Piezoelectric droplet deposition apparatus and method and control system optimized for high viscosity fluids
阅读说明：本技术 为高粘度流体优化的压电微滴沉积设备及方法和控制系统 (Piezoelectric droplet deposition apparatus and method and control system optimized for high viscosity fluids ) 是由 尼古拉斯·马克·杰克逊 安格斯·康蒂 沃尔夫冈·沃特 安德鲁·科克斯 迈克尔·雷迪什 于 2020-05-20 设计创作，主要内容包括：一种微滴沉积设备包括微滴沉积头、流体供应部和控制器,其中：微滴沉积头包括各自有喷嘴的一个或更多个流体室、有进入头的流体入口且终于一个或更多个喷嘴的流体入口路径及始于一个或更多个喷嘴且终于头的流体返回部的流体返回路径；每个流体室包括两个相对室壁,其包括压电材料且在施加电驱动信号时可变形以从喷嘴喷射微滴；流体供应部被配置成以根据流体入口和流体返回部之间测量的压差向流体入口供应流体；且控制器被配置成向压电室壁施加驱动信号使得一个或更多个喷嘴在20℃和90℃之间的喷出温度沉积具有在45mPa·s至130mPa·s范围内粘度的流体的微滴,且其中由流体供应部施加的压差导致以50ml/min和200ml/min之间的流量进入流体返回部的流体返回流。还提供操作微滴沉积设备的方法及执行该方法的控制系统。(Droplet deposition apparatus comprising a droplet deposition head, a fluid supply and a controller, wherein: the droplet deposition head comprises one or more fluid chambers each having a nozzle, a fluid inlet path having a fluid inlet into the head and terminating at the one or more nozzles, and a fluid return path starting at the one or more nozzles and terminating at a fluid return of the head; each fluid chamber comprises two opposing chamber walls comprising a piezoelectric material and deformable upon application of an electrical drive signal to eject a droplet from the nozzle; the fluid supply is configured to supply fluid to the fluid inlet in accordance with a pressure differential measured between the fluid inlet and the fluid return; and the controller is configured to apply drive signals to the piezoelectric chamber walls such that the one or more nozzles deposit droplets of the fluid having a viscosity in the range 45 to 130 mPa-s at an ejection temperature between 20 ℃ and 90 ℃, and wherein the pressure differential applied by the fluid supply results in a fluid return flow into the fluid return at a flow rate between 50ml/min and 200 ml/min. Also provided are methods of operating droplet deposition apparatus and control systems for performing the methods.)
1. Droplet deposition apparatus comprising a droplet deposition head, a fluid supply and a controller, wherein:
the droplet deposition head comprises one or more fluid chambers each having a nozzle, a fluid inlet path having a fluid inlet into the head and terminating at one or more nozzles, and a fluid return path starting at the one or more nozzles and terminating at a fluid return of the head;
each fluid chamber comprising two opposing chamber walls comprising a piezoelectric material and being deformable upon application of an electrical drive signal to eject fluid droplets from the nozzle;
the fluid supply is configured to supply fluid to the fluid inlet in accordance with a pressure differential measured between the fluid inlet and the fluid return; and is
The controller is configured to apply drive signals to the piezoelectric chamber walls such that the one or more nozzles deposit droplets of a fluid having a viscosity in a range from 45 to 130 mPa-s at an ejection temperature between 20 ℃ and 90 ℃,
and wherein a pressure differential applied by the fluid supply results in a fluid return flow into the fluid return at a flow rate of between 50ml/min and 200 ml/min.
2. Droplet deposition apparatus according to claim 1, wherein the fluid supply is configured to heat the fluid to respective temperatures in the range 20 ℃ to 90 ℃ and to provide the heated fluid to the fluid inlet at a corresponding viscosity of 45 mPa-s to 130 mPa-s.
3. Droplet deposition apparatus according to claim 1 or claim 2, wherein the droplet deposition head further comprises a heater configured to heat the fluid to an ejection temperature.
4. Droplet deposition apparatus according to any one of claims 1 to 3, wherein the viscosity of the fluid at 30 ℃ is in the range 60 to 660 mPa-s.
5. Droplet deposition apparatus according to any one of claims 1 to 4, wherein the fluidic resistance between the fluid inlet and the fluid return is equal to or lower than 800 mbar/(ml-min) per fluid chamber.
6. Droplet deposition apparatus according to any one of claims 1 to 5, wherein the maximum peak-to-peak voltage of the drive signal is less than or equal to 35V, so as to eject droplets of a volume between 7pl and 120pl at a droplet ejection speed of 11 m/s.
7. Droplet deposition apparatus according to any one of claims 1 to 6, wherein the fluid has an OncoSk number greater than 1.5.
8. Droplet deposition apparatus according to any one of claims 1 to 7, wherein the drive signal is applied according to a first, high-precipitation drive mode, and wherein the fluid within the chamber has a viscosity in the range 45 mPa-s up to and including 130 mPa-s at an ejection temperature between 20 ℃ and 90 ℃.
9. Droplet deposition apparatus according to any one of claims 1 to 7, wherein the drive signal is applied according to a second, three-cycle drive mode, and wherein the ejection temperature of the fluid within the chamber between 20 ℃ and 90 ℃ has a viscosity value in the range 45 mPa-s up to and including 65 mPa-s.
10. Droplet deposition apparatus according to claim 9, wherein the fluid has an OncoSk number greater than 1 and less than 2.
11. Droplet deposition apparatus according to claim 10, wherein the fluid has an OncoSk number greater than 1.5.
12. Droplet deposition apparatus according to claim 8, wherein the fluid has an Oncodel number greater than 0.44 and less than 4.
13. Droplet deposition apparatus according to claim 12, wherein the fluid has an Oncorky number greater than 0.44 and less than 2.5.
14. Droplet deposition apparatus according to claim 12 or claim 13, wherein the fluid has an OncoUkD number greater than 1.5.
15. Droplet deposition apparatus according to any one of claims 1 to 14, the head further comprising drive signal generation circuitry, wherein the controller is configured to receive a value of current drawn by the drive signal generation circuitry and to determine a modified peak-to-peak voltage of the drive signal in response to the value of current so as to modify the droplet velocity of the ejected droplets.
16. Droplet deposition apparatus according to claim 15, wherein the drive signal generation circuitry is configured to receive the modified peak-to-peak voltage from the controller and to generate a drive signal having the modified peak-to-peak voltage so as to modify the droplet velocity of the ejected droplet.
17. Droplet deposition apparatus according to any one of claims 1 to 16, wherein the first drive mode has a maximum peak-to-peak drive voltage that is lower than the maximum peak-to-peak drive voltage of the second drive mode to eject droplets of the same velocity.
18. A method for operating droplet deposition apparatus according to any one of claims 1 to 17, the method comprising the steps of:
supplying fluid to the fluid chambers of the droplet deposition head so as to cause a recirculation flow of fluid through each chamber at a flow rate of between 50ml/min and 200 ml/min;
providing heating to the fluid before and/or after the fluid is to be supplied to the fluid inlet of the head such that the fluid in the fluid chamber is at a predefined ejection temperature between 20 ℃ and 90 ℃ and corresponds to a viscosity in a range from 45 mPa-s to 130 mPa-s; and
applying a drive signal to the piezoelectric wall of one or more of the chambers so as to eject some of the fluid supplied to the chamber in the form of one or more droplets, and returning excess fluid supplied to the chamber but not ejected to a fluid return of the head at a flow rate of between 50ml/min and 200 ml/min.
19. The method of claim 18, further comprising a drive signal generation circuit providing a current signal to the controller based on a duty cycle of the actuation of the chamber wall, wherein the controller adjusts a peak-to-peak voltage of the drive signal in response to a current value so as to maintain a droplet velocity of the ejected droplet substantially equal to a predefined droplet velocity.
20. The method of claim 18 or claim 19, further comprising: heating the fluid in the fluid supply such that the heated fluid reaching the fluid chamber is substantially at the predefined ejection temperature.
21. The method of any one of claims 18 to 20, further comprising heating the fluid on the droplet deposition head such that the heated fluid reaching the fluid chamber is substantially at the predefined temperature.
22. A control system for performing the method of any one of claims 18 to 21, the control system comprising a controller and a drive signal generation circuit, wherein the controller is configured to receive the predefined droplet velocity and current value from the drive signal generation circuit and, in response to the current value and the predefined droplet velocity, determine a modified peak-to-peak voltage based on stored test data; and wherein the drive signal generation circuitry is configured to receive the modified peak-to-peak voltage and generate a drive signal having the modified peak-to-peak voltage such that the generated drive signal modifies a droplet velocity of a droplet ejected from the one or more nozzles of the one or more fluid chambers of the droplet deposition head, and wherein the controller is further configured to
Applying the drive signal to the piezoelectric chamber walls such that the nozzle deposits droplets of a fluid having a viscosity in a range from 45 to 130mPa · s at the predefined ejection temperature between 20 ℃ and 90 ℃.
23. The control system of claim 22, further comprising a heater and a heater controller, wherein the heater is configured to heat fluid provided to the chamber and the heater controller is configured to receive operational data from the controller, wherein the operational data is based on the predefined droplet velocity and a current value of the drive signal generation circuit, the heater controller further configured to control the heater based on the operational data so as to heat fluid in the chamber to substantially the predefined ejection temperature.
24. The control system of claim 22 or claim 23, further comprising a fluid supply controller for controlling a fluid supply configured to supply fluid to the fluid inlet in accordance with a pressure differential measured between the fluid inlet and the fluid return, wherein the fluid inlet path begins at the fluid inlet into the head and terminates at the one or more nozzles, and wherein the fluid return path begins at the one or more nozzles and terminates at the fluid return of the head, and wherein the pressure differential applied by the fluid supply results in a fluid return flow into the fluid return at a flow rate of between 50ml/min and 200 ml/min.
25. The control system of any one of claims 22 to 24, wherein the drive signal generation circuitry is included within head control circuitry.
26. The control system of claim 23 or claim 25 when dependent on claim 23, wherein the heater is positioned on a print head and the heater controller is included within the head control circuitry of the print head.
27. The control system of any of claims 22 to 26, wherein the drive signal generation circuitry is configured to generate the drive signal with a modified peak-to-peak voltage such that the generated drive signal results in a droplet velocity substantially equal to the predefined droplet velocity.
Technical Field
The present disclosure relates to a piezoelectric droplet deposition apparatus (piezoelectric droplet deposition apparatus) suitable for printing high viscosity fluids, a method of operating the apparatus, and a control system therefor. Droplet deposition apparatus may be particularly suitable for applications requiring a fluid of high molecular weight polymer composition, such as 3D printing and photopolymer ejection.
Background
The inkjet industry is constantly evolving to meet the demand for new and challenging applications requiring new capabilities such as increased productivity and reduced cost.
One long established principle is that piezoelectric ink jet print heads are limited to depositing droplets of fluid having a viscosity (Ohnesorge number Oh <1) below 30mPa · s due to the fluidic resistance of the flow through the nozzle, which leads to excessive drive voltage requirements or ink starvation (ink station) from failing to replenish the ink channel (ink channel). This limits the ability to print mechanically tough and flexible parts that require fluids such as resins that contain high molecular weight polymer chains and have viscosities much higher than conventional inkjet fluids.
SUMMARY
Aspects of the invention are set out in the accompanying independent claims, while specific embodiments of the invention are set out in the accompanying dependent claims.
In one aspect, the following disclosure describes a droplet deposition apparatus comprising a droplet deposition head, a fluid supply, and a controller; wherein: the droplet deposition head comprises one or more fluid chambers each having a nozzle, a fluid inlet path having a fluid inlet into the head and terminating at the one or more nozzles, and a fluid return path starting at the one or more nozzles and terminating at a fluid return of the head; each fluid chamber comprising two opposing chamber walls comprising a piezoelectric material and being deformable upon application of an electrical drive signal to eject fluid droplets from the nozzle; the fluid supply is configured to supply fluid to the fluid inlet in accordance with a pressure differential measured between the fluid inlet and the fluid return; and the controller is configured to apply a drive signal to the piezoelectric chamber wall such that the one or more nozzles deposit droplets of the fluid having a viscosity in a range from 45 to 130 mPa-s at an ejection temperature (jetting temperature) between 20 ℃ and 90 ℃, and wherein a pressure differential applied by the fluid supply causes the fluid return flow to enter the fluid return at a rate between 50ml/min and 200 ml/min.
A method of operating a droplet deposition apparatus and a control system for performing the method are also provided.
Brief Description of Drawings
Referring now to the drawings wherein:
FIG. 1 is a graph of viscosity versus temperature for standard fluids D and E, developer fluids (developer fluids) A, B, C, F, G and K, and commercially available fluids such as H, L and M;
FIG. 2 is a plot of the rate of change of viscosity versus temperature for the data of FIG. 1;
FIG. 3 is a graph of Weber number versus Reynolds number for streams A, B, C, D, E, F, G, H, K, L and M;
FIG. 4 is an image of an in-flight droplet for a first drive mode using a high viscosity fluid;
FIG. 5 is a schematic cross-section of a low fluidic resistance recirculating flow component of a printhead for testing fluid;
FIG. 6 is a schematic plan view taken along section D-D' of FIG. 5;
FIG. 7 is a three-dimensional view of a low fluid resistance piezoelectric printhead having flow features according to the principles of FIGS. 5 and 6;
FIG. 8 is a representation of a first mode of operating the droplet deposition apparatus, illustrating movement of the chamber walls to produce a first pattern;
fig. 9a is a diagram of drive pulses comprising sub-droplet pulses (sub-drop pulses) suitable for the first drive mode illustrated in fig. 8;
FIG. 9b is a graphical representation of the drive pulses applied to the first five chamber walls of FIG. 8;
FIG. 10 is an illustration of a first mode of operating a droplet deposition apparatus according to the same example as illustrated in FIG. 8, but using different input data;
FIG. 11 is a representation of a second mode of operating a droplet deposition apparatus, illustrating chamber wall movement for 3-cycle printing;
FIG. 12 is a graphical representation of the drive pulses applied to each cycle of FIG. 11;
FIG. 13 is a block diagram of droplet deposition apparatus described herein; and
fig. 14 is a block diagram of a control system for droplet deposition apparatus described herein.
In the drawings, like elements are designated by like reference numerals throughout.
Detailed Description
The function of the embodiments and their various embodiments will now be described with reference to fig. 1-14.
Unexpectedly, the inventors have unexpectedly discovered that it is possible to eject high viscosity fluids with certain types of piezoelectric inkjet print heads, provided certain specific conditions and combinations are employed. This allows stable jetting of high viscosity fluids with an oindole number greater than 1.
Contrary to expectations, it was found that a printhead could be designed with fluid recirculation through a nozzle having a pressure differential between its fluid inlet and fluid return, and having a sufficiently low fluidic resistance to allow ejection of high viscosity fluids. In such a printhead, it was found that recirculation through the nozzles allows a sufficiently high fluid flow, which ensures a constant supply of fluid to the pressure chambers and a continuous nozzle replenishment, refilling the nozzles faster than the viscous flow alone. The print head under test has a pressure chamber in which the opposing chamber walls contain piezoelectric material and these movable walls (active walls) are capable of deforming upon application of a voltage signal. Each wall is also a movable wall for the adjacent chamber, which means that each wall is shared between two chambers. The wall operates most efficiently in a V-shaped shear mode (chevron shear-mode) and the print head was found to be capable of ejecting fluids with an oindongled number (Oh) greater than 1 and even greater than 2. In a printhead for testing high viscosity fluids, the pressure chamber is elongated and open to an ink manifold (ink manifold) at opposite ends, and opposite sidewalls deform to eject droplets in an acoustic mode: two pressure pulses are generated from both ends of the chamber and they reinforce the droplet ejection pulse at the nozzle located at the center of the chamber. This acoustic operation maximizes the supply of fluid to the nozzle and minimizes the energy required to eject a droplet.
Furthermore, the first drive mode was found to be particularly suitable for ejecting very high viscosity fluids. The use of a near-resonant single cycle operation "High Laydown" (HL) "mode allows ejection of fluids in excess of 60mPa · s and up to about 126mPa · s, with an oindokel number Oh >2 up to 2.5, while showing stable droplet formation with little flying ink or satellite droplets (satelite). In other examples, the "high sedimentation (HL)" mode also allows ejection of up to about 130mPa · s of fluid with an oindole number of up to 3 or up to 4. The droplets imaged in flight for this drive mode are shown in figure 4. Furthermore, this first printing mode enables the piezo print head used for the tests described herein to print full layers of photopolymer up to a thickness of 80 μm in a single pass (single pass) at a scan speed of 423 mm/s.
Still additionally, by increasing the ejection temperature, i.e., the temperature of the fluid when it passes through the fluid chamber, it is possible to eject a fluid having a viscosity exceeding 600mPa · s at 30 ℃. This enables printing of specially formulated fluids with improved mechanical toughness and flexibility at high resolution and speed, and potentially enables some existing stereolithography 3D printing resins to be printed with piezoelectric droplet deposition heads.
Fluid parameter
Fluids tested using low fluid resistance inkjet printheads with open-ended recirculating pressure chambers were analyzed with respect to their properties with respect to temperature (to assess properties at potential ejection temperatures) and compared to standard inkjet fluids as follows. Fig. 1 shows a plot of viscosity versus temperature for five different fluids labeled A, B, C, D and E and corresponding to the fluids listed in table 3. Fluids a and B are high viscosity developing fluids manufactured by BASF, and have viscosities of 293mPa · s (fluid B) and 656mPa · s (fluid a) at 30 ℃. Fluid C, fluid F, fluid G, fluid L and fluid M, also made by BASF, have moderate viscosities of 74mPa · s, 156mPa · s, 108mPa · s, 63mPa · s and 119mPa · s, respectively, at 30 ℃. Fluid H manufactured by Delo has a medium viscosity of 182 mPas at 30 ℃ and fluid K has a medium viscosity of 72 mPas at 30 ℃. Fluids D and E are standard inkjet fluids having a viscosity of 32 mPa-s or less at 30 ℃. Fluid D (Sunjet ULX5832 cyan) is a standard UV ink; fluid E (Itaca MA5115) is a standard ceramic ink. The information label (key) of the fluid is provided in table 3.
Fig. 1 also shows viscosity limits (viscocity limit) L1, L2 and L3. L1 indicates a "conventional" limit of about 30mPa · s above which conventional ink jet heads are not believed to provide stable, good quality droplets. The inventors have found that, contrary to expectations, above L1, a much higher viscosity fluid can be ejected: up to about L2(65mPa · s) for one drive mode (3-cycle mode) and up to about L3(126mPa · s) for another single-cycle drive mode. In other examples, L3 may be 130mPa · s. The grey area of the fluid temperature above 90 c in fig. 1 indicates the maximum temperature above which the fluid may degrade and droplets cannot be ejected. This may be due, for example, to the thermal curing of the UV cured fluid within the printhead. The values used to plot fig. 1 are also listed in table 2 for ease of reference in the following description. The actual value of degradation depends on the particular fluid and the temperature of the fluid.
As can be seen from fig. 1, the platform (plateau) that increases the viscosity of the fluid not only moves the viscosity curve in the diagram of fig. 1 upwards, it also moves it to a higher temperature, which means that the platform itself starts to be moved to a higher temperature when the viscosity at the platform increases overall.
Two types of fluid recirculating print heads of the Xaar1003 series were used to test high viscosity fluids, which differed only in nozzle volume and jetted 7.5pl sub-droplets ("GS 6") and 15pl ("GS 12") sub-droplets for the first print mode (HL mode) and jetted 6pl sub-droplets ("GS 6") and 12pl ("GS 12") sub-droplets in the second print mode (3 cycle mode). In addition, the fluid flow paths for both print heads are the same. Each print head has 1000 nozzles, one for each pressure chamber, arranged in two parallel rows of 500 nozzles each. The pressure chamber is elongated and open to fluid flow at opposite ends of the pressure chamber without a change in cross-section of the pressure chamber. Each pressure chamber is bounded at opposite elongated sides by chamber walls comprising a piezoelectric material. When actuated by a drive pulse of a drive signal, the walls deform to cause ejection of a droplet from the nozzle. This configuration is also referred to as a "shared wall," which means that each piezoelectric wall is shared between two adjacent chambers. The piezoelectric material is polarized (poled) in a direction perpendicular to the elongation direction of the chamber and perpendicular to the row direction of the nozzles, i.e., in the case of the Xaar1003 head, in the direction of the nozzle axis. This results in shear mode distortion. This mode is made most efficient by constructing the piezoelectric walls such that they are formed of an upper portion polarized in one direction and a lower portion polarized in the opposite direction, such that the deformation is "V-shaped" when viewed along a cross-section of the chamber perpendicular to the direction of elongation. The Xaar1003 printhead family can operate in a "V-shaped" shear mode with effectively shared walls. The flow path of the Xaar1003 head will now be described in more detail with reference to fig. 5, 6 and 7.
Recirculating flow path
An example of such a print head is shown and described in WO 00/38928 with respect to open-ended pressure chamber recirculation. WO 00/38928 teaches that fluid can be injected into an inlet manifold and returned via a return manifold, wherein the manifold is common to and connected via each pressure chamber, so as to generate a fluid flow through each chamber and hence through each nozzle during operation of the printhead.
The fluid path of a print head 30 (such as Xaar 1003) is schematically illustrated in fig. 5 and 6, where fig. 5 is a cross-section through a flow member 20, the flow member 20 bisecting the pressure chamber 10 along the elongation of the pressure chamber 10 and along the section E-E' shown in fig. 6. For this type of print head, as Xaar1003, this is the direction perpendicular to the rows of nozzles 6. Meanwhile, fig. 6 is a plan view of the flow member along the section D-D' of fig. 5, i.e., looking up the flow member with the nozzle plate 16 removed.
The fluid enters the flow member 20 of the print head via an inlet port 22 provided in the manifold portion 19 of the flow member 20. The inlet port 22 is common to both rows of nozzles 6. In fig. 5, a row of nozzles 6 extends into the page (here in direction y). The fluid then travels as an inlet flow 42 through the common inlet 12 and splits into two flows that flow in opposite directions (here along x) through the different rows of pressure chambers 10 (indicated in fig. 6). The pressure chamber is shown bounded on one side by a wall 8 and having the same wall on the other side.
The manufacturing techniques for forming the pressure chamber 10 and the electrodes and the contacts of the electrodes are described in detail in, for example, WO 00/29217. In short, the chamber 10 is machined in a base component of piezoelectric material so as to define the piezoelectric channel wall 8. The two rows of chambers are formed in respective strips of piezoelectric material bonded to the planar surface of the substrate 15. For addressing each chamber wall, electrodes are provided on the wall of the chamber, so that an actuator is formed by the chamber wall 8, as is known for example from EP 0277703 a1, so that an electrical signal can be selectively applied to the wall. The disconnection of the electrodes allows the chamber walls of each row to be independently operated by means of an electrical signal applied via an electrical input (not shown). Thus, the chamber walls may act as actuator members capable of causing droplet ejection. The substrate 15 is formed with conductive traces (not shown) that are electrically connected to respective chamber wall electrodes and extend to the edges of the substrate 15 where the respective drive circuitry (integrated circuits) for each row of chambers is located.
The arrangement of the pressure chambers 10 is the same between the two rows of nozzles. The fluid travels through each pressure chamber and exits the pressure chambers such that one row flows into the common return 14a as a return flow 44a and the other row flows into the common return 14b as a return flow 44 b.
Each pressure chamber 10 has a nozzle 6 at or near its center, the nozzle 6 being arranged in a nozzle plate 16, the nozzle plate 16 delimiting the pressure chamber on one side. This is more readily seen in fig. 6, which shows a portion of two rows of nozzles, which in Xaar1003 each extend over 500 nozzles. Further, fig. 6 shows the nozzles of each row in a 3-cycle pattern. Three adjacent nozzles are successively offset along the direction of elongation of the pressure chamber in a repeating pattern for the nozzle groups of the subsequent three nozzles. The nozzles in each set of three nozzles may be referred to as a set A, a set B, and a set C nozzles. This grouping will be further described below with reference to the second drive mode (the 3-cycle drive mode of the shared wall print head).
Fig. 6 shows each chamber 10 bounded on each side by a chamber wall 8. The inlet 12 is shown with a flow indication of the common flow 42 in the droplet ejection direction (along z), which common flow 42 then splits to flow through each chamber 10. The return flow exits each chamber 10 and combines with other return flows from the same row to form a return flow 44. The return flow 44 passes through the common return 14 and into the common return port 24.
When the chamber walls are provided with a drive signal, the walls 8 deform and droplets are ejected from the nozzles 6. The flow through the nozzles contributing to the return flow 44 is greater than the flow ejected from the nozzles 6 in the form of droplets, which allows the print head to operate in a "recirculation" mode. To this end, a positive pressure is applied to the fluid entering the inlet port 22 via the inlet pipe 23 (shown in fig. 7), and a negative pressure is applied to the fluid returning via the return port 24 and the return pipe 25. In the case of Xaar1003, two return ports 24a, 24b are connected downstream to flow into one combined return pipe 25. The positive and negative pressures may be provided, for example, by external fluid supplies connected to inlet and return tubes of the printhead 30. Fluid recirculation as referred to herein is provided when the fluid flow through the chamber 10 is higher than the flow of ink ejection from the chamber, and in some cases may be five or ten times that flow.
It should be noted that the cross section of the (non-actuated) pressure chambers remains constant and each "open end" 18a, 18b of each pressure chamber 10 provides an opening into the pressure chamber 10 which has the same cross section as the pressure chamber itself. For the Xaar1003 print head series, the cross-section was 0.0225mm for a chamber length of 1.8mm2. For Xaar1003 print heads GS6 and GS12, the resulting fluidic resistance of the entire print head with its two parallel rows of manifolds was about 0.8 mbar/(ml · min). This means that the resistance of each manifold row is 1.6 mbar/(ml min) and each chamber has a fluidic resistance of 800 mbar/(ml min).
Fig. 7 illustrates the print head 30 in a three-dimensional perspective from below, so that the nozzle plate 16 with two rows of nozzles 6 can be seen, as well as the inlet pipe 23 and the combined return pipe 25 of the flow member 20. The tube is shown with covers 26, 28, such as those used during shipping.
Next, a first drive mode and a second drive mode, which are found suitable for ejecting a high viscosity fluid from a recirculation head (such as Xaar 1003), will be described.
High sedimentation/first mode
Fig. 8(a) and 8(b) show a method according to the first driving mode, which was previously described in detail in WO 2018/224821 and WO 2019/058143. In this mode, a sub-droplet is ejected from each pressure chamber 10, for each pressure chamber 10, the two walls move inwards in opposite directions (opposing sense) within the same drive signal. As a result, the ejected droplets all land along the same line of pixels (pixel line) on the media for the duration of the drive signal. As indicated by the bold horizontal lines in fig. 8(a) and 8(b), based on the input data, some of the chambers within the nozzle row are designated as firing chambers (in the illustrated example, the chambers 10(b), 10(c), 10(d), 10(h), 10(i), 10(l)) during the application of the drive signal, and droplets will be deposited during the application of the drive signal, while the remaining chambers (in the illustrated example, the chambers 10(a), 10(e), 10(f), 10(g), 10(j), 10(k), 10(m), 10(n)) are designated as non-firing chambers. As is evident from the figures, this designation results in one or more bands of contiguous firing chambers (bands), indicated by the bold horizontal lines, separated by one or more bands of contiguous non-firing chambers for one period of the drive signal.
In the case where this designation has been done, the walls of some of the chambers are then actuated by the drive signal. Fig. 8(a) and 8(b) show the head at corresponding points in the actuation cycle of the drive signal. More specifically, fig. 8(a) shows the points in an actuation cycle in which the wall is at one extreme of its motion, while fig. 8(b) shows the points of a portion of a subsequent cycle when the wall is at the opposite extreme. The corresponding drive signals of fig. 8(a) and 8(b) are illustrated in fig. 9.
Fig. 9(a) shows a close-up of a drive signal 60 consisting of sub-droplet pulses 61. For one pixel period 62, four sub-drop pulses are shown, within which pixel period 62 four sub-drops form a drop to land in a pixel along a pixel line. For the first mode or high settling mode, each sub-droplet pulse may cause ejection from an adjacent chamber. For example, for chamber 10(b), the first portion of sub-droplet pulse 63 to one wall of chamber 10(b) (e.g., the shared wall between chambers 10(b) and 10 (c)) and to the other wall (i.e., the shared wall of chambers 10(b) and 10 (a)) causes the wall of chamber 10(b) to move inward, as shown in fig. 8(a), and chamber 10(b) ejects the sub-droplet. The second portion of sub-droplet pulse 64 to one wall of chamber 10(b), e.g., the shared wall between chambers 10(b) and 10(c), and a similar pulse applied to the shared wall between chambers 10(c) and 10(d), causes the shared wall between 10(b) and 10(c) to move to the outside of chamber 10(b) such that both walls of chamber 10(c) move inward, as shown in fig. 8(b), and chamber 10(c) ejects a sub-droplet, while chamber 10(b) does not eject a sub-droplet. The next sub-droplet pulse repeats the wall motion until a total of four sub-droplets are ejected to form a drop that is deposited into a pixel on the media. Fig. 9(b) shows example drive pulses for chambers 10(a) through 10(e) of fig. 8, where firing chambers 10(b) through 10(d) receive drive signals, and non-firing chambers 10(a) and 10(e) do not. It can be seen that the drive signal for chamber 10(c) is opposite to the drive signals for chambers 10(b) and 10(d), as shown in fig. 8(a) and 8 (b). The timing of the drive signal sent to each chamber is also illustrated. The drive signal is activated by a pixel clock trigger (PCLK). The pixel clock is associated with an encoder of a movement mechanism of the print medium and allows a controller of the droplet deposition apparatus to determine the position of the pixel line on the medium and coordinate droplet ejection from the nozzles of the pressure chamber as a result of applying the drive signal. Upon receiving the pixel clock flip-flop, the controller that sends a drive signal to the cell causes the cell to receive the drive signal. After a predetermined time from the activation of the drive signal for the first pixel line, which predetermined time is related to the medium velocity and the chamber acoustics, the drive signal is sent again to cause the nozzle to eject a droplet into the second pixel line.
As is evident from comparing the two figures in fig. 8, for each of the excitation chambers 10(b), 10(c), 10(d), 10(h), 10(i), 10(l), the walls move in opposite directions.
With respect to the non-excited chambers, two different types of behaviour of their walls can be observed: for some non-firing chambers, particularly those of the strip adjacent to the firing chamber (in the example shown, chambers 10(a), 10(e), 10(g), 10(j), 10(k), 10(m)), one wall is moved while the other remains stationary; for other non-firing chambers, particularly those of the strip that are not adjacent to the firing chamber (chambers 10(f), 10(n) in the example shown), both walls remain stationary.
Attention is next directed to fig. 10(a) and 10(b) which show a first mode according to the same example as fig. 8(a) and 8(b), when this first mode is used for depositing droplets according to different input data. As with fig. 8(a) and 8(b), fig. 10(a) and 10(b) show the head at various points in the actuation cycle. As can be seen in fig. 10(a) and 10(b), different chambers 10 are designated as firing chambers and non-firing chambers based on the new input data. More specifically, it can be noted that this designation has resulted in a band of non-firing chambers consisting of only a single non-firing chamber, particularly chamber 10 (e).
As is evident from comparing the two figures, for each of the excitation chambers 10(b), 10(c), 10(d), 10(f), 10(g), 10(h), 10(i), 10(l), the walls move in opposite directions, as in fig. 8(a) and 8 (b).
However, with respect to non-excited chambers, three (as opposed to two) different types of behavior of their walls can be identified: for some non-firing chambers, particularly those of the strip adjacent to the firing chamber (in the example shown, chambers 10(a), 10(j), 10(k), 10(m)), one wall is moved while the other remains stationary; for other non-firing chambers, particularly those that are not adjacent to the strip of firing chambers (chamber 10(n) in the example shown), both walls remain stationary; for other non-excited chambers, especially chamber 10(e) in the single-chamber wide band of non-excited chambers, the walls move in the same direction.
It will be appreciated that moving the walls of each firing chamber, as shown in figures 8 and 10, results in the release of one or more droplets from the chamber in question upon application of one or more actuation pulses. The resulting droplets form a body of fluid disposed on a line on the medium, where the body of fluid is separated on this line by a respective gap for each of the bands of non-firing chambers (at least momentarily separated upon landing-the body of fluid may coalesce on the medium). It will be appreciated that the dimensions of each such gap will therefore correspond in size approximately to the width of the respective band of non-firing chambers.
As can be seen from the actuation sequences in fig. 8 and 10, for example, if the applied drive signal in fig. 10 directly follows the drive signal in fig. 8, some non-firing chambers may only require a small wall movement to provide a transition from the non-firing chamber to the firing chamber. Furthermore, it is possible that many walls of the non-firing chamber remain stationary. This may improve the lifetime of the head by reducing the number of wall movements by the wall in order to achieve a certain settling density of droplet fluid on the substrate.
The methods illustrated in fig. 8, 9 and 10 represent a high deposition drive mode, providing high rate throughput. The excitation chamber can be actuated at or near the resonant frequency and thereby achieve a "pumping power" (the amount of droplet fluid deposited per second for a head width per inch) significantly higher than 500 μ Ι/(s-inch), in several cases higher than 750 μ Ι/(s-inch), and possibly up to 1000 μ Ι/(s-inch). Both the reduction of the drive voltage and the more efficient use of the actuation wall increase the life of the head.
The printhead has a maximum acceptable drive voltage, thereby limiting the maximum pulse that can be imparted on the fluid and thus the maximum viscosity that can be ejected from the nozzle. The lower drive voltage produced by the near resonant one-cycle high settling drive mode (first mode) means that the viscosity can be further increased before the voltage limit of the printhead is reached.
As shown in fig. 8 and 10, application of a drive signal to each firing chamber that moves the opposing walls inward causes the release of one or more sub-droplets from the firing chamber. The resulting sub-droplets form bodies of fluid disposed on the media on a line of pixels, wherein the bodies of fluid each land in their respective pixels of the line of pixels, and are separated on this line by respective gaps between each of the stimulation bands (firing bands) of the bands of non-stimulation chambers (at least momentarily upon landing-the bodies of fluid can coalesce on the media). It will be appreciated that the dimensions of each such gap will therefore correspond in size approximately to the width of the respective band of non-firing chambers.
In order to locate the body of fluid so deposited in a line on the medium, it will often be convenient for the actuation of the excitation and non-excitation chambers to overlap in time. However, this is not necessary, for example where the nozzles of the head are offset in some manner, such as the jet groups A, B, C indicated in FIG. 6. Furthermore, in some cases they may be synchronized such that actuation of all chambers begins simultaneously (although they may of course also be synchronized to terminate simultaneously).
3 cycle mode/second mode
In the second drive mode, the print head is driven in a 3-cycle mode. The nozzles of each row are arranged in groups of three. The nozzles in each group are offset in a direction perpendicular to the row direction. The nozzles in different groups having the same offset distance with respect to the row direction are in the same spray group (cycle group), which provides three spray groups A, B and C, as indicated by the nozzles of groups A, B and C in fig. 6. In fig. 6, the offset is along x. During printing, as the print head moves relative to the media in the print direction (in fig. 6, this may be along the x-direction), the nozzles eject droplets into the pixel lines such that the group furthest downstream relative to the print direction is actuated first, the middle group is actuated second, and the group furthest upstream in the print direction is actuated last. The time between actuations for each set is related to the media velocity and the acoustic properties of the pressure chamber.
In 3-cycle printing in the second drive mode, droplets are ejected when both walls of the pressure chamber move inward to generate pressure pulses along the chamber. The adjacent chambers experience low pressure because their opposing chamber walls remain stationary. In FIG. 11(a), the first cycle of "group A" chamber wall movement is shown. For chambers 10(a) through 10(n), every third chamber is actuated and its walls move inward. These are chambers 10(a), 10(d), 10(g), 10(j) and 10(m) shown in bold numbers. The chambers deposit droplets into respective pixels of a line of pixels. Next, the second cycle (group B) is actuated as shown by chambers 10(B), 10(e), 10(h), 10(k), and 10(n) in fig. 11 (B). These B groups of chambers now deposit droplets into the corresponding pixels of the same pixel line. At the same time, the remaining chambers experience a low pressure (resulting in the intake of fluid). For actuated chambers 10(C), 10(f), 10(i) and 10(l) shown in bold numbers, the final cycle, C-cycle, is shown in fig. 11(C), during which the chambers are actuated to deposit droplets into corresponding pixels of the same pixel line. The pixel lines are now completely printed.
Fig. 12 illustrates the timing of the drive pulse sent to each chamber of group a, group B, and group C. As previously described, the drive signal applied in the pixel period 62 is initiated by the pixel clock flip-flop PCLK. After receiving the pixel clock flip-flop, the controller causes the chambers of group a to receive sub-droplet pulse 61 (shown for group C, but the same shape for all other groups). After a predetermined time from the initiation of group a sub-droplet pulses, which is related to the medium velocity and chamber acoustic properties, the sub-droplet pulses are sent to group B. After a predetermined time further elapses from the initiation of the sub-droplet pulse for group B, the sub-droplet pulse is sent to group C. The predetermined time is kept constant if the medium speed is not changed. Each of the three cycles results in the ejection of one sub-droplet per chamber. To complete the printing into a line of pixels, the cycle is repeated for the desired number of sub-droplets for that pixel.
Blow out test
Xaar1003 GS6 and GS12 printheads were used to test a variety of standard fluids with developing inks having high to very high jetting viscosities in the first and second modes. The ejection flow through the nozzle is determined by the number of sub-droplets ejected.
The GS12 print head can eject sub-droplets of a volume of 15pl each in high-settling (HL) or first mode, with this GS12 print head, printing at a full load of 4 sub-droplets per pixel (i.e. a total drop volume of 60 pl) provides an ejection flow (ejection rate) of about 100ml/min for a fluid of viscosity 65mPa · s at a pixel clock frequency of 28kHz when all nozzles are activated (or 100% load (duty)).
Reliable printing conditions were found at a low flow ratio of 1.5:1 of recirculation volume flow (recirculation rate) to drop ejection volume flow (ejection rate). This corresponds to a recirculation flow of 150 ml/min. The low fluid resistance path of the Xaar1003 printhead required a relatively low pressure differential DP of about 529 mbar (DP being the pressure differential between the inlet 23 and return 25 tubes to the printhead) to achieve a recirculation flow rate of 150ml/min for this 65mPa · s fluid. For a viscosity of 97 mPas and the same recirculation flow of 150ml/min, the DP needs to be 790 mbar. The higher end of the differential pressure to be applied (e.g., comprising 790 mbar) may require higher specification fluid supply components to reduce variability in the applied pressure, such fluid supply designs being within standard engineering capability. The values for fluid a are summarized in table 1A, which allows one to compare standard inkjet fluids, such as fluid D (Sunjet ULX5832) having a viscosity of 32mPa · s at 30 ℃, with non-traditional inkjet fluids, such as fluid a (BASF high viscosity developing fluid), fluid C (PEG400), fluid K (high viscosity developing fluid), fluid H (Delo katobiond OM6600), fluid L (BASF ultracure 3D WS07), and fluid M (BASF ultracure 3DST30 LV).
As can be seen in table 1A, fluids a, C, K, H, L, and M have higher viscosities than conventional inkjet fluids. Fluid a has a maximum viscosity of 656mPa · s at 30 ℃ followed by fluid H having a viscosity of 182mPa · s at 30 ℃. The fluid C, the fluid K, the fluid L and the fluid M have a viscosity in the range of 63mPa · s to 87mPa · s at 30 ℃. The fluid a was heated to different ejection temperatures of 60 ℃ and 70 ℃ to obtain viscosities of 97mPa · s and 65mPa · s, respectively.
Turning to GS6, when driven in the first mode, the head deposits a lower total drop volume of 30pl per pixel per nozzle, which is produced by 4 sub-drops of 7.5pl, i.e. half the total drop volume of GS12 using the first drive mode. Thus, at the same printing frequency of 28kHz, the jet flow at 100% load (when all nozzles are activated) of GS6 is halved compared to GS12 to about 50ml/min, and similarly, a flow ratio of 1.5:1 corresponds to a recirculation flow of about 75 ml/min. For a fluid with a viscosity of 65mPa · s, the pressure difference required to obtain this flow is about 250 mbar. For a fluid with a viscosity of 97mPa · s the desired DP is 370 mbar, and for a fluid with a viscosity of 126mPa · s the desired DP is 475 mbar. These values for fluid a and fluid B are summarized in table 1B. As previously described, fluid a provided viscosities of 65mPa · s and 97mPa · s at ejection temperatures of 70 ℃ and 60 ℃, respectively, and fluid B was used to provide a viscosity of 126mPa · s at an ejection temperature of 45 ℃ (the viscosity of 293mPa · s was reduced from 30 ℃). Fluid B is also a high viscosity developing fluid.
Turning to the results from using the 3-cycle mode or the second mode, the sub-droplet volumes for the GS6 head and the GS12 head were slightly lower compared to the first mode, and the printing frequency was only 6kHz due to the 3-cycle drive compared to the first mode at 28 kHz. In the 3-cycle mode, seven sub-droplets (more than with the first HL mode) are ejected to form a total drop volume that is deposited into the pixel.
For GS12, the sub-droplets each had a volume of 12pl, or a total droplet volume of 84 pl; for GS6, the sub-droplets had a volume of 6pl, and the total drop volume per pixel was 42 pl. This is also equivalent to a recirculation flow of 150ml/min and an injection flow of about 30ml/min for a 5:1 recirculation ratio of GS12 and a 10:1 recirculation ratio of GS 6.
Fluid a, fluid C, fluid D, fluid H, fluid K, fluid L and fluid M were tested with GS 12. Fluid a and fluid K are much higher viscosity developing fluids than the conventional inkjet fluid of fluid D: fluid a provided a viscosity of 656mPa · s and fluid K provided a viscosity of 72mPa · s at 30 ℃, compared to the viscosity of fluid D (Sunjet ULX5832 cyan) at 30 ℃. Different fluids require different pressure differences DP to maintain the recirculation flow at 150 ml/min. For example, fluid A requires a pressure difference DP of about 494 mbar in order to supply a recirculation flow of 150ml/min at a fluid viscosity of 65 mPas for a temperature of 70 ℃. On the other hand, the fluid K has a pressure difference of about 403 mbar at a fluid viscosity of 53mPa · s for a temperature of 35 ℃.
Further examples are: fluid C (PEG400) having a pressure difference of 774 mbar at a fluid viscosity of 95mPa · s for a temperature of 25 ℃; fluid H (Delo kabobond OM6600) having a pressure difference of 502 mbar at a fluid viscosity of 66mPa · s for a temperature of 45 ℃; fluid L (BASF Ultracur3D WS07) having a pressure difference of 479 mbar at a fluid viscosity of 63mPa · s for a temperature of 30 ℃; and a fluid M (BASF Ultracur3D ST30 LV) having a pressure difference of 742 mbar at a fluid viscosity of 91 mPas for a temperature of 27 ℃.
In contrast, fluid D was ejected at 45 ℃ and a viscosity of 17 mPas. With the same setting of frequency and number of sub-droplets per pixel, only a pressure difference DP of 129 mbar is required to provide a recirculation flow of 150 ml/min.
A summary of the jettable fluids and their properties for the second mode is provided in table 1A for the GS12 printhead and table 1B for the GS6 printhead.
Table 1A: GS12
Table 1B: GS6
Table 2: FIG. 1 data
Table 3: fluid information label
Thus, a droplet deposition apparatus 1 is provided comprising a droplet deposition head 30, a fluid supply 40 and a controller; wherein the droplet deposition head comprises one or more fluid chambers 10 each having a nozzle 6, a fluid inlet path having a fluid inlet 23 into the head and terminating in the one or more nozzles, and a fluid return path starting at the one or more nozzles and terminating at a fluid return 25 of the head. Each fluid chamber 10 comprises two opposing chamber walls 8, the chamber walls 8 comprising a piezoelectric material and being deformable upon application of an electrical drive signal 60 for ejecting fluid droplets from the nozzle 6. The fluid supply 40 is configured to supply fluid to the fluid inlet 23 according to a pressure difference measured between the fluid inlet 23 and the fluid return 25. The controller is configured to apply a drive signal to the piezoelectric chamber walls such that the one or more nozzles deposit droplets of the fluid having a viscosity in a range from 45 to 130 mPa-s at an ejection temperature between 20 ℃ and 90 ℃, and wherein the pressure differential applied by the fluid supply 40 results in a return flow of the fluid into the fluid return at a flow rate between 50ml/min and 200 ml/min.
Xaar1003 printheads have been operated with fluids (such as hot melt waxes) at an ejection temperature of 90 ℃. As previously mentioned, the upper limit of the ejection temperature (and beyond which the fluid degrades and becomes un-ejectable or unreliable) depends on the specific fluid properties.
In some embodiments of droplet deposition apparatus, the ratio of the recirculation volume flow (recirculation flow) to the drop ejection volume flow rate (ejection flow) may be 1.5:1 to ensure reliable printing conditions. Further, the 1.5:1 ratio may correspond to a recirculation flow rate of 150 ml/min.
In some embodiments, the viscosity of the fluid may be 65 mPa-s, which requires a pressure differential DP of about 529 mbar (DP being the pressure differential between the inlet pipe 23 and the return pipe 25 to the print head) to achieve a recirculation flow rate of 150ml/min for this 65 mPa-s fluid. In an alternative embodiment, the viscosity of the fluid may be 97mPa · s, which requires a pressure difference DP of 790 mbar.
The pressure differential may be applied by applying a positive pressure to the fluid inlet 23 and a negative pressure to the return 25. For a two nozzle row printhead, the two return ports 24a, 24b may be combined downstream to flow into one combined return 25.
In some arrangements, the fluid supply 40 may be configured to heat the fluid to a temperature in the range 20 ℃ to 90 ℃ and provide the heated fluid to the fluid inlet at a corresponding viscosity of 45mPa · s to 130mPa · s. The corresponding viscosity provided to the fluid inlet may in turn provide a predefined ejection viscosity of the fluid when it enters the pressure chamber 10. The predefined ejection viscosity is a viscosity previously determined to be suitable for ejection. The predefined ejection viscosity may correspond to a predefined ejection temperature, for example, an ejection temperature determined from measurements such as those provided in table 2. The bold values in table 2 show the temperature and the corresponding viscosity at the time of fluid ejection.
The droplet deposition head may further comprise heaters 58, 59, the heaters 58, 59 being configured to heat the fluid to an ejection temperature. Such a heater (heater 58) may be included within the fluid supply 40. Additionally or alternatively, an on-board heater (or heaters) 59 may be provided within the print head 30, the on-board heater 59 preferably being in close proximity to and in thermal contact with the pressure chamber 10.
As can be seen from fig. 1 (and table 1), the viscosity of the high viscosity fluid at the ejection temperature can be much higher than the conventional viscosity range of up to 30mPa · s and about 30mPa · s. For these fluids, the viscosity at 30 ℃ can be extremely high. Thus, in some cases, the viscosity of the fluid at 30 ℃ may be in the range of 60 to 660 mPa-s. A suitable ejection viscosity can be obtained by heating the fluid. Fluid a, fluid B, fluid F, and fluid G are high viscosity developing fluids formulated by BASF, and it is expected that routine experimentation can determine a suitable high viscosity fluid capable of being ejected at a suitable ejection viscosity, such as for fluid a, the viscosity of 656mPa · s at 30 ℃ is reduced to 65mPa · s at 70 ℃ and becomes ejectable. Such fluids may for example be high molecular weight and/or standard fluids and particle loaded variants using standard solvents. Another example is shown by fluid H, which has a relatively high viscosity of 182mPa · s at 30 ℃, which decreases to 48mPa · s at 50 ℃. Thus, it was found from experiments that for fluids having a viscosity ranging from 60 to 660mPa · s at 30 ℃ (or a viscosity ranging from 30 to 1392mPa · s at 20 ℃), each fluid has a corresponding viscosity ranging from 45 to 120mPa · s (fluid a at 90 ℃, fig. 1) at a temperature ranging from 20 to 90 ℃ (fluid C at 20 ℃, fig. 1). Similarly, from the experiments in table 1, for a fluid having a viscosity ranging from 30mPa · s to 1392mPa · s at 20 ℃, the temperature of the corresponding viscosity that allows ejection of the fluid can be determined; in this case, the ejection temperature from 20 ℃ to 90 ℃ provides a selection of ejectable viscosities in the range of 45 mPas (fluid D, 42 mPas at 25 ℃ and also ejectable at 20 ℃ with a viscosity of 55 mPas) to 120 mPas (fluid G, 108 mPas at 30 ℃ or fluid F, 119 mPas at 35 ℃), or up to 130 mPas (for fluid A (125 mPas at 55 ℃), fluid B (126 mPas at 45 ℃), fluid C (126 mPas at 20 ℃) and fluid H (129 mPas at 35 ℃).
With respect to the fluid path of the head, the fluidic resistance measured between the fluid inlet and the fluid return may be equal to or lower than 800 millibar/(ml-min) per fluid chamber. The pressure chambers used for the open-ended design cause the highest fluid resistance in the printhead. Such a fluid resistance may be equal to or lower than a constant cross-sectional area of 0.0225mm2(in the unactuated state) and a chamber length of 1.8mm, wherein the chamber length is in a direction perpendicular to the cross-sectional area.
Further, in some embodiments of head 30, operation of the head may represent an active mode of operation in which the maximum peak-to-peak voltage of the drive signal is less than or equal to 35V, so as to eject droplets of a volume between 7pl and 120pl at a droplet ejection speed of 11 m/s. In some embodiments, the peak-to-peak voltage may be less than 30V to eject droplets of a volume between 7pl and 120pl at a droplet ejection speed of 11 m/s. Further, in some embodiments, the peak-to-peak voltage may be less than 20V in order to eject droplets of a volume between 7pl and 120pl at a droplet ejection speed of 11 m/s.
Viscosity gradient
The rate of change of the viscosity curve was also evaluated. These are plotted in fig. 2. It can be seen that the viscosity gradient decreases with increasing temperature of the fluid, and that the standard fluid D, E decreases to a gradient below 1 at temperatures between about 35 ℃ and 40 ℃. At temperatures of about 50 ℃ or higher, the remaining high viscosity fluid drops to a gradient below 1. In particular, the high viscosity fluid A, B only drops below or reaches a viscosity gradient of less than 1 near the degradation limit of the fluid.
Olympic Table number
The inventors have found a very strong relationship between reliable printing and the oindiger number Oh. The olanzog number is defined as:
wherein
Eta is the viscosity of the liquid
ρ is the density of the liquid
σ is the surface tension
L is the characteristic length scale (usually drop diameter)
Re is Reynolds number
We is the Weber number
The reynolds number is defined as the ratio of the product of the fluid density ρ, the fluid velocity v (in this case the drop velocity upon ejection) and the characteristic linear dimension L (in this case the nozzle diameter) to the dynamic viscosity η of the fluid:
the weber number We is the ratio of the inertial force and the force generated by the surface tension σ of the fluid. Which is defined as
Whereas ρ above is the fluid density, v is the fluid velocity (in this case drop velocity upon ejection), and L is the characteristic linear dimension (in this case nozzle diameter).
Therefore, the oprozomib number for each fluid at different temperatures can be calculated and the inputs and values are listed in table 4. These values relate to a droplet ejection speed of 11m/s and for a GS12 nozzle diameter a length scale L of 3.50E-05m。
Fluid, especially for a motor vehicle
T，℃
nSpraying out，mPa·s
ρ，g/cm3
σ，mN/m
Oh
Re
We
A
60
97
1.1003
38.4
2.52
4.4
121.2
A
67
72
1.1042
37.0
1.90
5.9
126.3
A
70
65
1.1058
36.4
1.73
6.6
128.5
B
45
126
1.0816
41.4
3.67
2.5
81.9
B
56
76
1.0735
40.2
1.95
5.5
113.1
B
70
48
1.0623
38.5
1.28
8.5
116.9
C
25
95
1.0783
51.8
2.16
4.4
88.2
D
45
17
1.0725
21.9
0.61
23.8
207.5
E
43
15
1.3429
31.3
0.38
35.7
181.9
F
50
58
1.0781
40.7
1.49
7.1
112.1
F
55
48
1.0779
40.2
1.22
8.7
113.5
G
55
34
1.0752
40.5
0.88
12.0
112.5
H
45
66
1.0431
37.4
1.79
6.07
118.1
K
27
88
1.1479
33.8
2.38
5.03
144.0
K
35
53
1.1422
32.9
1.47
8.25
147.0
L
30
63
1.1131
28.5
1.88
6.86
165.3
M
33
98
1.0890
36.7
2.62
4.27
125.6
Table 4: FIG. 3 data
FIG. 3 is a graph of the Weber number We versus the Reynolds number Re for fluid A, fluid B, fluid C, fluid D, fluid E, fluid F, fluid G, fluid H, fluid K, fluid L, and fluid M. The information labels for the fluids are found in table 3. The three data points in the traditional "good" areas ("printable fluid" areas) are standard inkjet ink D, E (Itaca MA5115 and Sunjet ULX5832) and developing fluid G near standard viscosity 34mPa · s. For these three inks, the OnEgger number is less than 1 and Oh <1.
Data points from other successfully ejected fluids are located in the "too viscous" region to the left of the trend line for Oh ═ 1, i.e. for which Oh >1, but all lie above the line indicated by trend line T1, indicating a "insufficient energy for drop formation" region. Trend line T2 indicates the onset of "spitting" above which the droplet breaks up into a spray T2. On the right side of the trend line Oh ═ 0.1, i.e., Oh <0.1, satellite droplets tend to form together with the ejected droplets and print quality deteriorates.
It has been found that using a second drive mode (3 cycle mode) in combination with a low resistance head (such as a Xaar1003 print head) a series of fluids with an oinsorge number of 0.25< Oh <1.75 can be ejected. Using the first drive mode, i.e. the high sedimentation mode, it is possible to eject fluid with an even higher Oh value in the range of 0.44< Oh < 4. In other examples, a fluid having an oendozog number in the range of 0.44< Oh <3 or in the range of 0.44< Oh <2.5 may be ejected using the first drive mode.
Thus, in some embodiments of droplet deposition apparatus, the fluid properties may be such that the fluid has an ovonic number greater than 1.5.
Where the droplet deposition apparatus operates according to the first drive mode (or high deposition mode) as described above, the ejection temperature of the fluid within the chamber between 20 ℃ and 90 ℃ may have a viscosity in the range of 45mPa · s up to and including 130mPa · s. Such fluids may have an ovonic number greater than 0.44 and less than 2.5 when the first drive mode is used. In further examples, the fluid may have an ocl number greater than 0.44 but less than 4 or less than 3. Further, the fluid may preferably have an oindole number greater than 1.5.
Alternatively, where droplet deposition apparatus 1 is operated in the second, three-cycle drive mode, the ejection temperature of the fluid within chamber 10 between 20 ℃ and 90 ℃ may have a viscosity in the range of up to and including 65mPa · s at 45mPa · s. For such fluids, the endolog number may be greater than 1 and less than 2, and furthermore, the fluid may preferably have an endolog number greater than 1.5 and less than 2.
The first drive mode may have a maximum peak-to-peak drive voltage that is lower than the maximum peak-to-peak drive voltage of the second drive mode to eject droplets of the same velocity. In some embodiments, the peak-to-peak voltage of the drive signal 60 between the first drive mode and the second drive mode may be 10V for the same fluid at the same ejection viscosity and achieving the same droplet velocity.
Fluid supply and droplet velocity
Fluid may be supplied to and from the print head 30 via the inlet tube 23 and the return tube 25 in fig. 7.
The fluid supply 40 may, for example, include a heater 58, and the heater 58 may heat the fluid to an ejection temperature that is high enough to reduce the viscosity to within a suitable range, for example, appropriate for the drive mode being applied. Additionally or alternatively, an on-board heater 59 may be disposed on the printhead 30 near or at the layer comprising the fluid chamber 10 to provide and/or maintain the fluid at a stable ejection temperature.
It has been found that when using a drive mode such as the first mode, a high actuation rate of the piezoelectric wall 8 can result in significant heating of the wall and thus of the fluid within the chamber. The actuation rate is typically represented by a duty cycle. The duty cycle represents the percentage of nozzle firing per cycle of the printhead. Increasing the duty cycle to eject more droplets means sending a higher number of drive signals to the actuator. This increases the heat generated within the piezoelectric wall 8. This generated heat is dissipated into the fluid in the fluid chamber, thereby heating the fluid and changing its physical properties, such as decreasing viscosity, and location on the viscosity-temperature curve in fig. 1.
Going from a low duty cycle to a high duty cycle generally means that the viscosity of the fluid decreases, which instantaneously changes the Oncott number of the fluid and can change the location of drop stability due to the shift in position on the Weber number-Reynolds number graph of FIG. 3. Furthermore, the decrease in viscosity increases the droplet speed and thus can affect the landing position of the droplets, which leads to a decrease in print quality.
Print reliability can be affected without managing or dissipating heat generated by chamber wall actuation.
It is therefore desirable to dynamically (i.e. regularly during operation of the head) control the droplet velocity during duty cycle changes to ensure reliable print quality. This can be achieved by varying the drive voltage in response to changes in the temperature of the fluid, which changes the droplet velocity. For example, a lower drive voltage decreases droplet velocity. Furthermore, the actuating wall is driven less forcefully and the heat generated by the actuating wall is also reduced. Droplet velocity can be dynamically controlled by using a feedback loop between fluid temperature and drive voltage, and droplet velocity and to some extent fluid temperature and viscosity can be actively managed during operation of the printhead.
The power consumed by the drive signal generation circuit to generate the drive signal for actuating the chamber walls can be used as a measure of the heat generated by the actuating chamber walls. For example, as the duty cycle increases, the current drawn by the drive signal generation circuit to generate and increase the number of drive signals increases. Thus, measurement of the current drawn provides an appropriate measure of the heating effect of the fluid within the pressure chamber 10 by the actuating wall, and hence of the expected increase in droplet velocity when the fluid within the chamber is momentarily heated when a higher duty cycle signal is applied. Accordingly, the current drawn by the drive signal generation circuit 80 may be periodically measured and provided to the controller 54. The controller determines a new peak-to-peak voltage of the drive signal from the current values and provides the new value to the drive signal generation circuit 80. Drive signal generation circuit 80 generates subsequent drive signals (and sub-droplet signals) with new peak-to-peak voltages to ensure that the droplet velocity subsequently remains substantially equal to the predefined value of the droplet velocity.
The predefined value of droplet viscosity is a value previously determined to be suitable for operation of the droplet deposition apparatus. Preferably, the velocity of the ejected droplets is kept close to or substantially equal to the predetermined droplet velocity in order to ensure reliable print quality.
If the controller determines that a reduced peak-to-peak voltage is to be applied, then the resulting drive signal that applies the reduced peak-to-peak voltage will also reduce the heat generated by the actuated wall and to some extent alter the heating effect of the fluid by the chamber walls.
Thus, adjustment of the peak-to-peak voltage in response to the current drawn by the drive signal generation circuit may be used to some extent (although less than an effect on droplet velocity) to control the temperature of the fluid within the pressure chamber 10.
The controller 54 may compare the current value provided by the drive signal generation circuit 80 to test data that was previously generated and stored in the form of a look-up table accessible to the controller 54, for example. From the look-up table, the controller 54 selects a reduced peak-to-peak voltage value corresponding to the current value and the predefined droplet voltage, where the new peak-to-peak voltage has been previously determined in the test run to stabilize the droplet velocity for the new fluid viscosity expected to result from the measured current value. In this way the droplet velocity remains stable, which provides a reliably operating print head.
In some embodiments, the drive signal generation circuitry 80 may be located within the head control circuitry 56 of the print head 30. In this case, the determination of the modified peak-to-peak value may be performed by the head control circuitry upon receiving a measurement of the current drawn by the drive signal generation circuitry 80. Head control circuit 56 selects a modified peak-to-peak voltage value corresponding to the current value and the predefined droplet voltage and provides it to drive signal generation circuit 80.
In some embodiments of droplet deposition apparatus using recirculating printheads, a return stream of ink may be used to carry away heat generated by the actuating wall. For example, a feedback loop may exist between the print head providing a temperature reading of the fluid and the fluid supply that changes the recirculation flow rate in response.
When operating with high viscosity fluids, a droplet deposition apparatus comprising the low drag print head described above may be controlled by various components of the control system of the apparatus. These will now be described with reference to fig. 13 and 14, fig. 13 being a block diagram of a droplet deposition apparatus 1 and fig. 14 being a block diagram of a control system 90 for a droplet deposition apparatus.
Droplet deposition apparatus 1 includes a print head 30, a user interface 50, such as a PC, a fluid supply 40, and a controller 54. Controller 54 receives image data from user interface 50 and determines, for each pixel line, a pixel clock trigger and sub-droplet data for the droplets to be ejected from chamber 10 of fluidic component 20 within printhead 30. The controller provides pixel clock flip-flops and sub-droplet data to drive signal generation circuitry 80 included within head control circuitry 56 of printhead 30. The drive signal generation circuit generates drive signals 60 for each pressure chamber 10 and supplies them to the chambers 10(a) to 10(n) of the fluid member 20 included in the print head 30. Fluid is supplied to the fluid component 20 from the fluid supply 40. The fluid supply 40 comprises a fluid supply controller 52, the fluid supply controller 52 being arranged to adjust the flow of fluid through the head, for example to provide a predetermined recirculation flow. The fluid supply controller 52 is configured to control a pump (not shown) within the fluid supply to apply a desired pressure differential between the inlet and return tubes of the printhead 30. The desired pressure differential value may be provided by the user interface 50 to the fluid supply controller 52.
More specifically, the control system 90 may include a fluid supply controller 52 for controlling a fluid supply configured to supply fluid to the fluid inlet in accordance with a pressure differential measured between the fluid inlet and the fluid return, wherein the fluid inlet path begins at the fluid inlet into the head and terminates at one or more nozzles, and wherein the fluid return path begins at the one or more nozzles and terminates at the fluid return of the head, and wherein the pressure differential applied by the fluid supply results in a fluid return flow into the fluid return at a flow rate of between 50ml/min and 200 ml/min.
The fluid supply 40 may also include a heater 58 in thermal contact with the fluid to enable it to heat the ink to a predetermined temperature. The heater may be controlled by a heater controller included within the fluid supply 40, such as within the fluid supply controller 52. The user interface may provide a value of the predefined temperature to a heater controller that determines the temperature to which the fluid in the fluid supply will be heated to ensure the predefined temperature of the fluid as it enters the pressure chamber 10.
Additionally or alternatively, an on-board heater 59 may be disposed within the printhead and in close proximity to and in thermal contact with the fluid in the pressure chamber, or at a location near the inlet of the pressure chamber. The heater 59 may be controlled by a heater controller included in the head control circuit 56. User interface 50 may provide a value of the predefined temperature, for example via controller 54, to an on-board heater controller that determines the amount of heat that the heater will provide to ensure the predefined temperature of the fluid within pressure chamber 10. Providing a heater within the flow path of the fluid supports the ejection of high viscosity fluids and ensures that they remain at or above the ejection temperature of the fluid to provide a fluid of a predefined ejection viscosity.
Fig. 13 also shows a media encoder circuit (media encoder circuit) 70 included in droplet deposition apparatus 1. The media encoder circuit 70 provides a pixel clock signal to the controller 54 to allow the controller to determine when a droplet enters a line of pixels on the media, and thus the correct position of the droplet entering the line of pixels on the media, in the form of a pixel clock trigger. The pixel clock flip-flop is provided to a drive signal generation circuit 80, which drive signal generation circuit 80 controls the provision of drive signals to the actuating wall in response to the pixel clock flip-flop.
To stabilize droplet velocity during fluid temperature variations caused by duty cycle variations, droplet deposition apparatus 1 may therefore include drive signal generation circuitry 80, wherein controller 54 is configured to receive a value of current consumed by drive signal generation circuitry 80, and to determine a modified peak-to-peak voltage of drive signal 60 in response to the current value, so as to modify the droplet velocity of the ejected droplets. The modified peak-to-peak voltage may then be provided to a drive signal generation circuit that generates a subsequent drive signal having the modified peak-to-peak voltage. Accordingly, the drive signal generation circuitry 80 may be configured to receive the modified peak-to-peak voltage from the controller 54 and generate the drive signal 60 having the modified peak-to-peak voltage so as to modify a droplet velocity of a droplet ejected from one or more nozzles of one or more fluid chambers of the droplet deposition head. The controller may also be configured to apply drive signals to the piezoelectric chamber walls such that the nozzle deposits droplets of the fluid having a viscosity in a range from 45 mPa-s to 130 mPa-s at a predefined ejection temperature between 20 ℃ and 90 ℃.
In some embodiments, the drive signal generation circuitry 80 may be included within the head control circuitry 56. Further, instead of controller 54, head control circuitry 56 may be configured to receive a value of current consumed by drive signal generation circuitry 80 and determine a modified peak-to-peak voltage of drive signal 60 in response to the current value in order to modify a droplet velocity of the ejected droplet. The modified peak-to-peak voltage may then be provided to a waveform generation circuit 80, which generates the subsequent drive signal 60 with the modified peak-to-peak voltage.
Furthermore, a method for operating a droplet deposition apparatus 1 is provided. The method comprises the following steps: (i) supplying fluid to the fluid chambers 10 of the droplet deposition head 30 to generate a recirculation flow of fluid through each chamber 10 at a flow rate of between 50ml/min and 200 ml/min; (ii) providing heating to the fluid before and/or after the fluid is to be supplied to the fluid inlet 23 of the head, such that the fluid in the fluid chamber 10 is at a predefined ejection temperature between 20 ℃ and 90 ℃ and corresponds to a viscosity in the range from 45 mPa-s to 130 mPa-s; and (iii) applying a drive signal 60 to the piezoelectric walls 8 of the one or more chambers so as to eject some of the fluid supplied to the chambers in the form of one or more droplets, and to return excess fluid supplied to the chambers but not ejected to the fluid return 25 of the head 30 at a flow rate of between 50ml/min and 200 ml/min.
The method may further comprise the step of providing a current signal to a controller 56 of the droplet deposition apparatus 1 based on the duty cycle of the actuation of the chamber walls 8, wherein the controller 54, 56 determines a modified peak-to-peak voltage of the drive signal 60 in response to the current value in order to keep the droplet velocity of the ejected droplets substantially equal to a predefined droplet velocity. To avoid visible defects in print reliability, the droplet speed may be kept within +/-1V of the predefined droplet speed.
The method may further comprise the step of heating the fluid in the fluid supply 40 such that the heated fluid reaching the fluid chamber 10 is substantially equal to the predefined ejection temperature.
Alternatively or additionally, the method may further comprise the step of heating the fluid on the droplet deposition head 30 such that the heated fluid reaching the fluid chamber 10 is substantially equal to the predefined temperature.
To avoid visible defects in print reliability, the ejection temperature may be kept within +/-1 ℃ of the predefined temperature. In some embodiments, the ejection temperature may be maintained within +/-0.5 ℃ of the predefined temperature.
The method may be performed by the control system 90 of the droplet deposition apparatus 1. A block diagram of the control system is shown in fig. 14. The control system 90 includes the controller 54 and the drive signal generation circuit 80. Controller 54 is configured to receive the predefined droplet velocity and current value 92 from drive signal generation circuit 80 and, in response to the current value and the predefined droplet velocity, determine a modified peak-to-peak voltage based on the stored test data. The drive signal generation circuit 80 is configured to receive the modified peak-to-peak voltage and generate the drive signal 60 with the modified peak-to-peak voltage 94 such that the generated drive signal 60 modifies the droplet velocity of the ejected droplet. The generated drive signal 60 may modify the droplet velocity such that it is substantially equal to the predefined droplet velocity. To avoid visible defects in print reliability, the droplet speed may be kept within +/-1V of the predefined droplet speed.
In the embodiment shown in the block diagram of fig. 14, the drive signal generation circuit 80 may be on the print head, although this is not required.
In an alternative embodiment of the control system, the functions of controller 54 described above may be performed instead by head control circuitry 56, and the same block diagram of controller 54 may be envisioned as being replaced with controller 56.
In some embodiments, the control system may further comprise heaters 58, 59 and a heater controller 57, wherein the heaters are configured to heat fluid provided to the chamber 10, and the heater controller 57 is configured to receive operational data 96 from the controller 56, wherein the operational data is based on the predefined droplet velocity and the current value 92 of the drive signal generation circuit 80, and wherein the heater controller 57 is further configured to control the heaters based on the operational data 96 so as to heat fluid in the chamber to a substantially predefined ejection temperature. The heater 58 may be located within the fluid supply 40 and the heater controller 57 may be included within the fluid supply controller 52. Additionally or alternatively, the heater 59 may be located on the printhead 30, and the heater controller 57 may be included within the head control circuitry 56 of the printhead.
In some embodiments, the drive signal generation circuitry 80 may be included within the head control circuitry 56. In other embodiments, the drive signal generation circuit 80 may be included within the controller 54.
It will be appreciated that the droplet deposition apparatus referred to above and herein comprises an inkjet printer and that the droplet deposition head referred to comprises an inkjet print head. To avoid visible defects in print reliability, the droplet speed may be kept within +/-1V of the predefined droplet speed. Additionally or alternatively, the ejection temperature may be maintained within +/-1 ℃ of the predefined temperature, and in some embodiments, the ejection temperature may be maintained within +/-0.5 ℃ of the predefined temperature.
In some embodiments, the peak-to-peak voltage of the drive signal between the first drive mode and the second drive mode may be 10V for the same fluid at the same ejection viscosity and achieving the same droplet velocity.
The present disclosure also provides a droplet deposition apparatus comprising a droplet deposition head, a fluid supply, and a controller, wherein: the droplet deposition head comprises one or more fluid chambers each having a nozzle, a fluid inlet path having a fluid inlet into the head and terminating at the one or more nozzles, and a fluid return path starting at the one or more nozzles and terminating at a fluid return of the head; each fluid chamber comprising two opposing chamber walls comprising a piezoelectric material and being deformable upon application of an electrical drive signal to eject a droplet from the nozzle; the fluid supply is configured to supply fluid to the fluid inlet in accordance with a pressure differential measured between the fluid inlet and the fluid return; and the controller is configured to apply a drive signal to the piezoelectric chamber walls such that the one or more nozzles deposit droplets of the fluid having a viscosity in a range from 45 to 120 mPa-s at an ejection temperature between 20 ℃ and 90 ℃, and wherein the pressure differential applied by the fluid supply results in a return flow of the fluid into the fluid return at a flow rate between 50ml/min and 200 ml/min. Optional or preferred features of such droplet deposition apparatus are as described with respect to the embodiments above.
There is also provided a method for operating such droplet deposition apparatus, the method comprising the steps of: supplying fluid to the fluid chambers of the droplet deposition head to cause a recirculation flow of fluid through each chamber at a flow rate greater than the ejection flow rate; providing heating to the fluid before and/or after the fluid is to be supplied to the fluid inlet of the head, such that the fluid in the fluid chamber is at a predefined ejection temperature and corresponds to a viscosity in a range from 45 to 120 mPa-s; and a fluid return to the head that applies a drive signal to the piezoelectric walls of the one or more chambers to eject some of the fluid supplied to the chambers in the form of one or more droplets, and returns excess fluid supplied to the chambers that is not ejected. Optional or preferred features of such a method are as described with respect to the embodiments above. A control system for performing such a method is also provided.
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