Faster to Series Production Through Simulation

Transcription

1 36 INJECTION MOLDING Lightweight Construction [VEHICLE ENGINEERING] [MEDICAL TECHNOLOGY] [PACKAGING] [ELECTRICAL & ELECTRONICS] [CONSTRUCTION] [CONSUMER GOODS] [LEISURE & SPORTS] [OPTICS] Faster to Series Production Through Simulation FRP Lightweight Engineering Based on Thermoset Matrix Systems Benefits from Integrated Process Development Aside from high part costs, investment and development costs often constitute an additional obstacle to the implementation of lightweight solutions in fiber-reinforced plastic composites. In particular, parts that are produced by reactive processing require an integrated approach to product and process development. Systematic process simulation affords a way of considerably boosting efficiency during development. Consistent lightweight construction: For the latch cover (small photo) of the X-Bow sports car, an HP-RTM process was developed to near-series readiness (figures: KTM, Engel) Lightweight engineering with fiber-reinforced plastics (FRPs) imposes high demands on the development of new processes and parts: aside from the desired weight savings relative to solutions in steel or aluminum, it is important that cost-efficient processes for mass production be developed. However, this is difficult to achieve because the parts can be complex. The very breadth of the range of materials involved complicates a purely empirical approach to process development. Among the greatest challenges faced today by such reactive processes as high-pressure resin-transfer molding (HP-RTM) are how to continuously lower scrap rates and ensure consistently high product quality. That is why, when the

2 Lightweight Construction INJECTION MOLDING 37 materials and process parameters are being selected, it is important that issues such as downstream processing by bonding or joining processes or surface finishing be addressed. In addition, production-ready implementation of reactive processes is contingent on the availability of versatile yet customized automation. An ever-increasing role is being played by process simulation on the basis of experimentally determined material parameters. It can provide valuable contributions at nearly every stage of development that save time and costs and so make the development process more economical. One example of this is the near-series implementation of a high-pressure RTM process for the manufacture of latch covers for the KTM X-Bow sports car (T i t l e fi g - ure). The aim of this joint project by Engel Austria GmbH, Schwertberg, and KTM Technologies GmbH, Salzburg, both in Austria, along with other companies, was to implement the entire process workfl o w ( Fig. 1), from choice of materials and preliminary tests to mold design through to parts manufacture in under six months. The partner companies presented the results at the international plastics trade fair K Fig. 1. Even in the early development stages, simulation processes help to increase efficiency ( fi g u r e :K T M ) Fig. 2. A test platen mold helps to experimentally validate the filling simulation ( fi g u r e :K T M ) Material Selection: Various Factors Are Decisive The short duration of the project was made possible by the use of simulation methods at almost every stage of development. KTM Technologies performed preliminary tests using a platen mold. Engel Austria and Hennecke GmbH, Sankt Augustin, Germany, were responsible for technical implementation of the process and system engineering. All projects commence by selecting the materials with the specified properties of the end product in mind. In particular, mechanical parameters, raw material prices, part weight and/or the desired weight reduction relative to previous versions need to be considered. The choice of manufacturing process also influences the choice of material. Mass production by means of HP- RTM requires paying close attention to the availability and processing of semi-finished textile goods. Here, the largest role from the processing point of view is played by characteristics such as ease of draping and the permeability of the reinforcing materials, the viscosity, and the tailored reaction profile of the reactive matrix materials. When the materials are being selected, it may well prove useful to opt for a more expensive semi-finished part if that will yield a stable manufacturing process with short cycle times. The sometimes complex relationships between material properties and the costs of the end parts will be discussed in detail later. Preliminary Trials: Ensuring a Sound Data Basis Preliminary tests on a trial mold are an essential part of process development. Experimentally determined process and material parameters and variations thereof constitute a sound data basis for the inputs into the filling simulation, and afford a way of realistically simulating processes and validating the simulation methodology. Standard practice here is to experiment on test platen molds. These are used to evaluate various curing temperatures, process pressures and injection speeds for different materials and to assess the impact on the resulting fiber-composite test specimen. In most cases, it is already possible at this stage to draw conclusions about the compatibility of the binders used and the choice and dosage of release agent. The simulation method is then validated by conducting filling studies etc. to compare simulated and measured pressure profiles with each other. Comparison of the partial filling grade of a test platen mold and the simulated flow front reveals very good agreement (Fig. 2). Darcy s law states that the flow front velocity ν is a function of the permea-» Kunststoffe international 7 /2014

3 38 INJECTION MOLDING Lightweight Construction Practical Benefits Process simulation based on material parameters and preliminary experiments contributed in this specific case to the development of an FRP-parts manufacturing process using high-pressure RTM that was both economical and was executed in the comparatively short period of just six months. In particular, employing the draping and filling simulation for the gate and mold design in the early stages of production development yields a stable process much faster, and avoids later modifications to the mold that would prove time and cost intensive. Furthermore, the integration of the installation components into a central control unit plays a key role in achieving high process and quality assurance. This facilitates systematic control and seamless documentation of the process parameters over the entire production process. The Authors Dr. Lorenz Reith is a development engineer in the Center for Lightweight Composite Technologies at Engel Austria GmbH, St. Valentin, Austria; lorenz.reith@engel.at Dipl.-Ing. Katharina Fischer works in Research and Simulation at KTM Technologies in Salzburg, Austria; Katharina.Fischer@KTM-technologies. com Dr. Michael Fischlschweiger is Head of Development at the Center for Lightweight Composite Technologies at Engel; michael.fischlschweiger@engel.at Dipl.-Ing. (FH) Hans Lochner is Head of Technology Development and Prototyping at KTM Technologies; Hans.Lochner@KTM-technologies.com Dipl.-Ing. Peter Egger is Director of the Center for Lightweight Composite Technologies at Engel; peter.egger@engel.at Service Digital Version B A PDF file of the article can be found at German Version B Read the German version of the article in our magazine Kunststoffe or at Pressure 90 bar s 12 Time Fig. 3. Pressure profile in the cavity for different reinforcement fabrics used to manufacture test specimens: the lower permeability of non crimped carbon fiber fabric, compared with non crimped glass fiber fabric, leads to a higher maximum pressure in the gate area ( fi g u r e :K T M ) Pressure 100 bar bility K, the viscosity μ and the pressure gradient p. From v = [ K ] µ p at a constant flow-front velocity, higher viscosities and lower permeability (less-permeable semi-finished fiber parts) give rise to higher cavity pressures during the injection stage. The influence of different permeability values on the maximum pressure during injection is clear: the lower permeability of the non crimped carbon fiber fabric leads to a much higher maximum pressure in the gate area than is the case Pressure at gate, carbon Pressure at outlet, carbon Pressure at gate, glass Pressure at outlet, glass Pressure at gate, sheared Pressure at outlet, sheared Pressure at gate, unsheared Pressure at outlet, unsheared s 13 Time Fig. 4. Pressure profile in the cavity of sheared and unsheared textile reinforcing elements: at any given geometry, shearing of the material leads to lower permeability and hence to higher process pressures ( fi g u r e :K T M ) for the more permeable non crimped glass fiber fabric (Fig. 3). But the experiments are not limited to studies of variations in reinforcing material. They can also serve to estimate the influence of different lay-up sequences and/or fiber orientations. It should also be noted that the cavity pressure during the curing stage exerts a decisive effect on the dimensional accuracy and/or the shrinkage compensation as well as the surface finish. Generally, higher pressures yield a better surface finish. The process parameters determined from the preliminary trials serve at a later stage as the basis for commissioning the series-pro-

4 Lightweight Construction INJECTION MOLDING 39 Fig. 5. In addition to shearing of the material, draping trials serve to measure and visualize compressions and flow-channel formation in the textiles ( fi g u r e :K T M ) KTM Technologies studied the influence of fiber shear on the permeability and the pressure profile by making sample panels from sheared and unsheared semi-finished products. The pressure profiles that were recorded for the sheared and unsheared semi-finished fiber parts during the injection process clearly show that shearing of material of given geometry lowers permeability and so increases the process pressures needed for infiltration (Fig. 4). As this shearing of the material is for the most part also dependent on the forming rate and has a direct impact on local permeability, it is necessary to conduct a preliminary draping analysis and to transfer the resulting fiber warpage and/ or the fiber orientation if the filling simulation is to have high predictive quality. For uniform progression by the flow front and homogeneous part filling, there must be no defects in the preforms. Such defects in the final part can manifest themselves on one hand as resin accumulations at radii and edges, and, on the other, as localized areas of excessive fiber content with insufficient infiltration. The Institute of Textile Technology at RWTH Aachen University, Germany, conducted draping trials using an optical measurement system (manufacturer: Argus monitoring, Alsdorf, Germany). Transverse strain can serve to measure and visualize compression and flow-channel formation in the textiles in addition to material shear (Fig. 5). Mold Concept: Avoiding the Need for Downstream Optimization Fig. 6. The manufacturing cell for producing the latch covers by the HP-RTM process was first presented at K The exhibit highlights the compact design of the v-duo machine (figure: Engel) duction installation, and leads much more quickly to a stable series process. Preforming: Avoiding Defects With regard to achieving fully automated series production, not only the curing time but more importantly the duration and reproducibility of preform production are critical. A reproducible preform process with stable fiber orientation and constant permeability directly enhances the reproducibility of the injection process and hence the properties of the parts. Simply classifying the semi-finished products on the basis of areal weight would not be enough. Aside from the fiber volume content, the permeability of a fabric depends on fiber shear. In the context of mold-concept evaluation, process simulation is an efficient way to gain insights into the sometimes complex processes of infiltration of the semi-finished part and of mold filling before the final design and manufacture of production molds. KTM Technologies uses PAM-RTM software program from ESI Group, Paris, France, for this. The data input required for the filling simulation comes from process parameters such as volumetric flow, and injection and curing temperature in addition to the aforementioned material characteristics. The output values for this are obtained in the preliminary tests described earlier. The surfaces of the part serve as the geometric model, which is why the process simulation can yield information about mold filling prior to mold design. In particular, the dependence of part filling upon various gate- or fiber-side sealing strategies can be readily evaluated (Fig. 1). From this, the optimal positioning of vent units for avoiding air entrapment in the finished part can be derived. When combined with corresponding preliminary experiments, simulation-assisted mold design, which already incorporates the draping and filling simulation, proves to be particularly efficient. Costly mold modifications are avoided. Validation of the models and parameters employed is provided by the technical implementation process. As a result, the models used can be continuously developed and refined.» Kunststoffe international 7 /2014

5 40 INJECTION MOLDING Lightweight Construction Fig. 7. All installation parts are integrated into the control unit of the production installation (figure: Engel) Series Implementation: Integration of Control Unit Boosts Process Reliability The near-series manufacturing process for the X-Bow latch covers (Title figure) presented at K 2013 used the polyurethane system Elastolit (manufacturer: BASF Polyurethanes GmbH, Lemförde, Germany). This material, which can be variously used for high- and low-pressure RTM as well as for wet pressing and spraying processes, is characterized by its mechanical properties, which it imparts to the composite material, e. g., high fatigue strength and damage tolerance. The mold was provided by Langer GmbH & Co. KG, Illmensee, while the preforms came courtesy of Wethje GmbH Kunststofftechnik, Hengersberg, both in Germany. The polyol and isocyanate starting components were metered from a Streamline high-pressure metering machine (Hennecke). This was equipped with an MN10-RTM mixing head which has a cleaning piston with integrated displacement encoder for building up holding pressure. The clamping unit (type: Engel v-duo 700) was optimized for use with fiber composites (Fig. 6). Its distinguishing features are a compact design and good accessibility. An Engel viper 20 linear robot handled the preforms and removed and stored the finished parts. In the trade-fair exhibit, the metering unit was fully integrated in the new CC300 control unit of the Engel machine. This ensures central processing of all process parameters and generation of a consistent parts data record over the entire process (Fig. 7). Integration of the control unit thus enhances process reliability and contributes substantially to higher product quality. As well as that, process optimization is simplified, because the effects of changes in process parameters can be logged quickly and clearly. W