Progress towards In-Mold Assembly of Mesoscle Rigid Body Revolute Joints

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1 NSF GRANT # DMI NSF PROGRAM NAME: Mat. Proc. Progress towards In-Mold Assembly of Mesoscle Rigid Body Revolute Joints Satyandra K. Gupta 1, Kamlakar Rajurkar 2 1 University of Maryland, College Park, MD University of Nebraska-Lincoln, NE Arvind Ananthanarayanan, Wojciech Bejgerowski, and Hugh A. Bruck University of Maryland, College Park, MD Abstract: This paper describes our progress towards development of an in-mold assembly process for realizing mesoscale rigid body revolute joints. Such a capability will eliminate the need for performing postmolding assembly operations during manufacturing of mesoscopic 3D articulated devices. This paper presents an overview of our ongoing project which describes: (1) our experimental results for revolute joints, and (2) our plans for investigating a universal joint. 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 serious challenge. In-mold assembly has been established as a popular method to produce 3-D articulated devices with significant out of plane motion in the macroscale (with minimum feature sizes of a few millimeters) [Gouk06, Priy07, Anan07a, Bane07]. With the requirement for miniature assemblies in modern devices (such as hard drives, cameras etc.) extension of in-mold assembly to the mesoscale is being seen as an attractive and viable option. But the in-mold assembly process at the mesoscale presents several manufacturing challenges, as follows: Developing mold configurations that support molds with varying cavity shape to perform in-mold assembly Developing accurate positioning methods to realize cavity shape change to avoid damage to delicate mesoscale parts created during molding Developing a method to limit the adhesion at the interfaces and hence provide articulation Developing a method to successfully eject parts from the molds 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 presents an overview of our ongoing project which describes: (1) our experimental results for revolute joints, and (2) our plan for investigating a universal joint. Results reported in this paper demonstrate the technical feasibility of creating rigid body mesoscale revolute joints using in-mold assembly process. 2. In-mold Assembly of a Mesoscale Revolute Joint: An in-mold assembled rigid body revolute joint consists of a part with a core and another with a cavity. The key to creating a smooth running revolute joint at the macroscale, is to mold the core inside the cavity such that the core shrinks radially to provide the clearances required to revolve the joint. The cavity in the first stage component is made by using a side mold core of the requisite diameter. To initiate development of inmold assembly methods at the mesoscale, we began conducting some exploratory experiments with direct scaling down of macroscale methods. The CAD model of the mesoscale part that was molded is shown in Figure 1. From the exploratory experiments we identified defect modes which were unique to the in-mold assembly process at the mesoscale. Direct scaling down of macroscale process suggested molding of the mesoscale

2 cavity as part of the first stage part and the mesoscale core as part of the second stage part. But we realized that molding of a mesoscale cavity in the first stage posed several challenges due to deformation and misalignment of the side mold core during the injection molding process. Hence we decided to reverse the molding sequence (i.e., mold the core as part of the first stage part while molding the cavity as part of the second stage part). Since the polymer used for the first stage part needs to be of a higher melting point than that used for the second stage part in order to avoid adhesion, an exchange of polymers was made for the core part and the cavity part. Reversing the molding sequence led us to another challenge. We observed that the mesoscale core in the first stage part tends to deform due to the high pressure and temperature of the second stage injection. This is illustrated in Figure 2. Stage 1 ABS Pin diameter = 0.75 mm Stage 2 LDPE Figure 1: In mold assembled revolute joint. After some experiments, we discovered that radially supporting the core, as illustrated in Figure 3, reinforces the core which in turn avoids deformation due to second stage injection. Side core First stage Unsupported meso scale Feature Second stage part Bent pins due to second stage injection Shut off surfaces Second stage Injection Second stage part (LDPE) First stage part (ABS) Example Defective Component Figure 2: Deformation of mesoscale core due to second stage injection Side core First stage Injection Unconstrained pin Supported pin Second stage part Second stage part Bent pins due to second stage injection Shut off surfaces Side core used as a support structure Shut off surfaces Figure 3: Mold design iterations for second stage injection.

3 635 bar, Radial Support Length 635 bar, bar, bar, bar, Injection Pressure 635 bar, Figure 4: Specimens showing varying deformation with injection pressure and radial support lengths 0.4 Deformation (mm) bar 700 bar 800 bar Radial Support Length (mm) Figure 5: Deformation v/s Radial support length for different injection pressures One of the primary issues concerning repeatability of the process for manufacturing in-mold assembled mesoscale revolute joints is that of the radial support length. A higher support length ensures lesser deformation of the core. Whereas a high support length requires the overall length of the core to be higher which thereby increases the mold tooling cost. This brings us to the problem of optimizing the radial support length such that the deformation of the core is within acceptable functional limits of the revolute joint. In order to characterize the radial support length with respect to different injection pressures, we conducted experiments varying the radial support length and subsequently measuring the maximum deformation of the core after the second stage injection. The maximum deformation of the core was taken as the maximum deformation near the center of the core. Figure 4 shows the images of the first stage specimen after second stage injection for varied radial support lengths and injection pressures. Figure 5 shows a plot of the maximum deformation with the radial support lengths for different injection pressures. As expected, increasing the radial support length was found to decrease the deformation while increasing the injection pressure increased the deformation. In order to validate the results obtained from the experiments, we are currently developing numerical models of the rigid body articulated joints created by in-mold assembly processes.

4 Using the advances summarized in this report we have successfully molded a mesoscale revolute joint. Figure 6 and Figure 7 shows images of the first stage parts and in-mold assembled mesoscale revolute joint that we successfully manufactured in the Manufacturing Automation Lab at the University of Maryland. 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. The mold setup is shown on Figure 11, and the resulting part is shown on Figure 12. Notice that the insert used to form the first part is not being disassembled. Figure 6: First stage part Figure 8: Universal joint Figure 7: In-mold assembled mesoscale revolute joint 3. Mold Design for In-Mold Assembly of Universal Joints: The next step in our research is focused on exploring mesoscale molding of universal joints. A universal joint consists of two revolute joints. So far we have developed a morphing cavity method for inmold assembly of a universal joint, shown on Figure 8. Design of the molding process introduced many challenges connected mainly with the mold and mold inserts design and manufacturing. The process that we have designed contains three stages. In the first stage, the middle part (joint with four pins) will be injected into the mold insert inside the mold. The mold insert actually consists of two separate parts, which are held together and in place by the cylindrical inserts. The mold set-up is shown on Figure 9. The ready part inside the mold insert is shown on Figure 10. The material used in the first stage has to have a higher melting point than the material for the two remaining stages. After injection of the first part, the top half of the mold will be replaced with a modified one to form a cavity for one of the links of the universal joint. Also the cylindrical mold inserts will be moved away for the same matter, but maintaining support for the pins. Stage three will include replacement of the original bottom part of the mold with a half-mold similar to the one from the second stage to create a cavity for the third part, moving aside the maintaining cylindrical inserts and injecting the material. The mold setup is shown on Figure 13, and the resulting part inside the common mold insert is shown on Figure 14. Notice the two parts of the insert, which after the final injection will be disassembled to release the ready assembly. Manufacturing the molds and inserts will require us to address several challenges. These will mainly concern machining complex shaped mold inserts. The molding process will require the use of a two shot molding process. Between molding stages moldhalves will need to be changed and the rearrangement of the inserts will have to take place. The common plate for all three stages will be the two-part center insert. Figure 9: First stage mold setup

5 Figure 10: First stage part Figure 11: Second stage mold setup Figure 14: Third stage part 4. Manufacturing of Mold Insert: Fabrication of mold insert was achieved by Micro Electro Discharge Machining (micro EDM) process. The machining operation was divided into two stages: rough machining and finishing. In rough machining, a tool of 0.95 mm was used under the machining conditions of 100V and capacitance 3300 pf with each layer depth of 0.2 mm to remove the large amount of material. To achieve uniform tool wear, in X-Y plane, the width of the remaining part for finishing (W in Figure 15) should be larger than the sum of the tool radius and the discharge gap but less than the tool diameter. Figure 15: Dimensional parameters for EDM. Figure 12: Second stage part Figure 13: Third stage mold setup A tool of 0.7 mm diameter was used for finishing operation. The width was 0.5 mm. In Z axis, the remaining thickness (H in Figure 15) for finishing was 0.1 mm. The thickness of each layer during finishing was 10 μm. Therefore, the flatness of finished surface is less than 10 μm. The machining parameters for finishing operation were 80 V and capacitance of 1000 pf. To drill the deep hole in Insert 1, the planetary movement of tool was applied to obtain deep and sharp edge at the bottom of the hole. Figure 16 shows the set of meso mold generated by Micro EDM. These brass mold inserts were successfully used in our experiments. The cost of a two stage mold is comparable to the cost of a two cavity family mold. Hence, the use of the In-Mold Assembly process does not increase the tooling cost.

6 Figure 16: Mold inserts machined by EDM. 5. Conclusions: This ongoing projects aims to make contributions in the following areas: 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. Understanding of the interaction between molds and parts in the mesoscale in-mold assembly process. Process models for rigid body articulated joints created by in-mold assembly processes. Research Conference, Ann Arbor, MI, May A. Ananthanarayanan, C. Thamire, and S.K. Gupta. Investigation of revolute joint clearances created by in-mold assembly process. IEEE International Symposium on Assembly and Manufacturing, Ann Arbor, Michigan, July A.K. Priyadarshi, S.K. Gupta, R. Gouker, F. Krebs, M. Shroeder, and S. Warth. Manufacturing multi-material articulated plastic products using in-mold assembly. International Journal of Advanced Manufacturing Technology, 32(3-4): , March A.K. Priyadarshi and S.K. Gupta. Algorithms for generating multi-stage molding plans for articulated assemblies. Accepted for Publication in Robotics and Computer Integrated Manufacturing. 7. A. Banerjee, X. Li, G. Fowler, and S.K. Gupta. Incorporating manufacturability considerations during design of injection molded multi-material objects. Research in Engineering Design, 17(4): , March Our preliminary results show that it is possible to create mesoscale revolute joints. We have also developed a preliminary model to relate processing conditions to the mold design parameters for in-mold assembly. Acknowledgements: We would like thank Dr. Murali Sundaram and Dr. Zuyuan Yu from the University of Nebraska at Lincoln for collaborating with us and providing us access to micro-edm machine for mold insert machining. 5. References: 1. A. Ananthanarayanan, H.A Bruck, and S.K. Gupta. Interfacial Adhesion in Multi-Stage Injection Molded Components. SEM Annual Conference and Exposition, St. Louis, Missouri, June R.M. Gouker, S.K. Gupta, H.A. Bruck, and T. Holzschuh. Manufacturing of multi-material compliant mechanisms using multi-material molding. International Journal of Advanced Manufacturing Technology, 30(11-12): , A. Ananthanarayanan, S.K. Gupta, H.A. Bruck, Z. Yu and K.P. Rajurkar. Development of inmold assembly process for realizing mesoscale revolute joints. North American Manufacturing