GB2193381A - Carbon fibre reinforced plastic waveguide elements - Google Patents

Abstract

Carbon fibre reinforced plastic (CFRP) waveguide of composite construction representing a lightweight alternative to standard aluminium or brass waveguide. Layers 16, 16' of CFRP are applied to form a sandwich with a lightweight core 18 of honeycomb structure. The internal surface of the waveguide is coated with metal 13, 14 by electroplating. Particular waveguide components are constructed by building the composite sandwich around appropriately shaped formers 12. <IMAGE>

Description

SPECIFICATION Carbon fibre reinforced plastic waveguide elements This invention relates to carbon fibre reinforced plastics (CFRP) composite waveguide elements and to a method of manufacture of such elements. CFRP offers significant weight savings over the conventional materials for waveguide aluminium or brass. An antenna incorporating lightweight waveguides requires a less powerful drive system, and has lower inertia enabling fine and rapid control of its position. The mechanical and electrical properties of CFRP composite waveguide are ideally equivalent to or better than purely metallic waveguide, and the manufacturing process should be capable of repeatedly producing highly accurate components.

An object of this invention is to provide a lightweight waveguide meeting some or all of these requirements.

According to one aspect of the invention a waveguide element has walls composed of at least one layer of CFRP and a layer of material having a honeycomb structure, the inner surfaces of the walls being coated with metal on their inner surfaces. The metal may be copper which may be coated on a layer of nickel.

According to another aspect of the invention a method of manufacture of a CFRP waveguide element comprises the steps of shaping a former for the element, applying a layer of metal to the former, applying at least one layer of CFRP to the metal layer, applying at least one layer of material having a honeycomb structure, and removing the former. The method may further include applying a second layer of metal to the former before applying the CFRP. The metal may be applied by electroplating. The former may be aluminium and may be removed by dissolving in a suitable solvent. Alternatively the former may be stainless steel.

One embodiment of the invention will now be described with reference to the accompanying drawings of which: Figure 1 shows a waveguide run; Figure 2 shows a cross-section of the feed horn assembly section of the waveguide run; and Figure 3 shows a CFRP composite sandwich construction in accordance with the invention.

The invention is applicable to the complete waveguide run shown in Fig. 1. One component which benefits particularly from the advantages offered by a CFRP composite construction is the feed horn assembly 1. As seen in Fig. 2 this is a complex section comprising a twin venturi 3, bends 5, tapers 7, cavities 8, threaded holes 9 and strengthening ribs 10. This complex component and the simpler components can be equally successfully manufactured from a CFRP composite construction. From the position of the feed horn assembly overhanging the front of the aerial skirt 2, it can be seen that appreciable weight savings here lessen the load on the skirt and improve vibration characteristics.

There are many types of reinforcing fibres available in various forms, with an even wider choice of resin systems and the choice of each can be made with regard to the intended application. One suitable material is Fibredux (RTM) 913 epoxy resin pre-impregnated into a woven carbon fabric. The material can be processed at a relatively low temperature (120"C) and exhibits high environmental resistance.

Three possible alternative constructions to aluminium, each one offering a different potential weight saving with equal mechanical performance, are shown in Table I, for the Feed Horn Assembly shown in Fig. 2.

WAVEGUIDE THICKNESS WEIGHT WEIGHT SAVING CONSTRUCTION ~ mm Kg/m2 Aluminium 3.175 8.573 (Counercially pure).

High Strength 3.08 5.221 39.1% Woven Carbon Fibre.

High Modulus 2.75 4.863 43.3% Fibre Sandwich 4.78 3.086 64.0X Construction.

TABLE 1 The full sandwich construction is made up of Fibredux (RTM) 913 carbon fibre prepreg with a honeycomb core of a lightweight material such as aluminium or Nomex (RTM). An adhesive film is used between the honeycomb and the carbon fibre skins to ensure that a strong bond is achieved. It can be seen that this sandwich construction offers the greatest potential weight saving of 64%, but has a wall thickness of 4.78mm as opposed to 3.175mm in the allaluminium construction. Thus, if wall thickness is a critical factor for the component it may be necessary to use an alternative construction. However, in the case of the Feed Horn Assembly a 4.78mm wall thickness is acceptable.

The following stages make up the manufacturing process: (i) Manufacture of suitable internal formers.

(ii) Metallisation of formers.

(iii) Reinforcement of metallisation with CFRP, by a cycle of applying layers of adhesive and CFRP and curing, incorporating a honeycomb layer and inserts if required.

(iv) Dissolving/removing formers.

(v) Final machining, drilling and bonding of inserts.

The basic requirements of the former are to remain stable at 1200C, to withstand applied pressure during the cure cycle, to be compatible with a process whereby copper or other suitable metal can be deposited on the surface and to be dissolved or removed from the moulded assembly after the cure cycle.

Among the possible materials for the former are low melting point alloys, ferrous and nonferrous metals. Low melting point alloys are attractive because they are capable of being melted, recovered and re-used. One suitable choice is a 137"C eutectic material consisting of 60% Bismuth/40% Tin. It remains stable at the curing temperature of the prepreg (120"C) but when heated to 138"C or above it becomes liquid and flows from the assembly. The liquid is recovered and allowed to solidify before being re-used.

To enable the electroplating process to be carried out, the surface of the former must be treated with a graphite solution which becomes semi-bonded to the surface of the copper after the cure cycle of the CFRP and must subsequently be etched away. In practice this is difficult to achieve without damaging the copper surface. However, other methods of removing the graphite layer, or the negation of its requirement altogether may be possible. Of the ferrous metals stainless steel is preferred as it can easily be plated with a copper coating. Because the adhesion between the copper and stainless steel surface is poor, formers can be extracted from the assembly in one piece if the component shape is simpie. Of the non-ferrous metals, Aluminium Alloy HE30 (6082) meets all the requirements.It is readily plated and can be dissolved in a sodium hydroxide solution, thus enabling bends and complex forms to be moulded without the problems of extracting the former.

Possible metallisation processes include electroplating, electroless plating, arc spraying and flame spraying. The principle of arc spraying is to spray a thin coat of copper on to a former and to reinforce the copper layer with CFRP. The former is then removed leaving the copper layer bonded to the CFRP. However it is not possible to spray pure copper, unlike materials such as tin and zinc, on to a smooth surface. The former requires a 'key' for the copper to adhere to the surface. A typical method of providing this 'keyed' surface is by shot blasting. This type of finish is unacceptable where a very high quality surface is required, but arc spraying may be suitable in some circumstances. A preferred metallisation technique is electroplating. Formers made from aluminium and stainless steel can be readily plated and removed after reinforcing with CFRP.

Fig. 3 shows a typical cross-section of the sandwich construction.

The formers 12 are electroplated with a layer of copper 13 followed by a layer of nickel 14.

Because of the vigorous oxidation of copper and therefore difficulty in bonding to it, the layer of nickel 14 is introduced to aid subsequent bonding of the CFRP.

The nickel surface is then treated with a suitable primer such as DZ80 compatible with the adhesive film. The primer is applied, dried at 800C for 30 minutes then cured at 150"C for 1 hour. It is then abraded, degreased and allowed to dry at ambient temperature. After this pretreatment the method of reinforcement of metallisation comprises the following stages: (i) Lay-up an adhesive film on prepared metallised surface 14.

(ii) Lay-up inner prepreg layers 16, orientating each to produce a baianced structure.

(iii) Bag-up and cure the assembly for 1 hour at 120"C.

(iv) Lay-up a second adhesive film.

(v) Cut and fit honeycomb 18.

(vi) Bag-up and cure the assembly for 1 hour at 120"C and incorporating inserts 20 if required.

(vii) Lay-up a third adhesive film.

(viii) Lay-up outer prepreg layers 16', once again orientating each layer.

Bag-up and cure the assembly for 1 hour at 1200C.

Various stages of this process may be combined. The method affords close control over resin/fibre ratio, good repeatability and a high quality inner surface finish. Inserts and bosses can be incorporated at various stages to provide support and shaping. For the flange portion 15 of the waveguide element a layer of pre-cured CFRP 17 is substituted for the honeycomb layer 18 so as to give this additional strength.

After the reinforcement of the metallisation is complete the internal formers 12 are removed.

For stainless steel, which is suitably used to form sections such as the flared portion 4 of the feed horn assembly, the formers can simply be extracted and re-used. Formers can be made from aluminium by machining the raw material to form, for example, the twin bends. Certain grades of aluminium can be dissolved in a warm sodium hydroxide solution: the completed moulded assembly is immersed in a bath until all traces of aluminium are removed.

Post machining operations, such as milling, routing and drilling are then carried out. It is very important that the machining does not cause any burrs in the metallisation. If inserts have not been incorporated, they can be bonded in at this stage.

Claims (10)

1. A waveguide element having walls composed of at least one layer of carbon fibre reinforced plastic and a layer of material having a honeycomb structure, the walls being coated with metal on their inside surfaces.

2. A waveguide element according to Claim 1 wherein said metal is copper.

3. A waveguide element according to Claim 2 wherein the coating of copper is made over a coating of nickel.

4. A method of manufacture of a waveguide element comprising the steps of shaping a former for the element, applying a coating of metal over said former, applying at least one layer of carbon fibre reinforced plastic over said coating of metal, applying a layer of material having a honeycomb structure, and removing said former.

5. A method according to Claim 4 wherein at least two layers of metal are applied to the former.

6. A method according to Claim, 4 or 5 wherein said metal is applied by electroplating.

7. A method according to Claim 4, 5 or 6 wherein said former is made of aluminium.

8. A method according to any preceding claim wherein said former is removed by dissolving in a solvent.

9. A method substantially as hereinbefore described with reference to the accompanying drawings.

10. A waveguide element substantially as hereinbefore described with reference to the accompanying drawings.