Patent Description:
Image formation is a procedure whereby a digital image is recreated on a medium by propelling droplets of ink or another type of print fluid onto a medium, such as paper, plastic, a substrate for 3D printing, etc. Image formation is commonly employed in apparatuses, such as printers (e.g., inkjet printer), facsimile machines, copying machines, plotting machines, multifunction peripherals, etc. The core of a typical jetting apparatus or image forming apparatus is one or more liquid-droplet ejection heads (referred to generally herein as "printheads") having nozzles that discharge liquid droplets, a mechanism for moving the printhead and/or the medium in relation to one another, and a controller that controls how liquid is discharged from the individual nozzles of the printhead onto the medium in the form of pixels.

A typical printhead includes a plurality of nozzles aligned in one or more rows along a discharge surface of the printhead. Each nozzle is part of a "jetting channel", which includes the nozzle, a pressure chamber, and a diaphragm that is driven by an actuator, such as a piezoelectric actuator. A printhead also includes a drive circuit that controls when each individual jetting channel fires based on image data. To jet from a jetting channel, the drive circuit provides a jetting pulse to the actuator, which causes the actuator to deform a wall of the pressure chamber via the diaphragm. The deformation of the pressure chamber creates pressure waves within the pressure chamber that eject a droplet of print fluid (e.g., ink) out of the nozzle.

<CIT>, <CIT>, <CIT>, <CIT>, <CIT>, <CIT> and <CIT> disclose background art to the invention. <CIT> discloses a jetting apparatus with ink circulation through the printhead. Between the ink supply and ink recovery manifolds bypass channels are formed to prevent ink stagnation in the manifolds.

Embodiments described herein comprise a jetting apparatus comprising a flow-through type of printhead, where a print fluid is able to flow from a supply manifold through jetting channels to a return manifold, or vice-versa. The print fluid, which is not ejected from nozzles of the jetting channels, circulates through the jetting channels and into the return manifold. A printhead as described herein has one or more bypass manifolds that fluidly couple the supply manifold and the return manifold. The bypass manifold(s) helps reduce the pressure delta required for the printhead, and helps reduce a pressure difference between nozzles closer to an inlet port on the printhead and nozzles closer to an outlet port.

Some embodiments of the present invention are now described, by way of example only, and with reference to the accompanying drawings.

The figures and the following description illustrate specific exemplary embodiments. It will thus be appreciated that those skilled in the art will be able to devise various arrangements that, although not explicitly described or shown herein, embody the principles of the embodiments and are included within the scope of the embodiments. Furthermore, any examples described herein are intended to aid in understanding the principles of the embodiments, and are to be construed as being without limitation to such specifically recited examples and conditions.

<FIG> is a schematic diagram of a jetting apparatus <NUM> in an illustrative embodiment. One example of jetting apparatus <NUM> is an inkjet printer that performs single-pass or multi-pass printing. Jetting apparatus <NUM> includes a mounting bracket <NUM> that supports one or more printheads <NUM> above a medium <NUM>. Mounting bracket <NUM> may be disposed on a carriage assembly that reciprocates back and forth along a scan line or scan directions for multi-pass printing. Alternatively, mounting bracket <NUM> may be fixed within jetting apparatus <NUM> for single-pass printing. Printheads <NUM> are a device, apparatus, or component configured to eject droplets <NUM> of a print fluid, such as ink (e.g., water, solvent, oil, or UV-curable), through a plurality of orifices or nozzles (not visible in <FIG>). The droplets <NUM> ejected from the nozzles of printheads <NUM> are directed toward medium <NUM>. Medium <NUM> comprises any type of material upon which ink or another print fluid is applied by a printhead, such as paper, plastic, card stock, transparent sheets, a substrate for 3D printing, cloth, etc. Typically, nozzles of printheads <NUM> are arranged in one or more rows so that ejection of print fluid from the nozzles causes formation of characters, symbols, images, layers of an object, etc., on medium <NUM> as printhead <NUM> and/or medium <NUM> are moved relative to one another. Media transport mechanism <NUM> is configured to move medium <NUM> relative to printheads <NUM>. Jetting apparatus <NUM> also includes a jetting apparatus controller <NUM> that controls the overall operation of jetting apparatus <NUM>. Jetting apparatus controller <NUM> may connect to a data source to receive image data, and control each printhead <NUM> to discharge the print fluid on a desired pixel grid on medium <NUM>. Jetting apparatus <NUM> also includes one or more reservoirs <NUM> for a print fluid. Although not shown in <FIG>, reservoirs <NUM> may be connected to printheads <NUM> via hoses or the like.

<FIG> is a perspective view of a printhead <NUM> in an illustrative embodiment. Printhead <NUM> includes a head member <NUM> and electronics <NUM>. Head member <NUM> is an elongated component that forms the jetting channels of printhead <NUM>. A typical jetting channel includes a nozzle, a pressure chamber, and a diaphragm that is driven by an actuator, such as a piezoelectric actuator. Electronics <NUM> control how the nozzles of printhead <NUM> jet droplets in response to control signals. Although not visible in <FIG>, electronics <NUM> may include a plurality of actuators (e.g., piezoelectric actuators) that contact the diaphragms of the jetting channels. Electronics <NUM> also include cabling <NUM>, such as a ribbon cable, that connects to a controller (e.g., jetting apparatus controller <NUM>) to receive the control signals. Printhead <NUM> also includes attachment members <NUM>, which are configured to secure printhead <NUM> to a jetting apparatus, such as to mounting bracket <NUM> as illustrated in <FIG>. Attachment members <NUM> may include one or more holes <NUM> so that printhead <NUM> may be mounted within a jetting apparatus by screws, bolts, pins, etc..

The bottom surface <NUM> of head member <NUM> includes the nozzles of the jetting channels, and represents the discharge surface of printhead <NUM>. The top surface <NUM> of head member <NUM> represents the Input/Output (I/O) portion for receiving print fluids into printhead <NUM> and/or conveying print fluids (e.g., fluids that are not jetted) out of printhead <NUM>. Top surface <NUM>, which is also referred to as the I/O surface, includes a plurality of I/O ports <NUM>-<NUM>. Top surface <NUM> has two ends <NUM>-<NUM> that are separated by electronics <NUM>. I/O port <NUM> is disposed toward end <NUM>, and I/O port <NUM> is disposed toward end <NUM>. I/O ports <NUM>-<NUM> may include a hose coupling, hose barb, etc., for coupling with a supply hose of a reservoir <NUM>, a cartridge, or the like.

Head member <NUM> includes a housing <NUM> and a plate stack <NUM>. Housing <NUM> is a rigid member made from stainless steel or another type of material. Housing <NUM> includes an access hole <NUM> that provides a passageway for electronics <NUM> to pass through housing <NUM> so that actuators may interface with diaphragms of the jetting channels. Plate stack <NUM> attaches to an interface surface (not visible) of housing <NUM>. Plate stack <NUM> (also referred to as a laminate plate stack) is a series of plates that are fixed or bonded to one another to form a laminated stack. Plate stack <NUM> includes the following plates: one or more nozzle plates, one or more chamber plates, one or more restrictor plates, and a diaphragm plate. A nozzle plate includes a plurality of nozzles that are arranged in one or more rows (e.g., two rows, four rows, etc.). A chamber plate includes a plurality of openings that form the pressure chambers of the jetting channels. A restrictor plate includes a plurality of restrictors that fluidly connect the pressure chambers of the jetting channels with a manifold. A diaphragm plate is a sheet of a semi-flexible material that vibrates in response to actuation by an actuator (e.g., piezoelectric actuator).

<FIG> is a schematic diagram of jetting channels <NUM> within printhead <NUM> in an illustrative embodiment. This diagram represents a view along a length of printhead <NUM>. A jetting channel <NUM> is a structural element within printhead <NUM> that jets or ejects a print fluid. Each jetting channel <NUM> includes a diaphragm <NUM>, a pressure chamber <NUM>, and a nozzle <NUM>. An actuator <NUM> contacts diaphragm <NUM> to control jetting from a jetting channel <NUM>. Jetting channels <NUM> may be formed in one or more rows along a length of printhead <NUM>, and each jetting channel <NUM> may have a similar configuration as shown in <FIG>.

<FIG> are schematic diagrams of a jetting channel <NUM> within printhead <NUM> in an illustrative embodiment. The view in <FIG> is of a cross-section of a jetting channel <NUM> across a width of a portion of printhead <NUM>. A supply manifold <NUM> is configured to supply a print fluid to jetting channel <NUM> through a first restrictor <NUM>. Restrictor <NUM> fluidly couples pressure chamber <NUM> with supply manifold <NUM>, and controls the flow of the print fluid into pressure chamber <NUM>. A return manifold <NUM> is configured convey a print fluid out of a jetting channel <NUM> through a second restrictor <NUM>. Restrictor <NUM> fluidly couples pressure chamber <NUM> to return manifold <NUM>, and controls the flow of the print fluid out of pressure chamber <NUM>. Printhead <NUM> is a "flow-through" printhead or re-circulating printhead, which means that the print fluid may be re-circulated through printhead <NUM> past each nozzle <NUM>. By having a flow-through design, a print fluid is able to flow from supply manifold <NUM> to a return manifold <NUM> through jetting channels <NUM> in printhead <NUM>.

The arrow in <FIG> illustrates a flow path of a print fluid through jetting channel <NUM> in one direction. Although not shown in <FIG>, supply manifold <NUM> is coupled to a reservoir or the like, and return manifold <NUM> is coupled to the same or another reservoir. The print fluid flows from supply manifold <NUM> in printhead <NUM> and into pressure chamber <NUM> through a first restrictor <NUM>. One wall of pressure chamber <NUM> is formed with diaphragm <NUM> that physically interfaces with actuator <NUM>, and vibrates in response to actuation by actuator <NUM>. The print fluid flows through pressure chamber <NUM> and out of nozzle <NUM> in the form of a droplet in response to actuation by actuator <NUM>. Actuator <NUM> is configured to receive a drive waveform, and to actuate or "fire" in response to a jetting pulse on the drive waveform. Firing of actuator <NUM> in jetting channel <NUM> creates pressure waves in pressure chamber <NUM> that cause jetting of a droplet from nozzle <NUM>. The print fluid, which is not jetted from nozzle <NUM>, flows from pressure chamber <NUM> into return manifold <NUM> through a second restrictor <NUM>. The print fluid may flow through jetting channel <NUM> due to a pressure difference between a reservoir coupled to supply manifold <NUM> and a reservoir coupled to return manifold <NUM>.

The arrow in <FIG> illustrates a flow path of a print fluid within jetting channel <NUM> in a reverse direction. The print fluid flows from return manifold <NUM> and into pressure chamber <NUM> through the second restrictor <NUM>. The print fluid flows through pressure chamber <NUM> and out of nozzle <NUM> in the form of a droplet in response to actuation by actuator <NUM>. The print fluid, which is not jetted from nozzle <NUM>, flows from pressure chamber <NUM> into supply manifold <NUM> through the first restrictor <NUM>. The length of the first restrictor <NUM> may be the same as the length of the second restrictor <NUM> to allow for a reversal of flow in this manner.

Jetting channel <NUM> as shown in <FIG> is an example to illustrate a basic structure of a jetting channel, such as the diaphragm, pressure chamber, and nozzle. Other types of jetting channels are also considered herein. For example, some jetting channels may have a pressure chamber having a different shape than is illustrated in <FIG>. Also, the position of supply manifold <NUM>, return manifold <NUM>, and/or restrictors <NUM>/<NUM> may differ in other embodiments.

<FIG> is a schematic diagram of printhead <NUM> in an illustrative embodiment. The jetting channels <NUM> of printhead <NUM> are schematically illustrated in <FIG> as nozzles in two nozzle rows. Although the nozzles are shown as staggered in <FIG>, the nozzles in the two nozzle rows may be aligned in other embodiments. Also, there may be more or less than two nozzle rows in other embodiments. Head member <NUM> of printhead <NUM> includes supply manifold <NUM>, which is a groove, duct, conduit, etc., within head member <NUM> that is configured to convey or supply a print fluid to/from jetting channels <NUM>. Supply manifold <NUM> is fluidly coupled to I/O port <NUM>, and is also fluidly coupled to the jetting channels <NUM> indicated by nozzles <NUM> via fluid path <NUM>. Fluid path <NUM> is provided in the form of a restrictor (e.g., restrictor <NUM>), which is a passageway that fluidly couples a manifold to a pressure chamber and prevents a backflow of print fluid. When a print fluid is supplied to I/O port <NUM>, the print fluid flows through supply manifold <NUM> and is drawn into the jetting channels <NUM>. The major portions or sections of supply manifold <NUM> are disposed longitudinally within printhead <NUM> to fluidly couple with a row of jetting channels <NUM>.

Head member <NUM> of printhead <NUM> also includes return manifold <NUM>, which is a groove, duct, conduit, etc., within head member <NUM> that is configured to convey or receive a print fluid to/from jetting channels <NUM>. Return manifold <NUM> is fluidly coupled to I/O port <NUM>, and is also fluidly coupled to the jetting channels <NUM> indicated by nozzles <NUM> via fluid path <NUM>. Fluid path <NUM> is provided in the form of a restrictor (e.g., restrictor <NUM>). A print fluid may flow out of jetting channels <NUM>, through return manifold <NUM>, and out I/O port <NUM>. The major portions or sections of return manifold <NUM> are disposed longitudinally within printhead <NUM> to fluidly couple with a row of jetting channels <NUM>. Because the flow of print fluid through printhead <NUM> may be reversed, supply manifold <NUM> may act as a return manifold, and return manifold <NUM> may act as a supply manifold depending on the direction of flow of print fluid through printhead <NUM>.

Printhead <NUM> also includes one or more bypass manifolds <NUM> disposed between supply manifold <NUM> and return manifold <NUM>. Bypass manifold <NUM> is a groove, duct, conduit, etc., within head member <NUM> that fluidly couples two other manifolds directly. Thus, supply manifold <NUM> and return manifold <NUM> are fluidly coupled by jetting channels <NUM> (because they are a flow-through type), and are also fluidly coupled by bypass manifolds <NUM>. A bypass manifold <NUM> is a high-impedance passage, which means that the fluid resistance of bypass manifold <NUM> is greater than the fluid resistance of jetting channels <NUM> within printhead <NUM>. The length or width of bypass manifold <NUM> may be designed in a desired manner to ensure that the fluid resistance of bypass manifold <NUM> is greater than the fluid resistance of jetting channels <NUM>. Bypass manifold <NUM> helps reduce the pressure delta required for printhead <NUM>, and also helps reduce the pressure difference between nozzles <NUM> closer to I/O port <NUM> (which acts as an inlet) and nozzles <NUM> closer to I/O port <NUM> (which acts as an outlet).

The following embodiments set forth examples of the structure of head member <NUM>. <FIG> illustrate the structure of head member <NUM> in one illustrative embodiment. The structural elements in these figures are not drawn to scale, and are provided as an example. As an overview, head member <NUM> includes jetting channels for two rows of nozzles. Head member <NUM> also includes a supply manifold, a return manifold, and one or more bypass manifolds disposed between the supply manifold and the return manifold. As described above in <FIG>, head member <NUM> includes a housing <NUM> and a plate stack <NUM>. <FIG> is a bottom view of housing <NUM> in an illustrative embodiment. The bottom surface of housing <NUM> is referred to as interface surface <NUM>, which is the surface of housing <NUM> that faces plate stack <NUM> and interfaces with plate stack <NUM>. Housing <NUM> includes an access hole <NUM> at or near its center that extends from interface surface <NUM> through to top surface <NUM> (see <FIG>). Access hole <NUM> provides a passageway for actuators <NUM>, such as a plurality of piezoelectric actuators, to pass through and interface with a diaphragm plate (shown in <FIG>). In this embodiment, actuators <NUM> are arranged in two rows that are staggered.

Housing <NUM> also includes supply manifold duct <NUM>, which comprises a cut or groove along interface surface <NUM> configured to convey a print fluid. Supply manifold duct <NUM> is generally a loop around access hole <NUM> that forms the supply manifold for printhead <NUM>. Supply manifold duct <NUM> includes straight sections that are disposed longitudinally along the length of housing <NUM>, and also include sections that are disposed transversely. Housing <NUM> further includes return manifold ducts <NUM>, which also comprise cuts or grooves along interface surface <NUM> configured to convey a print fluid. Return manifold ducts <NUM> are disposed generally transverse on interface surface <NUM> toward the short ends of housing <NUM> to form the return manifold for printhead <NUM>. Supply manifold duct <NUM> is fluidly coupled to I/O port <NUM>, and one of return manifold ducts <NUM> is fluidly coupled to I/O port <NUM>.

<FIG> show one example of plate stack <NUM> that includes a diaphragm plate, an upper restrictor plate, chamber plates, a lower restrictor plate, and a nozzle plate. <FIG> is a plan view of a diaphragm plate <NUM> in an illustrative embodiment. Diaphragm plate <NUM> is a thin sheet of material (e.g., metal, plastic, etc.) that is generally rectangular in shape and is substantially flat or planar. Diaphragm plate <NUM> includes diaphragm sections <NUM> comprising a semi-flexible material that forms diaphragms for the jetting channels. Diaphragm plate <NUM> further includes supply manifold openings <NUM>, which comprise elongated apertures or holes through diaphragm plate <NUM> disposed longitudinally along a length of diaphragm plate <NUM>. Supply manifold openings <NUM> are disposed toward the long sides <NUM>-<NUM> of diaphragm plate <NUM> on opposing sides of diaphragm sections <NUM> to coincide with the longitudinal sections of supply manifold duct <NUM> of housing <NUM> and to form the supply manifold. In other embodiments, supply manifold openings <NUM> may include or comprise filters.

Diaphragm plate <NUM> also includes return manifold openings <NUM> that coincide, at least in part, with return manifold ducts <NUM> of housing <NUM>. Return manifold openings <NUM> comprise apertures or holes through diaphragm plate <NUM> disposed toward the corners of diaphragm plate <NUM>, where the long sides <NUM>-<NUM> of diaphragm plate <NUM> meet the short sides <NUM>-<NUM>. Diaphragm plate <NUM> also includes bypass manifold openings <NUM>. Bypass manifold openings <NUM> comprise elongated apertures or holes through diaphragm plate <NUM> that coincide with supply manifold duct <NUM> and a return manifold duct <NUM> of housing <NUM>, and are configured to fluidly couple supply manifold duct <NUM> with return manifold duct <NUM>. Bypass manifold openings <NUM> may be disposed longitudinally as shown in <FIG>, but may have different orientations to intersect with both the supply manifold duct <NUM> and a return manifold duct <NUM>. Although the position of bypass manifold openings <NUM> may vary as desired, bypass manifold openings <NUM> may be positioned toward the short ends <NUM>-<NUM> of diaphragm plate <NUM>, and extend inward toward diaphragm sections <NUM>.

<FIG> is a plan view of upper restrictor plate <NUM> in an illustrative embodiment. Upper restrictor plate <NUM> is a thin sheet of material that is generally rectangular in shape and is substantially flat or planar. Upper restrictor plate <NUM> includes restrictor openings <NUM>, which comprise elongated apertures or holes through upper restrictor plate <NUM> transversely disposed or oriented between a center of upper restrictor plate <NUM> and a long side <NUM>-<NUM> of upper restrictor plate <NUM>. Restrictor openings <NUM> are configured to fluidly couple pressure chambers of jetting channels with supply manifold openings <NUM> of diaphragm plate <NUM>. Restrictor openings <NUM> are aligned in two longitudinal rows that are staggered in this embodiment, but the configuration of the restrictors may vary as desired. Upper restrictor plate <NUM> also includes return manifold openings <NUM> that coincide, at least in part, with return manifold openings <NUM> of diaphragm plate <NUM>. Return manifold openings <NUM> comprise apertures or holes through upper restrictor plate <NUM> disposed toward the corners of upper restrictor plate <NUM>, where the long sides <NUM>-<NUM> of upper restrictor plate <NUM> meet the short sides <NUM>-<NUM>.

<FIG> is a plan view of an upper chamber plate <NUM> in an illustrative embodiment. Upper chamber plate <NUM> is a thin sheet of material that is generally rectangular in shape and is substantially flat or planar. Upper chamber plate <NUM> includes chamber openings <NUM> disposed toward a middle region of upper chamber plate <NUM>. Chamber openings <NUM> comprise apertures or holes through upper chamber plate <NUM> transversely disposed or oriented between a center of upper chamber plate <NUM> and a long side <NUM>-<NUM> of upper chamber plate <NUM>. Chamber openings <NUM> form pressure chambers for the jetting channels. Chamber openings <NUM> do not extend transversely as far as restrictor openings <NUM> (see <FIG>). Chamber openings <NUM> are aligned in two longitudinal rows that are staggered in this embodiment, but the configuration of the chambers may vary as desired. Upper chamber plate <NUM> also includes return manifold openings <NUM> that coincide, at least in part, with return manifold openings <NUM> of upper restrictor plate <NUM>. Return manifold openings <NUM> comprise apertures or holes through upper chamber plate <NUM> disposed toward the corners of upper chamber plate <NUM>, where the long sides <NUM>-<NUM> of upper chamber plate <NUM> meet the short sides <NUM>-<NUM>.

<FIG> is a plan view of a lower chamber plate <NUM> in an illustrative embodiment. Lower chamber plate <NUM> is a thin sheet of material that is generally rectangular in shape and is substantially flat or planar. Lower chamber plate <NUM> includes chamber openings <NUM> disposed toward a middle region of lower chamber plate <NUM>. Chamber openings <NUM> comprise apertures or holes through lower chamber plate <NUM> that are transversely disposed or oriented between a center of lower chamber plate <NUM> and a long side <NUM>-<NUM> of lower chamber plate <NUM>. Chamber openings <NUM> coincide with chamber openings <NUM> of upper chamber plate <NUM> to form the pressure chambers for the jetting channels. Chamber openings <NUM> are aligned in two longitudinal rows that are staggered in this embodiment, but the configuration of the chambers may vary as desired. Lower chamber plate <NUM> further includes return manifold openings <NUM>, which comprise elongated apertures or holes through lower chamber plate <NUM> disposed longitudinally along a length of lower chamber plate <NUM>. Return manifold openings <NUM> are disposed toward the long sides <NUM>-<NUM> of lower chamber plate <NUM> on opposing sides of chamber openings <NUM> to coincide with return manifold openings <NUM> of upper chamber plate <NUM>, and form the return manifold.

<FIG> is a plan view of lower restrictor plate <NUM> in an illustrative embodiment. Lower restrictor plate <NUM> is a thin sheet of material that is generally rectangular in shape and is substantially flat or planar. Lower restrictor plate <NUM> includes restrictor openings <NUM>, which comprise elongated apertures or holes through lower restrictor plate <NUM> transversely disposed or oriented between a center of lower restrictor plate <NUM> and a long side <NUM>-<NUM> of lower restrictor plate <NUM>. Restrictor openings <NUM> are configured to fluidly couple pressure chambers of jetting channels with return manifold openings <NUM> of lower chamber plate <NUM>. Restrictor openings <NUM> are aligned in two longitudinal rows that are staggered in this embodiment, but the configuration of the restrictors may vary as desired.

<FIG> is a plan view of nozzle plate <NUM> in an illustrative embodiment. Nozzle plate <NUM> is a thin sheet of material that is generally rectangular in shape and is substantially flat or planar. Nozzle plate <NUM> includes orifices that form nozzles <NUM> of the jetting channels. Nozzles <NUM> are arranged in two nozzle rows that are staggered or offset from one another, but may be arranged in a single row, two rows, in four rows, or another number of rows in other embodiments.

The flow resistance of bypass manifold <NUM> (see <FIG>) may be tuned or selected for optimal performance of printhead <NUM>. The following provides an example of selecting a flow resistance for bypass manifold <NUM>. Printhead <NUM> may be modeled as an electrical circuit <NUM> as shown in <FIG>. In electrical circuit <NUM>, Vs represents supply inlet pressure, Vr represents return outlet pressure, Vm represents a pressure delta across a manifold, Vc represents a pressure delta across a pressure chamber, Rm represents a manifold resistance, Rb represents a bypass resistance, C1 represents a first pressure chamber, and Cn represents a last pressure chamber. To solve for Rb, total V = Vs+Vr, total R = (Rm+Rb)/<NUM>, and total I = V/R = 2V/(Rm+Rb). Vc = Vc1 = Vcn = Rb*I/<NUM> = Rb*V/(Rm+Rb). Total pressure between inlet and outlet V, normalized by Vc, is V/Vc = (Rm+Rb)/Rb = <NUM>+Rm/Rb. Pressure across a manifold (i.e., the delta between C1 and Cn), normalized by Vc, is Rm*I/<NUM>/Vc = Rm*[2V/(Rm+Rb)]/<NUM>/[Rb*V/(Rm+Rb)] = Rm/Rb. Vc and Rm are constants, so Rb may be selected based on delta pressure between inlet and outlet (<NUM>+Rm/Rb) and/or delta pressure across a manifold (Rm/Rb). <FIG> is a graph showing resistance ratios in an illustrative embodiment.

Claim 1:
A jetting apparatus comprising:
at least one printhead (<NUM>) configured to jet droplets onto a medium, the at least one printhead including a plurality of flow-through jetting channels (<NUM>) each configured to jet a print fluid out of a nozzle; and
a controller (<NUM>) configured to control the at least one printhead (<NUM>);
wherein the at least one printhead (<NUM>) further includes:
a supply manifold (<NUM>) fluidly coupled to, and configured to supply a print fluid to, the flow-through jetting channels (<NUM>);
a return manifold (<NUM>) fluidly coupled to, and configured to receive the print fluid from, the flow-through jetting channels (<NUM>);
at least one bypass manifold (<NUM>) fluidly coupled between the supply manifold (<NUM>) and the return manifold (<NUM>); and
a housing (<NUM>) having Input/Output (I/O) ports (<NUM>, <NUM>) disposed at a top surface; and a plate stack (<NUM>) attached to an interface surface (<NUM>) of the housing that forms the flow-through jetting channels (<NUM>), wherein the housing and the plate stack form: the supply manifold (<NUM>) that is fluidly coupled to a first one of the I/O ports (<NUM>); and the return manifold (<NUM>) that is fluidly coupled to a second one of the I/O ports (<NUM>);
wherein the plate stack (<NUM>) comprises:
a diaphragm plate (<NUM>) that forms diaphragms (<NUM>) for the flow-through jetting channels (<NUM>);
an upper restrictor plate (<NUM>);
an upper chamber plate (<NUM>) and a lower chamber plate (<NUM>) that form pressure chambers for the flow-through jetting channels (<NUM>), wherein the upper restrictor plate (<NUM>) fluidly couples the pressure chambers to the supply manifold (<NUM>);
a lower restrictor plate (<NUM>), wherein the lower restrictor plate fluidly couples the pressure chambers to the return manifold (<NUM>); and
a nozzle plate (<NUM>) having nozzles (<NUM>) for the flow-through jetting channels (<NUM>);
wherein the housing (<NUM>) includes a supply manifold duct (<NUM>) along the interface surface that forms the supply manifold (<NUM>), and one or more return manifold ducts (<NUM>) along the interface surface that form the return manifold (<NUM>); and
the diaphragm plate (<NUM>) includes at least one bypass manifold opening (<NUM>) configured to fluidly couple the supply manifold duct (<NUM>) and the return manifold ducts (<NUM>) of the housing (<NUM>).