Micro-Inkjet Printer Head

Presented on April, 2009

One of the highly demanding products from MEMS application is Ink jet printer head which is core part of ink jet printer. In inkjet printer, ink is emitted from a series of nozzles in a print head, spraying drops of ink directly on the paper as the nozzles pass over a variety of possible media, i.e. paper or other specialised materials. The liquid ink in various colours is squirted at the paper to build up an image.

Role of Print Head

• The print head of the inkjet printer scans the page in horizontal strips, using a motor to move it back and forth, as another motor rolls the paper in vertical steps.
• A strip of the image is printed, then the paper moves on, ready for the next strip.
• To speed things up, the print head doesn’t print just a single row of picture elements or ‘pixels’ in each pass, but a vertical row of pixels at a time.
• Most inkjets use thermal technology, whereby heat is used to fire ink onto the paper.
• There are three main stages with this method.

1.The squirt is initiated by heating the ink to create a bubble until the pressure forces it to burst and hit the paper.

2.The bubble then collapses as the element cools, and the resulting vacuum draws ink from the reservoir to replace the ink that was ejected, as illustrated below.

3.The use of heat in thermal printers creates a need for a cooling process as well, which adds time to the printing process.

• Thermal inkjet print heads contain between 300 and 600 nozzles in total, each about the diameter of a human hair (approx. 70 microns).
• These deliver drops which contain around 8 – 10 picolitres (a million millionth of a litre), creating dot sizes of between 50 and 60 microns in diameter.
• Cyan, magenta and yellow inks are normally delivered via a combined print-head.
• Several small colour ink drops – typically between four and eight – can be combined to form dots of variable size, which gives inkjets a bigger palette of colours and smoother images.
• Inkjet printing mechanisms can be categorized as either continuous inkjet or drop-on-demand inkjet. Drop-on-demand inkjet printers selectively eject droplets of ink toward a printing media to create an image.
• Such printers typically include a print head having an array of nozzles, each of which is supplied with ink.
• Each of the nozzles communicates with a chamber which can be pressurized in response to an electrical impulse to induce the generation of an ink droplet from the outlet of the nozzle.
• Many such printers use piezoelectric transducers to create the momentary pressure necessary to generate an ink droplet.
• Such piezoelectric transducers are capable of generating the momentary pressures necessary for useful drop-on-demand printing but they are relatively difficult and expensive to manufacture since the piezoelectric crystals (which are formed from a brittle, ceramic material) must be micro-machined and precision installed behind the very small ink chambers connected to each of the inkjet nozzles of the printer.
• Additionally, piezoelectric transducers require relatively high voltage, high power electrical pulses to effectively drive them in such printers.
  • To overcome these shortcomings, drop-on-demand printers utilizing thermally-actuated paddles have been suggested.
    • Each paddle would include two dissimilar metals and a heating element connected thereto. •When an electrical pulse is conducted to the heating element, the difference in the coefficient of expansion between the two dissimilar metals causes them to momentarily curl in much the same action as a bimetallic thermometer, only much quicker.
    • A paddle is attached to the dissimilar metals to convert momentary curling action of these metals into a compressive wave which effectively ejects a droplet of ink out of the nozzle outlet.

PRESENT INVENTION

• According to a feature of the present invention, a drop-on-demand inkjet print head includes

1. a nozzle with an ink outlet,
2. an ink supply channel through which a body of ink is supplied to the nozzle, and
3. a member movable in the ink supply channel toward the nozzle outlet for causing a droplet to separate from the body of ink.

• A micro-actuator applies a mechanical force to the member.
• The micro-actuator includes a body of elastomer material having opposed first and second surfaces spaced apart in a first direction by a predetermined at-rest dimension.
• A charge mechanism is coupled to the first opposed surface of the elastomer material so as to apply an electrical charge in the first direction.
• The charge is spatially varied in a second direction substantially normal to the first direction so as to create spatially varied mechanical forces across the elastomer material such that the elastomer material exhibits spatially varied growth in the first direction.
• The member is associated with the second opposed surface of the elastomer material so as to move in the first direction in response to growth of the elastomer material.
• Epson Develops a Next-Generation Inkjet Print Head Using an Original Thin-Film Piezo Element
• The Micro Piezo print head is an inkjet print head which utilizes Epson’s original Micro Piezo technology.
• This technology uses electrical signals to change the shape of piezo elements and then fires ink droplets according to the physical force generated by the change in shape of these elements.
• Compared to other inkjet systems, Micro Piezo technology offers superior ink ejection performance, compatibility with a wide variety of inks, and durability.
• Epson is currently focusing its development efforts on creating a next-generation Micro Piezo print head to support expansion of the inkjet field beyond consumer printers and to strengthen its applications for business and industry. Through these efforts, Epson has achieved the following:

1. Created a piezo element with the world’s highest degree of distortion through film thickness reduction and materials development.
2. Raised density levels by developing and utilizing innovative thin-film processing techniques.
3. Designed a high-density ink reservoir through independent MEMS technology development.

Special characteristics of Epson’s original Micro Piezo print head

• This print head fulfills all the requirements for an inkjet print head—superior ink ejection performance, compatibility with a wide variety of inks, and durability.
• The print head also makes it possible to control not only water-based pigment ink with superior water and light resistance, but also a wide variety of ink types in miniscule quantities, and fire them with high precision.

1. Ink ejection performance

• Meniscus control, which limits vibration on the surface of the liquid in the nozzle, makes it possible to achieve improved gradation and faster print speeds in addition to the following advantages: –Perfectly spherical dots –High precision impact point control –Higher drive frequencies –Variable-sized ink droplets (MSDT: multi-sized dot technology

2. Ink compatibility

• Epson’s original pigment ink achieves high levels of color reproduction and durability. •Industrial applications such as solvent ink, color filters, and UV ink are currently under development.

3. Durability

• Because it utilizes a permanent print head, the technology is applicable to a wide range of uses ranging from consumer printers to industrial manufacturing equipment.

fig : Structure of the next-generation Micro Piezo print head