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How microwave technology is reforming 3D printing and opening up new applications for continuous fiber reinforced plastics with multidimensional printing paths.

Druckprobe eines freistehenden 3D-Gitters, das mit neuem Mikrowellendrucker gedruckt wurde

The brushstroke of an artist is as individual as his works. Each one is unique and made according to individual ideas. Figuratively speaking, Dr. Nanya Li from KIT's Institute for High Power Pulse and Microwave Technology (IHM) belongs to the master class of 3D printing art. He gives additive manufacturing, better known as 3D printing, the flexibility and freedom of form with which a painter maneuvers his brush. Dr. Li and his team from the Microwave Materials Process Engineering group have developed a printing system for fiber-reinforced thermoplastics that can print highly complex shapes and custom geometries at top speeds. Such sophisticated fiber composite components are typically used in lightweight construction for automobiles, aviation and medical technology: from lightweight aircraft bodies and customized vehicle interiors to high-performance sports equipment, walking aids or prostheses. "Object art" for industrial use, which could be even better developed by KIT with the new process. Li explains: "We have applied for a patent in Europe for our SERPENS printing process, which works with electromagnetic waves. Currently, a third-generation prototype of the 3D microwave printer including path planning and control system is available, and we are continuing to experiment with it."

SERPENS is short for 'Super Efficient and Rapid Printing by Electromagnetic-heating Necessitated System,' as the researchers have christened their process. In this case, the curved brush is a new type of microwave print head that can be moved and rotated freely in three-dimensional space in conjunction with a robotic arm. This freedom of movement is only possible thanks to the combination of a novel printing principle using microwave radiation and the possibilities developed at IHM for controlling the printing system.

Layerless freedom

Conventional 3D printing, in which parts and components are built up layer by layer from engineering plastics, has already found its way into industrial production. Dr. Guido Link, who as head of the "Materials Process Technology with Microwaves" working group provides technical support for the work, reports: "Prototypes and pilot series in particular are already being produced from the printer today. In this process, plastic filaments are heated, the print head moves within three axes over a printing plate and applies the heated, liquefied plastic layer by layer - until the printed object is layered to the final layer." The new technology from KIT breaks with these conventions and uses filaments made of plastic reinforced with continuous carbon fibers. This reinforcement allows the print head to be pivoted to any position in space during printing, while the print material can be ejected from the print nozzle in any conceivable direction. For example, complex components with undercuts or cavities can be produced. The fact that the liquefied material solidifies directly and even without a solid base is due to microwave technology.

Continuous fiber-reinforced thermoplastics (CFRTP) are melted uniformly with the aid of electromagnetic waves and freely molded with the pressure nozzle
Continuous fiber-reinforced thermoplastics (CFRTP) are melted uniformly with the aid of electromagnetic waves and freely molded with the pressure nozzle

Plastic melting with microwave radiation

In contrast to conventional 3D printers, the printing material in SERPENS is not heated using conventional methods via contact with a heating element. Instead, the special filamentary plastic filaments with integrated continuous carbon fibers are melted using electromagnetic waves as they pass through the print head. To do this, the research team has invented a resonant microwave applicator as a print head that liquefies filaments with very different diameters very quickly and uniformly. As it emerges from the print nozzle, the mass cools and immediately becomes dimensionally stable due to the fiber reinforcement. The process thus makes it possible for the first time to produce even free-standing lattice structures. As a result, previous process-related limitations for the design and construction of fiber-reinforced composite components are softened. Completely new shapes can be produced. Microwave technology not only brings advantages in computer-aided design (CAD), but also represents an energy- and cost-efficient alternative to thermal melting.

As is typical of microwaves, the electromagnetic waves generate a very homogeneous temperature distribution, so that the plastic component heats up evenly and quickly. This means that even large filament diameters of up to 5 millimeters can be liquefied and printed at high printing speeds of up to 100 millimeters per second. This plus of printing material in one cast in turn has a positive effect on the statics of the printed objects: The mechanical load-bearing capacity of the fiber composite components can be increased by a purpose-optimized inner support structure (Infi ll). Microwave technology offers enormous potential for the further development of 3D printing. Strong and lightweight composite components can be produced quickly and without high manual manufacturing effort. The freedom of form of SERPENS also opens up completely new applications that have not yet benefited from the advantages of additive manufacturing.

Printing material to match the system

The processing of plastic and carbon fibers in a single filament is another special feature of the SERPENS process. "The microwave printer requires prefabricated filaments in which the polymer surrounds the continuous carbon fibers. The fibers in the filament core not only serve the stability of printed components, but also support the melting of the filament as an inner conductor in the microwave print head," explains inventor Li. In order to be able to print with the prototype in the laboratory, the working group from IHM has developed its own production line for its filaments. This can be used to produce customized filaments with precisely fitting properties.

The third generation of the microwave printer relies on robotics for even more flexibility
The third generation of the microwave printer relies on robotics for even more flexibility

Small batch production made to measure

The industry already appreciates the benefits of additive manufacturing in rapid prototyping and one-off production: With fiber composite components from the printer, manufacturers can take into account individual customer wishes and respond relatively easily and quickly to adaptations. Compared to other processes such as mold casting, printing completely eliminates the need for cost-intensive tool or mold production. The new process from KIT significantly surpasses conventional plastic printing techniques in terms of time required and freedom of form, thus making industrial manufacturing by means of 3D printing even more widely applicable - right up to 'mass customization'. This gives a customer the ability to personalize certain features of a product while keeping costs at or near mass production levels.

The speed advantage of SERPENS allows economical processing of small batches as well as customized special products in series. From batch size one to mass production, SERPENS opens up new possibilities that the KIT researchers would like to transfer into production reality with the appropriate industrial partner. Dr. Li emphasizes, "Our system makes it possible to combine the trend toward individualization with the economical additive series production of fiber composite components with continuous fibers. If we can create a commercial printer with industry support, we will make fiber-reinforced plastics accessible and easy to use for more industries."

Further links

Images: Riccardo Prevete · Institute for Pulsed Power and Microwave Technology / KIT · Markus Breig / KIT

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