Press forming technologies
Project leader
Dr.ir. Harald Bersee
Dr.ir. Otto Bergsma
PhD. student
MSc. Valeria Antonelli
MSc. Giovanni Nino
MSc. Darko Stavrov
Researcher
Ing. Sebastiaan Lindstedt
Ing. Bert Weteringe
Contactperson
Dr.ir. Harald Bersee
Summary
The focus of the research in press forming technologies lies at the design and manufacture of large, typically > 2 m2, thermoplastic composite primary aircraft structures. The increasing size of aircraft forces the aeronautic industry to extend the limits of technology in order to be able to design and produce such aircraft, as is illustrated by the development of the Airbus A380 (550-800 seat aircraft). The state-of-the-art in aircraft design is the design and manufacture of primary structures from metal or thermoset composites. Over the last 10 years new composites have been developed based on thermoplastic matrices. These thermoplastic composites offer, compared to metals, significant weight reduction at no cost penalty, preservation of environment, preservation of natural resources/energy and improvement of work environment. Compared to thermoset composites they offer a reduction of manufacturing cost, preservation of environment and of natural resources/energy and improvement of work environment. The use of thermoplastic composites is currently, though still expanding, limited to non-structural interior parts like stow bins, and to small secondary structures like ribs for flaps. Also more complex welded structures like the fixed leading edge of the A340-600 have been developed, but primary structures have not been developed so far. The limited implementation of thermoplastic composites is caused by the lack of knowledge as well as the lack of proper design and process tools and manufacturing technology for thermoplastic composites.
Research projects
1. Diaphragm forming
Diaphragm forming of high temperature thermoplastic composites requires the use of an autoclave. This means that the laminates are deformed by applying a hydrostatic pressure and isothermally heated by a hot gas. Therefore, products of a reproducible, high quality can be realised with this process. However, the processing cycle is very long compared to press forming: roughly 1 hour instead of a couple of minutes for rubber forming. This restricts the process to the production of small series of high quality products. Most aerospace industries have autoclaves; the investment cost of diaphragm forming can therefore be kept low, while the products can be quite large due to the sizes of the used autoclaves. Hence the use of diaphragm forming for the production of small series of large shell elements can be very interesting for the aerospace industry.
Diaphragm forming in its current form, though offering cost reductions compared to thermoset processes, is a rather costly process. The diaphragms that are used for forming of the high temperature thermoplastic composites are made of superplastic aluminium or Polyimides. Both materials are very expensive and the diaphragms are disposed of after one cycle. The use of an autoclave is another cost raiser. This research project focuses on the development of techniques to reduce the costs of diaphragm forming. The approach is twofold:
Obtaining knowledge about the forming mechanisms occurring during diaphragm forming and subsequently, developing techniques to reduce the required forming pressure to atmospheric pressure. In this way the process can be performed outside the autoclave.
Development of diaphragms that can be used for numerous process cycles.
2. Smart blankholder for rubber forming
In case of rubber forming of complex products, the maximum shearing angle of the fabric will be exceeded, consequently resulting in wrinkles. These wrinkles are unacceptable for structural components. The common practice in rubber forming is to use a blankholder that pulls out these wrinkles. The conventional blankholders are reactive systems and use springs to pull out the fibres. The blankholder is clamped in the transport system of the press and thus also serves as a material handling device. During heating of the laminate, the stiffness reduces significantly due to the melting of the resins. This results in a large deflection of the laminate and thus in large temperature gradients. For the products currently being rubber formed, such as ribs for non-primary structures, brackets etc, this gradient is still acceptable. However, for the manufacture of large primary aircraft structures, the laminate will be significantly thicker. Subsequently the temperature gradient occurring during heating of the laminate will increase to an unacceptable level.
In this research project a new type of blankholder will be developed. The principle of the blankholder is active control of the laminate. The first prototype consists of 10 cylinders, upgradeable to 20, which can be controlled individually. The objectives for the development of an active blankholder are:
Increasing the homogeneity of the temperature distribution within the laminate during heating and transportation. This will be achieved by straightening the laminate, in which the tensile force is a function of the temperature and laminate lay-up.
Increasing the deformability of the laminates by applying a complex computer controlled tensile pattern on the laminate during forming. As a result products with a larger complexity or deep draw ratio can be manufactured.
3. Mould heating systems
In rubber forming the laminate is heated to the processing temperature after which it is transferred to the press unit and pressed into the mould. The typical processing temperature of the thermoplastic composites used for aeronautical applications (PEI, PPS) is ± 300 - 320 oC. The mould is kept at a significant lower temperature of ± 120 - 140 oC, similar to injection moulding. This is done in order to reduce the cycle time. However, the large drop in temperature results in an immediate cooling of the laminate. Consequently, the ability of the layers to slip and the fabric to shear is reduced. The objective of this research project is to develop a mould heating system, which enables the instant heating of the mould to high temperatures and cooling to room temperature without or with a minimum effect on the cycle time. In this way both the product quality can be improved and the complexity of the product to be formed can be increased.
4. Pressure sensors
Press forming of thermoplastic composites that are used for aeronautical applications require high processing temperatures (> 300 oC). In order to investigate the forming mechanisms of the press forming technologies, forces acting during the process should me measured. The current commercially available pressure sensors are, besides being expensive, too large for the moulds that are used in rubber- and diaphragm forming. In this research project, pressure sensors will be developed that can be operated at high temperature, can be placed inside small moulds and are accurate over a pressure range of 1 - 100 bar. The sensors also have to be low cost, thus enabling the use of numerous sensors in one mould.
5. Modelling of rubber mould behaviour
In rubber forming one of the moulds is made of an elastomer. The deformation of the rubber stamp creates a more or less hydrostatic pressure distribution. This compensates uneven thickness distributions within the laminate caused by intraply shearing of the fabric. The shape of the rubber mould is crucial for successful forming of the product. Currently, the design of the rubber mould is based on the craftsmanship of the manufacturing engineer and material supplier. This research project focuses on the investigation of the rubber mould, i.e. dependence of shape, type of rubber, processing parameters etc. on the product quality. The results will be translated into (a) parametric model(s) and combined with the models of the other sub-projects result in a model simulating the rubber forming process.
6. Modelling of intraply-shear and interply-slip behaviour
The main forming mechanism of fabric-reinforced composites is intraply shear. A simulation code "DRAPE" has been developed, which predicts the formability of fabrics. However, only the deformation of one fabric can be simulated. Processing effects like TEX, viscosity of the matrix, mould friction etc. cannot be taken into account. The interaction between the different fabric layers of a laminate on the intraply shear behaviour and the slip of the layers (interply slip) is also not taken into account. In this research project the effect of all these parameters will be modelled and combined with the existing simulation code.
7. Modelling of the heat transfer during rubber forming
The heat transfer during rubber forming is complicated and has a large effect on cycle time and product quality. The project will focus on the creation of models describing the thermodynamics during rubber forming of thermoplastic composites as well as the effects on the quality of the products. The emphasis will be on the manufacture of large primary aircraft structures. This differs significantly from the current manufacture of secondary structures, because of the application of carbon fibres instead of glass fibres, the thickness of the laminates (>7 mm instead of 2-3 mm) and the size of the surfaces (> 1 m2).
