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As 3D printing materials advance, it’s becoming possible to create stronger 3D printed parts. Porosity is an issue that afflicts many 3D printed parts, however, including those produced by FDM. One of FDM’s weaknesses is poor bonding between layers, which results in tiny pores and less strength than is ideal. Other factors can weaken parts, as well, depending on the material. Nylon is generally an excellent 3D printing material, but it tends to absorb humidity, which reduces both strength and stiffness.

In a new paper entitled “Failure Analysis of Additive Manufacturing Nylon Parts Coated with Protective Products,” which you can access here, a group of researchers explored the option of applying a protective coating over 3D printed nylon parts to reduce porosity and moisture absorption. They tested the strength and stiffness of the parts as well as the water absorption after applying the coatings, in hopes of opening up new applications for nylon 3D printed parts.

The researchers 3D printed two samples using a Mark Two 3D printer from Markforged. They then applied two different protective materials to the samples: the polyurethane elastomer 10Excellent (denoted as Exc) and the liquid silicone Rubson SL 3000 (Rub). Both are used in outdoor waterproofing applications. After being treated and dried for 24 hours, the 3D printed samples were placed over a filter paper pile immersed in water. Periodically, the samples were removed from the water and weighed. Compression and tension tests were then performed on the samples to determine yield stress and strain, plateau stress and strain, and ultimate stress and strain.

Specimens coated with Exc and Rub: a) compression samples; b) tensile samples

“Although silicone displays good weather resistance properties to nylon, the Rub product was found to poorly adhere to polyamide,” the researchers state. “The polyurethane Exc product presented much better water absorptions results in comparison with Rub product. It is known that pure nylon having a polar structure attract moisture as it bonds with hydrogen water molecule. However, polyamide in the presence of polyurethane, accomplish hydrogen bonding taking place among the coating molecules, which will reduce water absorption.”

Because of the hydrogen bonding that takes place between polyamide and polyurethane, the researchers continue, it is possible that the polyurethane coating acts as a plasticizer, affecting material properties such as strength and stiffness – decreasing them, in fact. But the decrease in strength and stiffness wasn’t significant, and the reduction of water absorption from the coating made the treatment worth it.

Failure of two tensile specimens coated with Rub product: a), b) with a raster angle of 90º; c), d) with a raster angle of 45º

“These results indicate that coating FDM parts with polyurethane based products is a new way to achieve polyamide parts with enhanced water absorption properties, which could contribute to a wide range of applications,” the researchers conclude. “Although a decrease in the strength and stiffness was detected of the nylon samples coated with polyurethane, they still show acceptable mechanical properties for non-load bearing applications, with a major reduction in the water absorption properties.”

Nylon 3D printed parts are being studied for use in medical applications, and are already being used in the automotive industry. Both of these applications could benefit from parts that absorb less moisture than untreated parts.

Authors of the paper include M. Miguel, M. Leite, A.R. Ribeiro, L. Reis, and M.F. Vaz.

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