the flow beyond the Venturi structure changes back to normal straight channel flow. The simulation results shown in Figure 4 are for 4 mm long channels with a cross section of 64 x 64 m2. The Venturi neck is 2.5 mm downstream from the channel inlet and the pressure minimum after the neck can be as wide as 100 m, which gives sufficient latitude to integrate the toner inlet.

We have shown experimentally as well as through detailed modeling work (Figure 5) that the jets stay well-collimated and maintain their high speeds for up to a few millimeters from the exit.

10 mm (b)

(a)

Figure 5: Collimated jets: (a) velocity vectors plot of air jets exiting the channel on the left and hitting the paper plane on the right (from CFD simulations), (b) density contrast visualization of He jets in air as they exit the channels on the left.

Particle dynamics inside the high speed jet is dominated by the drag force and the equation of motion at intermediate particle Reynolds numbers is given as

, (1)

where m, , d are the particle mass, velocity, and size, respectively and m, f, u are the gas viscosity, density, and speed, respectively. From equation (1) we can estimate the speed of the toner particles at the paper plane and, by including reflections off the side walls, the shape and size of the printed pixels.

As can be seen from Figure 6 the entrained particles rapidly accelerate to the speed of the air jet prior to exiting the channel. Because of the parabolic flow profile,

the end speed of the particles depends on the location of the particle within the channel, and the velocity distribution can be reduced by placing all the particles into the center of the channel. The resulting pixel from this set of toner particles is also shown in Figure 6. In this particular case the pixel covers about the same area as the channel cross-section. However, depending on the toner feed conditions, the particle spread can be much larger.

toner inlet

channel exit

x [mm]

parti

cle

velo

city

[m/s

]

(a)

(b)

Figure 6: (a): Velocities of toner particles after release from an inlet into the channel. The spread of velocities of the different particles is due to the different initial locations on the cross-sectional area of the inlet. (b): Pixel shape of the set of toner particles. The dashed line shows the projection of the channel cross-section on the paper plane. The pixel is about the size of the channel cross-section. Channel fabrication and testing

The fabrication of the devices involves standard micro-electromechanical system (MEMS) processing techniques including photolithography for patterning the channels and Venturi structures, followed by deep-reactive ion etching (DRIE) for creating the desired structures in silicon (Figure 7). Back-side etched holes in the silicon serve as the post-Venturi toner particle inlet ports. Alternatively, the channels can be etched into a photoresist that allows fabrication of structures with the required (high) aspect ratio, such as SU-8. The chips are then sealed through anodic bonding with a cover glass.

Inlet ports for air are drilled into the cover glass and connected to controlled pressurized air (Figure 8b).

Tonerinlets

ChannelsVenturi neck

100 m

Figure 7: SEM image of BAM channel array on Si waver with Venturi structures and toner inlets.

Test cells used for device testing and performance

characterization are shown in Figure 8. For continuous operation (i.e. without a gating mechanism that allows printing of individual pixels), a fluidized zone of the particles is created under the inlet ports using piezo actuation and low pressure air. This   sustained   ‘cloud’  of  particles in the test chamber and the continuous air jet created using the high pressure air enables continuous operation of the device for testing and characterization under various actuation pressures and corresponding particle speeds and throughputs. Figure 9 shows lines printed with such a device onto an intermediate roll before transfer to paper. The line width and quality compares favorably with that of a commercial Xerographic Machine (Xerox DC12) at comparable resolution.

toner

Figure 8: Test cell used for characterizing device performance: [a] Side view; [b] Top view

Line width = 116 m

DC12 Print On CX Paper

50 X

Line width = 125 m

Transfused 50 X

(a) (b)

Figure 9: Comparison of lines printed with a (a) Xerographic printer (Xerox DC12) and with (b) BAM print head using a transfuse role. GATING Xerographic toners are optimized for tribo charging, making them easily accessible for an electrostatic gating mechanism. The main concern, though, is the propensity

of the tribo charging mechanism to yield particles with either positive or negative charge. To minimize the impact of   “wrong”   sign   toner   on   the   image   quality,   we   have  chosen to implement the electro static gate using a traveling wave (TW) grid [3, 4] (Figure 10). Here, a traveling voltage pulse either pushes (one charge) or pulls particles (the other charge) along the same direction on the grid. By operating the grid in one or the other direction toner of any charge is either gated into the BAM channel or pushed back into the reservoir.

(a)

0V

-100V

0V

-100V

0V

-100V

1

2

3

(b)

Figure 10: (a) Schematic diagram of a 3-phase traveling wave grid for toner gating and transport. The blue arrows indicate air flow directions, the red arrows the toner gating direction (b) Voltage pulses applied to the different electrodes to drive the grid.

We have implemented and tested traveling wave grids by laser-drilling holes in multilayer sheets made with alternating dielectric and conductive materials. Figure 11 shows a 10 by 10 array of vertical toner movers that are laser drilled through a stack of 4 gold coated Kapton sheets. The aperture holes are 50 m in diameter, and allow continuous transport without significant blocking (500 V pulses at 10 Hz).

Figure 11: Left: 10 by 10 array of 50 m laser-drilled vertical traveling wave grid. Right: after continuous operation for several minutes.

To integrate the gating mechanism between the toner

reservoir and the BAM channels, we can use a similar

design as used for the continuous print head (Figure 8): A fluidized toner cloud in the reservoir feeds the particles to the gating mechanism, which is directly coupled to the BAM channels after the Venturi neck. Besides creating the fluidized bed with a piezo we can also use a lateral TW array as shown in Figure 12. This approach decouples the main toner supply from the fluidized bed and opens up opportunities for premixing toner particles before feeding them to the BAM channels.

BAM chipN2

Vertical ATOM

Lateral ATOM

Direction of toner motion

Figure 12: Schematic diagram on how to couple a fluidized bed of toner (generated by a lateral TW array) through a vertical TW gate to the BAM channel.

SUMMARY We have shown a novel printing technology that

combines the speed and simplicity of direct marking approaches like inkjet printing with the image quality and persistence of Xerography. Through a combination of simulations and proof-of-concept experiments we have demonstrated that this is a viable approach to put marks on paper using a compact print head design.

Because of the separation material from the delivery fluid, this technology can be used for many other marking needs, where particles (including droplets) need to be delivered at precise locations and precise amounts. This can range from printed electronics to 3D functional materials and bio medical application such as transdermal drug delivery. ACKNOWLEDGEMENTS

This project has not been possible without the help of a lot of co-workers, including Eric Peeters, Philip Floyd, Jaan Noolandi, Meng Lean. We also like to thank our colleagues Karen Moffat, Rick Veregin, and Maria McDougall from the Xerox Research Centre of Canada, who provided us with many samples of custom made toners for this project.

REFERENCES [1] Singh, M., Haverinen, H.M., Dhagat, P., Jabbour,

G.E.,  “Inkjet  Printing  – Process  and  its  Application”,  Adv. Mat., 22, 673-685, (2010).

[2] Schein, L.B. “Electrophotography  and  Development  Physics”, Springer Series in Electrophysics. 14. Springer-Verlag, Berlin (1988).

[3] Schmidlin, F.W., “A   New   Nonlevitated   Mode   of  Traveling   Wave   Toner   Transport”,   IEEE Trans. Industry Appl., 27, 480-487 (1991).

[4] Schmidlin, F.W., “Modes   of   traveling   wave   particle  transport  and  their  applications”,  J. Electrostatics, 34, 225-244 (1995).

CONTACT

*A.R. Völkel, tel: +1-650-8124198; avolkel@parc.com