Injection Molding of Soft Robots

Abstract

tional components such as magnets or tubing, or using new production methods such as directly 3D printing soft actuators. [1] The creation of internal voids and undercuts is inherently a challenge when molding. The traditional method is to glue two parts together, [2] or dip a cured half in uncured material. [3] Soluble or wax cores can allow for complex voids which are later removed out drain holes. [4][5][6] Soft cores are flexible (usually softer than the desired molding material), and enable voids to be created while being able to be reused multiple times. [7] Using a technique from industry, rotational molding that is normally used to make large plastic products such as kayaks, can be used to create internal voids in soft actuators, but control of the wall thickness becomes difficult for features such as bellows. [8] Alternatively, dip-coating can control the internal geometry but control of the external wall thickness is difficult. [9] Strain limiting layers are the most common type of reinforcements embedded into soft actuators, such as paper or fabric sheets to prevent extension of an actuator. [10,11] In order for embedded components to adhere to the silicone, components can be dipped and stuck to a structure, [11] or molded during the casting process. [10] Small magnets or nuts can also be embedded, but require a mesh to grip enough silicone to prevent the magnet or nut from pulling out during use. [12,13] 3D printers and materials have recently become more capable of directly printing soft actuators, potentially eliminating the need for mold making and casting. [14,15] Thermoplastic polyurethane (TPU), which is widely and cheaply used in shoes and clothing, has the potential to be directly printed producing robust bellows. [16] Recently, Voxel8's ActiveLab Digital Fabrication System (Voxel8, Somerville, MA, USA) has been able to print varying stiffness TPU with large overhangs while curing on-the-fly. Another method, embedded 3D printing, allows complex microchannels to be produced using sacrificial materials to create void space after silicone is cured. [17] Injection Molding Soft RobotsWe have developed a low-cost injection molding method ($10s-$100s), as seen in Figure 1, that incorporates the high pressure and in-line mixing capabilities of industrial liquid silicone injection molding machines into a versatile machine for prototype and low volume production. Our setup allows for To date, injection molding has not been a practical manufacturing method for soft robots due to machine costs, large volumes of liquid silicones required, and the inability to change materials quickly between shots. Injection molds are typically machined from metals to allow for high pressure and clamping forces, which further limits the ability to rapidly prototype soft robots when molds could cost thousands of dollars. To circumvent these issues, a lowcost injection molding system and process are pioneered. In this article, the apparatus, design process, economics, and workflow are described using standard stereolithogra...

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