GB2485758A

GB2485758A - Method of forming a composite component by machining a frangible separation line

Info

Publication number: GB2485758A
Application number: GB201015951A
Authority: GB
Inventors: Andrew Chiverton; Bob Allinson
Original assignee: GKN Aerospace Services Ltd
Current assignee: GKN Aerospace Services Ltd
Priority date: 2010-09-22
Filing date: 2010-09-22
Publication date: 2012-05-30
Anticipated expiration: 2030-09-22
Also published as: WO2012038747A1; EP2618992A1; GB2485758B; GB201015951D0

Abstract

A method of forming a composite component comprises the steps of machining, or cutting, a stack of pre-impregnated composite material 2 to form a first portion 6 defining a component and a second portion 7 defining the excess material wherein the machining step separates the first and second portions but maintains their relative positions whilst the stack is cured. The machining step forms a separation line 5 corresponding to the outline of the desired component shape; during curing, resin may flow into the separation line to form a frangible line extending around the periphery of the component. A shear load may be applied to fracture the frangible line and separate the first and second portions providing clean, sharp edges and reducing the finishing process steps required for a given composite component. The machining step may substantially separate portions with the exception of a thin retaining portion of the pre-impregnated fibres. The machining step is preferably performed by ultrasonic cutting, drag cutting or automated laser ablation.

Description

Net Edge Method

Field of the Invention

The present invention relates to an improved method of manufacturing composite components. Specifically the invention provides a method which improves manufacturing efficiencies and reduces the finishing process steps required for a given composite component.

Background

Characteristics of composite materials have meant that composite components are employed in an increasing range of applications from aerospace to automotive parts.

In the aerospace industry for example composite materials have been used for a number of years owing to their strength to weight ratio. The term composite materials (known also as composites') are used to describe materials comprising for example glass fibre or carbon fibres and an epoxy resin (or similar). These are also known as glass reinforced plastic or carbon fibre reinforced composites. The carbon fibre reinforced composite material offers improved properties such as lower weight, improved fatigue/damage resistance, corrosion resistance and negligible thermal expansion.

The use of these materials has increased throughout the aerospace industry predominantly because of the fuel savings which can be achieved over the life of an aircraft by reducing the overall sum weight of the components making up the aircraft.

Aerodynamic as well as structural components are formed of composite materials and particularly carbon fibre materials.

However, a limitation in the use of composite materials is the manufacturing cost.

Composite components have to be layed-up using a cloth or the like pre-impregnated with resin into a stack corresponding to the desired shape of the part to be formed.

I

The stack is then cured either at ambient temperature and pressure or at elevated temperature and pressure in an autoclave to create a hardened component.

Industry standard machinery is required to cut the pre-impregnated cloth so as to correspond to the desired shape. Once the part has been cured the component is then frequently machined in a fmishing process to the desired final dimensions and tolerances. This requires additional manufacturing equipment and consequently increases the lead time of manufacture and also the overall manufacturing costs.

The present invention aims to provide a method of manufacturing a composite component which negates or minimises the need for edge finishing steps in the manufacture of composite components.

Invention Summary

According to an aspect of the invention there is provided a method of forming a composite component comprising the steps of machining a stack of pre-impregnated composite material to form a first portion defining the component and a second portion surrounding at least a part of said first portion; wherein the machining step separates said first portion from said second portion; and maintaining the relative --position of said first to said second portion of said stack whilst said stack is cured.

According to a method of the present invention, the machining step separates the desired component from the surrounding material of the stack. This surrounding material is termed trim allowance' and is a sacrificial part of the stack. Depending on the desired edge finish of the component the second portion may extend around all of the first portion or only a portion thereof.

The machining creates a small gap or separation between the first and second portion which is maintained until the cure step of the manufacturing process. As the stack is cured resin from the pre-impregnated cloth flows into the gap separating the first and second portions.

Thus, according to the method of the present invention a discontinuity is created in the resulting cured component. The component comprises a first and second portion of cured resin and fibre and a fracture or break line formed of cured resin void of any cloth or carbon fibres separating the two. The method defines a frangible divide S between the portion of the stack defining the desired component and the portion of the stack defining the trim allowance.

The stack, once machined, is positioned in the curing apparatus in the same configuration as on or in the machining apparatus i.e. the relative position of the fir st portion to the second portion is maintained.

Surprisingly the inventors have established that completely separating the first and second components (that is cutting through the cloth or the like fibres) using the machining step (and before the cure step) and only then curing the component is provides significant advantages for manufacture.

Advantageously the fracture line can conveniently be broken by the application of a force along the machined line. The cured resin is conveniently brittle resulting in a surprisingly clean fracture through the resin and along the fracture line. Thus, the --20 desired component can be separated from the trim allowance portion of the stack and exhibits clean and straight edges requiring minimal if any further edge finishing.

The carbon material may advantageously be a Bismaleimide (BMJ) resin which can be conveniently utilised in high temperature environments. This particular resin requires a second (post) cure to harden it. This resin system is advantageously harder and more brittle and this therefore facilitates the breaking of the resin post (second) cure.

Advantageously the method of the present invention not only provides a clean fracture between the two portions but additionally maintains substantially unifonn cross-section dimensions of the component right up to component's edge.

The method may be applied to a variety of composite materials. For example the composite may be an epoxy or Bismaleimide (BMI) resin system with a woven glass, carbon or Kevlar composite cloth.

Still further the present prevents delamination of the layers forming the stack when trim allowance is removed. Conventional finishing techniques are prone to cause delamination of the layers reducing the structural integrity of the component and the edge fmish thereof.

The inventors have recognised that machining' the component from a stack. in a conventional manner and then subsequently curing the component causes flow of the resin around the periphery of the part. The resulting part does not therefore have sharp and clean edges but rather is formed with curve or sloping edges caused by resin flow at the component edge. In many applications it is advantageous for a component to exhibit the clean edges provided by the present invention.

Still further the nature of the edge finish obtained using the present invention negates the need for complex and expensive post-cure machining. Typically components are fmished or cut from a cured stack using a multi axis drilling and rout machine.

Machinery-of this type is expensive to purchase adding to theuriit cost of component manufacture and also adds an additional time component to the overall unit production time.

Thus, according to an aspect of the present invention there is provided a method in which a composite component can be made of a predetermined shape with a line in of weakness in its structure corresponding to the shape of the part to be made. Applying a breaking force to this line conveniently fractures the resin providing a surprisingly clean edge finish requiring no or minimal edge finishing.

The separating or fracture load may be applied at any time after the part has been cured. Maintaining the trim allowance portion of the stack allows for convenient storage and transportation of the component before the desired component is separated from the trim allowance. This advantageously ensures the part is not damage.

The second portion of the stack may itself be sub-divided into multiple parts wherein each part has its own separation or fracture line. This advantageously allows the trim allowance to be broke away from the desired part in a number of steps. This is S particularly advantageous when the desired part has a complex profile since multiple sub-components can be broken away from the desired part leaving the part undamaged.

The sub-portions may be separated by a fracture line which does not separate the two sub-parts in their entirety. Specifically, the separation lines separating adjacent sub-portions of the second portion of the stack (the trim allowance portion) may be.

provided with a small portion where the cloth fibres are retained i.e. the two adjacent parts are machined to be separated with the exception of a small portion or tab having a cross-section substantially smaller than the cross-section of the respective separation line. Thus, the sub-parts of the trim allowance can be conveniently secured together additionally protecting the component and allowing the trim allowance to be removed as one piece.

The invention can be conveniently used with both autoclave resin materials and also out of autoclave materials. The latter materials cure at ambient or neat ambient conditions.

The machining step according to the present invention can be achieved using any suitable manufacturing process. It should be recognised that the term machining' is not limited to a particular process or machine'. For example, the machining step may be achieved using an ultrasonic cutting apparatus or a drag cutting apparatus. The machining step may also be performed by ablation, for example laser ablation.

Viewed from another aspect there is provided a method of forming a composite component, said component comprising a hole extending therethrough, said method comprising the steps of machining a stack of pre-impregnated composite material to form a first portion defining the component and a second portion defining a hole to be formed in said component wherein the first portion is separated from said second portion; and maintaining the relative positions of said first and second portions of said stack whilst said stack is cured.

Thus, according to another aspect there is provided a method of forming a component s comprising a hole. The trim allowance is therefore arranged to be surrounded by the desired component and the frangible separating line defmes the perimeter of the hole to be formed in the part.

The stack may of course be formed with both a trim allowance surround at least a portion of the component and a trim allowance defining a hole to be formed. In such an arrangement the stack,niay be provided with two frangible lines created. by separating the component from the inner and outer trim allowance portions before curing.

Viewed from another aspect there is provided a method of manufacturing a component comprising the steps of laying-up a stack of pre-impregnated plies to form a composite stack, forming a separation tine within said stack corresponding to a desired component shape and curing said stack to form a frangible line extending along said line and around the periphery of said component.

2O -. . ---Aspects of the invention extend to aerospace and aerodynamic component manufactured according to the method and so cosmetic or facia panels manufactured according to the method.

Brief Description of the Drawings

Embodiments of the invention will now be described by way of example only with reference to the accompanying drawings in which: Figure 1 shows a laminate stack in a pre-cure condition; Figure 2 shows a stack and cutting machine and the resultant component shape; Figures 3A and 3B show a separation line between the two stack parts and the resulting post-cure frangible line; Figure 4 shows a method of separating the trim allowance from the component; Figure 5A shows a component with a sub-divided trim allowance; Figure 5B shows a retaining tab connecting adjacent trim allowance portions; Figure 6 shows a component comfrising ahole; and --Figure 7 shows a method of formed a component with a hole formed therein.

While the invention is susceptible to various modifications and alternative forms, specific embodiments are shown by way of example in the drawings and are herein described in detail. It should be understood however that drawings and detailed description attached hereto are not intended to limit the invention to the particular form disclosed but rather the invention is to cover all modifications, equivalents and alternatives falling within the spirit and scope of the claimed invention.

Detailed Description

Figure 1 shows a series of pre-impregnated layers 1 forming a stack 2. The dimensions of the stack are selected so as to provide sufficient material to surround the desired component, discussed below.

The material forming the stack may be any suitable pre-impregnated cloth material.

For example this process can be applied to any pre-impregnated material with either a fibre of carbon, glass or Kevlar impregnated with an epoxy resin or BMI resin system.

The stack may be formed of any suitable number of layers, each layer being optionally layed at a particular angle relative to the adjacent layer(s) dependent on the application. Additionally an optional peel ply layer may be applied to one or both outer planar surfaces. This may advantageously provide a mall finish to allow for painting of the part for example.

The process of laying-up the layers to form the stack is well know to the skilled person in the art and will not therefore be explained.

--

Figure 2 shows the stack 2 and a suitable cutting machine 3. This may for example be an ultrasonic cloth culling machine. The culling head is controlled to cut a path 5 through and along the composite stack 2. The path in effect divides the stack 2 into a first portion 6 corresponding to the desired component and a second portion 7 which is a sacrificial portion of the stack called a trim allowance portion.

As shown in Figure 2 the trim allowance portion has been divided into two portions by a further cut 8 extending to the periphery of the stack.

Once the component shape has been formed in the stack the stack is then moved to the cure apparatus. This might for example be an Autoclave, Oven, via heated tooling or heater blankets. In each case a vacuum would be applied to withdraw volatiles as is known in the art. in the case of an Autoclave an external pressure may also additionally be applied to improve consolidation of the component. The positions of the first and second portions of the stack are maintained within the curing apparatus i.e. the first portion defming the part remains surrounded by the second portion defining the trim allowance of second portion of the stack.

The curing process is well know to the person skilled in the art (both autoclave and out of autoclave) and will not therefore be explained here.

Figures 3A and 3B illustrate the separation line formed by the cutting process before and after the curing process. Figure 3A shows the separation formed by the cutting (alternatively termed machining) step of the method.

In Figure 3A it can be clearly seen that the machining step involves cutting through the entirety of the stack 2 i.e. to cut through all of the cloth material that constitutes the layers. is

Figure 3B shows the same cross-section after the cure step where the resin has been consolidated during the cure to flow and harden. The separation step creates a cavity 9 into which the resin can flow during the cure step. As the resin flows the cavity is filled up with resin which then hardens to form a frangible portion 10 or line extended arounthheathf hediiytha ttingtp: Figure 4 illustrates one method of separating the component 6 from the trim allowance 7. As shown the stack 2 has been cured and now comprises the frangible line 10 extending around the component and to the exterior of the stack.

The strength of the frangible line 10 depends on the resin used during the cure but will consistently be weaker than the component and trim allowance portions which each comprise consolidate resin and fibres (either carbon or glass).

To separate the component force 11 is applied to the trim allowance portion 7 whilst the part is restrained. The force causes the frangible line to snap releasing the trim allowance from the component. It will be recognised that the component could be removed from the trim allowance in a variety of ways using manual or automated techniques.

Breaking the brittle resin in this way results in a surprisingly clean separation of the two parts requiring no or minimal surface finishing such as deburring. This advantageously rninimises non-recurring costs for component manufacture and negates the need for edge finishing processes that might cause component damage.

Figure 5A shows another example of a component 6 and trim allowance 7. Here the trim allowance has been sub-divided into 4 portions 7A to 7D.

As shown in figure SB and as illustrated by the tab markers 12 in figure 5A the machining step has retained a small tab 13 connected adjacent trim allowance portions.

Thus, in this arrangement the frangible path 10 created during the cure step has interruptions 13 where a small portion of fibre is permitted to connect the adjacent trim allowance portions. It will be recognised that the tab 13 need only be small compared with the overall cross-section of the frangible line having a suitable cross-section sufficient to connect the two adjacent portions of trim allowance together without fracture. As shown in Figure 5B the tab 13 is small compared to the overall size of the cavity 9 prior to curing.

-Figure-6 shows a-more complex *shape manufactured according to the present -invention. Here the component 6 comprises an integral hole 14 within its boundary.

In this arrangement the component is itself machined to create a frangible line 15 within its boundary. As additionally shown the outer frangible line 9 defines the outer periphery of the part and in the inner frangible line 15 defines the inner boundary i.e. the hole.

The component can be separated from the trim allowance by breaking the frangible line 9 around the perimeter. To remove the central hole defining stack portion 16 the frangible line 15 can be broken by punching the portion 16 out to create the hole 14.

Alternatively the portion 16 may itself be provided with a central hole and further comprise frangible lines extending from the line 15 to the inner hole so that the edges can be broken off as described above. This is illustrated in Figure 7.

As shown in Figure 7 the inner hole portion 16 is surrounded by a first frangible line which can be broken to release the central portion 16. The central portion (shown as a hatched area) is itself provided with a sub hole 17 formed by the machining step.

The edges of this portion serve no purpose for the component i.e. this portion is also sacrificial. Four further frangible lines 1 8A to 1 SD are provided extending to the corners of the portion 16. Once the stack has been cured these lines 1 8A to I SD can themselves be fractured together with the frangible line 15 to create a net fmished inner hole exhibiting the same improved edge surface characteristics of the outer surface as described above.

Claims

Claims 1. A method of forming a composite component comprising the steps of: machining a stack of pre-inipregnated composite material to form a first portion defining a component and a second portion surrounding at least a part of said first portion; wherein the machining step separates said first portion from said second portion; and maintaining the relative positions of said first and second portion of said stack Jo whilst said stack is cured.
2. A method according to claim 1 further comprising the step of applying a load to the portion of the cured stack connecting the first and second portions so as to separate said first portion from said second portion.
3. A method of claim 2, wherein the load is a shear load.
4. A method according to any preceding claim wherein the second portion is also sub-divided into a plurality of sub-portions each machined prior to the cure step.--
5. A method as claimed in claim 4, wherein the sub-portions are machined such that each sub-part is substantially separated from an adjacent sub-part with the exception of a retaining portion.
6. A method as claimed in claim 5, wherein the retaining portion is formed by machining the second portion to define the sub-portions whilst retaining a portion of the pre-impregnated fibre to link adjacent sub-portions.
7. A method according to any preceding claim wherein stack is cured in an autoclave.
8. A method according to any of claims 1 to 6 wherein the stack is cured out of autoclave.
9. A method according to any preceding claim wherein the machining step is performed by means of one of: ultrasonic cutting, drag cutting or ablation.
10. A method according to claim 9, wherein the ablation is performed by means of automated laser ablation.
11. A method according to any preceding claim further comprising the step of de-.burring the peripheral edge of the cured component.
12. A method of forming a composite component, said component comprising a hole therethrough, said method comprising the steps of machining a stack of pre-impregnated composite material to form a first portion defming the component and a second portion defining a hole to be formed in said component wherein the first portion is separated from said second portion; and maintaining the relative positions of said first and second portions of said stack whilst said stack is cured.
13. A method according to claim 12, wherein the method further comprises the step of removing a portion of said second portion within the boundary defined by the -20 -machining-steji----
14. An aerospace component formed according to the method of any preceding claim.
15. A method of manufacturing a component comprising the steps of laying-up a stack of pre-impregnated plies to form a composite stack, forming a separation line within said stack corresponding to a desired component shape and curing said stack to form a frangible line extending along said line and around the periphery of said component.
16. A method as substantially described herein with reference to theaccompanying figures and description.