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Enabling high-volume production of large 3D printed composite parts with low porosity

Oak Ridge National Laboratory has installed the first AMCM test cell that integrates extrusion-based 3D printing and molding to produce low-porosity thermoplastic composite parts rapidly.

3D printing + molding of large composite parts: To achieve high volume production of large 3D printed composite parts with high performance, Oak Ridge National Laboratory has developed a system called AMCM, which combines a robot, an extrusion-based 3D printer, and a molding machine. The first test case is the propeller blade shown here, with the ultimate goal of being able to produce automotive battery boxes and other high-volume, complex shapes.

A U.S. Department of Energy (DOE) user facility has been installed at Oak Ridge National Laboratory’s (ORNL) 110,000-square-foot Manufacturing Demonstration Facility (MDF), dedicated to early-stage R&D in a wide range of areas, including manufacturing, robotics, and simulation (including composites).

One of these innovations is a large-scale additive manufacturing (BAAM) large-format 3D printer developed and commercialized by ORNL in partnership with Cincinnati (Harrison, Ohio, USA). Large-scale additive manufacturing has long been used primarily to make molds needed for infrastructure, aerospace, and automotive applications, as well as end-use components such as machine tool bases and ship structures.

Traditionally, a limitation of large-format additive manufacturing is that 3D printing tends to produce parts with rough and irregular surfaces and high porosity, thus hindering the application of additive manufacturing in producing many high-performance end-use parts. Several commercial efforts have been made to reduce porosity by adding secondary processing steps such as molding. For the past two years, an ORNL team has been developing its scalable two-step process to eliminate porosity in the high-volume production of large-format end-use parts while ensuring a production cycle of no more than 2.5 minutes per part.

Developing production cells for high-volume, large 3D-printed parts

The process developed by ORNL, called AMCM (Additive Manufacturing + Molding), combines a robotic extrusion printer with a subsequent molding step in one cell.

The AMCM process has been in development for more than two years, and the team initially used MDF’s existing large-format BAAM printer and molding machine to demonstrate the process combination’s benefits, such as lower porosity in the final part. However, the printer and molder were not next to each other. Although the two machines were not too far apart, a reheating step had to be added to the belt furnace before molding to re-soften the preforms to the required glass transition temperature (Tg) for molding. However, an additional 5 to 6 minutes of preheating per part would significantly increase the total cycle time, bringing the total production beat to 8 to 9 minutes per part.

Therefore, they found that a dedicated production cell combining additive manufacturing and molding into one system was needed to demonstrate that this technology could be used in a high-volume production environment.

To shorten the production cycle time for this technology, ORNL developed the current AMCM production cell concept, which was installed in November 2021. The production cell includes an extrusion print head mounted on a six-axis KUKA robot arm, a 500-ton die press, and a material drying system that allows the printer to deposit up to 150 pounds of material per hour.

ORNL’s lab-scale AMCM cell, to be completed in fall 2021, combines robots, an extrusion-based 3D printer, and a molding press to test the system’s performance limits.

To produce a part in the AMCM cell, the part shape is first extruded directly onto a die, resulting in a three-dimensional custom preform. This preform is then directly fed into the press via a conveyor belt and is instantly molded. By extruding the material at or slightly above its melting temperature, the design of the AMCM cell allows the preform to reach the press for molding before the temperature of the material drops below its glass transition temperature.

Printing in the Mold: To reduce cycle time, the AMCM system prints directly in the Mold, and the printed preforms are fed to the press via a conveyor belt.

The team used the AMCM unit to demonstrate the production of a simple flat panel of 20% carbon fiber reinforced ABS with a cycle time of 2.5 minutes to complete the printing, pressing, and drying process. In addition, the AMCM cell demonstrated the production of other flat parts, such as drone propellers.

The team hopes the printer mounted on the robotic arm can print parts of relatively large sizes or complex shapes. In contrast, the current lab-scale system is limited to a 41-inch by 48-inch Mold on a 500-ton press, and while the molds currently used are unheated, if larger parts are to be printed, the molds may need to be heated to ensure that the 3D printed preforms do not too quickly cool down, as well as the temperature can be maintained above the glass transition temperature during printing and transfer to the press. It is important to note that current parameters, such as extrusion temperature, can change when using a heated die, so there is no need to worry about the material cooling down too quickly.

Initial demonstrations of the cell focused on producing flat parts in a single material to test the speed and performance of the system. Still, the team’s ultimate goal is to print more complex non-flat shapes and multi-material parts that can use materials, including glass-fiber-reinforced ABS and glass fiber-reinforced nylon.

The advantage of this approach is that parts with less than 1.5 percent porosity can be made compared to traditional injection molding or extrusion molding (ECM) processes. The team is also integrating fiber control for 3D printing to align the fibers in the direction of the printed beads to obtain the low porosity of molded molding.

The process is highly controlled, eliminating all porosity and achieving focused fiber orientation and alignment.

First test case: UAV propeller blades ORNL expects its AMCM cells to produce commercially available parts, possibly large ones, in high volume. As a first test case, technicians fabricated a series of carbon-fiber-reinforced thermoplastic propeller blades for UAVs.

While the AMCM cell now installed at MDF is a lab-scale system, it is said to be easily scalable to a production-scale facility and to allow for digitization, digital design, and automation.

The team hopes to use the process for high-volume production of next-generation composite parts needed in automotive and urban air transportation, such as battery boxes or seat backs, as well as drone propellers. The system is expected to produce 120 parts per hour when fully optimized.

More 3D printing

The AMCM process is one of MDF’s main innovations but not the only 3D printing innovation for composites that MDF is developing. Overall, 3D printing has always been a major core area of focus at ORNL, including ongoing multi-material printing using its BAAM printer, printing with foam, printing with bio-based recycled or natural fibers, and printing wires into the center of 3D printed parts.

The first laboratory version of the Reactive Additive Manufacturing (RAM) printer was also installed at MDF. This is a large-format thermoset composite 3D printer developed by ORNL in collaboration with Magnum Venus Products (MVP) of Knoxville, Tennessee, USA, and printed with resins from Polynt, USA, and is being developed and marketed commercially at MVP’s nearby Knoxville facility.

At MDF, ORNL continues to optimize the material’s flow properties by using the RAM system to build demonstration parts. Other research using RAM printers includes a project with MVP to integrate a RAM printing system with an MVP lab-scale fiber winder to be installed at MDF. the project also involves printing demonstration parts using RAM printers and then winding fibers around the cured 3D printed parts as a way to strengthen the parts by adding material. ORNL is also working on a project ORNL is also working with Boeing on a project to 3D print molds using the RAM system.

Using Magnum Venus Products equipment, ORNL researchers are exploring the integration of fiber winding (which provides compressive strength) into 3D printed parts (to obtain complex shapes)

In collaboration with Orbital Composites, ORNL is also seeking to optimize multi-material robotic 3D printing technology. Researchers are working to make the system (capable of printing thermoplastic or thermoset filaments) capable of printing more accurately on top of non-planar surfaces, as well as printing continuous fibers onto the top of thermoplastic discontinuous fiber parts. To this end, ORNL is researching material feedstock, while Orbital aims to refine the machine and accompanying software. Applications under development include drone parts, molds, automotive, and wind turbine components.

Orbital Composites is working with ORNL to develop new thermoset and thermoplastic materials for its robotic 3D printing system, and several test machines have been installed at MDF.

Continued composite innovation: CMC, biomaterials, and more

With and without 3D printing, MDF is currently active in numerous composite materials, including carbon-carbon materials for extreme environments such as space re-entry structures and nuclear reactors. For example, Orbital Composites is experimenting with 3D printing carbon-carbon rocket nozzles with MDF at ORNL. ORNL is also working on molds of carbon-carbon materials that have the advantage of having a near-zero coefficient of thermal expansion. Another collaboration with project partner Sandia National Laboratories is working on carbon-carbon and ceramic matrix composites (CMC) for defense and aerospace applications.

The development of bio-based materials, particularly bio-based feedstocks for 3D printing, is also a focus, led by ORNL senior R&D scientist Dr. Soydan Ozcan. ORNL is working closely with the Center for Advanced Structures and Composites at the University of Maine to develop cellulose nanofibers (CNFs) and the application of these materials using various production processes. In a pilot project with the University of Maine, they made two molds using CNF materials on an LFAM printer with a build area of 60 feet by 22 feet at the University of Maine to produce seven offshore wind turbine blades 100 feet long. For this project, ORNL completed the development of materials and equipment.

Another team focused on sustainability is working on a lab-scale recycling method for composite parts, including wind turbine blades. The system uses a shredder and waterjet cutting machine to explore new methods of processing/shredding end-of-life components. Recycling cases are also evaluated, such as mixing recycled shredded material into 3D-printed pellets that are then sent back to the BAAM printer for printing.

A 30-minute drive from MDF, the U.S. Department of Energy’s Carbon Fiber Technology Facility (CFTF) at ORNL operates a full-scale carbon fiber production line, allowing technicians to test and optimize all aspects of carbon fiber production and the performance of the carbon fiber coming off the line. A BAAM printer and pultrusion system enable the direct manufacture of parts from newly made fibers. Ongoing research also includes using coal for manufacturing low-cost, high-quality carbon fibers and treating carbon fiber during production, making it naturally lightning-resistant, and eliminating the need for additional lightning strike protection for aircraft or wind turbine components.

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