Abstract:
A method of chemical milling which includes the following steps: (I) suspending an extrusion or casting formed from metal in an etching solution wherein initially the extrusion or casting is fully submerged in the etching solution by virtue of its weight and which thereafter is rendered buoyant by metal being removed from the extrusion or casting by the etching solution wherein the casting or extrusion is supported by a non-buoyant support which maintains the casting or extrusion in a fully submerged condition throughout its immersion in the etching solution; and (ii) withdrawing the extrusion or casting from the etching solution when required.

Description:
This application is a Divisional of patent application Ser. No. 08/765,617, filed Dec. 27, 1996, U.S. Pat. No. 5,961,771, which claims priority to Australian Patent Application PM6470, filed Jun. 27, 1994. 
    
    
     FIELD OF THE INVENTION 
     This invention relates to chemical milling apparatus and method and is suitably directed to chemical milling or etching of metal articles formed from aluminum or other suitable metal such as heat exchanger elements which may be formed by extrusion or casting. Such elements are usually formed from “primary structure” in contrast to heat exchanger elements which are formed from “secondary structure” which may include additional components welded or otherwise attached to the “primary structure.” 
     BACKGROUND OF THE INVENTION 
     Heat exchanger elements formed from primary structure are considered to be more thermally efficient than fabricated heat exchanger elements which constitute “secondary structure” and which may include a number of boundaries constituted by welds or joins. Thus not only is there a possibility of fluid leaks occurring in relation to these boundaries but also such boundaries provide zones of resistance to conduction of heat to heat transmission surfaces of the heat exchanger element. 
     Heat exchanger elements formed from primary structure are well known and references may be made for example to U.S. Pat. Nos. 3,202,212; 4,565,244; 3,743,252; 4,352,008; 3,556,959; 4,567,074; 3,137,785 and 3,467,180 which all show that formation of one piece extrusions as heat exchanger elements are not new. 
     Reference, however, may be made to GB Patent 2,142,129 which describes a heat exchanger element or core which was incorporated in a radiator for use in a central heating system. The heat exchanger element was in the form of a rectangular elongate hollow body which is provided on each of the opposite sides with a plurality of spaced heat radiating fins. However, in this reference, there was no particular use described of the heat exchanger element per se and reference was only made to a radiator incorporating the heat exchanger element. The radiator included a cover plate which extended across the free ends of each set of fins so as to provide a plurality of open ended channels through which air could flow which air was heated by a transfer of heat from a hot fluid which could flow through the rectangular hollow body. This reference also describes an embodiment using a multiplicity of rectangular hollow sections. In such embodiment, to form the radiator, each hollow section had to be welded to each other and the terminal hollow sections provided with cover plates. 
     A heat exchanger comprising a plurality of modules each having a plurality of channels spaced from each other and located in a central body part and also a plurality of fins extending outwardly on each side of the central body part is also described in U.S. Pat. No. 4,401,155. 
     Of particular interest, in relation to the present invention, is the chemical milling of aluminium extrusions of the type described in GB Patent 2,142,129. Thus, the invention is concerned particularly with the chemical milling of aluminium heat exchanger elements having an elongate hollow body bounded by a peripheral wall and also having a plurality of fins extending outwardly from the peripheral wall. However, it will be appreciated by the skilled addressee that the chemical milling apparatus and method may also be applied to other heat exchanger elements having different shapes as hereinafter described and which, for example, are described in the prior art aluminium extrusions referred to above. 
     In this regard, it is useful in relation to heat exchanger elements, to be able to carry out a chemical milling or etching process so as to reduce the weight of metal which therefore provides a lighter heat exchanger element. 
     Reference may be made to “The Chem-Mill Design Manual” prepared by the Chem-Mill and Coatings Division of Turco Industries Inc in the United States which provides basic information on chemical milling processes and describes that the essential requirement of conventional chemical milling processes is to provide the fabrication of light weight metal parts of high strength which before the advent of chemical milling were either prohibitively expensive or impossible to manufacture using conventional mechanical processes. 
     In the above publication, chemical milling is defined as a process used to shape metals to an exacting tolerance by the chemical removal of metal or deep etching of parts rather than by the use of conventional mechanical, milling or machining operations. 
     The advantage of chemically milling a metal article, such as a heat exchanger element, is to remove metal chemically so that the heat exchanger element may be shaped to a desired shape or tolerance. The amount of metal removed or the depth of etch is controlled by the time of immersion in an etching solution. The location of unetched or unmilled areas on the element may be controlled by masking or protecting these areas from the action of the etchant solution. 
     The etchant solution which is mainly used in conventional chemical milling or metal etching is a strong acid or a strong base which dissolves in a controlled manner all surfaces it comes into contact with. 
     However, a particular problem occurs in relation to etching of aluminium extrusions when such extrusions have a specialised, awkward or other shape which may render the extrusion buoyant in an etching solution. An example of such shape is when the external surface area is substantially in excess of the internal surface area and this may be exemplified by a heat exchanger element having an elongate hollow body and a plurality of outwardly extending fins as described above. The problem is the difficulty of maintaining the heat exchanger element submerged within the etchant solution during the etching process. In this regard, such a heat exchanger element as described above may initially sink to the bottom of an etchant vessel but as milling continues, a chemical reaction occurs at the aluminium surface of the extrusion whereby aluminium will be dissolved. Subsequently, the extrusion will have a tendency to float and thus become a buoyant body and therefore rise upwardly in the etchant solution to the surface. This problem creates considerable difficulties in regard to providing a continuous etching process so as to etch the aluminium extrusion to the required extent. 
     SUMMARY OF THE INVENTION 
     It therefore is an object of the invention to provide chemical milling apparatus and an associated method so as to alleviate or inhibit the abovementioned problem. 
     The method of the invention includes the following steps: 
     (i) suspending an extrusion or casting formed from metal in an etching solution wherein initially the extrusion or casting is fully submerged in the etching solution by virtue of its weight and which thereafter is rendered buoyant by metal being removed from the extrusion or casting by the etching solution wherein the extrusion or casting is supported by a nonbuoyant support which maintains the extrusion or casting in a fully submerged condition throughout its immersion in the etching solution; and 
     (ii) withdrawing the extrusion or casting from the etching solution when required. 
     The main factor causing the extrusion or casting to rise without the non-buoyant support is the extreme chemical reaction in the etching vessel which produces an aerated solution whereby hydrogen gas is evolved in the etching solution and bubbles of hydrogen gas (or other gas which is dependent on the etchant chemicals used) rise to the surface of the etching solution. A pressure may be generated in the etching solution of the order of 50 p.s.i. if the etching vessel has an open top or of the order of 100 p.s.i. if the etching vessel is provided with a cover or lid. In this latter situation the etching vessel may be provided with vents so as to allow the hydrogen gas to escape from the etching vessel. 
     Suitably, extrusions or castings which may be chemically milled by the method of the invention have a surface area relative to mass of between 794 sq mm per g and 1323 sq mm per g. 
     In this regard, it will be appreciated that the process of the invention may be applied to aluminium extrusions having an external area which is substantially in excess of an internal area. This may be exemplified by the extrusion comprising an elongate hollow body and a plurality of radiating fins as described above. However, it will be appreciated that the process of the invention will be applicable to any shape of extrusion or casting which may render the extrusion or casting buoyant in an etching process. Examples of other shapes include tubular shapes wherein the extrusion or casting has an outer wall and one or more inner walls which are all concentric with respect to each other. The extrusion or casting may also be plate-like in shape having a plurality of spaced ribs or comprise a corrugated plate or sheet. The process of the invention may also be applicable to an aluminium casting or an extrusion or casting made of a metal other than aluminium which undergoes a chemical reaction during a milling process. 
     The invention also includes, within its scope, the aforementioned support which, for example, in one example embodiment, supports the abovementioned casting or extrusion so that the fins are all aligned substantially vertically in the etching vessel. 
     Preferably, the support has primary engagement means for engaging with a top part or end portion of the casting or extrusion and also secondary engagement means for engaging with a bottom part or end portion of the extrusion or casting. Suitably, each of the primary engagement means and secondary engagement means are adjustable relative to the support so as to be able to engage with extrusions or castings of different lengths. To this end, each of the primary engagement means and the secondary engagement means are releasably attached to an elongate rod and are therefore movable toward or away from each other. In an alternative embodiment, one of the primary engagement means or the secondary engagement means may be fixed to the elongate rod and the other of the primary engagement means or secondary engagement means is releasably attached to the elongate rod. The elongate rod may also have a handle attached thereto at an upper end in use. 
     In a particularly preferred form, both the primary engagement means and the secondary engagement means are provided with one or more engagement finger(s) which may engage with an adjacent open end of the hollow body. In this arrangement, both the primary engagement means and the secondary engagement means may be provided a pair of engagement fingers which engage with the said adjacent open end. 
     In a particular arrangement, one or both of the primary engagement means and the secondary engagement means may include a central sleeve which is attached to the elongate rod and a plurality of mounting arms extending outwardly from the central sleeve wherein each of the mounting arms have a plurality of offset projections and which may constitute an engagement finger as described above. The central sleeve in relation to one or both of the primary engagement means and the secondary engagement means may be slidably attached to the elongate rod and may also be rotatable relative to the elongate rod and suitably locking means may be provided to lock the central sleeve in a desired position to the elongate rod such as a grub screw or other form of adjustable locking member. 
     The invention also includes within its scope, chemical milling apparatus for suspending one or more extrusions or castings in an etching solution so as to retain the extrusion(s) or casting(s) in a fully submerged position in said etching solution, said chemical milling apparatus including a conveyor belt to which is attached one or more non-buoyant supports for supporting a corresponding extrusion or casting. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     Reference may now be made to a preferred embodiment of the invention as shown in the attached drawings wherein: 
     FIG. 1 a  is a perspective view of an aluminium extrusion which may be chemically milled by the process and apparatus of the invention; 
     FIG. 1 b  is a perspective view of a support for holding the aluminium extrusion of FIG. 1 a  in an etching vessel as described herein; 
     FIG. 1 c  is a perspective view of the support of FIG. 1 b  holding the aluminium extrusion of FIG. 1 a  for subsequent immersion in-an etching vessel; 
     FIG. 2 a  represents the aluminium extrusion of FIG. 1 being held by the support of FIG. 2 in an etching vessel which is vented to atmosphere; 
     FIG. 2 b  is a similar view to FIG. 2 a  wherein the etching vessel has an open top; 
     FIGS. 3 a - 3   d  represent various forms of the engagement fingers as hereinbefore described; 
     FIGS. 4 a - 4   d  represents various means by which the engagement fingers of FIGS. 3 a - 3   d  may engage with the aluminium extrusion of FIG. 1 a;    
     FIGS. 5 a - 5   i  represent various forms of supports which may be utilised in the process and apparatus of the invention; 
     FIGS. 6 a - 6   f  represent various forms of support which may support a plurality of aluminium extrusions as shown in FIG. 1 a  simultaneously in the etching vessel; 
     FIG. 7 represent a conveyor belt assembly for transferring aluminium extrusions of the type shown in FIG. 1 to different vats which may be utilised in a chemical milling process are hereinafter described; 
     FIG. 8 is a micrograph showing part of the structure of a heat exchanger element after extrusion and prior to being subjected to the chemical milling process of the invention; 
     FIG. 9 is a micrograph showing part of the structure of a heat exchanger element constructed in accordance with the chemical milling process of the invention and which has been subjected to acid etching; and 
     FIG. 10 is a micrograph showing part of the structure of a heat exchanger element constructed in accordance with the chemical milling process of the invention and which has been subjected to alkaline etching. 
    
    
     DETAILED DESCRIPTION 
     The aluminium extrusion  10  in FIG. 1 a  includes a peripheral wall  11  of shallow rectangular configuration which has opposed side parts  12  and opposed end parts  13 . The peripheral wall  11  which is of a continuous nature defines a plurality of fluid channels  14  separated by partitions or webs  15 . There is also included a first array of fins  16  extending away from one side part  12  and a second array of fins  17  extending away from the opposed horizontal part  12 . End walls  18  may also be included. Fluid channels  14  may also be provided with open ends  19 . 
     In FIG. 1 b , the support  20  which is constructed in accordance with the invention includes primary engagement means in the form of a top clamp  21  and secondary engagement means in the form of a bottom clamp  22 . Each clamp  21  includes a central mounting plate  23 . Attached to top mounting plate  23  is a central sleeve or hub  24  through which may be inserted a locking pin  24 A which may be inserted through co-aligned apertures (not shown) in sleeve  24  and upright shaft or rod  25 . Locking pin  24  may also be replaced by a grub screw or other suitable locking member as desired. Rod  25  may also be provided with an upper handle  26 . Each clamp  21  and  22  may also include a plurality of horizontal arms  27  each of which at their outer free end may include offset projections or engagement fingers  28 . Sleeve  24  also has handle  28 A. 
     As shown in FIG. 1 c , extrusion  10  is supported by support  20  with fins  16  and  17  oriented vertically. 
     In FIGS. 2 a - 2   b , there is provided etching vessels  30  and  31  wherein etching vessel  30  is provided with vent  32  for passage of escaping gases or whereby etching vessel  31  is provided with an open top  33 . The etchant  34  is shown reacting with the aluminium of the extrusion  10  causing the aluminium to be dissolved in the etchant  34  with the evolution of bubbles  35  of hydrogen gas. 
     Without the agency of support  20 , each of extrusions  10  shown in FIGS. 2 a - 2   b  would rise to the surface and thus would be rendered buoyant by the reaction between the etchant and the aluminium. 
     FIGS. 3 a - 3   b  show the various types of offset projections or engagement fingers  28  that may be utilised with horizontal arms  27 . In FIG. 3 a , there is provided a pair of offset spaced projections  36  separated by a gap  37 . Projections  36  are extensions of parts  38  of arm  27  which parts are also separated by gap  38 . 
     In FIG. 3 b , there are provided spaced and opposed offset projections  40  each having a pointed end  41 . Projections  40  may be separated by gap  42 . 
     In FIG. 3 c , the engagement of projections  40  in the spaces  43  between adjacent fins  17  is also shown. 
     In FIG. 3 d , arm  27  may be provided with opposed lugs or offset projections  44  which may engage with an adjacent open end  19  of fluid passages  14  of extrusion  10  whereby projections  44  each having a pointed end  45  may engage on either side of web  15 . 
     In FIGS. 4 a - 4   d , it will be noted that fingers  28  may engage on either side of web  15  in FIG. 4 a , between adjacent fins  16  and  17  and thus on both sides of wall  11  as shown in FIG. 4 b , again on either side of web  15  as shown in FIG. 4 c , or as shown in FIG. 4 d  where pointed ends  41  may be located between adjacent fins  17  on opposed sides of peripheral wall  11 . FIG. 4 d  shows essentially the same arrangement as shown in FIG. 3 c.    
     In FIGS. 5 a - 5   i , the construction of clamps  21  and  22  may take many different forms. In each case, the support  20  may be pivotally attached by pivot joint  22 A to a conveyor belt  22 B described hereinafter in FIG.  7 . In FIG. 5 a , bottom clamp  22  is fixed while top clamp  21  is movable toward and away from clamp  22  as shown by the doubled headed arrow. In FIG. 5 b , a tension spring  46  may be inserted between clamps  21 - 22  as shown. Alternatively in FIG. 5 c , a compression spring  47  may be inserted between clamp  22  and pivot joint  22 A. In FIG. 5 d , clamp  22  may be part of a piston  48  movable toward or away from fixed cylinder  49  having clamp  21  attached thereto. Clamp  22  may be attached to cylinder  49  by attachment sleeve  48 A having associated therewith grub screw  48 B. Hose  50  may also be provided for passage of pressurised air or hydraulic fluid from a suitable source (not shown). 
     In FIG. 5 e , bottom clamp  22  may be attached to a worm gear  51  whereby bottom clamp  22  may move toward or away from housing  52  by movement of worm gear  51  which is controlled by motor  53  having power cable  54 . 
     In FIG. 5 f , there is shown a turnbuckle arrangement  54 A whereby either top clamp  21  or bottom clamp  22  is movable toward or away from each other as shown. In this arrangement, bottom clamp  22  is attached to a screw threaded shaft  55  of a different thread to screw threaded shaft  56  to which is attached clamp  21 . 
     In FIG. 5 g , both top clamp  21  and bottom clamp  22  comprise a pair of pivoted arms  21 A which are pivoted to pivot joints  21 B. There is also provided tension springs  57  which interconnect each of arms  21 A as shown. Tension springs  57  may also be replaced by a screw threaded equivalent or a pneumatic/hydraulic equivalent if desired. The arrangement shown in FIG. 5 g  may therefore provide a support  20  which can accommodate a pair of extrusions  10  of different lengths. In another variation, either of bottom clamp  22  or top clamp  21  may be fixed if desired. 
     In FIG. 5 h , bottom clamp  22  is fixed and top clamp  21  comprises pivoted arms  21 A attached to pivot joint  21 B. There is also provided springs  58  which may be dispensed with if desired. There is also provided solenoid valve  59  which may initiate movement of arms  21 A through pivot links  60  and  61 . 
     In FIG. 5 i , in another arrangement top clamps  22  may be provided with a cam lock  62  having an operating lever  63  whereby clamp  22  may be locked in position as required when lever  63  is in the position shown in phantom. In FIG. 5 i , top clamp  21  may be formed into two separate clamp components  22 C as shown with each clamp component  22 C movable along an associated shaft  64 . Thus each of top clamps  22 C are movable independently of each other while bottom clamps  22  are fixed. Instead of a cam lock  62  there may be used other suitable locking means such as a grub screw or locking pin as described previously. There also may be provided tension or compression springs (not shown). 
     In FIG. 6 a , there may be provided a support assembly  65  which may support a plurality of aluminium extrusions  10  (not shown) each of which may be suspended from opposed arms  27  of bottom clamp assembly  21  and top clamp assembly  22 . Each of arms  27  are attached to mounting plate  66 . 
     In FIG. 6 b , there may be provided another support assembly  68  comprising a number of different supports  20  each attached to a common carrier beam  68 A. 
     In FIGS. 6 c  and  6   d , it will be appreciated that any number of attachment arms  27  (e.g. 3 or 5) attached to a central mounting plate  23  which may be mounted to upright rod  25  through aperture  70 . 
     In FIG. 6 e , there may be provided a number of clamping arms  71  which are each pivoted at  72  to an adjacent clamping arm  71 . Each clamping arm  71  as shown by the arrows may be pivoted from an upper inoperative position to a lower operative position. Top clamp  21  comprising the movable clamping arms  71  is movable along rod  25  in the same fashion as discussed previously as shown by the double headed arrow. The lower clamp  22  is fixed and has a plurality of spaced clamping projections  76  as shown. 
     In FIG. 6 f , a number of aluminium extrusions  10  may be stacked vertically by the use of bottom fixed clamp  21 , intermediate clamp  77  and movable top clamp  22  each of which are associated with elongate rod  25 . 
     From the foregoing, it will therefore be appreciated that chemically milled heat exchanger elements may therefore be considerably lighter in weight and wall thicknesses may be considerably reduced when compared to the wall thickness of an extruded heat exchanger element. The wall thickness of a chemically milled heat exchanger element may vary from 0.1 to 1.0 mm compared to the extruded element which may have a wall thickness of 0.8 mm or greater. 
     If desired, differential wall thicknesses may be used in relation to the peripheral wall  11  which may, for example, be 1 mm and the fins  16  or  17  which may each have a thickness of 0.5 mm. 
     The chemical milling process to which the aluminium extrusion or casting  10  may be subjected to may include the following steps: 
     (i) cutting or shearing the heat exchanger element to the desired length; 
     (ii) passing the sheared heat exchanger element through an etchant solution; 
     (iii) passing the etched heat exchanger element through a de-smutting or residue removing solution which may remove residue produced by corrosion or erosion of metal such as aluminium; and 
     (iv) subsequently passing the de-smutted heat exchanger through a sealing solution which produces a chemical coating against oxidation or corrosion. 
     The heat exchanger element may be sheared or cut by any suitable means such as a guillotine or hand saw to the desired length which may depend on the relevant application. 
     Preferably before step (i), the heat exchanger element is immersed in a alkaline solution to remove contaminating surface layers such as oil, dirt or other impurities. 
     Subsequently the heat exchanger element may be immersed in a water rinse to remove any trace of alkaline solution. 
     In step (ii), the desired etchant may be a strong acid or strong base which dissolves in a controlled manner all surfaces it may come into contact with. A suitable etchant for aluminium is TURCOFORM etchant additive which may be used in combination with sodium hydroxide. 
     In the etching step as described above, the fluid channels if desired may be masked or blocked off so that only the fins and the external surface of the peripheral wall is etched. This may be provide for example the peripheral wall having double the wall thickness of the fins. This may be carried out when it is appreciated that the fins are being etched on both sides in comparison with the peripheral wall which is only being etched on one side. A suitable masking preparation is TURCOFORM MASK which is a hand strippable, protective coating. 
     Subsequently the etched heat exchanger element may be passed through a water rinse to remove all traces of etchant solution. 
     The cleaned heat exchanger element may then be passed through a de-smutting solution which may remove residue produced by corrosion or erosion of the metal which is suitable aluminium. The de-smutting solution may comprise an alkaline solution or detergent. 
     Subsequently the heat exchanger element may be passed through a water rinse to remove the de-smutting agent. 
     The heat exchanger element may then be passed to a sealing solution which chemically seals the surface layer of the metal (e.g. aluminium) against oxidation or corrosion. Preferably the sealing solution contains chromate ion (i.e. CrO 4   ═ ). 
     The heat exchanger may then be passed through a final water rinse to remove excess sealing solution. 
     In both the etching step and the sealing step, the solution may be to any suitable temperature in the range of 30-90° C. The time of immersion in the etchant solution may be in the range of 1-5 minutes and, more suitably, 3 minutes dependant upon the degree of etching required. 
     In the milling process, there may be utilised a plurality of vats or containers arranged in series or at spaced intervals wherein the heat exchanger element(s) may be passed. In one arrangement, there may be provided a conveyor having a conveyor belt or chain operated by drive rollers and idler rollers so that the chain or belt is passed through each vat. In this arrangement, an array of heat exchanger elements may be suspended from the chain at spaced intervals therealong. Each heat exchanger element may be suspended from the conveyor longitudinally, i.e. with the plurality of fins on either side of the peripheral wall all being oriented vertically. 
     In another alternative, there may be provided a gantry member which is located above the plurality of vats or tanks with an array of heat exchanger elements supported at spaced intervals along the length of the gantry member, e.g. by appropriate supports  20 . In this alternative, the gantry member may be provided with suitable means for dropping a support  20  and its associated heat exchanger element into a selected vat. After immersion in the vat the heat exchanger may then be elevated before being immersed in the succeeding vat. Means  91  may comprise, for example, a hydraulic ram assembly or pneumatic ram assembly wherein the piston (not shown) of the hydraulic ram assembly or pneumatic ram assembly is attached to support  20 . This may provide a totally computerised or automated milling system. 
     The heat exchanger element of the invention, after the milling process as described above, may remove the tool hardened surface, i.e. the surface created in the extruder die which creates a barrier to heat flow. Thus the tool hardened surface may be a relatively even or smooth surface with fine serrated lines all oriented substantially parallel to each other. In comparison, the milled element will have a pitted or cratered surface appearance of greater depth than the serrated lines (e.g. of the order of three times the depth). 
     The rough surface to the heat exchanger element reduces laminar adhesion of fluids passing over the surface which encourages turbulent fluid flow conditions. This results in greater absorption of heat into the coolant which in many cases is air flowing through the fins of the heat exchanger element. 
     The milled surface also substantially increases the surface area of the heat exchanger element especially in relation to the fins. The milling process may also be regulated so as to control the degree of pitting—i.e. one can have 10 holes or pits per sq mm to 20 holes or pits per sq m. 
     As stated above the walls of the heat exchanger element may be reduced to a thickness of between 0.01 to 0.8 mm without sacrificing structure integrity. Ultra-thin thicknesses of 0.01 to 0.05 mm may also be produced. 
     The pitted surface may be regularly pitted or pitted irregularly with the pits or holes having a depth of 20 microns to 150 microns and a maximum transverse dimension of from 5 microns-150 microns. 
     The reduction in thickness after milling when compared to the extruder may be of the order of 99%. 
     In FIG. 7, there is shown a conveyor belt assembly  78  including belt  22 B supported by idler rollers  79  located at spaced intervals whereby a support  20  may be attached to belt  79  at  81  and thus enable extrusion  10  to be passed through a series of vats corresponding to the chemical milling process described above wherein vat  82  is a pre-clean vat, vat  83  corresponds to a water rinse vat, vat  84  corresponds to a etchant vessel, vat  85  corresponds to a water rinse vat, vat  86  corresponds to a desmutting or residue removing vat, vat  87  corresponds to a water rinse vat, vat  88  corresponds to a vat containing chromate conversion solution and finally vat  89  refers to a water rinse vat. 
     In FIG. 7 shown in phantom, reference is also made to the alternative embodiment of the gantry member described above. The gantry member is indicated by reference numeral  90  and there is also provided reciprocatable means  91  for raising and lowering the support  20 . Such reciprocatable means may comprise, for example, a hydraulic ram assembly or pneumatic ram assembly. In either of these embodiments, support  20  may be attached to a piston (not shown) of the hydraulic ram assembly or pneumatic ram assembly. 
     In relation to micrographs of the heat exchanger element before the element has been chemically milled by the process of the invention, it will be noted that such heat exchanger element comprises a series of substantially parallel lines as indicated after passage through an extrusion die. These lines may range from 1 micron to 40 microns in width. A sample micrograph is shown in FIG.  8 . 
     In relation to the heat exchanger element which has been subjected to an acid etch by the process of the invention, it will be noted that the surface comprises a series of finely textured hills and valleys with there being 3-300 hills or valleys per square millimetre. The hills or valleys may also have an average of 0.1 mm in depth. A sample micrograph is shown in FIG.  9 . In relation to an alkaline etch of a heat exchanger element achieved by the process of the invention, there is shown a far more coarser structure of hills and valleys with there being provided 1 to 200 hills or valleys to the sq mm. Each of these hills and valleys may also have an average of 0.2 mm depth. A sample micrograph is shown in FIG.  10 . 
     From the foregoing therefore, it will be appreciated that the invention the texture or structure of the heat exchanger element after being processed by the milling process of the invention may have its texture pre-determined according to a particular desired application. 
     The texture may be controlled by the temperatures attained in the etchant vat. Thus, for example, for relatively low pressure applications one would require a coarser texture and this would have applicability in applications such as vehicle radiators. 
     On the other hand, for high pressure applications, one would require a finer structural texture and this could apply to condensers for air conditioning and refrigeration for example. 
     In this regard, it will be appreciated that a coarser texture means a lesser number of hills or valleys per sq cm and a finer texture means a greater number of hills or valleys per sq mm.