WO1991005653A1

WO1991005653A1 - Process for the manufacture of a tubular composite part

Info

Publication number: WO1991005653A1
Application number: PCT/NL1990/000156
Authority: WO
Inventors: Ronald Hermanus Bult; Jan Klaas Waninge; Jacques Cornelus Maria Plouvier
Original assignee: Stamicarbon B.V.
Priority date: 1989-10-19
Filing date: 1990-10-17
Publication date: 1991-05-02
Also published as: AU6631690A; NL8902588A

Abstract

The invention relates to a process for the manufacture of a tubular composite part whose longitudinal axis exhibits a curve and/or whose wall exhibits a variable cross section relative to the longitudinal axis in which process, a tubular semi-finished product, comprising an uncured or a partially cured resin and fibre reinforcement and further comprising an inflatable former, is introduced into a mould, a medium is brought into the inflatable former under pressure, the resin being cured and the composite part thus obtained being removed, characterized in that the fibre reinforcement comprises at least a knitted or braided sleeve. Composite parts according to the invention can be used in numerous applications where complex structural parts are required that combine high pressure-resistance, flexural, torsional and tensile strength. Such an application is sometimes referred to as a space frame. Examples are the supporting parts in a car body or in a boat or aircraft, in construction, rucksack frames, bicycle frames, lampposts and sports equipment, such as bobsleighs, etc.

Description

PROCESS FOR THE MANUFACTURE OF A TUBULAR COMPOSITE PART

The invention relates to a process for the manufacture of a tubular composite part whose longitudinal axis exhibits a curve and/or whose wall exhibits a variable cross section relative to the longitudinal axis, in which process a tubular semifinished product, comprising an uncured or partially cured resin and fibre reinforcement and further comprising an inflatable former, is introduced into a mould, a medium is brought into the inflatable former under pressure, the resin is cured and the composite part thus obtained is removed.

Such a process is known from DE-A-2.631.374. Here, a composite part is produced by winding a resin-impregnated non-woven fabric around a rubber balloon or tube, placing the balloon or tube in a heated mould, heating the mould until the resin softens, inflating the rubber balloon and subsequently heating the mould until the resin cures. On curing, the pressure in the rubber balloon or tube is reduced to atmospheric pressure and the rubber balloon or tube is removed from the composite part. A disadvantage of a process as described in DE-A-2.631.374 is that it is impossible to produce a tubular composite part of complex geometry and good mechanical properties. A non-woven fabric substantially consists of randomly distributed fibres that exhibit a mutual coherence in that they are bonded to one another by means of adhesion or fusion. When such a two-dimensional non-woven fabric is bent in more than one plane, it will tear or, at best, the fibres will be redistributed. Since the fibres of the non-woven fabric no longer possess any strong coherence, if at all, in heating, forming or inflation, they will, in forming, reorient themselves already at a low pressure to conform to a geodesic, the geodesies of several fibres overlapping one another rather than lying adjacent to one another. A geodesic is the shortest possible line between two points on a surface. This leads to serious anisotropy in the local strength of the composite part produced. This precludes use of the method for the production of composite parts as described in DE-A-2.631.374 for the production of structures with a complex geometry and good properties.

A further disadvantage of the composite part described in DE-A-2.631.374 is that the production process is cumbersome and inefficient.

The object of the invention is to provide a process for the manufacture of a composite part that does not have the aforementioned limitations.

This is achieved according to the invention in that the fibre reinforcement comprises at least a knitted or braided sleeve.

In a braided or knitted sleeve the fibres form an interlocking structure. This is advantageous over the fibres in a non-woven fabric, which are held together by an adhesive or through fusion. A disadvantage of adhesion or fusion is that the fabric looses its flexibility to a great extent and, thus, is less easy to process.

Since the fibres that form part of the sleeve have a mutual coherence, they remain uniformly distributed across the surface in more than one direction during the forming process. Alternatively, a sleeve may be used consisting of a woven fabric, but this has the disadvantage that a woven fabric is not inherently stretchable, which means that a woven fabric used must always have a cross section at least equal to the greatest cross section of the product.

A process in which a knitted sleeve is used to produce a tubular composite part is described in FR-1.153.826 but no mention is made there of a semifinished product of an uncured or partially cured resin and fibre reinforcement nor of an inflatable former. The sleeve in FR-1.153.826 is permanently applied around a tube joint and is subsequently impregnated with resin. This is irrelevant to the invention.

An application of a knitted sleeve for the production of a composite part is further described in GB-A-2.051.662, in which use is also made of an inflatable second sleeve, but no reference is made to a semifinished product of an uncured or partially cured resin and fibre reinforcement. In GB-A-2.051.662 the sleeve is wetted with resin by placing the sleeve in a mould, inflating the inflatable second sleeve and subsequently injecting the resin. A disadvantage of this is that the wetting of the fibres of the sleeve is much poorer than when use is made of a semifinished product according to the invention. Furthermore, air pockets are prone to develop; flow profiles are formed in the composite part that adversely affect the mechanical properties; and the resin must have a very low viscosity. This limits the range of suitable resins; Nor is it well possible to obtain complex geometries with a process according to GB-A-2.051.662 because the resin will not reach, or not reach to a sufficient degree, the extremities of the geometry.

GB-A-2.169.845 describes a process for manufacturing composite parts with the aid of a knitted material but here the knitted material is not used in the form of a sleeve nor is use made of a semifinished product of an uncured or partially cured resin and fibre reinforcement.

Surprisingly, the invention appears to afford the possibility of producing composite parts with complex geometries and, in comparison with the state of the art, a relatively large wall thickness at curvatures. With wound non-woven fabrics there appeared to be a limitation regarding the ratio of the wall thickness and cross section of the composite part at a curvature. This problem arises to a much lesser extent when use is made of a sleeve according to the invention and a greater wall thickness may optionally be selected.

The medium introduced into the inflatable former will normally be a gas or a liquid such as air or water. In place of a gas or liquid it is possible to introduce into this tubular semifinished product a foam or foaming material, which need not be removed from the composite part after curing.

The flexible former may be, for instance, a rubber bag or hose as described in DE-A-2.631.374. Preferably, the flexible former consists of a bag made of flexible film, whose circumference is greater than the tube circumference of the sleeve. The bag may, optionally, be left in the composite part after curing. This is advantageous in that it simplifies the production process. It is possible to produce the sleeve on-line, already containing the inflatable film just described, and to then impregnate the sleeve. The impregnated sleeve may subsequently be stored and processed later but may also be introduced into the mould immediately thereafter for curing. Composite parts according to the invention differ from those produced according to the state of the art in that in the composite parts according to the invention the fibre reinforcement is uniformly distributed in convex locations too and in that the fibres are well impregnated in all locations. It is also known to wind filaments around a rubber bag - this is known as the filament winding method - and to then inflate the bag. A composite part produced in this manner has disadvantages similar to those of the aforementioned state of the art, DE-A-2.631.374, namely that in convex and concave locations the fibres will be distributed according to a geodesic. With the filament winding method it is usually only possible to wind filaments according to a geodesic pattern. This means that projections, ri s, depressions, etc. are not adequately covered with filaments. The sleeve according to the invention may, as described above, consist of a braided or knitted sleeve. The fibres of which the sleeve is composed and which provide the reinforcement in the composite part may consist of glass, carbon, aramide or any other material. The fibres may consist of twisted short fibres or of continuous endless filaments. A method for producing suitable sleeves is described in, for example, EP-A-243.147.

It is possible to impregnate the sleeve with resin after the sleeve has been applied round the flexible former. This can be done, for example, with the aid of a brush or through immersion, but it can also be done in a mould by pressing and/or sucking the resin through it (the so-called Resin Transfer Moulding). In impregnation use may be made of pressure to promote the exclusion of air bubbles and the flowing of the resin between the fibres. The temperature may be increased to reduce the viscosity of the resin and thus improve the wetting of the fibres. These methods are known to a person skilled in the art.

Preferably, the sleeve is impregnated with resin before being combined with the flexible former. This can optionally be done on-line as described above.

A further possibility in on-line impregnation is that the sleeve is passed over a hollow tube, from which resin is pressed. The resin can be pressed therefrom via holes in the wall of the hollow tube or in a different manner. Around the hollow tube may be a flexible material that is saturated with the resin and in turn transfers said resin to the sleeve because the sleeve exhorts pressure on the flexible material as it is passed over it. There may be a second hollow tube in the hollow tube, through which the flexible former is passed to end up inside the sleeve after impregnation. Preferably, the resin is combined with the sleeve on-line.

The sleeve" impregnated with resin that has not yet been cured is also called a 'prepreg' . Preferably, the prepreg is dry and not tacky after the so-called B-Stage.

The resin may be chosen from, for example, unsaturated polyesters, vinyl ester resins, acrylate-urethanes, phenol-formaldehyde resins, melamine-formaldehyde resins or any other resin.

The rubber bag around which the sleeve can be applied may consist of any type of rubber that is resistant to the resins, temperatures and pressures applied. For reasons of costs it will usually be a requirement that the rubber bag be reusable.

If use is made of a bag of a flexible film less stringent requirements will be imposed on the film because the film will usually be used once only. The film must be resistent to the resins, temperature and pressures used for the length of time that is required to manufacture the composite part. The material of the film may be chosen from any type of material that is capable of resisting an air or water pressure and that is resistant to the resins and temperatures used. Preferably, the film consists of polyamide.

The bag of film must have a diameter that is at least as large at the largest diameter of the inside of the composite part. At the smaller cross sections the film will eventually be wrinkled against the inside wall of the composite part. However, this is not a disadvantage because a thin film on the inside has little effect on the outside surface. An advantage of a film of the kind described above over a rubber bag is that when use is made of a film the internal pressure is the same everywhere, whereas when the rubber bag is stretched when pressurized, the pressure that is exerted on the sleeve will not be the same everywhere if the sleeve has different cross sections, which difference in pressure will result in differences in wall thickness and properties. If the rubber bag is not stretched but is to wrinkle at the smaller cross sections, this may lead to coarser irregularities on the inside and possibly even on the outside because a rubber bag, partly in view of the requirements described above, will usually be thicker than a film.

The mould in which the composite part is formed may be any mould. In the case of a thermosetting resin, it must be possible to heat the mould, either by pipes running through it or by placing the entire mould in a heating area. Between the mould may be maintained at the temperature required to cure the resin but may also be cooled between two production runs. If the resin is curable with the aid of light, the mould must transmit light or it must be possible to apply a light source in the mould. This would, for example, be possible in the flexible former.

A possible embodiment of a mould to be used in a process according to the invention is a mould wit: a cεvity on the inside that corresponds to the outside dimensions of the composite part to be produced. In the mould is an opening that is connected with the inside of the former, through which a gas or liquid can be fed under pressure to raise the pressure in the former. Preferably, there are also a number of openings on the inside of the mould for the discharge of air or any other gas that may still be in the mould, outside the former, which gas would otherwise cause irregularities in the outside wall of the composite part, and for the discharge of any excess resin.

Composite parts produced with a method according to the invention may have any desired shape but the invention is particularly advantageous if a composite part is produced that consists of a hollow, non-branched pipe whose longitudinal axis is not straight. The longitudinal axis of a composite part according to the invention may exhibit sharper angles than the longitudinal axis of a composite part according to the state of the art could. A further advantage is that composite parts can be produced that have sharper angles at the same cross section and wall thickness than was so far possible.

Composite parts according to the invention can be used in numerous applications which require complex structural parts that combine high pressure-resistance, flexural, torsional and tensile strength. Such an application is sometimes referred to as a space frame.

Examples are the supporting parts in a car body or in a boat or an aircraft, in construction, rucksack frames, bicycle frames, lampposts and sports equipment, such as bobsleighs, etc.

Example

A sleeve braided from 96 continuous glass filaments of 1200 tex with an angle of 45/135° relative to the longitudinal axis, which sleeve has a circumference of 150 mm and a length of 60 cm, is slipped round a film tube of polyamide with a length of 65 cm, a wall thickness of about 50 μm and a circumference of 170 mm. Two more identical sleeves are then slipped round this to obtain a three-layer sleeve around a film tube. The sleeves are impregnated at room temperature with a mixture of two unsaturated polyester resins of the type Stypol^R 40-3910 and Stypol^R 40-1080 supplied by Freeman Chemicals Limited, England, in a ratio of 60 : 40. In addition, the resin mixture contains, per 100 parts of resin:

1.5 parts of Trigonox-c of Akzo

3.5 parts of Zn stearate

2.8 parts of MgO

2 parts of a viscosity reducing agent. 100 parts of this resin mixture are used per approximately 330 parts of sleeve. The resin is applied to the sleeve with the aid of a brush. The film tube is sealed at two ends by means of a stopper with a square cross-sectional shape and a circumference of 163 mm. A tube is inserted through the stopper at one of the ends, which tube is connected with a pump that pumps in nitrogen with a pressure of 5 bar. The impregnated sleeve around the film tube is placed in a mould consisting of two halves, the two parts forming a longitudinal cavity with a square cross-sectional shape with sides of 40 mm and rounded corners, which longitudinal cavity in the middle exhibits a curve of 90° with a radius of 50 mm. Via an opening at the end of the longitudinal cavity the tube through the stopper can ensure contact between the film tube and the pump. During the curing a pressure of 5 bar is maintained. There are openings in two diagonally opposed sides of the cavity, through which gas and/or liquid can be extracted by means of a pump which is connected therewith. Because a vacuum is created, the two halves of the mould are pressed together.

The temperature of the mould is increased to 150°C and is maintained for 15 minutes.

The pressure in the film tube is lowered to atmospheric pressure and the mould is opened.

The result is a tubular composite part with a square cross-sectional shape of 40 x 40 mm and a wall thickness of 2 mm.

The inside wall of the composite part exhibits no wrinkles, not even in the curve, but does exhibit texture. When the composite part is sawn in two at the curve it can be seen that the reinforcing fibres are well distributed across the cross section of the tube and also at the location of the curve. The outside wall of the composite part is also smooth and exhibits no folds.

Claims

C L A I M S

1. Process for the manufacture of a tubular composite part whose longitudinal axis exhibits a curve and/or whose wall exhibits a variable cross section relative to the longitudinal axis in which process, a tubular semi-finished product, comprising an uncured or a partially cured resin and fibre reinforcement and further comprising an inflatable former, is introduced into a mould, a medium is brought into the inflatable former under pressure, the resin being cured and the composite part thus obtained being removed, characterized in that the fibre reinforcement comprises at least a knitted or braided sleeve.

2. Process according to claim 1, characterized in that the resin is chosen from the group consisting of unsaturated polyesters, vinyl ester resins, acrylate-urethanes, phenol-formaldehyde resins and melamine-formaldehyde resins.

3. Process according to claim 1 or 2, characterized in that the sleeve contains fibres that are chosen from the group consisting of glass, carbon, polyethylene and aramide.

4. Process according to any one of claims 1-3, characterized in that the flexible former consists of a balloon of flexible film with a circumference larger than the inside circumference of the sleeve.

5. Process according to claim 4, characterized in that the balloon consists of a polyamide film.

6. Use in a space frame of a composite part obtained with the aid of a process according to any one of claims 1-5.

7. Product obtainable with the process according to any one of claims 1-6.

8. Product according to claim 7, characterized in that the hollow composite part contains at least a portion of a tubular film on the inside.

9. Process or product as substantially set forth in the description and/or the example.