Reducing High Porosity and Improving Sealing Properties in Extrusion-Based 3D Printing

While there’s been a lot of ongoing research into what causes structural defects (porosity) and other flaws in metal 3D printed parts, let’s not forget that 3D printed objects made with other kinds of materials can also suffer from issues. A small team of researchers from the Russian Academy of Sciences’ N.D. Zelinsky Institute of Organic Chemistry is working to get rid of porosity in PLA objects that are created using extrusion-based 3D printing, and also evaluate the quality of 3D printed objects…and they learned something interesting about the shape of 3D printed objects in the process.

Functional assessment of 3D printing quality for objects of different shapes.

3D printing continues to develop at a rapid rate, and so too do the materials used to 3D print pieces and parts for a wide range of industries. Extrusion-based 3D printing technologies, such as fused deposition modeling (FDM), are among the most popular methods, but in order for the quality of these 3D printed parts to remain consistent, we must find a way to reduce high porosity and poor sealing properties.

If the walls of your print are not thick enough, warping can occur, which is obviously not what one hopes for in a successful print. According to the Zelinsky Institute research team, the effective sealing of liquids and gases “is a necessary requirement for the design of devices used in research, industry and a variety of practical applications.”

Researchers Evgeniy G. Gordeev, Alexey S. Galushko, and Valentine P. Ananikov have just published a paper, titled “Improvement of quality of 3D printed objects by elimination of microscopic structural defects in fused deposition modeling,” which explains their latest research efforts.

The abstract reads, “Additive manufacturing with fused deposition modeling (FDM) is currently optimized for a wide range of research and commercial applications. The major disadvantage of FDM-created products is their low quality and structural defects (porosity), which impose an obstacle to utilizing them in functional prototyping and direct digital manufacturing of objects intended to contact with gases and liquids. This article describes a simple and efficient approach for assessing the quality of 3D printed objects. Using this approach it was shown that the wall permeability of a printed object depends on its geometric shape and is gradually reduced in a following series: cylinder > cube > pyramid > sphere > cone. Filament feed rate, wall geometry and G-code-defined wall structure were found as primary parameters that influence the quality of 3D-printed products. Optimization of these parameters led to an overall increase in quality and improvement of sealing properties. It was demonstrated that high quality of 3D printed objects can be achieved using routinely available printers and standard filaments.”

The team used a desktop Picaso Designer PRO 250 3D printer to 3D print objects, using PLA material, in a variety of shapes, such as spherical, pyramidal, cylindrical, and conical, and then analyzed the objects. They developed an experiment to evaluate the quality of the 3D printed products, which began with connecting them with a flexible pipe to an air compressor.

Printed cylindrical tubes of different wall thickness

Then the objects were put inside a transparent, water-filled glass container, before low internal gas pressure was applied through the pipe. This caused bubbles to emanate from the objects’ pores, which then permeated its walls.

“The intensities and densities of the bubble flows corresponded to the linear dimensions and densities of the pores, respectively,” the paper reads. “The larger the diameter of the pore, the more intense the formation of air bubbles through this pore. The quantitative density of air bubbles on the surface of the printed part corresponds to the density of the through channels inside the wall.”

This approach could be used on all of the objects regardless of shape, and taught the researchers a valuable lesson: a 3D printed object’s wall permeability is very dependent on its geometric shape, even more so than the extrusion temperature or the type of the polymer used.

The paper concludes, “The present study shows that 3D printing is suitable for the production of completed and functionally consistent products with good sealing properties from a wide range of polymers even by using inexpensive personal 3D printers. Of course, each specific case of manufacturing by 3D printing can require a separate optimization of parameters, especially when changing the model of a 3D printer or filament material. The product properties can be affected by the feeder construction, presence/absence of a closed case, heating mode of the working platform, extruder cooling system, etc. Despite that, with proper optimization of printing conditions, commercial desktop 3D printers can be suitable for the production of sealed containers for various applications. The proposed quality assessment procedure allows the gradual improvement of the quality of 3D printed objects by elimination of structural defects.”

As research from the Russian Academy of Science increasingly examines aspects of 3D printing, such efforts will continue to contribute to global understanding of additive technologies.

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