WO2002072922A2

WO2002072922A2 - Composite mandrel

Info

Publication number: WO2002072922A2
Application number: PCT/EP2002/002635
Authority: WO
Inventors: Flemming Lausen Christensen; Orla Lang Soerensen
Original assignee: Giantcode Tools A/S
Priority date: 2001-03-12
Filing date: 2002-03-09
Publication date: 2002-09-19
Also published as: GB0106044D0; WO2002072922A3; AU2002256659A1

Abstract

A method of forming a mandrel for electrodeposition by forming a shaped solid shell on a model, optionally separating the shell from the model, casting a backing mass directly or indirectly against the shell, causing the backing mass to solidify and bond to the shell, if not already separated from the model body, separating the shell from the model body to expose the plating face of the backing mass-bonded shell, optionally working, for example by milling, grinding, polishing, etching or machining, the plating face of the backing mass-bonded shell to adjust the shaping thereof, and if not already so activated, activating at least one region of the plating face of the backing mass-bonded shell to receive build-up of plating metal by electrodeposition. A composite mandrel of this type is also disclosed.

Description

Composite Mandrel

This invention relates to bodies and apparatus for use in electrodeposition processes. In particular it concerns a method of forming a body having a shaped surface for receiving build-up of plating metal deposited thereon by electrodeposition, as well as to electrodeposition apparatus comprising a shaped layer in the form of a solid shell having a plating face and a non-plating face, the non-plating face being bonded to a solid backing mass of material different from that of the shell, the backing mass material having a coefficient of thermal expansion not greatly dissimilar to that of the plating metal.

In this specification and claims, all coefficients of thermal expansion are as measured at 20°C.

Background to the Invention

In electrodeposition processes, for example electroplating or electroforming processes, a plating metal is deposited on an electrically conductive shaped surface of a body, often known as a "mandrel". If the material of which the body of the mandrel is constructed is itself non-conductive, the deposition surface may be rendered conductive by applying a thin conductive coating, for example of silver-based paint, or chemically reduced silver or copper

In practice, the choice of mandrel material is often limited. For example, a monolithic mandrel must be constructed from a material which can be surface shaped, for example by casting or milling, to the required tolerances of the desired shaped plating surface. The mandrel material should also be resistant to acid or chemical degradation under the conditions of the electrodeposition process. Furthermore, especially for electroforming large surface area metal shells, the mandrel as a whole must be self-supporting and substantially non- deformable under the electrodeposition conditions, otherwise there will be a risk of deformities and/or internal stresses in the deposited metal shell. In the latter connection it is undesirable to construct the mandrel of a material which has a coefficient of thermal expansion (CTE) greatly different from that of the plating metal (which is typically about 13-14 μm per m per °C (μm*m^"1*°C¹)), since the different thermal expansion characteristics of the mandrel and plating layer during the heat-generating deposition process may also lead to undesirable deformities and/or internal stresses in the plated shell. Materials which are capable of being milled or cast for the desired plating surface shape do not in general have the required CTE properties.This invention makes available a method for constructing a stable composite mandrel from an easily shaped material which ultimately defines the shaped surface for plating metal deposition, and a backing material having the desired self-supporting, substantially non- deformable, acid and chemical resistant, and acceptable CTE properties. The resultant mandrel is useful in electrodeposition apparatus, for example for the construction of tools for forming sheet metal or plastics parts.

Description of the Invention

The present invention provides a method of forming a body having a shaped surface for receiving build-up of plating metal deposited thereon by electrodeposition, which method comprises

forming on a shaped surface region of a solid model body a correspondingly shaped plastics layer in the form of a solid shell having a plating face and a non-plating face, the face in contact with the model being the intended plating face,

optionally separating the shell from the model body,

casting a backing mass directly or indirectly against the non-plating face of the shell, causing the backing mass to solidify and bond to the non-plating face of the shell,

if not already separated from the model body, separating the shell from the model body to expose the plating face of the backing mass-bonded shell,

optionally working, for example by milling, grinding, polishing, etching or machining, the plating face of the backing mass-bonded shell to adjust the shaping thereof, and

if not already so activated, activating at least one region of the plating face of the backing mass-bonded shell to receive build-up of plating metal by electrodeposition.

The first step is to form on a shaped surface region of a solid model body a correspondingly shaped layer in the form of a solid shell having a plating face and a non-plating face, the face in contact with the model being the intended plating face. The model may in principle be made of any solid material the surface of which may be shaped to the desired tolerances, such as wood including laminated wood, foamed or unfoamed plastics, metal or metal alloy, ceramic materials,wax or clay. However, polyurethane foam will often be the preferred material, since it is cheap and highly amenable to shaping using standard CAD ("computer aided design") software and hardware. The shaped surface of the model body will be the reverse image of the eventual plating face of the final mandrel. The correspondingly shaped plastics layer in the form of a solid shell may be formed on the shaped surface of the model by any method compatible with the chosen material of the model body.

Thus, a curable layer of resin may be coated on the shaped model surface; or a fibre mat impregnated with curable resin may be built up on the model surface; or a mixture of fibres and curable resin may be sprayed or cast onto the shaped surface.

Alternatively, it will often be convenient to form the solid shell by vacuum forming a sheet material onto the shaped surface of the model body. Plastics sheet materials for vacuum forming are well known and include ABS ("acrylonitrile- butadiene-styrene"), polypropylene, polyethylene or acrylic resin plastics sheets.

In other cases it may be convenient to form the solid shell by positioning the model body adjacent but spaced apart from a negative thereof, having a surface shape which is a reverse image of the shaped surface of the model body, thereby defining a cavity between the two bodies which is the thickness of the desired shell, then filling the cavity with a hardenable fluid plastics material such as a resin, hardening the fluid plastics material or allowing it to harden, then separating the negative from the hardened shell. In such a method, filling the fluid hardenable plastics material into the cavity may be assisted by applying a partial vacuum to the cavity while injecting the fluid plastics, Separation of the negative may be assisted by the application of a release layer, for example of wax, on the surface thereof prior to filling the cavity.

For shell forming methods which use curable resins, suitable materials include epoxy resins, such as Araldite® from Ciba, Series 2000, grades 20-12 and 20- 13, and Axson Fast Cast Resin F16 and F18. As these resins have CTE of about 100, it may be desirable to reduce their thermal expansion capacity, which can be done by incorporating a sufficient amount of a fine ceramic filler, e.g. Mineral Filler (hydrated alumina) RZ 30-150 from Axson, in amounts in the range of 10- 65% by weight, preferably 40-65% by weight (incorporation of 65% of the filler will result in a CTE of about 25 μm*m^"1*°C^"1). The resin may be prepared for use by mixing resin and hardener, at the same incorporating the filler. With a high filler content in the range mentioned, the resin will have a thickened, but still pourable consistency. For vacuum forming from sheet plastics materials, these sheet plastics materials may also contain a filler reducing their thermal expansion capacity, whatever the method chosen for its formation, the side of the solid shell abutting the shaped surface of the model body will ultimately become the plating face of the eventual mandrel. The other (currently exposed) side of the solid shell is designated the non-plating face. If desired, a release layer, for example of wax, may be coated on the shaped surface of the model body prior to forming the solid shell layer thereon, to facilitate separation of the solid shell from the model body.

The plastics layer forming the shell may be one which is electrically conductive, for example a resin material containing electrically conductive particles e.g. of graphite, or fibres e.g. of carbon. Such materials have the advantage that they do not require activation to receive build-up of metal during electrodeposition.

The next step is to cast a suitable backing mass against the non-plating face of the solid shell. The solid shell may be separated from the model body prior to casting, or casting may take place with the shell in place on the model body. The latter may be preferred in cases where the shell is inherently flexible or is of large area, since the rigidity imparted by the model body may help to minimise distortion of the shell during casting and solidifying the cast backing mass. The castable backing mass may be of any material compatible with the material of the solid shell. However, the backing mass will form part of the eventual composite mandrel, with the plating face of the shell eventually forming the shaped deposition layer of the mandrel. Hence, the backing mass is preferably chosen with the requirements of CTE not substantially dissimilar to that of the plating metal, self supportability and substantial non-deformability, in mind. Resistance to acid degradation, or at least treatability of exposed surfaces to inhibit acid degradation is also a useful characteristic of suitable backing mass materials. A backing mass CTE will normally be in a range from 0.4 times the CTE of the plating material to 2.5 times the CTE of the plating metal, such as from 0.4 times the CTE of the plating metal to 2.2 times the CTE of the plating metal, in particular from 0.5 times the CTE of the plating metal to 2.0 times the CTE of the plating metal, often preferably from 0.5 times the CTE of the plating metal to 1.8 times the CTE of the plating metal such as from 0.6 times the CTE of the plating metal to 1.5 times the CTE of the plating metal, a preferred range being from 0.6 times the CTE of the plating metal to 1.25 times the CTE of the plating metal.

When the plating metal is nickel having a CTE of 13 μm*m^"1*°C^"1, the range of CTE of the backing material acceptably matched thereto, given as μm*m^"1*°C^"1, can be expressed as 4.0-35, such as 4.0-30, in particular 6.5.-27.5, often preferably 6.5-25, such as 7-20, e.g., 7-17.5, with a preferred range being 8-15. It is generally preferred that the CTE of the backing material is lower than that of the plating metal rather than higher than that of the plating metal. The CTE of the backing material will also in most cases preferably be lower than the CTE of the plastics material of the shell.

Examples of castable backing masses include metals and metal alloys, concrete or mortar, DSP materials, (DSP: Densified Systems containing ultrafine Particles) and aggregate-filled polyurethane, acrylic, polyester or epoxy resins.

DSP materials are particularly useful, that is materials based on densely packed particle systems with ultrafine particles homogeneously distributed between the densely packed fine particles by means of an effective dispersing agent, these materials typically being fibre-reinforced. Examples of such materials are disclosed, e.g., in US Patents Nos. 5,234,754 and 4,588,443. (It may be noted that US Patent No. 4,923,665 discloses the application of a metal coating on DSP materials, e.g. by casting DSP materials against a metal layer such as a nickel layer formed by electrodeposition, and that International Patent Application Publication No. WO 82/01674 relates to a shaping tool made in this manner.) A type of DSP material which has been found suitable for the purpose of the invention is a mortar having a cement-based matrix, the cement particles constituting the densely packed fine particles, the ultrafine particles being silica fume particles, the dense and homogeneous structure having been obtained using an effective amount of a concrete superplasticizer, the mortar preferably being reinforced with fibres such as steel fibres.

The backing mass may be cast indirectly against the non-plating surface of the shell via an intermediate body or layer which increases the rigidity and/or strength of the bond between shell and solidified backing mass relative to the rigidity and/or strength of the bond achieved in the absence of such intermediate body or layer. Prior to casting the backing mass, keying elements such as studs or aggregate chips may be formed on or fixed (for example by gluing) to the non- plating face of the shell, whereby when the backing mass is cast directly or indirectly against that face the shell becomes keyed into the backing mass.

The next step in the method of the invention is to prepare the plating face of the solid shell, now backed by the solidified backing mass, for receiving the plating metal in an electrodeposition process. Accordingly, if the model body has not been separated from the plating face of the solid shell prior to casting and solidifying the backing mass, it should now be removed to expose the plating face of the solid shell. In some cases the plating face of the solid shell may not be an exact reverse image of the shaped surface of the model body. For example it is well known that vacuum forming may fail to conform the sheet material precisely to its substrate shape in cases where the geometry of the substrate shape is complex or awkward. To some extent the loss of tolerances in vacuum forming may be minimised by making allowances in the design of the shaped surface of the model. However, one of the advantages of the present method is that the plating surface of the solid shell, backed by the backing mass, may be correctively re-milled to produce the desired degree of tolerance relative to the ideal shape. This is because the materials from which the solid shell may be formed are in general easily worked, e.g. by milling, (plastics materials, including those with filler incorporated), which is not always the case where the plating surface is of a mandrel material which is suitable for a monolithic mandrel, such as iron, or steel.

Where the shell is not electrically conductive, part of the preparation of its plating face for receiving the plating metal in an electrodeposition process is activation of the plating surface. This may be done in conventional manner by spraying an electrically conductive coating, for example of silver-based paint, onto the surface, or by forming a conductive layer of chemically reduced silver or copper thereon

The end product of the method of the invention is a composite mandrel for use in an electrodeposition process, having a solid shell with an upper plating surface adapted to receive build up of plating metal, the shell being backed by a solid backing mass. The method of the invention is applicable to the preparation of mandrels having two plating surfaces. In such cases, a second shell having a plating face and a non-plating face is formed in the same manner as the first on a shaped surface region of the same or a different solid model body and is optionally separated from that model body, and the first and second shells are arranged in a desired spaced relationship with opposed non plating faces, and the backing mass is cast between the spaced shells such that when it is solidified both shells become bonded thereto. The shells may be maintained in spaced relationship by spacing posts extending between their opposed non-plating faces.

It will be noted that for the preparation of a mandrel having two plating faces, one of which is a reverse image of the other, the method described above for forming a shell by filling a cavity between a model body and a negative body may be particularly useful. In such cases the surface of the negative is of course the reverse image of the plating face of the model body. Accordingly, a shell may be formed on the model body surface by filling the cavity between the model body and the negative, hardening the filled plastics, and separating the negative, all as previously described. This produces a shell backed by the model body, with the non-plating face of the shell exposed. The procedure may then be repeated with another model body and negative, but this time the model body is separated from the shell, leaving a shell backed by the negative, but this time the exposed non- plating face is in effect the image of the plating face of the first shell. The two shells are then arranged in a desired spaced relationship with opposed non- plating faces, and the backing mass is cast between them as described in the preceding paragraph.

Aspects of the invention include electrodeposition apparatus including a cathode body formed as a composite mandrel of the kind formed by the method of the invention, described above. Such apparatus includes an anode body comprising plating metal, a cathode body having at least one exposed surface region for receiving build-up of the plating metal by electrodeposition, an electrolyte solution comprising a salt of the plating metal, and means for passing current between the anode and cathode bodies via the electrolyte solution CHARACTERISED IN THAT

the cathode body comprises a shaped layer in the form of a solid shell having a plating face and a non-plating face, the non plating face being bonded to a solid backing mass of material different from that of the shell,

the backing mass material has a coefficient of thermal expansion in the range 4.0 μm/m/°Cto 35 μm*m^"1*°C^"1 ,

the plating face of the backing mass-bonded shell has at least one region adapted to receive build-up of plating metal by electrodeposition.

In this aspect of the invention, the examples of ranges of backing mass material CTE mentioned above, and the preferred ranges, also apply. In one aspect of the invention the cathode body of such apparatus comprises two shells bonded to and spaced from each other by the solid backing mass, and spacing posts may extend through the backing mass between opposed non- plating faces of the shells.

The invention will now be illustrated by reference to the following drawings, wherein:

Figure 1 is a diagrammatic perspective view of a model body having a shaped upper surface.

Figure 2 is a cross sectional view of the model body of Figure 1 through A-A'.

Figure 3 shows the cross sectional view of the model body of Figure 1 with a solid shell vacuum formed onto the shaped upper surface and with keying elements glued to the exposed surface of the shell.

Figure 4 shows the model body of Figure 3, with shuttering erected to contain a backing mass cast against the shell.

Figure 5 shows the composite mandrel formed by removal of the shuttering from Figure 4 and separation of the model body from the mandrel

Figure 6 shows a diagrammatic cross sectional view of a double sided mandrel formed by casting a backing mass between two spaced model bodies formed in the same manner as the model body of Figures 1-4.

Figure 7 shows the double-sided mandrel of Figure 6 after separation of shuttering and model bodies. Referring first to Figs. 1 and 2 a model body 1 of foamed polyurethane has a generally planar upper surface 2, on which two shape features are defined, namely a recess 3 and a protuberance 4. That surface has been shaped as the substantial reverse image of an intended plating surface of a mandrel for use in an electrodeposition process. CAD design equipment has been used to design and store the shape of the surface of the model body, and compatible associated milling equipment has been used to mill the shape from a foamed polyurethane board material. The polyurethane model body is of suitable density to serve as a model in a process of vacuum forming a given sheet material over the shaped surface. Tolerances in the design of the shaped surface of the model body have been scaled to take account to some extent of the predicted result of vacuum forming thereon a given thickness of the intended sheet material.

In Fig. 3, a sheet of vacuum formable ABS plastics, with a thickness in the typical range of 1mm-5mm has been vacuum formed over the shaped surface 2 of the model 1 , to form a solid shell 5. The shell 5 will ultimately form part of the intended mandrel for electrodeposition, the lower face of the shell which is in contact with the model body shaped surface 2 being the intended plating surface. On the upper exposed non-plating surface of the shell, irregularly shaped stone aggregate keying elements 6 have been randomly distributed and glued firmly to the surface by epoxy resin.

In Fig. 4, shuttering 7 has been erected around the model body 1 with ABS plastics shell 5 and glued aggregate 6 still in place on the non-plating surface of the shell. A backing mass 8 of DSP cement- and silica fume-based material, such as that marketed under the trade mark DENSIT® by Densit A/S, Aalborg, Denmark, 1.5% by volume of steel fibres of diameter 0.4 mm and length 12.5 mm having been incorporated in the mass, has been cast against the non plating surface of the shell 6, and allowed to solidify. The stone aggregate keying elements 6 key the shell 5 into the backing mass 8. In Fig. 5, the shuttering 7 has been removed and the polyurethane model body has been separated from the plating surface 10 of the shell 5 backed by backing mass 8. The composite of shell 5 and backing mass 8, generally indicated as 9, is the intended mandrel for electrodeposition of plating metal on the plating surface 10. The shell 5 being tightly keyed to the massive backing mass 8, the composite mandrel 9 is very strong and stable. Prior to use, the plating surface 10 of the mandrel may be correctively milled to fine tune the tolerances of key dimensions not accurately reproduced during the vacuum-forming step. This corrective milling step is made easy by the tractability of the ABS plastics shell of the mandrel. The plating surface 10 of the composite mandrel 9 being electrically non-conductive, it must be treated to render it conductive prior to use, for example by spray coating with silver based paint. The DSP backing mass has CTE properties not dissimilar from that of the usual plating metals, such as nickel and nickel/cobalt alloys. The ABS plastics shell 5 is keyed firmly to the backing mass so its thermal expansion and contraction characteristics are closely aligned with those of the backing mass. Hence the problem of differential expansion of plating metal layer and mandrel is reduced or eliminated. The DSP backing mass may be rendered substantially resistant to acid and chemical degradation in the plating bath by masking vulnerable surfaces with masking tape or by applying a resistant coating.

Referring now to Figure 6, an assembly of two model bodies, 1 and 11 is shown. The model body 1 is that shown in Fig 3, with the vacuum formed ABS shell 5 formed on the shaped surface 2 of the model body, and with stone aggregate keying elements 6 randomly distributed and glued to the non plating surface of the shell 5. Again the surface of the shell in contact with the model body 1 is an intended plating surface of the intended double-sided mandrel. Model body 11 also has a differently shaped surface 22, onto which a second ABS shell 15 has been formed, also with stone aggregate keying elements 16 randomly distributed and glued to its non plating surface. Model bodies 1 and 11 are arranged in spaced relationship, with opposed non-plating surfaces. Shuttering 17 is erected around the assembly and a backing mass 8 of DSP material is cast against the non plating surfaces, between the model bodies, and allowed to harden.

Not shown in Fig. 6 are optional spacing posts which may be fixed perpendicular to the opposed non plating faces of the shells 5 and 15 and extending therebetween. Such spacing posts are useful for maintaining the desired spacing distances between the two shells during casting of the backing mass.

In Figure 7 the shuttering 17 has been removed from the assembly of Fig.6, and the polyurethane model bodies 1 and 11 separated from their respective ABS shells 5 and 15. . The composite of shells 5 and 15 and backing mass 8, generally indicated as 19, is the intended double faced mandrel for electrodeposition of plating metal on the plating surfaces 10 and 20. Corrective milling of the plating surfaces and treatment to render them electrically conductive is as described in relation to the mandrel 9 of Fig 5.

Claims

Claims:

1. A method of forming a body having a shaped surface for receiving build-up of plating metal deposited thereon by electrodeposition, which method comprises

optionally separating the shell from the model body,

casting a backing mass directly or indirectly against the non-plating face of the shell,

causing the backing mass to solidify and bond to the non-plating face of the shell,

2. A method as claimed in claim 1 wherein a second shell having a plating face and a non-plating face is formed in the same manner as the first on a shaped surface region of the same or a different solid model body and is optionally separated from that model body, and

the first and second shells are arranged in a desired spaced relationship with opposed non plating faces, and

the backing mass is cast between the spaced shells such that when it is solidified both shells become bonded thereto.

3. A method as claimed in claim 2 wherein the shells are maintained in spaced relationship by spacing posts extending between their opposed non- plating faces.

4. A method as claimed in any of claims 1 to 3 wherein keying elements are formed on or fixed to the non-plating face of a shell whereby when the backing mass is cast directly or indirectly against that face the shell becomes keyed into the backing mass.

5. A method as claimed in any of claims 1 to 4 wherein the backing mass is cast indirectly against the non-plating surface of a shell via an intermediate body or layer which increases the rigidity an/or strength of the bond between shell and solidified backing mass relative to the rigidity and/or strength of the bond achieved in the absence of such intermediate body or layer.

6. A method as claimed in any of claims 1 to 5 wherein exposed surfaces of the backing mass are inherently resistant to acid degradation or are treated to be resistant to acid and/or chemical degradation.

7. A method as claimed in any of claims 1 to 6 wherein the model body is of foamed or unfoamed plastics, laminated wood, metal alloy, or ceramic material.

8. A method as claimed in claim 7 wherein the model is of foamed polyurethane resin.

9. A method as claimed in claim 8 wherein the plastics layer forming the shell is electrically conductive.

10. A method as claimed in any of claims 1 to 9 wherein a shell is formed on the model body by vacuum forming a sheet material on the shaped surface region of the model body.

11. A method as claimed in claim 10 wherein the sheet material is of ABS (acrylonitrile-butadiene-styrene), polypropylene, polyethylene or acrylic resin.

12. A method as claimed in any of claims 1 to 9 wherein a shell is formed on the model body by applying a fluid hardenable coating material to the shaped surface region of the model body, and causing the applied coating to harden.

13. A method as claimed in claim 12 wherein the hardenable coating material is a spreadable or sprayable lacquer.

14. A method as claimed in claim 12 wherein the hardenable coating is a sprayed mixture of polyester resin and glass fibres.

15. A method as claimed in claim 12 wherein a shell is formed by positioning the model body adjacent but spaced apart from a negative thereof, having a surface shape which is a reverse image of the shaped surface of the model body, thereby defining a cavity between the two bodies which is the thickness of the desired shell, then filling the cavity with a hardenable fluid plastics material, hardening the fluid plastics material or allowing it to harden, then separating the negative from the hardened shell.

16. A method as claimed in any of claims 1 to 9 wherein a shell is formed on the model body by applying a flexible hardenable sheet material to the shaped surface region of the model body, and causing the applied sheet to harden.

17. A method as claimed in claim 16 wherein the hardenable sheet material is a curable resin-impregnated fibre mat.

18. A method as claimed in any of claims 1 to 17 wherein the plastics material from which the shell is formed contains a filler reducing the thermal expansion of the plastics material.

19. A method as claimed in any of claims 1 to 18 wherein the backing mass is a ceramic.

20. A method as claimed in claim 19 wherein the ceramic is a chemically bonded ceramic.

21. A method as claimed in claim 20, wherein the chemically bonded ceramic is a cement-based paste, mortar or concrete.

22. A method as claimed in claim 20, wherein the chemically bonded ceramic is a lightweight or low shrinkage concrete.

23. A method as claimed in any of claims 1 to 18, wherein the backing mass is made from or comprises a DSP material.

24. A method as claimed in claim 23, wherein the DSP material is a cement- based mortar or concrete.

25. A method as claimed in claim 24, wherein the DSP material is reinforced with steel fibres.

26. A method as claimed in any of claims 1 to 18 wherein the backing mass is an aggregate-containing resin matrix.

27. A method as claimed in claim 26 wherein the resin matrix is of polyurethane, acrylic, polyester or epoxy resin.

28. A method as claimed in any of claims 1 to 18 wherein the backing mass is a metal alloy.

29. Electrodeposition apparatus including an anode body comprising plating metal, a cathode body having at least one exposed surface region for receiving build-up of the plating metal by electrodeposition, an electrolyte solution comprising a salt of the plating metal, and means for passing current between the anode and cathode bodies via the electrolyte solution CHARACTERISED IN THAT the cathode body comprises a shaped layer in the form of a solid shell having a plating face and a non-plating face, the non plating face being bonded to a solid backing mass of material different from that of the shell,

the backing mass material has a coefficient of thermal expansion in the range 4.0 Dm/m/DC to 35 μm*m^"1*°C^"1,

30. Apparatus as claimed in claim 29 wherein the cathode body comprises two shells bonded to and spaced from each other by the solid backing mass.

31. Apparatus as claimed in claim 30 wherein spacing posts extend through the backing mass between opposed non-plating faces of the shells.

32. Apparatus as claimed in any of claims 29 to 31 wherein a shell is keyed into the backing mass by keying elements formed on or fixed to the non-plating face of the shell.

33. Apparatus as claimed in any of claims 29 to 31 wherein the backing mass is bonded indirectly to the non-plating surface of a shell via an intermediate body or layer which increases the rigidity an/or strength of the bond between shell and backing mass relative to the rigidity and/or strength of the bond achieved in the absence of such intermediate body or layer.

34. Apparatus as claimed in any of claims 29 to 33 wherein exposed surfaces of the backing mass are inherently resistant to acid and/or chemical degradation or are treated to be resistant to acid and/or chemical degradation.

35. Apparatus as claimed in claim 34 wherein the shell material is electrically conductive.

36. Apparatus as claimed in any of claims 29 to 35 wherein a shell is a vacuum formed sheet material.

37. Apparatus as claimed in claim 36 wherein the sheet material is of ABS (acrylonitrile-butadiene-styrene), polypropylene, polyethylene or acrylic resin.

38. Apparatus as claimed in any of claims 29 to 35 wherein a shell is a fluid hardenable coating material which has been caused to harden.

39. Apparatus as claimed in claim 38 wherein the hardenable coating material is a spreadable or sprayable lacquer.

40. Apparatus as claimed in claim 38 wherein the hardenable coating is a sprayed mixture of polyester resin and glass fibres.

41. Apparatus as claimed in any of claims 29 to 35 wherein a shell is a flexible hardenable sheet material which has been caused to harden.

42. Apparatus as claimed in claim 41 wherein the hardenable sheet material is a curable resin-impregnated fibre mat.

43. Apparatus as claimed in any of claims 29 to 42 wherein the plastics material from which the shell is formed contains a filler reducing the thermal expansion of the plastics material.

44. Apparatus as claimed in any of claims 29-43 wherein the backing mass material has a coefficient of thermal expansion in the range 4.0 μm*m^"1*°C^"1to 30 μm*m^"1*°C^"1.

45. Apparatus as claimed in claim 44, wherein the backing mass material has a coefficient of thermal expansion of at least 6.5 μm*m^"1*°C^"1.

46. Apparatus as claimed in claim 45, wherein the backing mass material has a coefficient of thermal expansion in the range 8-15 μm*m^"1*°C^"1.

47. Apparatus as claimed in claim 45 or 46, wherein the backing mass material has a coefficient of thermal expansion which is lower than that of the plating metal.

48. Apparatus as claimed in claim 45 or 46, wherein the backing mass material has a coefficient of thermal expansion which is lower than that of the material of the shell.

49. Apparatus as claimed in any of claims 29 to 48 wherein the backing mass is a ceramic.

50. Apparatus as claimed in claim 49 wherein the ceramic is a chemically bonded ceramic.

51. Apparatus as claimed in claim 50, wherein the chemically bonded ceramic is a cement-based paste, mortar or concrete.

52. Apparatus as claimed in claim 51 , wherein the chemically bonded ceramic is a lightweight or low shrinkage concrete.

53. Apparatus as claimed in any of claims 29 to 48, wherein the backing mass is made from or comprises a DSP material.

54. Apparatus as claimed in claim 53, wherein the DSP material is a cement- based mortar or concrete.

55. Apparatus as claimed in claim 54, wherein the DSP material is reinforced with steel fibres.

56. Apparatus as claimed in any of claims 29 to 45, 47 and 48 wherein the backing mass is an aggregate-containing resin matrix.

57. Apparatus as claimed in claim 56 wherein the resin matrix is of polyurethane, acrylic, polyester or epoxy resin.

58. Apparatus as claimed in any of claims 29-48 wherein the backing mass is a metal alloy.