Using composite materials is hardly a new phenomenon. A trip to the British Museum will show you that the Ancient Egyptians constructed their tombs using bricks made from a primitive, yet effective, mix of straw and mud.
Today the moulds used for forming composites, also known as tools, can be made from virtually any material. For parts cured at low temperature or for prototyping where tight control of dimensional accuracy isn't required, materials such as fibreglass, high-density foams, machineable epoxy boards or even clay or wood models are often suitable.
For parts cured at higher temperature, or if high dimensional accuracy is required, or if the mould is expected to be used for high-volume part production, then higher-performance tools must be used. Materials for these tools include Invar (a nickel steel alloy), steel, aluminium, nickel, and carbon fibre.
Selection of the tooling material is typically based on the coefficient of thermal expansion (CTE), expected number of cycles, end item tolerance, desired surface condition, method of cure, glass transition temperature of the material being moulded, moulding method, the available curing equipment, and cost.
Steel and aluminium had traditionally been the materials of choice for high-performance tooling, but they can have major drawbacks when used to make composite parts. During autoclave cure, the CTE mismatch between the tool and the part is often too extreme for compatibility. Higher-priced metal alloys, such as Invar, can offer closer CTE matches but the high cost of machining and, for larger parts, the sheer size and weight of the tools makes them difficult to machine, move and store.
A new direction
Composite tooling, made of similar material to the final part, can offer a high-performance result without the high costs of metal. Once an art known only to a few dedicated aerospace and Formula 1 (F1) technicians, composite tooling is now widely in use, from weekend club race car builders to aerospace industry leaders like Boeing and Airbus. After decades of development and refinement, composite toolmaking has become less of an art and more of an easily repeatable, high quality process with predictable results.
“The use of industrial composites has been under development for over 50 years,” explains Jed Illsley, European Sales Manager for Amber Composites. “First, wet lay-up methods were used with resin and dry fabric — but obviously this wasn't an accurate method of controlling the amount of resin or the impregnation of resin in the fabric. By pre-impregnating the fabric with resin, under carefully controlled conditions with precision machinery, a material is created with known and repeatable engineering characteristics. This is the prepreg we are familiar with today. More recently tooling prepreg has become the standard method for producing precision composite moulds by employing the same proven principles.”
By slowly improving the characteristics of the material — including the handleability, the life at ambient temperature, the right amount of tack (stickiness) — the tooling route is now becoming easier and more cost effective. A new generation of composites are being used in an increasing number of efficiently engineered applications.
A small team of experts founded Amber Composites in the UK in 1988 to develop and manufacture high performance composite prepreg. Prepreg technology was still in its infancy then and the early team was involved in developing some of its first commercial applications. From its corporate headquarters in Nottinghamshire (the setting for the revolution in industrial textiles and a logical place for the birth of the carbon fibre industry), Amber worked closely with F1 racing teams and a number of other high performance engineers. This demanding customer segment drove the development of the tooling materials that are available today. Today, Amber serves a wide variety of industries worldwide including aerospace, automotive, marine, and communications.
Vacuum bag moulding A process using a single side mould set that shapes the outside surface of the panel. On the lower side is a rigid mould and on the upper side is a flexible membrane or vacuum bag. The flexible membrane can be a reusable silicone material or an extruded polymer film. Then, vacuum is applied to the mould. This process can be performed at either ambient or elevated temperature with ambient atmospheric pressure acting upon the vacuum bag.
Autoclave moulding A process using a single-sided mould set that forms the outer surface of the panel. On the lower side is a rigid mould and on the upper side is a flexible membrane made from silicone or an extruded polymer film such as nylon. Reinforcement materials can be placed manually or robotically. They include continuous fibre forms fashioned into textile constructions. Most often, they are pre-impregnated with the resin in the form of prepreg fabrics or unidirectional tapes. In some instances, a resin film is placed upon the mould and dry reinforcement is placed above. The membrane is installed and vacuum is applied. The assembly is placed into an autoclave pressure vessel. This process is generally performed at both elevated pressure and elevated temperature. The use of elevated pressure facilitates a high fibre volume fraction and low void content for maximum structural efficiency.
Resin transfer moulding (RTM) A process using a two-sided mould set that forms both surfaces of the panel. The lower side is a rigid mould. The upper side can be a rigid or flexible mould. Flexible moulds can be made from composite materials, silicone or extruded polymer films such as nylon. The two sides fit together to produce a mold cavity. The distinguishing feature of resin transfer moulding is that the reinforcement materials are placed into this cavity and the mould set is closed prior to the introduction of matrix material. Resin transfer moulding includes numerous varieties which differ in the mechanics of how the resin is introduced to the reinforcement in the mould cavity.
Other techniques Other types of moulding include press moulding, transfer moulding, pultrusion moulding, filament winding, casting, centrifugal casting and continuous casting. There are also forming capabilities including CNC filament winding, vacuum infusion, wet lay-up, compression moulding, and thermoplastic moulding.
Low-temperature-cure epoxy tooling prepregs, such as HX50 and HX70 from Amber Composites or LTM series from Advanced Composite Group, have now become benchmark products in aerospace, automotive, marine, industrial and motorsport applications. With their reliability long-proven, steady improvement has resulted in materials that are easy to handle and apply, provide Class-A surface finish, and offer surprising longevity of tool life. Some systems now enable 200°C end use temperatures, and recent advancements in materials have enabled composite tools to be compatible with an ever wider variety of processing methods.
Illsey points to the example of F1 car builders and America's Cup boat builders.
“F1 teams need a large number of small, complicated parts. Yacht builders may need a single 30 m long part. Both require extremely precise dimensions. You can find both using a similar HX tooling system with great results.”
Today's high-performance contemporary yacht moulds are frequently made using tooling prepreg, as are the moulds in military and unmanned aerospace applications. Even in the commercial aerospace industry, after a long development cycle, composites are now widely used. Entire wing sections on the new Airbus A350, for instance, will be made from carbon fibre, and a considerable proportion of the tooling will utilise composite prepreg. The new Boeing 787 Dreamliner is composed of over 50% composites and some very large production tools are made of composite tooling prepreg.
More recently, companies are also able to consider developing very low cost composite tooling through the development of prepreg that can be processed without the need for autoclave pressure and without increasing resin volume. These are commonly known as out of autoclave (OOA) products. This technique is helpful when making large parts and when the integrity of the pattern/master cannot withstand the pressure of an autoclave.
Benefits of composite tooling
Compared with traditional metal tooling, composite tooling can provide a lower cost of production and easier handling and storage. For performance parts requiring accurate dimensions, composite tooling offers a CTE closer to the part CTE, helping the part maintain dimensional integrity during cure.
Compared with a few years ago, composite tooling is more widely available, more user-friendly, and more efficient to process. Prepreg is now available with excellent drapeability enabling accurate reproduction of small radii corners, a wide range of tackiness, a wide range of curing temperatures, and a wide range of dimensions.
“A great example application was the recent development of the ALMA telescopes to be placed in Chile”, says Illsley. “The telescope is comprised of a set of segments, each of which have to be identical and precise. In order to meet these exacting accuracy requirements, they used our HX50 tooling prepreg. And, to address the outlife challenges faced when working with such a large tool, we supplied the tooling prepreg in pre-cut squares.”
Another reason composite tooling is gaining popularity is the availability of a proven package solution. Customers can buy a complete set of materials including tooling paste or blocks, adhesive, release agent, primer, sealer, tooling prepreg and component prepreg — all from a single source and all proven to work together seamlessly. Axson Technologies is a full-service supplier that has seen rapid growth from its complete solution packages.
Multipreg HX42: An epoxy resin system that can be pre-impregnated into high performance fibres such as carbon, and glass. It is an exceptional and very well-proven system in aerospace applications that exhibits a high end-use temperature and extended outlife. After a suitable post-cure an end-use temperature of 190°C (374°F) is achieved
Multipreg HX50: Like HX42, HX50 is a volatile-free epoxy resin system that can be pre-impregnated into a wide range of high performance fibres. It allows fast and low-temperature curing and exhibits excellent handleability. After a suitable post-cure an end-use temperature of 180°C (356°F) is achieved.
Multipreg HX70: An epoxy resin system that can be pre-impregnated into high performance fibres such as carbon and glass. It is a highly reactive system that offers shortened cure cycles and low temperature cures. After a suitable post-cure an end-use temperature of 180°C (356°F) is achieved.
Multipreg HX90N: HX90N is a nano-modified epoxy resin system which improves the surface flatness and finish of tools. After a suitable post-cure an end use temperature of 180°C (356°F) is achieved.
“We recently supplied a shipyard in Germany during production of an Admiral's Cup racing yacht. A complete package of tooling materials including SC175 epoxy paste, EC85 Surface Coat, HX50 tooling prepreg and EG42 Gelcoat enabled them to decrease the time and cost of delivering a precision yacht — with excellent results,” says Axson Technologies' CEO Lionel Puget.
Paste has played an important role in such progress, particularly for large moulds. With paste, a relatively low-cost CNC milling machine can rough-cut an inexpensive low-density pattern, then place a layer of epoxy or polyurethane paste over the pattern, and once the paste is cured, machine the paste to the desired dimensions creating a low-cost pattern for the tool. Tooling prepreg is then placed over the precisely cut paste to create the tool.
This method of toolmaking has substantially cut the time and cost of building large precision parts which would have previously been made using a highly skilled and time consuming wooden construction.
New developments such as HX90N tooling prepreg have been designed to handle special requirements for low temperature curing while providing excellent surface finish. HX90N provides a unique combination of exceptionally low thermal expansion (60-70% lower than comparable materials) with a flawless surface finish off aluminium or epoxy patterns and high end-use temperatures.
The exceptional properties of HX90N make this material ideally suited to demanding tooling prepreg applications. It is already helping toolmakers meet increasingly severe and diverse application requirements across the aerospace, automotive and mass transit industries.
What all this means is that both small and large composite structures are now faster, cheaper and more accurate than ever to create, allowing designers to utilise the benefits of composite materials in an ever increasing range of high performance engineering applications.
Amber Composites' new low-temperature-cure HX90N tooling prepreg features a specially designed nano-modified epoxy resin to provide benefits that Amber believes were previously unattainable. HX90N provides a unique combination of exceptionally low thermal expansion (60-70% lower than comparable materials) with a flawless surface finish off aluminium or epoxy patterns and high end-use temperatures.
HX90N improves the surface flatness and finish of tools made from various pattern materials, including aluminium and epoxy tooling board. By providing moulds with a flatter and pinhole-free surface finish, the product reduces both the preparation time necessary to achieve Class A surfaces and the quantity of primer needed, which decreases the weight of the finished product.
Tool life is extended due to the material's greater resistance to surface pitting. The material also offers good drapability, allowing complex mould shapes to be more easily formed.