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.

Ricoh recirculating ink printhead

US 2015/0306875 A1
WO 2015/163487 A1
Inkjet head that circulates ink
Ricoh Printing Systems America, Inc.

The benefits of circulating ink through the actuator chambers of ink jet printheads is now well known. The chambers are easily and quickly primed, air bubbles can be displaced, inks with a tendency for pigment settling or separation can be used, and fresh ink is brought to the nozzles reducing the requirements for nozzle maintenance.

Oct-15aInk passes from an inlet manifold 302 through passageway 225 into the actuator chamber 221. In this design a second passageway 223 close to the nozzle allows ink to return to a separate manifold 304. Ink being supplied to the printhead is fed under a positive pressure, and a negative pressure is applied to the return path.

Oct-15bOverall though, the average pressure is slightly negative by arranging the negative pressure to be higher than the positive. Therefore there is enough differential pressure to achieve the continuous flow through the actuator chambers and a net negative pressure at the meniscus.

The ink flow paths and pressure chambers within the printhead are made up from a stack of etched stainless steel plates that are bonded together. Note that as the inlet and outlet manifolds are both the same side of the actuator chambers, then the overall width of the printhead is minimised.

3D printing using Memjet printheads

US 2014/0345521 A1
Printing system for forming three dimensional objects
3D Systems, Inc.

This is a continuation patent dating back to 2011 and filed by Kia Silverbrook. However the assignee is 3D Systems, the leading 3D printing machine manufacturer. The basic idea is to use stationary Memjet printheads to deposit layers of material to build up structures. The printhead technology proposed is not the bubble jet system which has been commercialised, but a moveable element system. Nov-14aFor instance Silverbrook has demonstrated a moving nozzle prototype in the past. A wide range of materials can be deposited up to a viscosity of 10 centipoise. The printhead is heated so that low-melting point metals such as indium (156C) and alloys of indium and gallium can be used, polymers with melting points of 120-180C, and sacrificial waxes with melting points above 80C.

Printheads 402 and 404 deposit drops onto a moving conveyor. Multiple printheads are used to deposit separate materials. In the figure 4 printheads 402a-402d are being used to deposit 4 different materials on the first layer. Some materials may need some form of processing, for instance forced cooling, evaporative drying, UV curing, precipitation reactions etc. Here the first two materials are processed by curing station 406. The second two materials have a different curing requirement and so a second curing system 408 is used. The second layer is built using seven diff erent materials by printheads 404a-404m. In this case 3 different curing systems are used.

Nov-14bNote also that the layers are off set slightly from each other. This enables small cavities to be constructed that are self-supporting and do not need a sacrificial support material. This avoids the extra processing and complication of removing the support material later.

The pitch of drop separation and the height of a layer is 10 microns. Larger cavities can be created to allow the insertion of dies such as integrated circuits, memory, LED and so on.

The process speed envisaged is 208 mm/sec. and the print width 295 mm. Up to 1,000 layers are envisaged allowing products to be printed up to 10 mm thick. This requires a minimum of 1,000 printheads, but if multiple materials per layer are used something like 8,000 printheads may be needed.

The patent application talks of applications such as flat panel TVs and PDAs, perhaps dating the document and also demonstrating how fast product development has been. Production speeds are 0.37 and 432 per second respectively for those two examples.

Although this process sounds incredible and optimistic, remember the patent application has been acquired by a market leader. It will be interesting to see what comes of it.

New silicon-MEMS printhead from STM

US 2014/0313264 A1
Method for manufacturing a fluid ejection device and fluid ejection device
STMicroelectronics S.r.l.

STM is one of the largest manufacturers of printheads in the world, making a large proportion of the dies for HP. In 2014 they announced projects to develop piezo MEMS devices, including page wide array ink jet printheads.

The manufacturing processes for a thin film piezo printhead is shown below. Previous designs have used 4 wafers but this design only uses 3, with subsequent cost savings and reduced manufacturing complexity as now only two alignment and bonding steps are required.

Oct-14Here the printhead is shown with the piezo actuator deflecting into the actuator chamber 232. The components made from the three wafers are designated 100, 200 and 300. The central wafer 200 is made from SOI (silicon on insulator) type. The top layer is process to form the diaphragm,piezo actuator and the bottom part the actuator chamber. Layer 300 can also be made from a SOI wafer, and forms the structure of the printhead with ink feed channels 316. The third wafer forms both the nozzle plate and the sealed chamber for the actuators. The full manufacturing process is shown in the patent.

Oct-14b

Compared to previous designs, the saving of a wafer is achieved by processing the nozzle layer and actuator capping structure in one piece. This is then brought into contact with the actuator wafer in the step shown. Bonding occurs due to the adhesive layer 230 coated onto the contact parts of the top structure.

Konica Minolta’s silicon MEMS printhead

EP 2 716 461 A1
Inkjet head and inkjet drawing device provided with same
Konica Minolta, Inc.

This patent application seeks to overcome two problems. When designing a high density printhead with multiple rows of nozzles it is difficult to provide electrical connections to actuators as the wiring must pass between other chambers. In addition the electrical contacts and piezo actuators may be sensitive to moisture.

April-14aHere a section through the printhead is shown. At the bottom is a silicon nozzle plate 11 and a spacer layer 12 made from glass. Next is a silicon on insulator (SOI) substrate, with layer 13 forming the chambers and thin layer 14 forming the diaphragm. Thin film piezo actuators 31 are formed on the diaphragm layer. At the top of the printhead is the wiring substrate layer 21 which is also silicon. On the surface of the wiring substrate are the wiring

Connections between the external exible circuit board 25 with driver chips 24 and the actuators. The printhead assembly is connected to the wiring substrate layer 21 by a resin adhesive layer 30. This is patterned to provide cavities for the actuators and the ink channels. above the printhead is ink manifold 40.

April-14bOn the left the structure is shown in more detail. The wiring on the substrate 21 is more visible. Connections are made from the upper surface to the lower surface with vias. Both top and bottom surfaces of the wiring substrate are protected by films 26 and 28. As the wiring substrate only has ink passages through it, almost the whole surface is available for wiring tracks to the multiple actuator rows.

On top of the piezo actuator is a gold bump 16. At the corresponding point on the wiring substrate is a solder bump 29. Heating the printhead to above the solder melting point of 139C makes the connection. the printhead and wiring substrates are fixed together by the resin adhesive layer 30 in the presence of dry nitrogen. This results in the actuators being encapsulated in a dry inert environment, preventing both the electrical connections and actuators from deterioration in the presence of moisture.