Active Mixing of Reactive Materials for 3D Printing

Abstract

The mixing of materials during additive manufacturing is a major benefit which allows one to compositionally and spatially tailor material properties, for example to locally control the reactivity in fuel: oxidizer systems known as thermites. This work characterizes an active mixing printhead used in conjunction with a 3D printing process known as Direct Ink Writing. Besides compositional control, a major benefit of this approach is that it offers a safe method for working with these materials which can otherwise be hazardous once mixed. Custom fuel and oxidizer inks are fed at fixed volumetric rates into an active mixing head, and both the rotational speed of the mixing impeller as well as the fuel:oxidizer ratio are varied. Upon ignition, the propagation speed increases with the rotational speed of the mixer and plateaus above a critical value of approximately 750 RPM. The critical mixing speed is corroborated by computational fluid simulations and an analytical expression that considers the inks' complex fluid behavior. Additionally, varying the composition results in a wide range of propagation speeds with peak reactivity corresponding to a fuel-rich formulation ( = 1.5). A test article incorporating a fast and slow-burning region demonstrates how spatial composition can manipulate the reactivity. This is the author manuscript accepted for publication and has undergone full peer review but has not been through the copyediting, typesetting, pagination and proofreading process, which may lead to differences between this version and the Version of Record. Please cite this article as

Direct ink writing advances in multi-material structures for a sustainable future

“…For instance, work with platelet-based materials suggests that platelet roughness at a submicron level can have a large contribution to the mechanical response of composites. 71,72 It can be more difficult to manipulate structure at the microlevel and this may require modications of the printing system as in the use of magnetic elds to manipulate particle orientation 63 or systems designed to actively mix 43,44 the cartridge in situ during printing while controlling the feeding of the different components to enable compositional variations along the structure 73,74 (Fig. 3).…”

Designer Direct Ink Write 3D‐Printed Thermites with Tunable Energy Release Rates

“…This added versatility would be essential for a number of applications, such as soft robotics propellants, solid rocket propellants, and flame-encoded infochemistry systems. [15] Typically, the flame propagation velocity is used as a macroscopic surrogate of reactivity for the rate and intensity of complex reactions happening on various length scales within a thermite. [16] This rate can be tuned and modified using microstructure, [17] chemistry, [18] synthesis technique, [19] or particle size and mixing.…”

3D printed gradient index glass optics

“…Hence, the micromixer used in this work was optimized for longer residence time to ensure complete mixing within the filament. The design of this mixer has been described in detail and tested previously for mixing of reactive materials by Golobic et al (38). Here, we simulate and validate the suitability of this mixer design for printing glass optics from viscoelastic nanoparticle inks.…”

“…The syringes were then mounted onto a DIW system, where they were extruded using linear actuators (Parker). The inks were mixed in-line using a custom active mixing system [previously described by Golobic et al (38)], where both inks were fed into a mixing chamber and blended with a rotating spindle at approximately 1500 rpm while being extruded through a 610-m nozzle. The nozzle was held in a fixed position, while the material was deposited onto a silicone sheet, which was clamped to a threeaxis motion positioning stage (Aerotech).…”

“…The CFD simulations were performed for two identical inks at a mixing ratio of 1:1 and a range of rotational speeds of the mixer. The output was characterized by a coefficient of variation (COV) over the cross-sectional area of the nozzle outlet (37,38). Experimentally, the quality of mixing was tested with fumed silica nanoparticle-based inks (34).…”