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Research Papers

Design of Revolute Joints for In-Mold Assembly Using Insert Molding

[+] Author and Article Information
Arvind Ananthanarayanan, Leicester Ehrlich, Jaydev P. Desai

Biomimetics Robotics Laboratory, Massachussetts Institute of Technology, Cambridge, MA 02140, e-mail: arvinda@mit.eduAdvanced Manufacturing Laboratory, Department of Mechanical Engineering,  University of Maryland, College Park, College Park, MA 20742, e-mail: lester.ehrlich@gmx.netRobotics, Automation, and Medical Systems Laboratory, Department of Mechanical Engineering,  University of Maryland, College Park, MA 20742, e-mail: jaydev@umd.edu

Satyandra K. Gupta1

Advanced Manufacturing Laboratory, Department of Mechanical Engineering and the Institute for Systems Research,  University of MA, College Park, MA 20742, e-mail: skgupta@eng.umd.edu

1

Corresponding author.

J. Mech. Des 133(12), 121010 (Dec 14, 2011) (10 pages) doi:10.1115/1.4005327 History: Received January 24, 2011; Accepted August 05, 2011; Published December 14, 2011; Online December 14, 2011

Creating highly articulated miniature structures requires assembling a large number of small parts. This is a very challenging task and increases cost of mechanical assemblies. Insert molding presents the possibility of creating a highly articulated structure in a single molding step. This can be accomplished by placing multiple metallic bearings in the mold and injecting plastic on top of them. In theory, this idea can generate a multi degree of freedom structures in just one processing step without requiring any post molding assembly operations. However, the polymer material has a tendency to shrink on top of the metal bearings and hence jam the joints. Hence, until now insert molding has not been used to create articulated structures. This paper presents a theoretical model for estimating the extent of joint jamming that occurs due to the shrinkage of the polymer on top of the metal bearings. The level of joint jamming is seen as the effective torque needed to overcome the friction in the revolute joints formed by insert molding. We then use this model to select the optimum design parameters which can be used to fabricate functional, highly articulating assemblies while meeting manufacturing constraints. Our analysis shows that the strength of weld-lines formed during the in-mold assembly process play a significant role in determining the minimum joint dimensions necessary for fabricating functional revolute joints. We have used the models and methods described in this paper to successfully fabricate the structure for a minimally invasive medical robot prototype with potential applications in neurosurgery. To the best of our knowledge, this is the first demonstration of building an articulated structure with multiple degrees of freedom using insert molding.

Copyright © 2011 by American Society of Mechanical Engineers
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Figures

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Figure 1

Mold design solutions for realizing mesoscale in-mold assembled revolute joints [(8),10]

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Figure 2

Clearances in in-mold assembled macroscale revolute joints [9]

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Figure 3

Mold design strategy for creating mesoscale in-mold assembled revolute joints [9]

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Figure 4

Mold design concept for in-mold assembly using insert molding

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Figure 5

Critical dimensions in insert molded revolute joint

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Figure 6

Shrinkage of polymer resulting in jamming of joint

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Figure 7

Weld-line formation in insert molded revolute joint

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Figure 8

Multi material molding for introduction of sleeve at joint interface

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Figure 9

Interfacial frictional torque v/s thickness ratio for two joint sizes

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Figure 10

Hoop stress v/s thickness ratio

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Figure 11

In-mold assembled miniature revolute joint fabricated using insert molding

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Figure 12

Radial stress relaxation due to presence of sleeve made of soft polymer

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Figure 13

Functioning in-mold assembled macroscale revolute joint fabricated using insert molding

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Figure 14

9 degree of freedom robot structure manufactured using insert molding

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Figure 15

Cost comparison based on production time estimate for fabricating a 9 DOF robot structure using different manufacturing strategies

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