Production of UD-Tape Based Thermoplastic Composite Parts
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Tape layup, pre-consolidation and compression injection molding are the primary steps for the processing of UD tapes. In addition to the cycle time, the resulting material scrap in the complete process chain for FRP plays a significant role for evaluating the suitability of the process for the largescale manufacturing. FORCE (Functionalized Oriented Composites) represents a process chain for the high-volume production with a cycle time of 60 – 90 s and material scrap rate less than 5 %.
Driven by the stringent emission norms and the need to produce efficient vehicles, the automotive industry is actively involved in developing cost-efficient lightweight designs based on fiber reinforced plastics (FRP). A major breakthrough was the BWW i3 model featuring carbon fiber-reinforced passenger cell. The currently used resin transfer molding proves to be a hurdle for the production volume of more than 100,000 cars per year. The thermoplastic composites are an excellent alternative to overcome the problem as there is no need for the curing step required in the case of thermoset resins. Apart from that, the thermoplastic FRP allows a high degree of functional integration in combination with the injection molding.
UD-Tapes as a Basis
Due to the basic concept, the processing of different fiber orientation in one layer would result in increased cycle time.
In case of the organosheets, depending on the complexity of the two dimensional preform, the typical scrap rate amounts to 25 to 30 %. Moreover, the high material cost of the organosheets (5 to 25 €/kg) makes it uneconomical for high-volume production. Above all, the extreme lightweight designs are limited due to the orthotropic nature of the organosheets.
Innovative Tape Laying Technology
The two independent vacuum tables, a fixed and a turnable, are used for the layup and stacking respectively. The UD tapes are cut to the required length and laid on the fixed table with a vacuum assisted pick-and-place unit. The individual layers are then stacked on the second table using a layer pick-and-place unit. The orientation of the layers is achieved by the rotating the second table to the desired angle. Finally, the layers are fixed locally with ultrasonic welding. Parallel to this process, the layup of the consequent layers take place on the first table to save cycle time. With the current prototype, preforms of maximum 1.500 mm x 1.500 mm2 can be laid.
Up to four tapes of different widths (5 mm to 165 mm) or materials can be processed in a single layer. This enables to reduce the amount of scrap of the expensive semi-finished product . Additionally, the tape pick-and-place units can handle two or more separate tape stripes in a single step, which allows to produce preforms with an opening (e.g. window frame).
The modular construction of the machine and the strong cooperation with the machine manufacturer M.A.i. (Kronach-Neuses, Germany), the machine can be modified to suit a variety of requirements. This includes for example the number of tape roll lines, the dimensions of the layup tables or the number of ultrasonic welding heads.
On the negative side, it is necessary to consider that the vacuum based pick-and-place transportation has limitations in processing tapes with high warpage or waviness. In addition to that, due to the basic concept of this machine, the processing of different fiber orientation in one layer would result in increased cycle time and prove to be uneconomical.
Consolidation in a Double Belt Press
In FORCE-con, the layer stackings are processed through nine individually controlled zones with a maximum temperature of 250 ± 5 °C during which the thermoplastic matrix is melted and the layers are pressed together in a calendar unit. The conveyer speed can be adjusted between 0.2 and 10 m/min. All common thermoplastic automotive plastics (PE, PP, PA6 as well as PC and PMMA) can be processed. Preforms up to a maximum width of 1500 mm can be preconsolidated. If required, the preform contour can be trimmed with waterjet cutting.
Forming and Overmolding
Next to FORCE-Con, the pre-consolidated preforms are heated above the melting point of the thermoplastic matrix homogeneously in a paternoster convection oven. The oven can reach a maximum temperature of 290 ± 2 °C and can handle depending on the preform geometry up to 32 parts at the same time.
In the following step, the handling of the heated preforms to the lower mold half fixed on a sliding table takes place with a robot gripper system. After the transfer is complete, the lower mold slides in to the press position and the mold is closed to form and consolidate the preform to its final shape. The two injection units (screw diameter of 90 and 105 mm) mounted adjacent to the vertical press can be used to integrate functional elements like ribs, bosses and snap fit joints. One of the two units is equipped with MuCell technology for physical foaming of the injection molded structure allowing additional weight reduction.
Shorter transfer times can be achieved when the IR heating instead of the convection oven is used. This is due to the fact that the IR units are mounted close to the clamping unit and the transfer of the preforms take place using a linear handling system. The IR system consists of 36 individual heating fields (each 250 mm x 250 mm) divided into 4 separately controlled modules. The temperature profiles of the modules can be adjusted to reduce the hotspots in the overlapping regions of the heating fields. It is also possible to heat the preforms either from one or both the sides.
It is possible to fully exploit the advantages of the UDUD tapes by allowing load-specific lightweight designs in short cycle times.
Cost Saving Using a Mold Base
The mold base consists of two base plates with complete 8x hot runner manifold and a variable hydraulic ejection in the base plate. With customer specific mold inserts, a fully functional compression injection mold can be realized. Mold inserts with a maximum size of 2,000 mm x 1,400 mm can be used. The 8 hot runner nozzles with hydraulic needle shut-off system facilitates the manufacturing of large components. Apart from that, the hot runner in the upper mold half can be heated up to 450 °C enabling the processing of high temperature plastics like PES, PEI or PEEK. The mold base provides a potential cost saving of nearly 60 % for a prototype mold.
Conclusion and Outlook
The FORCE process chain developed at Neue Materialien Bayreuth GmbH enables to realize high volume production of thermoplastic composite parts. Additionally, it makes possible to fully exploit the advantages of the UD tapes by allowing load-specific lightweight designs and functional integration in short cycle times. Currently, the UD tapes can be processed at 2 s per stripe. Based on the preform geometry 30 % minimum scrap is realizable.
The described process chain could facilitate the automotive industry to overcome the bottleneck related to mass production of composite parts using resin transfer molding technique and could lead to a paradigm shift to save weight and increase overall efficiency using thermoplastic composites to achieve the emission goals set by the EU legislation.
Within the scope of ongoing research activities, apart from conventional thermoplastic tape materials the processing of wood veneers and other natural fibers products with the FORCE technology are investigated. Possible components could be furniture or innovative concept for automobiles .
In addition to that, the currently available process chain is being developed to include two additional modules — FORCE Melt for continuous melt impregnation of rovings and FORCE Sim for the simulation of the individual process steps in the complete process chain.
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