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Processing of class-a surface smc (sheet moulding compounds)

Development of a material/process system for the reliable
manufacture of smc parts with class-a surface performance.
novel filler, polymeric and reinforcing materials as well
as novel processing methods will be addressed.

SMC (Sheet moulding compound) composites have been shown to be a cost-effective alternative to metals for applications within the transportation industry. SMCs are lightweight materials with excellent properties and suitable for high rate processing and arefor these reasons already applied in many vehicle applications. They suffer, however, from major drawbacks in reliability of surface appearance and performance, limiting further use of these materials in such applications. This project is thus aimed at developing a new generation of SMC composite materials for aesthetic and performance- demanding applications for a high production capacity, such as car body panels. The surface quality required, generally referred to as Class-A surface, cannot suffer from surface defects and local or global part deformation. This has proven to be difficult to attain with current SMC materials in production without requiring a high discard rate of non-qualifying parts. New SMC materials, which should not require costly surface finishing processes, must furthermore comply with the demanding durability requirements, rigidity, impact and environmental resistance. Finally, processing reliability and reproducibility of SMC production needs to be improved. SMC materials are characterised by their composite nature combining dissimilar fibrous, particulate and resin materials. The individual characteristics of each of these components and their interactions have an important influence on the processability and final properties of SMC products, including surface quality. As an example, the flow behaviour of the fibre network and initial fibre bundle dimensions influence the fibre distribution in the compression-moulded products. In particular, the sizing chemistry, which has to ensure multiple and essential characteristics to the fibre, such as strand integrity, wetting potential and limited styrene solubility to ensure a uniform glass distribution and minimum warpage, plays a major role. This controls defect distribution and lack of local or global dimensional stability. Furthermore, the resin solidification kinetics similarly can affect the flow behaviour of the composite system and, most importantly, control its interaction with low profile additives and the development of internal stresses, which are at the origin of many surface defects. Finally, the filler content, dimensions and compatibility also strongly influence the rheological behaviour and thus material homogeneity and internal stress development. Several factors thus affect the surface quality and performance of produced parts. These depend on the material-process-property relations and Class-A surface SMC materials can thus only be achieved by a tightly controlled material and process combination. The goal of this project is, therefore, to acquire an understanding of the mechanisms controlling the development of defects and, by focused material and process tailoring, to ensure reliable production of high-quality parts. To achieve these goals it is necessary to: (a) improve the SMC stability (b) control the material flow during moulding (c) control solidification shrinkage (d) optimize the fibre and filler contents and ratio, and (e) define a surface quality dedicated processing window. Key properties i.e. suspension rheology, fibre impregnation fibre distribution, solidification rate and uniform shrinkage, which all depend on material characteristics as well as processing conditions, need to be controlled and/or tailored. The project has been built in such a manner to allow identification of key parameters affecting the surface quality by considering the 4 steps characterizing SMC manufacturing: (i) material preparation (ii) filler and fibre-matrix interaction (iii) matrix solidification, additive phase separation and (iv) processing control. These 4 themes include the control of parameters such as: fibre network characteristics, sizing chemistry, resin composition, mat pre-impregnation and maturation for: (i) wettability characteristics, impregnation dynamics and fibre flow for (ii) curing kinetics and rheological evolution, internal stress build-up and use of low profile additives for (iii) and (iv): the occurrence of localised and long range defects, establishment of a well defined processing window and characterization of process reliability for (iv). These themes will each be addressed by all participants in accordance with their respective competencies. However, since material and processing issues need to be considered as an ensemble, the study will also focus individually on each of the SMC material's main components, i.e. fibre network, resin and low profile additive composition and fibre-filler/matrix interactions. These transversely defined tasks aim to relate component characteristics to processability and surface quality. Defect formation mechanisms, surface quality characterization, process window definition, as well as the use of a common standard reference material will ensure a continuous interrelation between these tasks. This should contribute towards an understanding of defect initiation mechanisms and allow focused tailoring of material characteristics. The resulting new generation of SMC materials with optimized process control should lead to the reliable production of high-performance Class-A surface SMC composites. The partners are leaders in Europe in: * fibre manufacturing for SMC applications (VETROTEX INT. (VTX)); * SMC resin formulation (UNION CARBIDE (UC)); * processing science, rheology and internal stress control (Laboratoire de Technologie des Composites et Polymeres at the ECOLE POLYTECHNIQUE FEDERALE DE LAUSANNE (LTC)) and * surface and interface science (Centre des Materiaux de Grande Diffusion at the ECOLE DES MINES D'ALES (CMGD)). Keywords: SMC (Sheet Moulding Compounds), surface quality, transport.
Acronym: 
SURFAS
Project ID: 
2 373
Start date: 
01-10-2000
Project Duration: 
36months
Project costs: 
2 350 000.00€
Technological Area: 
Market Area: 

Raising the productivity and competitiveness of European businesses through technology. Boosting national economies on the international market, and strengthening the basis for sustainable prosperity and employment.