Piezoelectric Inkjet Printing

History

The physics underlying inkjet printing was described by Lord Rayleigh in 1878, when he analyzed how a liquid jet breaks up into droplets. Early continuous-jet work followed decades later; a continuous-jet patent was described by Elmqvist in a US patent application dated 1948 (US 2,566,443). These continuous-jet devices predate and are distinct from piezoelectric drop-on-demand (DOD) printing.

An early drop-on-demand concept using a piezoelectric element is documented in the work of Hansell at RCA. US Patent 2,512,743, "Jet Sprayer Actuated by Supersonic Waves," was filed on 1 April 1946, granted 27 June 1950, and assigned to RCA. It described a piezoelectric disc with a conical nozzle and is widely cited as an early DOD concept, though it was not commercialized.

Practical piezoelectric DOD emerged in the early 1970s. Steven Zoltan patented a tubular single-nozzle acoustical-wave drop generator in 1972 (US 3,683,212, Clevite Corporation) — the "squeeze tube" design. Two further actuator geometries are consistently attributed in the technical literature: a bend-mode (flat, planar diaphragm) design associated with Kyser and Sears at Silonics, and a push-mode design associated with Stemme, whose patent was granted in 1973. Precise inventor precedence for the different actuation modes is debated across sources, so individual "firsts" beyond the firmly documented Hansell and Zoltan anchors are best described as attributed rather than definitive.

Early commercial products soon followed. The Siemens serial character printer of 1977 used piezoelectric DOD and is cited as the first commercial DOD inkjet printer (the model is variously given as PT-80 or PT-80i). Silonics shipped piezoelectric DOD printers using wax- and oil-based inks around 1975.

How it works

A piezoelectric material — typically a lead-zirconate-titanate (PZT) ceramic — changes its dimensions when an electric field is applied across it. A piezoelectric printhead exploits this to eject ink on demand:

• A small ink chamber sits behind a nozzle and is fed from an ink supply.
• A voltage pulse is applied to a piezoelectric actuator that forms or is attached to a wall or diaphragm of that chamber.
• The actuator deforms, rapidly reducing the chamber volume and generating an acoustic pressure wave in the ink.
• That pressure pulse ejects a droplet through the nozzle. When the actuator relaxes, the chamber refills by capillary action, ready for the next drop.

The dimensional change of the piezoelectric element is extremely small, so printheads are engineered to convert a tiny displacement into an effective pressure pulse. Shaping the drive waveform — the rise and fall of the voltage pulse and its timing against the returning pressure wave — controls drop size and velocity and suppresses unwanted "satellite" droplets. Many modern heads are grayscale, firing several small sub-drops that merge to produce multiple selectable drop sizes per nozzle.

Several actuation modes are documented as categories in the literature. In squeeze (tube) mode, a tubular piezoelectric element constricts a capillary (the Zoltan-type design). In bend mode, a piezoelectric layer bonded to a diaphragm flexes to push on the chamber (the Kyser–Sears planar design). In push mode, a piezoelectric element pushes directly on a chamber wall. In shear mode, the electric field is applied perpendicular to the direction in which the material is poled, so the wall shears sideways. Xaar's shear-mode architecture uses a patented "shared wall" arrangement in which each wall is shared between two adjacent channels and can fire both independently, and a "chevron" design in which two oppositely poled pieces of piezoelectric material are fused so that they deflect into a chevron shape to pressurize the channel.

Evolution

In the 1970s, piezoelectric printing used single-nozzle and small-array glass or tube heads, exemplified by Zoltan's drop generator, the Siemens serial printer, and Silonics' oil- and wax-ink machines.

In the 1980s, Epson introduced an on-demand piezoelectric office printer, the IP-130K (launched in Japan in June 1984) and the SQ-2000 (launched outside Japan in October 1984). These used a glass printhead with piezoelectric elements and 24 nozzles, and Epson describes them as the original model for its later inkjet printers. During the same period, thermal (bubble-jet) inkjet arrived as the competing DOD approach, and shear-mode research at Cambridge laid groundwork for what became Xaar.

The key breakthrough came in the early 1990s, when Epson addressed the high drive voltages that piezoelectric heads had required by developing the multi-layer piezo (MLP) element — a capacitor-style stack of many thin ceramic layers (on the order of 20 micrometres each) that produces a large shape change at lower voltage. This enabled compact, high-nozzle-count heads. Epson began Micro Piezo development around 1990 and reached mass production at the end of 1992; the first product to use a Micro Piezo head was the Stylus 800 (MJ-500) in March 1993, followed by the Stylus Color in 1994.

From the 1990s onward, grayscale, shear-mode, and shared-wall industrial heads proliferated through vendors such as Xaar and the Spectra–Dimatix–Fujifilm lineage, and piezoelectric printing spread into wide-format and textile work. In the 2010s, Epson introduced its higher-density PrecisionCore print chips, MEMS-fabricated silicon piezoelectric heads appeared, and the technology expanded into functional and materials-deposition printing.

Advantages

The advantages of piezoelectric inkjet are qualitative and follow from its mechanical, heat-free ejection:

• No heating of the ink. Because a droplet is ejected mechanically rather than by boiling the ink, the fluid is not vaporized. This avoids kogation — the residue buildup from heated ink that limits the life of thermal printheads.
• Wider ink compatibility. Inks do not need a volatile, vaporizable component, so solvent, UV-curable, oil- and wax-based (hot-melt), textile, ceramic, and many functional or specialty fluids can be jetted.
• Durable, often long-lived printheads. Manufacturers such as Epson describe their piezoelectric heads as non-degrading and reusable, with only the ink cartridges replaced, in contrast to thermal designs in which the heater element is repeatedly stressed.
• Fine control over droplets. Tuning the drive waveform gives consistent drop shape and size and supports multiple drop sizes per nozzle (grayscale printing), which aids image quality.

Disadvantages

The trade-offs are likewise qualitative:

• Higher printhead manufacturing cost. The piezoelectric (PZT) material and the more complex fabrication make piezoelectric heads more expensive to produce than thermal heads — a point repeatedly noted in comparisons of the two technologies.
• Ink formulation constraints. Jettable fluids must fall within a fairly strict viscosity and surface-tension window to avoid spray or satellite droplets, which makes ink development demanding.
• Waveform and design complexity. Reliable, satellite-free ejection depends on carefully engineered drive electronics and on timing the drive pulse against the pressure wave inside the chamber.

Modern use

Piezoelectric drop-on-demand is the workhorse of both consumer and industrial inkjet. In the consumer market it underlies Epson's document and photo printers. In industry it is used for wide-format graphics and signage, textile and garment printing, ceramic tile decoration, product coding and marking, label printing, and packaging.

Because of its ink flexibility, piezoelectric printing is also the standard approach for deposition and functional printing. Applications include printed electronics and circuit patterning, color-filter fabrication for displays, and materials-science and laboratory dispensing (for example, Fujifilm Dimatix materials printers). It also underlies some material-jetting 3D printers.

Relationship to other technologies

Within drop-on-demand printing, piezoelectric is the primary alternative to thermal (bubble-jet) inkjet. Thermal heads use a resistive heater to boil a small amount of ink and eject a droplet with the resulting vapor bubble, while piezoelectric heads eject the droplet mechanically with no heat. The practical difference is that thermal heads are cheaper to manufacture but restrict ink chemistry — they need a vaporizable carrier and are subject to kogation — whereas piezoelectric heads cost more but handle a far wider range of inks and tend to have longer head life.

Both drop-on-demand types differ from continuous inkjet (CIJ), which streams charged droplets continuously and deflects and recycles the unused ones. Continuous inkjet is used mainly for high-speed coding and marking.

Relationship to manufacturers

• Epson — developed the proprietary Micro Piezo and later PrecisionCore printhead technologies and is the most prominent consumer and photo piezoelectric brand.
• Xaar — makes industrial shear-mode heads and holds patents on its shared-wall and chevron architectures, commercializing shear-mode research from Cambridge around 1990.
• Fujifilm Dimatix (formerly Spectra) — produces industrial and materials-deposition piezoelectric printheads and is among the makers with their own printhead fabrication.
• Historic and associated names include Siemens (its 1977 DOD serial printer), Silonics (Kyser–Sears planar-actuator heads), and RCA (the Hansell concept).

Related printer families

• Epson Micro Piezo printhead line, first shipped in the Epson Stylus 800 (1993) and the Epson Stylus Color (1994).
• Epson PrecisionCore printhead technology.
• Epson SQ-2000 / IP-130K (1984), an early piezoelectric office printer.
• Xaar industrial printhead families using shared-wall and chevron shear-mode designs.
• Fujifilm Dimatix / Spectra printheads, including materials printers used for research and deposition.
• Siemens serial DOD printer (1977; model given as PT-80 or PT-80i).

Timeline

1. 1878

   Lord Rayleigh describes how a liquid jet breaks up into droplets.

2. 1946 / 1950

   Hansell (RCA) files (1946) and is granted (1950) US 2,512,743, an early piezoelectric drop-on-demand concept; never commercialized.

3. 1948

   Elmqvist continuous-jet patent application (US 2,566,443), distinct from piezoelectric DOD.

4. 1972

   Steven Zoltan patents a tubular (squeeze-tube) piezoelectric drop generator (US 3,683,212).

5. early 1970s

   Bend-mode (Kyser & Sears) and push-mode (Stemme, patent granted 1973) actuator designs attributed; precedence debated.

6. ~1975

   Silonics ships piezoelectric DOD printers using wax- and oil-based inks.

7. 1977

   Siemens ships a piezoelectric DOD serial printer, cited as the first commercial DOD inkjet printer.

8. 1984

   Epson SQ-2000 / IP-130K piezoelectric office printer.

9. ~1990

   Xaar founded to commercialize Cambridge shear-mode research; Epson begins Micro Piezo development.

10. 1992

    Epson reaches Micro Piezo mass production using multi-layer piezo elements.

11. 1993

    Epson Stylus 800 (MJ-500), the first product with a Micro Piezo head.

12. 1994

    Epson Stylus Color launches.

13. 2000s–2010s

    Shear-mode and shared-wall industrial heads (Xaar), the Spectra–Dimatix–Fujifilm lineage, Epson PrecisionCore, and expansion into wide-format, textile, ceramic, and functional printing.