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:
Fig 1: In-mold assembly significantly reduces part count

Main Results

In-Mold Assembled Articulated 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.


Fig. 2: Macro scale in-mold assembled articulating joints

Design and Development of In-Mold Assembly Techniques for Manufacturing a Swashplate: The swashplate on an micro air vehicle (MAV) has an important functionality for direction control of the MAV. Hence it is important that this device should operate at very high rotational speeds while being as light as possible. Currently swashplate is manufactured by a machining and assembly process. Machining and assembly involves high labor costs for each part since each component has to be machined individually. This causes very high unit costs of the swashplate due to increase in the manufacturing and labor time. In the in-mold assembly method there is a high initial cost of mold manufacturing. However, once the molds are machined the molding cost per unit part is very low. In this process, the outermost ring and the innermost ring (Fig. 3(a) and 3(c)) are molded as first stage parts. These parts are now inserted into the second stage mold and the molten plastic is injected forming the middle ring (Fig. 3(b)) hence completing the in-mold assembly of the swashplate. The swashplate thus manufactured was tested at a rotational speed of about 15000 rpm with two loads of 2 g each eccentrically attached to the swashplate at a radial distance of 1.5 inches. The swashplate passed this test without any damages to the part. Hence we believe that the in-mold assembled swashplate can be used as a replacement for the current machined and assembled swashplate which would in turn decrease both the overall cost and the weight of the MAV. Hence we developed in-mold assembly method for making swash plate.


Fig. 3: In-mold assembly of the swashplate

Multi-Material Compliant Mechanisms: 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. 4 illustrates some of the joints that have been developed in Manufacturing Automation Lab (MAL).

Fig. 4: In-mold assembled compliant joints developed in the lab

Meso Scale Revolute Joints: In-mold assembly process at the mesoscale presents several manufacturing challenges. Results developed as part of this work demonstrate the technical feasibility of creating rigid body mesoscale revolute joints using in-mold assembly process. This work firstly describes a mold design with varying cavity shape to perform in-mold assembly. This mold design uses an accurate mold piece positioning method to avoid damage to delicate mesoscale parts during the cavity change step. Secondly, a mold insert fabrication process for making mold inserts with the desired surface characteristics for mesoscale molding is described. Finally, methods to limit the adhesion at the interfaces and hence create articulated revolute joint are described. Using the advances reported as part of this work we have successfully molded a mesoscale revolute joint. 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. Fig. 5 illustrates the meso scale in-mold assembled revolute joint that was manufactured in MAL.

Fig. 5: In-mold assembled meso scale revolute joint

Characterization of Clearances Due to Soft Mold Pieces: Several issues are involved in the proper functioning of the rigid body joints that are manufactured using the in-mold assembly operation. A clearance fit in the revolute joint has to be ensured in order to have appropriate functioning of the rigid body joint. This clearance is provided by controlled in-mold shrinkage between the second stage part and the second stage mold. Experimental investigation was conducted in order to understand the differences in expected shrinkage values for soft molds as compared to hard molds.  Aluminum is considered the hard incompressible mold whereas ABS is the soft compressible mold. The data from the experiments clearly shows that there is a considerable difference between shrinkage values for ABS sleeve mold and Aluminum sleeve mold. We have developed a detailed computational model of thermo-mechanical behavior of the second stage parts in the mold.

Publications:

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/