Flexible Production of Thermoset FRP Components
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The Institute of Plastics Processing at RWTH Aachen University has developed a process for the economical production of continuous fiber-reinforced, thermoset components. For this purpose, the researchers have expanded the double diaphragm forming process for the processing of thermosets. The process offers a high degree of flexibility even for small and medium series.
Automation for Small Series Production
Endless Fiber-Reinforced Plastics (FRP) are used in a wide variety of applications due to their outstanding potential for lightweight design. The production scope ranges from large series applications to manual, individualized parts with lot size one. The production of FRP parts in general is influenced by the increasing diversification and individualization of products. This desire for individualized production at low prices presents manufacturers with challenges in the areas of product development, process flexibility and cost efficiency.
Looking at the current FRP market, it becomes clear that despite the high growth rates in the field of thermoplastic reinforced plastics (TPC), thermoset matrix systems still account for the majority of applications. Highly automated production processes for processing long fiber-reinforced molding compounds — the annual production quantity of Sheet Molding Compound (SMC) and Bulk Molding Compound (BMC) is 274 kt  — and the manufacture of components using Resin Transfer Molding (RTM) with a production quantity of 141 kt  are large areas of application. However, due to high tooling costs, these processes are currently not suitable for the production of small and varied series as well as individualized products. For the large number of applications in small quantities, manual open processing methods with a production volume of 237 kt still account for a significant proportion of the total production volume [1, 2] of Glass Fiber-Reinforced Plastics (GFRP) of 1096 kt. These processes are characterized by a low degree of automation, long cycle times and human-dependent reproducibility, especially in the areas of manual lamination and prepreg processing.
Thermoset matrix systems still account for the majority of applications.
The aim of current work at the IKV is to provide a flexible process chain that makes it possible to react quickly and flexibly to new market and customer requirements, to improve reproducibility and automation compared to manual processes and at the same time to minimize the costs for plant and tool technology in the sense of individualized production. The Reactive Double Diaphragm Forming process (R-DDF) currently developed at RWTH Aachen University offers precisely this process flexibility combined with a high degree of automation.
Double Diaphragm Forming
The spatial separation of the process steps “Impregnation” and “Forming and Curing” as well as the automated process technology via the shuttle system make it possible to process resin systems that, with a gelation time after approximately 3 min, react much faster than open processing methods.
The following section presents selected results on the influence of the impregnation time on the accuracy of the forming process.
Due to their excellent mechanical and thermal properties, epoxy resin systems are used for numerous structural applications in the aerospace and automotive sectors. Due to the high economic relevance of this material, an epoxy system is selected for the investigations. An epoxy resin system of type Epikote 05475 from Hexion with the reaction agent Epikure 05443 from the same manufacturer is used for the tests.
Resin systems that react much faster can be processed.
In the R-DDF process, the processing time includes the process preparation times, i.e. mixing, impregnation and forming times. A cycle time of 5 min, composed of impregnation duration and reaction time, is aimed for, which is why the processing time should be between 180 and 300 s, which is given for the present system.
The influence of the semi-finished fiber products used is investigated using various semi-finished products. A Fabric with 2/2 twill weave (GK 2-2) from P-D Interglas Technologies and a 0°/90° biaxial non-crimped fabric (Draoptex) from the company Gustav Gerster is used. The twill-woven fabric product has a base weight of 390 g/m2 and the non-crimped fabric has a base weight of 1029 g/m2. In order to achieve a comparable component thickness, five layers of the twill-woven fabric are stacked, or two layers of the non-crimped fabric, with the structure 90°/0°/0°/90° selected here for symmetry reasons.
Impregnation Time and Forming Accuracy
The high potential results in particular from the low tooling costs.
It can be seen that for an impregnation time of 70 s almost no resin is pressed out of the fibre bundles and the pure resin area has thus disappeared. With regard to the resulting pure resin areas, the best test results are achieved, which is why the impregnation time of 70 s is kept constant for further investigations. However, it should be noted that the quality of the forming accuracy also deviates from industrial quality at this process point by 5 mm. However, it should be noted that very strong flank angles of a modular tool were used.
The R-DDF process promises great potential for the cost-effective, flexible production of fiber-reinforced components with thermoset matrix. The high potential results in particular from the low tooling costs compared to conventional liquid impregnation processes such as the RTM process with simultaneous high automation capability and short cycle times if less than 300 s. Cost-effective starting materials can be used, which is a clear advantage in terms of product variability, especially in comparison to pre-impregnated semi-finished products. An impregnation bag minimizes the cleaning effort between two production cycles, as the liquid resin system is never in direct contact with machine components. Furthermore, the impregnation bag can remain on the component as a protective film during further transport.
The examination of forming accuracy reveals the current shortcomings in the draping of the semi-finished products. However, it should be noted that the modular truncated pyramid tool represents a chicane geometry with steep flank angles. It can be assumed that flatter geometries with lower degrees of deformation can still be formed with the method. In order to nevertheless increase the forming accuracy of three-dimensional geometries, further investigations are carried out using a matched die or another tool concept  with an adaptive cavity surface. Furthermore it is necessary to transfer the extension of the R-DDF process to the processing of carbon fibers, which have different draping and impregnation properties than glass fibers.
Main Influencing Factors During the Forming Process
The main factors influencing the component quality of three-dimensional components in the R-DDF process are the inherent reaction kinetics of the resin system, the parameters for impregnating the dry semi-finished products (temperature, pressure and impregnation time) and the forming parameters (forming pressure, curing temperature). Here, heating temperature and pressure are limited by material-side or machine-side boundary conditions. Due to the characteristic viscosity curve of a thermosetting resin system during processing, the impregnation time is a particularly important parameter.
Mold Concept with Adaptive Cavity Surface
By combining modular and adaptive tooling technology, new degrees of complexity and flexibility can be achieved in the manufacture of thermoset components.
A central challenge in production with a geometrically adaptive tool is the adjustment of the cavity surfaces for each new component cycle. Within the framework of the mold concept to be developed for the repair strategy, passive approaches to the adjustment of the cavity surface are pursued [3, 4].
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