WO2003091028A1

WO2003091028A1 - Jet printing method and apparatus

Info

Publication number: WO2003091028A1
Application number: PCT/GB2003/001793
Authority: WO
Inventors: Alan Lionel Hudd; James Edward Fox; Richard Andrew Wilkinson; Jagvi Ramesh Patel
Original assignee: Xennia Technology Limited
Priority date: 2002-04-26
Filing date: 2003-04-28
Publication date: 2003-11-06
Also published as: GB0209538D0; AU2003227889A1

Abstract

Disclosed is a method of jet printing comprising receiving a liquid for printing, cooling the liquid, and jet printing the cooled liquid from an inkjet print head wherein by virtue of cooling the liquid, the viscosity of the liquid is increased and one or more operating characteristics of the jet printing is thereby improved. There is also disclosed jet printing apparatus comprising means to receive a liquid for printing, a jet print head and cooling means operable to cool the liquid prior to jet printing by the jet print head and thereby increase the viscosity of the liquid and improve one or more operating characteristics of the print head dependent on the viscosity of the liquid to be jet printed.

Description

Jet Printing Method and Apparatus

Field of the Invention

The invention relates to jet printing of liquids, for example inks, using inkjet print heads.

Background to the Invention

Inkjet print heads are in common use to print liquids such as, but not limited to, inks. Known inkjet print heads include continuous inkjet (CIJ) and drop on demand inkjet (DOD) print heads, such as binary and raster continuous inkjet, and piezo and thermal drop on demand inkjet print heads, which maybe integrated into printing systems.

Inkjet printing has gained wide acceptance in the printing industry because it is a relatively inexpensive form of printing and can produce high quality printed images. However, inkjet printing is complex to implement in practice. Although there are a broad range of techniques describing the non-contact printing process of propelling droplets from an inkjet printing device onto a recording medium, there are basically two classes of inkjet printing technology. They are continuous inkjet printing (CIJ) and drop on demand printing (DOD). For example, U.S. Patent No. 5,463,416 discloses a method of operating a drop on demand inkjet printer.

In common with all drop on demand printing techniques, liquid, such as an ink, is drawn from a liquid reservoir into a capillary channel in a print head. Within the channel, an energy pulse disrupts a portion of the liquid into a droplet that is expelled onto a recording medium, such as paper. The energy pulse may be provided by a piezo-electric element, which vibrates to produce the pulse. Alternatively, the external stimulus may be provided by a thermal element, which heats up the liquid and forms a bubble that creates a pressure wave, forcing a liquid droplet out of the channel. The distribution of the liquid droplets is controlled to form the desired image. The droplet volume and the resolution of the resulting liquid pattern or image are controlled by the inkjet printer system.

The viscosity of the liquid to be printed has important implications for the quality of print, speed of printing, reliability and other operating characteristics of inkjet print heads of the types listed above. Such inkjet print heads will only print liquids within a particular range of viscosities. For example, a print head specified as having a designated minimum limit for the viscosity of the liquid to be printed of 4 centipoise (cps) will typically give poorer print quality and reliability when used to print liquids with a lower viscosity. If too high a viscosity liquid is used, no drops will be generated as the amount of energy generated in the firing process will be insufficient to eject droplets.

In some applications, for example plastic transmission laser welding, liquids which would be preferred for their utility in those applications may have properties incompatible with jet printing. For example, they may have a viscosity sufficiently low that it would impair the operating characteristics of an inkjet print head or even cause the inkjet print head to not function at all.

The aim of the present invention is to improve the operating characteristics of an inkjet print head when jet printing a liquid which, due to its composition, has a viscosity sufficiently low to impair the operating characteristics of the inkjet print head. In some embodiments, the invention seeks to enable the jet printing of a liquid which, due to its composition, has a viscosity sufficiently low to reduce the operating characteristics of an inkjet print head to below specified operating characteristics.

It is known to include high viscosity liquids or volatile polymers within a liquid in order to increase the viscosity of the liquid. However, many high viscosity liquids have low evaporation rates, leading to undesirably long drying times, or a need for excessive post- heating to remove the volatile components completely. It is also known to include involatile polymers or other solid materials within a liquid in order to raise the viscosity of that liquid, so that it can be inkjet printed. However, these materials will be left behind once the liquid has dried, and may interfere with the properties of the desired deposited material.

According to a first aspect of the present invention there is provided a method of jet printing comprising receiving a liquid for printing, cooling the liquid, and jet printing the cooled liquid from an inkjet print head wherein by virtue of cooling the liquid, the viscosity of the liquid is increased and one or more operating characteristics of the jet printing is thereby improved.

The invention therefore enables or improves the jet printing of liquids which, by virtue of their low viscosity, could not be satisfactorily jet printed at the temperature at which they are received (typically ambient temperature). This reduces or removes the need for additives to increase viscosity and allows a greater range of liquids to be adequately jet printed. By controlling the cooling of the liquid it is also possible to concomitantly control operating characteristics such as drop volume, drop velocity, image quality, latency and de-cap time. Preferably, the liquid is cooled so that the liquid which is printed is of a constant temperature. This is preferably achieved by cooling the inkjet print head, or part thereof, to a constant temperature below ambient temperature.

The liquid may be received and cooled before being introduced to the inkjet print head, for example it may be received into and cooled within a reservoir. However, the liquid is preferably cooled within the inkjet print head. This may be achieved by cooling a portion of the inkjet print head through which the liquid passes prior to being jet printed. The liquid may be cooled before being introduced to the inkjet print head, and then temperature controlled (preferably cooled further) within the inkjet print head. This reduces the heat which must be dissipated from the inkjet print head, but can be more complex and expensive to implement. The liquid may be cooled to below ambient temperature. Preferably, the liquid is cooled at least 5 °C below ambient temperature. The technique is applicable for liquids which have a viscosity that increases when they are cooled. Preferably, the liquid has a substantial increase in viscosity on cooling.

Preferably, the viscosity of the liquid is raised to enable a specified operating characteristic of the inkjet print head. For example, the viscosity of the liquid may be raised to a value sufficient to allow the print head to print at a specified minimum firing frequency, such as 1kHz. Several commercial inkjet print heads have a recommended minimum liquid viscosity of around 8cPs, but will operate reliably at 1kHz when printing a liquid with a viscosity of at least 4cPs, Thus, the liquid may have a viscosity at 25 °C of less than 4 cPs but be cooled sufficiently to raise the viscosity to at least 4 cPs. Typically, the liquid has a viscosity at 25 °C of less than 3 cPs, but is cooled sufficiently to raise the viscosity to at least 4 cPs.

Preferably, a gas of lower moisture content than ambient air is brought into contact with a cold surface of the inkjet print head to reduce or preferably prevent condensation where it may damage the inkjet print head or impair print quality. The liquid may therefore be cooled to below the dew point of ambient air.

The gas of lower moisture content than ambient air may be air which has had some water vapour removed therefrom by a dehumidifier. Preferably, the dew points of the gas of lower moisture content than ambient air has a dew point of below 0°C. Preferably, the gas of lower moisture content than ambient air has a dew point of below -15 °C. This has been found to be effective in avoiding condensation when printing a liquid at 6°C in ambient air at 25°C, with 50-60% relative humidity and a dew point of 12°-14°C.

Preferably, the gas of lower moisture content than ambient air is brought into contact with cold surfaces of the inkjet print head whilst the cooling device is cooling. Preferably also, the gas of lower moisture content than ambient air is brought into contact with cold surfaces of the inkjet print head before the cooling device begins cooling, for example for a period of 5 minutes before the cooling device beings cooling. Preferably also, the gas of lower moisture content than ambient air is brought into contact with cold surfaces of the inkjet print head for a period of time after the cooling device finishes cooling, for example a period of 20 minutes.

Examples of low viscosity liquids for which the method is applicable include solutions of of biological materials for drug production, screening or testing, materials for plastic transmission welding, and specific chemicals for reaction or formulation to produce a specific mixture or blend.

The liquid may be a solution of an Infra-Red activatable dye suitable for laser welding.

According to a second aspect of the present invention there is provided jet printing apparatus comprising means to receive a liquid for printing, an jet print head and cooling means operable to cool the liquid prior to jet printing by the jet print head and thereby increase the viscosity of the liquid and improve one or more operating characteristics of the print head dependent on the viscosity of the liquid to be jet printed.

Preferably, the print head is an inkjet print head which includes the cooling means. For example, cooling means may comprise a thermoelectric device, such as a Peltier device in contact with the inkjet print head, configured to cool liquid which enters the inkjet print head for jet printing. Although any type of cooling device, for example a conventional refrigerator device, can be used, a Peltier device is preferred as they are small, simple devices which can be attached directly to the print head rather than requiring connecting tubes to supply refrigerant. Preferably, however, the Peltier device is attached to a base plate of the inkjet print head through a heat transfer block, such as an aluminium block in thermal communication with both the Peltier device and the base plate of the inkjet print head. Peltier devices typically have very high lifetimes (often in excess of 100,000 hours) and so provide a high mean time before failure with rrnnimal maintenance. Preferably, the apparatus includes temperature monitoring means, such as a thermocouple, in thermal communication with the print head. Preferably, the cooling device acts to maintain the temperature of the liquid at a constant temperature below ambient temperature.

The apparatus preferably includes means for directing gas or air with humidity below that of ambient air onto a cool region of the inkjet print head to allow the liquid to be cooled to below the dew point of ambient air whilst reducing or preventing condensation on said cool region of the inkjet print head.

Preferably means for bringing gas with humidity below that of ambient air into contact with a cool region of the inkjet print head comprises air dehumidifying means. Preferably also, the inkjet print head, or part thereof, is at least partially enclosed within a housing including an air space and reduced humidity air from the dehumidifying means is introduced into the air space.

The means operable to cool the liquid may protrude from the housing. The housing may have barrier means, such as a lip, to prevent moisture which condenses on the housing and/or any part of the means operable to cool the liquid which protrudes from the housing, from interfering with the printing process, or from wetting a substrate onto which the liquid is printed.

The liquid may be an ink, but may equally be any other liquid printable by inkjet print heads.

According to a third aspect of the present invention there is provided an inkjet print head mounted with a cooling device and jetting a low viscosity fluid. Preferably, the cooling device allows the print head temperature to be maintained constant and below ambient temperature.

Preferred features correspond to the second aspect. Brief Description of the Drawings

An example embodiment of the invention will now be described with reference to the accompanying drawings in which:

Figure 1 is a side view of an inkjet print head assembly according to the present invention.

Detailed Description of an Example Embodiment

Figure 1 is a side view of an inkjet print head assembly, shown generally as 1, according to the present invention.

Inkjet print head assembly 1 comprises a conventional drop on demand inkjet print head 2, such as the XaarJet 500 (XaarJet is a trademark) available from Xaar Ltd. of Cambridge, United Kingdom. Liquid supply coupling 4 receives liquid for printing. Liquid is conducted through conduits (not shown) then jetted through nozzles 6 towards a substrate (not shown).

Inkjet print head 2 includes a rear plate 8. An aluminium heat transfer block 10 is located in thermal communication with the rear plate 8. A thermally conductive grease or adhesive is provided between the rear plate 8 and the aluminium heat transfer block 10 to ensure thermal communication.

A thermoelectric device in the form of a Peltier device 12 is in contact with aluminium heat transfer block 10 on one face, and a heat sink 14 on the other face. A fan 16 is provided for cooling heat sink 14. A thermocouple 18 is provided in the aluminium heat transfer block to monitor the temperature of the heat transfer block, and thus the print head. A suitable Peltier device is an ST1.4-127-045L with modified THP68, available from Thermo Electric Devices, Moreton-in-Marsh, United Kingdom. This device is a 72W device and can be controlled with a Carel IRDRAIOOOO thermostatic controller, available from Tempatron Ltd, Reading, United Kingdom.

The print head 2 is encased within a stainless steel housing 20 which has an inlet 22 for receiving dehumidified air, and an aperture 24 through which heat transfer block 10 is sealedly mounted. Housing 20 also has an elongated slit aperture 26 through which ink may be jetted from nozzles 6 on to a substrate.

The housing 20 has a lip 28 which protrudes from the base of the housing, below the heat transfer block 10.

A plate 30 is located below fan 16 to prevent turbulence from fan 16 and deflecting the drops jetted through aperture 26 from the print head 2.

In use, Peltier device 12 transfers heat from the heat transfer block 10 to the heat sink 14, where it is dissipated to the surrounding air aided by fan 16. The temperature of the heat transfer block 10 is therefore reduced to below ambient temperature. As the rear plate 8 of the inkjet print head 2 is in thermal contact with the heat transfer block 10, the temperature of the rear plate is reduced. The rear plate is in direct contact with the internal print head channels and liquid path, and so the liquid within the print head is cooled. As printing takes place, fresh liquid is drawn into the print head and is also cooled.

The temperature of the heat transfer block 10 and therefore the rear plate of the print head 8, is monitored by means of the thermocouple 18. A controller (not shown) monitors this temperature, and controls the power supply to the Peltier device 12 to thermostatically maintain the liquid within the print head at a constant temperature. As a result of being cooled, the viscosity of the liquid within the inkjet print head is higher than the liquid which is drawn in at ambient temperature. If the viscosity of the liquid, when at ambient temperature, is below the minimum viscosity specified for the inkjet print head, and the liquid is cooled sufficiently, the viscosity of the liquid that is jet printed can be equal or greater than the minimum viscosity specified for the inkjet print head.

Air at a rate of 10 litres per minute is reduced in humidity by a dehumidifier (not shown). The dehumidifier is a high efficiency dessicant compressed air dryer, for example the Pneudri DM002 supplied by Domnick Hunter Limited of Gateshead, United Kingdom. An oil-free compressor is used to supply air to the air dryer. The resulting air has a dew point of -15 °C to -20 °C as a result of its low moisture content.

The resulting air is introduced through inlets 22 into a space within the container 20. The reduced humidity air displaces the air which is already within the space, through aperture 26. The volume of the space is around 0.75 litres and so the reduced humidity air flows through the space in around 4 or 5 seconds.

As a result of the low humidity, and correspondingly low dew point, of the air within the casing 20, the temperature of the liquid to be printed can be reduced to below the dew point of ambient air without damage by condensation to the print head, its circuitry, or the quality of the resulting print.

The apparatus described above can print a liquid at 6°C in ambient air of up to 80% humidity at 25 °C without condensation forming on the print head.

Under these conditions, some condensation deposits on the portion of the heat transfer block 10 outside of the container 20, or on the container 20 itself near the heat transfer block 10. However, this condensation will run down onto the lip 28 which prevents it running onto the base of the container 20 where it may drip onto to the substrate or otherwise interfere with printing. Lip 28 drains water to a location away from the substrate. Example Application

One specific industrial application of jet printing that is made possible with the ability to print a low viscosity liquid is plastic transmission laser welding. Plastic laser transmission welding is a broadly applicable technique and is utilised in a wide range of market segments, such as packaging, textiles, automotive components, medical products, consumer and electronics products.

An example plastic welding technique is described in International Patent Application No. WO00/20157 to TWI, Cambridge, United Kingdom. Briefly, this method involves deposition of an IR activated dye onto a substrate, which is then brought into contact with a second substrate, the dye is heated using a laser and this causes the two substrates to melt and fuse together. The benefit of the IR activatable dye over other materials such as carbon black or even other welding processes is that it produces a colourless weld. Previously, it has been difficult for an inkjet print head to deposit the IR activatable dye as this material when dissolved in a volatile solvent gives a low viscosity solution. Addition of polymers or low volatility solvents to increase viscosity are not desirable as they may lead to reduced weld strength and long drying times. The low viscosity of the liquid resulting from the low solids content and the need for volatile solvents make it difficult for inkjet print heads to reliably form and expel droplets. Thus prior to the present invention, inkjet print heads have not to our knowledge been used in a commercially successful system for printing of IR materials for laser welding.

Jet printing with an inkjet print head is attractive for plastic transmission laser welding because of the following benefits over and above traditional welding techniques:

1.) High line quality with sharp edge acuity, which promotes strong welds.

2) Small dot size and narrow line width, which gives the ability to create smaller welds.

3) High dosing accuracy due to small droplet size, so only the required amount of liquid is deposited, which reduces wasted material and consequently has cost and environmental benefits. 4) High dot placement accuracy, which means that the liquid is only deposited in the area specifically desired.

5) The ability to print variable patterns on parts of different shapes and sizes, even 3- dimensional objects, which means that each part printed maybe different, allowing for smaller batch sizes and production runs with a high degree of customisation.

6) Uniformity or evenness of the printed dye layer, which produces a more uniform and stronger weld.

7) The ability to print an area (rather than just a line or point) with jet printing and also the ability to vary the coverage thickness, also gives enhanced flexibility and applicability to the system.

The ability of such a system to produce a strong, high quality weld is a function of the quality of the printed image and requires the absence of other materials or impurities in the dried liquid, which may weaken the weld created. Maximizing the quality of the jet printing and the purity of the printing liquid enhances the ability to produce strong, high quality welds.

In the present example, the temperature of the print head (and consequently the liquid which is printed) is reduced to raise viscosity. The temperature is also thermostatically controlled to minimise fluctuations in viscosity, in this way high quality jet break up can be achieved, controlling drop volume, drop velocity and image quality. As purity is also a key requirement, the present invention provides a system whereby a liquid that comprises only the IR activatable dye and volatile solvents maybe reliably printed. The absence of any other non-volatile components in the liquid mean that only IR dye is present in the dried image and no other materials which may weaken or compromise the weld strength are present. Further more, such a system functions highly reliably and so can be used in an industrial environment, where it may function 24hrs a day, 7 days a week.

Alternative welding techniques suffer from the following problems; compromising the integrity of the base material, generating particulates and not being well suited to covering an area. One example of an alternative welding technique is ultrasonic welding. In this form of welding, ultrasonic energy is directed by an energy director/horn to the desired point(s) where welding is required. The region of weld is fixed by the hardware comprising the energy director/horn and is a disadvantage of this system. To perform ultrasonic welding at multiple positions simultaneously or rapidly requires multiple energy directors or rapid movement of the part to be welded. The directed energy vibrates the polymer surfaces in contact at the point(s) where the energy is directed, which generates heat and melts the polymers forming a weld. ^' In the process small particulates of the polymer are often generated in the midst of the work medium, which is undesirable for both weld strength and clarity. For these reasons, an alternative to ultrasonic welding is desirable and necessary when the flexibility of welding in 3 -dimensions on a part is desired without the need for multiple energy directing systems or unwieldy substrate movements.

Jet printing of IR activatable dye provides a suitable alternative technique in that the point(s) or surfaces where welding is desired are defined by where the liquid is deposited, and that may be a point or a line or a plane or a ruled surface or a non-ruled surface or even a discontinuous surface, and there may be multiple such surfaces in a given substrate where the welding is desired and which are all simultaneously addressed with a single piece of laser welding equipment. In applications where particulate formation of the substrate material is not acceptable such as intravenous bags, blood filters, filters, containers of liquids for ingress into living entities, catheters, tubes and some food products inkjet printing of IR activatable dye eliminates the particulate formation intrinsic to ultrasonic welding and the polymer weld obtained is colourless.

One example of an alternative IR dye dispensing technique is liquid dispensing, where a syringe system is used to draw a line of liquid on the object in the areas to be welded. Liquid dispensing is limited in the minimum line width that can be achieved and also is not well suited to covering an area. Thus a practical and reliable way to perform deposition of an IR activatable dye for welding in very narrow lines and over an area is desired.

One example of an application for which liquid dispensing is not suitable because of the described deficiencies is the welding of Microfluidic devices. Microfluidic devices typically have precise, complex patterns involving both narrow lines and areas that require welding. Very small inter-channel barriers must be welded without contamination of the micro- channels, to avoid toxicological issues. Use of liquid dispensing to produce suitably thin lines or the area fill required is not practicable. Also, the complexity of the shapes mean that covering all of the areas to be welded by liquid dispensing would be a slow process requiring a multi axis motion system.

With the system of the current invention, it is also possible to weld difficult materials such as olefins and PNC substrates, which currently have a limited number of joining methods. Also, a wide range of common materials maybe welded such as, but not limited to, ABS, CA, ENA, HIPS, PA, PC, PCTG, PEEK, PEI, PEN, PET, PETG, PMMA, PPO, PS, PSU, PVDF, acetal, polyurethane and polyester.

The following illustrative examples are included to describe the liquid and properties used in the inkjet printing system of the invention, in order to print an IR activatable dye suitable for laser welding.

Example 1

A liquid formulation consisting of-

0.4% A194 dye (supplied by Gentex Corp, PA, USA)

59.2 1 -methoxy propan-2-ol

25.4 % Isopropyl alcohol

5.0% Cy clopentanone

10.0% Cyclohexanol

was cooled to 6°C by the apparatus described above and jetted.

Where the viscosity of the liquid, measured using a Brookfield DVII-f- viscometer with UL adaptor.was 2.2cPs at 25 °C and 4.1cPs at 6°C. This liquid jetted reliably for 4hrs without nozzle loss in a XaarJet500 print head at 6°C, whereas at 25 °C it was difficult to achieve prime for all nozzles and nozzle loss occurred rapidly on jetting.

This liquid has a drying time of 12.6s on polycarbonate with a 300W IR lamp.

The weld strength for polycarbonate was measured as an average 51.6Nmm^"2 with failure occurring each time in the parent material rather than the weld. The weld strength was measured on an Instron 4466 Universal Tensile Tester at test speed 5mm/min, gauge length 102mm.

Example 2

A liquid formulation consisting of-

0.4% A194 dye (supplied by Gentex Corp, PA, USA)

49.8% 1 -methoxy propan-2-ol

49.8 % Diacetone alcohol was cooled to 6°C by the apparatus described above and jetted.

The viscosity of the liquid, measured using as Brookfield DNII+ viscometer with UL adaptor was 2.5cPs at 25 °C and 4.4cPs at 6°C.

This liquid had a drying time of 56s on polycarbonate with a 300W IR lamp

The weld strength for polycarbonate was measured as an average 49.6Νmrn² with failure occurring each time in the parent material rather than the weld. The weld strength was measured on an Instron 4466 Universal Tensile Tester at test speed 5mm/min, gauge length 102mm.

Comparative example 1

A liquid formulation consisting of-

0.4% A194 dye (supplied by Gentex Corp, PA, USA) 97.6% Acetone

2.0% Pioloform BN-18 (supplied by Wacker-Chemie GmbH, Germany)

The weld strength on polycarbonate was rated as 3 on the following 4-point scale. The strength of the weld produced from a liquid without the Pioloform polymer is 1.

1 - good weld, broke in parent material

2 - strong weld, broke in weld

3 - weak weld, broke in weld

4 - no weld

Comparative example 2

A liquid formulation consisting of-

0.4% A194 dye (supplied by Gentex Corp, PA, USA)

61.7% 1 -methoxy propan-2-ol

25.9% Isopropyl alcohol

10.0% Cyclohexanol

2.0% Polypropylene glycol 4000

Where the viscosity was 2.8cPs at 25 °C, measured using as Brookfield DVII+ viscometer with UL adaptor.

A drying time could not be determined as the polyproylene glycol prevents the system from fully drying and even after several hours the ink can still be smeared when tested.

Claims

1. A method of jet printing comprising receiving a liquid for printing, cooling the liquid, and jet printing the cooled liquid from an inkjet print head wherein by virtue of cooling the liquid, the viscosity of the liquid is increased and one or more operating characteristics of the jet printing is thereby improved.

2. A method according to Claim 1, wherein the liquid is cooled so that the liquid which is printed is of a constant temperature.

3. A method according to Claim 1 or Claim 2, wherein the liquid is cooled within the inkjet print head.

4. A method according to Claim 3 wherein a portion of the inkjet print head through which the liquid passes prior to being jet printed is cooled, thereby cooling the liquid within the inkjet print head.

5. A method according to any one preceding Claim, wherein the liquid is cooled to below ambient temperature.

6. A method according to Claim 5, wherein the liquid is cooled at least 5 °C below ambient temperature.

7. A method according to any one preceding Claim, wherein the viscosity of the liquid is raised to enable a specified operating characteristic of the inkjet print head.

8. A method according to Claim 7 wherein the viscosity of the liquid is raised to a value sufficient to allow the print head to print at at least 1kHz.

9. A method according to Claim 7 or Claim 8 wherein the liquid has a viscosity at 25 °C of less than 4 cPs, but is cooled sufficiently to raise the viscosity to at least 4 cPs.

10. A method according to any one preceding Claim, wherein a gas of lower moisture content than ambient air is brought into contact with cold surfaces of the inkjet print head to reduce or prevent condensation where it may damage the inkjet print head or impair print quality.

11. A method according to Claim 10, wherein the liquid is cooled to below the dew point of ambient air.

12. A method according to Claim 10 or Claim 11, wherein the dew points of the gas of lower moisture content than ambient air has a dew point is below 0°C.

13. A method according to any one of Claims 10 to 12, wherein the gas of lower moisture content than ambient air is brought into contact with cold surfaces of the inkjet print head before the cooling device begins cooling.

14. A method according to any one of Claims 10 to 13 wherein the gas of lower moisture content than ambient air is brought into contact with cold surfaces of the inkjet print head for a period of time after the cooling device finishes cooling.

15. A method according to any one preceding Claim, wherein the liquid comprises an infra-red activatable dye suitable for laser welding.

16. Jet printing apparatus comprising means to receive a liquid for printing, a jet print head and cooling means operable to cool the liquid prior to jet printing by the jet print head and thereby increase the viscosity of the liquid and improve one or more operating characteristics of the print head dependent on the viscosity of the liquid to be jet printed.

17. Jet printing apparatus according to Claim 16, wherein the print head is an inkjet print head which includes the cooling means.

18. Jet printing apparatus according to Claim 16 or Claim 17 wherein the cooling means comprise a thermoelectric device.

19. Jet printing apparatus according to Claim 18 wherein the cooling means comprises a Peltier device in contact with the inkjet print head, configured to cool liquid which enters the inkjet print head for jet printing.

20. Jet printing apparatus according to any one of Claims 16 to 19, wherein the apparatus includes temperature monitoring means in thermal communication with the print head.

21. Jet printing apparatus according to any one of Claims 16 to 20, wherein the cooling device acts to maintain the temperature of the liquid at a constant temperature, which is less than ambient temperature.

22. Jet printing apparatus according to any one of Claims 16 to 21, wherein the apparatus includes means for directing gas or air with humidity below that of ambient air onto a cool region of the inkjet print head to allow the liquid to be cooled to below the dew point of ambient air whilst reducing or preventing condensation on said cool region of the inkjet print head.

23. Jet printing apparatus according to Claim 22 wherein the means for bringing gas with humidity below that of ambient air into contact with a cool region of the inkjet print head comprises air dehumidifying means.

24. Jet printing apparatus according to Claim 22 or Claim 23, wherein the inkjet print head, or part thereof, is at least partially enclosed within a housing including an air space and reduced humidity air from the dehumidifying means is introduced into the air space.

25. Jet printing apparatus according to Claim 24 wherein the means operable to cool the liquid protrudes from the housing, and the housing has barrier means to prevent moisture which condenses on the housing and/or any part of the means operable to cool the liquid which protrudes from the housing, from interfering with the printing process, or from wetting a substrate onto which the liquid is printed.

26. Jet printing apparatus according to any one of Claims 16 to 25 wherein the liquid is an ink.

27. An inkjet print head mounted with a cooling device and jetting a low viscosity fluid.

28. An inkjet print head according to Claim 27 wherein the cooling device allows the print head temperature to be maintained constant and below ambient temperature.