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.


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:

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.

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.



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