New Mold Concept for Fiber-Plastic Composite Components
In the ResWer project a new mold design with innovative tempering was developed and implemented by means of advanced molding and casting processes to obtain reproducible fiber plastic composite parts more quickly, less expensively and at higher quality. New casting alloys were processed, and additive manufacturing was used as well.
Limited natural resources and ever-increasing energy prices demand more efficiency in the life cycle of products manufactured using molds, as well as in the manufacturing of the mold itself. There is still room to increase resource efficiency in mold-making, from mold design and production up to actual use. In addition to constantly refining and optimizing the techniques used in manufacturing, an efficient utilization of energy and resources above all entails implementing new manufacturing methods.
Components made of fiber-reinforced plastics are generated in a sophisticated technique combining process engineering and shaping. Most of these components are still made manually. This procedure is not conducive to large-batch production. However, intelligent manufacturing and die (mold) concepts extend the suitability of these techniques for larger scale production. Optimal mold and die tempering is essential for economical industrial production of laminates with demanding properties.
The die temperature is an essential parameter in the manufacturing of fiber reinforced plastic parts. Tempering of the cavity is crucial when fiber composite components with a large-surface area have to be produced. Tempering in a narrow temperature range is required both in the feeding and curing stages. Tempering involves both heating and the intentional cooling down of the cavity as well. The mold is conventionally heated electrically, in an inductive way or through additional media. The cycle times can be reduced by an active cooling, using water and nitrogen. The channels required are conventionally drilled or milled in the mold. In these techniques, one disadvantage is the relatively large and variable distance of the heating-cooling channels to the die contour, which negatively affects both manufacturing process and product, above all at critical points in the die, the so-called hot spots.
The goal was to develop new dies (molds) for the manufacturing of large-area fiber composite parts with an innovative tempering system, combining the geometrically free design of casted molds with additive manufacturing techniques. The task was to engineer technologies and manufacturing process chains for a new type of lamination molds suitable for large-batch production, based on pre-cast, near-net shape preforms. The results regarding design and technology were to be implemented in practice in a mold demonstrator for the automotive industry. In the project, the composite mold demonstrator had a size of approximately 1400 mm x 200 mm and was developed to produce a benchmark part for electric mobility application. High speed and high performance cutting strategies and parameters were intentionally chosen for the mold materials and specific mold regions and issues, such as cavity, mold structure, or tempering.
Based on the 3D-CAD files both the temperature characteristics and the flow inside the mold inserts were simulated. The cavity in the mold represents the structure which the tempering medium then streams through.
To avoid the dead water zone as in variant 1, another configuration of connecting elements was developed, mainly with a centric inlet and two lateral outlets. Variant 3 shows a significantly more homogeneous temperature distribution along the mold’s longitudinal axis (without dead water zones), which results in a more uniform surface temperature.
Manufacturing of the Demonstrator Mold
Selecting the casting alloy plays a key role when transferring the results obtained through theoretical considerations (simulation) into practice (demonstrator mold). Stainless steels, such as 1.4008, are typical mold materials due to their high resistance to strength and corrosion. Aluminum alloys are also widely in use thanks to their sound thermal conductivity. Both materials have a relatively high thermal expansion that can cause problems when the component is ejected from the mold. Especially for large and complex workpieces, the result can be inaccurate parts.
To minimize this, tests with INVAR 36 (1.3912) were performed. INVAR 36 (1.3912), made of 64 % iron and 36 % nickel, is acceptable in terms of thermal conductivity; its thermal expansion is about seven times lower than that of steel. However, when using this alloy rather than conventional stainless steel, tool life is significantly lower.
The experimental INVAR 36 castings made over the course of the project provided very good results on a laboratory scale. When casting INVAR 36, some special requirements have to be considered, such as the molten alloy’s strong affinity towards gas absorption, which can cause pores during solidification. The alloy is also not hard and strong enough, giving rise to deformations in the manufacturing process — above all in large-surface area molds.
The material was subjected to many investigations as well as casting tests to guarantee the optimal casting quality of the functional surfaces by optimizing the melting and pouring technologies. After successful casting tests, in principle, there is no obstacle to using the INVAR 36 material for cast molds or other castings for which low thermal expansion is desired.
Tempering involves both heating and the intentional cooling down of the cavity as well.
Nevertheless, steel 1.4008 was chosen for the demonstrator mold. This decision was made because casting parameters based on experience were available for this material, allowing for successful pouring even in the case of a complex component like the demonstrator mold. Furthermore, the results achieved can also be directly compared in terms of tempering with other molds, allowing for direct conclusions regarding the efficiency of the new mold concept. As an outlook for further projects the use of INVAR is also planned, from which further positive effects can be expected in the mold. However, ongoing parameters must first be investigated regarding the material’s reliable usability in studies.
Solidification in the casting mold was simulated, based on this material and the desired casting geometry. The simulation was performed in several iterations. As a result, a casting system — consisting of gate, filter, run, feeder — was developed, reducing the risk of casting defects in the mold to a minimum.
Implementation in a Near-series Test Mold
Thermal fluid analysis was performed by means of simulation; the flow conditions (course, direction) were analyzed in both mold halves and optimized for heating or cooling down the mold when processing the fiber composite components. In the first design iteration, with piping outside the mold, a shortfall was observed: flow through the mold halves was not as uniform as desired. A redesign was done, and the result obtained was analyzed with a thermal fluid flow simulation again. The goal of the redesign was to optimize the channel cross sections and minimize the occurrence of dead water zones. The final results for both mold halves were significantly better.
Four contour ejectors were integrated in the lower mold for easier component extraction because the component is relatively high and has an arched shape. The function of the ejectors has to be guaranteed over the entire temperature range during the manufacturing process of the fiber composite components. The four ejectors were experimentally made using different technologies for a comparison aimed at identifying the manufacturing technique with the best ratio between tempering performance and manufacturing costs. One cast, one milled and two made by additive manufacturing — laser beam melting — contour ejectors were produced.
For high functional integration, several aspects were considered when designing the cast body of the mold. Material recesses for the ejectors were also considered. The ribbings were designed according to the flow characteristics of the medium, using the simulation results, and the structural elements were designed for casting and the required stiffness.
The blowhole behavior illuminated by simulation, and the cooling simulation as a whole, showed that complete pouring of the part, free of volumetric defects is technically impossible, even if the mold maker has considered all mold structure requirements and has designed the mold in the best possible way for casting. Thus, the challenge for the casting technology was to minimize the unavoidable volumetric defects and to relocate them to non-critical mold sections which are far from finish-machining areas and those with high mechanical stresses.
Near-contour tempering channels were integrated into the design of the contour ejectors to be produced by additive manufacturing. The channels were integrated to test this very efficient tempering solution in a mold. The ejector inserts were made of maraging hot work steel 1.2709 by laser beam melting, with minimal allowance for finishing and fitting in the lower mold.
The experimental INVAR 36 castings made over the course of the project provided very good results on a laboratory scale.
The goal of the redesign was to optimize the channel cross sections and minimize the occurrence of dead water zones.
The industrial test mold was manufactured in the same way (process chain) as developed for the demonstrator. After manufacturing, the mold halves were completed and assembled at the ZVZ tool making company. Machining did not disclose any discontinuities, such as pores or blowholes, which indicates a correctly designed casting mold and process.
Tryout of the Near-series Test Mold
The application test was performed under near-series conditions of use, after successful completion of the test mold. For the test trials, the mold was heated to operational temperature. Heating of the molds demonstrated significant time savings and was sped up by the new tempering system. The mold temperature distribution after heating was recorded by probes and thermal imaging, and documented. The medium had a constant 125 °C temperature, which also resulted in a uniform temperature distribution in the mold as a whole, thanks to the innovative tempering sytem.
This research and development project was funded by the German Federal Ministry of Education and Research (BMBF) within the Funding Action “SME — Innovative: Resources and Energy Efficiency” (funding number 02PK2430 — 02PK2433) and managed by the Project Management Agency Karlsruhe (PTKA). The author is responsible for the contents of this publication.