Investigation of Revolute Joint Clearances Created by an In-Mold Assembly Process

Ananthanarayanan, Arvind; Thamire, Chandrasekhar; Gupta, Satyandra K.

doi:10.1109/isam.2007.4288458

Cited by 3 publications

(5 citation statements)

References 17 publications

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“…After cooling, the pin, which is made of an injection molded polymer shrinks away from the hole [7]. By controlling the injection molding parameters, it is possible to obtain appropriate friction in the in-mold assembled revolute joint at the macroscale [11]. However, the reversed molding sequence at the mesoscale may lead to jammed joints due to shrinkage of the polymer.…”

Section: Background: In-mold Assemblymentioning

confidence: 99%

“…In order to control the shrinkage stress at the joint interface, it is important to understand and model the parameters resulting in shrinkage in injection molded parts. Shrinkage in injection molded parts has been studied by several researchers over the past several years [11,13,14]. The injection pressure holding phase in the injection molding cycle ensures that initial shrinkage in the polymer melt is compensated by additional polymer flow in the cavity.…”

Section: Selection Of Design Parametersmentioning

confidence: 99%

“…Hence, shrinkage studies conclude that shrinkage of the polymer melt in the cavity begins after the gate freezes. Studies have concluded [11] that the main parameters responsible for the shrinkage are the crystallinity and thermal contraction. For amorphous polymers, the effect of crystallinity is very low compared to the thermal contraction.…”

Section: Selection Of Design Parametersmentioning

confidence: 99%

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

Ananthanarayanan

Ehrlich

Desai

et al. 2011

Journal of Mechanical Design

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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.

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Section: Background: In-mold Assemblymentioning

confidence: 99%

Section: Selection Of Design Parametersmentioning

confidence: 99%

Section: Selection Of Design Parametersmentioning

confidence: 99%

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

Ananthanarayanan

Ehrlich

Desai

et al. 2011

Journal of Mechanical Design

Self Cite

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“…In addition, these components often have lower thermal conductivities and stiction. Recent experimental studies have shown that the shrinkage values are fundamentally different as a result of the presence of these soft and insulating mold pieces [5]. Therefore, we need to develop a fundamental understanding of the influence of thermal conductivity, rigidity, and stiction on the transient interaction of the mold and polymer melt.…”

Section: Introductionmentioning

confidence: 99%

“…For example, shrinkage of a polymer around an embedded electronic component during processing induces residual stresses that accelerate fatigue [17]. Also, in a multi-stage molding process, polymers are molded on top of each other altering the interaction between the mold and polymer melts [5]. There are ways to control the shrinkage response of the polymer by changing materials, component geometry, or processing conditions [8,9].…”

Section: Introductionmentioning

confidence: 99%

Thermoplastic Polymer Shrinkage in Emerging Molding Processes

et al. 2008

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In this paper, in situ experiments have been designed using the full-field deformation technique of Digital Image Correlation (DIC) to characterize non-uniform shrinkage in thermoplastic polymers commonly used in traditional and emerging molding processes. These experiments are capable of characterizing the differences in strains that develop due to thermal gradients and stiction as the polymer shrinks from the molten to the solid state during molding processes. The experimental set-up consists of simulated open molds, a heating stage, thermocouples for temperature measurements, and a video imaging system for DIC. From these experiments, it has been shown that there is a large increase in shrinkage strain associated with the transition of the polymer from the molten to the solid state in a mold with reduced side rigidity, and as it is cooled below the Vicat softening point. Changing the cooling rate from air-cooled to quasi-steady state can eliminate the transition at the Vicat softening point. Furthermore, substantial decreases in shrinkage strain are observed when the polymer is melted in an open mold without mold release, while using mold release produces results similar to that observed with reduced side rigidity. A simple one-dimensional model reasonably explains and predicts the observed trends in the shrinkage behavior due to temperature differences through the thickness of the polymer melt when using high conductivity molds as well as constraint in the polymer melt near the mold resulting from stiction.

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Characterization of a reverse molding sequence at the mesoscale for in‐mold assembly of revolute joints

Ananthanarayanan

Gupta

Bruck

2010

Polymer Engineering & Sci

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In-mold assembly and miniature molding are two technologies that have seen considerable advances in the past few years. These two technologies can be combined to realize miniature assemblies. Our previous work has shown that a significant change in the mold design is necessary to enable in-mold assembly at the mesoscale. Specifically, parts having the pins in the joint need to be molded before parts having holes. This sequence is reverse of the sequence typically used in macroscale assemblies. Moreover, special features are needed in the mold to prevent bending of the pin due to the pressure exerted by the second stage melt. This paper will present methods to characterize the reverse molding sequence from the perspective of: (1) plastic bending of the premolded component, and (2) joint jamming during the mesoscale in-mold assembly process. In this paper we also show that at the mesoscale, the joint jamming is prevented because of the deformation of the pin under the compressive loading during the second stage molding. We also describe features in the mold that can control the pin diameter deformation and hence control the joint parameters. We present experimental data and computational models to enable the choice of the right in-mold assembly process taking the part sizes into consideration.

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Investigation of Revolute Joint Clearances Created by an In-Mold Assembly Process

Cited by 3 publications

References 17 publications

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

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

Thermoplastic Polymer Shrinkage in Emerging Molding Processes

Characterization of a reverse molding sequence at the mesoscale for in‐mold assembly of revolute joints

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