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RTM speeds production of automotive composites

11 October 2012

Out-of-autoclave processes, such as resin transfer moulding (RTM), as well as other fast curing epoxy systems, are accelerating the take-up of lightweight automotive composites as their rapid manufacture matches the mass production needs of this demanding industry. Klaus Ritter reports.

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. Much of this progress must be attributed to the latest epoxy resins, which are enabling the growth of cost-competitive manufacturing techniques and the delivery of high quality components at mass production rates.

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 mass produced vehicle parts.

Epoxy resins reinforced with fibres produce composite materials with mechanical performance and corrosion resistance properties that are comparable to, if not better than, metal for structural applications. Composite materials offer high mechanical strength, stiffness and better impact resistance.

Substituting conventional materials with composites can reduce vehicle weight, resulting in better fuel efficiency, improved power-to-weight ratios and ultimately overall better vehicle performance.

A major challenge, as far as the use of composite materials is concerned, has been the need 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 only moderate investment costs, EES allows the easy and exact moulding of complicated shapes and the possibility of extremely short cycle times of only 45-60 seconds at 145 -160°C.

Innovative designs
Huntsman Advanced Materials has recently been involved in various ground-breaking automotive composite projects with different global manufacturers. For example, an Araldite RTM system was selected by Lamborghini for the production of the first carbon fibre chassis on its Aventador LP700-4 model.

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 a cost effective method for the quick and repeatable production of structural parts with high mechanical and thermal properties that are comparable to autoclaved prepregs. For the mass production of parts, however, standard RTM processes needed to be improved and a faster RTM method developed.

Moving from low to high pressure systems
Epoxy RTM systems used in automotive applications are dual-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 via a static, helicoidal mixer at a relatively low pressure, most often lower than 15bar. The mixture then feeds the mould containing the reinforcement, which is 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 50bar. The chemicals meet and mix thoroughly in the small cylindrical cavity, converting their kinetic energy into turbulence.

The computerised control determines the precise amount of liquid materials required by each different mould and calculates a precise injection time for pump output. When the machine’s control sends an injection command, the piston sealing the mixing chamber is operated hydraulically, the mixing chamber is opened and the liquid components are delivered through pressure-inducing nozzles.

The laminar liquid flow 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 high pressure – in excess of 50bar.

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 and avoiding the need for flushing with solvents or detergents.

With standard RTM, part production in the mould takes between 15 and 85 minutes. In most cases, a post cure is required to develop the material and ensure its ultimate performance. Indeed, with the latest developments in high pressure RTM there is now the potential to produce finished parts at intervals between 4.5 and 13 minutes, which equates to a significant time saving of up to 85 percent.

RTM has traditionally provided high-quality parts as well as the flexibility that allows designers to produce composite components with complex shapes that simply would not have been possible with conventional materials. The latest material developments and processes in high pressure mixing are now taking RTM to the next level, providing a commercially viable process that meets increasing demands for fast processing and shorter cycles for volume production markets.

The is a promising future for epoxy composites in the automotive industry. Huntsman Advanced Materials now offers epoxy resin systems for both standard and high pressure RTM and these systems are already proven to offer advanced processing properties and significant production time savings.

Klaus Ritter is with Huntsman Advanced Materials

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