Variable power preheater for an ink printer

A preheater is placed between a supply tray station and a print zone of an ink printer. Power to the preheater is varied so that the preheater is heated to a fist relatively high temperature during the time that the recording medium is advanced from the supply station to the print zone. When the recording medium enters the print zone, the medium is moved at a slower indexing speed, and the power to the preheater is reduced to a second level. The result is a more uniform application of preheat to the recording medium.

BACKGROUND OF THE INVENTION AND MATERIAL DISCLOSURE STATEMENT 
This invention relates generally to ink printers and, more particularly, to 
printers in which an aqueous ink is applied to a porous recording medium 
such as paper. 
Liquid ink printers of the type frequently referred to as continuous stream 
or as drop-on-demand, such as piezoelectric, acoustic, phase change 
wax-based or thermal, have at least one printhead from which droplets of 
ink are directed towards a recording medium Within the printhead, the ink 
is contained in a plurality of channels. Power pulses cause the droplets 
of ink to be expelled as required from orifices or nozzles at the end of 
the channels. 
In a thermal ink-jet printer, the power pulse is usually produced by a 
heater transducer or a resistor, typically associated with one of the 
channels. Each resistor is individually addressable to heat and vaporize 
ink in the channels. As voltage is applied across a selected resistor, a 
vapor bubble grows in the associated channel and initially bulges from the 
channel orifice followed by collapse of the bubble. The ink within the 
channel then retracts and separates from the bulging ink thereby forming a 
droplet moving in a direction away from the channel orifice and towards 
the recording medium whereupon hitting the recording medium a dot or spot 
of ink is deposited. The channel is then refilled by capillary action, 
which, in turn, draws ink from a supply container of liquid ink. 
The ink jet printhead may be incorporated into either a carriage type 
printer, a partial width array type printer, or a page-width type printer. 
The carriage type printer typically has a relatively small printhead 
containing the ink channels and nozzles. The printhead can be sealingly 
attached to a disposable ink supply cartridge. The combined printhead and 
cartridge assembly is attached to a carriage which is reciprocated to 
print one swath of information (equal to the length of a column of 
nozzles), at a time, on a stationary recording medium, such as paper or a 
transparency. After the swath is printed, the paper is stepped a distance 
equal to the height of the printed swath or a portion thereof, so that the 
next printed swath is contiguous or overlapping therewith This procedure 
is repeated until the entire page is printed. In contrast, the page width 
printer includes a stationary printhead having a length sufficient to 
print across the width or length of a sheet of recording medium at a time. 
The recording medium is continually moved past the page width printhead in 
a direction substantially normal to the printhead length and at a constant 
or varying speed during the printing process. Partial width array printers 
are disclosed, for example, in U.S. Pat. No. Re. 32,572 and U.S. Pat. No. 
4,638,337. A page width ink-jet printer is described, for instance, in 
U.S. Pat. No. 5,192,959. These patents are hereby incorporated by 
reference. 
Many liquid inks and particularly those used in thermal ink jet printing, 
include a colorant or dye and a liquid which is typically an aqueous 
liquid vehicle, such as water, and/or a low vapor pressure solvent. The 
ink is deposited on the substrate to form an image in the form of text 
and/or graphics. Once deposited, the liquid component is removed from the 
ink and the paper to fix the colorant to the substrate by either natural 
air drying or by active drying. In natural air drying, the liquid 
component of the ink deposited on the substrate is allowed to evaporate 
and to penetrate into the substrate naturally without mechanical 
assistance. In active drying, the recording medium is exposed to heat 
energy of various types which can include infrared heating, conductive 
heating and heating by microwave energy. 
Active drying of the image can occur either during the imaging process or 
after the image has been made on the recording medium. In addition, the 
recording medium can be preheated before an image has been made to 
precondition the recording medium in preparation for the deposition of 
ink. Preconditioning of the recording medium typically prepares the 
recording medium for receiving ink by driving out excess moisture which 
can be present in a recording medium such as paper. Not only does this 
preconditioning step reduce the amount of time necessary to dry the ink 
once deposited on the recording medium, but this step also improves image 
quality by reducing paper cockle and curl which can result from too much 
moisture remaining in the recording medium. 
Various drying mechanisms for drying images deposited on recording mediums 
are illustrated and described in the following disclosures which may be 
relevant to certain aspects of the present invention. 
U.S. Pat. No. 5,005,025, to Miyakawa et al., describes an ink jet recording 
apparatus for recording which fixes ink through evaporation of an ink 
solvent. The apparatus includes a heating member extending both upstream 
and downstream with respect to a recording area and a conveying direction 
of the recording sheet. The heating member contacts the recording sheet to 
assist in the fixation of the ink. 
U.S. Pat. No. 5,406,321, to Schwiebert et al., describes an ink jet printer 
and a paper preconditioning preheater therefore. The paper preconditioning 
preheater has a curved surface and a multi-purpose paper path component to 
accomplish direction reversal for the paper. The paper contacts the 
preheater which dries and shrinks the paper to condition it for a printing 
operation. The preheater is a thin flexible film carrying heater elements 
which is suspended in air to provide extremely low thermal mass and 
eliminate the need for long warm up times. 
U.S. Pat. No. 5,296,873 to Russell et al. discloses a paper preconditioning 
preheater in the form of a preheated drive roller which engages the medium 
and drives it into the print zone. 
Copending application U.S. Ser. No. 08/523,322 assigned to the same 
assignee of the present invention, discloses a segmented heater which 
includes a curved preheater segment and a planer segment positioned in the 
print zone. All of the above-identified references are hereby incorporated 
by reference. 
A continuing problem with printer designs which include a recording medium 
preheating function is the unevenness of the medium warm up as it moves 
from a medium supply station into the ink print zone. Referring to the 
prior art design shown in FIG. 1, a recording medium 10 is moved from a 
supply tray 12 by feed roller 14. Medium 10 is conveyed across the top 
surface of a heater 16 which is powered by a power supply 18 creating a 
current in the heater, which may be, but is not limited to, a foil or 
other type of heating mechanism, and increasing the temperature thereof 
The medium is moved into a print zone 20, where an ink jet cartridge 22 
comprising a printhead 24, connected to an ink reservoir 26, is pulsed by 
input signals from drive circuit 28 to cause ink droplet ejection in an 
image-wise pattern on the medium. The medium is advanced into the print 
zone by drive roller 30 cooperating with a pinch roller 32. The cartridge 
is moved by a carriage (not shown) back and forth (into and out of the 
page) and the medium is incrementally advanced following formation of each 
image line. 
Medium 10 is typically advanced quickly along the path extending from the 
lip of the supply tray (point A) to the nip formed by roller pairs 34, 36 
(point B). This initial rapid advance of the medium is done to minimize 
interprint time and reduce the first print out time. Once the leading edge 
of the medium enters the print zone, the medium moves through the print 
zone at the normal medium indexing time (the medium is held stationary 
until a line is recorded and then indexed forward). It will be appreciated 
that the first leading portion of the medium (distance from A to B) will 
be warmed less than the trailing portion of the medium (distance A to the 
trailing edge). This unequal heating of the paper can lead to differential 
print quality problems for the first portion of each recorded medium. 
SUMMARY OF THE INVENTION 
According to the invention, a more even heating of the recording medium is 
provided by increasing the power to the preheater during the time that the 
forward portion of the medium is moving from a feed position into the 
print zone. Thus, the first portion of the medium is heated at a greater 
rate to provide a more uniform heating of the entire medium. 
More particularly, the present invention relates to a printing machine for 
printing on a recording medium moving along a path from a supply station 
to a print zone, comprising: 
a printhead for depositing ink on the recording medium; 
a preheater disposed adjacent to said path, for preheating the recording 
medium, 
a power supply for applying power to said preheater and 
a controller for varying the power output of said power supply so as to 
supply a first power input to the preheater during a first time period 
when the medium is moving from the supply station to the print zone and a 
second relatively lower power input to the preheater during a second time 
period that the medium is moving through the print zone.

DESCRIPTION OF THE INVENTION 
Although the present invention discussed here may be used for drying an 
image which is created by any type of liquid ink printer, the following 
description is directed towards an environment of a thermal ink jet 
printer such as that shown in FIG. 1 and modified as in FIG. 2. 
FIG. 2 shows a preferred embodiment of the invention wherein the prior art 
design of FIG. 1 is modified by varying the output of a power supply 18' 
to selectively increase the power applied to a preheater 16. 
As in the FIG. 1 prior art design, a recording medium 10, which can be a 
paper sheet, is moved out of supply tray 12 by feed roller 14. The tray is 
spring biased by conventional means to force the top sheet of the stack 
into contact with the feed roller. The first portion of sheet 10 moves 
along preheater 16, which can be any conventional heater such as, for 
example, a foil type heater. The preheater preheats the sheet and removes 
excess moisture from the paper resulting in a more dimensionally stable 
sheet as well as improving ink absorption into the paper. (Transparencies 
and certain coated papers do not require preheating and, in fact, can be 
damaged by excess preheating because of softening.) As the leading edge of 
the sheet passes beneath a sensor 40, a first signal is sent to controller 
42 which controls the power output of power supply 18'. The power output 
is at a first high preheat power level, which brings the current applied 
to preheater 16 to a predetermined level resulting in the paper being 
heated at a first predetermined level When the leading edge of sheet 10 
passes beneath a second sensor 44, a second signal is generated and sent 
to controller 42. Controller 42 sends a signal to power supply 18' 
reducing the power output to a second preheat level lower than that of the 
first output power level resulting in the paper being heated at a second 
level lower than said first level This power cycling sequence is repeated 
with succeeding sheets of paper being fed from tray 12. 
The print zone 20 is the area directly beneath the printhead 12 where 
droplets of ink 21 are deposited by an array of ink nozzles printing a 
swath of information and arranged on a front face of the printhead. The 
front face of the printhead is substantially parallel to the recording 
medium. A carriage traveling orthogonally to the recording medium deposits 
the ink droplets upon the recording medium in an imagewise fashion. The 
medium is supported by a platen member 47. The printhead 24 receives ink 
from attached ink tank 26. The image deposited upon the recording medium 
can include text and/or graphic images, the creation of which is 
controlled by controller 42, in response to electrical signals from drive 
circuit 28. A printer of this type is disclosed in the patents referenced 
supra; e.g., U.S. Pat. No. Re. 32,572 and U.S. Pat. No. 4,638,337. Before 
the paper 10 has been released from drive roll 30 and the pinch roll 32, 
an exit drive roll/pinch roll combination 48 captures the leading edge of 
sheet 10 for transport to output tray 50 which holds printed recording 
medium. 
Typically, the normal print time per swath is determined by the firing rate 
of the printhead and the width of the printhead. The nominal power input 
to the paper depends on print speed and other factors such as type of ink 
and the medium. The preheat time decreases as the paper feed speed 
increases (to minimize the interprint time). The amount of time the paper 
is exposed to the heater requires additional power to compensate. The 
increase in power is approximately inversely proportional to the advanced 
rate of feed of the paper (rate of movement from point A to point B) 
versus the nominal advance speed through the print zone. Or, stated 
another way, the ratio of the first power level to the second power level 
is directly proportional to the ratio of the first time period t.sub.1 to 
the second time period t.sub.2. For example, assumed that the nominal 
advance speed through the print zone is one inch/second, and the power 
level for the second preheat level is 10 watts. If the advance rate is six 
inches/second, then the first power output level is 60 watts; thus, the 
power applied to preheater 16 upon generation of signal inputs from sensor 
40 is 60 watts heating the paper advancing at six inches/second to a 
first, relatively high, temperature level. The power to preheater 16 drops 
to 10 watts upon generation of a signal from sensor 44 and remains at that 
level until the next sheet is moved out of tray 12 and is sensed by sensor 
40. These parameters are variable depending on the heat transfer 
characteristics of the preheater. 
To summarize the above, the printing quality of the image recorded on 
medium 10 is improved by preheating the first portion of the medium as it 
advances to the print zone at a higher temperature than the temperature 
applied to the remaining portion of the sheet. The first temperature is 
maintained for a shorter time than the second temperature with the result 
that an appropriate uniform preheat temperature is applied to the entire 
sheet. 
Various modifications may be made consistent with the invention. As an 
example, the platen support member 47 may be separately heated to provide 
additional moisture removal from the medium. Although the preheater is 
shown as a planar member, it may also take other configurations such as a 
curved preheater of the type disclosed in aforementioned copending U.S. 
Ser. No. 08/523,322. Further, in regard to the control system in 
controller 42, it is well known, and normally preferable, to program and 
execute imaging, printing, document, and/or paper handling control 
functions and logic with software instructions for conventional or general 
purpose microprocessors. This is taught by various prior patents and 
commercial products. Such programming or software may, of course, vary 
depending on the particular functions, software type, and microprocessor 
or other computer system utilized, but will be available to, or readily 
programmable without undue experimentation from, functional descriptions, 
such as those provided herein, or prior knowledge of functions which are 
conventional, together with general knowledge in the software and computer 
arts. "Object oriented" software development environments, such as C++, 
can even provide portable source code. Alternatively, the disclosed system 
or method may be implemented partially or fully in hardware, using 
standard logic circuits or a single chip using VLSI designs. 
While the embodiment disclosed herein is preferred, it will be appreciated 
from this teaching that various alternative, modifications, variations or 
improvements therein may be made by those skilled in the art, which are 
intended to be encompassed by the following claims: