MULTIPIN STRUCTURES

Abstract: A heat sink comprises a thick base plate (1), a plurality of parallel pins (2) running outwardly from the base plate and spaced close to each other to define a labyrinthine air path, and a guide plate (3) in which an axial fan (4) is mounted to direct air parallel to the pins towards the base plate (1) which is mounted on the object to be cooled. The heat sink is produced by solidifying a mould (15) of resin coated sand around a tool (11-14), then withdrawing the tool, then gravity die casting through the mould, then destroying the mould.

Description: MULTIPIN STRUCTURES This invention relates to raultipin structures comprising a plurality of pins (e.g. elongate members or rods), connected at at least one end, and extending longitudinally in a direction (for example parallel one to the other). In a further aspect, this invention relates to heat exchangers including such multipin structures. A further aspect of the invention comprises a method for the manufacture of such multipin structures, and heat exchangers made therefrom. One example of a heat exchanger is a heat sink comprising a structure made of a material with good thermal conductivity (generally a metal), which presents a relatively large surface area, and is placed in contact with an article which it is desired to cool. The heat sink conducts heat away from the portion thereof in contact with the article and onto the large surface area, at which heat is dissipated to the surrounding atmosphere. Heat sinks are widely used in electrical applications, where resistive losses, for example, often generate substantial heat, which may lead to one or more of thermal expansion, cracking, fatigue, chemical reaction, and alteration or deterioration in electrical performance of components. Heat sinks may also be used for non-electrical application. The performance of the heat sink depends, amongst other things, on the rate at which heat is removed from the surface area of the heat sink by the surrounding atmosphere and this in turn depends upon motion of the atmosphere; air currents improve the performance of the heat sink. Accordingly, in one aspect, we provide a method of manufacture of a heat sink comprising a plurality of features co-extensive in a first direction, defining air channels generally transverse to said first direction, in which the air channels are such as to render the air flow transverse to said direction generally turbulent, the features being interconnected by means for transmitting heat thereto. For example, the interconnection means may be a plate connecting the heat sink to the article to be cooled. The features may for example be pins. A turbulent air flow is much more efficient at dissipating heat than a laminar flow, and so this has a high efficiency. In a preferred embodiment, there may be provided means for guiding air currents to follow a path transverse to said first direction; for instance, the guide means may be a plate or plates transverse to the direction of the features. The heat sink may be adapted for cooperation with means for generating an air current to the features; in a preferred embodiment the means may be arranged to direct the air current in the first direction (for example, axially of the pins if present) and the air flow may be redirected outwardly transverse of said first direction. In a particularly preferred embodiment, the air flow is directed towards the connecting means, and the air is then redirected to flow turbulently through said features transverse to the first direction. In another aspect, the invention provides a heat exchanger structure comprising a plurality of substantially co-extensive aligned pins at least partially, interconnected at both ends by at least one support member, the pins being packed so that the air flow through the pins is substantially non-laminar. This effect is preferably enhanced by densely packing the pins in a radially staggered manner so that the airflow is predominantly not straight. A preferred embodiment of this aspect further comprises means for generating an air stream outwardly transverse of the pins; the means is in a preferred embodiment arranged to direct the air flow axially towards the object to be cooled, the air flow being redirected radially to flow out transverse to the pins. Preferably, in either of the above aspects the surface of the features or pins is rough, so as to increase the effect of surface area and/or further break up the air flow. Such heat sink or heat exchange structures could in principle be fabricated by individual machining and assembly of parts, and for some structures investment casting could also be used. However, whilst these solutions are technically feasible they are completely ruled out on commercial grounds. Cheaper methods, such as die casting techniques used in the prior art, cannot be used to fabricate the labyrinthine assemblies of extending parallel features (e.g. closely packed arrays of pins) because since these features are surrounded by others (e.g. plates) a conventional mould cannot be employed to cast them together as one piece. Even investment casting is unsuitable where the pins are densely packed, or need to be parallel, because the ceramic moulding produced in investment casting would be trapped by the dense array of pins and containing plates and could not be removed. Investment casting normally requires tapered features which limits pin length. The known sand casting technique, in which a former is pressed into loose sand to leave an impression into which molten metal is poured, is likewise unsuitable for forming pins with parallel sides since a tapered former is required to permit removal of the former from the sand, and relatively long indentations are not practical with loose sand. Further, enclosed shapes such as plates interconnected by rods cannot be produced using loose sand. The technique is known for producing cores for casting, from initially loose sand coated with thermosetting resin, by providing a female tool, heating the tool, introducing resin coated sand into the heated tool so that the sand adjacent to the tool solidifies as the resin cures, and then pouring out the remainder of the sand. A thin walled hollow mould of sand is thus created with an outer conformal surface. However, this technique is unusable for manufacturing multiple elongate aligned features, because the mould would be trapped between the features. A further aspect of the invention therefore manufactures elongate metal structures by forming a mass of flowable, hardenable material such as resin coated sand around a tool, hardening the mass, removing the tool so as to provide channels in the mould corresponding to the shape of the structures, introducing molten casting material (e.g. metal) into the mould, and then destroying the mould. In this way, complex metal shapes can be produced because they need not be withdrawn from the mould, at a low cost. The tooling to make the sand mould is also relatively inexpensive. As the resin coated sand mass can be degraded back to sand, it can be extracted from the complex shape which would not be possible with a ceramic investment casting mould. In a further aspect of the invention, we provide a method of casting multipin or similar structures comprising forming a one-shot mould through which the features or pins run, pouring molten metal through the mould so as, on cooling, to form the features, and then destroying the mould. The mould may, for example, be formed from resin coated sand. The sand used may be chosen so as to provide the surface quality of the features as required - rough sand is preferred for a heat exchanger. Connecting plates can thus be formed on either side of the features if required. A narrow bleed-channel can be provided to allow metal to flow through any pin that does not need to join to the second connecting plate. This technique allows the formation of complex, densely packed arrays of parallel features (e.g. pins), potentially at a very low cost. The limits caused by the taper required in conventional processes are avoided. Other aspects and preferred embodiments of the invention, together with corresponding advantages, will be apparent from the following desciption, claims and drawings. A number of embodiments of the invention will now be illustrated, by way of example only, with reference to the accompanying drawings in which; Figure 1 illustrates a heat sink structure according to a first embodiment of the invention; Figure 2a-2g illustrate stages of a process for manufacturing structures, according to an embodiment of the invention; and Figure 3 illustrates a heat sink structure according to a second embodiment of the invention; Figure 4 illustrates a heat sink structure according to a third embodiment of the invention; Figure 5 illustrates a heat sink structure according to a fourth embodiment of the invention; Figure 6 illustrates a heat exchanger according to a fifth embodiment of the invention; Figure 7 illustrates a heat sink structure according to a sixth embodiment of the invention; and Figure 8 illustrates an alternative construction to that of Figure 7; Figure 9 illustrates a heat sink structure according to a seventh embodiment of the invention; Figure 10 illustrates a heater according to an eighth embodiment of the invention; and Figure 11 illustrates a spatial light. modulator projection system in which the invention may be employed. FIRST EMBODIMENT Referring to Figure 1, a heat sink according to one embodiment comprises a base plate 1, a plurality of pins 2 comprising parallel sided circular cross section rods upstanding from the base plate 1 with parallel axes, and a guide plate 3 capping the pins 2. For example, the base plate and guide plate may be generally circular, of diameter 90 mm, the base plate having a thickness of 5 mm, the diameter of the pins being approximately 2 mm and their length being approximately 15 mm. The pins and plates 1-3 are a unitary metal casting, typically of a die-castable alloy (for example an aluminium based casting alloy such as M6, or a zinc based alloy) widely available. The plate 1 is relatively thick and is arranged to be mountable to a component to be cooled (mounting fittings such as holes being provided for this purpose as necessary), for example to the spatial light modulator device, or lamp, of a projection display system as described in our earlier UK Applications 9201810.0, 9204798.4, and 9208076.1, and our earlier International Applications PCT/GB91/02032, PCT/GB91/02033, PCT/GB92/00002, and PCT/GB92/00132, (incorporated herein by reference in their entirety). Heat from the component to be cooled therefore heats the plate 1 and the pins 2. The pins 2 are disposed in a densely packed formation on the plate 1. Preferably, the spacing between the pins is on the order of (and preferably approximately equal to) the diameter of the pins. Preferably there is no clear line-of-sight path from the centre of the plate 1 to the outside of the plate 1; in other words, the heat sink is optically opaque. It would, of course, be possible for a few direct line-of-sight paths to exist, but the performance is found to be significantly better if none are present. Additionally, the arrangement of the pins is such as to further reduce the number of direct paths preferably from the interior of the plate 1 to the exterior via the pins; for instance, the pins may be arranged in a hexagonal array, or in concentric circles each circle offset within its neighbours. Thus, air within the centre of the array of pins can only reach the outside of the heat sink by labyrinthine paths (and through a few narrow remaining direct channels if present). The upper plate 3 may as in the illustrated embodiment include a central aperture, in which is mounted an air impeller 4; for example, an axial fan such as the R.S. Components stock no. 506-160 minature DC 12 volt axial fan. The axial fan 4, when energised, therefore directs an air stream towards the plate 1 parallel to the pins 2, so that a relatively smooth flow (with a spiral component from the fan) towards the plate 1 occurs. On reaching the plate 1, the air flow from the impeller 4 is heated by contact with the plate 1 and the pins below the impeller 4. Multiple contacts with the pins between the impeller 4 and plate 1 redirects the airflow radially of the plate 1 towards the outside of the heat sink. The air contacts the pins 2 and cools the pins. The pins 2 also break up the flow of air, causing turbulence and eddy flows around the pins so that the air currents reach a larger part of the surface area of each pin than had the air flow been purely laminar. The close spacing of the pins 2 keeps the air flowing through the pins 2 in a turbulent state, and greatly enhances the heat exchange efficiency of the heat sink. The guide plate 3 acts to contain the air flow and keep the air moving transverse to the pins . It may also itself act as a surface for dissipating heat, and additional heat dissipation features could be mounted to it if desired. It may also act as a secondary heat distributor, improving heat distribution over the pins of the heat sink where the heat applied to the plate 1 is localised. This embodiment therefore provides a compact, lightweight (lOOg) heat sink with a much higher efficiency than conventional structures of the same size and weight; typically, on the order of ten times the efficiency. A method of making the structure shown in Figure 1 according to an embodiment of the invention will now be described with reference to Figure 2. As a preliminary measure, a tool for fabricating moulds is made. The tool comprises essentially a replica of the heat sink structure of Figure 1, comprising a plurality of pins 12 running through a first plate 11 and a second plate 13. The pins 12 are conveniently interconnected by a plate 14 to which they are fastened. The plates 11, 13 slide on the pins 12 and mate together to enclose a forming chamber in the tool. The tool needs to be resistant to moderate temperatures (on the order of 100°C) and pressures (on the order of 10 tonne/metre 2 ) necessary for the forming of the mould. For example, epoxy fibreglass board may be used to make the tool. Referring to Figure 2a, loose resin-coated sand of the type used for sand casting (for example 150 micron coated sand available from Hepworth Minerals and Chemicals Limited in the U.K. ) is introduced into the forming chamber, and the chamber is closed, heated to a curing temperature (for example around 100°C) and subjected to pressure as shown in Figure 2b, so that the resin part-cures throughout the sand mass, to form a "green" solid mould. The chamber is then opened by sliding out the pins 12 from the sand mass using the plate 14 and separating the plates 11, 13 and the sand mass comprising the moulding core 15 is extracted. The sand core 15 is then baked further as shown in Figure 2d to fully cure the resin; typically at 150°C for 4-24 hours. It is also possible to part-cure the resin without pressure, using only heat, but this is a slower process resulting in a less dense core. The fully cured core 15 is then placed within a die casting chamber defined by two mould halves 16a, 16b between which the core 15 is located, and hot metal is flowed into the die to penetrate the core, through the channels which were formed around the tooling pins as shown in Figure 2e. The die metal is preferably gravity die cast, in other words poured downwards into the die to rise through the core to avoid the expense of injection die casting apparatus. This does not, however, preclude the use of for example low or high pressure die-cast apparatus. On solidication of the metal after cooling, the mould halves 16a, 16b are separated and the component and core 15 are withdrawn. The sand core cannot be separated from the cast component since, firstly, the second plate 3 completely encloses the sand core and, secondly, the surface of the pins 2 engages the rough sand edges of the holes in which they are formed and thirdly the pins are preferably close packed. The sand core is therefore destroyed; for example by air baking the component and core in an oven at an elevated temperature (for example 500°C) for a period of, for example, 24 hours sufficient to oxidise the resin, and thus destroy the cohesion of the sand core; a sharp tap is then sufficient to dislodge the sand from between the pins 2, to leave the component as shown in Figure 2f. The texture of the sand core provides a rough finish on the pins which assists in increasing the surface area, and consequently the heat dissipation performance, of the pins. This roughening effect is improved if coarser grades of sand are used. By curing the entire sand mass during the stage of forming the sand core, the holes through the core are stable and pins of considerable length relative to their diameter can be formed, without requiring the substantial taper and short lengths of the pin which would be produced if prior art casting techniques were employed. After the residual sand has been removed, the casting may be annealed (in practice, the annealing stage may be combined with the above described resin oxidation baking stage), and any necessary holes or other details are machined, and surface finish is applied (e.g. by sand blasting, anodising or otherwise as required). Unnecessary metal produced by the die casting process, shown in Figure 2f, is machined away to result in the finished structure of Figure 2g. At the same time as the parallel pins are formed in the core 15 it is also possible to form other features. For example, as well as the pins, blank stubs with narrow vent channels running parallel to the pins can be provided entering into the resin coated sand core. These therefore create blind holes aligned with the pins. After the core 15 is cured, threaded inserts (for example those made by PSM International) may be inserted into the blind holes so as to be brazed to the die cast component during the die casting process. Likewise, blind holes with narrow vent channels could be provided for creating mounting blocks or other structural features of the heat sink or heat exchanger; for instance, thick pillars or bosses, or for forming shorter pins (e.g. to clear a fan impeller 4) .. Part or all of plate 3 may be arranged to be removed during machining to leave, for example, a recess to mount the fan of Figure 1. SECOND EMBODIMENT Referring to Fig. 3, in a heat sink of a second embodiment, an array of pins 2 (shown as a rectangular array) is provided between a pair of plates 1, 3; in this embodiment both plates are relatively thick to act to distribute heat over the pins. A further pair of thin walled parallel plates Ila, lib are provided enclosing the sides of the pin array, to channel air through the pins. In structures according to this embodiment, where air is directed transversely through the array of pins, it is preferred to provide a containment duct of this type comprising the walls 1, 3, Ila and lib so as to ensure that the air travels to the pins 2 furthest away from the fan 4 and is not dissipated before it reaches them. The walls Ila, lib could, of course, be made thicker if so desired in order to further dissipate heat directly. The pins 2 located closest to the fan 4 are preferably, as shown, of narrower diameter than those further away; this construction is found to reduce ambient sound generated by the fan, by reducing the amount reflected back towards the fan by the pin array. The fan is also less likely to stall with this arrangement. The pin diameter could also be varied for other purposes, such as for example to vary the rate of heat dissipation in different areas of the array of pins so as to give greater heat dissipation where a greater surface area is provided. In the structure of Fig. 3, a component to be cooled can be mounted to either of the thick walls 1,3. If the thickness of the walls Ila, lib is extended, either of these walls may likewise be used to mount a component or further heat-dissipating features. The walls could likewise be extended to provide mounting features. The structure of Figure 3 can be formed in the same way as that of Figure 1, the walls being formed in the same casting step. The fan 4 could alternatively be mounted on one of the side walls 1, 3 or Ila, lib to blow transverse to the pins. THIRD EMBODIMENT Referring to Fig. 4, in a heat sink according to a third embodiment of the invention, as in the second embodiment the fan 4 is directed to provide air transverse to the array of pins 2, which is likewise provided within a duct comprised of four walls la, 3a, lb, 3b. In this embodiment, the array of pins comprises two sub-arrays 2a, 2b of pins at different inclinations so as to provide a two-dimensional array. As shown, the two sub-arrays 2a, 2b may be provided normal to each other; this is thought likely to provide the most efficient turbulence in three-dimensions. It is found that providing two arrays defining a grid in two-dimensions creates turbulence in three-dimensions, which is very efficient at heat removal. In this embodiment, all four walls la, 3a, lb, 3b are preferably relatively thick to spread heat equally across all pins in the array. Variations in pin size and packing density, wall thickness and provision of mounting features are all possible as described in the second embodiment. It would be possible to provide the fan axial to one set of pins (as in the first embodiment) rather than being transverse to both as shown. Structures of the type shown in Fig. 4 can also be manufactured according to the process described with reference to Fig. 2 above, although the shape of the core and of the die casting mould is necessarily more complex. Equally, such structures could be made in several stages (for example, two stages in which an array of pins are formed according to the method of Fig. 2) and the resulting sand cores secured together prior to casting. FOURTH EMBODIMENT Referring to Fig. 5, in a heat sink according to a fourth embodiment of the invention a heat spreader plate 1 is relatively thick and supports two arrays of parallel pins 12a, 12b; one array being supported by either side of the plate 1. A pair of plates 13a, 13b are provided at the other sides of the arrays of pins 12a, 12b to guide air through the array. A central aperture 14 within one of the arrays 12b of pins is provided, aligned with a recess 15 and threaded hole 16 into which a heat generating component is fitted and secured; in this embodiment, the heat generating component may be a lamp in a projection display system of our above referenced applications. As in the above two embodiments, air is directed transversely to the arrays of pins 12a, 12b by a fan (not shown) mounted adjacent the heat sink. Using two arrays of pins at either side of a heat spreading plate doubles the surface area and hence dissipation of the heat sink; in the embodiment shown in Fig. 5 (which is approximately to scale) the heat dissipation efficiency is above 0.08°C/ att. Structures according to this embodiment of the invention are conveniently made by using the process of Fig. 2 to manufacture two cores, one for each of the arrays of pins 12a and 12b. The die cast mould for manufacturing this embodiment is arranged to mount the two cores parallel. Pin arrays 12a and 12b need not be identical, thus each is adjusted for optimum air flow and efficiency. It is also possible, although not so easy, to make structures comprising more than two coaxial arrays of pins stacked similarly; in this case, multiple cores may be made and aligned in the die casting mould. FIFTH EMBODIMENT Referring to Fig. 6, in a fifth embodiment of the invention a heat exchanger for exchanging heat between two fluids is provided as a pair of ducts 100a, 100b sharing a thin heat transmitting wall 101, and defined by further walls 113a, 113b to provide two separate channels through which two fluids can flow. If a relatively hot fluid is passed through a first duct 100a, heat will quickly and effectively be extracted by the pins 112a, which are spaced and arranged so as to create turbulence in the fluid based on its density, viscosity and speed. The heat is transmitted along the pins through the thin wall 101 to the second array of pins 112b, which are likewise arranged and spaced to create a turbulent flow in the second fluid passing through the second duct 100b, and thereby heat from the first fluid in the first duct 100a is efficiently transferred into the fluid in the second duct 100b. The duct sizes 100a,100b and pin arrays 112a and 112b will often be different to optimise the heat exchange efficiency in each channel. Plate 101 will have a thickness commensurate with its load as a heat spreading member and dependent on the difference between the pin arrays 112a and 112b. Such heat exchangers may be used in many applications, such as power stations, for heating or cooling fluids or extracting heat energy from fluids. This embodiment may be made by the same method as that employed in the preceding embodiment. SIXTH EMBODIMENT In the embodiments of Figs. 3-5, for example, the air blown through the pin array may be heated to the temperature of the pin array or close thereto within a relatively short distance of the fan 4, so that there is less cooling of the pins furthest away from the fan 4. Referring to Fig. 7, in a sixth embodiment of the invention, the first pin array 22a is provided, and a fan 4 directs air transversely through the pin array. Next to the pin array 22a is provided a duct 30 defined by an internal wall 31 and external wall 33. Some air from the fan 4 is directed through the duct 30. The pin array 22a does not extend over the entire length of the heat sink device of Fig. 7, and at the end of the pin array 22a the thin wall 31 extends inwardly across the heat sink, to broaden the duct 30. In the broadened part of the duct 30 lies a second pin array 22b, extending over the remaining portion of the length of the heat sink. Thus, unheated air from the fan 4 is lead through the duct 30 to the second pin array 22b to cool portions of the heat sink which are located distantly from the fan 4. Thus, in general, this embodiment of the invention provides a heat sink including a first set of heat dissipating features 22a and a second set of heat dissipating features 22b spaced along the heat sink further from the fan 4, with a connecting duct 30 bringing air directly to the second set 22b bypassing the first set 22a, and an exit path or duct 30a bypassing the second set 22b with heated air from the first set 22a. The wall 31 and walls 32, 33 can be formed in a process generally as shown in Fig. 2 together with the pins in a single die casting operation. Referring to Fig. 8, in a second example of the structure according to this embodiment of the invention, the duct 30 runs centrally between two pin arrays 22a, 22b from which it is separated by a pair of walls 31a, 31b (relatively thin metal cast walls, as in the above example) and a pair of fans 4a, 4b each direct air into a respective pin array and into the duct 30. After passing through the pin arrays, the air is channelled by the walls 31a, 31b and vented out transverse relative to the axis of the fan through apertures in the outer walls 32, 33 of the heat sink. The unheated air from the duct 30 may then be used to cool other components; for example, as shown, power components 50a, 50b, and a further heat sink 60. Air from the duct 30 is directed through and over the power components 50a, 50b to cool them and thence the heat sink 60. It will accordingly be seen that a structure according to Fig. 8 may be used both as a heat sink for cooling components attached thereto (not shown), and as a source of cool air for cooling other components. As in the preceding embodiment, the array of pins 22a, 22b and the dividing walls creating the duct 31a,31b are preferably cast in a single operation, preferably according to Fig. 2. It would, of course, also be possible to provide two dimensional pin array structures of the type illustrated in Fig. 4 in each of the pin arrays 22a, 22b. Structures according to this embodiment could also be used in other applications; for example, as counterflow heat exchangers. In the case of Fig. 7, different fluids would then pass through the different pin arrays 22a, 22b. SEVENTH EMBODIMENT Because of the large amount of turbulence created by the heat sinks of the preceding embodiments, created by the high density of the pin arrays, turbulence is created even at very low rates of air flow; this is not the case in prior art heat sinks which rapidly lose turbulence as the air speed falls and consequently undergo a marked loss of efficiency at low air flows. Because the heat sinks according to the above embodiments are useful over a wide range of air flow rates, to provide different rates of cooling, in this embodiment of the invention a controllable cooling device is provided which comprises a heat sink according to any of the preceding embodiments (a heat sink according to Fig. 1 is illustrated) with a control unit 10 supplying a variable energisation to the fan 4 to control the fan speed and hence rate of air flow. Also provided is a temperature sensor 15, mounted for example on the heat sink or on the component to be cooled or in the atmosphere close to the heat sink. The control unit 10 compares the signal from the temperature sensor 15 (for example a thermistor) with a predetermined correct temperature level, and increases or decreases the power supplied to the fan 4 to reduce the difference between the two in a feedback manner. The reference signal with which the signal from the temperature sensor 15 is compared may be made manually variable, by providing for example a potentiometer to set the level of the reference signal as part of the control unit 10. In this case it is then possible to set the desired running temperature for the device to be cooled manually. The same principle could be employed in other applications, such as heat exchangers in general. This embodiment of the invention is particularly advantageous in enabling the fan 4 to be run at the minimum speed necessary to keep the component temperature below a desired maximum, thus enabling the noise made by the fan to be reduced to a minimum level and reducing the power used. EIGHTH EMBODIMENT Insofar as the aspects of the invention described above provide a structure in which an air stream from an air impeller 4 is heated by a heat exchanger structure, they may also be used in this embodiment of the invention to provide a fan heater. Referring to Fig. 10, in one example of such a heater, the structure generally corresponding to that of Fig. 1 is employed with a pair of plates 1, 3 between which a plurality of pins 2 are arranged in an optically dense array. A fan 4 directs air into the array of pins 2. Within the heat spreading plate 1 is cast a resistance heater 70 comprising a heating element in a refractory sleeve (for example, a MgO sleeve, typically of the type employed in immersion heaters) . A power supply 80 energises the heater 70 with heating current. A power supply 90 energises the fan 4. In use, the heating element 70 heats the plate 1 and the pins 2, and the fan 4 directs air onto the pins to be heated. Because of the efficient heat transfer, the exit air is at essentially the same temperature as plate 1 and pins 2 and so the exit air temperature can be accurately controlled by controlling the temperature of plate 1 which can be measured more simply than the exit air temperature. Preferably, one or both of the power supply 80 and the power supply 90 are controllable so as to vary the air flow rate and/or the temperature of the plate 1. As in the embodiment illustrated in Fig. 9, a thermal sensor may be provided to control the power supply 90 to maintain a constant temperature (or, if preferred, the power supply 80) . The embodiment shown in Fig. 10 can be produced by the method shown in Fig. 2, by placing the resistance coil in the die casting mould before pouring die casting metal in. Provided the heating element 70 has a higher melting point than the temperature of the die casting alloy, the heating element is cast in place in the plate 1. It is therefore possible to cast a simple one part heater enabling precise temperature control and high efficiency, providing an effective heater at a low price. Furthermore, the characteristic that the temperature of the fluid (e.g. air) drawn through the heater is almost the same as that of the heater is useful in simplifying control of the heater. Instead of employing the structure of Fig . 1, any of the other structures of, for example, Figs. 3 and 4 may be employed. The latter may be preferred where it is desired to provide a directional air or other fluid flow. This embodiment may for example be used to advantage in electric shower heating units, providing a more safe system. PROJECTION SYSTEM Referring to Fig. 11, in a spatial light modulator projection system a white light source 216 comprising a high power lamp generates light along an incident light path. A colour selective surface 230a inclined at an angle to the incident light beam reflects a blue beam of light to a spatial light modulator device 220a (for example a deformable mirror device or DMD) driven in accordance with a blue video signal. The transmitted red and green components of the beam for the light source 216 are again split at a second angled colour selective surface 230b and a red component is reflected to a spatial light modulator device 220c driven in accordance with a red video signal. The transmitted green light beam is modulated by a third spatial light modulator device 220b driven in accordance with a green video signal. The modulated beams from each of the three spatial light modulator devices 220a-220c are recombined at the colour selective surfaces 218a, 218b to form a single colour image which is projected, by a projection lens 240, onto a screen 210 for, for example, cinema display. The light source is necessarily of very high power; for example several kilowatts, and a considerable proportion of the total energy from the light source 216 may be at infra-red wavelengths. Our earlier U.K. applications and PCT applications referenced above provide further details. The light source 216 and spatial light modulator devices 220a-220c can therefore reach relatively high temperatures, and it is therefore highly desirable to provide a heat sink for each so as to avoid thermal stresses, chemical degradation of glass bonded components and change of dimensions of optical components. The heat sink of Figure 5 is preferably provided to mount the light source 216, and the heat sink of Figure 1 is provided with an integral casting of one of the plates of the position adjusting mechanism of our earlier UK Application 9201810.0 (Agents Ref 3203701) refiled as PCT Application PCT/GB93/00172 to mount each spatial light modulator device 220a-220c. It will be readily apparent from the foregoing description that the structure and method above described are only examples, which may be modified in many ways. For instance, the dimensions of the structure may be varied; the structure may be used for applications other than heat sinks; the structure may be produced by other methods; and the method may be used for structures other than heat sinks. The pins need not, of course, have a constant cross section or a circular cross section; more complex cross sectional shapes may be preferable as providing greater surface areas for heat loss and/or more turbulence. Pins could, of course, have differing sizes and shapes if required. The invention can, as mentioned above, be applied to heat exchanging structures in general, for exchanging heat with fluids, for heating, cooling, refrigeration or other purposes. For example, an array of pins may be provided within the (cast iron) cooling jacket surrounding a piston in an internal combustion engine, the jacket comprising a pair of coaxial cylinders, the pins running radially between the cylinders to form a cylindrical array; this structure corresponds to that of Fig. 1 with the two plates 1, 3 transformed to cylindrical surfaces. In general, the invention extends to such arrays of aligned (e.g. radially aligned) connected features. The method aspect of the invention can be employed for making similar structures for purposes unconnected with heat exchanging. A fan could be omitted altogether, but a lower efficiency results if there is no forced air flow. Other modifications and substitutions may of course be made within the general spirit of the invention as above described. It will be appreciated that this invention relates to a device to aid heat transfer to/from a fluid. This may be from one fluid to another, such a device typically being called a heat exchanger, or from a solid object (e.g. a transistor) to a fluid, the device then being typically called a heatsink. The principle in any such system is that means must be provided to effectively transfer heat between the fluid(s) and the device. The laws of physics dictate that large surface areas, rough surfaces and turbulent fluid flow are needed for efficient heat transfer. This invention relates to a method of making a heat exchange structure in a simple and cost effective way, including fluid guides, mountings, etc. Practically, any version described may simply be adapted for heatsink or heat exchanger use and so the distinctions made in the description are for clarity only. Similarly, while air is normally referred to, any fluid compatible with the chosen materials may be used.

Claims: CLAIMS 1. A method of manufacturing a structure comprising a plurality of connected elongate generally aligned members comprising the steps of: providing a tool shaped to correspond to said members; introducing a flowable material around the tool; setting the flowable material to form a mould; withdrawing the tool from the mould so as to leave a plurality of channels in the mould substantially corresponding to the shape of the aligned members; flowing a liquid castable material into the mould; solidifying the castable material; reducing the cohesion of the set flowable material; and destroying the mould. 2. A method according to claim 1 in which the flowable material comprises aggregate particles coated with a curable coating. 3. A method according to claim 2 wherein the flowable material comprises resin coated sand. 4. A method according to claim 3 wherein the step of reducing the cohesion comprises the step of oxidising the resin. 5. A method according to any of claims 1 to 4 further comprising casting, together with said members, a pair of end plates interconnected by said members. 6. A method according to claim 5 in which the tool includes a structure effective to produce one or more bleed-channels, effective to allow egression of casting material from any channel corresponding to an elongate structure which is not required to be connected to one of the end plates. 7. A method according to any one of claims 1 to 6 further comprising inserting prefabricated objects into the mould to cast the casting material around the objects. 8. A method according to any one of the preceding claims in which the castable material is cast by gravity die casting. 9. A method according to claims 1 to 8 wherein the casting material comprises a die castable metal or metal alloy. 10. A method according to claim 9 further comprising the step of annealing the metal or metal alloy. 11. A method according to claim 10 when dependent on claim 4 wherein the steps of oxidising the resin and annealing the metal or metal alloy take place simultaneously. 12. A method according to any one of the preceding claims in which the elongate generally aligned members are pins. 13. A method according to any one of the preceding claims in which the elongate generally aligned members are substantially parallel. 14. A method according to any one of the preceding claims in which the structure includes at least two arrays of connected elongate generally aligned members, one tool being produced for each array, and the mould being arranged such that the arrays are mounted in the required configuration relative to each other. 15. A method according to any one of the preceding claims including the step of providing a heating element in the mould before the step of flowing the liquid castable material into the mould. 16. A method of casting metal objects comprising curing a mass of resin coated sand around a tool to provide a mould including an orifice around the tool, and pouring molten metal into the orifice. 17. A method of casting a structure which comprises a pair of plates interconnected by at least one elongate connecting member by casting the structure as a one piece casting in a one piece degradable mould. 18. A method according to any one of the preceding claims in which the structure is a heat exchange structure. 19. A method according to any one of the preceding claims including the step of forming at least one internal wall within the structure so as to form one or more ducts. 20. A structure comprising a plurality of connected elongate generally aligned members manufactured by a process according to any one of claims 1 to 19. 21. A structure according to claim 20 comprising an integral piece of metallic material. 22. A structure according to claim 20 or 21 forming a heat exchange structure. 23. A structure according to claim 22 including a temperature sensor. 24. A structure according to claim 22 or claim 23 including a heater element. 25. A projection system including a heat exchange structure according to any one of claims 22 to 24.

Inventors: KAVANAGH MARTIN, KAVANAGH, MARTIN
Applicants: KAVANAGH MARTIN, RANK BRIMAR LTD, RANK BRIMAR LIMITED
Application year: 1993
Main classification: B22C9
IPC codes: B22C7, B22C9, B22D19, B22D25, F28F3, F28F13, H01L23