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HP Pre Drupa Briefing - Wide Scan Printing Technology explained

Important to the new Latex Printing Technology announced by HP at their Pre Drupa event is their Wide Scan Printing Technology, a system of modular print heads that can be added by the operator to increase the width of print.

HP Scalable Printing Technology
An important element of print quality is precise dot size and dot placement on the print media. As printheads scan across the media, they must deliver drops of ink reliably and within specifications on weight, speed and trajectory. The design and construction of an inkjet print head is critical to drop ejection performance.

HP Scalable Printing Technology (''SPT'') is the result of HP's investment of $1.4 billion and four years of research and development. SPT print heads incorporate fluidic design and electronic control features that are needed to meet the demands of commercial and industrial inkjet printing.

Print heads used in HP Wide Scan Printing Technology are built on a silicon substrate (“chip'') using SPT. SPT uses integrated circuit manufacturing processes for the on-chip electronic circuits that provide print head control, signal-processing, and monitoring functions. These processes are also used for precision assembly of the fluidic (ink flow) structures that form the drop generator. With SPT, thousands of identical drop generators can be produced on a single chip, and this ensures high print quality with consistent drop weights, drop speeds, and drop trajectories.

A cross-sectional detail of a single SPT drop generator is shown in Figure 1 . To get a sense of scale, the entire fluidic structure (shown in a tan colour) is less than the thickness of a human hair.

Fluidic structures are built up from the surface of the silicon substrate by applying a thin layer of photo-sensitive epoxy material (shown in a tan colour in Figure 1 ). The epoxy is exposed through an optical mask and then chemically developed to remove material. Repeating this process produces ink channels and filters, drop generator chambers and orifices (nozzles). These features are defined and aligned together with sub-micron precision to become an integrated structure that is assembled in-place and without adhesives.

Figure 1: Detail of a Drop Generator built using HP Scalable Printing Technology
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Because the SPT process can define and produce very small physical features, ink filters can be placed in the ink supply for each drop generator (see the ''pillars'' in Figure 1 ). This improves print head reliability by preventing unwanted particles in the ink from clogging the drop generators. The epoxy materials used in SPT resist chemical interaction with ink to give long operational life.

A single SPT print head chip is shown in Figure 2. The chip is mounted onto a ceramic substrate that provides electrical connections and ink supply. Wire bonds at each end on the chip attach conductors that carry power and signals to operate the chip and to monitor its performance. The chip prints with two arrays of 1,056 nozzles, each with its own ink supply, for a total of 2116  nozzles on-chip and the capability of printing two colours of ink. With its nozzles closely- spaced at 1200 per inch, the print head can print 1,200 dpi (vertical) in a single pass.

SPT delivers a high level of electronic integration: the 2112 drop generators on this chip are powered and controlled by only 28 electrical interconnections. This allows print head chips to be stacked closely together to make wide printing arrays and fewer electrical connections between the print head and the printer make an important contribution to overall system reliability.

HP Wide Scan Print Heads
Each print head used in HP Wide Scan Printing Technology prints a swath 4.25 inches (108mm) wide. These wide print heads are made by precisely placing and aligning five of the 2112-nozzle print head chips, shown in Figure 2, on a ceramic substrate as seen in Figure 3. A small overlap between the nozzle arrays of neighbouring chips further minimises any visible effects on print quality between chips.

Figure 2: Print head "Chip" shown mounted on a Ceramic Substrate (Multi-Layer Ceramic Module)

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The ceramic substrate is a stack of six ceramic circuit boards that are fused together into an integrated assembly. This multi-layer ceramic module has power and ground planes and routes electrical signals to the five print head chips.
10,560 nozzles are operated with only 76 electrical interconnections to the printer. The module provides precise chip-to-chip alignment on a dimensionally- stable, flat, and rigid substrate. Slots through the layers supply the two inks.

The print head array prints two colours of ink with 5,280 nozzles per colour. With a 12pl drop volume, operating all 10,560 nozzles at 24 KHz could theoretically deliver 180 ml of ink per minute.
The multi-layer ceramic module is mounted on a plastic print head body, which provides mechanical, electrical, and ink interconnections to the printer and contains two independent pressure regulators for ink delivery to the print head array.

The print heads used in HP Wide Scan Printing Technology have been designed together with HP Latex Inks for low user maintenance, reliable performance, and maximum productivity. HP Latex Inks are formulated to keep critical components of the print head clean to optimise drop ejection performance and to provide consistent, dependable results.

When necessary, the user can remove a used print head and install a new one without the expense, printer down-time, and inconvenience of a service call.

Figure 3: Print head Array and Print head Module used in HP Wide Scan Printing Technology
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HP Wide Scan Printing Technology
HP Wide Scan Fringing Technology is based on HP Wide Scan Print heads and the HP Optical Media Advance Sensor. Working together, these technologies deliver high quality at high speed using a wide print swath and accurate, longer moves of the print medium between scans of the print carriage. Accurate media positioning takes advantage of the throughput potential of wide print swaths enabling printers based on HP Latex Printing Technologies to deliver breakthrough print speeds in the price and performance categories for which these printers are intended.

HP Wide Scan Printing Technology is a scalable platform for industrial printing. Using print heads each printing two colours of ink, 4-, 6-, and more colour writing systems can be built. For example, a 6-colour writing system could use three HP Wide Scan Print heads and have a total of 31,680 nozzles (5,280 per colour). In a 2-pass, unidirectional print mode, this writing system could deliver approximately 800 square feet/hour of dry, ready-to-use outdoor- quality prints. In a 4-pass, unidirectional print mode, indoor-quality prints are produced at approximately 400 square feet/hour.

Figure 4:
img032.gif HP Optical Media Advance Sensor
The print medium is held stationary as the print heads scan to produce an image. After each scan is completed, the print is fed through the print zone, and the process repeats until the entire image is printed. Moving media in these discrete steps is called ''media advance''.

Precise and accurate motion control of media advance is a critical element of image quality.

Media position errors at a fraction of a dot row can be visible as banding - either a white line across the image or a colour band that is lighter or darker than its surroundings. While multiple-pass print modes can help to hide these bands, image quality improvement is gained at the expense of productivity.

For example, in a 4-pass print mode, the print heads scan four times over the some area on the print and the media is advanced 1/4 of the print head width between passes.

HP Wide Scan Printing Technology delivers high productivity by using 4.25-inch (108mm) print head arrays to cover a large area with ink on each print head scan. But, wider print swaths require the media drive system to move media farther while still keeping position errors within absolute tolerances.

Most inkjet printers make indirect measurement of media advance by using an encoder to measure the rotation of a mechanical element in the media drive system, such as a gear or drive roller. The media motion control system can be calibrated to convert this rotational signal into an estimate of media position and velocity. But, although the rotational position may be measured accurately, there are a number of variable factors that affect the conversion of rotary motion of drive train elements into linear motion of the media, and these can lead to media advance errors.

Even when manufactured to high precision, gear trains and the media drive rollers have some out-of-round eccentricity, backlash, and other imperfections. And, each type of media has its own mechanical characteristics (e.g., thickness, stiffness, and friction coefficient) that affect how rotation of media drive elements is translated into linear motion.

Figure 5: Banding may be experienced when 'not' using HP Optical Media Advance Sensor
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Furthermore, media characteristics may vary batch-to-batch and with temperature and humidity in the print shop.

The media drive system for an individual printer can be calibrated automatically for a specific type of media by printing and then scanning a special test pattern. This process compares the intended motion to the actual linear motion of the media surface.

This information is stored in the printer and used by on algorithm that compensates for the measured discrepancies. For best results, this calibration must be repeated periodically and it is particularly necessary when changes occur in temperature and humidity in the print shop and between different batches of media.

If the linear motion of the media surface could be measured directly, then control of media advance could be based on actual motion. This would give the highest-possible accuracy by eliminating the effects of temperature, humidity, and many variables in the media drive system and in media mechanical properties.

To deliver high image quality at the productivity potential of HP Wide Scan Print heads, HP invented and developed a proprietary technology for direct, precision measurement of media motion: the HP Optical Media Advance Sensor. The sensor and its installation are shown in Figure 5.

The HP Optical Media Advance Sensor is a tiny, custom ''machine vision'' system built into the printer platen. The sensor looks through the platen at the back side of the print media. Since the microscopic structure of the media surface provides the signal used for position measurement, no printed tracking patterns or artificial marks are required.

The sensor module, only 3 inches (75mm) high, consists of an optical assembly and printed circuit board.

Figure 6: HP Optical Media Advance Sensor and CAD Rendering of Installation in the Printer Platen
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The optical assembly has a hardened glass window that is normally in contact with the back side of the media, an array of bright red light-emitting diodes (LEDs) to provide illumination, and a lens system to project a magnified image of media surface texture onto a digital image sensor.

The small printed circuit board at the bottom of the module holds a custom image sensor designed for high-speed imaging and fast data transfer, control electronics for the sensor and LEDS, and a programmable memory chip with factory calibration data for the sensor and optics.

The HP Optical Media Advance Sensor works by taking digital images on the microscopic structure on the back side of the media as it moves through the printer. Two images of the same area on the surface are taken about 3mm apart as the media moves continuously. The LEDS flash to freeze the motion.

The images are compared and a precise physical offset between images is computed in real-time by sophisticated image correlation algorithms processed by a dedicated Digital Signal Processor (DSP).

With a three-sigma measurement accuracy better than +/-0.45% the HP Optical Media Advance Sensor is 2-times to 10-times more accurate than a conventional rotary encoder on the final media drive roller.

Because the Optical Media Advance Sensor provides direct measurement of linear media motion, it is not affected by mechanical tolerances in the media drive system and changes in media thickness, stiffness, and coefficient of friction. This means consistent quality in images, area fills, and graphics over a wide range of temperature and humidity conditions, and dependable performance over the life of the printer in production printing environments.