Singapore: Effects of Porosity on Mechanical Properties in FDM 3D Printing

Authors Xue Wang, Liping Zhao, Jerry Ying His Fuh, and Heow Pueh Lee lead a complex discussion about porosity in 3D printing in their recently published article, ‘Effect of Porosity on Mechanical Properties of 3D Printed Polymers: Experiments and Micromechanical Modeling Based on X-ray Computed Tomography Analysis.’

With a focus on FDM 3D printing and the defects it can cause due to voids and pores create in 3D printed thermoplastics, the authors investigate and experiment to understand the true impacts on mechanical properties. They also propose a micromechanical model to predict elastic qualities. In explaining the process of FDM 3D printing, the authors state the following:

“The fabrication process itself is a thermal one introducing heterogeneities in micro/meso length scale, especially voids and pores, of which the size, shape, and spatial distribution are highly dependent on the process parameters. Such voids and pores may affect the internal structure of the deposited materials and, in turn, affect the mechanical properties of the final product.”

While there are already many different research studies in existence regarding porosity, the researchers here are specifically interested in how the mechanical properties are diminished. Tests are performed, using a non-destructive method, with X-ray computed tomography (XCT), using a chromatic X-ray cone to show 3D features and characterize the following:

• Size
• Density
• Shape
• Spatial location of pores

Schematic illustration of a cone-beam X-ray computed tomography (XCT) system.

Three samples were created for testing:

“To utilize the DIC technique, high contrast speckle patterns in black and white were applied to the specimens before the tensile tests. Each specimen was then clamped at the two ends in the universal testing machine by two pairs of hydraulically controlled jaws, which were separated at a speed of 2 mm/min until the specimen fractured. During the whole test process, the specimen images were captured at a rate of 10 Hz by high resolution DIC cameras, and simultaneously, the load data used for calculating the stress (that is, tensile load divided by the cross-sectional area of the specimen) were recorded by the testing machine at the same rate,” explained the researchers.

In terms of pore size, the researchers found that most of the pores were below 0.2 mm, making up 99 percent of the pores and indicating that PLA may cause a ‘major population’ of small pores. In each sample, the pores covered nearly the whole specimen.

The PLA also demonstrated anisotropic behaviors as the researchers investigated tensile properties. This was expected, as was the fact that samples printed with a 0.48 mm extrusion width exhibited better mechanical properties.

“The actual pore size distributions were used to generate the RVEs and in turn to predict the macroscopic elastic properties. The prediction results for the elastic properties showed good agreement with the corresponding experimental data where the percentage difference was not larger than 7.9%. The predicted elastic properties also agreed well with two existing numerical works. The proposed micromechanical model has demonstrated itself as a potential tool for predicting elastic properties for the future designer. This provides a possibility of saving the material from undergoing destructive testing,” concluded the researchers.

Porosity is an ongoing topic among 3D printing users on nearly every level, from examining porosity in titanium to improving sealing qualities and also examining lattice structures. Find out more about experiments on 3D printed polymers regarding porosity here. What do you think of this news? Let us know your thoughts! Join the discussion of this and other 3D printing topics at 3DPrintBoard.com.

Pore distribution for UPL = 1 mm characterized by X-ray computed tomography (XCT) of three-dimensional visualization in (a) and two-dimensional visualization in (b) the transverse plane, (c) the sagittal plane, and (d) the coronal plane. The red slim rectangular in (b) and (c) mark the selected pores of large size between two adjacent layers of the specimen. The distance between the l inearly aligne d large pores in (b) and (c) was measured as 0.4 mm, which is twice of the layer thickness of 0.2 mm.