The present invention relates to a process for the manufacture of polymer and/or composite products and to the related equipments.
Polymers and polymer composites have the advantages of weight saving, high specific mechanical properties and good corrosion resistance which make them indispensable materials. Nevertheless, manufacturing costs are sometimes detrimental, since they can represent a considerable part of the total costs. Furthermore, the production of complex shaped parts is still a challenge for the composite industry. Parts with relatively simple geometries are common place today for composites; pre-peg manufacturing, autoclave, filament winding, pultrusion, etc. are examples of well-developed technologies. But the production of complex 3-dimensional parts usually requires injection moulding or compression moulding of short fibre composites (xe2x80x9cengineering compositesxe2x80x9d). The drawback of short fibre reinforced composites is their considerable lower intrinsic specific mechanical properties. Assembly technologies used to obtain complex geometry for advanced composites are sometimes inefficient and not cost-effective.
The proper selection of a material system and process for manufacture of composite parts depends on a number of factors including material processability, design, part performance, and manufacturing economics [see in particular J.-A. E. M{dot over (a)}nson, New demands on manufacturing of composite materials, in High-performance composites, Ed. K. K. Chawla, P. K. Liaw, S. G. Fishman, TMS, Warrendale, Pa., (1994)].
As shown in FIG. 1, which shows a mapping of composite processing techniques with respect to their ability for complex shaping and annual production volumes, the number of parts to be produced and the required part size and complexity influence the selection of a suitable manufacturing process [see in particular W. J. Lee and J.-A. E. M{dot over (a)}nson, xe2x80x9cFactors Influencing Process Selection and Processingxe2x80x9d (Proceedings: Polymer Composite Applications for Motor Vehicles, SAE International, Detroit, USA, Feb. 25, 1991) 35.]. For low number of parts to be manufactured, a process which requires lower investment in tooling and equipment costs but longer cycle times and more labour may be favoured.
The limited potential for complex shape forming offered by advanced composite materials leaves little scope for design freedom in order to improve mechanical performance and/or integrate supplementary functions. This has been one of the primary limitation for a wider use of advanced composites in cost-sensitive large volume applications. Contrary to most traditional composite applications, many applications in, for example, the mechanical industry are small and are of more complex three-dimensional shapes, which are normally produced by casting techniques.
Traditional injection moulding, provides almost unlimited possibilities for shaping. Here, considerably greater design freedom for mechanical performance is achieved with a considerably cheaper material. However, the intrinsic mechanical properties are also lower, given the short fibre materials used, and so the potential for both load bearing and weight saving is diminished.
In most cases performance improvement has been achieved by the development of material systems with improved intrinsic properties (fibre types, resin systems and fibre content). These developments have also driven up raw material costs and the interest in branches with cost-sensitive applications has been reduced, slowing down the introduction of composite applications.
Increased design freedom nevertheless opens many possibilities for engineering solutions which may considerably increase the interest for composite materials in structural applications. FIG. 2 gives an indication of this dilemma. Most of the manufacturing techniques used today for composite materials may be placed on the exponentially-shaped band as indicated in the diagram.
Given the demands of many industries today, it is obvious that. the desired direction for future development is towards the upper right-hand corner of the diagram. The development of material systems with both high intrinsic properties and improved formability for complex shapes may only be ensured by close interaction during the development of the material preforms and of the processing techniques.
In this view, several material systems and manufacturing techniques are today under development, aiming to improved complex shape forming of advanced composites under attractive manufacturing conditions. Novel material systems using either pre-impregnated preforms or post-shaping impregnation are being closely studied by several research groups [see in particular A. G. Gibson and J.-A. E. M{dot over (a)}nson, xe2x80x9cImpregnation Technology for Thermoplastic Matrix Composites,xe2x80x9d Journal of Composites Manufacturing, 3 (4) (1992), 223-233, F. Neil Cogswell, Thermoplastic Aromatic Polymer Composites (Oxford: Butterworth-Heinemann, 1992) and J.-A. E. M{dot over (a)}nson, xe2x80x9cProcessing of Thermoplastic-based Advanced Compositesxe2x80x9d, Advanced Thermoplastics and their Composites, ed. H-H. Kausch (Munich: Carl Hanser Verlag Gmbh. 1992), 273]. Powder preform techniques have so far been the most explored route to improved complex formability with thermoplastic-based composites, but automated tape placement methodologies have also shown promise [see in particular K. V. Steiner, E. Faude, R. C. Don and J. W. Gillespie Jr., xe2x80x9cCut and Refeed Mechanics for Thermoplastic Tape Placement,xe2x80x9d (Proceedings of the 39th International SAMPE Symposium, Anaheim, Calif., 1994), 2627].
The potential conversion routes for composites, from fibre and matrix to finished products are illustrated on FIG. 4. Traditionally each processing steps (FIG. 3) are developed by separate companies and semi-products delivered to the next link of the processing chain.
It is by no means clear that an optimal matrix material for a composite, in terms of fibre-matrix adhesion, is suited for the other demands set upon a composite part. For instance, environmental resistance and tribological performance may not always be given by a typical matrix material. Furthermore, a high fibre content will normally have a negative influence on the surface finish of the product. The free-forming potential of neat polymers or short fibre systems will always be superior to that of continuous fibre materials. In addition, a well-known strategy to reduce cost is to reduce the number of sub-components in complex structures while integrating multiple functionality. To meet several of these requirements with one material or one processing technique may not be possible.
Considering these points it is clear that higher flexibility of the composite materials is required in many applications to increase the material""s attractiveness to design engineers. A more integrated approach using complementary materials and processes in the fabrication of the part would be desirable.
The logical step seemed therefore to integrate as far as possible the individual processing operations illustrated on FIG. 4.
The aim of the present invention is to propose novel processing techniques and equipments permitting to remedy to the disadvantages of the existing techniques and in particular to reduce manufacturing costs by the suppression of intermediate processing and assembly stages.
Therefore, the invention concerns a novel processing technique where neat polymers, reinforcements and/or preforms and/or composites are combined in a single operation, by combining several processes into a single step or as a sequence of steps in rapid succession, where simultaneous material and process integration is achieved.
As shown in FIG. 7 this integrated processing technique offers larger design freedom, performance integration and multifunctionality in complex shaped composite parts.
An object of the present invention is an integrated processing unit allowing an automated combination of processing steps such as tow placement, consolidation, internal stress release, press-forming, horizontal and vertical injection moulding, inner-gas-forming, slit-die extrusion and liquid injection for integrated multi-component or multi-functional parts.
Another object of the invention is a sheet impregnation unit comprising several stations for manufacturing of composites preforms such as fibre lay-up, powder lay-up, impregnation, preconsolidation and lofting.
Still another object of the invention is a multi-task robotised unit permitting an automatic control synchronisation between all processing units of the equipment, such in particular tow placement, composite preparation, combination of preforms and manipulations of tools, preforms and processed parts.
Still another object of the invention is a tow impregnation unit arranged to deliver tow preformdirectly into the moulds.
Integrated processing is the use of an automated sequence of forming operations in rapid succession. A comparison of processing cycles for conventional and integrated processing to produce a single complex component is illustrated on FIG. 5, which refers to conventional processing, and FIG. 6, which refers to integrated processing, where 1 refers to a first processing step, 2 to a second processing step, 3 to bonding of processed components issued of said first and second steps, T represents a transfer to another equipment and F the finishing operations.
Further opportunities lie in the combination of materials of high mechanical performance with engineering plastics in the same component.
Integrated processing provides the means of using advanced composites for high performance, and engineering plastics for geometry and appearance, in demanding structural components with maintained design benefits and attractive manufacturing costs.
The integrated processing facilitates an automated consecutive processing cycle, including both high and low pressure forming, techniques. Furthermore, the equipment permits pre-impregnated preforms, drapable preforms for post-shaping impregnation and short fibre compounds to be processed. It is obvious that inter-material bonding and management of internal stress generation play an important role in the process. Initial work performed on classical presses has shown that by careful selection of materials and processing conditions sufficient bond strength may be obtained at cycle times in the order of minutes [G. D. Smith, S. Toll and J.-A. M{dot over (a)}son, xe2x80x9cA study on Interface Healing in Polypropylene Processingxe2x80x9d (Flow Processes in Composite Materials ""94) and G. D. Smith, S. Toll and J.-A. E. M{dot over (a)}son, xe2x80x9cIntegrated Processing of Multi-Functional Composite Structures,xe2x80x9d (Proceedings of the 39th International SAMPE Symposium, Anaheim, Calif., 1994), 2385].
The following objectives have been considered:
Integration of Materials
Investigation and modification of compatibility between integrated polymeric materials (neat polymers, preforms and composites)
Optimisation of adhesion mechanisms during integrated processing sequences
Evaluation of bond strength and management of stress state in multi-component parts
Prediction of dimensional stability and durability of integrated parts
Integration of Manufacturing Processes
Integration of fast, low pressure processing techniques
Combination of the process windows of all integrated materials
Integration and automation of process sequences
Improvement of equipment flexibility and cost-efficiency
The performance of a structural part does not depend only on geometric parameters. For composites more than for other traditional materials, material properties and functional requirements are part of any optimum design. According to the invention, the integration of various material types into complex shaped parts introduces new perspectives in parts design.
Up to now, combinations of several polymers have been studied by numerous industries for different applications. For example multimaterial injection has-been used for automotive multicolor lens or food containers. Electronics and hygiene/cosmetics industries are other application sectors. Coextrusion is another example of a technique successfully developed for multilayer packaging.
The integration processing according to the invention is appealing and one can now combine reinforced polymers and composite preforms with surrounding neat polymers. By a judicious disposition of high tailorable intrinsically stiff composites the load transmission is optimised. Then non-structural components are moulded to keep the composite in place and to fulfil additional requirements like surface properties, and/or other supplementary functions.
Different types of materials are envisaged for integration, neat and modified thermoplastic and/or thermoset polymers, reinforcements, particles and fibres reinforced polymers, composites, metals. Subsequently and as illustrated in FIG. 8, various functions can be integrated in one part: complex geometry, load transmission and stiffness 4, connection using inserts and integration of sub-structures 5 (channels, doors, fastening elements . . . ), wear and corrosion resistance, surface finishing 6 and heat insulation or transfer.
Consequently, owing to material integration, multi-functionality and fine tailoring of performances (FIG. 9) are proposed by the developed integrated processing:
bulk performances: stiffness and strength, damping
surface performances: surface protection, surface finish, tribological properties
shape performances: continuous or discontinuous shape
function performances: temperature shielding, close tolerances, multi-functionality.