Faster, stronger, fiber reinforced

3D printing, also known as “additive manufacturing”, has evolved in recent years from the simple production of lowcost plastic molded parts to a wide variety of sophisticated engineering manufacturing processes. In addition to highly diverse plastics and metals used for additive manufacturing, the nature of the printing processes themselves has reached an increasingly high level of technical specificity. For the production of prototypes and small series, this type of component manufacturing is unrivaled because it can be implemented quickly and cost-effectively. Complex components for technical applications place increased demands on printing techniques in order to meet the requirements for strength, weight and other physical properties.

Accordingly, various printing processes have already been able to establish themselves. In laser sintering, a laser beam fuses powdery starting materials layer by layer to form the finished object. In selective laser melting, metal powders are fused in a similar process. Stereolithography and polygraphy use liquid photopolymers that are cured layer by layer. However, the FDM process is the most common printing technique, which has already spread to the consumer sector. FDM stands for “fused deposition modeling”, simply translated as “melt layering”. A continuous ribbon of a thermoplastic polymer is melted in the print head, which is heated to around 200 °C, and the object is then built up layer by layer. The simple handling of this technique and low printing costs undoubtedly predestine the FDM process for a wide range of applications, especially for the production of prototypes or in the consumer sector, where the technical requirements are not too high. However, significant disadvantages of the process are, on the one hand, the limited printing speeds for technical use, since the molten thermoplastic always has to cool down before a new layer can be applied. Secondly, the strength of the printed objects is limited, which makes them of only limited use for technical applications: the reason for this is the low molecular orientation of the polymers after curing. Printing processes that combine an increase in printing speed with increased component strengths are therefore indispensable for technical applications.

This is where fiber-reinforced 3D printing inevitably comes into play. In addition to the above-mentioned advantages, this technology also enables the optimization of strength-to-weight ratios when using suitable reinforcing fibers and polymers,because the fibers used give the printed objects greater strength without increasing the weight.

Two fundamentally different concepts are currently established in fiber-reinforced 3D printing: Printing with short fibers or with continuous fibers. Short fibers are added directly to the printing polymer and still enable a simple printing technique, while the strengths of the printed bodies are only moderately improved. Printing with continuous fibers is demanding, as two nozzles are needed for simultaneous printing. One nozzle extrudes the thermoplastic, the other the reinforcing fibers. It is possible to introduce continuous fibers in a defined manner in the load direction of the component. This is the outstanding advantage over the use of short fibers, with which only components of lower strength can be produced. Printing with continuous fibers is well suited to the targeted reinforcement of small-scale components. Highstrength reinforcing fibers made of carbon, glass or aramid are often used here. Reinforcing fibers can either be completely fused into the matrix polymer from the second extrusion die or, in another process, “ironed” into the printed object as a tape, as it were.

A completely different approach is being pursued at the Competence Center High Performance Fibers at the DITF in the working group of Dr. Erik Frank. In the “FaserFab” research project, prepared yarns are used as the exclusive printing material for fiber-reinforced 3D printing. Multiple individual components are no longer necessary. Printing speeds can be reduced while at the same time lowering costs. Carbon fibers, which are developed at the DITF and optimized for printing applications, are used as particularly highstrength reinforcing fibers. For 3D printing, the fibers are prepared as yarn or tape. The polymer matrix is part of the yarn. The polymer, in this case PA6, is applied as a wrapping yarn to the core carbon fibers. Fiber-reinforced printing can then be carried out with just one spinneret – with a continuous feed of a continuous yarn. In the case of the wrapping yarn, only the wrapped fiber is melted in the printing nozzle and pressed into the core fiber.

Further wrapping yarns are to be developed and optimized in this research project. For example, initial trials with PET core fibers and wrapped PLA yarns have already been successful. Furthermore, PA6.6 core fibers will be combined with PLA. Metal fiber multifilaments and glass fiber yarns will also be prepared as core yarns and optimized for smooth feeding into the 3D printer. The DITF produce the yarns themselves on rewinding machines. The core yarn materials, which are so diverse, can cover a wide range of requirements for the printed components. In addition to the use of core yarns, another approach is being pursued: Here, the prepared printing filament is to be a bicomponent fiber. This fiber consists of a high-melting core surrounded by a melting sheath polymer. The bicomponent fibers can also be produced in-house at the DITF's own spinning facilities. The core is made of PET, the sheath of PBT. Both materials have different melting points, so that only one component is melted via a defined temperature control in the printer, while the other remains as a reinforcing fiber.

The basic requirement for fiber-reinforced 3D printing is to achieve a high degree of fiber filling in the component. This is because only fiber bundles that are optimally densely packed guarantee the highest component strengths. The wrapping and bicomponent yarns described are particularly well suited for achieving high fiber fill ratios. In the extrusion die, they allow processing under high pressures, which are necessary to compact the core fibers. Initial laboratory tests have already yielded promising results: PET core fibers with a wrapping yarn made of PLA enabled components with a high degree of fiber filling and already significantly increased strengths compared to a comparative body made of pure PET. However, the die geometry and temperature profile still require improvements, especially with regard to faster printing speeds. It is foreseeable that the novel yarns to be developed at the DITF will increase production speeds and expand technical application areas for 3D-printed components. Wellknown printer manufacturers have already registered their interest in the development of the printing yarns, as they expect to gain a competitive advantage in this rapidly developing market.