Vehicle manufacturers are aggressively adopting lightweight composite material strategies to meet requirements for increased design freedom and flexibility, high strength, stiffness and reduced vehicle weight. Huntsman explores how the latest epoxy resins are enabling the growth of cost-competitive manufacturing techniques and high quality solutions in the automotive composites market for mass production.
In mass-market vehicle manufacturing the use of epoxy composites has become widespread. From the early days where composites were used for non-structural exterior applications such as car boots and bonnets, they are now being used for structural applications as well as the mass production of vehicle parts.
Epoxy resins reinforced with fibres produce composite materials with mechanical performance and corrosion resistance properties that are comparable to, if not higher than metal for structural applications. Composite materials offer high mechanical strength, stiffness, better impact resistance and weight saving.
Substituting conventional materials with composites can reduce vehicle weight; produce better fuel efficiency and improved power-to-weight ratios that lead to an overall better performance.
A major challenge
A major challenge to the use of composite materials has been the ability to reduce production cycle times. Because of this, many manufacturers are re-evaluating their chosen processing methods with a view to speeding up production rates and total production volumes, while also aiming to keep quality at a premium and investment costs down.
Aerospace-derived prepreg materials for autoclave cure were most frequently used within the racing and high-value sectors of the automotive industry. However, for composites to be cost-effectively applied in mass production, both the manufacturing processes and resin systems needed to be adapted accordingly.
New epoxy liquid resins have subsequently been developed for a wide variety of out-of-autoclave processes, such as Resin Transfer Moulding (RTM). Other fast curing epoxy systems and specifically expandable epoxy systems (EES) are also available for wet compression moulding. Involving moderate investment costs, EES allows the easy and exact moulding of complicated shapes and the possibility of extremely short cycles times up to 45-60 seconds at 145 -160°C.
Huntsman Advanced Materials has recently been involved in various groundbreaking automotive composite projects with different global manufacturers. For example, an Araldite RTM system was selected for the production of the first carbon fibre chassis from Lamborghini on the Aventador LP700-4.
The Araldite resin and Lamborghini’s ‘RTM-Lambo’ technique created a robust and lightweight chassis with an excellent power-to-weight ratio. In combination, this system offers, in comparison to prepreg technology, a cost effective solution for the repeatable production of structural parts with high mechanical and thermal properties that are comparable to autoclaved prepregs – proof that RTM is fast becoming an excellent industrial solution.
For the mass production of parts, however, standard RTM processes needed to be improved and a faster solution for RTM developed.
Epoxy RTM systems used in automotive applications are bi-component systems and most commonly consist of a formulated resin and a formulated hardener. When the process starts, the chemical components are warmed to the required temperature in storage tanks and kept separate in continuous re-circulation through the circuit of the dosing machine up to the inner part of the mixing head. This ensures their viscosity remains constant throughout the process.
The low viscosities of Huntsman’s resin systems play an essential role in the entire process from the dosing and mixing right through to impregnation and good fibre wet-out in the mould.
Standard RTM is also known as ‘low pressure RTM’ because the system components (resin and hardener) are usually mixed through a static, helicoidal mixer at a relatively low pressure, most often lower than 15 bars. The mixture then feeds the mould containing the reinforcement, also at low pressure.
In comparison, for ‘high pressure RTM’, the mixing is managed 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 50 bars. The chemicals meet and mix thoroughly in the small cylindrical cavity, converting their kinetic energy into turbulence.
The laminar flow of liquids leaves the head through an injection nozzle that fits a hole drilled into the mould. All the blended material is therefore quickly transferred into the cavity, without waste. This transfer operation is also performed under very high pressure, higher than 50 bars. The control panel then sends a signal to the hydraulic pack, which quickly closes the mixing head and brings it to the rest position. This quick action cleans out the cylindrical wall of the mixing chamber, removing any residual liquid. Therefore, these heads do not require any flushing with solvents or detergents.
With standard RTM, part production in the mould takes between 15-85 minutes. In most cases, a post cure is required to develop the ultimate performance.
With the latest Huntsman’s developments for high pressure RTM (Araldite LY 3585/Aradur 3475) there is now potential to produce finished parts in much less than 5 minutes, which equates to a significant time saving up to 85%.
Several reasons such as part dimension or reinforcement permeability make that not all parts will require the same latency during production. This is why Huntsman offers “resin system versatility”. The system composition can be adjusted thanks to several hardener variations to optimize the cycle time and to meet a wide range of latency / injection window without affecting the system mechanical performance.
Dynamic Fluid Compression Moulding (DFCM), a complementary process to Standard High-Pressure RTM (HP-RTM)
Where structural composite processes, and RTM or HP-RTM in particular, are still considered too expensive, Huntsman’s latest DFCM technique will not only reduce costs, but offers the potential for significant performance benefits and enhanced design freedom against existing methods, such as wet compression molding generally considered as fast and cheap process.
To clearly understand the savings this technique offers, the lowest HP-RTM part-to-part cycle time achievable today is in the order of 3 minutes, whereas using the novel DFCM process, this is reduced to just one minute.
The process itself follows steps recognisable from standard compression moulding, with a wetted fibre stack inserted into a mould, followed by a short press curing step and demould. However, the new process differs from standard compression moulding by using a proprietary technique operating a dynamic compression to obtain a part quality far from the conventional process, reaching autoclave prepreg quality in both, fiber volume content (up to 65%) and porosity level
The innovative DFCM process is not limited to the so-called 2.5 D, simple or shallow draw parts produced with conventional compression molding process, but it also allows design of 2.5+ D parts characterised by medium draw, in comparison to deep draw or 3D parts achievable with RTM process.
Process simulation is a virtual environment to visualise what is happening in a manufacturing process. With cure and flow simulation Huntsman support to properly select materials and to identify optimum process parameters according to the process requirements.
For cure simulation, each product, curing cycles can be adapted in wide ranges to applications’ requirements. Precise mathematical modeling is used to characterise resin cure and rheology. These mathematical models are used to predict the processing performance of a product.
An optimum cure cycle can be determined considering material, tooling and processing requirements. The given example, here below, of a pressure vessel demonstrates the capabilities of process simulation. Vessels for high pressure are thickwalled composite structures. During processing of thickwalled structures, exothermal peaks cannot be avoided. By cure simulation, the exothermal peak can be predicted to allow process engineering considering selection of resin, processing parameters or used materials.
Complete filling without voids is essential for maximum part performance. So by applying flow simulation, Huntsman support process engineers to evaluate injection strategies and to find optimum processing parameters. Furthermore the impact of process induced filling variability can be visualized to assess the filling stage with regards to robust processing. The shown example here below points out the areas of last filling in an RTM-example with and without presence of preferential flow channels caused during closure of the tool. In this example the injection port is on the left and a circumferential channel distributes the resin in the preform.
Huntsman Advanced Materials now offers epoxy resin systems for both standard and high pressure RTM with superior performance and the possibility to adjust reactivity to part size and process window requirements (e.g. injection time). These systems are already proven in offering advanced processing properties and significant production time saving.
Complementary to RTM and HP-RTM solutions, the new DFCM process will result in measurable performance benefits for manufacturers and component suppliers in key global industry sectors. It can radically reduce production cycle times reaching performance and part quality levels comparable with autoclave technology.
Huntsman’s capabilities in cure and flow simulation support customers to properly select materials and to identify optimum process parameters according to the process requirements.
The future of epoxy composites in the automotive industry is greater than ever before. Offering an extensive range of innovative formulated systems that are proven to meet the processing and performance requirements of the automotive market for composites, Huntsman Advanced Materials is able to cater to the sector’s needs, both in high-end applications and mass manufacturing, more effectively than ever before.