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Multi-material 3D printing is becoming more common with newer 3D printers, but there are still challenges in getting the layers of multiple materials to adhere properly. In a paper entitled “Enhanced Bonding of Immiscible Polymers via Intermixed Co-extrusion in Fused Deposition Modeling,” a pair of researchers describe a method designed to improve the bonding strength of parts 3D printed with dissimilar materials.

“Since most multi-material FDM systems use separate nozzles for each of the material, this limits the capability of printing true FGM (functionally graded material) parts because these printers produce components with very distinct transition from one material to another,” the researchers point out. “Thus, the devices printed with these multi-material FDM systems are vulnerable to delamination or other types of failures when loaded mechanically or thermally because of potential material bonding issues.”

Therefore, the researchers developed a multi-material FDM 3D printing system with a single nozzle capable of depositing multiple materials, changing the composition continuously while printing. They designed and manufactured a custom tri-extruder system with three inert channels. The system can be used to print with or without intermixing, therefore it is capable of printing by both side-by-side co-extrusion and intermixed co-extrusion.

The tri-extruder head has four main components: two guideways, a split melt chamber and a nozzle. The guideways have a heat sink that ensures that the filaments do not reach a temperature above their glass transition temperature.

“Two inlets are angled at 45° to the horizontal plane from the left and right side towards the melt-chamber and nozzle, and the third inlet is arranged at 90° to the horizontal plane and concentric with to the melt chamber and nozzle exit,” the researchers explain. “The left and right inlet are used for thermoplastic materials whereas the top inlet is intended for future work. Hence, the top inlet was kept closed using a M6 screw during the entire work here.”

The tri-extruder was installed on a Geeetech 3D printer. To turn on the intermixing, a static helical intermixer is inserted into the nozzle which is then screwed to the melt chamber. When the molten polymers flow through the intermixer, they mix by chaotic advection. ABS and HIPS were used as the materials for the study.

The tensile strengths of the side-by-side printed parts and the intermixed parts were indistinguishable, showing that intermixing does not have any adverse effect on mechanical strength. However, the side-by-side printed parts split after tensile tests while the intermixed parts cracked uniformly across the cross section.

“Tests with extrudate samples confirm that the intermixed co-extrudates have random but well consistent mechanical keying, resulting in similar ultimate strength of side-by-side co-extrudates,” the researchers conclude. “More importantly, the planar samples show that intermixing enhance the adhesion strength of two adjacent deposited beads by more than 40%.”

The results suggest that the intermixing may result in higher mechanical strength, as long as all other parameters are kept the same. Future work will include further mechanical tests and the 3D printing of functionally graded materials.

Authors of the paper include Mohammad Abu Hasan Khondoker and Dan Sameoto.

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