In high-volume auto manufacture, epoxy composites are used for structural applications on luxury marques and more and more on mass produced vehicle parts. Being able to reduce production cycle times is a key issue in the use of composites, and one that the industry is reviewing in order to speed up rates and overall production. Olivier de Verclos, Technical Support Leader at Huntsman Advanced Materials, illustrates latest developments in epoxy systems for automotive mass production.
Standard RTM has been the process of choice for many years in the supercar industry, enabling the manufacture of highly complex composite parts. The part production time requirement of mass production has proved a difficult hurdle to overcome. High pressure RTM and compression moulding are often chosen for fast part production. The two different processes complement each other. Whilst HP-RTM enables fast production of complex parts, compression moulding allows an even faster production but is restricted to low complexity parts. According to de Verclos, the pressure of part production costs will drive OEMs to design low complexity components and to increase the integration of compression moulding in their production. The growth in pressure vessels technology in automotive will also require the standard filament winding process to either become faster or be replaced by other faster processes.
Compression moulding – a complementary solution to HP-RTM
Moving from standard RTM, also known as ‘low pressure RTM’ pressure, to ‘high pressure RTM’ enables part production time to be reduced. In the standard RTM process the system’s components are mixed through a static mixer. In HP-RTM, by contrast, the mixing is carried out by the high speed counter-flow of the reactive components with appropriate injectors or ‘jets’ in a cylindrical mixing chamber (whose diameter can be as small as 4-5mm) at high pressure, most commonly above 50bar. The chemicals meet and mix thoroughly in the small cylindrical cavity, converting their kinetic energy into turbulence.
The compression moulding process enables faster part production for the following reasons. Firstly, it eliminates the injection process. Instead, the resin system is filmed in the ‘liquid lay down’ process onto a stack of fabric outside the mould. Secondly, with no injection step, the mould can operate at a higher temperature and the resin system can cure faster. Indeed in HP-RTM, the mould temperature should be low enough to ensure latency and low viscosity of the resin system during the injection. Faster production coupled with lower investment, compared to HP-RTM, results in a reduction in cost in part production provided the parts are low complexity parts.
Huntsman says that its resin system and hardener Araldite LY 3585 / XB 3458 has been a benchmark for RTM part production with a 7.5min cycle time. A 4min production time is also claimed to be feasible in compression moulding, resulting in a reduction of 45% in production time, compared to HP-RTM. To further reduce part production time and also to increase latency during mould filling to produce large parts, the company has introduced the new Araldite LY 3585 / Aradur 3475 system to meet both these requirements. According to the company, it allows parts to be produced in 5min and 3min respectively in HP-RTM and compression moulding as well as exhibiting a 1min latency at 110°C mould temperature. The system’s viscosity and reactivity at 40-60°C enables low temperature standard RTM or infusion technology to be used for prototyping developments.
A key driver for the automotive industry is the crash resistance behaviour for which prototypes can be manufactured using low temperature injection and infusion. There are several parameters that play a role and the elongation at break of the resin system matrix is considered to be of high importance. In addition to reducing part production time, the new system is claimed to give high tensile elongation of 8-10%.