The present invention relates to a method of making a tubular body of fiber reinforced plastic material with a desired inner or outer pe- ripheral surface with close tolerances. Large diameter tubular bodies to be used in telescopic systems and in other structures where close fits are needed must be produced with peripheral surfaces having a high degree of smoothness and with close tolerances. Traditionally, such tubes are manufactured so that the outer or inner surfaces in question are oversized or undersized, respectively, and such surfaces are then ground to the desired tolerances. Such grinding process does not allow for very close tolerances and for a very high degree of smoothness within the sub-micron region. Furthermore, the grinding process may interfere with an adjacent layer of fibers in a disadvan- tageous manner.

These disadvantages are overcome by the present invention, which pro¬ vides a method of making a tubular body of fiber reinforced plastic material with a desired inner or outer peripheral surface with close tolerances, said method comprising forming a tubular body sample of unhardened fiber reinforced plastic material with an outer peripheral surface slightly undersized in relation to said desired outer periph¬ eral surface, or with an inner peripheral surface slightly oversized in relation to said desired inner peripheral surface, pressing the peripheral surface of the sample into tight engagement with a mould surface complementary to said desired inner or outer peripheral sur¬ face and with the desired smoothness and dimensions, when the plastic material is in its unhardened condition, and hardening the plastic material while in engagement with the mould surface. The peripheral sample surface which is pressed into engagement with the mould sur- face will adopt the size or diameter of the mould surface without affecting any of the reinforcing fibers.

In order to avoid any risk of inclusion of air and in order to im¬ prove the close mutual engagement between the sample surface and the

mould surface, the body sample is preferably exposed to vacuum while pressed into engagement with the mould surface.

The method according to the invention may be used whether the tubular body is circular cylindrical, cylindrical or non-cylindrical. The reinforcing fibers may be monofila ents or stable fibers. When mono- fila ents are used, either wet, dry, or preimpregnated, they may be wound on to a mandrel with a lay-up pattern applicable to the contem¬ plated application of the tubular body to be produced. However, the pattern of the monofilaments in the various layers cannot always be chosen arbitrarily, when it is desired to obtain close tolerances for an outer peripheral surface of the tubular body. Thus, layers of monofilaments extending at substantially right angles to the longitu¬ dinal axis of the tubular body sample may counteract the necessary expansion of the sample into engagement with the mould surface to an unacceptable degree. Thus, according to the invention at least some of the reinforcing fibers are helically wound monofilaments, which at least in the inner layers of the sample define an acute angle with the longitudinal axis of the body sample.

The body sample surface may be pressed into engagement with the mould surface in any suitable manner, for example by mechanical means. In the preferred embodiment, however, the necessary pressure is exerted by a pressurized fluid, such as pressurized gas or liquid. When it is desired to form an inner surface of a tubular body sample to a desi¬ red smoothness and desired dimensions, the sample may be compressed around a mandrel-like mould member on which the mould surface is formed. When, however, the desired surface is an outer peripheral surface, the body sample may be enclosed in a space, which is defined between an inner peripheral mould surface of a mould and an adjacent inner surface of a tubular, flexible wall or membrane, and to which space vacuum is supplied. The outer peripheral surface of the body sample will then be pressed into engagement with the mould surface by atmospheric pressure acting on the flexible wall or membrane. This pressure is, however, preferably increased by exposing the outer sur¬ face of the flexible wall or membrane to a pressurized fluid.

The plastic material used as a matrix material may be of the ther- osetting, the thermoplastic or the elastomeric type, such as poly¬ ester, epoxy, PEEK, PES, or polyimide, and the reinforcing fibers or filaments may be glass, Kevlar*, carbon, and any other suitable fila- nent material. In case the body sample is made from a fiber rein¬ forced thernosetting plastic material, a unit comprising the mould, the flexible wall or membrane, and the body sample arranged there¬ between may be arranged in an autoclave with elevated temperature and pressure, whereby the heat treatment for setting or hardening the plastic material and the pressure for expanding the tubular body sam¬ ple may be obtained at the same time.

The present invention also provides a mould for use in carrying out the method described above, said mould comprising a mould body with a smooth peripheral mould surface, and a tubular, flexible wall or mem- brane, and connecting means for connecting a mould cavity defined between the mould surface and the flexible wall or membrane to the vacuum source.

The invention will now be further described with reference to the drawings, wherein Fig. 1 illustrates a tubular sample body of fiber reinforced plastic material and comprising various layers of filaments wound around a mandrel,

Fig. 2 is a side view and partially sectional view of a mould with a sample body arranged therein, and Fig. 3 is a diagram illustrating processing of the body sample in an autoclave.

Fig. 1 shows a tubular sample 10 of a reinforced plastic material. The sample 10 has been made by winding various layers 11-21 of fibers or monofilaments around a mandrel 22 in a matrix of a suitable plas- tic material. The thickness of the layers 11-21 is controlled very accurately to secure that the outer diameter of the circular cylin¬ drical tubular sample 10 is about 1/10 of a mm smaller than the diameter of an inner circular cylindrical mould surface 23 of a tubu¬ lar mould member 24 shown in Fig. 2.

The mandrel 22 with the tubular sample 10 formed thereon is now ar¬ ranged within a tubular mould member 24 with the outer peripheral surface 25 of the sample 10 opposite and closely adjacent to the mould surface 23, which has a smoothness and a diameter exactly cor- responding to the desired smoothness and outer diameter of the outer peripheral surface 25 of the tubular sample 10. The mandrel 22 is now removed from the mould member 24 and from the sample 10, and tubular layers of various helping materials are arranged within the inner bore of the sample 10, namely a peel ply 26, a bleeder sheet 27, a release film 28, and a breather layer 29. The free end portions 30 of the release film 28 is clamped between the end surfaces of the mould member 24 and a clamping ring 31. A tube or hose 32 of a flexible material, such as silicone or rubber, has its free end portions 33 clamped between the first clamping ring 31 and a second clamping ring 34 whereby a mould space 35 is air-tightly defined between the mould surface 23 and the tube or hose 32. This mould space, which contains the tubular sample 10, the peel ply 26, the bleeder sheet 27, the release film 28, and the breather layer 29, may be connected to a vacuum source, not shown, by means of a vacuum tube 36.

The complete mould assembly with the tubular sample 10 enclosed therein as shown in Fig. 2 may now be arranged in an autoclave in which the mould assembly is heated and exposed to a superatmospheric pressure, while the mould space is evacuated through the vacuum tube 36. The elevated pressure within the autoclave tends to press the hose 32 towards the mould surface 23 of the mould member 24 and thereby to press the outer peripheral surface 25 of the sample 10 into tight engagement with the mould surface 23 by expanding the tubular sample. The expansion of the tubular sample is facilitated by the heating of the sample 10, which tends to soften the matrix mate- rial of the sample. Such heating is part of a normal curing of the matrix, if the matrix is formed by a thermosetting plastic material. In case the matrix is a thermoplastic material, the heat is restrict¬ ed to what is necessary for softening the plastic material so as to facilitate expansion of the tubular sample. After cooling, the matrix material is hard, and the outer peripheral surface 25 of the sample 10 has obtained the desired smoothness and the desired accurate diameter. The tubular member thus produced may now be trimmed to the

5 desired length and is suitable for use in telescopic systems in which tight fit is required, such as pneumatically or hydraulically ac¬ tuated extensible and retractable telescopic systems.

It is understood that the mould assembly with the tubular sample ar- ranged therein is not necessarily arranged in an autoclave. Alterna¬ tively, the ends of the mould assembly could be closed by end walls so as to define an air-tight space within the hose 32. This space may then be connected to a source of pressurized air or gas while the mould space 35 is evacuated through the vacuum tube 36. At the same time the tubular sample 10 arranged within the mould space may be heated in any desired manner, if necessary.

EXAMPLE 1

A carbon fiber reinforced epoxy plastic tube was made of a material marketed by Fiberite under the tradename Fiberite HY-E-1082K and with the following dimensions and prescribed tolerances:

Length: 3892 mm +3.0 mm/-0.0 mm

Outer diameter: 178.2 mm +0.0 mm/-0.1 mm Inner diameter: 171.5 mm +0.2 mm/-0.0 mm

Furthermore it was prescribed that the maximum surface roughness Ra is 0.4 μm.

A sample was made by winding the material on to a mandrel 22 by a conventional winding process. The first three layers arranged on the mandrel were prepreg sheets with non-continuous fibers extending at substantially right angles to the longitudinal axis of the mandrel, i.e. at an angle of 90° and these first three layers were applied to the mandrel with an appropriate overlap to counter for the later ex¬ pansion. The remaining layers were made of helically wound continuous filaments with varying orientation. Thus, the lay-up configuration was as follows from the inner layer to the outer layer:

3x90", lx+45\ lx-45*. 4x0", lx+45", lx-45 ^β , lx+45', lx-45 ^β , 4x0°, lx+45 ^β , lx-45 ^β , lx+87", lx-87 ^β , lx+87\ lx-87 ^β , and 2x0°.

This means that the tubular sample was built up from 25 layers, in each of which the carbon filaments extended in a direction which de- fined the angle indicated with the longitudinal axis of the mandrel or sample. The tubular sample thus produced was transferred to a ■ould assembly as that shown in Fig. 2, where the peel ply 26 was of the type marketed by Airtech International Inc. , Carson, California, under the tradename 234TFNP, the bleeder sheet 27 was of the type marketed by the same company under the tradename Airweave 'S' , the release ilm 28 was of the type marked by Richmond Technology Inc. , Redlands, California, under the tradename A 5000, the breather layer 29 was of the type marketed by Fothergill Tygaflor Ltd. , U.K. , under the tradename Tygavac NW 339, and where the tube or hose 32 was made of silicone rubber of the type marketed by Aerowac Systems Ltd. ,

Keighley, West Yorkshire, U.K., under the tradename 1453D. This mould assembly containing the tubular sample prepared as described above was arranged within an autoclave while the vacuum tube 36 was con¬ nected to a vacuum source. The mould assembly with the tubular sample therein was processed in the autoclave for a period of about 3 hours, and the temperature and the pressure within the autoclave and the vacuum applied to the mould space 35 were varied in response to time as shown in the diagram of Fig. 3. As will be seen from this diagram the maximum pressure within the autoclave was about 9.5 bar while the maximum temperature was about 125 ^β C. It also appears from the diagram that vacuum was applied to the mould space 35 only in the initial processing period, and that the mould space was put under atmospheric pressure when the pressure within the autoclave was raised to 9.5 bar.

EXAMPLE 2

A tube as that described in Example 1 is made. However, the three innermost layers with non-continuous filaments or fibers extending at right angles to the longitudinal axis of the mandrel are replaced by three layers of continuous, helically wound filaments extending in a

direction defining an angle of 20°. The use of continuous filaments in these layers allows for a fully automatic winding process using either wet filament, vacuum impregnated filament or prepregged rov¬ ing. The 20° layers will expand easily during processing in the autoclave.