Composites resin transfer moulding
Carbon composite transmission casings are currently the sole preserve of Formula One and top-level sportscar racing, owing to the very high costs involved in developing and producing a functional casing. A transmission casing is an exceptionally complex component that has to withstand various torsional and shock loadings, particularly when used as a structural chassis member. To produce a composite unit that can survive such loadings, extensive FEA analysis is required to ensure that the correct types of fibre are used and that they are orientated correctly in the lay-up process. This requires both a high degree of design expertise and very skilled fabricators to ensure consistent mechanical performance.
In recent years, our understanding of the way composites behave under load has improved immeasurably, and the tools needed to predict this behaviour accurately are becoming more widely available. But the cost of materials and the manufacturing processes required for their construction – notably the use of pre-preg fabrics and the need for an autoclave – remains very high. However, with the increasing use of hybrid powertrains, which currently have a tendency to increase overall vehicle weight, it is likely that composite transmissions will start to appear in more and more racing machines as engineers strive to keep weight under control. That could see the adoption of alternative manufacturing processes that allow for more cost-effective production of such parts.
One such process, resin transfer moulding (RTM), is seeing growing use in other areas of industry, notably aviation. RTM uses a two-part mould, with dry carbon fibre weaves laid in the mould halves. The mould is closed, and epoxy resin is injected into it under pressure, while a vacuum is applied at strategically placed vents, ensuring the resin is distributed throughout the fibres. Depending on the precise manufacturing requirements, the part can then be cured at elevated temperatures. The fabrics used are laid up and orientated in a similar way to pre-preg construction methods, with different orientations and tows used to tailor the structural properties. For example, unidirectional tapes may be added in areas that need extra reinforcement.
The advantage of RTM is that parts can be produced with a very high degree of repeatability, and the exact level of resin distribution in the material is closely controlled. Also, the cost of the basic fabrics is much lower than with pre-preg, and it is not subject to the same storage and shelf-life issues (pre-preg needs to be kept refrigerated to prevent the resin curing).
While no-one (to the best of my knowledge) has yet produced an automotive transmission casing using this process, there have been a number of applications developed for rotary winged aircraft transmissions, which need to withstand similarly extreme loadings as a racing gearbox.
As is often the case, aerospace leads the way in the development of such techniques, the focus in that industry being the same as racing in that parts must be as light as possible; however, the production requirements differ owing to the fact that they also need to be produced in volume. There is also the matter of funding for research into making techniques such as RTM viable. While teams in Formula One undoubtedly have large budgets, they pale into insignificance when compared to those of companies such as Boeing or Bell Aerospace (who have produced resin transfer moulded transmission casings).
Composite racing transmission casings are currently produced in only very small numbers, so traditional hand lay-up and autoclaving are still the most viable manufacturing method. The cost and resources needed to produce moulds for RTM are considerable, and any design changes would require the production of a new mould. However, it would not be inconceivable that RTM could be a viable approach for producing casings cost-effectively for a spec series, where a lot of units are needed and the design does not evolve over a season. As the technology continues to mature it will be interesting to see if any manufacturers adopt this approach.
Written by Lawrence Butcher