The structural feasibility of 3D-printing houses using printable polymers

Abstract

At this point in time, 3D-printing techniques in general, but especially applied for the building industry, still are in a phase of early experiments. One of the experimental attempts is to print a full-scale, three-story high, house in Amsterdam, using an up scaled version of a FDM-printer that is able to print blocks of 1.8 x 1.8 x 3.0 meters using printable polymers. The paper focuses on answering the initial structural question that appears around this project, which is a research to the behavior of the currently applied printing material. The outcome of this research will be used to give recommendations for the structural design of printable geometries. The material research emphasis on obtaining the material properties that are most essential to be known for this particular printing material and the application of the material within this particular building project. These basically are the mechanical (strength) properties of the material and its thermal behavior. Since the FDM-printer lays down the material layer by layer, the hypothesis was that the material would show anisotropic behavior. Therefore the strength properties are researched in different orientations relative to the direction of the printed lines. Furthermore, it was expected that the strength properties would differ for the horizontal plane and vertical plane in which there can be printed, as the resolutions in both planes differ as well. For the vertical printing plane, the material indeed shows clear anisotropic behavior, as the tensile, shear and flexural strength values and the failure modes parallel and perpendicular to the printing direction differ significantly. Material that is printed within the horizontal printing plane shows more isotropic behavior than material that is printed in the vertical plane, due to more and better adhesive connections between the different layers. For the compressive strength it holds that not much difference is noticed between the different orientations, especially because the tested samples are composed of multiple printed layers in both the horizontal and vertical plane. This result leads to an important recommendation to compose printable building-blocks out of 3D-elements instead of only 2D-plate elements, which so far has been the case. This actually leads to a stronger, more isotropic, more homogeneous and therefore better predictable material behavior. Throughout the research this recommendation is further confirmed by outcomes of the absorption test, geometry tests and insulation requirements. The absorption test shows that the material becomes watertight in case multiple printing layers are applied in the horizontal direction. The performed bending and pressure tests on printed geometries demonstrate that geometries, which are build up by a single layer, fail due to local effects: they either fail on local buckling or local bending of an individual member of the geometry or they fail at the location of local inaccuracies, which often occur within printed geometries. The stress level at which these failure modes take place can be significantly increased by composing individual geometry members out of multiple printed layers. Finally, to meet the requirements for heat and sound insulation, a certain wall and floor thickness is required which only can be achieved by printing multiple layers within the horizontal printing direction. The performed temperature-strength test and DSC-test show that current thermal behavior of the material is the most important point of concern regarding the applied printing material. The material is applied in the rubber phase, it softens at 60 degrees Celsius and at a surface temperature of 40 degrees Celsius, the material has lost already about 70% of the material strength it has at room temperature. It is obvious that further research on the improvement of the temperature behavior of the building material is essential to make printable polymers suitable for structural applications. Also the comparison with general structural materials and polymers applied in the construction practice, confirms that the printing material in its current form, is not structurally applicable. Furthermore, the comparison shows that the stiffness of the material needs to be improved, as the Young’s Modulus is relatively low. Although the essence of further research should lie on the improvement of the printing material, it still can be valuable to continue the structural design process parallel to the material development. Based on the performed material research, recommendations are given for design improvements. These recommendations can be used as a starting point for a possible future study to the structural design of printable geometries, chambers and complete houses.