Development of Multi-Stage Molding Methods for Manufacturing of Mesoscopic 3D Articulated Devices

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1 NSF GRANT # DMI NSF PROGRAM NAME: Mat. Proc. and Mfg. Development of Multi-Stage Molding Methods for Manufacturing of Mesoscopic 3D Articulated Devices Satyandra K. Gupta Mechanical Engineering Department and Institute for Systems Research University of Maryland, College Park, MD Arvind Ananthanarayanan and Hugh A. Bruck Mechanical Engineering Department University of Maryland, College Park, MD Abstract: 3D articulated devices involve moving parts with significant out-of-plane motion. While manufacturing technologies exist for scaling down 2D articulated devices, a scalable and cost effective manufacturing method does not currently exist for making mesoscopic 3D articulated devices. Even though individual mesoscopic parts can be easily fabricated, assembling them into devices remains a major challenge. We are developing a multi-stage molding process that will eliminate the need for performing post-molding assembly operations during manufacturing of mesoscopic 3D articulated devices. This paper presents an overview of our ongoing project and describes our preliminary results in the areas of adhesion modeling and development of mold designs for realizing mesoscale revolute and universal joints. 1. Introduction 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 mesoscopic 3D articulated devices will be highly desirable because their unique kinematic behavior can result in significant performance gains. While manufacturing technologies exist for scaling down 2D articulated devices, 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 mesoscopic 3D articulated devices). 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 manufacturing technologies (e.g., electro discharge machining) provide a way to create molds with very small features. Such molds can be used to create parts that are submillimeter in size. We envision that by combining recent advances in mold insert fabrication and multi-stage molding [Kuma02, Gupt04, Li04], we can create a new molding process to enable economically viable fabrication of mesoscopic 3D articulated devices. Mesoscale mold insert fabrication technology will allow us to scale down part sizes, while multi-stage molding will eliminate the need for post-molding assembly and hence significantly lower the part count. Because of these benefits, the combination of the two technologies will make it possible for mesoscopic articulated devices to be produced in an economically viable manner. Development of a novel molding process requires us to address several research challenges. These challenges include: (1) developing a method to limit the adhesion at the interfaces and hence provide articulation, (2) developing mold configurations that support molds with varying cavity shape to perform in-mold assembly, (3) developing a method to successfully remove parts from molds, and (4) developing a process for making mold inserts with the desired characteristics. Our ongoing project aims to develop an improved understanding of the multi-stage mesoscale molding and mold insert fabrication processes in the areas of adhesion control, mold design, part ejection, and mold insert surface characterization to address these challenges. This paper reports our preliminary results in the area of mold design and adhesion modeling. 2 Review of Related Work

2 2.1 Molding of Small Parts One of the most scalable processes known to human kind is the molding process. Once a mold has been created, parts can be manufactured using the molds quickly and in high volume. As the volume of production goes up, the cost of making individual parts goes down due to amortization of the mold cost over a larger number of parts. Moreover, the geometric complexity of the objects being molded does not significantly affect the part cost. Micro and meso molding of polymers is a promising process that has gained significant popularity in last few years [Hane00, Heck04, Broc02]. Parts with features sizes as small as 10 microns are being routinely molded [Yama94, Goll97]. The following five different processes are used in molding of small parts: injection molding, reaction molding, hot embossing, compression molding and thermo forming. Out of these processes, injection molding is the most popular process due to faster cycle times. The proposed work will mainly use injection molding process. While significant success has been achieved in molding micro parts, there are still technical challenges associated with micro molding. These challenges are mainly in the area of mold flow simulations and thermal management [Yao02, Zhao03, Naka03]. As a part of the proposed work, we do not plan to push the limits of micro molding process and hence we will not do any new research in mold flow simulations and thermal management areas. We will instead rely on existing knowledge in these fields. 2.2 Characterization and Modeling of Interfaces in Heterogeneous Structures For an interface between two heterogeneous materials, it is common to determine the critical stress at which the interface will begin to debond, known as the interfacial bond strength. The bond strength of an interface is typically attributed to five main adhesion mechanisms: adsorption and wetting, interdiffusion, electrostatic attraction, chemical bonding, and mechanical interlocking. For a perfectly planar interface, the effects of the mechanical interlocking and interdiffusion mechanisms can be eliminated. When a perfectly planar interface is fabricated by joining two dissimilar materials, this bond strength is characteristic of the joint strength attributed to the remaining three mechanisms and indicates the maximum load that must be applied normal to the interface that is required to separate the two materials. Once an interface begins to separate, the load required to continue separating the interface is determined by the resistance of the interface to crack propagation. To determine this resistance, analytical models have been developed for K-dominant stress fields near crack tips growing along debonded interfaces, and have been verified by experiments [Liec92]. We have previously used advanced full-field deformation measurement techniques to investigate the effects of chemical bonding and mechanical interlocking adhesion mechanism, mesoscopic geometric complexity, on the interfacial strength of heterogeneous polymers [Bruc04]. From this investigation, we isolated the effects that geometric complexity had in converting the failure mechanism from K-dominance to ligament failure which is controlled by the strength of the weaker base material. This resulted in up to a 30% increase in interfacial strength, and removed the need to include K-dominant failure on the mechanical response of geometrically complex interfaces. Thus, in order to design geometrically complex joints for mesoscopic 3D articulated devices it will be necessary to understand the effects of chemical bonding and mechanical interlocking adhesion mechanisms on the interfacial strength and the mechanical response of the interface. For joints without geometric complexity, there will be a need to understand the effects of these adhesion mechanisms on fracture toughness as well. Without this knowledge, it would be impossible to develop the process models that are capable of predicting the type of forces required for the Part Removal method and for articulating joints without damaging them. 3. Preliminary Results 3.1 Development of Mold Designs to Realize Mesoscale Joints We have successfully developed a multi-stage molding process that allows us to produce mesoscale revolute and universal joints. Unlike traditional multi-stage molding where adhesion is used to completely constrain the motion between two components, our process creates non-binding mechanical interfaces to allow for selective degrees of freedom between components. Figure 1 shows design of rotor hub that contains mesoscale universal joints at both of its ends. This rotor structure is produced using a four stage molding process. This fourth stage is needed for creating blades that are not shown in this figure. The rotor is fully assembled inside the mold. To avoid chemical bonding between the parts present in the mold and the new injected material, the melt temperature has to be carefully selected. We achieved good results at 130 C for polyethylene. We used the same material for every stage to study the extent of the problem. The molded rotor system exhibited the desired degrees of freedom.

3 Universal Joints (a): CAD Model of the Rotor Hub (b): Molded Rotor Hub Both joints are easily movable. The range of motion of ±45 was realized and the size of the part was kept as small as possible. The weight of the rotor system without blades is 0.3g. 3.2 Adhesion Modeling During the multi-stage molding process, when the second material is injected on top of an already molded material, the two materials tend to adhere to each other. To create articulated devices, we will need to limit the adhesion at the interfaces so that we can obtain free moving articulated devices. At the same time, to produce compliant joints, we have to ensure appropriate strength at the interface to achieve in-mold assembly. Therefore, adhesion between materials during multi stage molding is one of the prime issues that needs to be dealt with while addressing the broader issue of multi stage injection molding for in-mold assembly. There are three mechanisms that might be responsible for the interfacial strength in specimens created using multi-stage molding: 1) Adhesion from polymer crosslinking. Figure 1: A rotor hub with mesoscale universal joints (blades are not shown in this figure) 2) Plastic welding at interface. 3) Geometrical interlocking. Polymer crosslinking requires chemical activity between the surfaces, which is usually obtained in the melt state. This is possible at higher injection temperatures, where the injected melt can possess enough energy to locally melt the solid piece in the first stage. If the melting is not sufficient to activate the surfaces, it is still possible for the surfaces to substantially deform near the interface and have the two surfaces intertwine to create plastic welding. Finally, if there is sufficient roughness or curvature at the interface, it is possible for the two solid pieces in the second stage to interlock with the solid piece in the first stage. Each of these can be isolated, since mechanical interlocking can only exist at lower melt temperatures where there is not sufficient energy to activate or melt the solid piece in the first stage. Thus, a straight interface with a smooth finish, as is produced after the first stage, will eliminate this mechanism. When there is sufficient energy to melt the solid piece in the first stage, distortions of greater than 1 micron in the straight interface should be visible under an optical microscope. Distortions below this level induced by roughening the

4 surface of the solid piece in the first stage with sandpaper have typically shown to have negligible effect on the interfacial strength. Therefore, when there is interfacial strength and no distortions appear at the interface, then the effects can be attributed to polymer crosslinking. In this research investigation, mechanical testing and microstructural examination of the interface between the first and second stage parts have been conducted. The variation of interfacial strength with processing conditions, in particular the injection temperature, has been quantified and the primary mechanisms that affect interfacial strength identified as polymer crosslinking and shrinkage stress using two different specimen configurations, lap-shear and cylindrical pull-out, in order to determine the appropriate processing conditions for a given part geometry and desired level of interfacial strength. It was also determined that a lap shear specimen with a redesigned notch could provide insight into interfacial strength due to adhesion from polymer crosslinking, while the cylindrical pull-out specimen could provide insight into geometric effects of shrinkage stress on interfacial strength. We are planning to conduct additional experiments to better understand the adhesion phenomena in multi-stage molding. 4. Conclusions This ongoing projects aims to make contributions in the following areas: Improved understanding of adhesion phenomenon in mesoscale molding. Significantly improved process models of multistage molding process for creating articulated geometries. Proven tooling configurations for creating mesoscale revolute, spherical, and prismatic joints. A novel manufacturing technology that will (1) eliminate need for post-fabrication assembly operations in mesoscale devices, and (2) reduce the number of component parts leading to a finished product that may be less expensive to build, lighter in weight, and more resistant to malfunction. Our preliminary results clearly show that it is possible to create mesoscale revolute and universal joints. We have also developed a preliminary model to relate processing conditions to the interfacial strength in products created by multi-stage molding. References [Broc02] Brock R. It's a small world after all - A leaner approach to micro molding. Plastics Engineering, 58(8), [Bruc04] H. Bruck, G. Fowler, S.K. Gupta, and T. Valentine. Towards bio-inspired interfaces: Using geometric complexity to enhance the interfacial strengths of heterogeneous structures fabricated in a multi-stage multi-piece molding process. Experimental Mechanics, 44(3): , [Goll97] Goll C, Bacher W, Bustgens B, Maas D, Ruprecht R, Schomburg WK. An electrostatically actuated polymer microvalve equipped with a movable membrane electrode. Journal Of Micromechanics And Microengineering, [Gupt04] 7(3): , S.K. Gupta and G. Fowler. A step towards integrated product/process development of molded multi-material structures. In Tools And Methods Of Competitive Engineering, Lausanne, Switzerland, April [Hane00] Hanemann T, Heckele M, and Piotter V. Current status of micromolding technology. Polym. News, 25, [Heck04] Heckele M, Schomburg WK. Review on micro molding of thermoplastic polymers. Journal Of Micromechanics And Microengineering, 14(3):1-14, [Kuma02] M. Kumar and S.K. Gupta. Automated design of multi-stage molds for manufacturing multi-material objects. ASME Journal of Mechanical Design, 124(3): , [Li04] X. Li and S.K. Gupta. Geometric algorithms for automated design of rotaryplaten multi-shot molds. Computer Aided Design, 36: , [Liec92] Liechti, K.M. and Chai, Y.S., "Asymmetric Shielding in Interfacial Fracture Under In-Plane Shear", Journal of Applied Mechanics, 59, (1992). [Naka03] [Yama94] Nakao M, Yoda M, Nagao T. Locally controlling heat flux for preventing micrometre-order deformation with injection molding of miniature products, CIRP Annals-Manufacturing Technology, 52(1): Yamaguchi K, Nakamoto T, Abbay PA, et al. The accuracy of micro-molds produced by a UV laser-induced polymerization. Journal Of Engineering For Industry, 116(3): , 1994.

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