Development of In-Mold Assembly Process for Realizing Mesoscale Revolute Joints

In-mold Assembly process at the mesoscale presents several manufacturing challenges. Results reported in this paper demonstrate the technical feasibility of creating rigid body mesoscale revolute joints using In-Mold Assembly process. The following new results are reported in this paper. First, we describe a mold design with varying cavity shape to perform In-Mold Assembly. This mold design uses an accurate mold piece positioning method to avoid damage to delicate mesoscale parts during the cavity change step. Second, we describe a mold insert fabrication process for making mold inserts with the desired surface characteristics for mesoscale molding. Finally, we describe methods to limit the adhesion at the interfaces and hence create articulated revolute joint. Using the advances reported in this paper we have successfully molded a mesoscale revolute joint. To the best of our knowledge, this is the first demonstration of In-Mold Assembly process using a varying cavity shape mold to create a mesoscale revolute joint. INTRODUCTION 3D articulated devices involve moving parts with significant out-of-plane motion. There are many applications such as hard disks, cameras, photonics, cell phones, micro air vehicles, and drug delivery systems where the ability to scale down size and deploy mesoscale (size range of 0.1mm to 1mm) joints will be highly desirable because their unique kinematic behavior provides significant performance gains. While manufacturing technologies exist for scaling down 2D articulated devices [1], a scalable and cost effective manufacturing method does not currently exist for making 3D articulated devices. Even though individual parts can be easily fabricated, assembling them into devices remains a challenge (e.g., current assembly methods require manual assembly under a microscope to realize mesoscale 3D articulated devices). Therefore, despite their superior performance characteristics, mesoscopic 3D articulated devices are not used in practice due to throughput and cost considerations. Recent advances in micro mold insert manufacturing technologies such as micro electro discharge machining (EDM) provide a way to create mold inserts with very small features. Such molds can be used to create parts that are sub-millimeter in size. By combining recent advances in micro EDM and In-Mold Assembly methods [2, 3], we can create a new molding process to enable economically viable fabrication of mesoscopic 3D articulated devices. Microand meso-molding of polymers is a promising process that has gained popularity during the last few years [4, 5, 6]. Parts with features sizes as small as 10 microns are being routinely molded [8, 7]. While significant success has been achieved in molding micron-sized parts, there are still technical challenges associated with micro-molding. These challenges are mainly in the area of mold flow simulations and thermal management [9, 10, 11]. Our work builds on these successes. To the best of our knowledge a scalable In-Mold Assembly method for creating mesoscale revolute joint has not been demonstrated so far. Please note that overmolding is not recommended because inserting a molded component into a new mold is not feasible due to the small component dimensions. Development of a molding process that combines the benefits of mesoscale molding and In-Mold Assembly requires us to address several challenges. These challenges include: (1) developing mold configurations that support molds with varying cavity shape to perform InMold Assembly, (2) developing accurate positioning methods to realize cavity shape change to avoid damage to delicate mesoscale parts created during molding, (3) developing a process for making mold inserts with the desired surface characteristics, (4) developing a method to limit the adhesion at the interfaces and hence provide articulation, and (5) developing a method to successfully remove parts from molds. This paper reports our progress towards successfully realizing mesoscale revolute joints. The paper describes how mesoscale mold design and mold insert fabrication are different from macroscale In-Mold Assembly process. This paper also describes the mold design, the mold insert fabrication method, and processing parameters that are used for successfully realizing mesoscale revolute joints.