Development of
In-Mold Assembly Methods for Producing Articulated Structures
Main Participants: Satyandra K.
Gupta, A. Ananthanarayanan,
W. Bejgerowski, H. A. Bruck, R. M. Gouker, D. Mueller, F. Krebs, A. Priyadarshi,
M. Shroeder, and S. Warth
Sponsors: This project is being sponsored by National Science Foundation
and US Army.
Keywords: Injection Molding, In-Mold Assembly, Multi-Material Molding,
Articulated Joints, and Mesoscale Molding.
Motivation
3D articulated devices involve moving parts with significant
out-of-plane motion. There are many applications where the ability to scale
down size and deploy mesoscopic (size range of 0.5 mm
to 10mm) 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 mesoscopic
parts can be easily fabricated, assembling them into devices remains a major
challenge. The proposed project aims to enable development of a new molding
technology that will eliminate the need for performing post-molding assembly
operations during manufacturing of mesoscopic 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
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 making and in-mold assembly, we can create a
new molding process to enable economically viable fabrication of mesoscopic 3D articulated devices.
Some of the key challenges that this project aims to address are:
- In-mold Assembled Macroscale Articulating Joints
- In-mold Assembled Compliant Joints
- In-mold Assembled Mesoscale Revolute Joints
- Characterization of Clearances in In-mold Assembly
Fig 1: In-mold assembly significantly reduces part count
Main Results
In-mold
Assembled Macroscale Articulating Joints
In-mold assembly can be used to create plastic products with
articulated joints. This process eliminates the need for post-molding assembly
and reduces the number of parts being used in the product, hence improving the
product quality. However, designing both products and molds is significantly
more challenging in case of in-mold assembly. We have developed a model for
designing assemblies and molding process so that the joint clearances and
variation in the joint clearances can meet the performance goals. We have also
developed proven mold design templates for realizing revolute, prismatic, and
spherical joints. We have developed mold design methodology for designing molds
for products that contain articulated joints and will be produced using in-mold
assembly process. The results of this work can be seen here.
Fig. 2: Macro scale in-mold assembled articulating joints
In-mold
Assembled Compliant Joints
Multi-material compliant mechanisms enable many new design
possibilities. Significant progress has been made in the area of design and
analysis of multi-material compliant mechanisms. A feasible and practical way
of producing such mechanisms is through multi-material molding. Devices
based on compliant mechanisms usually consist of compliant joints.
Compliant joints in turn are created by carefully engineering interfaces
between a compliant and a rigid material. We have developed feasible mold
designs for creating different types of compliant joints found in multi-material
compliant mechanisms. We have also developed guidelines essential to
successfully utilizing the multi-material molding process for creating
compliant mechanisms. Fig. 3 illustrates some of the joints that have
been developed in Advanced Manufacturing Lab
(AML). More details of this work can be seen here.
Fig. 3: In-mold assembled compliant joints developed in the lab
In-mold Assembled Meso Scale
Revolute Joints
In-mold assembly process at the mesoscale presents several manufacturing challenges. One of the major challenges faced during in-mold assembly at the mesoscale is occurrence of defects due to plastic deformation of the premolded components during the second stage melt flow. This defect is illustrated in Fig. 4. Another challenging aspect is prevention of damage to delicate mesoscale parts during the cavity change step. Results developed as part of this work demonstrate the technical feasibility of creating rigid body mesoscale revolute joints using in-mold assembly process. To overcome the challenges described above we have developed two alternate mold designs to accomplish in-mold assembly at the mesoscale.
Fig. 4 The plastic
deformation of premolded component during in-mold assembly at the mesoscale
The first strategy involves use of radial supports to
constrain the premolded component during the second stage injection. These
constrains if chosen appropriately can inhibit the plastic deformation due to
the force applied by the second stage melt flow. This strategy is illustrated
in Fig. 5. We have also developed finite-element based computational modeling
methods which can be used to select the right radial support length to inhibit
the deformation due to the second stage injection. More details of this work
can be seen here. 
Fig. 5: In-mold assembled meso scale revolute joint using radial support strategy
In an alternate strategy, we have developed a multi-gated mold design solution for the second stage injection. This multi-gated strategy allows bi-directional filling of the second stage polymer. Introduction of an opposing force to neutralize the drag force applied on the premolded component will enable us to overcome some of the technical challenges posed by the unidirectional filling strategy. As part of this work, we reported an alternate mold design solution. This is illustrated in Fig. 6. The force caused by the flow of the second stage polymer melt is fully neutralized if the gates are placed exactly equidistant to the mesoscale premolded component. However, it is important to understand the effect of misalignment in the gate placement on the plastic deformation of the premolded component in order to understand the parameters that will control manufacturing tolerances. As part of this work, we have developed a computational model to characterize the plastic deformation of mesoscale premolded components resulting from the use of the bi-directional filling strategy. More details of this work can be seen here.
Fig. 6: In-mold assembled mesoscale revolute joint using bi-directional filling strategy
Characterization
of Clearances Due to Soft Mold Pieces
Another
significant challenge in in-mold assembled revolute joints is how to obtain
adequate clearances. Macro-scale revolute joints can be formed by first molding
the hole and then molding the pin inside the hole. As the pin shrinks during
the solidification process, it moves away from the hole and provides the
clearance for the joint to function. The value of clearance in the macro-scale
joint can be controlled by carefully selecting the process parameters and the
material for the pin.
However
at the mesoscale, due to a reversed molding sequence, the physics of the
clearance formation is significantly different from the macroscale.
Use of this reversed molding strategy results in joint jamming at the macroscale. However the reversed molding sequence leads to
formation of good quality joints at the mesoscale. Hence in order to understand
this discrepance which occurs due to the size effects
of the joint, we conducted experimental and computational studies to understand
this physics. Our findings indicate that the mesoscale premolded component
undergoes plastic deformation resulting in a change in effective diameter. This
deformation and the resulting final joint dimensions can be controlled by
appropriately controlling the support cavity length of the side core. This
phenomenon is illustrated in Fig. 7
Fig. 7 Controlling
joint dimensions in mesoscale in-mold assembled revolute joints
We have developed computational modeling
methods to relate the support cavity length with the final joint dimensions.
Using this model we can choose the appropriate support cavity length to enable
fabrication of in-mold assembled mesoscale revolute joints with desired joint
characteristics. The details of this work can be seen here.
Journal
Publications
·
A. Ananthanarayanan, S. K. Gupta and H. A.
Bruck. 2009, “Modeling and
Characterization to Minimize Effects of Melt Flow Fronts on Premolded Component
Deformation during In-Mold Assembly of Mesoscale Revolute Joints”,
submitted to ASME Journal of Manufacturing Science and
Engineering
· A. Ananthanarayanan, S. K. Gupta and H. A. Bruck. 2009, “Characterization and Control of Plastic Deformation in Mesoscale Premolded Components to Realize In-mold Assembled Mesoscale Revolute Joints”, Polymer Engineering and Science, 49(2), pp. 293-304,
· A.K. Priyadarshi, S.K. Gupta, R. Gouker, F. Krebs, M. Shroeder, and S. Warth. 2007 “Manufacturing multi-material articulated plastic products using in-mold assembly”, International Journal of Advanced Manufacturing Technology, 32(3-4):350--365.
· R.M. Gouker, S.K. Gupta, H.A. Bruck, and T. Holzschuh. 2006, “Manufacturing of multi-material compliant mechanisms using multi-material molding”, International Journal of Advanced Manufacturing Technology, 30(11-12):1049-1075.
Conference
Publications
·
A.
Ananthanarayanan, S.K. Gupta and H.A. Bruck. 2009, "Characterization and
control of pin diameter during in-mold assembly of mesoscale revolute
joints" Transactions of North
American Manufacturing Research Institution of SME.
·
A.
Ananthanarayanan, S.K. Gupta and H.A. Bruck. 2008, "Characterization and control
of plastic deformation in premolded components in in-mold assembled mesoscale
revolute joints using bi-directional filling strategy" All India Manufacturing Technology and
Development Research. Chennai, India.
·
A.
Ananthanarayanan, S.K. Gupta and H.A. Bruck. 2008, "Mechanical
characterization of cold weld-lines and meld lines in mesoscopic
revolute joints for bioinspired structures" SEM Annual Conference and Exposition.
Orlando, Florida.
·
A.
Ananthanarayanan, C. Thamire, and S.K. Gupta. 2007,
"Investigation of revolute joint clearances created by an in-mold assembly
process" IEEE: International
Symposium on Assembly and Manufacturing. Ann Arbor, MI.
·
A.
Ananthanarayanan, S.K. Gupta, H.A. Bruck, Z. Yu, and K.P. Rajurkar.
2007, "Development of in-mold assembly process for realizing mesoscale
revolute joints" Transactions of
North American Manufacturing Research Institution of SME.
·
A.
Ananthanarayanan, H.A. Bruck and S.K. Gupta. 2006, "Interfacial adhesion
in multi-stage injection molded components" SEM Annual Conference and Exposition. St Louis, MO.
Contact
For additional information and to obtain copies of the above
papers please contact:
Dr. Satyandra K. Gupta
Department of Mechanical Engineering and Institute for Systems Research
2135 Martin Hall
University of Maryland
College Park, Md-20742
Phone: 301-405-5306
FAX: 301-314-9477
WWW: http://www.glue.umd.edu/~skgupta/