Detecting drops by measuring temperature of actuator chamber

US 2015/0321470 A1
Base, liquid discharge head, printing apparatus, and method for determining liquid discharge status
Canon Kabushiki Kaisha

With increased interest in high resolution page arrays, a major issue is maintaining print quality in the event of nozzle failures. Nozzles may fail for a variety of reasons — clogging due to debris or the ink viscosity increasing locally. Air bubbles may get trapped inside the actuator chamber, or the nozzle plate can become contaminated. Many of these failures can be overcome by performing some nozzle maintenance, while firing adjacent nozzles can compensate for more permanent failures.

While it is possible to regularly perform nozzle maintenance, this can be problematic with single pass page array printing. It is possible to spit drops onto the substrate either in between pages, or even in the images or background areas. but any more extensive maintenance, for instance cleaning the nozzle plate or purging ink from the nozzles, requires a halt to printing while the maintenance station and printhead are brought together.

Therefore it is becoming more desirable to know the status of all nozzles all of the time. With this knowledge nozzle maintenance can be carried out when required, rather than at regular intervals. In addition missing nozzle compensation algorithms can be used to maintain high print quality in the event of nozzle failures.

Popular techniques for determining missing nozzles are optical systems to either detect drops in flight or dots on the substrate after printing. Neither of these processes are easy to implement.

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Here Canon explores building detection circuits under the heaters in each actuator chamber to detect partial or full drop generation failures. The detection circuits track temperature to determine if the drops are fired or not. In the figure heater 104 is generating a bubble within an actuator chamber. Underneath the heater is the temperature detection circuit 105 which records the temperature rise and decay. If no drop is generated then the ink remains in the chamber and the heater is completely uncovered by the bubble. If a drop is generated then some ink is left above the heater. These different conditions can be detected thermally.

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On the left the temperature detection circuits are shown. These are underneath the heater, separated by an insulating film. There are two circuits, 105a under the centre of the heater and 105b under the periphery of the heater. Both circuits have serpentine-shaped tracks to increase the circuit length and therefore the resistance, making it easier to measure variations in voltage in the circuit. The connections to the circuits 110 are taken out by vias to tracks on a layer underneath the detection circuits, so they are less affected by the temperature changes.

The two circuits for each heater 105a and 105b are connected in a detection circuit. The outputs of differentiators 121 and 122 feed the comparator 123 resulting in Vout. If there is a drop discharge failure there is always a bubble on the surface of the actuator chamber above the heater, so the temperature gradually lowers. If a drop is generated then heat is removed from the chamber by the ink drop, so the temperature of the surface of the chamber decreases much more rapidly.

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The graph shows the output voltage Vout from the comparator. If V2≥V1 then the output is high. If V1>V2 then the output is low. If no drop is generated then V1>V2 is always true so the output stays low. The time for the measurement Tk is chosen to be when the ink fragment left from the discharge should remain on the heater, and before the refill process.

The detection circuits are integrated with the silicon substrate of the printhead die. The detection process is very sensitive yet accurate. no complicated signal processing is required, so detection is very fast.

Metallic effects from a thermal ink jet ink

US 2014/0170384 A1
Neutral gray reflective ink
Hewlett-Packard Development Company

Judging by the number of patents filed, it appears that the ink jet printing of expanded colours and appearances are sought after for home and office decorative printing. Reflective inks, in particular, have been disclosed with a particular emphasis on metal flakes – materials currently used in more traditional metal-effect printing processes. Thermal printheads have significant constraints regarding viscosity and particle size and so large metal flakes are a non-starter and a silver nano-particle approach would be expensive and questionable from a toxicological perspective.

The patent suggests that to look metallic, then only a certain threshold value for specular reflection needs to be achieved. It is suggested that 10% of the incident light must be specularly reflected, but greater than this is always desirable. With this specular reflectivity requirement relaxed, alternative non-metallic materials can be considered, such as magnetite. Magnetite (Fe3O4) can be made up into a stable nano-dispersion, suitable for thermal ink jet, and will give a metallic lustre when printed on glossy paper.

However, magnetite has an inherent yellow-orange hue and so to achieve a neutral silver effect a mixture of blue, cyan or magenta dyes must be added to counteract this. The exemplified ink jet formulations first prepared the magnetite dispersion in water by heavily bead milling a 5.6 wt% mixture of the oxide in water with a polyether alkoxysilane reactive dispersant at 0.5 wt%, relative to the pigment. This was then used (at 36.3 wt%) to make a simple thermal ink jet ink with 14 wt% humectant, 0.1 wt% neutralised styrene acrylic binder, 0.3 wt% surfactants, 5.4 wt% of a cyan and magenta dye mixture and water.

June-14aThis ink was loaded into a HP black cartridge from a Photosmart 8450 printer to print colour test patches. The chart left shows the colour coordinates of the neutral formulation above, compared against the non-colour corrected magnetite dispersion and a comparative silver nano-particle ink. It can be seen that the yellow tint has been fairly well counteracted in comparison to the metal ink samples.

The patent goes on to discuss the importance of the media in achieving a metallic effect and that the pore size of the surface of the media must be smaller than that of the magnetite pigment particles to ensure they remain on the surface as a contiguous layer. Fig 1 below shows this schematically with the dyes (16) absorbing into the media under the ceramic particles (14). There is an obvious issue to such a configuration in that the robustness of the printed image will be compromised if the pigment is left unprotected on the surface, but this is addressed in the patent application US 2014/0170395 A1.

June-14bAlthough it seems that the ultimate specular reflectivity of this approach is not quite that of a metallic silver ink, there do seem to be significant advantages in using such benign and low cost iron oxide materials.

Thermal inkjet head with improved jet straightness

US 2013/0293638 A1
Fluid ejection device having firing chamber with mesa
Hewlett-Packard Development Company, L.P.

There have been several thermal ink jet printhead designs in the past that have attempted to improve jet straightness.  As drops are formed, the bulk of the ink moves rapidly to the substrate.  Some of the ink is contained in the tail, which breaks off.  Often the tail will fragment into fine drops which either slowly follow the main drop to the substrate or fall back on the nozzle plate.  In addition erratic tail break-off can lead to mis-directed drops.

The proposed solution is to redesign the heater chamber floor with a central column or mesa and a circular heater.  The silicon substrate is etched to form a circular pit 230 around a central column 250.  The heater 205 is formed on the outer wall of the pit.  The heater, electrodes 208 and the exposed surface of the substrate are coated with an insulator, which also increases the robustness of the structure.

The column is centrally positioned under the nozzle.  It is thought that the tail then breaks more centrally than otherwise.   In addition, because the heater is not at the centre of the chamber, it is far less likely to be damaged by the collapsing bubble.  Therefore the overcoat layer, which has a protective function as well as being an insulator, can be reduced in thickness and therefore increase the efficiency of the device.