3D printed CFRP molds for RTM flaps, exoskeletons, etc. | World of Composites

The Chairman of Carbon Composites of the Technical University of Munich is advancing the production of composite additives through large-scale extrusion, continuous fiber printing, and integration of heating into tools. #Clean sky#continuousfiberAM#white
The Chairman of Carbon Fiber Composites (Lehrstuhlfür Carbon Fiber Composites or LCC) was established in 2009 at the Department of Mechanical Engineering of the Technical University of Munich (TUM, Munich, Germany), funded by SGL Carbon (Wiesbaden, Germany). Its mission is to conduct research and development in carbon fiber reinforced polymer (CFRP) materials, processes and applications, including simulation and test projects. The “chairman” in the German university system is the smallest unit composed of professors and their teams. The LCC team is led by Professor -Ing. Klaus Drechsler, currently employs 30 full-time researchers. Although SGL’s funding expires in 2016, LCC continues to maintain its leadership in composites research and development, including making molds for the lower half of the Multi-Function Body Demonstrator (MFFD), see the sidebar below, and now also includes 3D printing/additive manufacturing.
The blog started when CEAD (Delft, The Netherlands) told me that LCC had purchased one of its extruder-based AM flexbot systems for 3D printing using robots. However, I later revealed in an interview with Patrick Consul, a researcher at LCC, that there is a treasure trove of projects in Clean Sky 2, including COMBO3D, to 3D print thermoplastic composite RTM molds to produce thermoset composite aircraft flaps, and use EMOTION to produce the tool The lower part of the Thermoplastic Composite Multi-Function Fuselage Demonstrator (MFFD) used to mold Clean Sky 2, and the second part used to demonstrate how to produce the same fuselage by direct in-place reinforcement (taken out of the autoclave) tool. I found the whole discussion about how TUM achieves this and where it is headed is interesting.
Please refer to my February 2020 blog: “Proof of LM PAEK welding for the multi-functional body demonstrator”. This sidebar is an excerpt from Bas Veldman, MFFD program manager of GKN Fokker (Hogefen, The Netherlands), the paper “Development of a Multifunctional Body Demonstrator” published in February 2020.
The shell of the lower half of the MFFD consists of a thermoplastic skin and is reinforced with welded stringers, clips and frames. The manufacturing process to be demonstrated includes three key steps:
The thickness changes of the 8-meter-long and 4-meter-wide lower fuselage parts of the Clean Sky 2 multifunctional airframe demonstrator. Picture source: Figure 4, “Development of the Multifunctional Airframe Demonstrator” by Bas Veldman.
Using a concave consolidation mold provides a simpler (flat) surface than a convex tool, while directly controlling the aerodynamic limitations on the quality of the skin’s outer surface.
The MFFD project also required a study of the autoclave process, which started in parallel with a large multifunctional body demonstrator, followed by a smaller size demonstrator. In particular, in-situ thermoplastic composite manufacturing that achieves consolidation during lamination will be considered because it can completely eliminate the autoclave step, thereby simplifying the manufacturing process into one step.
The Clean Sky 2 project for the production of MFFD lower shell molds is a thermoplastic fuselage reinforcement mold for autoclave reinforcement or EMOTION (https://cordis.europa.eu/project/id/864474). “It sounds simple,” said Patrick Consul of the Technical University of Munich, the project coordinator. “But it’s actually very challenging because it needs to be heated to 400°C. This is much higher than Invar 36 does not have a significant CTE (Coefficient of Thermal Expansion) temperature range.” Invar alloys are often used for composite molding tools due to their low expansion during high-temperature curing cycles. However, Invar’s CTE does increase with increasing temperature. The consul pointed out: “The challenge will be to control the thermal expansion of the 8m x 4m mold and fuselage skin during the heating and cooling process during the curing process.”
This small (1m x 1m base area) test tool for the EMOTION project uses different materials to make the shell and backing structure so that it can be used at a temperature of 400°C. The shell is fixed in place by fasteners acting as tension springs, so the backing structure can be moved to minimize deformation due to the difference in CTE between the shell and the shell. This floating shell allows the use of ordinary steel as the backing structure instead of the more costly alloys and is based on the concept of partner Ostseestaal.
“We have 8-9 people engaged in additive manufacturing within LCC,” the consul said. He started using LCC to purchase a laser-assisted thermoplastic composite tape placement machine from AFPT (Dörth, Germany) in 2012 and started the history of this work. Later, the equipment added a Coriolis Composites (Quéven, France) composite machine to realize automatic fiber placement (AFP) thermosetting prepreg. The first desktop printer based on filament was purchased in 2017. “I also joined in 2017 to assist in the development of new applications and research projects, including writing proposals to Clean Sky 2.” He explained that the Clean Sky 2 project was submitted in response to a public call for proposals (CFP) Awarded by the theme manager’s proposal. LCC successfully completed the proposals for CFP08 COMBO3D project and CFP09 EMOTION project. Both are discussed below. “We have other projects, some of which are studying the use of continuous fiber reinforced materials for 3D printing,” Consul said. “Others use laser-integrated print heads to preheat composite substrates to print on already consolidated CFRP parts, or to increase the interlayer shear strength (ILSS) between printed layers. Another project explored Tool applications, for example, you have a small number of dedicated composite parts.”
He continued: “These projects have been supported by numerical simulations to predict the behavior of additively manufactured parts during and after printing.” “LCC is also involved in a project at Imperial College London, which aims to use 3D Printing CFRP to make exoskeletons. Another project studied the lattice structure to optimize the stiffness, strength or energy absorption properties of 3D printed parts.”
The Apium P220 printer uses a heating plate around the print head to maintain the required high temperature to ensure that the semi-crystalline PEEK and PEKK thermoplastic polymers fully crystallize, thereby ensuring good mechanical properties. This advantage is further enhanced by minimizing the temperature gradient and thermal stress in the printed matter. Image source: Apium
“We first used a simple FDM (Fused Deposition Model)-based 3D printer from Apium (Karlsruhe, Germany) to print special test fixtures that require high stiffness,” Consul explained. “The printer is 3 meters long and 1 meter wide. It has high material output and can reduce printing time. Although it cannot use continuous fibers, it has been carefully designed to be used for printing on PEEK, PEKK and chopped carbon fiber reinforced PEEK. It Not only a heating bed is used around the print head, but also a heating plate is used, which helps to produce a uniform temperature distribution in the print, thereby reducing thermal stress and ensuring crystallization.”
“We still had a Markforged (Cambridge, Massachusetts) printer for a while, and then we got Anisoprint (Luxembourg, Esch-sur-Alzette),” Consul said. Although the Markforged printer did enable FDM with continuous fiber, the LCC team chose not to keep it. “The problem we encountered was that the system was very closed,” the consul explained. “Markforged’s slicer software is difficult to use for research, because it severely limits our ability to work. We can only print a layer of continuous fiber material on it, and then print a layer of short fiber reinforced thermoplastic filaments. Generate code The slicer will not accept our G code. Therefore, there is no way to tell the machine to print in each layer of continuous fiber the way we want, etc.”
When asked about this, Markforged explained that its system was never intended for research, but was designed to be simple and powerful, widely used in manufacturing parts, utilize rule-based fiber optic paths and require less effort and Time to configure the slicer. Its Eiger software does allow users to configure layers individually. Markforged welcomes feedback and prioritizes improving user experience for customers.
“Anisoprint using the fully open G-code slicer allows us to access the process more easily,” Consul said. “It can also print continuous carbon fiber, not only in 2D layers, but also in 3D curves out of the plane. We have an Anisoprint print head for small Kuka robots, which can produce 1 meter x For a one-meter part, this part is larger than most desktop computers, but it is very small for a robotic system.”
But what about Anisoprint’s dual matrix thermoset thermoplastic (TS-TP) materials? As I explained in my blog on Anisoprint in 2019: “…it first impregnates the continuous fiber reinforcement with a thermosetting polymer and then extrudes it into the molten thermoplastic filament during the printing process.” Consul The reply stated that the adhesion between TS filament and TP is very good. “In our first trial, we can get a higher fiber content compared to the Markforged printer, but this is because we can modify the G code. The Anisoprint print head pushes the TS filament into the TP and then places it. We are planning to The printer is used for exoskeleton parts. This is an easy way to integrate high-strength fibers.”
The next development is large-scale printers, first with short fiber reinforced TP, and then continuous fiber TP. “We first developed the extruder mounted on the robot, and then our interest in the CEAD machine rose rapidly. We hope to integrate continuous optical fiber into these two systems next year.”
However, if you have already installed an extruder on the robotic arm, why use a CEAD machine? “The original extruder was the Beta version of the Dyze Pulsar pellet extruder. The maximum output of PAEK is about 2kg/hr, which is about 1kg/hr, but it is difficult when the carbon fiber loading exceeds 20%,” Consul said. “However, because the robot often accelerates and decelerates during the printing process, the average output is low. For COMBO3D, the initial small-scale half mold already requires at least 36 kg, so we need about 48 hours to print half. We must continue Someone accompanies it, because errors can occur at any time, such as blockages in the material feed line or warped parts.”
Consul said that because the size of the final COMBO3D demonstrator is about 10 times the size of the initial small part, it is not feasible to use this first extruder system for printing. “In addition, some of the materials Vigers provided to us have higher viscosity, and this first extruder could not provide enough torque to extrude those polymers. With the CEAD printer, our maximum output is about 12.5 kg/hr, after several hours of testing, our average output stabilized at around 5-6 kg/hr. This allows us to print half of the small-scale molds within 8 hours and allows us to use higher Fiber content to reduce warpage, thus making the whole process easier to control.”
“So even though Pulsar allows us to use pellets, achieve high material output and take advantage of the robot’s freedom,” he continued, “CEAD printers extend our capabilities to larger parts, higher fiber content, and wider polymerization. The range of objects. Pulsar bridges the gap between our filament-based printers and CEAD, not only in terms of output, but also in terms of nozzle size and details of printed results.”
The goal of the Clean Sky 2 project is to 3D print a mold to produce a demonstrator composite flaperon (1.5 x 3 meters) for large passenger aircraft, which is part of work package A-3.1: High and Low Speed ​​Multidisciplinary Wing. The goal of using additive manufacturing is to shorten the lead time for tool production. The project started in January 2019 and will end in March 2021. Partners include Alpex Technologies (Millsberg, Austria), light metal specialist Leichtmetallkompetenzzentrum Ranshofen (Ranshofen, Austria) and Victrex (Thornton Cleveleys, UK) supplier polyaryl ether ketone (PAEK).
Figure 9 on the left side of the 8th “Suggestion Collection” file issued by Clean Sky 2 shows the Fleuron demonstrator in COMBO3D. However, it seems to more accurately match the flap size and geometry of the A320 wing, as shown on the right. Image courtesy: Clean Sky 2 CFP 08, page 23. 195 and Mike Arnot’s “How an Airplane Wing Works”, 2019.
Another key part of the project is to prove that the carbon fiber/epoxy flieron can be made with resin transfer molding (RTM) instead of autoclave-cured prepreg. The RTM parts will be cured at the same 180°C as the autoclave parts. To ensure thermal stability, the tool will use short carbon fiber reinforced PAEK for printing with a melting temperature of 305°C.
In order to shorten the curing cycle, 3D printing molds will integrate active temperature control. “Compared with autoclaves, we must be able to heat and cool molds faster,” the consul pointed out. “We will use a grid of electric heating elements 3 mm below the mold surface, and integrate a printing channel for heating oil or air 6 mm below the mold surface. In this way, we can heat the mold surface very quickly, but also The entire mold space can be heated using the channel. Our goal is to make the heating and cooling rate 50% faster than the autoclave, and we are confident that we can make the heating and cooling rate at least 30% faster.”
In tailor-made fiber placement (TFP), wires or other heatable filaments can be stitched to various substrates. Image source: Qpoint Composite
Which heating elements are integrated into 3D printing? “We have used tailor-made carbon fiber roving fiber placement (TFP) on the glass fiber fabric as a heating element to achieve different heating zones in the CFRP mold for helicopter rotor blades,” Consul explained. “This is to complete the Clean Sky 1 project, which we have already done in cooperation with QPoint Composite (Dresden, Germany). We will use a similar concept on the COMBO3D mold.”
The simulation can support the entire development of the printing tool. By implementing heating and cooling systems in thermal simulations, tool design can be optimized. The manufacturing process will also be simulated to support the printing process by generating knowledge about the temperature distribution during the printing process and associating it with path planning.
“The delivery of the CEAD system was very fast-it took only 6 months from order to installation,” Consul said. “This time, we have completed the material test, designed the printing plate mold and started printing small parts. We are worried that it will take a long time for CEAD AM Flexbot to make the process run normally, but it worked well in the first test.”
Tablet tools? He explained: “This is a small mold that has integrated cooling channels and heater elements to produce CFRP plates for sample testing.” “This test is to ensure the quality of our 3D printed CFRP RTM molds Compared with the current CFRP produced by Saab using its autoclave process.”
Unfortunately, the project was interrupted by COVID-19, “but now everything is going well and we are catching up,” Consul said. So what about using PAEK for 3D printing molds? “The only problem at the moment is that the Tg [glass transition temperature] of PAEK is 130-140°C, which is lower than the 180°C curing of the demo part,” Consul pointed out. “So, before creep occurs or the surface is damaged, we still have to see how many cycles can be completed at the curing temperature.”
Before processing the internal channels, the two printed “blanks” of the RTM mold half of the COMBO3D project. The white cup on the side of each blank covers the wires for internal heating, which extend out and can be connected to a power source. Each part weighs about 35-40 kg and is part of the heavier PAEK parts 3D printed so far.
Consul said: “We are applying for an aerospace tool project. The project will start next year. The German GKN Aerospace Deutschland (Munich) will use CEAD machines and polyethersulfone (PESU).” [Note: German GKN is A330 and A350 and Bombardier Global The ailerons of the business jet series manufacture composite wing flaps. ] “CEAD will become part of the development of continuous carbon fiber machines. We also plan to use CEAD machines to manufacture RTM prototype molds internally, otherwise they will be subcontracted. It will bring us benefits for speeding up our work.”
The consul pointed out that PESU has a higher Tg than PAEK. “It is also amorphous, not semi-crystalline, so it should be easier to print, although creep resistance may be an issue. We are trying to avoid using PEI (polyetherimide). In the past, we studied PEI The problem of adhesion with epoxy resin. RTM6 [epoxy resin for RTM] dissolves PEI, thereby promoting good adhesion between the two. This is precisely where you need to release the 3D parts instead of adhering parts Unwanted in the print mold.”
While discussing some of the other projects that LCC pursues in additive manufacturing, I asked Consul what restricts CFRP to the exoskeleton? He replied: “Because the exoskeleton needs to be adapted to each wearer, the parts are specialized, so the volume is not large.” “Our project aims to enable the production of patient-specific lower extremity exoskeleton components, which can be used in less than 24 hours. Produce patient-specific exoskeletons within hours. These exoskeletons will support the rehabilitation of stroke patients based on their biophysical characteristics and unique medical rehabilitation needs.” (Please refer to the CW article “C-FREX Exoskeleton Depends on CFRP …”)
People’s perception is that 3D printing can print any part, but you must first have experience in 3D printing and composite materials. For example, there are often problems of layer-to-layer adhesion and anisotropy that must be understood. The design of 3D printed composite parts will be different from current parts made of plastic and metal. ”
He pointed out that using 3D printed composite materials to produce high-performance aircraft parts still needs to overcome some problems, such as: how do you design these parts for additive manufacturing? How do you determine the value-added parts of additive manufacturing? “Aerospace tools are the main application area currently being studied by our team,” Consul said. “3D printed molds are very suitable. These molds must be able to withstand all the same curing conditions as the parts, but compared to flying parts, the risks associated with the mold are small. The rib reinforcement of the wing is a good example. There are many different parts, but the number of cycles for each part is not many. Therefore, this is a good way to accumulate technical experience. It is too early to try the structural parts that will be flying. We will target this long-term goal, but the printing tools will enable We can make the 3D printing process have sufficiently stable and high performance, and have the ability to accurately predict these parts and processes in simulation.”
There are many ways to make composite parts. Therefore, the choice of method for a particular part will depend on the material, part design, and end use or application. This is a selection guide.

Post time: Feb-27-2021
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