Introduction to Advanced Composites and Prepreg Technology

Printable copy (SM1010)

General

This page has been prepared to act as a general introductory guide for anyone with limited knowledge of the advanced composites industry. The aim of this guide is to familiarise the reader with advanced composites, but particularly prepreg technology.

Introduction

The terms ‘advanced composites' and 'prepreg' have become accepted as the generic names for a generation of materials manufactured from high performance fibres pre-impregnated with a suitable resin matrix. So what are advanced composites or prepregs used for? They are used to manufacture components, structures and tooling by means of heat and pressure, although the Group's LTM resin systems allow the manufacture of high performance composites and structures at low temperatures.

Advantages of Composites (Prepregs)

'Advanced composites' or 'prepregs' offer engineers the freedom to design structures of optimum performance.

Composites have several advantages over conventional metallic alloys. The most significant of these are:

• Due to the high specific modulus and strength of the materials, very strong and stiff structures can be designed, with substantial weight savings.
• The ability to align the fibre orientation with the direction of principle stresses and, therefore, achieve high structural efficiency.
• Exceptional environmental degradation and corrosion resistance properties.
• Very low coefficient of thermal expansion, with the added possibility of designing the material to give desired thermal expansion in a particular direction.
• Improved vibration damping properties.
• Easy repairability of damaged structures.
• Ability to manufacture complex shapes at lower costs compared with fabricated or machined metallic alloys.
• Time and cost reductions on tooling and manufacturing of one-offs, prototypes and short length production runs.
• Excellent fatigue life, i.e. carbon fibre composites can be designed to be essentially fatigue free.
  Improved energy absorbing safety structures.

The following diagram provides a comparison of several material characteristics. It can be seen that advanced composites provide the advantages of lower weight, greater strength, and higher stiffness.

Reinforcement Properties

Fibre Properties

• Carbon:
  These fibres are available in high strength, intermediate modulus, high modulus, and ultra high modulus grades. They are mostly used in high strength, high stiffness applications where the benefit from weight saving outweighs the additional material cost.
  
  
• Glass:
  Most commonly used is E-glass, with S-glass and Quartz being lesser used, but higher strength alternatives. These are used on weight critical items where the cost of the item is of equal importance to the weight saving. Glass is much denser than carbon and has lower strength and stiffness values.
  
  
• Aramid:
  Most commonly known as Twaron™ or Kevlar™. Aramid is normally applied to areas or components where there is a likelihood of an impact. Aramid has the ability to absorb and dissipate energy and has excellent abrasion resistance.
  
  
• Polyethylene:
  Better known as Dyneema™ or Spectra™, Polyethylene is normally used in impact areas. Polyethylene has similar energy absorption characteristics to Aramid fibres, but is a much lighter, although more expensive fibre than Aramid. Polyethylene has little or no compressive strength and begins to shrink and lose its properties at a fairly low temperature (around 100°C).
  

Fabric Properties

Fabrics consist of fibres in at least two directions. Fibres running along the length of a roll are called warp fibres, and those across the width are called weft fibres. These fibres are interlaced with each other in varying configurations to give different fabric styles. The following sketch shows how fibre orientations relate to the weave of the cloth.The more commonly used fabric styles are as follows:

• Plain Weave

A plain weave fabric is where warp fibres are interlaced each time they meet weft fibres. This is achieved by passing then alternatively above and below each other. While the resulting fabric is very stable, it can be difficult to distort and get it to conform to sharp profile changes.

 

 

 

 

 

• Twill Weave

In ‘twill’ weaves the fibres pass over and under a number of fibre bundles, e.g. a 2 x 2 twill would have fibres passing over two bundles and then under two bundles. Subsequent fibres are offset by one fibre bundle which creates a ‘herring bone’ or diagonal pattern on the cloth. Twill weave fabrics have a much more open weave that lends itself to being distorted and draped to readily conform to a required profile. This weave style is fully balanced both top and bottom and, therefore, does not require inverting in a multiply laminate.

 

 

• Satin Weave

The construction of a satin weave fabric is such that a fibre bundle passes over a number of fibre bundles (the exact number depending on the type of satin weave) and then under one fibre bundle. This produces a much flatter fabric that can be readily easily distorted or draped to suit complex surface profiles. However, due to its construction, stain weaves are one sided or unbalanced (fabric with one side consisting of mainly warp fibre whilst the other is mainly weft). The result of this imbalance can be distortion in the item being manufactured. It is normal practice to invert half the plies within the laminate.

 

• Stitched Multiaxial

The construction of a stitched multiaxial consists of several layers of unidirectional fibres in different orientations, i.e. 0°, +45°, -45°, 0°. These layers are then stitched together to form a fabric that can be handled in very much the same way as a woven cloth. Stitched multiaxial fabric can be obtained in heavier aerial weights that are usually not practical or economical with woven cloth. Multiaxials exhibit excellent drape characteristics and can readily conform to complex shapes, with the added advantage of rapidly building up the required laminate thickness. One drawback with multiaxials is that extreme care has to be taken to ensure a balanced laminate is obtained and that the weight/thickness can prove difficult to tailor around fine details.

Matrix Properties

The matrix supports and bonds the fibres in a composite. It keeps the fibres in the correct position and orientation, and transfers applied loads to the fibres. The matrix normally governs the maximum service temperature of a composite, but one exception to this is Polyethylene fibres, which have a temperature limit of approximately 80°C.

The following matrices are commonly used in advanced composites:

• Epoxy

These are available in many different forms and can be processed using numerous techniques, such as wet lay up, vacuum oven/autoclave, press moulding, resin transfer moulding, filament winding and pultrusion. They have excellent mechanical properties, good environmental resistance and high toughness. They can be formulated to suit many applications, providing characteristics such as high service temperature, impact resistance, improved hot/wet mechanical performance, and fire retardancy.

• Phenolic

This type of resin system is most commonly used where fire resistance/prevention/low smoke and toxicity outweigh all other criteria (e.g. aircraft interior panels). Phenolic resins exhibit excellent fire resistance characteristics and are relatively cheap. Phenolic resins can be difficult to process, with some acid catalysed variants attacking composite tool surfaces. Compared to epoxy systems, phenolic resins have lower mechanical properties.

• Bismaleimide (BMI)

These are relatively expensive systems, but they have excellent mechanical properties at elevated service temperatures. Bismaleimide resins can be difficult to process due to their inherently high cure temperatures and the low viscosity achieved during curing.

• Cyanate Ester

Cyanate ester resins have the capability to retain their mechanical properties at extremely high service temperatures (up to 350°C), but they are also expensive. These systems can absorb water, which can give rise to problems with blistering. Processing is similar to that applied for epoxy resin systems.

Fabrication and Production Processes

As mentioned, the production processes for the manufacture of components from advanced composites generally requires two elements.

• Heat to activate the curing mechanism in the resin system and also to reduce the resin viscosity, which assists flow in the case of prepreg composites.
• Pressure to compact and consolidate the laminate being moulded.

There are many methods available for generating heat and pressure with adequate control. Some of these methods will be outlined in this document.

In all these applications, before moulding can commence, the material must be laid up with controlled fibre orientation either on or in a mould tool.

Vacuum bagging techniques have been developed for fabricating a variety of components, but mainly for complex shapes, double contours and relatively large components. The technique is employed either to consolidate a wet lay-up or a prepreg lay-up during cure. The process is principally suited for moulding low cost components too large and/or complex to be pressurised by other means. The technique utilises a flexible sealed bag under which a vacuum is drawn, hence applying an even pressure up to 1bar (14psi) to the lay-up in the mould. The assembly is then heated in an oven to promote the resin flow and curing processes. This method, which requires low cost equipment and tooling, is capable of producing advanced composite components of reasonable quality, ones that are acceptable for many applications.

An autoclave is simply a large, heated pressure vessel. Autoclave moulding is similar to the vacuum bag process with the exception that the lay-up is subjected to additional pressure, usually up to 7bar (100psi), whilst heat is applied to cure the resin. Vacuum is usually applied during the initial stages of the cure cycle to remove volatiles and trapped air without causing excessive resin flow. Autoclave pressure may be maintained during the entire heating and cooling cycles.

The pressure exerted on the lay-up is normally within the range 3.5 to 7bar (50 to 100psi). The autoclave moulding process produces laminates of high quality with minimum void content, and control of laminate thickness is much better than that achieved by the vacuum bag moulding method. The capital equipment costs are high, however, and the output relatively low, which restricts the use of the autoclave moulding process to higher cost markets where high quality is essential. Many large primary structural components for aircraft, such as fins, wing spars and skins, fuselages and flying control surfaces, are manufactured by this method, as it guarantees reproducability.

• Press Moulding

As with vacuum bag and autoclave moulding, the prepreg fabric is laid into the mould, or pre-formed into a separate, shaped charge to facilitate rapid loading in a hot tool.

The moulds/tools are usually manufactured from machined or cast metal, and are produced as matched male and female halves, the space between them defining the shape and wall thickness of the component being made.

Consolidation pressure is generated hydraulically and the cure cycle may be controlled by various means of heating.

The tool may be heated directly by electricity, oil or steam, with passages being built into the tool in the case of direct oil or steam heating. When using electricity, cartridge heaters may be installed within the mould. On small components, the heat may be supplied via heated platens in the press itself.

Very accurate moulding tolerances are achieved using this method and a high degree of automation can be applied. Cure cycles can also be very accurately set and controlled, so the process can produce components of very high quality and consistency.

Due to the high costs of capital equipment and tooling, this method is best suited to high volume production. Less expensive nickel electro-formed, glass fibre or sprayed metal tooling can be used for short production runs.

• Pressure Bag Moulding

This method is an extension of vacuum bag and autoclave moulding and uses a rubber bag, often silicone, on top of the lay-up. A hot, compressed gas or fluid can be used to apply heat for the curing process and pressure for consolidation. The process may also be vacuum assisted. This method is often applied to simple sections such as tubes.

• Filament Winding

In this process continuous fibres or tape are wound onto a rotating mandrel. The fibres are fed via a translating head with an accurately controlled fibre feed angle to the axis of the rotating mandrel. Resin can be drip dispensed at an accurate rate onto the fibre, or the fibre can be passed through a resin bath. Tape can also be introduced in prepreg form. Consolidation pressure is achieved through tensioning the fibres as they are wound onto the mandrel.

Examples of products manufactured by this method include power transmission tubes (torque shafts), control rods and landing gear struts. Other applications include pressure vessels, rocket motor cases and oil well lining tubes.

• Thermal Expansion Moulding

Thermal expansion moulding is generally used to mould integrally stiffened structures with complex forms. Prepreg layers are wrapped over blocks of rubber or foam and the lay-up assembly is restrained in a metal cavity. The assembly is then heated. As the temperature increases a high differential thermal expansion takes place between the metal and rubber, i.e. the rubber expands much more than the metal. Since the metal is restraining the assembly, very high pressures are generated which consolidate the lay-up.

This method requires very little capital equipment, and the tooling is simple and low cost. Components with very complex shapes can be moulded in a single cure cycle, thus reducing the number of joints and parts, and therefore significant weight and production cost savings can be achieved.

• Pultrusion

This method is an equivalent of plastic extrusion. Low cost constant cross sections such as top hats, round and flat bars, channels and Z-sections up to 12m long may be produced. The process is carried out by pulling dry aligned fibres or sewn cross-plied or fabric tapes through a resin bath, and then through a heated die. Alternatively prepreg tape can be used, but this is not common due to the increased costs involved.

Capital equipment costs are rather high for buttressing but since it is for high production rates the system/part cost is low. It is difficult to pultrude epoxies, hence release agents are added to the resin which can result in a variable end product.

Manufacturing With Prepregs

• Introduction to Prepregs

A prepreg consists of a reinforcement material pre-impregnated with a resin matrix in controlled quantities. The resin is partially cured to a B-stage, and in this form is supplied to the fabricator, who lays up the finished part and completes the cure with heat and pressure. The required heat and pressure will vary with the resin system and the intended application.

• Manufacturing Prepreg

• Vacuum Bag and Autoclave Processing

• Processing Parameters

The cure cycle is a process where the resin within the prepreg is changed from a liquid to a solid by the application of heat. There are a number of stages to this cycle.

• Cure Temperature/Time

For each prepreg resin system there is a range of options for cure temperature/duration, and there is also a minimum cure temperature. For each given cure temperature there will be a corresponding cure time. The oven/autoclave, the component and the tooling should all reach and remain above the given cure temperature throughout the specified cure cycle. Thermocouples are used to monitor the temperature of the component and tooling.

• Heat Up Rate

The heat up rate dictates how quickly the component/tool is brought up to the cure temperature. This is governed by numerous factors: matrix viscosity and reaction rate, thickness of laminate, and tool mass and conductivity. For highly reactive matrices and thick laminates, the heat up rate will be low in order to avoid exothermic reactions.

• Cooling Rate

The cooling rate is controlled in order to avoid sudden temperature drops that may induce high thermal stresses in the component.

• Vacuum/Pressure

At specific times throughout the cure cycle, vacuum and pressure (autoclave only) are applied and removed.

Low Cost/Low Temperature Oven Construction

Advanced Composites Group produces a range of LTM prepregs that cure at low temperatures, making it possible to manufacture very cheap temporary ovens. This type of oven allows a very cost effective method of producing numbers of very large components or tools, which might otherwise be prohibitively expensive and would permanently occupy too much production space.

• Type 1

This consists of the commonly sold ‘poly tunnel’ type of ‘greenhouse’ construction. It is, essentially, a series of metal tubes that have been formed into a semi-circular arch. These are then covered with a layer of polythene sheeting, which is then clad with a layer of insulating material.

Possible materials that could be used for insulation are:

• tube of polythene filled with fibreglass loft insulation

• specially prepared insulation blankets

• continental quilts

Depending on the size of the oven, the temperature required and the output of the available heaters, the oven should be heated by one or two fan-assisted space heaters. These heaters should be positioned such that they do NOT blow hot air directly onto the item being cured.

• Type 2

The construction of a type 2 oven comprises a simple wooden frame covered in polythene film. This is then covered with an insulating layer. Due to the shape, this type of oven can make use of slabs of insulating material such as ‘rockwool’. Heating and air circulation are as the Type 1 oven. With the addition of some cheap plywood, the oven can be made into a fairly robust and semi-permanent facility.

Master Models

The final component will dictate what type of tool will be required, which in turn will dictate the type of master model and the material used to produce this master model. The final component cure temperature, surface finish, size, tolerances required, number of tools required, and life expectancy of master must also be considered.

The following lists the various materials available for master model construction together with their associated advantages/disadvantages:

Wood:

• Only usable at low temperature.
• Easily hand worked.
• Low material cost.
• Very labour intensive.
• Not accurate.
• Pattern susceptible to movement due to warping through moisture loss or ingression.
• Moisture can inhibit prepreg curing.
• Different properties along and across the grain.
• Not generally suitable for autoclave moulding.
• Requires sealing to produce suitable moulding surface

Headers/Infill:

• Relatively cheap construction.
• Will not endure excessive pressure/temperature.
• Unstable.

Urethane Tooling Block:

• Different properties along and across the grain.
• Not generally suitable for autoclave moulding.
• Requires sealing to produce suitable moulding surface.
• More accurate.
• More stable than wood in the short term.
• Unstable in the long term due to uptake of moisture.
• Requires surface sealing.
• Cure inhibition problem with epoxy prepregs.
• Pre-release treatment or gel coated tools required to avoid the reaction problem.
• Heavy fabricated support structure required for large models.
• Capable of withstanding autoclave cures.
• Can be machined or hand worked.
• 60°C maximum service temperature.
• Very high Co-efficient of Thermal Expansion (CTE).

Epoxy Tooling Block:

• More expensive than wood/urethane.
• Surface coating only required for high gloss finish, no reaction problems with ACG tooling blocks and Low Temperature Moulding (LTM) prepregs.
• Very accurate and extremely stable over long periods.
• Care required with heating/cooling rates to prevent thermal shock.
• Capable of withstanding autoclave cures.
• Large pattern requires adequate support structure.
• Typically 5-axis CNC machined.
• 120 to 130°C maximum service temperature.
• High CTE.

Metal:

• Can be steel/aluminium cast and/or fabricated, as well as electrodeposited.
• Excellent surface finish, no requirement for surface sealing.
• Extremely heavy, requires heavy lifting equipment for large pattern.
• Large thermal mass to heat up and cool down.
• CNC or machining is expensive and slow, robust machine required.
• Adequate long-term stability if protected from corrosion and correct grade used.
• Cast metals can be porous, welded joints can leak (resulting in a non-vacuum integral master and resultant tool).
• Long lead-times.

Tooling

• Coefficient of Thermal Expansion Considerations

To exert the maximum control on the dimensions of the curing component, the mould tool material’s growth rate needs to be predicted and controlled. The use of composite tooling whose fabric reinforcement matches most closely the reinforcement materials of the curing component is the simplest means of exerting maximum control on differential dimensional changes during cure. The table below gives comparative coefficient of thermal expansion (CTE) values for the most commonly used tooling materials. Where there is a marked mismatch between the CTE of mould tool and component material, bridging of the component material can arise during heat-up and distortion of the part can occur during cool down.

• Thermal Limit

The ability of the mould tool material to withstand repeated thermal cycling at elevated temperatures is most important. With metallic mould tools this is in most cases not an issue. The following table provides an indication of the safe thermal limits of common tool materials.

• Additional Fixtures

The manufacture of the component mould tool is one operation that has to be undertaken to enable a composite component to be successfully manufactured. Additionally, there may be a requirement to provide tool edge bars, trimming fixtures, drilling jigs, checking fixtures, and assembly jigs. If laminated composite tooling is used, all such tools and jigs may be produced with relative ease from a single master model, and by adjusting the cure temperatures used in their manufacture, it is possible to make them dimensionally accurate at different end use temperatures. However, this is not always the case with other mould tool materials.

Prepreg Nomenclature

• Fibre Areal Weight (FAW) The weight of the fabric used in a prepreg (g/m2).
• Fibre Volume Fraction (Vf) The percentage, by volume, of fibre in the prepreg.
• Flow The ability of resin to move under pressure, allowing it to fill all parts of a laminate.
• Gel Time The time required at a given temperature for a resin to progress from a liquid to a solid, indicated by a rapid increase in resin viscosity.
• Glass Transition Temperature (Tg) The temperature at which a phase change occurs in the matrix. This gives an indication of the maximum end application temperature.
• Resin Weight (RW) The percentage, by weight, of resin in the prepreg.
• Shelf Life The length of time the prepreg can be stored under specified conditions and continue to remain suitable for its intended function.
• Tack Measurement of the capability of an uncured prepreg to adhere to itself and to mould surfaces.
• Tack Life The length of time the prepreg can be stored at room temperature (20°C) and continue to have sufficient tack.
• Viscosity This, which is a measurement of the flow characteristics of a resin, is influenced by temperature and heat up rates. A product with a low viscosity is one which pours freely, e.g. water, while one with a high viscosity is one which exhibits a resistance to flow, e.g. syrup.
• Void Content This is the percentage, by volume, of voids in a cured laminate.
  Volatiles These are materials, such as water or solvents, which are capable of being driven off as a vapour at room or elevated temperatures.

Prepreg Storage and Safety

Prepregs should be stored, wrapped and sealed in polythene, at -18°C for maximum shelf life. The material must be fully thawed before breaking the polythene seal in order to avoid moisture contamination.

Prepregs are low-risk in terms of handling hazards, but the usual precautions should be applied. Gloves and protective clothing should be worn. Use mechanical exhaust ventilation when heat curing prepreg systems.

Material Safety Data Sheets (MSDSs) are available for each ACG product.

Vacuum Bagging

Vacuum bagging is used to consolidate the laminate and to remove air and volatiles from within the laminate structure. A composite laminate fabricated from prepregs should be subjected to vacuum pressure at various stages during the lay-up procedure. This process, called ‘debulking’, ensures that all the plies laminated so far are properly consolidated and conform to the mould profile. Any bridging at corners will become a lot more obvious and hence can be rectified. Debulks also ensure that subsequent plies do not compound any slight bridging problem, and having consolidated the previous laminated plies, that they are long enough to reach fully into female features. After a laminate lay up has been completed it is vacuum bagged for final cure.

Consumables for Vacuum Bagging

The principle of vacuum bagging a composite laminate is the same whether you are debulking or bagging for final cure. The main differences occur within the consumable pack that is used. This consists of a combination of the following materials.

Reusable rubber bags are often used in place of the disposable bag. These types of vacuum bag are obviously more expensive than disposable bagging film, but they are cost effective on production items because they are not as time consuming to apply, and they will last, if treated correctly, for a large number of cure cycles.

Following are the procedures and consumable packs to be used for different processes utilising vacuum bagging. Vacuum infusion processes have not been included due to the proprietary nature and licence requirements of these processes.

• Vacuum Bag for Wet Lay-Up

Optional consolidation for high quality laminates made by wet lay up method.

1. Ensure the laminate preform is well consolidated and there is no evidence of bridging.
2. Cover the preform with peel ply, overlapping the preform by 25 to 50mm.
3. Cover the preform with a layer of pin-pricked release film, overlapping the preform by 25 to 50mm. On parts requiring more than a single width of release film, it is appropriate to create an overlap of 50mm and secure the film in position using small pieces of flash tape.
4. Cover the whole surface with a layer of lightweight breather extending up to but avoiding contact with the sealant tape. Cut and tailor the breather as required to avoid bridging. The specific weight or number of layers of breather required will depend on the amount of resin required to be bled from the laminate.
5. Lay sufficient bagging film over the assembly to enable it to conform to the component/tool without bridging. Use pleats and tucks as required. Seal the edges of the bag onto the sealant tape. Note: For simple small parts, the tool may be envelope bagged.
6. Position vacuum fittings ensuring the vacuum pump fitting is the greatest distance from the vacuum gauge fitting and that all the fittings are off the component. Use additional breather under vacuum fittings to ensure contact with main breather ply. For large or complex tools, a minimum of one vacuum port per m2 will be required.
7. Apply vacuum slowly, adjusting bagging as required to prevent bridging and obtain a minimum of 710mm (28in) Hg.

Vacuum Bag for Debulk

1. Cover entire laminate +50mm overlap with pin-pricked release film and tailor around all details, ensuring release film is not bridged.
2. Cover release film with one layer of peel ply. Cut and tailor around all details. Where required hold in place with flash tape, again ensuring no bridging.
3. Cover peel ply with one layer of lightweight breather ensuring it extends to beneath proposed vacuum connectors and 25mm past peel ply. Cut and tailor around all details. Where necessary hold in position with flash tape.
4. Position sealant tape around periphery of laminate onto tool/pattern surface.
5. Position bagging film over the lay-up ensuring sufficient excess to avoid bridging around details.
6. Seal bagging film around periphery positioning vacuum ports off the laminate area, or into a tuck in the vac bag if flanges are not large enough to accommodate vacuum ports.
7. Slowly draw a vacuum, adjusting the bag to avoid bridging.
8. Check vacuum ports and bag for leaks, and seal if necessary.
9. Connect one vacuum port to a gauge and attain a minimum of 660mm (20in) Hg vacuum. If bridging is observed, partially vent vacuum and re-apply after adjustment to the bag.
10. Re-check the bag to ensure there is no bridging.
    When vacuum pressure holds 660mm (26in) Hg minimum connect the vacuum pump and leave under vacuum pressure for 20 minutes maximum.
11. After the specified time disconnect the vacuum pump, remove the vacuum gauge and remove the consumables pack for subsequent vacuum debulks.

If a laminate was left for a long period the above procedure would utilise a solid release film as opposed to a pin-pricked one. This would avoid resin loss and prevent the breather sticking to the laminate.

Vacuum Bag for Autoclave or Simple Oven Cure

1. Ensure the laminate preform is well consolidated and there is no evidence of bridging.

2. If required, cover the preform with peel ply, overlapping the preform by 25 to 50mm.

3. Cover the preform with a layer of non perforated release film, overlapping the preform by 25 to 50mm. On parts requiring more than a single width of release film, it is appropriate to create an overlap of 50mm and secure the film in position using small pieces of flash tape.

4. Place single glass tows at 0.5m intervals around the laminate under the release film, and extending beyond it by a minimum of 15mm. If the release film overlap is less than 25mm, seal the edges with flash tape ensuring the strings extend past the tape.

5. Cover the whole surface with a layer of heavyweight breather, extending up to but avoiding contact with the sealant tape.

6. Cut and tailor the breather as required to avoid bridging.

7. Lay sufficient bagging film over the assembly to enable it to conform to the component/tool without bridging. Use pleats and tucks as required. Seal the edges of the bag onto the sealant tape. Note: For simple small parts, the tool may be envelope bagged.

8. Position vacuum fittings ensuring the vacuum pump fitting is the greatest distance from the vacuum gauge fitting and that all the fittings are off the component. Use additional breather under vacuum fittings to ensure contact with main breather ply. For large or complex tools, a minimum of one vacuum port per m2 will be required.

9. Apply vacuum slowly, adjusting bagging as required to prevent bridging and obtain a minimum of 50mm (28in) Hg.

10. Disconnect from the vacuum pump and check the leakage rate does not exceed 50mm (2in) Hg per 10 min. Do not proceed with the cure until this test has been completed successfully.

11. Repeat this vacuum integrity check when the assembly has been connected inside the autoclave.

Laminate Design

• Fibre Orientation

The use of advanced composites, particularly prepregs, provides the ability to achieve accurate control over fibre orientation. By aligning fibres with the direction of principal stresses provides a high degree of structural efficiency, cutting down on material where it is not required. In turn this can bring about large weight savings.

• Stiffened Panels

Thin laminate panels can be stiffened by the use of a number of different elements. These include Z, I, J, and top hat stiffeners.

• Sandwich Construction

Using sandwich construction in the design of components substantially increases the stiffness of the structure with very little weight increase. Thin high strength materials such as carbon fibre are bonded to a suitable low density core material. This gives a similar result to an I-section beam, where the maximum amount of material is placed in the most effective location to provide bending stiffness, or stability under compressive loading. Although there are a number of core materials available, advanced composite sandwich structures mainly use honeycomb cores of metallic or non-metallic nature.

• Properties of a Sandwich Construction

Core Materials

A wide variety of core materials exist, with varying costs, structural properties, and temperature capabilities.

• Balsa
  • Usually end-grain, and supported by a fibre tissue. Balsa exhibits high compressive properties, as well as good thermal and acoustic insulation. However, it has a high density, and it can absorb large quantities of resin if not pre-sealed.
• Foam
  • PVC (polyvinyl chloride); Exhibits a good balance of static and dynamic properties as well as resistance to water absorption. It has an upper temperature limit of approximately 80°C, and is resistant to many chemicals. It is the most commonly used foam in advanced composites.
  • PS (polystyrene); Low mechanical properties. Not commonly used.
  • PU (polyurethane); Moderate mechanical properties. Can experience deterioration at foam/skin interface with time. Commonly used as fill in stringers.
  • Acrylic; Best mechanical properties of existing foams. Higher temperature capability, but expensive.
  • SAN (Styreneacrylonitrile); Similar to PVC, but tougher.
  • PEI (Polyetherimide); Thermoplastic foam with excellent fire resistance. Temperature capability up to 180°C. Expensive
• Honeycomb
  • Aluminium: Provides one of the highest strength/weight ratio of any core material. Low cost. Potential corrosion problem if used in conjunction with carbon skins.
  • Nomex: High mechanical properties. Good fire resistance. Expensive.
    Thermoplastic: Numerous types exist. Relatively low stiffness, and there is a difficulty in bonding to skins.

Methods of Construction

There are a number of methods by which sandwich structures may be produced, two of which are:

• Two stage cure:

Facing skins are first produced on a suitable tool by the vacuum bag oven cure or autoclave method. After machining or forming the core to the required shape (if necessary), the skins are positioned on either side of the core in an assembly tool, with an adhesive film layer at the two interfaces. The assembly is then vacuum bagged and cured either in an oven or in an autoclave.

• Single stage cure:

In this method the lay-up is carried out on one tool. The prepreg is laid up usually followed by a layer of adhesive film (not always necessary). The core is then positioned, followed by an adhesive layer (if required) and closing prepreg plies. The whole assembly is then vacuum bagged and transferred to an oven or an autoclave for curing. Alternatively in can be processed in a press. Single cycle cured sandwich construction components are cheaper to produce, although some reduction in the properties can be expected. This method is acceptable for most applications unless the structural requirements are very tight.

Applications

Product Range

Advanced Composites Group provides a complete range of products to serve the advanced composites industry. Web-based product selector guides are available via the Group web site for specific market areas.

Prepregs (Component and Tooling)

• LTM series – Typical cure temperatures: 20 to 80°C
• MTM series – Typical cure temperatures: 80 to 135°C
• HTM series – Typical cure temperatures: >135°C
• VTM series – Typical cure temperatures: 65 to 180°C

Film Adhesives/Resin Films

Syntactic Films

Tooling Block

• Syntactic Epoxy and Urethane

Tooling Ancillaries

• Backing structures, sealers, adhesive

For further information on these products, or any of your composite materials requirements, please contact one of our Technical Sales Representatives.