Automated Design and Fabrication of Multi-Stage Molds for Making Multi-Material Objects



Main Participants: Satyandra K. Gupta, Ashis Gopal Banerjee, Greg Fowler, Malay Kumar, Xuejun Li, and Alok K. Priyadarshi

Sponsors: This project is sponsored by the National Science Foundation. We also received in-kind support from Spatial Technologies and  Black and Decker. 

Keywords: Mold Design, Multi-Material Molding, Multi-Stage Molds, and Geometric Reasoning.


Motivation

Multi-material (also known as heterogeneous) objects refer to the class of objects in which different portions of objects are made of different materials. Multi-material objects are different from traditional objects that are assemblies of several single-material components. Multi-material objects allow significant reduction in assembly operations. Furthermore, the product quality can be improved, and the possibility for manufacturing defects can be reduced.

One of the most scalable processes known to mankind is the molding process. Once a mold has been created, it can be used to manufacture large volumes of objects quickly. As the volume of production goes up, the cost of making individual objects goes down due to amortization of the mold cost over a larger number of objects. Over the last few years, a wide variety of multi-material molding processes have emerged for making multi-material objects. In multi-material molding, multiple different materials are injected into a multi-stage mold.

There are several  injection molding techniques that can be used to mold multi-material objects. In case of Co-injection, Sandwich and Bi-injection molding, the mold remains the same throughout the process. The mold is not opened until the last material is completely injected. However, in case of over-molding the molds for injecting different materials are completely different. After injecting one material into the mold, the partially finished object is removed and transferred to a different mold for injecting another material. Multi-shot injection molding is a multi-stage injection molding processes, in which some pieces of the mold are removed after the injection of the first material, and some new pieces are added to form a new cavity into which the second material is injected.

Currently very limited literature exists that describes how to design molded multi-material objects and molds. Very few designers have the required expertise and experience to design multi-material objects and their molds. Therefore, developing molded multi-material products is currently a very time consuming process. Multi-material objects can be considered to be an assembly of single material components. However, traditional design-for-manufacturing and design-for-assembly guidelines cannot be applied to molded multi-material objects due to the following reasons. In traditional molding processes, components are first fabricated and then assembled together outside the mold. In multi-material molding processes, fabrication and assembly steps are performed concurrently inside the mold. Therefore, use of existing knowledge in the design of molded products often results in selection of a wrong shape. Correcting these mistakes delays the product development process unnecessarily. Therefore, manufacturability analysis during the product design process is necessary to help the designer in designing molded multi-material objects. Similarly, designing molds for multi-material molded parts is a very time consuming activity. We believe that automation of the mold design and manufacturability analysis will significantly reduce the product design time and improve the product quality.

Main Results and Their Anticipated Impact

Manufacturability Analysis:  We have developed a systematic approach for performing manufacturability analysis during design of molded multi-material objects. Our contributions in this area include: We expect that our work will be helpful in reducing the product development time associated with the molded multi-material objects.

Molding of Multi-Material Compliant Mechanisms: We have shown that multi-material molding is a promising manufacturing process for creating multi-material compliant mechanisms. Several different types of interfaces that can be used in compliant mechanisms are described. Experimental characterization results for these interfaces show that these interfaces have the required motion range and do not break under the loads needed to produce the motion. Furthermore, experimental characterization results show that certain geometrically complex interfaces can be modeled as simple interfaces without having any serious effect on the accuracy of an assembly analysis. We have developed several different compliant joints that provide one to three degrees of freedom. For each joint design, we have also developed a feasible mold design to realize that joint. Finally, we show how a complex device consisting multiple different compliant joints can be fabricated by combining mold pieces for individual joints into an overall mold.

Our case studies clearly show that compliant joints can be used to reduce part count and eliminate assembly operations. Along with these manufacturing benefits, multi-material compliant mechanisms can be used to create geometrically complex structures. While there are several different methods to create multi-material compliant mechanisms, the multi-material molding method is superior because of it’s adaptability to traditional molding technologies. This adaptability enables one to create very complex structures at highly reduced costs. 

Automated Mold Design:  We have developed geometric algorithms for automatically generating mold stages for multi-shot molding process. Based on these algorithms we have implemented a system and have tested our implementation successfully with a number of different two-material objects. In addition to being useful in rotary-platen mold design, these algorithms can be used to design traditional injection molds for single material components. These algorithms are based on a novel concept that utilizes partitioning of the gross mold by surfaces and combine the resulting solids to form the final mold pieces. We have tested our implementation successfully with a number of different single material components. We expect that these algorithms will provide the necessary foundations for automating the design of multi-stage molds and therefore will help in significantly reducing the mold manufacturing lead-time associated with these types of molds. 

Models for Comparing Traditional Molding with Multi-Material Molding Processes: We have developed two independent yet complementary models for comparing traditional injection molding and assembly operations with bi-material multi-shot injection molding.  The first model uses a cost-based metric for evaluation and comparison where the second model uses a set of relevant performance aspects as a basis for comparison. The cost estimation model uses a set of semi-empirical formulas to evaluate the various important cost drivers associated with each process. The second model is performance evaluation model. It offers guidelines for measuring several different attributes related to performance. The designers can then compare the values of these performance aspects side-by-side and select the preferred process based on their own subjective ranking schemes.

Additional Material


Related Publications

The following papers provide more details on the above-described results.
Some of these papers are available at the publications section of the website.

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
University of Maryland
College Park, Md-20742
Phone: 301-405-5306
FAX: 301-314-9477

WWW: http://www.glue.umd.edu/~skgupta/