Print station in an ink jet printer

A serial drop-on-demand ink jet printer having an ink cartridge mounted for reciprocation on a carrier which traverses paper advancing through the printer. The cartridge includes a nozzle array for the ejection of ink droplets onto the paper, and the carrier includes a tab upstream from the nozzle array, in the direction of paper travel, which touches the top surface of the paper and sets the printhead-to-paper gap. The paper is pushed against the tab and maintains a minimum gap to prevent paper from coming into contact with the nozzle array. The paper is urged against the tab into the proper position relative to the nozzle array by a deflectable guide which acts on the back surface of the paper. In the printing zone of the ink jet printer, there is therefore no support structure for paper beneath the printhead, and advantageously a trough is provided in this area extending the width of the paper path so that if ink is ejected in the absence of paper, the ink is ejected into the trough where it will dry over time without being contacted by subsequent sheets of paper being fed through the printer.

BACKGROUND OF THE INVENTION 
In an ink jet printer, a sheet of paper or other transfer medium is 
advanced past a print station at which droplets of ink are applied to the 
paper from a nozzle array. After paper has traversed the printing station 
where the nozzle array is located, it is ejected from the printer and the 
next sheet of paper is advanced through the print station. 
In one form of ink jet printer, a serial drop-on-demand printer, a 
printhead having a nozzle array of limited width is reciprocated 
transversely to the direction of paper travel in order to place ink 
droplets on the paper to effect printing. Typically today, a disposable 
ink cartridge including the nozzle array is mounted for reciprocation on a 
carrier which traverses the advancing paper. Usually, the nozzles are 
pointed downward; and a surface, referred to as a platen, lies below the 
nozzle army to support the paper. The paper rests on the platen, with the 
carrier kept a specified distance from the top surface of the platen in 
order to maintain a gap between the paper and the printhead. Accurately 
maintaining this gap is essential to controlling the ink drop dot location 
on the paper, the spot size and misdirected satellite drops. 
One problem with this approach to maintaining the gap between the paper and 
the printhead is that a number of parts size tolerances can additively 
create too large a variation in the paper-to-printhead gap. In some 
instances, for example, the printhead carrier and platen are fixed 
relative to a paper guide; and the accumulation of tolerances between the 
printhead, the carrier, the guide surface, and the paper all contribute to 
printhead-to-paper spacing variations. 
In another form of print station, a spacer on the printhead or on the 
carrier rides on the paper and pushes it against the platen. This has the 
drawback of increased frictional forces both in the reciprocation of the 
printhead and in the movement of the paper past the print station. 
Another phenomenon of prior print stations of this type is that ink can be 
ejected from the printhead onto the platen or other parts of the printer 
which will be contacted by subsequent sheets of paper passing through the 
printer. For example, a print job might call for a printed page on A4 size 
paper, but an envelope or narrower sheet of paper is actually being fed 
through the printer. In this case excess ink is ejected beyond one or both 
lateral edges of the undersized print medium. Other situations can arise 
where there are holes in paper, paper is fed at an angle, or a paper jam 
occurs. Conditions such as these result in ink being ejected onto the 
platen and other parts of the printer. Subsequent sheets of paper will 
pick up the ink from the platen if the ink is not cleaned prior to further 
printing operations. 
To overcome these shortcomings, the present system employs a type of a 
"platenless" print station. 
In accordance with one aspect of the invention, in a serial drop-on-demand 
ink jet printer, in order to eliminate some of the more significant 
tolerances in spacing the paper from the nozzle array on the printhead, a 
small tab is provided on the carrier upstream from the nozzle array. The 
bottom surface of the tab touches the top surface of the paper and sets 
the printhead-to-paper gap. This eliminates the tolerances between the 
carrier and a guide surface, the thickness of a guide surface, the 
thickness of the paper or print medium, and the variation in how the paper 
lines up against the bottom of the guide surface. Instead, the paper is 
pushed against the tab and maintains a minimum gap and therefore 
eliminates the possibility of paper coming into contact with the nozzle 
array and causing a smear. 
In order to push the paper against the tab and into proper position 
relative to the nozzle array, a deflectable flap urges the paper toward 
the tab, with the paper-contacting flap surface ending upstream from both 
the tab and the nozzle array. Frictional forces on the reciprocating 
printhead and those which would interfere with the paper drive are greatly 
reduced from those generated in the use of a rigid support surface for the 
paper. 
In addition, with the absence of support structure for the paper beneath 
the printhead in the vicinity of the nozzle array, a trough is provided 
extending the width of the maximum width of paper to be printed upon 
beneath the reciprocating path of the printhead. The trough lies below the 
paper support structure so that if ink is ejected into the trough, it will 
merely dry over time without being contacted by subsequent sheets of paper 
being fed through the printer. This trough could also be used to 
intentionally eject droplets of ink from the nozzle array in the printer 
for maintaining and cleaning the printhead.

DETAILED DESCRIPTION 
The ink jet printer to be described herein will be described and claimed 
with reference to printing on a sheet of paper or sheets of paper passing 
through the printer. It will be understood that this designation of print 
medium is meant to include other forms of print media such as envelopes, 
transparencies, card stock and the like. In the figures, FIGS. 1 and 2 
each show an ink jet printer with portions removed to facilitate 
illustration of the present invention. 
With reference to the figures, an ink jet printer 10 includes a printhead 
assembly made up of a cartridge 11 and a carrier 12. The cartridge 11 is 
removably secured in the carrier 12 by a spring-loaded latch (not shown). 
The carrier 12 is reciprocated by a drive belt (not shown) secured at belt 
attachment points 13 and 14. The carrier 12 is moved back and forth along 
a guide rod 16 (shown with a portion removed in FIG. 2), with a pair of 
feet 17 riding in a groove 18 in a guide rail 19 which is secured to the 
side frames 20 (one of which is shown in FIG. 2) of the printer. Each foot 
17 includes a surface which rides along a sloped surface defining one side 
of the groove 18 as indicated at 21 (FIG. 3). A nozzle plate 22 on the 
bottom of a downwardly extending portion 23 of the cartridge 11 contains 
an array of nozzles (not shown) for ejecting ink droplets in a downward 
direction. In the illustrated form of the invention, the portion 23 of the 
cartridge 11 includes a thermal printhead chip which receives ink from the 
body of the cartridge 11 in nozzle chambers having individual heaters 
associated therewith. 
A sheet of paper S is advanced through the printer from a paper supply (not 
shown) and fed to the vicinity of the printhead by several pairs of 
rollers 24, 26. The rollers 26 extend through openings 25 in the mid-frame 
in order to engage the paper S. The paper then moves below a deflector 27 
and above a portion 28 of the mid-frame of the printer. The rollers 24, 26 
are mounted on a pair of shafts 38, 40 which, along with the deflector 27 
and the mid-frame are supported by the side frames 20 of the printer. 
As the paper is advanced by the rollers 24 and 26, the front edge of the 
paper moves beneath the guide rail 19 and encounters a paper flap 29. The 
deflector 27, the mid-frame including its portion 28, the guide rail 19 
and the paper flap 29 extend at least the width of the paper path along 
which the sheets of paper S travel. The paper flap 29 is a laminate of 
polycarbonate and acrylic secured in the mid-frame by projecting fingers 
30, 35 molded into the mid-frame. The fingers 30 are received in openings 
in the flap 29, and the fingers 35 extend over the rear edge 42 (FIG. 4) 
of the flap 29, holding it in place. 
The paper flap urges the sheet of paper S against the underside of the 
guide rail 19. As the paper moves beyond the flap 29, and before it is 
contacted by a carrier tab 32, its top surface is slightly closer to the 
nozzle array 22 than the final printhead-to-paper gap. As the printhead 
carrier 12 reciprocates across the paper, the tab 32 extending from the 
bottom of the carrier 12 deflects the paper downwardly and maintains it at 
a substantially fixed distance from the nozzle array 22 during printing. 
As best seen in FIG. 5, the paper flap 29 deflects downwardly with the 
paper as the carrier tab 32 reciprocatingly encounters the paper. 
Therefore at any point in the traverse of the carrier, the paper S and the 
paper flap 29 will be deflected downward to set the printhead-to-paper gap 
in the vicinity of the carrier tab 32. Away from the carrier tab, the 
paper remains in contact with the underside of the guide rail 19 at a 
reduced gap, with the paper flap undeflected. 
Advantageously, papers of various thicknesses can still be maintained at 
the desired printhead-to-paper gap. The carrier tab 32 sets the gap to the 
top surface of the paper, and differing thicknesses of paper are 
accommodated by flexure of the paper flap 29. 
As shown in FIG. 3, there is no platen or paper support structure lying 
below the nozzle plate 22 of the printhead cartridge 11. In addition, 
there is formed beneath the path of travel of the nozzle array 22 a trough 
33, defined by a portion of the mid-frame, for receiving ink from the 
printhead when ink is ejected in the absence of paper. A felt gutter pad 
34 is provided in the trough 33 to facilitate the drying of ink which is 
ejected into the trough. The felt pad also guards against wet ink being 
able to run out of the trough if the printer is tilted while being 
transported. 
Paper moving beyond the surface 31 of the mid-frame, located downstream 
from the printhead, is engaged by a roller 36 and a star roller (not 
shown) above the roller 36 to move the paper out of the printer.