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Essentium Materials, a company developing an isotropic FDM printing system, has asked researchers at Texas A&M University and Texas Tech University to evaluate the technology's effectiveness. Experiments have shown that the new technique increases the tensile strength between layers by 275%.
As it turns out, the technique is based on the principle of microwave welding rather than induction, as was previously thought. The idea is to coat a standard filament with carbon nanotubes, and then print and weld the deposited layers by heating with microwave radiation. The researchers' report revealed many interesting points. To begin with, the scientists used conventional PLA plastic in their experiments, which is due to the extremely simplicity of working with this material and a decrease in the likelihood of deformations during heat shrinkage, which could introduce inaccuracies in subsequent strength tests. In theory, the technology is applicable to almost any standard material.
A special coating is obtained by mixing carbon nanotubes into a solution of the plastic used (in this case polylactide). Chloroform was used as a solvent. The resulting solution is applied to a standard filament, and after drying, the bar is ready for printing. When applying the mortar, it is necessary to take into account the thickness of the resulting coating in order to avoid problems with pushing the bar into the hot end. At the same time, the film must be sufficiently saturated with nanotubes for optimal heating under the influence of microwaves, but not too much, since in this case there is a "mirror" effect - the material reflects more microwave radiation and absorbs less (see illustration below).
It is also noteworthy that the researchers tested both the application of the solution to the filament and the fabrication of a rod by coaxial extrusion of pure polylactide and nanotube-saturated plastic. The latter option turned out to be quite effective, which opens up the possibility of large-scale production of specialized filaments.
The question arises: why not just produce a composite of polymer and nanotubes instead of dancing with tambourines and coatings? The fact is that in this case, under microwave irradiation, uniform heating of 3D-printed models will occur, and this will lead to deformations or even spreading. Essentium Materials technology, on the other hand, involves spot heating to the melting point of the joints between layers. That is, it is welding. Hence the name - "Locally-Induced Radio Frequency Welding" or "Localized radio frequency welding".
Experiments in 3D printing on conventional desktop 3D printers (the company itself prefers Stackers) have shown that the special coating tolerates extrusion well, without mixing with the bulk of the filament. Optimal results were obtained at 240 ° C - the upper heating threshold of polylactide, which is insufficient for pyrolysis, but provides maximum interlayer adhesion. Of course, when working with other materials, you will need different temperature conditions. The researchers decided against blowing because cooling increases the risk of delamination.
After printing and microwave treatment, various strength tests were carried out, including tensile tests of the obtained samples and checking the level of adhesion between individual layers. In the illustration above, it can be seen that if the products from the control group are delaminated rather accurately, torn breaks are obtained in the welded samples, which just indicates the mixing of the layers to be joined. In general, the strength of interlayer adhesion has increased by 275%, and the effects of welding are manifested not only between layers, but also threads laid in the horizontal plane.
