Incremental printing of symbolic information – Ink jet – Controller
Reexamination Certificate
1999-10-14
2002-04-16
Yockey, David F. (Department: 2861)
Incremental printing of symbolic information
Ink jet
Controller
C347S059000, C347S067000
Reexamination Certificate
active
06371589
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The invention relates to a printhead used in equipment for producing black and colour images on a printing medium, generally though not exclusively a sheet of paper, using the thermal ink jet technology, and to a device and associated method of operation for regulating the energy supplied to the emission resistors of the head.
2. Related Technological Art
Equipment of the type described above is known in the art, such as for example printers, photocopying machines, facsimile machines, etc., and particularly printers used for the printing of a document, by way of printing means generally taking the form of fixed or interchangeable printheads.
The composition and general method of operation of a thermal ink jet printer, as also those of the relative ink jet printhead, are already widely known in the art and will not therefore be described in detail here, a more detailed description being provided only of some of the characteristics of the heads that help give a better understanding of the present invention.
A typical ink jet printer schematically consists of:
a system, selectively activated by a motor, for supplying and feeding the sheet of paper whereon the image is to be printed, in such a way that the feeding is performed in a determined direction in discrete steps (line feed),
a movable carriage, sliding on ways in a direction perpendicular to that of the sheet feeding, selectively activated by a motor to perform a forward motion and a backward motion over the entire width of the sheet,
printing means, generally for example a printhead, removably attached to the carriage, comprising a plurality of emission resistors deposited on a substrate (usually a silicon wafer), and disposed inside emission cells or chambers filled with ink, each individually connected to a corresponding plurality of nozzles, through which the head can emit droplets of ink, and to a main tank containing the ink,
an electronic controller which, on the basis of the information received from a computer whereto it is connected and of the presetings made by the user, selectively commands the above-mentioned motors and also the printhead, resulting in the emission therein, by way of the selective heating of the resistors, of the droplets of ink against the surface of the sheet, thus generating a visible image.
According to a recent evolution of the known art, the printheads also comprise, in addition to the emission resistors, the active driving components that selectively supply the energy for heating the emission resistors, generally in the form of MOS transistors integrated within the semiconductor substrate, i.e. produced using known techniques of the silicon wafer integrated semiconductor circuit technology.
From the electrical point of view, these integrated driving components, as they all have substantially identical geometrical and electrical properties, and their associated emission resistors, are typically laid out according to working arrangements known in the sector art in a matrix of rows and columns, so as to minimize the number of connections and contacts between the head and the electronic controller.
The energy is supplied by the MOS transistors to the emission resistors, selectively enabling a current supplied by a voltage power supply unit to flow through the said resistors, all the emission resistors being connected to this power supply unit. Inside the emission resistor, this current is transformed into thermal energy by the Joule effect, resulting in its heating rapidly to a temperature of more than 300° C. A first portion of this thermal energy is transferred to the ink present in the emission chamber surrounding the resistor, vaporizing it with the resultant enucleation of a vapour bubble and thus causing the expulsion of a droplet of given volume through the nozzle connected to that emission chamber. A second portion of this thermal energy is lost by conduction through the common substrate (the silicon wafer) whereupon are deposited the emission resistors, increasing the temperature T
s
of the substrate and thus of the head as a whole and of the ink contained therein, with respect to the ambient temperature.
The phenomenon of droplet emission may be better understood with reference to the graph in
FIG. 1
, illustrating the experimentally proven trend represented by the curve
30
of the volume VOL of the droplet of ink emitted by a nozzle, in relation to the thermal energy E supplied to the emission resistor in the cell connected to the nozzle, for a given constant value of the temperature T
s
of the substrate.
As shown by the graph, below a value E
s
(threshold energy) the droplet is not formed, since the resistor does not reach a high enough temperature to vaporize the ink surrounding it. If the energy E supplied to the resistor is increased from value E
s
to value E
g
(knee energy), the volume VOL of the droplets emitted increases substantially proportionally to the increase in energy E supplied to the emission resistor; beyond the value E
g
, the volume VOL on the other hand remains substantially unchanged with the increase in the energy E supplied to the resistor. This zone is the zone normally used as the working zone.
The knee energy E
g
of a thermal ink jet head is a characteristic of the geometrical and manufacturing configuration adopted, apart from being also dependent on the working temperature T
S
of the substrate (Si wafer), as seen above. With all other conditions being equal, it varies from head to head as a result of deviations entering the manufacturing processes. In particular, for the heads with integrated driving components, it depends largely on the following parameters typical of the manufacturing process:
thickness of the field oxide SiO
2
(Locos—local oxidation of the Silicon substrate),
thickness of the protective passivation (BPSG—Boron/Phosphorous silicon glassivation),
thickness of the SiN and SiC protective layers on the emission resistors,
thickness of the Ta anti-cavitation layer,
resistance value and geometrical dimensions of the emission resistors,
the RON value of the integrated MOS active drive components.
Use is made of the asymptotic characteristic of the pattern of the volume VOL of the droplets in relation to the energy E supplied to the emission resistor in defining the typical working value E
I
for the energy E to be supplied to the emission resistor (energy operating point). In current practice, for example, a value is taken for E
I
that is considerably higher than E
g
, so that any limited fluctuations of the thermal energy E supplied to the emission resistor (for various reasons, for example the natural tolerances of the power supply voltage value and of the duration of the current pulse supplied to the emission resistors by the printer the head is fitted on) or deviations of the value E
g
due to the tolerances of the head manufacturing parameters, do not entail significant variations of the volume VOL of the droplets emitted.
This is due to the fact that the energy operating point of the emission resistors is in any case inside the asymptotic portion of the curve
30
, thereby avoiding the occurrence of unstable operating conditions, which could on the other hand occur if E
I
were to drop below E
g
and the droplet volume were to become variable.
However, use of a value of E
I
that is considerably higher than E
g
also involves a series of negative effects, on account of the rise in temperature of the head due to the portion of thermal energy that is not used for emission of the droplet of ink. Among these negative effects are:
the volume of the droplets of ink emitted by the nozzles, for a like working energy value E
I
, increases with the rise in temperature of the substrate (and therefore of the ink) causing, as illustrated above, a corresponding variation of the diameter of the elementary dots printed on the paper and uniformity of the printout deteriorates accordingly. The phenomenon may be so marked that the characters printed at the top of a page may differ signifi
Conta Renato
Menegatti Angelo
Banner & Witcoff , Ltd.
Olivetti Tecnost S.p.A.
Yockey David F.
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