JEC COMPOSITES MAGAZINE - Issue #43 - September 2008 - 31

Modelling Seamless modelling paste in the marine industry High series volumes and cost reduction increasingly drive boat producers to look at automotive production methods. Seamless modelling paste is one example of how technology has been successfully transferred from the car industry. The immediate benefits within the marine industry are clear: greater design freedom, reduced specialist labour, and faster plug production. However, some benefits are less obvious and not yet fully explored by boat makers. For example, the increased accuracy of a boat built from precision SMP plugs means that parts fit together more accurately. This brings an overall reduction in assembly costs, reduced adhesive consumption and can reduce the weight while increasing the mechanical strength of the boat (since bond-lines are reduced). Another example where this technology is already finding applications is in short series or single boat manufacture and customisation. The growth of the superyacht business has increased demand for complex, oneoff designs. SMP allows direct transfer of design complexity to the finished part. The freedom permitted by the coupling of CAD/CAM and CNC technology also means full-detail scale models can be rapidly made for client demonstration. For such short production runs, producers are now exploring the direct manufacture of moulds using SMP, eliminating completely the master plug. form a seamless sheet of material. However, thixotropy must be high enough so that the paste does not slump or move even when applied to a vertical surface. RenPaste® SMP uses a unique system where liquid components are mixed together to form a highly thixotropic paste. This is achieved using special chemistry which activates the thixotropy once the resin and hardener components are combined. Furthermore, by changing the dynamic mixer speed on the application machine, the operator has the possibility to adjust the exact viscosity of the mixed paste in accordance with the type of application. New automated application techniques have increased the importance of controlled rheology, since high pumping and deposition rates are increasingly demanded. RenPaste® 4666 was developed to meet the specific rheological needs of automated production of large parts, such as those used in the marine industry. Its unique programmable rheology enables rapid automated application with high pump rates, but also ensures that detailed hand application is possible. A performance-driven product The apparent simplicity of an SMP paste hides a number of complex, interdependent characteristics which are critical to the performance of the product. Companies which pioneered the use of these products learned the hard way not to underestimate the complexity of producing parts using this process. Fortunately, improvements in products, application machines and technical support have now made it easier for inexperienced users to achieve consistent results with this technology. Most thermoset materials show some shrinkage as they cure. Dimensional stability is critical for the manufacture of plugs, so measures are taken to avoid shrinkage. Epoxy resins are a popular choice of chemistry, since they display inherently low shrinkage. They may then be combined with a number of solid fillers, which enables further reduction of shrinkage during curing. Using these techniques, the RenPaste® seamless modelling pastes typically display linear shrinkage of less than 0.05% during cure (compared to 0.2-0.5% for unfilled epoxy resin, 0.2-1% for unfilled polyurethane, and 1-5% for unfilled polyester resin). Graph. 1: Liquid resin and hardener components are mixed to produce a highly thixotropic paste through interaction of special fillers in one component with a cationic polymer in the other The Curing Reaction The curing reaction of thermoset materials is accompanied by the evolution of heat, normally referred to as the exotherm. Due to the high thickness of SMP (up to 40mm), measures must be taken to avoid excessive exotherm, since this can lead to distortion, cracking and even melting of the expanded polystyrene sub-structure. RenPaste® SMP limits reaction exotherm, while still maintaining a rapid cure speed. Hardness of the material builds quickly after cure and then stabilises to allow rapid milling. Rheology: a crucial factor SMP is typically extruded at 10-40mm thickness. The extruded paste must fuse together whilst still liquid to No43 September 2008 / JEC Composites Magazine

JEC COMPOSITES MAGAZINE - Issue #43 - September 2008

Table of Contents for the Digital Edition of JEC COMPOSITES MAGAZINE - Issue #43 - September 2008