Embedded 3D Printing of Thermally‐Cured Thermoset Elastomers and the Interdependence of Rheology and Machine Pathing

Abstract

polyether ether ketone (PEEK) can be directly printed into high-performance parts. [1][2][3][4][5] However, there are still large classes of materials that have been difficult to adapt to 3D printing. For example, thermally-cured thermosets such as epoxies and silicones are widely used in many applications for their combination of mechanical properties, chemical resistance, and thermal stability. [6] However, these polymers are often two-part systems that must be mixed and then take minutes to hours to crosslink and fully cure. These thermosets remain in a liquid state for a prolonged period and are thus challenging to 3D print with high fidelity because they flow and do not retain their intended geometry.Extrusion-based direct ink writing (DIW) has had success printing thermosets such as epoxies and silicones but typically requires modification of ink composition and rheological properties to make them thixotropic, or that are already thixotropic, to permit printing in air. [7,8] Additionally, DIW faces the same geometrical constraints that limit related fused deposition modeling (FDM) type approaches, such as overhangs and free-standing structures that are difficult to print without the use of support material. These constraints on the materials and geometries that can be 3D printed severely limit the complexity of parts that can be fabricated using slow-to-cure liquid prepolymers and soft materials.Freeform reversible embedding (FRE) 3D printing is a technique that was recently developed to print soft and liquid materials and overcome these challenges. [9] First described in separate papers by the Feinberg and Angelini groups in 2015, FRE and related embedded 3D printing techniques involve extruding prepolymers into a microgel-based support bath that possesses a yield stress. [10,11] Unlike typical FDM approaches in which the filament is extruded onto a platform, in FRE the material of choice (often referred to as the ink) is extruded directly into the support bath and held in place until it is cured. The support bath also greatly diminishes the effects of gravity and generally eliminates the need for any additional printed support structures. Despite these advantages, there are still challenges that are unique to the FRE process Thermoset elastomers are widely used high-performance materials due to their thermal stability, chemical resistance, and mechanical properties. However, established casting and molding techniques limit the overall 3D complexity of parts that can be fabricated. Advanced manufacturing methods such as 3D printing have improved design flexibility and reduced development time but have proved challenging using thermally-cured thermosets due to their viscosity, slow gelation kinetics, and high surface tension. To address this, freeform reversible embedding (FRE) 3D printing extrudes thermosets such as polydimethylsiloxane (PDMS) elastomer within a carbomer support bath, but due to the liquid-like state of the prepolymer during extrusion has been limited to hollow structures. Here, FRE printi...

Dissecting the Interplay Mechanism among Process Parameters toward the Biofabrication of High‐Quality Shapes in Embedded Bioprinting

“…Pluronic-based Pluronic, modified Pluronic [22,61] Alginate [32,37,62,175,176] Gelatin or GelMA -based [32,37,103,155,176] Collagen [32,37] Polyelectrolyte complex [177] PDMS, Silicone [178] 𝛼-calcium phosphate / Pluronic P123, 𝛼-calcium phosphate / hydroxypropyl methylcellulose [179] Silicone-based Carbon conductive grease [72] Photocrosslinkable PVA [180] Water/carboxylic acid-functionalized silica nanoparticles [181] PDMS, Silicone [81,180,182,183] Xanthan gum [65,184] Pluronic [184] Hyaluronic acid (HA)-based HA, modified HA [59,60,79,185] Spheroids [186] GelMA [79] Modified HA / PEDGA / Agarose μP [ 133] Carbopol PVA [180] PDMS, Silicone [49,87,117,129,180,[187][188][189] Alginate [143,[190][191]…”

“…Methylcellulose, Pluronic F-127, PVA [ 77] Gelatin μP/ Gelatin Chemical Carbon conductive grease, [ 72] Silicone-based [ 27,81,[181][182][183]218] , Pluronic-based, [ 73] Xanthan gum [ 65] Silicone-based or Mineral oil-based Silicone-based [ 49,87,129,[187][188][189] , Epoxy [201] Carbopol Pluronic-based, [ 61] Silicone-based [ 178,209] , 𝛼-calcium phosphate / Pluronic P123, 𝛼-calcium phosphate / hydroxypropyl methylcellulose [ 179] Pluronic-based Xanthan Gum, [ 43] PEG, [161] Fibrinogen, HA, Fibrinogen / HA, [ 154] Calcium phosphate, [ 163] Silk fibroin, [ 164] Alginate, [ 136] Collagen, Fibrinogen [ 56] Gelatin μP-based Carbohydrazide-Modified Gelatin [ 110] Oxidized alginate/Gelatin μP Silicone-based [ 168,169] Alginate μP Silicone-based [ 219] Pollen, Water…”

3D printing of polyvinyl alcohol hydrogels enabled by aqueous two-phase system

“…Microparticulate embedding media allowed freeform fabrication of thermoplastic polymers, 39,40 and curable resins. [41][42][43] PVA can be converted to a hydrogel by two simple routes with embedding media: (1) with a salt solution (e.g., sodium sulfate, sodium chloride), [44][45][46] and (2) with an alkali hydroxide solution (e.g., sodium hydroxide, potassium hydroxide). 47 When two aqueous solutions containing appropriate concentrations of specific polymers and salts are mixed, they separate into two immiscible polymer-rich phases and a salt-rich phase with water as a solvent in both phases.…”

A Novel Low-Temperature Extrusion Method for the Fused Filament Fabrication of Fluoroelastomer Compounds

“…These challenges converge to make 3D printing fully compounded thermoset elastomers via FFF/FDM a complex undertaking. Some developments have occurred recently in the additive manufacturing of curable thermoset elastomers, such as Nitrile Butyl Rubber (NBR) [17], Hydrogenated Butyl Rubber (HNBR) [18], and silicones [19][20][21][22][23][24]. However, most reports in the literature employ either the material extrusion approach with custom print heads (a ram-type material extruder, single and twin screw extruder) or the costly vat photopolymerization approach [17][18][19][20][21][22][23][24].…”

“…Some developments have occurred recently in the additive manufacturing of curable thermoset elastomers, such as Nitrile Butyl Rubber (NBR) [17], Hydrogenated Butyl Rubber (HNBR) [18], and silicones [19][20][21][22][23][24]. However, most reports in the literature employ either the material extrusion approach with custom print heads (a ram-type material extruder, single and twin screw extruder) or the costly vat photopolymerization approach [17][18][19][20][21][22][23][24]. To enable the general consumer to print with high-performance thermoset elastomers, such as NBR, HNBR, silicones, fluoroelastomer (FKM), and perfluoroelastomer (FFKM), it is desirable to find a method that would enable the printing of curable compounds using commercially available 3D printers.…”