Negshell casting: 3D-printed structured and sacrificial cores for soft robot fabrication

Soft robot fabrication by casting liquid elastomer often requires multiple steps of casting or skillful manual labor. We present a novel soft robotic fabrication technique: negshell casting (negative-space eggshell casting), that reduces the steps required for fabrication by introducing 3D-printed thin-walled cores for use in casting that are meant to be left in place instead of being removed later in the fabrication process. Negshell casting consists of two types of cores: sacrificial cores (negshell cores) and structural cores. Negshell cores are designed to be broken into small pieces that have little effect on the mechanical structure of the soft robot, and can be used for creating fluidic channels and bellows for actuation. Structural cores, on the other hand, are not meant to be broken, and are for increasing the stiffness of soft robotic structures, such as endoskeletons. We describe the design and fabrication concepts for both types of cores and report the mechanical characterization of the cores embedded in silicone rubber specimens. We also present an example use-case of negshell casting for a single joint soft robotic finger, along with an experiment to demonstrate how negshell casting concepts can aid in force transmission. Finally, we present real-world usage of negshell casting in a 6 degree-of-freedom three-finger soft robotic gripper, and a demonstration of the gripper in a robotic pick-and-place task. A companion website with further details about fabrication (as well as an introduction to molding and casting for those who are unfamiliar with the terms), engineering file downloads, and experimental data is provided at https://negshell.github.io/.

The fabrication of soft robotic structures is often a tedious process with multiple 2 steps [1][2][3]. Soft robots benefit greatly from complex mechanical structures and 3 geometric features to achieve actuation and sensation. Unfortunately, the soft and 4 compliant nature of the materials used for soft robots, such as silicone or 5 urethane-based elastomer, are only compatible with few fabrication methods such as 6 injection molding and casting. Casting has tremendous benefits in terms of the example, the ubiquitous PneuNets [4] actuator relies on expanding internal bladders, 12 while some sensing modalities require fluids embedded within the soft robot [5]. Casting 13 these internal features is not a straightforward process. The fabrication process often 14 requires multiple steps: first, a "core" that will create the internal features must be 15 casted around and removed, then, another section is additionally casted to complete the 16 body or another piece of the body is bonded onto the first part. Because these cores 17 have to be removed during the fabrication process, they either: 1. have geometry 18 constraints, such as minimal undercuts and overhangs, to prevent difficulty of removal, 19 or 2. are made to be dissolved later, also known as lost-wax casting [3]. Lost-wax 20 casting enables more complex geometry for the core, but the core itself must be cast 21 from wax, which is prone to shrinkage and breakage. Aside from wax, 3D-printable 22 dissolvable materials can be used to create arbitrarily-shaped molds and cores, such as 23 polyvinyl alcohol (PVA) which dissolves in hot water or acrylonitrile butadiene styrene 24 (ABS) which dissolves in acetone [6]. However, dissolving the material takes a large 25 amount of time (more than 3 hours, according to [6]) and requires the core to be 26 accessible from the outside of the soft robot to drive liquid solvent through.

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3D printing soft robots is another heavily-researched fabrication process. 3D printing 28 enables internal and external features to be fabricated directly and with multiple 29 materials simultaneously. However, some solutions are prohibitively slow for mass 30 manufacturing while others have limitations in geometry due to the absence of support 31 material [7,8]. Resin-jetting, multi-material printers such as the Objet Connex 32 (Stratasys, Ltd.) series have both support material and elastomeric-like resins, but suffer 33 from poor mechanical properties when compared to typical silicone rubbers [9]. Some 34 stereolithography (SLA) [10,11], digital light projection (DLP) [12][13][14] or continuous 35 liquid interface production (CLIP) printers have high-resolution and fast print times, 36 but can only print with one material at a time, which leads to support structures that 37 need to be printed from the same material and then later removed. Enclosed 38 chamber-like features, such as for bellows and deformation sensors, printed with 39 SLA-style printers also suffer from cupping artefacts which can lead to ruptures [15]. 40 High resolution 3D printing with SLA-like methods does have its benefits, however, 41 in the casting process, as the features in the outer shell molds can be as complex and 42 intricate as the designer desires due to SLA's high-resolution. Furthermore, resin based 43 printing can create thin-walled parts and small cavities for use in microfluidic 44 devices [16]. In this paper, we combine the benefits of casting liquid silicone rubber and 45 SLA-based resin printing by 3D printing thin-walled cores for the internal features of 46 soft robots that are meant to be left in place after casting. This greatly reduces the 47 steps required for fabrication as casting is only done once -since the core is never 48 removed. These thin-walled cores can be used as sacrificial elements such as for creating 49 expanding bellow chambers and fluidic channels, or used as passive structural elements 50 such as fingernails or bony features in soft robotic fingers. Another benefit is that since 51 the the thin-walled cores are devoid of material, they can occupy space in the form of 52 air instead of heavy silicone rubber, which is useful for creating features that require 53 high stiffness and low weight. We characterize these sacrificial thin-walled cores 54 (negative-space eggshells, or negshells) and non sacrificial cores (structural cores) casted 55 in silicone rubber through mechanical testing. We also provide the fabrication steps for 56 3D printing our cores and molds, and guidelines for designing soft robots that employ 57 our fabrication method along with an example to show the efficacy of the cores when 58 combined. We also present an example application in the form of a three-finger gripper, 59 as shown in Fig.1, performing simple pick-and-place tasks while mounted on a robotic 60 arm. Negshell casting used for a three-finger soft robotic gripper. Pictured are three stages of negshell casting from the rightmost to the leftmost finger: 1. 3D-printed structural and negshell cores, 2. The cores placed in 3D-printed molds to be injected with silicone rubber, and 3. A completed finger ready to be actuated.

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In this section, we describe the design of each component for negshell fabrication. We 63 contextualize and describe sacrificial and structural cores. Next, we describe how these 64 cores interact with the remainder of the mold and how they are affixed into place for 65 casting. We then present the design of an example application, a bellow-jointed finger 66 incorporating both kinds of cores, and demonstrating how this relatively common design 67 theme can be miniaturized and fabricated faster and more reliably.

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Soft robotic features such as expanding bellows or sensor cavities require a void to be 70 left after fabrication that is later filled with fluids. As described previously, these voids 71 are often created by solid geometry (cores) that represents the void that is later 72 removed either by means of dissolving (lost-wax casting) or removed during the casting 73 process. In our fabrication process, we replace these cores with 3D printed thin-walled 74 volumes (negative-space eggshells or negshells) that are meant to be left in place or 75 broken into small pieces that minimally mechanically effect the surrounding structure.

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The negshell cores are simply the outer surface of the desired core with a thickness of 77 0.4 mm, which is the minimum supported wall thickness for the Formlabs' resin we use 78 to print them. This minimum thickness is used to maintain the shape of the volume 79 during handling while being fragile enough to break under minimal force. To help 80 promote breakage, a cross-hatch pattern, shown in Fig.2, is cut throughout the surface, 81 as shown in Fig.3. The patterned minuscule slots also serve as channels for uncured 82 resin to escape during the print process. Finally, 1.5 mm diameter holes are created for 83 support structures for suspending the cores in the mold, which is further explained in 84 the Outer molds and support columns section. 85 We design our negshell cores as parts in SolidWorks, but the process can be adapted 86 to any modern CAD software. We start by modelling the solid representation of the 87 desired core. Then, the core is hollowed out first by copying the entire external surface 88 followed by thickening it to 0.4 mm inwards. Finally, the hollow shell is perforated using 89 Due to the SLA printing process, the parts will be covered with uncured resin after 99 printing. The residual resin must be removed prior to usage. Typically, parts are 100 cleaned by submerging them in 95% isopropanol (IPA) for a 5 to 15 minutes and 101 scrubbed gently with towels or brushes. However, we found that submerging negshell 102 parts in IPA can cause them to swell and crack prematurely due to the absorption of 103 IPA. Instead, we use a paper towel doused with IPA to wipe away the residual resin and 104 the parts are immediately dried with a dry paper towel. The parts can also be further 105 cured using ultraviolet (UV) light in a UV enclosure to attain higher strength and cure 106 February 28, 2020 4/17 any remaining resin. Finally, the support structures are removed using flush-cutters.

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Structural cores 108 Casting is often performed by injecting or pouring a homogeneous material and having 109 it set. Casting multiple materials is possible, but requires a multiple-step process, such 110 as overmolding: casting additional elastomer on top of an existing injection-molded 111 rigid part in a separate mold. Overmolding is often used for electric power tools and 112 cable assemblies to add compliance to an otherwise rigid part. For soft robots, having 113 the entire structure comprised of a single material limits the design space due to the 114 high compliance that is homogeneous throughout. By embedding semi-rigid, passive 115 structures inside the soft robots' body, the localized mechanical properties at those 116 structures can be tuned. We achieve this by utilizing cores similar to negshell cores but 117 do not have the perforations. Such structures provide a semi-rigid internal skeleton that 118 provides a stiffer backing to exert force akin to fingernails in humans, or provide 119 lightweight void space for areas that connect one structure to another that would 120 otherwise be heavy and undesirably non-rigid if made entirely from elastomer. 121 We design and build these structural cores in the same manner as the negshell cores: 122 a hollow shell with minimum wall thickness. The thickness can be tuned to create 123 varying stiffness in the resulting part. We present a characterization of these parameters 124 in Characterization section.

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Outer molds and support columns 126 The cores must be suspended in the outer mold at its intended location during the 127 casting process, as shown in Fig. 4. It should be noted that this is a limitation of this 128 fabrication method as the structure necessary to suspend the cores prevents the internal 129 features from being completely sealed. However, the support structures can be made  The support structures (or standoffs) are round 2.5 mm columns that reach from the 133 inner surface of the outer mold up to the core. At the end of the column, there is a 134 smaller column with a beveled tip approximately 1.5 mm in diameter that will 135 penetrate and secure the corresponding hole on the core, as shown in Fig.4. Depending 136 on the geometry of the core, the support columns can be placed at multiple locations 137 around the mold and core. To aid in the ease of removal of the outer mold after casting, 138 the columns should be oriented in the same pulling direction when removing the mold, 139 although this design rule can be somewhat relaxed due to the compliance of the 140 elastomer.

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The outer mold can be as simple as two halves of a shell that defines the external 142 features of the desired part, or a complex multiple part mold to accommodate undercuts 143 or overhangs and features that would be otherwise impossible to be demolded with a 144 two-part mold. As our focus of this paper is on the internal features, we designed most 145 of our parts to be cast in simple two-part molds. The molds are often split at the 146 central plane of the desired part that creates the least undercuts. In this paper, our  near the bellows due to the low thickness. The liquid silicone was then poured into a 30 176 mL syringe and the silicone was injected into the mold via the Luer lock port at the tip. 177 The Luer lock port at the other end serves as a vent but can also be used with another 178 empty syringe to draw a vacuum to aid in the flow of liquid silicone. Both syringes were 179 left in place during curing to prevent silicone from flowing out of the mold. After The specimen is placed at the center of the platen. As the actuator moves the platen down, the specimen is compressed and the resulting force is read from the load cell beneath.

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The results show that the cores start to break from 20 -35 N of force and had a wide 221 range of variance between each sample, as shown in Fig.11. This is, however, well within 222 range of the force a human can exert with a pinch grasp -and is also demonstrated in 223 S1 Video.   Negshell cores, once broken, are intended to minimally affect the mechanical properties 227 of the surrounding elastomer and provide the same function as a removed core. To 228 demonstrate this, we cast three different types of negshell specimens (top perforation, 229 side perforation and perforation at right angles) and compare them to specimens with 230 the equivalent resulting geometry that were fabricated with a traditional casting 231 process: casting with a solid core in halves and then bonded together. Each specimen 232 was first subjected to a preload of 1 mm of deformation followed by a 0.5 Hz triangle 233 wave of compression with a peak-to-peak displacement of 4 mm for five cycles. The 234 average stress-strain curve of each type of negshell specimen is shown in Fig.12. The 235 stress in the specimen is calculated by dividing the force read from the load cell by the 236 area of the base of the specimen. From the stress-strain curve, it can be implied that 237 the Top and Right Angles specimens behave similarly to the specimens without the 238 cores. The Side specimens have slightly higher overall stiffness, as shown in stress-strain 239 curve and the resulting elastic modulus. The average elastic modulus (E) (derived from 240 an approximate linear fit of the stress-strain curve) of the different core types that is 241 shown in Fig.12 confirms that the the broken negshell cores contribute an insignificant 242 amount of stiffness to the casted samples. To demonstrate that structural cores with thin walls can modulate the overall stiffness 245 of soft robotic elements, we casted specimens with structural cores with three different 246 thicknesses: 0.4 mm, 0.5 mm and 0.6 mm and compare them to a equivalent 247 homogeneous specimen without a core. Each specimen was subjected to a 0.5 mm 248 displacement preload followed by a 1 mm peak-to-peak displacement triangle wave at 249 0.5 Hz for five cycles. We did not exceed this displacement range due to the amount of 250 force reaching the upper limits of the load cell in some samples. The average in Fig.13. It can be seen from the resulting elastic moduli that the stiffness of the overall 253 structure can be increased by over 500%, while reducing the weight by 67% (Fig.10). structure. An example of actuation against the load cell is shown in S2 Video.

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The results from the blocked force experiment, in Fig.14, show that the finger with 266 the structural cores can exert a maximum force of 1.83 N with 64 kPa of air pressure 267 while the finger without the cores can only provide 1.52 N of force with the same 268 pressure -a 20% increase in force output with the structural cores. Furthermore, the 269 structural cores reduce the overall weight of the finger by 1.8 grams or 14%.

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Each phalanx of the gripper can be independently actuated by applying fluid pressure 280 on each of the two bellow-joints: the proximal joint and the distal joint. The proximal 281 joint connects the structural core in the base to the middle structural core and the 282 distal joint connects the middle structural core to the finger tip, as shown in Fig.15.

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Each joint consists of four fan shaped 0.6 mm thin chambers with a diameter of 284 approximately 16 mm, similar to the bellow-jointed finger in the previous section. A 3 285 mm tubular negshell core connects the bellows to the exterior of the finger through two 286 holes in the base -one for each joint, which is later used for inserting and bonding a 3 287 mm silicone tubes to connect the chambers to a pressure source. The middle structural 288 core is a half-cylinder with a thickness of 0.6 mm. The core at the fingertip is a hybrid 289 between negshell and structural cores, where the "pulp" of the fingertip core is to 290 become soft and compliant to emulate soft human fingertip tissue while the immediate 291 structural core connected to the negshell core emulates the hard backing provided by 292 the bony structure and fingernails in humans, as shown in the inset in Fig.15. We intend 293 for the fingertip to become a sensor, but it is beyond the scope of this paper. The mold 294 is similar to the mold presented in the previous sections with one difference where the 295 base is also used as part of the mold. Geometry similar to a structural core is integrated 296 into the base which is used to retain the finger on the base, as shown in Fig.15. 297 We fabricated the gripper by first 3D printing the cores, outer molds and base using 298 Clear resin on a Formlabs Form 3 printer. The parts were then cleaned and processed in 299 the same manner as in the previous sections. The molds were then prepared by 300 applying a light coat Pol-Ease 2500 to aid in demolding. One half of the mold was then 301 assembled onto the base followed by the insertion of the various cores. The other half of 302 the mold was then carefully closed and several M3 screws and nuts clamped the two 303 halves shut. Approximately 30 grams of Plat-Sil Gel 25 was mixed, vacuum degassed 304 and loaded into a syringe, then the liquid elastomer was injected into the port at the 305 fingertip. Another syringe was used at the base to draw a vacuum or increase pressure 306 to help with residual air bubbles. Once the elastomer set, the two halves were released 307 and Sil-Poxy was used to seal the holes created by the standoffs, and to bond silicone 308 tubes to the fluid channels for the bellows and fingertip. Each finger can be done one at 309 a time, or all three at once if three sets of molds were printed. Finally, each negshell 310 core is broken by hand and the gripper is ready to be mounted to the WAM Arm tool 311 plate using M6 hexagon cap screws. Overview of three-finger gripper's fabrication. The 3D-printed base, which serves as a part of the mold for the fingers, provides a rigid connection from the fingers to the tool plate of the WAM Arm. Two seperate negshell cores are used for each joint of the fingers. A structural core aids in rigidity and force transmission between the joints. The fingertip contains a hybrid structural-negshell core with renders a soft finger pad and solid backing, as shown in the inset. The base also contains a retention feature geometrically similar to the structural cores, where silicone will be mechanically trapped and locked in place due to the overhang.

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There are a total of six bellow-joints in the gripper, which can be individually actuated 314 by modulating the pressure within the bellows of each joint. Pressure modulation can 315 be done in multiple ways such as using multiple pulse-width modulated (PWM) valves 316 and a pressure source [17] or multiple leadscrew driven syringes [9]. However, as robust 317 actuation and dexterous manipulation is beyond the scope of this paper, we simply 318 combined all the fluid channels together into a single channel so all joints are 319 pressurized with a single pressure source, which in this case is a hand-actuated syringe. 320 However, individual actuation of each bellow using a hand-driven syringe is shown in  As a demonstration of real-world usage of the negshell casted three-finger gripper, we 324 performed a brief demonstration by using the gripper mounted on a Barrett WAM Arm 325 to perform pick and place tasks of everyday items. We pre-recorded the arm movement 326 using a "Teach and Play" program that is then played back each time the arm performs 327 a pick and place task. The arm starts from its home position, moves to position the 328 gripper directly above the object to grasp, and moves the gripper down to encompass 329 the object with the fingers. The fingers are then closed using the syringe and the arm 330 moves the object with the gripper towards a bin. Finally, the fingers are opened and the 331 object is dropped into the bin. 332 We tested grasping a total of 28 objects varying in shape, size and density, with 333 most shown in Fig.17 e.g. a water bottle, a lightbulb, and a 5 mm hex key. Some 334 objects were handed to the robot, such as the water bottle and pen, while others were 335  Pick and place demonstration. The three-finger gripper is shown attached to a Barrett WAM Arm. The arm goes through a pre-recorded motion while the gripper is manually actuated with a syringe (not shown). 1. The arm positions the gripper to encompass the object to be picked up, 2. All bellows in the gripper are pressurized at once using a syringe, grasping the object, 3. The arm moves the object up over the bin, 4. The object is dropped into the bin upon the release of pressure. Inset: the objects that were successfully picked and placed.

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Negshell casting opens up new possibilities for casted soft robotic designs by integrating 342 3D-printed parts both into the structure and the casting process -cutting down the 343 casting process while increasing the design space significantly. The internal geometry of 344 soft robotic structures can be much more complex than what could previously be done 345 through casting. However, negshell casting does have limitations. Since the cores must 346 be suspended during casting, some support structure must exist, which requires sealing 347 the holes created by the structures. Though straightforward, sealing the holes is an 348 additional step that is done manually and prone to human error. In the future, we hope 349 to address this step to further streamline the fabrication process for the sake of 350 scalability and manufacturability.

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Sensing, although mentioned in the previous sections, has not been demonstrated in 352 this paper. However, the mechanical characterization of the negshell specimens show 353 that, under compression, the broken negshell cores have little to no effect on the 354 structure, thus negshell casting could possibly be used for creating sensor channels or 355 structures. An example of this is shown in the fingertip of our three-finger gripper, 356 where the fingerpad is deformable chamber that can be connected to a pressure sensor 357 for simple tactile sensing. Sensing will "close the loop" for our soft robotic grippers and 358 thus we will explore sensing and closed-loop control in the near future.

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3D-printing soft robots, especially with complex internal structures, is still an active 360 research topic. Work such as [18], still requires molds and manual labor within each We have presented a novel fabrication technique, negshell casting, that leverages SLA 371 3D-printing to create complex cores for use during an otherwise traditional casting 372 processing. We present two types of cores: negshell cores and structural cores. Negshell 373 cores are sacrificial cores that are used to create soft or flexible internal structures such 374 as channels and bellows while structural cores are used for modulating stiffness and for 375 weight reduction. We showed that the negshell cores have little effect on the mechanical 376 structure of silicone elastomer and structural cores can increase the stiffness of silicone 377 structures significantly. Our demonstration of the bellow-jointed finger and the 378 three-finger gripper shows that negshell casting can be used for creating soft robotic 379 actuators. In the near future, we will explore how negshell casting can enable more 380 sophisticated and integrated sensing capabilities in soft robotic designs.

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Supporting information 382 S1 Video. Example of breaking a specimen by hand before mechanical 383 testing.

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S2 Video. Blocking force test for the two compared bellow-jointed fingers. 385 S3 Video. Actuating individual bellows in the fingers. Each joint of the finger 386 is actuated to their respective extents and three samples of delicate grasping is shown. 387 S4 Video. Pick-and-place task of various objects done with the example 388 three-finger gripper. 10× sped up footage of the three-finger gripper prototype 389 mounted on the end of WAM Arm grasping various objects and dropping them in a bin. 390