To understand the various factors that affect the green strength, extensive characterization on the PEI-sand interface was conducted and it revealed unique interfacial interactions between PEI and silica sand. These properties, along with a low molecular weight (~800 g/mol), allow PEI to be dispersed in a solvent mixture with a wide range of solid loadings, which enables fine control of the polymer content within the green part to tailor the strength for various applications. PEI’s hyperbranched structure also provides low viscosity, high solubility, and limited crystallinity, making it ideal for the piezoelectric drop-on-demand (DOD) inkjetting process used in BJAM 14, 15, 16. The functionality enables the PEI binder to form strong green parts with silica sand, allows for reactive secondary infiltration, and exhibits washout functionality. In the following study, we report a versatile binder, hyperbranched polyethyleneimine (PEI), for BJAM that has strong interfacial interactions due to the presence of its primary and secondary amine groups.
Thus, it is important to develop binders having strong interfacial interactions with the sand that will lead to strong green parts. The low mechanical strength of the no-bake system can be linked to limited interfacial interactions between the polymer and the sand.
These binders impart relatively low mechanical strength, making the manufacturing of functional tooling challenging 13. The current state-of-the-art binder for silica sand is a no-bake or self-hardening binder system, based on a furfuryl alcohol polymerization or phenol-formaldehyde reaction. In BJAM, the binder is responsible for the mechanical strength of the green parts. However, the mechanical weakness of the green parts has been a major bottleneck, preventing the widespread use of BJAM of silica sand for tooling. Silica sand is an attractive powdered material that can be readily processed using BJAM, and it is especially suited for creating tools and dies due to its low coefficient of thermal expansion (CTE) coupled with its low cost 10, 11, 12. The green part can then be post-processed in various ways, including sintering or polymer infiltration, to produce dense materials with higher mechanical strength 9. The process forms a porous preform (i.e., a green part) after curing at elevated temperature. Selective inkjetting is used to deliver the binding polymer into the layers of powder to bind the powdered materials together (Fig. BJAM is an AM process that uses a layer-by-layer polymer binding process to rapidly manufacture complex three-dimensional tools and dies at significantly higher throughput when compared with directed-energy binding AM methods 6, 7. Binder jet additive manufacturing (BJAM) is well suited for manufacturing these tools and dies due to its high production rates, low operator burden, high resolution, and ability to process low-cost feedstocks, such as sand 6. The strong printed parts coupled with the ability for sacrificial washout presents potential to revolutionize AM in various applications including construction and tooling.Īdditive manufacturing (AM) enables the creation of unparalleled structures out of common materials 1, 2, 3, 4, 5 and provides a path to producing customized tools and dies for industrial manufacturing. Furthermore, we demonstrate that PEI in the printed parts can be reacted with ethyl cyanoacrylate through a secondary infiltration, resulting in an increase in flexural strength to 52.7 MPa. We report the discovery of a versatile polyethyleneimine (PEI) binder for silica sand that doubled the flexural strength of parts to 6.28 MPa compared with that of the conventional binder, making it stronger than unreinforced concrete (~4.5 MPa) in flexural loading. Adoption of BJAM has been limited due to its inability to form strong green parts using conventional binders. BJAM utilizes inkjet printing to selectively bind these powder particles together to form complex geometries.
Binder Jet Additive Manufacturing (BJAM) is a versatile AM technique that can form parts from a variety of powdered materials including metals, ceramics, and polymers.