Apparatus for making components of large area by RTM

Details Apparatus for making components of large area by RTM Apparatus for making components of large area by RTM

1998-06-19

2001-07-10

Davis, Robert (Department: 1722)

Plastic article or earthenware shaping or treating: apparatus

Distinct means to feed, support or manipulate preform stock...

Female mold type means

C264S571000, C425S389000

Reexamination Certificate

active

06257858

ABSTRACT:

The invention relates to an apparatus and a method of making large-area components from fibre composites by injecting a resin mixture into a mould with interacting mould elements between which the preformed fibre layers are inserted, one element being resiliently deformable and the other dimensionally stable, the two elements being joinable in vacuum-tight manner at their edges, wherein the mould is provided with connections for means for injecting the resin mixture and a negative pressure source for lowering the internal pressure of the mould chamber below atmospheric, and wherein an external pressure above atmospheric is applied to the closed mould in a pressure vessel from all sides, and the resin mixture is injected into the mould at a pressure and until cured is kept at a pressure which is lower than the external pressure acting on the mould by a difference determining the volumetric fibre content of the component.
Processes of this type are also described as RTM (resin transfer moulding) processes.
Horst Wurtinger's article on “Impregnating methods for making large-area fibreglass/plastic components” in the journal “Kunststoffe”, volume 54 (1964) number 12, pages 797 to 803, describes a mould in which the connections for the means for injecting the resin mixture and the negative pressure source are arranged in the dimensionally stable mould member, in fact at opposing edges of the dimensionally stable mould member, and respective elongated grooves are arranged on the inside of that member, into which the connections discharge and which are used for uniform transverse spreading of the resin at both sides of the connections. Thus the resin, spread on both sides of the connection by the groove, is said to flow through the fibre layers, advancing in a front parallel with the groove. These grooves form on the surface as linear projections which have to be removed by abrasion.
Another article by Horst Wurtinger on “Production of large-area fibreglass/plastic parts by an injection process” in the journal “Kunststoff-Rundschau”, volume 9 (September 1970), pages 475 to 479, gives a pressure of 200 torr (corresponding to 270 hPa) as the lower limit for working at low air pressure in the mould, on the grounds that any further lowering of pressure leads to foaming of the resin and excessive bubble formation in the laminate.
U.S. Pat. Nos. 5,316,462 and 5,439,635 show another conventional version of the RTM process. Here the apparatuses work with sub-atmospheric against atmospheric pressure to make fibre-reinforced composite materials, i.e. without external pressure being applied separately. They have feed pipe elements and a strongly structured interior of the apparatus, which is juxtaposed with the composite to be formed. This structure, forming intersecting channels, improves the resin spreading. In this conventional version of the RTM process the resin flows through the fibre layers at right angles. For this purpose it is necessary and helpful first to spread the resin as evenly as possible over the whole available area before the first fibre layer, in order to obtain a uniform flow in all zones. Complicated shapes cannot be made in this way and there is always a danger of inclusions, shrinkage and uncontrolled flow directions in the resin. The resin advances along a ragged front and there is a very wide contact area from the resin to the sub-atmospheric pressure or vacuum region. The resin also overshoots the last fibre layer uncontrollably. The first and last fibre layer are extremely rough and have to be pulled off and/or smoothed as a general rule.
The above-mentioned advanced version of the RTM process operates with differential pressures as theoretically proposed by Wurtinger. Consequently the resin can flow parallel with the fibre layers rather than at right angles. This makes the front along which it advances small and clearly defined. It is unnecessary and in fact undesirable to spread the resin before it flows in. The problem with the very low sub-atmospheric pressures theoretically proposed by Wurtinger is however that, although these are desirable for large moulds with complex divisions, bubbling and foaming of the resin again occur at the contact zones and, although the inflow areas are small, trouble spots appear.
The problem of the invention is to develop the above-mentioned apparatus and the above-mentioned method further, so that even very large components can be made, e.g. wing shells for aircraft, of a great length of up to 25 m and more and with large volumes of fibre, including those provided with rib-like projections on the inside such as stringers and rib supports.
According to the invention the problem is solved in an apparatus, in that pipe elements, lying on the surface of the fibre layer arrangement opposite the dimensionally stable mould member, are provided for injection of the resin mixture and connection to the negative pressure source, the pipe elements having a length of the same order as the length or width of the large-area components and having a wide-area bearing surface, being provided with a cavity extending longitudinally of them above the bearing surface and being provided with a narrow slot in the bearing surface, which slot extends substantially over the whole length of the pipe element and is connected to the cavity along its length, the width of the slot being narrower than the diameter of the cavity.
The problem is solved in a method, in that the resin mixture on the surface of the fibre layer arrangement opposite the dimensionally stable mould member is injected by means of a linear gate from an elongated slot in a pipe element, the length of which is of the same order as the length or width of the large-area components, and a connection to the negative pressure source is provided by an elongated slot in another pipe element. the width of the slot being narrower than the diameter of a cavity in the pipe element.
Preferred embodiments are the subject of the other claims.
A special advantage of the method of the invention and the apparatus of the invention is that the dimensionally stable mould member, which determines the external surface, may be constructed without connections firstly to the means for injecting the resin mixture and secondly to the negative pressure source, so that no sprues form, and that the resiliently deformable mould member may also be constructed without such connections. When the resin mixture has cured the pipe elements are joined to the component by narrow stalks, which are broken off in the immediate vicinity of the surface of the component during removal of the pipe elements.
An embodiment of the invention is illustrated in the accompanying drawings and described below with reference to them. In the drawings:
FIG. 1
illustrates the principle of the method, by showing a cross-section through an autoclave with a mould inserted therein;
FIG. 2
is a graph showing the injection pressure, plotted against the injection time;
FIG. 3
is a cross-section through a pipe element;
FIG. 4
is a cross-section through a simple shell;
FIG. 5
is a plan view of an aircraft wing;
FIG. 6
is a cross-section taken along the line VI—VI in
FIG. 5
, and
FIG. 7
is an isometric representation of a point of intersection of stringers and a rib support.


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patent: 5015168 (1991-05-01), Boime et al.
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patent: 63-183831 (1988-07-01), None
patent: 3-162933 (1991-07-01), None

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