Typewriting machines – Sheet or web – Including friction-feed means
Reexamination Certificate
2001-01-31
2004-03-09
Colilla, Daniel J. (Department: 2854)
Typewriting machines
Sheet or web
Including friction-feed means
C400S636000
Reexamination Certificate
active
06702493
ABSTRACT:
TECHNICAL FIELD OF THE INVENTION
The present invention relates generally to inkjet apparatus, including inkjet printing mechanisms, and more particularly to an improved mechanism for avoiding print head crashes in such apparatus.
BACKGROUND OF THE INVENTION
Inkjet printing mechanisms may be used in a variety of different inkjet apparatus, such as plotters, facsimile machines, copiers, and inkjet printers collectively referred to in the following as printers, to print images using a colorant, referred to generally herein as “ink”. These inkjet printing mechanisms use inkjet cartridges, often called “pens” or “print heads” to shoot drops of ink onto print media, which can be used in the form of cut sheets or rolls of print media, which may include paper, vinyl, films, canvas or the like, in a variety of different dimensions.
Some inkjet print mechanisms carry an ink cartridge with an entire supply of ink back and forth across the sheet. Other inkjet print mechanisms, known as “off-axis” systems, propel only a small ink supply with the print head carriage across the print zone, and store the main ink supply in a stationary reservoir, which is located “off-axis” from the path of print head travel. Typically, a flexible conduit or tubing is used to convey the ink from the off-axis main reservoir to the print head cartridge. In multi-color cartridges, several print heads and reservoirs are combined into a single unit, with each reservoir/print head combination for a given color also being referred to herein as a “pen”.
Each pen has a nozzle plate that includes very small nozzles through which the ink drops are fired. The particular ink ejection mechanism within the print head may take on a variety of different forms known to those skilled in the art, such as those using piezo-electric or thermal print head technology. For instance, two earlier thermal ink ejection mechanisms are shown in U.S. Pat. Nos. 5,278,584 and 4,683,481, both assigned to the present assignee, Hewlett-Packard Company. In a thermal system, a barrier layer containing ink channels and vaporization chambers is located between a nozzle orifice plate and a substrate layer. This substrate layer typically contains linear arrays of heater elements, such as resistors, which are energized to heat ink within the vaporization chambers. Upon heating, an ink droplet is ejected from a nozzle associated with the energized resistor.
By selectively energizing the resistors as the print head moves across the sheet, the ink is expelled in a pattern on the print media to form a desired image (e.g., picture, chart or text). The nozzles are typically arranged in one or more linear arrays. If more than one, the two linear arrays are located generally side-by-side on the print head, parallel to one another, and substantially perpendicular to the scanning direction. Thus, the length of the nozzle arrays defines a print swath or band. That is, if all the nozzles of one array were continually fired as the print head made one complete traverse through the print zone, a band or swath of ink would appear on the sheet. The height of this band is known as the “swath height” of the pen, the maximum pattern of ink that can be laid down in a single pass.
For placing further print swaths on the print media, a print media feed mechanism is employed to advance or index the medium in the print zone in a second direction, called the media direction, which is usually substantially perpendicular to scanning direction of the print head.
Thus, to print an image, the print head is scanned back and forth across a print zone at a very close distance above the sheet, with the pen shooting drops of ink as it moves. On one hand, for instance, the distance between the printhead and the paper must be as small as possible, for example less than 1.7 mm, in order to obtain an accurate positioning of the ink dots projected from the printhead and to avoid spraying artefacts.
However, when a lot of ink is placed on some print media (especially on low cost paper based media) the print media may be subject to a phenomenon known as “cockle”. In existing printers, cockle results from the print media swelling and expanding as it absorbs water contained in the ink, whilst the print media is simultaneously constrained against lateral expansion due to being gripped at given locations along the scan axis (i.e. along the axis of movement of the print head), between the pinch wheels and the main drive roller. Thus, the effect of wet cockle increases with the amount of ink deposited on the paper.
This results in the formation of undulations or wrinkles in the plane of the print media. As a consequence, the distance between the print media and the print head decreases at some localized points. This phenomenon is especially noticeable when printing area fills of more than 200%. By this it is meant that in a given area of print media, the amount of ink deposited during the printing operation is two or more times the quantity of ink that is required to cover that area. This problem is further exacerbated by high temperatures and high levels of humidity.
If the degree of cockle is particularly severe, a “bubble” in the media may form. If the height of the media bubble is sufficient, the plot may be damaged as ink on the plot is smeared by the print head. Indeed, in more severe cases, a media crash may occur as the print head impacts against the print media itself. A media crash may seriously affect the subsequent print quality or throughput of the printer due to damaging the operation of individual nozzles of the pen. In some cases a media crash may necessitate the replacement of the pen.
This problem is often of particular concern where a plot is printed on a single sheet of print media, where the problem may be particularly pronounced in the trailing edge, of the sheet of print media; i.e. the last area to be printed. This situation is illustrated schematically in FIG.
1
.
FIG. 1
a
illustrates a sheet of print media
10
during a print operation. The print media
10
is resting on a platen
400
as it is driven through a printer drive mechanism
30
, consisting of a drive roller and opposing pinch wheels, in the direction indicated by the arrow. As can be seen from the figure, the media has expanded laterally, as indicated by arrows “A” after having passed through the print zone
40
and as a result of having absorbed the moisture in the ink deposited on it. However, where the media
10
is gripped between the drive roller and opposing pinch wheels of the printer drive mechanism
30
it is constrained against such lateral expansion. However, as can be seen from the figure, the edges of the print media
10
which have yet to pass through the printer drive mechanism
30
have a tendency to align themselves at the same angle as the edges of the print media
10
at the print zone side of the printer drive mechanism
30
. In the figure, this is indicated by arrows “B”. This has the effect of causing the edges of the print media
10
which have yet to pass through the printer drive mechanism to move towards each other, thus causing the raised zones, which together resemble a “wave” in the print media
10
. The wave is indicated by arrow “C”. The form of the wave is more clearly shown in
FIG. 1
b
which is a cross sectional view of the print media shown in
FIG. 1
a
, taken along lines X—X.
FIG. 2
illustrates a section of the printer drive mechanism
30
of
FIG. 1
, illustrating the interrelationship between the platen
400
, the drive roller
300
and a series of pinch wheels
310
of the drive mechanism
30
. As the print media (not shown) passes the drive roller
300
and the series of pinch wheels
310
, it may retain the wave shape “C” that it had acquired, as shown in
FIG. 1
, where it is not constrained between the pinch wheels
310
and the main drive roller
300
; i.e. at the locations
70
. This results in the formation of media bubbles in these areas. With increased quantities of ink deposited on the sheet of print media
10
, such media bubbles expand. This causes the hei
Alonso Xavier
Hierro Lluis
Sole Macia
Colilla Daniel J.
Lippman Peter I.
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