Patent Application
20080084447
Preferred embodiments of the invention will now be described by way of example only with reference to the accompanying drawings in which:
A single unit cell 30 is shown in
The heat that diffuses into the ink and the underlying wafer prior to nucleation has an effect on the volume of fluid that vaporizes once nucleation has occurred and consequently the impulse of the vapor explosion (impulse force integrated over time). Heaters driven with shorter, higher voltage heater pulses have shorter ink decap times. This is explained by the reduced impulse of the vapor explosion, which is less able to push ink made viscous by evaporation through the nozzle.
Using the drive circuitry 22 to shape the pulse in accordance with the present invention gives the designer a broader range of bubble impulses from a single heater and drive voltage.
The time taken for the bubble to nucleate is the key parameter. Higher power (voltages) imply higher heating rates, so the heater reaches the bubble nucleation temperature more quickly, giving less time for heat to conduct into the heater's surrounds, resulting in a reduction in thermal energy stored in the ink at nucleation. This in turn reduces the amount of water vapor produced and therefore the bubble impulse. However, less energy is required to form the bubble because less heat is lost from the heater prior to nucleation. This is, therefore, how the printer should operate during normal printing in order to be as efficient as possible.
Lower power will increase the time scale for reaching the superheat limit. The energy required to nucleate a bubble will be higher, because there is more time for heat to leak out of the heater prior to nucleation (additional energy that must be supplied by the heater). Some of this additional energy is stored in the ink and causes more vapor to be produced by nucleation. The increased vapor provides a bigger bubble and therefore greater bubble impulse. Lower power thus results in increased bubble impulse, at the cost of increased energy.
This permits the printhead to operate in multiple modes, for example:
a normal printing mode with high power delivered to each heater (low bubble impulse, low energy requirement);
a maintenance mode with low power delivered to each heater to recover decapped nozzles (high bubble impulse, high energy requirement);
a start up mode with lower power drive pulses when the ink is at a low temperature and therefore more viscous;
a draft mode that prints only half the dots (for greater print speeds) with lower power drive pulses for bigger bubbles to increase the volume of the ejected drops thereby improving the look of the draft image; or,
a dead nozzle compensation mode where larger drops are ejected from some nozzles to compensate for dead nozzles within the array.
A primary objective for the printhead designer is low energy ejection, particularly if the nozzle density and nozzle fire rate (print speed) are high. The Applicant's MTC001US referenced above provides a detailed discussion of the benefits of low energy ejection as well as a comprehensive analysis of energy consumption during the ejection process. The energy of ejection affects the steady state temperature of the printhead, which must be kept within a reasonable range to control the ink viscosity and prevent the ink from boiling in the steady state. However, there is a drawback in designing the printhead for low energy printing: the low bubble impulse resulting from low energy operation makes the nozzles particularly sensitive to decap. Depending on the nozzle idle time and extent of decap, it may not be possible to eject from decapped nozzles with a normal printing pulse, because the bubble impulse may be too low. It is desirable, therefore, to switch to a maintenance mode with higher bubble impulse if and when nozzles must be cleared to recover from or prevent decap e.g. at the start of a print job or between pages. In this mode the printhead temperature is not as sensitive to the energy required for each pulse, as the total number of pulses required for maintenance is lower than for printing and the time scale over which the pulses can be delivered is longer.
Similarly, temperature feedback from the printhead can be used as an indication of the ink temperature and therefore, the ink viscosity. Modulating the drive pulses can be used to ensure consistent drop volumes. The printhead IC disclosed in the co-pending PUA001US to PUA015US (cross referenced above) describe how ‘on chip’ temperature sensors can be incorporated into the nozzle array and drive circuitry.
The invention has been described herein by way of example only. Ordinary workers in this field will readily recognize many variations and modifications which do not depart from the spirit and scope of the broad inventive concept.
