A Cartridge for a Cartridge Heater

Abstract

A cartridge 1 for a cartridge heater is provided for use between the heater and a surrounding bore of an object to be heated, the cartridge having a generally hollow cylindrical or part cylindrical sh

Description

The present invention relates to a cartridge for a cartridge heater. Cartridge heaters are well known in the art as providing heating elements which are inserted into bores of an object to be heated. They are particularly designed to provide localised heating in order to heat the object from the inside. In normal operation, a heat transfer medium such as air or a paste is used between the cartridge heater and the object in order to allow good heat transfer between the cartridge heater and the bore. If the cartridge heater is used in a vacuum environment, there is little / no air, and it is not possible to use a paste as it will not remain within the bore under vacuum. Another known solution is to clamp the cartridge within the bore. However, this is only possible if there is space to fit the clamp. The present invention is particularly aimed at addressing this problem, although the solution can apply to non-vacuum implementations. Without a heat transfer medium, it is not practical to use a conventional cartridge heater in a vacuum environment in view of the tolerances involved. If the heater is too tight a fit in the bore, it cannot be removed. If there is too much of a gap, the heat transfer into the object is impaired which overheats the heater. There is not enough margin of error between these two extremes to allow for a conventional heater to be used in a vacuum environment. The current solution in a vacuum environment is to use a split-shield heater. This is a generally cylindrical heater which is split by a gap along a central longitudinal plane forming two parts with generally semi-circular cross sections. This gap opens as the element is heated to provide good contact between the heater and bore to provide effective heat transfer. The gap will then close on cooling to allow removal of the heater. This solution is effective in a vacuum environment as it does not require a paste. However, a split shield heater is typically at least three times the cost of a conventional cartridge heater. The present invention aims to provide a solution to this problem which is particularly effective in a vacuum environment. According to a first aspect of the present invention, there is provided a cartridge for a cartridge heater according to claim 1. The present invention provides a sheet of metal with engagement portions such that the cartridge provides an effective thermal bridge between the cartridge heater and the bore thereby providing effective heat transfer without requiring a paste. Further, the solution can be implemented without a clamp so can be used when there is insufficient space for the clamp. The cartridge provides a degree of flexibility which can be configured to allow it to be bent radially outwards on insertion of the heater, and to bend further upon the application of heat to fill a substantial portion of the gap between the heater and object enhancing the thermal contact. The flexibility of the cartridge allows room for manoeuvre in order to allow the heater and the cartridge to be readily removed from the bore once cooled. Further, the intermittent contact between the cartridge and the bore / object caused by the engagement portions allows for venting of the bore which is useful even in a vacuum environment as there may still be a small amount of residual pressure which could create problems if it cannot be vented from the bore. The present invention can be used with a conventional cartridge heater. This allows a conventional cartridge heater to be used in a vacuum environment thereby avoiding the need for the much more expensive split shield heater. 09 04 25 The metal preferably has a relatively high thermal conductivity, preferably greater than 350W / mK. The preferred material is copper as this is an economical choice for the desired thermal conductivity. However, a more expensive material such as silver can also be used. 5 The cartridge may be formed as a number of segments arranged around the central axis and which are separated by regions forming the engagement portions having a smaller radius of curvature than the radius of curvature of the segments to provide a segmented structure. This provides a way of providing the desired shape with the engagement portions. 10 This is relatively easy to manufacture as it can be formed from a single strip of metal bent to the required shape. As an alternative, the engagement portions may be formed as raised regions which are punched in the sheet. These raised regions can be formed as dents in the sheet, alternatively, may be punched holes, the edges of which will be raised by the punching 15 process. The cartridge may be formed as a closed cylinder. However, the cartridge preferably has an axially extending gap along its full length. This allows easier insertion of the cartridge which can be compressed to close the gap thereby reducing the outer diameter upon insertion of 20 the cartridge into the bore. The cartridge can then be released and expand to fit the bore. The cartridge is designed with a number of requirements in mind. There is a desire to have good thermal contact between the cartridge and the heater on one side and the bore on the other side over as wide an area as possible in order to provide optimal thermal contact. On 25 the other hand, the cartridge cannot be too large that it will prevent insertion or removal of the heater. Therefore, the difference between the minimum inner diameter of the cartridge and the maximum outer diameter of the cartridge in an unstressed state is less than 10% and more preferably less than 8% of the maximum outer diameter of the cartridge. This provides a relatively small variation in the outer diameter of the cartridge providing for good 30 thermal conductivity but allowing the insertion and removal of the heater as described above. The present invention also extends to combination of a cartridge heater and a cartridge according to a first aspect of the invention surrounding the heater. The invention further extends to an assembly comprising the cartridge and heater with an object with a bore, the cartridge and heater being within the bore with the engagement portions abutting the bore. In this case, the gap between the heater and the bore, calculated as half of the difference between the outer diameter of the heater and the inner diameter of the bore, in an unheated state is no more than 1.5 times the thickness of the sheet. This allows the cartridge to fill a significant proportion of the gap thereby providing enhanced thermal conductivity. The invention further extends to a method of heating an object, the method comprising inserting a cartridge according to the first aspect of the invention into a bore in the object, inserting a cartridge heater into the cartridge and activating the cartridge heater. As mentioned above, the cartridge can be used in a situation, but is particularly suitable when the bore is exposed to vacuum. Examples of the cartridge according to the present invention will now be described with reference to accompanying drawings, in which: Fig 1 is a perspective view of a first cartridge prior to assembly; Fig 2 is a schematic cross section in a horizontal plane showing the cartridge in situ in an object and with the heater inserted; Fig 3 shows the detail in the area III in Fig 2; Fig 4 perspective view in part of an object with the cartridge and heater inserted; Fig 5 is a perspective view a second example of the cartridge; and Fig 6 is a perspective view showing the heater and a cooling assembly. As shown in Fig 1, the cartridge 1 has a generally cylindrical configuration. It is expected that the cartridge can be made for any copper sheet, for example CU CW004A or CW021 A. The cartridge is single sheet of material. This may be a mesh but preferably a single continuous sheet. This is bent into the generally cylindrical configuration shown in Fig 1 which has an axial gap 2 and a plurality of axially extending segments 3 separated by ridges 4 providing engagement portions. The cartridge 1 designed to be inserted into the bore B of an object W to be heated by the cartridge heater as shown in Fig 4. The object may be an aluminium alloy (e.g. AIMg4,5 Mn0,7) for heating up to 150°C or stainless steel for heating up to 500°C. In order to insert the cartridge, it is initially compressed to close up the gap 2 thereby decreasing its diameter to allow it to be easily inserted in the bore B. Once in place, the resilience of the material of the material causes it to expand to the bore B as shown in Fig 2. Once in place, the heater H is inserted into cartridge 1. In the present case, the heaters are rated at 100W and heat to 105 °C. However, the cartridge heater H can be up to 500W or more for bigger shapes, and heat to ~600°C. This may have a cylindrical cross section as shown or may be elliptical or part cylindrical (i.e. generally cylindrical with a portion missing) The outer diameter of the cartridge in an unstressed configuration is larger than the maximum diameter of the bore. The minimum diameter of the cartridge in an unstressed configuration is less than the outer diameter of the heater. This ensures good contact between the elements when the three are assembled together. Fig 2. Is a schematic drawing in the sense that, although the heater H is shown in place, the cartridge 1 is shown in the configuration that it would take up prior to the insertion of the heater H. With the heater inserted, the element 3 would be deflected radially outwardly such that the cartridge 1 takes up a much more circular configuration than that shown in Fig. 2 which follows the curvature of the bore B much more closely. Upon heating, the heater H and the object W expand towards one another such that a radial gap G between them is not much larger than the thickness T of the cartridge 1 as shown in Fig. 3. The presence of the axial gap 2 allows this movement to be accommodated to some extent as the axial gap 2 will close as the cartridge 1 expands in this way. If no axial gap is present, in cartridge can be modified, for example by using a thinner sheet or to limit the radial deflection described above. In this position, there are relatively large areas of direct contact between the engagement portions 4 and the object W. Similarly, there are areas of direct contact between the heater H and the cartridge 1 in the areas halfway between adjacent engagement portions 4. This direct contact provides heat transfer from the heater H into the cartridge 1 and then into the object W. As well as providing the thermal conduct between the heater and the object, the relatively thin material of the cartridge and its relatively high thermal conductivity provides heat distribution around the cartridge and avoiding the localised overheating. The relationship between the cartridge 1 and gap G is further illustrated in Fig 3 in which the outer diameter of the heater prior to heating is indicated by reference numeral 10 while the maximum diameter which is achieved upon heating is indicated by reference numeral 11. The maximum diameter for the bore is designated by reference numeral 12 and the minimum diameter is indicated by reference numeral 13. The cartridge 1 is shown schematically in an unstressed state. It will be appreciated from this that, when heated, the cartridge 1 will fill most of the gap G. Even when there is no direct contact in a particular location with the heater H or object W, the gap between the two is extremely small such that there is good heat transfer even in non-contacting regions. A second cartridge 20 is shown in Fig 5. In this case, a sheet is indented to have a plurality of outwardly projecting dimples 21 which are shown aligned in axial rows generally corresponding to the engagement portions of the first example, but could be arranged in any suitable pattern. Instead of producing dimples 21, the sheet could be punched through to create a plurality of holes with raised circumferential edges which would achieve the same effect. The dimpled / punched sheet is then bent into the cylindrical configuration shown in Fig 5. This is again provided with an axial gap 22. The dimples may project outwardly as shown or may project inwardly or be a combination of both. In the case of only inwardly extending dimples, the parts of the cartridge between the dimples effectively represent engagement portions which extend radially outwardly further than the adjacent parts of the cartridge. This cartridge is used in the same way as the previously described cartridge. In this case, rather than axially extending areas of contact between the cartridge and the object, contact in the second example is at discrete areas at each of the dimples 21. Otherwise the operation is as described previously. Fig 6 shows a heat sink arrangement for regulating the temperature of the heater H. A pair of thermally conducting wires extend from the end of the heater H to a ceramic block 31 which will absorb and dissipate the heat.

Claims

1. A cartridge for a cartridge heater for use between the heater and a surrounding bore of an object to be heated, the cartridge having a generally hollow cylindrical or part cylindrical shape and being formed of a sheet of metal arranged around a central axis, the sheet having a plurality of engagement portions which extend radially outwardly further than the adjacent parts of the cartridge to engage, in use, with the bore wherein the difference between the minimum inner diameter of the cartridge and the maximum outer diameter of the cartridge in an unstressed state is less than 10% of the maximum outer diameter of the cartridge..

2. A cartridge according to claim 1, wherein the metal has a thermal conductivity of greater than 350W / mK.

3. A cartridge according to claim 2, wherein the metal is copper4. A cartridge according to any preceding claim comprising number of segments arrangedaround the central axis and which are separated by regions forming the engagement portions having a smaller radius of curvature than the radius of curvature of the segments to provide a segmented structure.

5. A cartridge according to any of claims 1 to 3, wherein the engagement portions are formed as raised regions punched in the sheet.

6. A cartridge according to any preceding claim, wherein the cartridge has an axially extending gap along its full length.

7. A cartridge according to any preceding claim, wherein the difference between the minimum inner diameter of the cartridge and the maximum outer diameter of the cartridge in an unstressed state is less than 8% of the maximum outer diameter of the cartridge.

8. A combination of a cartridge heater and a cartridge according to any preceding claim surrounding the heater.

9. An assembly comprising the cartridge and heater according to claim 8 and an object with a bore, the cartridge and heater being within the bore with the engagement portions abutting the bore.

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