GB2542749A

GB2542749A - Liquid heater and cooler

Info

Publication number: GB2542749A
Application number: GB1420901.9A
Authority: GB
Inventors: Marcin Zaba Jerzy; Brian Tomlin Matthew; Michael Fost Ian; Lecheheb Ammar
Original assignee: MATRICODE Ltd
Current assignee: MATRICODE Ltd
Priority date: 2014-11-25
Filing date: 2014-11-25
Publication date: 2017-04-05
Also published as: GB201420901D0

Abstract

A heater assembly suitable for heating ink of an ink jet printing system, comprising a heat exchanger unit and a heating elements, the heat exchanger 15 has channels on one surface that are sealed by a cover plate 18, through which the inks flow, and on the opposing surface of the heat exchanger the heater element 13 is attached, such that heat from the heating element is transferred to the ink via the heat exchanger. Preferably the heat exchanger is made from stainless steel, aluminium, brass, copper or thermally conductive polymer. The heating elements are preferable positive temperature coefficient heating elements such as heating wires embedded in aluminium plates or chips. The heat exchanger channels and the heating element may be covered by thermally insulating covers 14, 17. The removal of the cover plate 18 and insulating cover 17, allows the ink channels to be easily cleaned. Alternatively the heat exchanger has a cooling elements (23, Fig 6) attached, such as a peltier element (28, fig 6), heat sink (27, fig 6), fan (24, fig 6), to cool the ink.

Description

Title: Liquid Heater and cooler Field of the Invention

The present invention relates to a heater and cooler apparatus for heating and cooling of fluids. More particularly, the invention relates to heating and cooling of ink in an ink jet printer positive or recirculating system to maintain the ink at the printhead(s) at optimum operating temperature.

Background to the invention

Inkjet Piezo Drop On Demand (DOD) or impulse inkjet systems operate to eject ink drops for printing by the application of pressure pulses to the ink. These pressure pulses are generated by the electrical actuation of a piezo element that is attached to one or more sides of each of the printhead channels in communication with and supplying ink to the various nozzles. Because of this fundamental principle of operation these ink channels must be free of air bubbles to ensure the pressure pulse is not absorbed by these air bubles which would then result in failure of the system to eject printing droplets. Also because of this operating principle the pressure of the ink supplied to these nozzles must be maintained within a desired operating range (normally very narrow) and be stable at all times. The operating pressure range is normally with the cappillary range of the printhead ink channels supplying the nozzles and the desired operating pressure at the nozzles is slightly negative being in the range of -5mb to -25mb +/-2mb. Therefore, the ink supplied to these nozzles must be free of pressure fluctuations or pulsations that are larger than +/-2mb. Pressure pulsations are normally generated by the ink recirculating systems using positive displacement pumps such as gear pumps and diaphragm pumps.

From the foregoing, it is clear that in order to obtain a reliable operation and good quality printing, especially in an ink jet printing system of the DOD type, it is necessary that the ink supplied to one or to multiple printheads, which each incorporates one or more arrays of several nozzles, is maintained stable and within the desired preset pressure and temperature range under all specified operating conditions. If the temperature and or pressure at the nozzles deviates and goes outside of the required operating range, ejected printing drops become non-uniform leading to degraded print quality. Furthermore, ink can then start weeping out of the nozzles leading to eventual failure of the system. The opposite can also occur whereby nozzles can be starved out of ink if the temperature and/or pressure at the nozzles is outside the operating range. In particular, if the pressure at the nozzle goes below the minimum range of the required negative pressure. In such case, the air get ingested into the nozzle channels and the printhead deprimes, as it is known in the ink jet industry. When one or multiple nozzles are deprimed, then these cannot eject printing droplet of ink as the nozzle exit get blocked by the air bubble which absorbs pressure pulses and conterract any ink volume displacement.

Industrial printing environment are subjected to wide temperature ranges. One of the operating conditions that contributes to the ink supply pressure drifting outside of the specified safe operating pressure range is variation in ink viscosity. This in turn, leads to variation in the ink pressure drop in the pipes and in the printhead channels. Therefore, the ink supply pressure could become either lower or higher than the desired lower or upper operating pressure respectively depending on ink temperature and thus ink viscosity and flow resistance. For example, when the environmental room tempeature is low such as 5°C during cold mornings starts or in cold environment, the ink viscosity could be as high as 50 centipoises. For a piezo DOD (such as the Xaar 1001 and the 1002) to operate and be able to eject droplets of ink, the viscosity of the ink reaching the nozzles must be in the range of 7 to 25 cp at the jetting temperature. Therefore, accurate control of the ink temperature and therefore ink viscosity is necessary for the industrial reliable operation of Piezoelectric DOD digital inkjet printing systems.

Prior Art

Several methods of the prior art have been proposed to heat the ink in such printing systems. These prior art heating methods have generally involved the use of coils of metal pipes that are attached to heater elements. There are several disadvantages with these prior art methods. For instance, the contact surface area between heating elements and the coil of pipe is neither high nor reliable. The resulting heater is not efficient with enery loss in the spaces between the coil of pipe. This construction results in the overheating of the ink in certain areas of the coil leading to crusting, coagulation and damage of the ink constituents. Furthermore, the time necessary to heat the ink to the desired temperature is unacceptably long, particularly in cold operating environments or during mornings starts leading to loss of productivity. Moreover, the heaters of the prior art are unserviceable if the coil of pipe become clogged with dried or crusted ink. This is as a result of the flowing ink not being to spend enough time in the heat exchanger coil of pipe, if the coil of pipe is made long in an attempt to remedy such prior inefficient system, then the construction becomes large, bulky and impractical.

For the foregoing, it is apparent that there is a need for a unitary heater and heat exchanger apparatus that overcomes the above listed shortcomings of the prior art.

Objectives of the invention

The present invention relates to an inkjet printing apparatus and more particularly to heating of the ink to achieve and maintain a desired operating temperature. More particularly, the invention is directed at an integrated unitary heater for use in positive and re-circulating piezoelectric drop on demand digital printing ink supply systems. The unitary heater may also be suitable for use in other applications where heaters are used for thermal management and control, such as dry bath incubators and systems for controlling the ink temperature in lithographic printing presses.

The other object of this invention is to provide a recirculating ink system incorporating a novel ink heating system enabling efficient, safe and accurate heating of the ink to quickly attain and maintain the ink temperature at the desired optimum level.

Still another object of the present invention is to provide an energy efficient heater unit that provides for increase in the rate of raising the ink temperature. Therefore, shortening the ink jet printing start up process and in the process shortening the period during which the heater is on further reducing energy consumption.

Another object of the invention is to provide an enegy effficient heater which is capable of heating the ink to its desired temperature whitout damagings its constituents and thus ensuring the system reaches its operating ready state faster than prior art teachings.

Summary of the invention

The present invention provides for a heater unit for raising and maintaining the ink temperature of an ink jet printing system at a desired value. The heater unit, comprising: A heat exchanger unit defined by a rectangular block having lengthwise a flat face on one side and machined cast or inkjet 3D printed channels on the other opposite side of the block. The heat exchanger unit having machined or cast channels in the form of labyrinth on one side and on the other side, a flat surface to which is attached a Positive Temperature Coefficient (PTC) heater elements based heater. A 200 Watts heater in the form of a rectangular thin plate incorporating Positive Temperature Coefficient (PTC) heating elements. The heater plate is an aluminium extrusion with embedded PTC heating wires. The heater is attached to the flat face of the heat exchanger block. A heat exchanger insulating cover plate

Two heat exchanger cover plates A mains power supply for supplying power to the heater

Ink inlet and outlet ports.

When the printer is switched on at start-ups, the ink is first circulated in a short closed loop path by drawing the ink from the ink supply tank using a diaphragm pump whose motor speed is set to run at constant speed. The ink is then conveyed in a short, small plastic pipe to the inlet port of the heat exchanger to raise the temperature of the ink if necessary. The ink flows through the heat exchanger, through the proportional valve and back to the ink supply tank. When the desired temperature has been reached, the proportional valve is adjusted to start regulating the ink flow to the printhead. Thus, a dc driven electromagnetic solenoid valve is opened to allow ink to flow on the supply line to the printheads. Simultaneously, the proportional valve on the return line is being controlled to regulate the flow of ink in the return line until the desired vacuum is achieved at the printheads.

The system is provided with two temperature sensors. a. One in the printhead manifold and utilised to measure the ink temperature and compare this to the set the target desired temperature of the ink for a given printhead and a given ink type in use b. The other in the re-circulating ink system downstream of the heater and used as part of the closed loop Proportional Integral Differential (PID) ink heater control A mains power supply provides the necessary power to drive the heater. The electrical power supplied to the heater is driven by Pulse Width Modulation (PWM) via a solid state relay so that the power is controlled in accordance with the PID system.

When the ink reaches the desired temperature, an electromagnetic solenoid valve is open to divert ink to the printheads.

Brief description of the drawings

Fig. 1 is a drawing of a heater or cooler in accordance with the present invention

Fig. 2 is a drawing of heat exchanger in accordance with the present invention

Fig. 3 depicts a drawing of a heat exchanger of the prior art

Fig. 4 shows a Positive Temperature Coefficient (PTC) heating elements assembly used with the heater assembly of the present invention

Fig. 5 is an exploded view of the whole heater assembly of the present invention

Fig. 6 is an exploded view of a cooler unit assembly in accordance with the present invention

Fig. 7 shows a cooler (peltier) element and associated heat sinks and fan of the present invention.

Fig. 8 is a preferred schematic circuit diagram showing an ink printing system in accordance with the present invention

Fig. 9 is another alternative ink circuit diagram showing the ink flow paths of the industrial inkjet printer in accordance with the present invention

Fig. 10 and Fig.11 show a thermal simulation carried out using two alternative heat exchanger geometries

Fig.12 shows velocity vector field results of the heat exchanger design in accordance with the present invention.

Fig.13 to Fig.16 show thermal simulation and temperature rise prediction for a heater in accordance with the present invention.

Fig. 14 and Fig. 15 show the surface temperatures of the top and bottom sides of the heat exchanger respectively,

Fig. 16 shows a cross-section of the heat exchanger and the temperature spread across it.

Fig.17 to Fig.20 show thermal simulation and temperature rise prediction for a preferred heater in accordance with the present invention

Fig.17 shows the velocity profiles being the same as in the previous case of the heater being made out of aluminium

Figures 18 and 19 show the surface temperature results of the heat exchanger on the top and bottom sides respectively

Figure 20 shows section view of the temperature spread across the stainless steel heat exchanger

Detailed Description of the drawings.

The embodiment of the present invention is better understood by referring to fig.1 through to Fig.21

The heater (fig. 1) raises and controls the temperature of the ink to ensure the ink is supplied to the printheads (55) of Figures 8 and 9 at the required temperature. This temperature control system incorporates a mains power driven heater (fig. 1) controlled by a PWM using a real state relay (not shown). The heater unit is designed to be efficient and to ensure that it does not overheat the ink at any time. This way, the rate at which the ink is heated up is high. The ink attains the desired temperature quicker than prior art teachings and thus the time required for the printer system to reach ready state is made short.

The prevention of over temperature and overheating is achieved by the use of Positive Temperature coefficients (PTC) heating elements 6 of fig. 4. These combine heating and temperature limiting functions in one unitary element. Therefore, temperature limiting control is provided by hardware rather than firmware/software making it intrinsically safe.

The efficiency of the heater or cooler unit is obtained by the combination of the high density packing of the PTC heating elements and the use of ink channels (fig. 2) arranged either in a labyrinth (fig. 2 and fig.11) form or in the form of parallel channels (fig. 10) cast or machined in a metal plate 15 (fig. 5). In this manner, these ink channels provide a long path and large surface heat transfer area. Therefore, the ink flowing through these channels spends longer time exposed to heat transfer thus providing efficient heating of the ink without overheating, “baking”, coagulating or damaging the ink constituents as occurs in conventional prior art systems (fig. 3) where ink flows through coiled tubes 4 (fig.3) attached or sandwiched between heater elements. As a result of this prior art conventional construction, surface contact between heater elements and tube is not assured and the ink path through which ink flows is not long enough as the provision of a big coil of tube will result in a construction that is bulky, large, requiring more space to fit and difficult to reproduce accurately resulting in a variable unreliable, heat exchanger.

The channels of the heat exchanger of the present invention can be made by machining or by casting of stainless steel to ensure chemical resistance and compatibility with the constituents of the inks such as Methyl Ethyl Ketone (MEK). The heat exchanger and its channels might also be capable of being made by ink jet 3D printing of thermally conducting materials such as stainless steel or thermally conductive polymer. The heat exchanger material needs to also be compatible with the chemicals components of the inks such as solvents oils and colouring agents in use.

The heater unit of the present invention is constructed by simply bolting the flat surface 7 shown in fig. 4 of the high density packed PTC elements 6 of FIG. 4 which are embedded in an aluminium extrusion 8 of fig. 4. onto the heat exchanger channel plate reverse flat side surface 13 of fig. 5 using ordinary bolts 11 (fig. 5). High performance thermally insulating covers 14 and 17 of fig.5 are used to cover the heater element and heat exchanger respectively to minimise energy loss from the heater 12 (fig. 5) and heat exchanger and thus ensure maximum heat generated is transferred to the circulating fluids. The channels side of the heat exchanger is hermetically sealed using an ‘o’ ring 16 (fig. 5) and a high performance thermally insulating cover 17 bolted onto the heat exchanger using plate 18 and bolts 9. Provided in the thermally insulating heat exchanger cover are fluids inlet and outlet ports 1 and 2 shown in fig. 1. These ports are provided with Festo type nuts 10 (fig. 5) to facilitate and secure connection of the plastic ink or liquid tubes. Heat exchanger thermal simulation models fig.10 to fig.21 show that such heater unit construction to be sufficient to provide an efficient heater unit. However, one could use a thin thermal gasket or paste between the heater and the heat exchanger to increase further the efficiency of the heater unit. Furthermore, and surprisingly, the heat exchanger thermal simulation models showed that lengthwise parallel channels (fig.10) provided less efficient heater unit than when the channels are formed width wise and arranged in a zig-zag manner (fig. 11). The construction of a heater or cooler in accordance with the present teaching results in an apparatus that is easily accessible and easy to maintain and service for instance if the channels get blocked or if the ink type or colour needs to be changed.

The present invention applies also to a cooler unit (fig. 6 and fig. 7) whereby the heating element sub-assemblies are replaced by thermoelectric (peltier) cooler 23 of fig. 6 with its thermoelectric elements 37, heat sinks 40 and fan 39 shown in fig. 7. The cooler share similar heat exchanger 22 (FIG. 6) teachings and construction as those of the heater described earlier. Although the main description of the present invention is related to a heater, the present invention applies to a cooler that share the main advantages of the heater unit described here.

Fig. 6 shows an exploded view of an example of a cooler in accordance with the present invention. Fleat exchanger 22; heat exchanger cover 20; securing plate 19 and securing nuts 36 and pipe securing nut 35; Cooler assembly 23; ‘o’ring seal 21; fan 24; peltier element 28; heat sink 34, heat sinks and fan cover 33; heat exchancher locating pins31; cooler assembly flat face 27 that mate to cooler heat exchanger 22. Fan securing nuts 26 and fan guard 25; heat sink securing nuts and bolts 29 and 30

Fig. 10 and Fig.11 show a thermal simulation carried out using two alternative heat exchanger geometries. The objective of the thermal simulation was to determine how much the ink temperature rises from the inlet to the outlet of each of the heat exchangers as the ink flows to the ink jet printheads. Therefore, comparing and determining the most efficient heat exchanger geometry and design. In use, for example, in an ink jet printer, the design requirement on the heater is to be able to raise, in the worst case, the ink temperature from 5°C to 40°C. Hence if the ink flowing into the heater is at a temperature of 5°C, it should exit the heater at a temperature of 40°C. Therefore, The heat exchanger must be able to transfer heat into the ink, which is typically flowing at 150cc/min, and raise its temperature by 35°C, i.e. dT = 35°C . The thermal simulation, in this case, was simulating ink entering the heater at 20°C. The results, Fig.10 and Fig. 11, using four heater elements each dissipating 50W into the heat exchanger, show that both heat exchanger geometries exceed the requirement of raising the ink temperature by dT of more than 35°C. However, the preferred geometry of the present invention is that of zig-zig or labyrinth channels of Fig. 11 as it is much more efficient at heat transfer. In this thermal simulation example, the heat exchanger with the parallel channels raises the ink temperature by 49°C whilst the heat exchanger with zig-zag or labyrinth channels is able to raise the ink temperature by 68°C. In both cases, a 2mm thick aluminium cover plate was implemented as a lid. Both heat exchanger and cover plate are made from Aluminium 6061-T aluminium alloy.

Fig. 13 to Fig. 16 show thermal simulation and temperature rise prediction for a heater in accordance with the present invention. The heater assembly and construction in this example is made out of a 2mm thick aluminium cover plate lid. Both heat exchanger and cover plate are made from Aluminium 6061-T aluminium alloy.

In Fig. 13, the thermal simulation shows that a heater in accordance with an example of heater construction of the present invention using aluminium as heater exchanger and cover plate will raise the temperature of an ink entering the heater at a flow rate of 150cc/min from 5°C to 52°C using a 200 W heater. This simulation shows that the whole heater length could be reduced by cutting the labyrinth at plane A-A shown in figure 13. Alternatively, the heat from the four 50W heater elements could be reduced to easily achieve the required ink exit temperature of 40°C

Fig.14 and Fig.15 show the surface temperatures of the top and bottom sides of the heat exchanger respectively, whilst fig. 16 shows a cross-section of the heat exchanger and the temperature spread across it. These figures show that the ink temperature is evenly spread along the length of the heat exchanger with no hot spots present. Hence, preventing “backing” and degrading of the chemicals components of an ink.

Fig.17 to Fig.21 show thermal simulation and temperature rise prediction for a preferred heater in accordance with the present invention. The heater assembly and construction in this example is made out of a 2mm thick nylon 66 cover plates as a lid. The heat exchanger was made from Stainless Steel 302 alloy.

Fig.17 shows the velocity profiles being the same as in the previous case of the heater being made out of aluminium. This is in line with expectations as the input boundary conditions are the same and the only difference between the two examples is the materials. The predicted ink outlet temperature for the stainless steel heat exchanger was 51 °C. Hence the predicted temperature rise based on a 200W heat input is (51 -5) = 46°C. This is higher than the required 35°C in the example of a heater in use in an ink jet printer. Fig. 17 shows the temperature rise of the ink.

Based on the predictions of this simulation, the required temperature rise could be achieved by removal of the last 3.5 labyrinth cut-out loops i.e. by reducing the length of the heat exchanger up to the loop after the point showing 44°C probe reading.

Figures 18 and 19 show the surface temperature results of the heat exchanger on the top and bottom sides respectively. The low thermal conductivity of stainless steel compare to aluminium alloy is illustrated by pronounced surface temperature gradients. Figure 20 shows section view of the temperature spread across the stainless steel heat exchanger.

The predicted ink temperature rise for a stainless steel heat exchanger with a nylon 66 cover plate was 46°C. Thus a heater in accordance with the present invention and whose heat exchanger is fabricated out of stainless steel will meet the specified performance requirements of an ink jet printer system. The heater performance, in this example, exceeds the requirements for an ink jet printer. This is desirable because the warm up and make ready times of the printer or other systems can be reduced. Alternatively the power input could be reduced so that the outlet ink temperature just meets the specification and save energy.

Claims

1. A fluid heater unit for raising or lowering and maintaining the ink temperature of an ink jet printing system at a desired value. The heater or cooler unit, comprising: A heat exchanger unit defined by a rectangular block having lengthwise a flat face on one side and machined cast or ink jet 3D printed channels on the other opposite side of the block. The fluids or ink channels arranged in the form of labyrinth, zig zig or parallel to each other on one side of the heat exchanger block and on the other side of the heat exchanger, a flat surface to which is attached a heater elements that provide the necessary heating source.

2. A heater of claim 1, wherein the heat exchanger is manufactured out of stainless steel, aluminium, brass, copper or thermally conductive polymer.

3. A heater of claim 1 and claim 2 where the heat exchanger fluids channels are contiguous.

4. A heater of claim 1 wherein a thermally insulating cover plate is hermetically attached, bolted or assembled on top of the fluids channels to form the heater assembly

5. A heater of claim 1, wherein the heater assembly is made out of Positive Temperature Coefficient (PTC) heating elements or chips.

6. A heater assembly of claim 5, wherein the PTC heating elements are embedded and firmly clamped between aluminium plates.

7. A PTC based heater of claim 5, wherein the PTC heater assembly is bolted onto the flat face of the heat exchanger

8. A heater of any preceding claims, wherein a thermally insulating cover is used to cover insulate the heater element to minimise energy loss.

9. A heater of any preceding claims for heating ink in an ink jet printer.

10. A heater of claim 1 for heating of fluids and liquids.

11. A fluid cooler unit for lowering and maintaining the ink temperature of an ink jet printing system at a desired value. The cooler unit, comprising: A heat exchanger unit defined by a rectangular block having lengthwise a flat face on one side and machined cast or ink jet 3D printed channels on the other opposite side of the block. The fluids or ink channels arranged in the form of labyrinth, zig zig or parallel to each other on one side of the heat exchanger block and on the other side of the heat exchanger, a flat surface to which is attached peltier cooler elements that provide the necessary cooling source for extracting heat energy out of the flowing fluids or inks.

12. A fluid cooler of claim 11, wherein, the cooler block is provided with heat sinks and fans on the opposite side of the peltier cooler elements.

13. A cooler of claim 11, wherein the heat exchanger is manufactured out of stainless steel, aluminium, brass, copper or thermally conductive polymer.

14. A cooler of claim 11 wherein the heat exchanger fluids channels are contiguous.

15. A cooler of claim 11, wherein the cooling function is provided by thermoelectric devices or peltiers.

16. A cooler of claims 11 to 15, wherein heat sinks and a fan or fans are provided on the reverse side of the thermoelectric units to dump the energy extracted from the ink or fluids and the heat exchanger to lower the ink or fluids temperature.

17. A cooler of claims 11 to 16 for cooling ink in an ink jet printer.

18. A cooler of claims 11 to 16 for cooling of fluids and liquids