Patent Description:
Known aqueous ink printing systems print images on uncoated and coated substrates. Whether an image is printed directly onto a substrate or transferred from a blanket configured about an intermediate transfer member, once the image is on the substrate, the water and other solvents in the ink must be substantially removed to fix the image to the substrate. A dryer is typically positioned after the transfer of the image from the blanket or after the image has been printed on the substrate for removal of the water and solvents. To enable relatively high speed operation of the printer, the dryer heats the substrates and ink to temperatures that typically reach about <NUM>° C for effective removal of the liquids from the surfaces of the substrates.

Coated substrates exacerbate the challenges involved with removing water from the ink images as low porosity clay coatings can prevent ink from wicking into the media substrates. Additionally, temperature gradients can form in the substrates as they pass through the dryer or dryers. Temperature gradients greater than <NUM>-<NUM> degrees C can cause the water and solvents in the ink to evaporate at different rates. The non-uniformity of the evaporation rate can cause ink to flow on the substrate surface, which concentrates pigments in the ink along the temperature gradient and produces ghost images in solid density coverage areas.

Current media transport belts that carry substrates through the dryer or dryers in a printer pass over a perforated platen covering a vacuum platen. The platen helps support the belt and the substrates on the belt. Some known belts have holes so as the belt passes over the perforated platen covering the vacuum plenum, a vacuum can exert a pull on the media substrates through the perforated platen and the holes in the belt to hold the substrates in position for printing and drying. The substrate areas that are adjacent the holes in the belt are cooler than the substrate areas adjacent the belt material because the void in the belt does not transfer heat energy to the back side of the substrate as the belt material. Instead, the vacuum pulls an air flow through the voids, which cools the portions of the substrates opposite the voids. The resulting temperature differential between these two types of areas in the substrates produces the image defects shown in <FIG>. As shown in the figure, the darker circles to which the arrows point are the areas that were adjacent the holes of the media transport belt. Vacuum forces inside the holes pull the media against the vacuum hole edges which increases the thermal conduction between the belt and media back side. This increased thermal conduction produces a temperature differential on the media surface. The water and solvents evaporate more quickly in these areas resulting in a higher concentration of ink pigments and dyes there. The ink pigments and dyes are drawn from surrounding areas in the image and lighter density boundaries arise. As shown in the figure, the lighter circles within the darker circles are the areas that were adjacent the holes in the media transport belt. Some media transport belts are an arrangement of a plurality of belts that pass over the perforated platen covering the vacuum plenum. Because a plurality of belts is provided, each belt is narrower than a width of the media carried by the belt arrangement in the cross-process direction. The temperature of the areas of the substrates that extend beyond the edges of the belts in the belt arrangement is less than the temperature of the substrate areas covering the belts so image defects can arise from this temperature differential. These areas are the straight lines to which the arrows point in <FIG>. Likewise, the inter-document gaps on the belt or belts between successive media substrates in the process direction are not covered by the substrates so these inter-document gap belt areas are heated to a different temperature than the covered areas of the belt. Since the substrates are not synchronized with the rotation of the media transport belt, an inter-document gap area of the belt during one revolution of the belt is covered by a substrate during a subsequent revolution of the belt. This phenomenon produces thermal bands of different temperatures that extend in the cross-process direction and follow one another in the process direction. These thermal bands result in different ink evaporation rates and possible image defects.

Media transport belts made of porous fabric have been developed to eliminate the vacuum holes and address the image defects arising from temperature differentials in the substrates and media belts. Unfortunately, the needling pattern, stitched seams, or ripples that occur in the fabric of these belts provide non-uniform contact points between the belt and the substrate. The non-uniform contact results in non-uniform thermal conduction between the belt and media which produces temperature differentials with the attendant image defects, particularly in solid ink coverage areas in the ink image. A media transport belt that works with a vacuum system to hold media substrates in place without producing image defects arising from temperature differentials in the substrates and the belt or belts carrying the substrates would be beneficial. <CIT> describes an inkjet recording apparatus including a fixer for fixing ink that has landed on a recording medium.

According to the invention, there is provided a dryer for an inkjet printer according to accompanying claim <NUM>.

A new printer includes a dryer that works with a vacuum system to hold media substrates against the belt without producing image defects arising from temperature differentials in the substrates. The printer includes at least one printhead configured to eject drops of an ink onto substrates moving past the at least one printhead to form ink images on the substrates, and a dryer having a heater, a media transport belt cooler, and a media transport belt. The media transport belt is configured to move the substrates past the heater after the ink images have been formed on the substrates and the media transport belt cooler being positioned to remove heat energy from the media transport belt after the media transport belt has passed the heater and the substrates have separated from the media transport belt.

A new dryer for an aqueous ink printing system works with a vacuum system to hold media substrates against the belt without producing image defects arising from temperature differentials in the substrates. The dryer includes a heater, a media transport belt cooler, and a media transport belt. The media transport belt is configured to move substrates past the heater after ink images have been formed on the substrates and the media transport belt cooler being positioned to remove heat energy from the media transport belt after the media transport belt has passed the heater and the substrates have separated from the media transport belt.

The foregoing aspects and other features of a media transport belt that works with a vacuum system to hold media substrates against the belt without producing image defects arising from temperature differentials in the substrates are explained in the following description, taken in connection with the accompanying drawings.

For a general understanding of the present embodiments, reference is made to the drawings. In the drawings, like reference numerals have been used throughout to designate like elements.

<FIG> depicts a block diagram of an aqueous printer <NUM> that is configured to print images on substrates carried by a new media transport belt configured to work with a vacuum system to hold media substrates against the belt without producing image defects arising from temperature differentials in the substrates. The printer <NUM> includes a media supply <NUM>, a pretreating unit <NUM>, a marking unit <NUM>, a drying unit <NUM>, and a media receptacle <NUM>. The media supply <NUM> stores a plurality of media sheets <NUM> for printing by the printer <NUM>. The media sheets <NUM> may, in some embodiments, be clay-coated or other types of treated paper.

The pretreating unit <NUM> includes at least one transport belt <NUM>, which receives the media sheets <NUM> from the media supply <NUM> and transports the media sheets <NUM> in a process direction <NUM> through the pretreating unit <NUM>. The pretreating unit <NUM> includes one or more pretreating devices <NUM> that condition the media sheets <NUM> and prepare the media sheets <NUM> for printing in the marking unit <NUM>. The pretreating unit <NUM> may include, for example, one or more of coating devices that apply a coating to the media sheets <NUM>, a drying device that dries the media sheets <NUM>, and a heating device that heats the media sheets <NUM> to a predetermined temperature. In some embodiments, the printer <NUM> does not include a pretreating unit <NUM> and media sheets <NUM> are fed directly from the media supply <NUM> to the marking unit <NUM>. In other embodiments, the printer <NUM> may include more than one pretreating unit.

The marking unit <NUM> includes at least one marking unit transport belt <NUM> that receives the media sheets <NUM> from the pretreating unit <NUM> or the media supply <NUM> and transports the media sheets <NUM> through the marking unit <NUM>. The marking unit <NUM> further includes at least one printhead <NUM> that ejects aqueous ink onto the media sheets <NUM> as the media sheets <NUM> are transported through the marking unit <NUM>. In the illustrated embodiment, the marking unit <NUM> includes four printheads <NUM>, each of which ejects one of cyan, magenta, yellow, and black ink onto the media sheets <NUM>. The reader should appreciate, however, that other embodiments include other printhead arrangements, which may include more or fewer printheads, arrays of printheads, and the like.

With continued reference to <FIG>, dryer <NUM> includes a media transport belt <NUM> that receives the media sheets <NUM> from the marking unit <NUM>. The drying belt <NUM> is tensioned between an idler roller <NUM> and a driven roller <NUM>, which is driven by an electric motor <NUM>. The dryer <NUM> is configured to expose the printed substrates to heat having an adequate temperature to remove the water and solvents in the aqueous ink on the substrates without producing image defects arising from temperature differentials. To accomplish this goal, the media transport belt <NUM> in dryer <NUM> is configured with the structure described in more detail below. The heater <NUM> is positioned within the dryer <NUM> to direct heat toward the substrates passing through the dryer <NUM>. The heater <NUM> can be one or more arrays of various types of radiators of electromagnetic radiation, such as infrared (IR) radiators, microwave radiators, or more conventional heaters such as convection heaters. After passing through the dryer <NUM>, the substrates are carried by the belt <NUM> to the output tray <NUM>. The pre-treating unit <NUM>, the marking unit <NUM>, and the dryer <NUM> are operated by a controller <NUM>. The controller is configured with programmed instructions stored in a memory operatively connected to the controller so the controller performs functions in the printer by operating various printer components when the controller executes the stored programmed instructions. Although only one controller is shown in <FIG> for simplicity, multiple controllers can be used for the various functions and these controllers can communicate with one another to synchronize the functions that they perform.

<FIG> is a side view of the dryer <NUM> that is configured with a new media transport belt <NUM> and vacuum plenum <NUM> that is not covered with a vacuum platen. Instead, the vacuum plenum <NUM> is a five-sided box with side plates <NUM> and a bottom plate <NUM> but no top platen having holes or slots in a plate placed over the box and over which the media transport belt typically slides. The plenum has flanges <NUM> around the top surface as shown in <FIG>. The flanges support the media transport belt <NUM> and provide a surface for the media transport belt <NUM> to seal the top of the vacuum plenum so vacuum air flow is directed through the belt holes and not lost around the plenum edges. This configuration removes the metal vacuum plenum plate having vacuum holes that were a source of temperature differentials as noted previously. In some embodiments, the length of the vacuum platen in the process direction is sufficiently short that no media belt support is required across the vacuum platen in the cross-process direction. That is, the tension roller <NUM> can keep the media transport belt <NUM> sufficiently taut in the process direction between the idler roller <NUM> and the driver roller <NUM> that no other support is required in the vacuum plenum to keep the belt relatively flat. As used in this document, the term "process direction" means the direction of media transport belt movement in the printer and the term "cross-process direction" means the axis that is perpendicular to the process direction in the plane of the media transport belt. The plenum <NUM> and media transport belt <NUM> are wider in the cross-process direction than the width of the widest media that can be printed by the printer <NUM>. This configuration ensures that the media substrates cannot extend over the flanges <NUM>, which can be a source of temperature differentials in the substrates as noted previously.

In some embodiments, the length of the vacuum plenum <NUM> in the process direction requires one or more belt supports <NUM> that extend between the flanges <NUM> as shown in <FIG> is a top view of the dryer transport from the perspective of the heater <NUM> looking down toward the media transport belt <NUM> as the belt moves over the open plenum <NUM>. The process direction is shown in the figure by the letter P and the arrow. To prevent temperature differentials, the belt supports <NUM> have a continuous surface, which means that no holes or other voids are in the surface of the supports that contact the belt <NUM>. Thus, another source of temperature differential in the process direction is removed. The supports can be stationary structures or they can be idler rollers that rotate as the belt contacts and moves over the supports. The supports can be perpendicular or angled relative to the belt travel. The belt supports <NUM> also extend across the entire width of the vacuum plenum <NUM> to maintain continuous contact and provide a uniform thermal heat sink with the media transport belt <NUM> in the cross-process direction within the plenum.

The media transport belt <NUM> is configured to be thin and comprised of a material that is transparent to or reflective of the heat energy produced by the heaters <NUM>. As used in this document, the term "thin" means a belt thickness substantially less than the thickness of belts used in previously known dryers so the thermal mass of the belt is reduced from one having the same length and width. In one embodiment, the belt thickness is in the range of about <NUM> to about <NUM>. By keeping the belt relatively thin, its thermal mass is minimized. The importance of a minimal thermal mass is discussed below. In one embodiment in which the heaters are IR heaters, the belt <NUM> is made from polyimide rather than silicone, which is used in previously known belts. Polyimide, polyethylene, and polypropylene are relatively transparent to IR but some sources of these materials include a number of additives in the materials that may absorb IR. These additives may require additional dryer configuration adjustments as described below. The media transport belt <NUM> also includes vacuum holes <NUM> (<FIG>) that have a small diameter. In one embodiment, the holes are <NUM>-<NUM> in diameter and are at least less than <NUM> in diameter. Holes in this range are adequate to apply a vacuum force to capture and hold media substrates transported by the belt without generating temperature differentials at the surface of the media substrate.

In a known printer having a dryer that uses one or more silicone belts with openings greater than <NUM>, the IR radiators are activated at <NUM>% of their power level twenty-three seconds prior to the arrival of the substrates at the dryer. The silicone belt absorbs this heat energy as its temperature peaks at <NUM> degrees C. One hundred blank substrates are fed through the dryer to stabilize the belt temperature since the substrates absorb the IR energy. Thus, this known belt has a temperature that stabilizes in a range of about <NUM> degrees C to about <NUM> degrees C. At these temperatures, temperature differentials arise in the belt around the vacuum holes and the belt edges and produce artifacts in some colors of the ink image.

To reduce these differentials and attenuate their effects on the ink images, a media transport belt cooler <NUM> has been developed. In an alternative embodiment of the media transport belt cooler, not falling within the scope of the claims, a fluid applicator <NUM>, which is operatively connected to a fluid source <NUM>, applies a fluid, such as water, to the belt at a position below the vacuum plenum <NUM> (<FIG>). The fluid applicator <NUM> can be a roller that applies the fluid by contact with the belt, a spray head that directs a mist toward the belt, and the like. The applied fluid evaporates before the belt reaches the idler roller <NUM> and contacts the substrates. To aid in the evaporation process, the cooler <NUM> includes a fan <NUM> or other source of air flow, such as a chiller, that directs ambient or chilled air toward the belt to aid in evaporation of the fluid from the belt and the cooling of the belt. This combination lowers the temperature of the previously known silicone belt to a range of about <NUM> degrees C to about <NUM> degrees C and keeping the belt in this temperature range removes most of the effects of the temperature differentials in the ink images. The controller <NUM> operates the fan <NUM> and the fluid applicator <NUM> to adjust the speed of the fan and the amount of fluid applied to the belt. These operations are performed using data supplied to the controller <NUM> through the user interface <NUM>. For example, the type of paper, which identifies the thermal mass of the paper, the presence or absence of coatings, and the like, can be used by the controller to operate the fan at one of a number of predetermined speeds and adjust the amount of fluid applied to the belt. Thus, the addition of a belt cooler along a portion of the belt free from the substrates and not exposed to the heater <NUM> can be effective in attenuating artifacts in ink images dried by a known dryer in previously known printers. The smaller thermal mass of the media transport belt <NUM> described above further enhances the effect of the belt cooler <NUM> since that IR reflective or transparent belt absorbs less heat energy to be dissipated by the belt cooler. Specifically, the IR transparent polyimide belt temperature peaks at about <NUM> degrees C rather than <NUM> degrees C for the thicker silicone belt. The applied fluid coupled with the air flow from the fan <NUM> cools the belt surface temperature to about <NUM> degrees C, which is more effective for preventing image artifacts than the <NUM> degree C temperature achieved with the silicone belt.

In this embodiment, the media transport belt cooler <NUM> has a fan <NUM> and the fluid applicator has been replaced with a metal heat sink <NUM>, which is relatively thin and is made from a metal that makes the heat sink flexible, such as aluminum. The heat sink <NUM> is operatively connected to an actuator <NUM>. The controller <NUM> operates the fan <NUM> and the actuator <NUM> to adjust the speed of the fan and the amount of belt area contacting the heat sink. These operations are performed using data supplied to the controller <NUM> through the user interface <NUM>. For example, the type of paper, which identifies the thermal mass of the paper, the presence or absence of coatings, and the like, can be used by the controller to operate the fan at one of a number of predetermined speeds and adjust the position of the heat sink with respect to the belt to increase or decreases the amount of belt area contacting the heat sink. Thus, the addition of a belt cooler along a portion of the belt free from the substrates and not exposed to the heater <NUM> can be effective in attenuating artifacts in the ink images dried by the dryer <NUM>. As the heat sink absorbs heat energy from the media transport belt, the temperature of the belt drops before the belt reaches the idler roller <NUM> and contacts the substrates. The heat sink configuration of <FIG> also lowers the temperature of the previously known silicone belt to a range of about <NUM> degrees C to about <NUM> degrees C and keeping the belt in this temperature range removes most of the effects of the temperature differentials in the ink images. Thus, the addition of a media transport belt cooler positioned to cool the belt after the belt has passed the heater <NUM> and after the substrates have been separated from the media transport belt can be effective in attenuating artifacts in ink images dried by the known dryers in previously known printers. The smaller thermal mass of the media transport belt <NUM> in dryer <NUM> described above further enhances the effect of the belt cooler <NUM> shown in <FIG> since that IR reflective or transparent belt absorbs less heat energy to be dissipated by the belt cooler. Specifically, the IR transparent polyimide belt temperature peaks at about <NUM> degrees C rather than <NUM> degrees C for the thicker silicone belt. The heat sink <NUM> and fan <NUM> of the cooler <NUM> cools the belt surface temperature to about <NUM> degrees C, which is more effective for preventing image artifacts than the <NUM> degree C temperature achieved with the silicone belt alone.

A process for operating the dryers of <FIG> and <FIG> is shown in <FIG>. The process begins with the retraction of the cooler components from the belt (block <NUM>). The type of media for the print job is identified by, for example, receiving it as a print job parameter from the user interface, and the controller determines whether belt cooling is required for the print job (block <NUM>). If the cooler is not needed, it remains retracted (block <NUM>). If belt cooling is required for the type of media to be printed, then the weight of the media is identified and compared to a predetermined threshold (block <NUM>). In one embodiment, the weight of the different types of media that can be printed by the printer are stored in a memory and the predetermined threshold is <NUM> grams per square meter. If the weight of the media exceeds the predetermined threshold, the cooler remains retracted (block <NUM>). If the weight of the media is less than or equals the predetermined threshold, then the fan is activated and either fluid is applied to the belt or the heat sink is moved into engagement with the belt (block <NUM>). As long as the belt temperature remains below a predetermined threshold, which in one embodiment is <NUM> degrees C (block <NUM>), the cooler remains engaged with the belt (block <NUM>). When the temperature of the belt equals or exceeds the predetermined threshold, then the fan speed and either the amount of fluid applied or the area of the heat sink engaging the belt is compared to the maximum set point for these parameters (block <NUM>). If these set points are not at their maximum, then the fan speed and either the amount of fluid applied or the area of the heat sink set points are increased and the operation of the cooler is adjusted accordingly (block <NUM>). This loop of checking the belt temperature and the operational set points for the cooler (blocks <NUM>, <NUM>, and <NUM>) continues until the maximum set points are reached and the cooler is operating at those set points. If the maximum set points are reached without the belt temperature staying below the predetermined threshold, then a signal is sent to the user interface to alert the user to an occurrence of a possible image defect (block <NUM>). At the end of the print job, the belt cooler is retracted (block <NUM>) and the fan continues to direct air onto the heat sink in the heat sink embodiment until the heat sink temperature falls below a predetermined threshold (block <NUM>), which in one embodiment is <NUM> degrees C.

As noted previously, thin polyimide media transport belts with low thermal mass gain and loose heat energy at significantly higher rates than thicker silicone belts. Thin belts heat and cool rapidly resulting in higher temperature differentials between areas of the belt in the inter-document gap that absorb more heat energy than belt areas covered by the substrates. This effect produces multiple cross-process direction bands of temperature differentials around the circumference of the belt. The belt cooling embodiments mentioned above are effective at minimizing the temperature differentials between the areas exposed to the heater <NUM> and those areas covered by the media.

Claim 1:
A dryer (<NUM>) for an inkjet printer, comprising:
a heater (<NUM>);
a media transport belt cooler (<NUM>); and
a media transport belt (<NUM>), the media transport belt being configured to move substrates past the heater after ink images have been formed on the substrates and the media transport belt cooler being positioned to remove heat energy from the media transport belt after the media transport belt has passed the heater and the substrates have separated from the media transport belt,
wherein:
the media transport belt cooler (<NUM>) further comprises:
a metal heat sink (<NUM>), the metal heat sink being configured to contact the media transport belt (<NUM>) after the media transport belt has passed the heater and the substrates have separated from the media transport belt; and
a fan (<NUM>), the fan being configured to direct air flow over the heat sink;
the media transport belt (<NUM>) is comprised of a material that is reflective or transparent of heat energy generated by the heater (<NUM>);
the media transport belt (<NUM>) is comprised of one of a polyimide, a polyethylene, and a polypropylene;
the media transport belt (<NUM>) has holes (<NUM>), each hole in the media transport belt has a diameter that is less than <NUM>; and
the media transport belt (<NUM>) has a thickness less than <NUM> so the thermal mass of the media transport belt is less than a silicone belt of a same length and a same width as the media transport belt,
the dryer further comprising:
a vacuum plenum (<NUM>) having sides (<NUM>) and a bottom (<NUM>) that form a structure having a U-shaped cross-section that encloses a volume of air that is adjacent the media transport belt; and
a vacuum source that is operatively coupled to the volume of air in the plenum (<NUM>) to pull a vacuum through the holes (<NUM>) in the media transport belt (<NUM>),
a top surface of the vacuum plenum comprising flanges (<NUM>) that support the media transport belt and provide a surface for the media transport belt to seal the top of the vacuum plenum so vacuum air flow is directed through the belt holes.