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Processing and Manufacturing of Interior Components Hans-Dieter Berg* INTRODUCTION The heading "Processing and Manufacturing of Interior Components" comprises a broad range of processing of very different materials, the majority of which are nonmetallic. The diversity of these materials and the components in which they are used requires considerable variance in manufacturing techniques. In addition to technical requirements, a large proportion of interior components must also meet decorative requirements. In most cases, the process of decorating is a separate operation that must be repeatable (for purposes of repair and refurbishment) for the large-area components of the passenger cabin (e.g., sidewalls, walls, and stowage bin doors). Therefore, interior components generally consist of a combination of different materials and semifinished products. An example of a simple structure is components manufactured by injection mouthing (e.g., parts of the passenger service unit) where component function and decorative aspect are combined by direct dyeing. An example of a complex structure is decorative sandwich panels. Today, the major part of the passenger cabin consists of this type of components. Normally, they are made of a sandwich structure with a rear and front top layer of f~bre-reinforced plastics (e.g., phenolic resin) with a Nomex hexagonal honeycomb core. Very often, the decoration comprises decorative laminates, which again consist of several layers (PVF foils, embossing, paints, adhesives). If we consider the fire/smoke toxicity (FST) behaviour of such a structure, it is first of all important that the specifications require the inspection of the overall structure. The FST behaviour of the individual elements, such as that of fore composites in such a sandwich structure, is only a part of the overall FST result. We know, for example, that an excellent heat release result for a prepreg system is not reflected by the heat release of the overall system. On the other hand, a heat release result measured on an individual laminate can clearly exceed the limit value of 65/65 (kW min/m2~/(kW/m2) without the limit value of the overall structure being exceeded. It is also known that different manufacturing processes, for example the process for manufacturing a closed sandwich panel, can result in completely different heat release values although identical materials are used. A sandwich panel, such as for a cabin partition, manufactured in a vacuum process, normally shows lower results than an identical pane! manufactured in a multistage press. Furthermore, this phenomenon depends on aging. *Material and Processes Nonmetallics, Interior and Equipment, Deutsche Aerospace Airbus, Bremen, Germany. 213
214 Improved Fire- and Smoke-Resistant Materials These interrelations clearly show that the manufacture of interior components that are subject to the various FST requirements can be successfully performed only in close cooperation between the developers of materials and the suppliers. designers. materials and process . engineers, and production departments of the aircraft manufacturers. There will be over challenges in the future that will make this process of optimizing the material properties, the processing methods, and component performance even more complex. In the first place, this concerns Me necessity to reduce manufacturing costs for the interior by 30 percent and more, a requirement that is implied by the passenger aircraft market. Furthermore, a weight reduction of approximately 10 percent is also expected for the interior for reasons of operating costs and environmental protection (fuel consumption). Added to this are increasing technical requirements and, above all, increasing requirements with regard to human safety, which means not only the safety of passengers in the case of a fire but also the health of those people working in the production department. Last but not least, the question of future disposal of interior components and materials remains to be answered. The above-stated objectives can surely no longer be achieved by marginally improving state-of-the-art materials and processing methods; it is necessary to achieve completely new solutions which, in a way, permit leaps in development. STAT¢OF-TH~ART PROCESSING AND MANUFACTI)RING This section is a summary of materials as well as processing and manufacturing techniques used today. It is obvious that materials and processes are closely related in plastics processing, so that one cannot be described without also dealing with the other. In this case, the term "interior" will be limited to the passenger cabin, the cargo compartment, cockpit, lavatories, and galleys. Equipment parts will only be mentioned in passing. A look at the interior of today's wide-body aircraft shows that more than 80 percent of the passenger cabin interior consists of sandwich structures mainly comprising f~bre-reinforced phenolic resin top layers with Nomex~ honeycomb cores and a decoration of decorative laminates, simple PVF foil, textiles, or varnishes. The manufacturing techniques typical for these components are described below. Flat Sandwich Structures The following components are typical: floor panels, partitions, walls (galleys, lavatories), and cargo liners.
Hans-Dieter Berg 215 The most common manufacturing process for these parts is "one-shot curing" in a flat press. Often, multistage presses are used. The prepregs are incorporated in the structure in "wet" condition and are cured on the sandwich core. Normally, the matrix resin is simultaneously used as adhesive for honeycomb bonding. Individual components can be cured in separate fixtures or several components simultaneously in the so-called Hmultitooling" process; in the latter case, the individual components are subsequently cut out of large panels. It is possible to implement edge sealing by previous incorporation of poking compounds or foam parts. Subsequent milling-out of the honeycomb in the edge area and hIling are also common. Metallic and nonmetallic reinforcements can be insured or subsequently incorporated in the con sold pane} (e.g., inserts for attachment). The curing conditions are as follows: cure temperature-120-~80 °C; cure time 30-90 mini cure pressure 2 3 bars. Curved Sandwich Structures The following components are typical: window panels, ceiling panels, stowage bins (parts), stowage bin doors, and door failings. These parts, too, are mainly manufactured in the one-shot curing process. The most common process for high production rates is the "crushed core" process. The "wet" prepreg of the front and rear top layer as well as the Nomex honeycomb core, and, if necessary, inserts and reinforcements, are placed in a heated, divided press tool; the press tool is then completely closed. In this case, the honeycomb core is given a thickness oversize of up to 30-40 percent. When the fixture is closed, the Nomex. honeycomb is inevitably crushed, simultaneously building up an increased pressure acting against the top layers, which results in considerably improved surfaces for polycondensated phenolic systems as compared to, for example, normal pressing processes. A similar process is the "package" process during which all materials are also placed "wet in wet" in a divisible closed, heated fixture without crushing the honeycomb. The cure times for the techniques stated are approximately 10-15 minutes, the cure temperatures up to 175 °C. The cure pressures can partially exceed 20 bars. The conventional autoclave technique or the vacuum-bag-furnace technique, too, can be used for curved components, as described above, which are to be manufactured only in small quantities.
216 Improved Fire- aru] Smoke-Resistant Materials Decoration Conventional decoration procedures are the application of decorative varnishes, decorative foils, or textiles. In exceptional cases, other materials can also be applied, such as leather and wood imitations. They are limited by the FST requirements. If required, the component surface is filled and ground or activated. Decorative varnish is normally applied in three steps: First, a filler is sprayed and ground; then a smooth coating varnish is sprayed; finally, a structural top varnish is applied. The procedure for the application of decorative foils depends on · whether the foils have already been provided with adhesive or not, and · whether a heat-activated or a pressure-sensitive adhesive is used. If a heae-activated adhesive is used-independent of whether it is applied to the component or provided on the decorative foil it is heated by a radiant heater field and the foil is pressed onto the component by means of a vacuum bag. The application temperatures are approximately 85-105 °C, the holding time approximately 3-7 minutes. A pressure-sensicive adhesive is often used for flat and very large components or in the case of repairs; it can be applied, for example, by pressing with a rubber roll. The simplest form of application of a decoration is by single-layer foil, e.g., a PVF foil, which can also be dyed. These foils can be placed direc~v in the fixtures for curing of sandwich components and are bonded by the matrix resin. O Textiles used for decoration are mostly applied by pressure-sensitive adhesives or by dispersion adhesives curing at room temperature; the procedure corresponds to the one used for the application of decorative foils. Monolithic Here composites, with thermosetting or thermoplastic matrix systems, as well as thermoplastic thermoformed parts and also injection moulding parts are used in addition to sandwich structures. Interior parts of thermosetting composites are manufactured by the conventional press, autoclave or vacuum-bag techniques. Thermoplastic composites processed by thermoforming or folding technique with local heating are also used to a limited degree. The decoration of these parts is performed analogously to the processes described for sandwich parts. Thermoplastic thermoformed parts (e.g., door failings) or injection moulding parts (e.g., passenger service units) are normally directly dyed and provided with the desired surface structures. But for decoration purposes, it may be necessary also to varnish these thermoplastic parts. The processing techniques correspond to the standard processes for these materials. Last but not least, metallic parts should also be mentioned; however, their use is limited. Depending on their application, these parts are decorated with decorative foils, varnishes, or textiles according to the processes described.
Hans-Dieter Berg FUTURE DEVELOPMENTS IN PROCESSING AND MANUFACTURTING 217 When thinking about future developments with regard to processing and manufacturing, the tensions inherent in the following triangular relation must be accepted: Technology =~.~ .. - - ~Ul~gy Economy None of these three fields can any longer be considered separately. it will only be possible to implement well-balanced overall solutions. Specifically, improving the economy of the pro- duction of interior components by new manufacturing technologies will no longer be possible without considering effects on the environment in its broadest sense (e.g., the implications for waste disposal and for industrial and health protection). On the other hand, investments in the processing of environmentally acceptable products may well be a sales argument additional to those of the relevant competitive advantages and economic effects. The above-stated three cornerstones that will frame future developments are already perceptible today in the field of processes and manufacturing procedures for interior components. Economic and Technological Developments The market situation requires a drastic reduction of the manufacturing costs for aircraft and thus of the costs for their interior in the future. Developments on the interior sector considered here will focus on the large-area sandwich structures of sidewall panels, ceiling panels, stowage bins, partitions, and walls for galleys and lavatories due to their high share in the interior. The reduction of the flow times for manufacture and the significant simplification and mechanization of assembly sequences offer an important potential for the reduction of manufacturing costs. However, as far as correlations to passenger safety in the case of fire are of interest in this connection, developments with an effect on the materials and the component configuration are more important. From today's point of view, there will be no alternative to the basic sandwich design, even in the future. For weight reasons, the sandwich design will remain necessary, in particular for the even larger passenger aircraft being discussed today (600 and more passengers). This type of structure simply offers weight optimization at a high stiffness level for the large-area interior parts that are subjected to comparatively small loading. It is evident that future manufacturing technologies must be based on such a construction. The potential for cost reduction can be found in the application of new matrix systems, new core materials,
218 Improved Fire- arm Smoke-Resistar~t Materials procedures for integration of the decoration into component manufacture, simplified designs, and integral manufacturing techniques for the reduction of assembly work. In particular, The new matrix systems should require lower cure temperatures, shorter cure times, and lower cure pressures, and they should be storable at room temperature. The potential for using catalytically curing resins must be applied to interior components. Lower cure pressures can be used for significant cost reductions in the prepa ration of tools. The application of foams as an alternative to Homed honeycomb cores in structural areas that are less critical with regard to structure mechanics offers cost reduction potentials. _. .. . c7 rrhe expenditure tor decorations can be reduced by using dyed thermoplastic top- layer materials for sandwich components provided with surface structures. · Simplified designs and integral manufacturing techniques permit, for example, the manufacture of the essential body of a stowage bin from a flat semifinished sandwich product by special folding technique. In the area of monolithic components, alternative technologies such as the resin transfer moulding or the resin film infusion process can also be reviewed for their cost reduction potential. Tt is evident that thermoplastic thermoforming materials and injection moulding materials are increasingly being used for these components in the established processes. Ecological Aspects As pointed out above, development will no longer be exclusively oriented towards economical and technological aspects. The use of working substances considered too dangerous will be more and more restrained. Carcinogenic, mutagenic, and teratogenic products, as well as products harmful to the environment, will be removed step by step from production. In the future, there will no longer be any CFCs, halogenated hydrocarbons, or solvents. Fire retardants such as antimony trioxide or toxic bromine compounds will be replaced by products that are less dangerous to health. On a longer-term basis, the replacement of phenol- formaldehyde resins will also be necessary. Today, Be disposal of waste from plastics manufacture has already become a considerable cost factor. Maior efforts are presently being made in this area to find cost -- - r - - ~ ~ . ~ . _ a, . _ ~ ~ ~ . ~ ~ _ ~ ~ . ~ reducing solutions. one possibility is to recycle these products. on a long-term basis, however, materials and processes that offer unproblematic parallel disposal will have to be put into place.
Hans-Dieter Berg 219 CONCLUSION Identifying trends and directions in aircraft fire safety and suggesting promising research directions requires that those engaged in processing and manufacturing recognize and accept the inherent mutual tensions between technological, ecological, and economic demands. None of these three fields can any longer be considered separately; it will only be possible to implement well-balanced overall solutions. The task of balancing boundary conditions and effects that are partly contradictory, however, is becoming more complex instead of simpler. All participants must therefore cooperate closely to meet this challenge.
