WO1993017870A1

WO1993017870A1 - Temperature control system

Info

Publication number: WO1993017870A1
Application number: PCT/GB1993/000520
Authority: WO
Inventors: Mark Smith
Original assignee: Willett International Limited
Priority date: 1992-03-12
Filing date: 1993-03-12
Publication date: 1993-09-16
Also published as: GB9205343D0

Abstract

The present invention relates to a method for regulating the temperature of a fluid ink flowing in an ink jet printer, notably a continuous jet ink jet printer, characterised in that at least part of the ink is passed through a heat pump which extracts and/or supplies heat to the ink so as to maintain the temperature of the ink within a desired temperature range. The heat pump is preferably a Peltier effect device. The invention also provides a heat pump apparatus and an ink jet printer incorporating that apparatus.

Description

TITLE: TEMPERATURE CONTROL SYSTEM

The present invention relates to a temperature control system, notably to one for use with an ink jet printer.

BACKGROUND TO THE INVENTION:

In continuous ink jet printers, ink is fed under pressure by a circulation pump from a reservoir to a print head where it is ejected as a jet of ink from a nozzle orifice. The jet of ink is broken up into a series of substantially uniformly sized and spaced apart droplets by the application of vibration and/or pressure pulses to the ink and/or the nozzle assembly, for example by means of a piezoelectric crystal acting directly on the ink or through a wall of the ink chamber immediately upstream of the nozzle orifice. The flight path of the droplets is controlled by charging the jet of ink so as to form charged droplets which then pass through a deflecting electric field. By varying the charge on the droplets and/or the strength of the deflecting field, the droplets are diverted to varying extends from their straight line flight to deposit at the desired position on a substrate. Those droplets which are not to be printed are not deflected and are collected in a gutter or other catching means and the ink is then re-cycled to the reservoir, usually by means of a venturi or other pump in the re-cycle line. For convenience, the term continuous jet ink jet printers will be used herein to denote such printers.

Typically, the circulation pump generates a higher flow of ink than is required to be printed onto the substrate at any one moment. The excess flow ensures that there is an adequate flow of ink at the required pressure to the print head at peak demand. Furthermore, part of the excess ink flow is often fed to the venturi pump to generate the suction used to draw ink and solvent vapour from the gutter for re-cycle* to the ink reservoir. The excess ink is circulated through the ink flow system of the printer either via a pressure relief valve and by-pass circuit directly back to the ink reservoir or to the venturi pump and typically only from 1 to 5% of the ink from the circulation pump passes through the print head.

The ink being circulated is heated by the action of the gears of the circulation pump and in operating the venturi pump so that the temperature of the ink often varies by up to 30° C from ambient temperature. Part of this heat is dissipated by heat loss from the system and in evaporation of solvent from the ink in its flight from the print head to the gutter. Operation of the printer thus reaches an equilibrium above ambient temperature at which the heat losses balance the heat input to the ink. However, this heat loss varies with the ambient temperature, so that the actual operating temperature of the printer varies from day to day and fluctuates during each day.

With a typical ink composition based on the use of a ketone or other evaporative solvent, the effect of temperature change on the viscosity of the ink can be marked. For example, the viscosity of an MEK based ink at 20°C may be 4cPs but this is reduced to 3cPs at 30°C. The viscosity of the ink affects its jetting properties and hence the operation of the print head. In order to compensate for the differences in viscosity at the different operating temperatures of a printer, it is customary to vary the pressure at which the ink is fed to the print head so as to achieve uniform droplet size and flight. This introduces complications in the operation of the printer, in some means must be provided for varying the pressure of the ink and some means for assessing what variation in pressure is required must also be provided, for example an assessment of the flight time of the droplets from the nozzle to the substrate.

The viscosity of the ink is also affected by loss of solvent from the ink as it travels between the nozzle and the gutter and back to the reservoir. It is therefore usually necessary to provide some form of solvent replacement system to compensate for this and this system is usually regulated by measuring the viscosity of the ink and adding solvent in response to changes in the viscosity observed. ^• Again, the viscosity observed will be affected by the operating ^* temperature of the printer and it is usually necessary to apply some temperature compensation factor when calculating the amount of solvent required to be added.

We have now devised a method by which the effect of variation of ambient temperature on the operating temperature of the printer can be reduced without the need to interrupt the flow of the ink. The method enables the operator to select and maintain an optimum operating temperature which can be above or below ambient temperature. Fluctuations in the viscosity of the ink due to temperature fluctuation can therefore be substantially eliminated. This reduces the need for temperature compensation during any viscosity monitoring operation and also simplifies the operation of the printer.

SUMMARY OF THE INVENTION:

Accordingly, the present invention provides a method for regulating the temperature of a fluid ink flowing in an ink jet printer, characterised in that at least part of the ink is passed through a heat pump which extracts and/or supplies heat to the ink so as to maintain the temperature of the ink within a desired temperature range.

The invention also provides an ink jet printer through which a fluid ink is to flow, preferably one in which the ink is be circulated within the ink flow system of the printer, characterised in that a heat pump is provided in the ink flow path so as to extract heat from and/or supply heat to the ink so as to maintain the temperature of the ink within a desired temperature range. The invention can be applied to any form of ink jet printer in which a fluid ink is to flow. The ink can be circulated within a flow system within the printer, as with a continuous jet ink jet printer of the type described above. However, the invention can be applied to drop on demand printers in which the ink is fed from a reservoir to a nozzle for printing individual droplets directly onto a substrate passing the nozzle. The flow of ink through the nozzle is regulated by a valve, transducer or other means so that the ink is ejected through the nozzle orifice as and when it is desired to apply a droplet of ink to the substrate. The required image being formed from an array of nozzles which are operated in the desired sequence to print a dot matrix alpha-numeric or other image on the substrate. In such drop on demand printers, the ink does not circulate through the nozzle and back to the reservoir as with a continuous jet printer. However, it may circulate through a manifold which feeds the valve or other flow control means.

For convenience, the invention will be described hereinafter in terms of a continuous jet ink printer of the type described above and with respect to a fluid ink. The term fluid ink is used herein to denote an ink which is fluid at ambient temperature and which does not, apart from the evaporation of a minor proportion of the solvent or carrier medium, change its phase to or from a solid or vapour during use thereof. Typically, the inks for present use are aqueous or organic solvent or carrier based and remain in the fluid phase throughout their flow through the ink system of the printer.

In the invention, heat is applied to or extracted from the ink circulating in the ink flow system at any suitable point by the heat pump so as to maintain the desired temperature in the ink and hence the desired operating temperature for the printer. Since a substantially uniform operating temperature can be achieved, it is not longer necessary to vary the pressure at which the ink is fed to the nozzle orifice to compensate for temperature variations. Furthermore, since it is not necessary to apply any temperature compensating factors in adjusting the solvent content of the ink in response to monitoring the viscosity of the ink, measurement of the viscosity is simplified and can be used as the sole control on the composition of the ink to achieve uniform operation of the print head.

The operation of the heat pump can be used to compensate for excess heat in the system or to supply heat where loss of heat to the environment is excessive using the same piece of equipment, thus simplifying the equipment requirements. The heat pump can act directly upon the flow of ink and it does not require that the ink flow be interrupted or diverted, thus enabling the temperature regulation to be carried out on line. The heat pump can be of sufficient capacity to accommodate wide ranges of heating or cooling requirements so that the temperature of the ink can be regulated over a wide range of ambient temperatures and wide fluctuations of heating or cooling requirement. The heat pump also acts directly upon the ink enabling a rapid response to temperature fluctuations to be achieved.

The heat pump can be of any suitable type and can be one which merely supplies heat to the ink so as to maintain a uniform above ambient temperature. However, it will usually be preferred that the heat pump exhibit a Peltier effect so that it can both supply heat to and extract heat from the ink so that the operating temperature can be selected at either above or below ambient temperature. Thus, the heat pump can be a Carnot type heat engine which converts mechanical energy into heat energy. However, it is preferred that the heat pump is of the thermoelectric type and can take a wide range of forms of such heat pumps, for example a solid state crystalline semi¬ conductor heat pump, such as one made from a quaternary alloy of bismuth, tellurium, selenium and antimony. Many forms of suitable heat pump are available commercially and may be selected depending upon the physical shape and size requirements as well as the heat load they are to handle.

The heat pump can be of any suitable form, for example having a tubular operating element which is fitted around a duct through which ink then flows in indirect thermal communication with the element via the duct wall. However, it is preferred that the operating element of the heat pump be in direct thermal contact with the ink to optimise heat transfer between the ink and the heat pump and to minimise the reaction time of the heat pump to changes in the temperature of the ink.

In a particularly preferred embodiment of the invention, the heat pump is in the form of a planar operating element and this forms one wall of a thermally insulated chamber through which ink flows to provide a simple add-on component which can be inserted into any suitable point in the ink circulation system. In such a component, the heat pump is typically provided by a Peltier effect sheet, for example a sheet of a suitable solid state semi-conductor alloy, and the cooling or heating effect is achieved by altering the direction of flow of an electric current through that sheet.

As indicated above, the operative element of the heat pump forms one or more walls of an insulated chamber through which at least part of the ink flows. Alternatively, the heat pump element can be incorporated into a split sleeve or other device which is clamped onto a suitable ink duct in the ink flow system and acts on the ink flow through the duct via the duct wall, the duct then acting as the chamber for the ink flow through which heat is applied to or removed from the ink. The chamber is preferably also provided with one or more heat sinks in thermal contact with the operative element of the heat pump to assist removal of heat from the heat pump when it is cooling the ink passing through the chamber. Typically, such heat sinks are provided with fins or the like over which air is passed by a fan or other means to aid removal of heat from the chamber .

The control of the flow of electric current or other energy source through the operative element of the heat pump and the operation of the fan or other means for blowing air over the heat sinks can be regulated in response to a conventional temperature sensor in the ink reservoir or in the ink flow system using conventional techniques and equipment.

Thus, in a preferred embodiment of the invention, the ink flows through a device comprising: a. a rectangular, tubular or other shaped housing formed from a thermally insulating material, such as aluminium or other metal sheet having a thermally insulating outer surface, or a thermally insulating plastic; and b. one or more heat pump operative elements incorporated into at least one wall of the housing; the housing defining a chamber having an ink inlet and an ink outlet through which ink may flow in thermal contact with the heat pump element(s); the heat pump element being connected to an electric current supply whose amplitude and direction of flow is regulated in response to a temperature sensor detecting the temperature of the ink in the flow system of the ink jet printer.

The heat pump or a device incorporating it can be incorporated into the flow system of the ink jet printer at any suitable point, for example in the return ink line from the venturi pump returning ink from the gutter, or in the line feeding ink from the circulation pump to the printer head. If desired, two or more heat pumps may be used at different points in the ink flow path or in parallel or series at the same point in the flow path.

The invention thus provides a means by which the temperature of the ink can be readily controlled both above and below ambient temperatures by means of a simple device which can readily be inserted into the ink flow system of an existing ink jet printer or as part of the flow system of new printers.

DESCRIPTION OF THE DRAWING:

The invention will now be described by way of illustration only with respect to a preferred form thereof as shown in the accompanying drawing, which is a diagrammatic part exploded view of a device incorporating the heat pump element for insertion into the flow line between the venturi pump serving the gutter of a continuous jet printer and the ink reservoir.

DESCRIPTION OF THE PREFERRED EMBODIMENT:

The device comprises a generally rectangular housing 1 defining an internal chamber having an ink inlet 2 and an ink outlet 3. The chamber walls are typically made from a metal inner wall, having an outer plastic or other thermally insulating layer, or from a thermally insulating plastic, so as to reduce incidental heat losses or gains through the walls of the chamber.

One wall of the chamber is provided with a heat pump element 4 which is removably mounted on the housing so that one face of the heat pump is in thermal contact with the interior of the chamber. Preferably, as shown, the housing 1 is formed with one open face 5 and the heat pump element 4 is mounted on the housing to form a wall of the chamber at that face and in direct thermal contact with ink within the chamber. The element 4 is preferably a sheet of a solid state semi-conductor alloy which exhibits Peltier effect properties so that the element 4 will both heat and cool the ink in the chamber.

The above device is mounted in the ink flow system of the printer, for example in the ink return line between the venturi pump and the ink reservoir, so that ink flows through the chamber in thermal contact with the heat pump element 4.

In operation, ink flows through the chamber. A temperature sensor, not shown, senses that the ink temperature is in excess of the required limit. This actuates a switch to cause current ^' to flow through the heat pump element 4 in a direction to cool the internal, chamber side, face of the element and thus extract heat from the ink flowing through the chamber until the temperature sensor detects that the temperature has been reduced to the desired value. The current flowing through element 4 may have a fixed value, so that the element 4 extracts heat at a constant rate from the ink. Alternatively, the amplitude of the current may be varied by a control means, not shown, so that initially heat is extracted at a high rate to correct any gross deviation of the temperature from the desired value, but at a slower rate as the required temperature for the ink is approached, thus reducing the risk of overshooting the desired temperature. The operation of the current supply may be intermittent so that the element 4 operates only when the ink temperature departs by more than a pre-set amount from the required value; or the element^' 4 can be operated continuously at a low level of heat extraction or supply to accommodate small variations in temperature, with the amplitude of the current being increased to compensate for larger variations in temperature as and when these occur.

Air from a fan, not shown, is blown over a finned heat sink, not shown, attached to the outer face of the heat pump element 4 to aid dissipation of heat from the face of the element.

When the temperature sensor detects that the ink temperature drops below the desired value, the flow of current through the element 4 is reversed to supply heat to the ink flowing through the chamber.

The heat pump thus provides a simple means without any moving parts for regulating the temperature of the ink. Furthermore, since the element 4 can be designed to operate at low voltages and can be totally sealed within the wall of the housing 1, the device presents a low fire risk and can be used with inks containing flammable solvents.

Claims

CLAIMS :

1. A method for regulating the temperature of .a fluid ink flowing in an ink jet printer, characterised in that at least part of the ink is passed through a heat pump which extracts and/or supplies heat to the ink so as to maintain the temperature of the ink within a desired temperature range.

2. A method as claimed in claim 1, characterised in that the ink jet printer is a continuous jet printer.

3. A method as claimed in either of claims 1 or 2, characterised in that the heat pump acts to cool the ink to maintain a predetermined above ambient temperature.

4. A method as claimed in any one of the preceding claims, characterised in that the heat pump is a Peltier effect device.

5. A method as claimed in any one of the preceding claims, characterised in that the heat pump comprises a planar operating element which forms part of a wall of a chamber through which the inks flows in thermal communication with the said operating element.

6. A method as claimed in claim 5, characterised in that the chamber through which the ink flows is provided by a device comprising: a. a rectangular, tubular or other shaped housing formed from a thermally insulating material; and b. one or more heat pump operative elements incorporated into at least one wall of the housing; the housing defining a chamber having an ink inlet and an ink outlet through which ink may flow in thermal communication with the heat pump element(s); the heat pump element(s) being connected to an electric current supply whose amplitude and direction of flow is regulated in response to a temperature sensor detecting the temperature of the ink in the flow system of the ink jet printer.

7. A method as claimed in any one of the preceding claims, characterised in that the heat pump is provided with means whereby heat extracted from the ink can be dissipated from the heat pump.

8. A method as claimed in claim 1, substantially as hereinbefore described with respect to the accompanying drawing.

9. An ink jet printer through which a fluid ink is to flow, characterised in that a heat pump is provided in the ink flow path so as to extract heat from and/or supply heat to the ink so as to maintain the temperature of the ink within a desired temperature range.

10. An ink jet printer according to claim 9, substantially as hereinbefore described with respect to the accompanying drawings.

11. Apparatus suitable for use in the method of claim 1, characterised in that it comprises: a. a rectangular, tubular or other shaped housing formed from a thermally insulating material; and b. one or more heat pump operative elements incorporated into at least one wall of the housing; the housing defining a chamber having an ink inlet and an ink outlet through which ink may flow in thermal communication with the heat pump element(s); the heat pump element(s) being connected to an electric current supply whose amplitude and direction of flow is regulated in response to a temperature sensor detecting the temperature of the ink in the flow system of the ink jet printer.