3D printed parts with orientation

AddiSim: Phenomenological design strategy for additively manufactured plastic components.

Directional material properties of printed test specimens

3D printed components are also increasingly being used in structural applications. In this context, it is necessary to be able to describe the mechanical properties in detail and to prepare suitable methods for the design. These points are being investigated in the AddiSim project involving selectively laser sintered (SLS) PA12 components. A major focus is on determining the influence of the position and orientation of the components in the build chamber. Parts with identical construction but printed in different orientations have very different mechanical properties. The aim is to provide a simple method for the safe design of 3D printed parts.

 

The aim of the ongoing AddiSim project is to develop a simplified method for the structural simulation of SLS-produced components based on mechanical investigations. Using this procedure, extensive results on the behavior of the printed material are determined based on a detailed measurement of the build chamber. The method generated at Fraunhofer LBF includes important findings for the safe and lightweight dimensioning of SLS-produced components. Manufacturers of printed components do not often have fully equipped mechanical testing laboratories at their disposal. Nevertheless, they are expected to develop lightweight components. This problem can be solved using the simplified design methodology.

The SLS process is outlined in the schematic sketch (Fig. 1). After the contour of the entire component is sintered, the component cools in the powder cake (Fig. 2.1). It is usually removed manually. This step involves removing the parts from the powder cake, sandblasting, blasting with compressed air (Fig. 2.2) and a spray shower to remove any remaining loose powder residue (Fig. 2.3). Any powder which is at an appropriate distance from components in the powder cake can be reused as recyclate in the next print job with a 1:1 ratio of fresh powder to recyclate, as recommended by the manufacturer. Powder close to the component cannot be used as a recyclate due to its previous temperature history. 

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Figure 3: Absolute deviation from the nominal dimension, printing direction of the test specimen depending on the orientation (top right)

Length measurement dependent on the orientation in the build chamber

In order to locally determine the mechanical behavior of the material in detail, the installation space is divided into 27 measurement cells. A tensile test specimen is aligned with one of the three main directions x, y and z of the printer in each of these cells. All printed test specimens are measured for dimensional deviation. The thickness, width and length of each specimen are recorded. The length measurement results in an absolute deviation of up to 0.3 mm (Fig. 3). The test specimens oriented in the x direction (green) exhibit their greatest deviation from the nominal dimension in the thickness dimensions. Similarly, the specimens oriented in the y direction (blue) show their highest deviation in the width and the specimens oriented in the z direction (red) in the length. It is evident that the dimensional stability on the printing level is significantly higher than in the build direction. 

Figure 4: Stress-strain curve of the differently oriented test specimens

Mechanical behavior

Quasistatic tensile tests are carried out to determine and evaluate the mechanical behavior of the test specimens produced. The tensile tests with low strain rates show no significant orientation dependence (Fig. 4). In the non-linear range, however, a clear dependence is evident. The stress-strain curves in the x and y directions appear to be isotropic, whereas a pronounced anisotropy is evident in the z direction. Specimens oriented in the z direction show a reduction in ultimate strain and maximum stress compared to specimens oriented in the x and y directions.

Simulation approaches for the reliable design of additively produced structures

Based on LBF data, no directional dependence is expected at low strain rates. Therefore, an isotropic approach can be selected to describe the mechanical material behavior.

In the plastic range, the researchers find a direction-dependent material behavior, which is strongly anisotropic in the z direction, compared to the x and y direction. This can be simulated by an orthotropic material model. For this purpose, the orientation of the component in the build chamber must be transferred to the simulation model.

The reported tests do not prove a position dependence for the examined printer and as such this is not taken into account in the simulation.

The simulation approach will be discussed in the coming year with regard to its imaging quality on the basis of validation experiments that are still pending. The project results obtained will make it possible to support customers in acquiring detailed knowledge on the mechanical behavior of SLS components. In addition, Fraunhofer LBF offers methods for the design of these components using structural simulations, which will be refined during the course of the project. The objective of being able to use this strategy for the design of reliable additively produced components in the future has been achieved.

Sponsors and partners

The results presented were obtained in connection with IGF project 21105 N of the German Research Association for Plastics (Forschungsgesellschaft Kunststoffe e. V.), entitled “Phenomenological strategies for the consideration of process-specific material properties in the simulative design of additively produced plastic components”. It was funded by the German Federal Ministry for Economic Affairs and Energy (BMWi) via the German Federation of Industrial Research Associations (AiF) as part of the program for the promotion of joint industrial research (IGF) on the basis of a resolution of the German Federal Parliament (Bundestag).
We would like to express our sincere thanks for this funding, as well as to the German Research Association for Plastics and the representatives of the committee accompanying the project.

 

»As Managing Director, I am delighted to be involved in the current research. Through direct dialog, we discuss issues relevant to our day-to-day work and provide printing time, for example. This means that in return, we receive additional information about our own printers.«

Felix Wendt, Managing Director Fiberthree GmbH