BACKGROUND

[0001] High speed inkjet web printers, commonly referred to as digital web presses, print on a continuous web moving through the printer at hundreds of feet per minute.

DRAWINGS

[0002] Figs. 1-6 illustrate one example of a moisture sensing system for an inkjet web printer. Figs. 1 and 4 are elevation views. Figs. 2 and 5 are plan views. Fig. 3 and 6 are details from Figs. 1 and 4, respectively. Figs. 1-3 show a serpentine web path for one pattern of printed images. Figs. 4-6 show a serpentine web path for another pattern of printed images.

[0003] Figs. 7 and 8 illustrate examples of an inkjet web printer with a moisture sensing system to measure the moisture content of the unprinted web and a moisture sensing system to measure the moisture content of the printed web.

[0004] Fig. 9 illustrates one example of a moisture control process for an inkjet web printer.

[0005] The same part numbers refer to the same or similar parts throughout the figures. The figures are not necessarily to scale.

DESCRIPTION

[0006] Knowing the moisture content of the web in a high speed inkjet web printer is useful in assessing the condition of the unprinted web as well as the printed web, for example to help control printing, drying and finishing. It is particularly desirable to measure the moisture content of the web during a printing operation without slowing or stopping the web. A new technique has been developed to measure the moisture content of the web in real time during printing by simultaneously sensing moisture collectively in multiple layers of the moving web. Sensing multiple layers at the same time presents more total moisture to the sensor for more accurate measurements of relatively low moisture content. Examples of the new technique may be implemented with less expensive sensors to measure the usually low moisture content of a web moving at printing speeds, for cost effective moisture sensing in high speed inkjet web printers. [0007] In one example, a moisture sensing system for an inkjet web printer includes a moisture sensor and a serpentine web path with multiple layers of the web in the sensing zone at the same time. The amount of moisture presented to the sensor varies according to the number of layers of the web that are present simultaneously in the sensing zone. More web layers in the sensing zone means more total moisture to sense. In one implementation, the serpentine web path is defined by a series of rollers that may be adjusted to vary the length of the web path to correspond with a repeating pattern printed on the web, so that the pattern appears simultaneously in the sensing zone for each of the multiple layers of the web. For example, it may be desirable for dryer control to monitor the moisture content of a region of high density ink printed repeatedly on the web. The length of the serpentine web path is adjusted to place the high density “target” in the sensing zone on each layer of the web. Moisture measurements for the target may be used to control the dryer to achieve the desired moisture content for the printed web.

[0008] These and other examples described below and shown in the figures illustrate the claimed subject matter but do not limit the scope of the patent, which is defined in the Claims following this Description.

[0009] As used in this document: “and/or” means one or more of the connected things; a “computer readable medium” means any non-transitory tangible medium that can embody, contain, store, or maintain instructions for use by a processor and may include, for example, circuits, integrated circuits, ASICs (application specific integrated circuits), hard drives, random access memory (RAM), read-only memory (ROM), and memory cards and sticks and other portable storage devices; and “light” means electromagnetic radiation of any wavelength.

[0010] Figs. 1-3 illustrate one example of a moisture sensing system 10 for an inkjet web printer. Referring to Figs. 1-3, system 10 includes a series of rollers 12-20 that define a serpentine path 22 for a paper or other printable web 24. The direction of web 24 along path 22 is indicated by arrows 26 in Fig. 1 . System 10 also includes a moisture sensor 28 positioned with respect to path 22 so that light 30 emitted by sensor 28 in a sensing zone 32 passes through multiple layers 34 of web 24 moving along serpentine path 22. Moisture sensor 28 is omitted from the plan view of Fig. 2 to not obscure underlying features.

[0011] Sensor 28 may be implemented, for example, as a microwave sensor 28 with an emitter 36 and a receiver 38 opposite emitter 36 across path 22. Microwaves 30 from emitter 36 pass through web layers 34 to receiver 38. A microwave sensor 28 with a split-cavity resonator could be used. Sensor 28 is not limited to a microwave sensor. Sensor 28 may be implemented with any suitable moisture sensor and moisture sensing technology, for example, NIR (near infrared) sensors. Microwave sensors are comparatively inexpensive and may be desirable where a serpentine web path 22 delivers enough total moisture to sensing zone 32 for accurate measurements.

[0012] Although web path 22 places five web layers 34 simultaneously in sensing zone 32 in this example, other configurations are possible. The serpentine web path may be configured to place more or fewer web layers simultaneously in the sensing zone depending on the type of sensor and the moisture content of the web in the sensing zone.

[0013] As shown in Fig. 2, web 24 is printed with a repeating pattern 42 of images 44. In the example shown in Figs. 1-3, the length of serpentine web path 22 corresponds to the repeating pattern 42 so that one of the images 44 appears in sensing zone 32 in each layer 34 of web 24. Each image 44 is depicted by heavy lines along web 24 in the detail of Fig. 3. The position of one or multiple rollers 12- 20 may be adjusted to lengthen or shorten serpentine web path 22 for different image patterns. For example, in the example shown in Figs. 4-6, roller pairs 12/14, 16/18, 17/19, and 13/15 are moved inboard compared to Figs. 1-3 for a pattern 46 of more closely spaced images 48 in Fig. 5. As shown in Fig. 6, one of the images 48 appears in sensing zone 32 in each layer 34 of web 24.

[0014] The actual length of any particular web path and the corresponding spacing of rollers to place a target image in the sensing zone on each layer of the web may vary depending on the pattern, the size of the sensing zone, the number of web layers, and the number and size of the rollers. Although serpentine web path 24 is oriented horizontally in the figures, a vertical or other orientation may be desirable for some implementations..

[0015] The position of one or multiple rollers 12-20 may be changed horizontally and/or vertically to lengthen or shorten web path 22. The roller configuration may be different from that shown in Figs. 1 and 3, for example to change the spacing between web layers 34 and/or to expose more or fewer layers 34 to a sensor 28. While the patterns 42, 46 with images 44, 48 shown in Figs. 2 and 5 suggest printed content, images 44, 48 represent generally any region of interest including, for example, reference marks printed specifically for measuring moisture content and/or unprinted parts of the web. The location of a sensing zone 32 along web path 22 may be different from that shown in the figures. In some implementations, the location of a sensing zone 32 may be adjusted by moving sensor 28, for example for different print patterns and/or for different web configurations. In some implementations, system 10 may include multiple sensors 28 with corresponding multiple sensing zones 32.

[0016] Fig. 7 illustrates one example of an inkjet web printer 50 with moisture sensing systems 10A and 10B. Each sensing system 10A, 10B includes a moisture sensor and a serpentine web path, for example sensor 28 and path 22 in Figs. 1-3. Referring to Fig. 7, printer 50 includes a web supply 52, an arched printing unit 54, a dryer 56, and a web take-up 58. In this example, a web 24 moves from supply 52 into a duplex printing unit 54 that prints on one or both sides of web 24, through dryer 56 to dry the printed web, to take up 58. Printer 50 includes a first moisture sensing system 10A upstream from printing unit 54 to measure the moisture content of the unprinted web. Printer 50 also includes a second moisture sensing system 10B downstream from printing unit 54 and dryer 56 to measure the moisture content of the printed and dried web. In this example, printer 50 also includes a web inspection station 60 commonly used in digital web presses to inspect the printed web.

[0017] Other suitable printer implementations for a moisture sensing system 10 are possible. For one example, a web printer may include more or fewer moisture control systems and/or at locations different from those shown in Fig. 7. For another example, a web printer may include two separate printing units and respective dryers for printing and drying each side of the web, with one or multiple moisture sensing systems for each printing unit and respective dryer.

[0018] Fig. 8 is a block diagram of a printer 50, such as the example printer 50 shown in Fig. 7. Referring to Fig. 8, printer 50 includes a web supply 52, a printing unit 54 to print on a moving web 24 during a printing operation, a dryer 56 to dry the printed moving web, a web take-up 58, and moisture sensing systems 10A, 10B each to simultaneously sense moisture collectively in multiple layers of the moving web. A first moisture sensing system 10A is located upstream from printing unit 54 to measure the moisture content of the unprinted web. A second moisture sensing system 10B is located downstream from printing unit 54 and dryer 56 to measure the moisture content of the printed and dried web. Each moisture sensing system 10A, 10B includes a serpentine web path 22 and a moisture sensor 28 configured to measure the moisture content of web 24 by simultaneously sensing moisture collectively in multiple layers of web 24 moving along serpentine path 22, for example as describe above with reference to Figs. 1-3.

[0019] Printer 50 also includes a controller 62 operatively connected to printing unit 54, dryer 56, and moisture sensing systems 10A and 10B. Controller 62 represents the processing and memory resources and the programming, electronic circuitry and components needed to control the operative elements of printer 50. Controller 62 may include distinct control elements for individual printer components. In the example shown in Fig. 8, controller 62 includes a processor 64 and a computer readable medium 66 with moisture control instructions 68 that represent programming to control printing unit 54 and dryer 56 based on measurements from moisture sensors 28.

[0020] Sensor 28 in sensing system 10B measures the moisture content of the moving printed web during a printing operation and communicates the measurements to controller 62. Processor 64 on controller 62 executing control instructions 68 controls dryer 56 to maintain the desired moisture content in the printed web based on measurements from system 10B. For example, if controller 62 determines the moisture content of the moving printed web is above a wet threshold based on measurements from system 10B, then controller 62 increases drying during the printing operation to lower the moisture content of the printed web. If controller 62 determines the moisture content of the moving printed web is below a dry threshold based on measurements from system 10B, then controller 62 decreases drying during the printing operation to raise the moisture content of the printed web. The way in which controller 62 increases or decreases drying may vary depending on the type of dryer 56. For a forced air dryer, for example, controller 62 may change the air temperature, fan speed, and/or printing speed to increase or decrease drying. For an IR or UV dryer, for example, controller 62 may change the intensity of the light to increase or decrease drying.

[0021] Sensor 28 in sensing system 10A measures the moisture content of the moving unprinted web during a printing operation and communicates the measurements to controller 62. Processor 64 on controller 62 executing control instructions 68 controls printing unit 54 to maintain the desired print performance based on measurements from system 10A. For example, if controller 62 determines the moisture content of the moving unprinted web is outside the expected range based on measurements from system 10A, then controller 62 adjusts one or multiple printing parameters to compensate for the unexpected moisture content.

[0022] Fig. 9 illustrates one example of a moisture control process 100 for an inkjet web printer. Referring to Fig. 9, process 100 includes measuring the moisture content of a moving web during a printing operation (block 102), for example by simultaneously sensing moisture collectively in multiple layers of the moving web as described above with reference to Figs. 1-3, and, based on the measuring, determining the moisture content of the moving printed web is above a wet threshold (block 104), for example as described above with reference to Fig. 8. Then, in response to determining the moisture content is above a wet threshold, process 100 includes reducing the moisture content of the moving web during the printing operation (block 106), for example as described above with reference to Fig. 8. [0023] "A" and "an" used in the claims means one or more. For example, “a moisture sensor” means one or more moisture sensors and subsequent reference to “the moisture sensor” means the one or more moisture sensors.