Automotive industry’s push to use ever more long fiber thermoplastic is driven by better mechanical properties offered by the longer fiber lengths. To materialize on this advantage the industry employed various processes such as LFT-D ECM (Long Fiber Thermoplastic – Direct Extrusion Compression Molding). LFT-D ECM process has its advantage in supporting the highvolume production, but it has limitations such as variation in fiber length in the finished parts, flow induced anisotropic properties and limited fiber loading. For end-user-based industry such as automotive, safety and reliability of components is of utmost importance. Consistency in the mechanical properties of the reinforced thermoplastic material is an important aspect governing the reliability.

Continuous fiber reinforcements can significantly improve the mechanical properties of thermoplastic injection-molded components compared to short and long fiber reinforcements. By aligning the continuous fibers in the main load paths, the reinforcement can be optimally exploited. The 3D skeleton winding process (3DSW) is a robot-based filament winding technique in which defined load application points are connected with wound closed loop structures. The introduction of thermoplastic commingled yarns in the winding process allows an efficient fiber impregnation to produce fiber skeletons that can be overmolded in an injection molding process to locally reinforce the final component. The combination of a robot-based winding process and injection molding as a process for large-scale production enables the use of thermoplastic materials for complex structural applications in higher quantities.

A long fiber reinforced glass mat thermoplastic (47 vol.% glass fiber with PA 6, Tepex Flowcore) was selected for the manufacture of demonstrator automobile parts through compression molding. Thermal modeling to predict charge temperature within the production cycle is essential for determining process windows and hence, optimizing process steps. The key inputs for thermal modeling include charge surface heat transfer parameters during each step. In this study, these heat transfer parameters were characterized and validated. The sample core temperature was recorded during each process step, then heat transfer parameters were estimated by fitting experimental data to a one-dimensional thermal model. The estimated parameters were validated by applying them in the modeling of additional samples. Overall, the model predictions show good agreement with experimental results. This suggests that heat transfer parameters have been accurately captured. By knowing these parameters, charge thermal history through the entire molding process can be evaluated. As a result, the time window of each process step can be assessed and adjusted for better part molding outcomes. Furthermore, these parameters can be adopted in thermomechanical simulations of the forming process.

Single-walled carbon nanotube (SWCNT)/epoxy composites were cured under external electric fields and the influence of the processing parameters (electric field magnitude and frequency, SWCNT concentration and curing temperature) on the electrical response of the system was evaluated. A mold for the electric field application was designed and manufactured, allowing in situ measurements of the electrical resistivity of the composite, during and after the curing process. The resulting electrical properties revealed a strong dependence on the processing parameters. By rising the curing temperature, the solid bulk resistivity was decreased by one order of magnitude. Further reduction was observed with electric fields, up to an additional order of magnitude. Such improvements can be related with the decrease in viscosity and improvement of interconnected-nanotube paths within the polymer matrix. The effect of the electric field on the rotation and interconnection of the SWCNTs was investigated using a classical mechanics model based on the dielectrophoretic theory for the liquid state. The influence of inter-nanotube distances on the bulk electrical properties was calculated at different particle concentrations, using finite element models of the microstructure. This processing technique presents promising results for enhancing the electrical conductivity of polymer composites with carbon-based nanoparticles.

The publicly funded research project SMiLE "System-integrated multi-material lightweight design for e-mobility" is developing a detailed understanding of efficient lightweight construction using an intelligent combination of fiber-reinforced plastic (FRP) with metal. A central research focus is the use of composites with a thermoplastic and a thermoset matrix for structural components in an automotive series production. For the processing of semi-finished continuous-fiber-reinforced thermoplastics in combination with long-fiber-reinforced thermoplastics (LFT), the novel local advanced tailored LFT-D process was developed. This process enables manufacturing of local ribs on a laminar insert, thus reducing the wall thickness while enabling a higher stiffness. The continuous-fiber-reinforced structures are overmolded only locally. Thereby the entire overmolding can be avoided and thus a further reduction of the components weight be achieved. Pursuing a holistic approach, advanced simulation models were developed for the new compression molding process. For the production of carbon fiber-reinforced plastic (CFRP) components based on thermoset matrix systems, the innovative pressure controlled resin transfer molding (PC-RTM), developed within the project SMiLE, is used. This technology offers a high-volume production process for high-performance and cost-effective CFRP parts with integrated foam cores and metallic inserts for structural applications. The key to success is an innovative process control as well as the use of optimized materials. Simulation methods developed and applied in the SMiLE Project – gave the possibility to optimize the overall process chain for serial production. Background and Requirements The wide variety of available mobility concepts, increasing individualization and megatrends such as urbanization are a challenge to urban mobility, and require new approaches [1]. In this context, battery electric vehicles (BEVs) can be a promising ecological and economical solution for the automotive industry. However, the battery weight increases the car’s weight significantly, by approximately 200 kg [2]. The excess weight has to be compensated by reducing the residual weight of the vehicle. This can be achieved through new concepts and vehicle designs for BEVs in a holistic multi-material-design approach [3] combining innovative processes, materials and methods.

For a realistic process simulation of sheet molding compound (SMC), comprehensive and reliable viscosity data are essential. As the mean fiber-length in SMC is similar to the sample size of shear rheometers, an alternative rheology tool and method was developed. Therefore, a molding tool for the compression molding process was equipped with pressure sensors. By means of this tool, the pressure distribution was measured and thus the rheological behavior of two semistructural SMC systems consisting of a hybrid UPPH resin with glass or carbon fibers were estimated. The characterization of the carbon SMC is performed by using a previously developed compressible rheological shell-model [5]. This shell model describes the SMC rheological behavior during 1D flow by considering the elongation flow in the core and the thin resin rich outer lubrication layer. In contrast, the glass fiber SMC shows a significantly different pressure distribution and thus different flow behavior. It shows a significant pressure peak at the flow-front, which decreases after a certain flowlength and then follows the classical SMC pressure behavior. This cannot be modeled by the compressible shell-model, nor any other available SMC related model. Therefore, a new approach has to be developed. By comparing these two SMCs with their different fiber types, concepts to model the flow behavior of the hybrid glass SMC is presented.

High pressure resin transfer molding is a method for processing continuous fiber reinforced composites at industrial production rates. This paper examines the more common HP-IRTM variant, where the 'I' stands for injection. To achieve a composite with the best mechanical properties, a combination of the fiber, resin and processing parameters must be understood. Two different matrix materials, epoxy and polyurethane, and two different fibers, glass and carbon, are processed on a KraussMaffei HP-RTM system at the Fraunhofer Project Center in London, Ontario, Canada. Several processing parameters are investigated during the manufacturing of these polymer matrix composites including the press force during injection, the press force during cure, and the injection rate. Subsequently, the manufactured parts are characterized and their mechanical properties are evaluated. The results of this study shed light on the critical properties and process settings in HP-RTM production.

Hybrid material systems result from the specific combination of different materials. For optimized design of hybrid materials for lightweight structures, a profound knowledge regarding the interaction of its constituents is essential. This paper systematically analyzes and assesses adhesively bonded hybrid material systems consisting of different sheet metals in terms of their underlying physical mechanisms under uniaxial tensile loading. Hybrid solutions constructed of Interstitial-Free steel and twinning-induced plasticity steel as well as combinations of Interstitial-Free steel with an aluminum alloy are investigated in the course of this work. It is shown that the hybridization-induced mechanisms, bridging effect, multiple neck formation and localization hindrance, contribute to changes in the strain paths of the individual layers of the hybrid material system. These changes of the strain behavior enable stabilization of plastic instabilities of specific layers and, based on that, an up to 25% improvement in uniform elongation of these layers in comparison to the monolithic material. By consistent implementation of a design strategy based on the presented mechanisms for the stabilization of plastic instability, material combinations with improved ductility and tensile strength can be obtained.

Recycling concepts for composites are gaining importance facing constantly growing production rates in multiple sectors. Today, a high percentage of the carbon fiber waste is generated during production. The unimpregnated share of waste cannot be reused as high-performance non-crimp fabrics, since shape, size and orientation of the patches were changed during the previous production step. The approach presented here shows the direct processing of chopped dry non-crimp fabrics cutting waste to Bulk Molding Compounds. The manufactured BMC materials are able to compete with materials from state-of-the-art Sheet Molding Compounds produced using virgin carbon fibers in terms of mechanical performance.

The high pressure resin transfer molding (HP-RTM) process has the potential for high-volume production of continuous fiber-reinforced components in the automotive industry. The development of robust equipment, new process variants and highly reactive matrix systems lead to significant reductions of the cycle time in recent years. The paper addresses the manufacturing of carbon fiber reinforced plastic (CFRP) laminates using different matrix systems. To evaluate the matrix influence on the material properties, matrix dominated test methods were selected for evaluation of the mechanical properties. The test plates were made with the HP-RTM process at constant process parameters using carbon fiber fabrics. Only the matrix-specific supplier instructions for processing of the matrix systems (mixing ratio, temperature of components in HP-RTM equipment and mold) were varied in the study. Three Polyurethane (PU) systems and one Epoxy (EP) system were used for the characterization of the matrix dominated propertie...

Recent trials using the Huntsman VITROX ® resin chemistry has proven the feasibility of using a polyurethane system for the production of sheet moulding compound (SMC). The resin system has been used successfully in both the direct process (D-SMC) and as conventional SMC. An unfilled heavy tow carbon fiber moulding compound has been investigated using different tow geometries and fiber loadings. Mechanical characterization has demonstrated strength and stiffness properties similar or greater than industry reported values for comparable material systems. This PUR system exhibits a low mixed viscosity and achievable fiber contents in excess of 47% by volume. The reactivity of the PUR system allows for cure times of less than 120 seconds at mould temperatures below 130 C with excellent demoulding characteristics. This material system has potential to meet the needs of industry trends towards high volume production of structural SMC’s with low VOC values. Introduction Sheet moulding compound (SMC) composites have developed over time to include many resin systems such as polyesters, vinyl esters, epoxies and hybrid resins. A shift from conventional polyester and vinyl ester systems to the new systems has happened in response to the demands of industry for high performance and styrene free applications. The work presented in this paper covers developments using the Huntsman VITROX polyurethane resin system in the Direct SMC (D-SMC) and conventional SMC process to produce a carbon fiber moulding compound. Polyurethane exhibits attractive properties including high toughness and strength when compared to conventional resins used in SMC. Its reactivity can be utilized to produce a fast curing resin system for short cure times. This reactivity has traditionally been coupled with a rapid increase in resin viscosity. The SMC process by nature requires a predictable, stable viscosity build during compounding to ensure precise resin throughput from the doctor box and fiber wet-out in the compaction rollers. This has prevented the use of polyurethanes in the SMC process until recently. Developments of the VITROX resin system by Huntsman Polyurethanes has produced a system with decoupled viscosity and cure profiles. This shift in the processability of PU allows for a low mixed viscosity that can be maintained throughout the SMC compounding phase, followed by maturation and snap cure profile in the mould. These characteristics make PU a candidate for high volume manufacturing of structural SMC components. This report covers the investigation of a carbon fiber moulding compound using the Huntsman Vitrox resin system with Zoltek Panex35 carbon fiber. The materials were moulded both in the direct way, using rapid maturation, and conventionally by cold storage and discontinuous moulding. Experimental SMC Production SMC was compounded using the Dieffenbacher D-SMC line at the Fraunhofer Project Center for Composites Research. For these trials the resin was mixed and added to the doctor boxes manually. Mixing was done using a 2 hp -Cowles type mixer with a saw tooth dispersion blade. The resin temperature and mixing time was monitored during the process. After material compounding, the SMC sheet was conveyed through the maturation section of the machine to rapidly mature the material. The material was then conveyed through a tempering section to stabilize it prior to moulding. A schematic of the D-SMC line is shown in Figure 1 and a picture of PU-CF material coming out of the D-SMC line in Figure 2. Figure 1: Schematic of the D-SMC line at the Fraunhofer Project Center Figure 2: Compounded PU-CF SMC material Formulation Overview In this trial two types of carbon fibers were used with a fixed resin formulation. Zoltek provided 50K carbon fiber rovings with distinct tow shapes: W-13 (flat wound) and T-13 (standard format). Each of the fiber types were tested at 3 fiber loadings 50, 55 and 60 wt%. All formulations were subject to the same processing conditions during both compounding and moulding. The naming convention of the compounds produced for the trial is given in Table 1. Table 1 – Overview of compounds produced PU-CF SMC Compounds Fiber\Loading 50 w% 55 w% 60 w% Zoltek PX35 W-13 W13-50 W13-55 W13-60 Zoltek PX35 T-13 T13-50 T13-55 T13-60 Compression Moulding Compression moulding was carried out directly after SMC production and its in-line rapid maturation. Panels were moulded for both mechanical characterization and for comparison of flow properties. Compression moulding process parameters are listed in Table 2. Some of the material was also placed in a freezer for moulding at a later date. Table 2 – Overview compression moulding parameters PU-CF SMC Compression Moulding Press Force [kN] 2100 Closing speed [mm/s] 1 Vacuum 30 sec Curing time [sec] 300 Mould temperature [C] 120 Results and Discussion Tensile and Flexural Characterization Tensile and flexural properties were measured following ISO 527 and ISO 14125 standards respectively. The results of the testing are shown in Figures 3 to 6. Furthermore, comparison was made between commercialized carbon fiber vinyl ester SMC manufactured by Polynt Composites, SMCarbon24 CF50-50K. Figure 3: Tensile Strength of PU-CF SMC and VE-CF commercialized material Figure 4: Tensile Modulus of PU-CF SMC and VE-CF commercialized material Figure 5: Flexural Strength of PU-CF SMC and VE-CF commercialized material Figure 6: Tensile Modulus of PU-CF SMC and VE-CF commercialized material In general it can be seen that both tensile and flexural strength and modulus increase with increase in the fiber content. The results of the mechanical characterization in tension and flexure showed that PU-CF SMC exhibits similar strength to the commercialized carbon fiber vinyl ester formulations. In general performance of two carbon fiber tow structures is similar, where tow shape T13 showcases enhanced tensile properties. Furthermore for the tow shape T13 the difference in mechanical values in flow and cross flow direction is bigger than for the tow shape W13. This coincides with the observations in the flow study, where tow shape T13 was found to be more efficient at mould filling.

Investigations on PA6-GF50 integral foams have been carried out using different material systems: longfiberand shortfiber-reinforced PA6 as well as unreinforced PA6 as a reference material. Both chemical and physical blowing agents were applied. Breathing mold technology (decompression of the mold) was selected for the foaming process. The integral foam design, which can be conceived as a sandwich structure, helps to save material in the neutral axis area and maintains a distance between load-bearing, unfoamed skin layers. For all test series an initial mold gap of 2.5 mm was chosen and the same amount of material was injected. In order to realize different density reductions, the mold opening stroke was varied. The experiments showed that, at a constant mass per unit area, integral polyamide 6 foams have a significantly higher bending stiffness than compact components, due to their higher area moment of inertia after foaming. At a constant surface weight the bending stiffness in these experiments could be increased by up to 600 %. Both shortfiber- and longfiber-reinforced polyamide 6 showed an increase in energy absorption during foaming.

Glass and carbon fiber reinforcements in injection molded parts have been used for many decades in combination with thermoplastics. Where short- or long-fiber pellets are used, all areas of the part are nearly equally reinforced by fibers. With local continuous-fiber reinforcements it is possible to reduce fiber usage to the most highly loaded areas of the components along the lines of flux. This method, which draws on principles applied in nature, strengthens the parts with only a slight weight increase compared to non-reinforced parts. The combination of injection molding as a process for large-scale production with the high mechanical properties of continuous-fiber-reinforcements enables the production of high-strength components at reasonable costs. The paper presents the investigation of a process development with injection molded components in combination with wound fiber structures. Fundamental experiments with tensile loaded wound fiber structures regarding to their design influences are presented. On this basis a reinforcement structure for a demonstrator was developed and examined.

Investigations on PP-LGF30 foam sandwiches have been carried out using different manufacturing processes: standard injection molding, MuCell (R) and LFT-D foam. Both chemical and physical blowing agents were applied. Precision mold opening (breathing mold technology) was selected for the foaming process. The integral foam design, which can be conceived as a sandwich structure, helps to save material in the neutral axis area and maintains a distance between load-bearing, unfoamed skin layers.The experiments showed that, at a constant mass per unit area, integral foams have a significantly higher flexural rigidity than compact components, due to their greater area moment of inertia after foaming: with an increase of the wall thickness from 3.6 mm to 4.4 mm compared to compact construction, the flexural rigidity increased by 75 %. With a final wall thickness of 5.8 mm an increase of 300 % was measured. Compared to non-reinforced components that show significant embrittlement during foaming, the energy absorption capacity (impact strength) of LFT foam components remains almost constant.

The paper addresses new variants of the high pressure resin transfer molding (HP-RTM) process namely high pressure injection RTM (HP-IRTM) and high pressure compression RTM (HP-CRTM) for manufacturing of carbon fiber reinforced composites with high fiber volume content. Both these processes utilize high-pressure RTM equipment for precise dosing and mixing of highly reactive epoxy resin and amine hardener with relatively high throughput rates. The paper addresses results of a study which investigated cavity pressure measurement for both the HP-RTM process variants using a specially designed highpressure RTM mold. The investigations indicate that the cavity pressure built up is a characteristic of the selected process variant. Further the relationship between the applied press force and the cavity pressure in HP-CRTM process was studied.

Thermoplastic foam injection molding (FIM) in combination with insert molding (IM) offers a possibility to generate sandwich panels in a one-step process. The prepared face sheets are first positioned inside the mold. A preheating process is carried out using quartz infrared emitters, which are mounted on a linear robot, before the mold is closed. The injection of the gas/melt mixture is combined with an embossing of the mold to further improve the face-core-adhesion. Finally, to initiate the foaming process, adjust the extent of foaming of the core and achieve the desired component dimensions, a mold opening stroke is carried out.The process described was performed with different facing materials, layer dimensions and overall wall thicknesses. Drawn PP fabrics (Curv (R)) as well as PP/GF70 tapes and consolidated sheets (unidirectional) were used to generate sandwich panels in a range of 5 to 6.4 mm thickness. PP was also chosen to form the foamed core which, in combination with the Curv (R) face sheets, produces a fully recyclable self-reinforced polymer (SRP) composite.Detailed process descriptions and the results of bending tests demonstrate the high potential. Other focuses are the preheating process and the foam structure.

This paper analyses the lightweight potential of long-fiber-reinforced and local continuous-fiber-reinforced foam injection molded components. Using the LFT-D foam process [7, 13] and breathing mold technology [11, 12], long-glass-fiber-reinforced polypropylene foams were manufactured with a constant weight per unit area and varying density reductions. As the area moment of inertia increases with the wall thickness to the third power, in these investigations small density reductions were sufficient to increase the flexural rigidity by several hundred% compared to a compact reference sample. The fiber length advantage generated by the direct process (injection molding compounding) also meant that even at higher density reductions the ductility was not reduced by the foaming. In order to achieve even better mechanical properties, foam injection molding can be combined with local continuous-fiber reinforcement. To demonstrate the potential, sandwich integral foam components with local continuous-fiber-reinforced facing were produced in-situ in an injection mold and characterized. Fully-consolidated tape blanks and self-reinforced PP fabrics were positioned on both sides of the cavity and heated. Afterwards a gas-loaded melt was injected between them. A short embossing stroke generated sufficient interfacial adhesion between the facing layers and the core material, and the foaming process was initiated by the pressure drop resulting from the precision opening of the injection compression mold.

4 wt% multiwalled carbon nanotubes (MWCNTs) were incorporated into a miscible blend of polyphenylenether/polystyrene (PPE/PS) on a twin-screw extruder at a screw speed of 600 rpm. The masterbatch obtained was diluted at 400 and 600 rpm to obtain lower MWCNT loadings in PPE/PS. Electron microscopy & optical microscopy images show very good MWCNT dispersion even at high filler loadings of 4 wt%, but slightly larger agglomerate size fractions are observable at higher screw speeds. While MWCNT addition enhanced the thermal stability of PPE/PS, a small change in glass transition was observed on the composites at different filler concentrations compared to PPE/PS. The specific heat capacity at glass transition decreases considerably until 2 wt% MWCNT and levels down thereafter for both processing conditions pointing to enhanced filler-matrix interaction at lower loadings. Storage modulus of the nanocomposites was enhanced significantly on MWCNT incorporation with reinforcing effect dropping considerably as a function of temperature, especially at lower filler contents. The modulus and the tensile strength of PPE/PS were only marginally enhanced in spite of excellent MWCNT dispersion in the matrix. Electrical percolation occurs at 0.4 wt% MWCNT content, and the electrical conductivity of 0.5 wt% MWCNT reinforced PPE/PS was close to 12 orders in magnitude higher compared to PPE/PS.

1, 3 and 5 wt.% multi walled carbon nanotubes (MWCNT) reinforced polycarbonate (PC) composites were processed in a twin-screw extruder (L/D=52) with two different screw speeds, throughputs and screw configurations. Extruded strands were characterized for dispersion and measurement of electrical resistivities while the pelletized extrudates were injection molded to produce samples for mechanical and further electrical property measurements. The absolute resistance of the melt was recorded with an online melt resistance setup developed by our group. The volume resistivity of pure PC (10(17) Omega.m) was lowered to 10(4) - 10(5) Omega.m on an injection molded PC-1 wt. % MWCNT composite. 3 wt.% MWCNT incorporated composites showed volume resistivity less than 1 Omega.m independent of process conditions. At lower filler contents the volume resistivity of injection molded samples were higher than those observed on the extruded strands and this effect diminished with increasing MWCNT loadings; owing to the loss of CNT network contacts due to shear induced filler orientation and core-skin effects. The quality of dispersion was exceptional for all filler concentrations at any process condition owing to the affinity of MWCNT towards PC due to the lower interfacial energy difference between the reactants and high polarity of PC. The modulus and strength of the composites increased with filler addition, however at 5 wt.% filler loading the strength of the composites processed with lower SMEs was less than that observed on the 1 wt.% MWCNT reinforced PC composite. The elongation of the composites at maximum tensile strength were comparable to that of neat PC except for composites with 5 wt.% MWCNT loading processed with lower SMEs. Composites with identical filler loadings which were processed with higher SMEs showed higher notched impact strength values principally because of the ability of very well dispersed filler fractions to inhibit crack propagation. The significance of the results obtained in this work stems from the fact that we were able to produce composites with substantial improvements in electrical properties with improvements/without significant loss in mechanical properties. This is one of the few exceptions to results achieved with commercially available MWCNT reinforced polymer composites processed on a large scale.