Method of additive manufacturing and dual material elastomeric filament

Description (excerpt)

RELATED APPLICATION This application claims benefit to U.S. Provisional Patent Application No. 63/562,752 filed Mar. 8, 2024, which is incorporated by reference herein its entirety for all purposes. STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT This research was financially supported with the HEROES (Harnessing Emerging Research Opportunities to Empower Soldiers) program through a cooperative agreement through DEVCOM Soldier Center (#W911-QY-20-2-0005). The government has certain rights in the invention. GOVERNMENT INTEREST The invention described herein may be manufactured, used and licensed by or for the U.S. Government. Certain aspects of the invention have been previously disclosed by the inventors in the following: 1) Presentation titled: “ENHANCED STRUCTURAL STABILITY AND MECHANICAL STRENGTH OF THERMOPLASTIC MECHANICAL STRENGTH OF THERMOPLASTIC ELASTOMER BY ADDITIVE MANUFACTURINGELASTOMER BY ADDITIVE MANUFACTURING” presented by Nikhil Patel at APS March Meeting 2023/Mar. 5-10, 2023/Las Vegas, NV; and 2) Journal paper: N. A. Patil, K. Joshi, J. Lee, K. E. Strawhecker, R. Dunn, T. Lawton, E. D. Wetzel, J. H. Park. “Additive Manufacturing of Thermoplastic Elastomer Structures using Dual Material Core-Shell Filaments.” Additive Manufacturing. v82 n104044. 2024 (published online 15 Feb. 2024), which are incorporated by reference herein in their entireties. The latter primarily formed the basis for the '752 provisional application. BACKGROUND 3D printing enables the direct production of three-dimensional solids from a digital computer aided design (CAD) file. The most common print method-based on number of trained users, number of printers sold, and volume of material printed—is fused filament fabrication (FFF). In the FFF process, a spool of thermoplastic filament is fed into a print head where the filament is heated and then extruded from a small-diameter nozzle, where it is then deposited onto a build platform (also referred to as a print bed). The print head and bed are part of a computer-controlled motion control system, such as a belt-driven gantry system, so that the extrusion can be deposited systematically into a 3D solid. Typically, the CAD file of the part is provided to a slicing software that programs the motion control system to deposit the thermoplastic in perimeter and fill traces that populate one layer at a time, with each layer built vertically on top of the prior layer. The vertical build direction is referred to as the z-direction. FFF printers are inexpensive and easy to operate. The spool feedstocks are usually time stable, environmentally benign, and even recyclable. Parts are highly accurate geometrically. One key challenge of FFF is that the mechanical properties of the parts are limited. Conventional rigid thermoplastics that are commonly FFF printed include ABS, nylon, PLA, and polycarbonate. Parts printed with these materials tend to be mechanically weak, particularly when loaded parallel to the build direction (z-direction). This weakness arises because the thermoplastic of one layer has cooled by the time the next layer is deposited as a hot melted extrudate. The interface between these two materials is only hot and soft for a few seconds, which is not enough time for the processes of wetting and molecular reptation necessary to form a molecularly entangled, high strength interface. As a result, parts loaded in the z-direction fail at low loads, and fail in a brittle manner. Additionally, these rigid parts tend to have poor cold weather toughness. All polymers are known to go through a ductile to brittle transition as they are cooled, because lower temperatures repress molecular mobilities that are necessary for molecular rearrangements in response to load. Therefore, conventional FFF parts are not generally suitable for use in cold climates, where the parts would be both weak and brittle. Additionally, there is a need for FFF printing of soft elastomeric materials. Soft materials are useful for medical devices; body-worn devices that need to be comfortable against the skin; gaskets, o-rings, and sealing sleeves; and flexible structures that can be bent, folded, or collapsed. FFF printing introduces filament to the print head by pulling or pushing the filament, typically with traction wheels that grab and advance the filament. The force on the filament can be significant, as this filament force creates the flow pressure that drives melt extrusion from the print head nozzle. If the stiffness of the filament is too low, it will buckle during the feeding process. When the filament buckles, pressure in the print head reduces and extrusion will slow or stop. As a result, the printed p