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 have identified the following four types of manufacturability
problems: infeasibility of molding sequences, undesired friction during
mold opening and closing, undesired material flash on finished faces,
and excessive interface deformation. These problems are unique to
multi-material molding and do not occur in traditional injection
molding.
- We have developed algorithms for determining the occurrence of
possible manufacturability problems for each of the above described
problems. Our algorithms can also generate high-level suggestions for
redesign. Currently our algorithms do not attempt to modify the
geometry of the object. Such modifications will be carried out by human
designers based on the redesign suggestions generated by our algorithms.
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.
- A. Banerjee, X. Li, G. Fowler, and S.K. Gupta. Incorporating
manufacturability considerations during design of injection molded
multi-material objects. Research in
Engineering Design, 17(4):207--231, March 2007.
- A.K. Priyadarshi, S.K. Gupta, R. Gouker, F. Krebs, M. Shroeder,
S. Warth. Manufacturing Multi-Material Articulated Plastic Products
Using In-Mold
Assembly. International Journal of
Advanced Manufacturing Technology, 32(3-4):350--365, 2007.
- R.M. Gouker, S.K. Gupta, H.A. Bruck, and T. Holzschuh.
Manufacturing of multi-material compliant mechanisms using
multi-material molding. International
Journal of Advanced Manufacturing Technology,
30(11-12):1049--1075, 2006.
- A.G. Banerje and S.K. Gupta. A step towards automated design of
side actions in injection molding of complex parts. Geometric Modeling and Processing
Conference, Pittsburgh, PA, July 2006.
- A.K. Priyadrashi and S.K. Gupta. Finding mold-piece regions
using computer graphics hardware. Geometric
Modeling and Processing Conference, Pittsburgh, PA, July 2006.
- X. Li and S.K. Gupta. Geometric algorithms for automated design
of rotary-platen multi-shot molds. Computer Aided Design,
36(12):1171--1187, 2004.
- H. Bruck, G. Fowler, S.K. Gupta, and T. Valentine. Towards
bio-inspired interfaces: Using geometric complexity to enhance the
interfacial strengths of heterogeneous structures fabricated in a
multi-stage multi-piece molding process. Experimental Mechanics,
44(3):261--271, 2004.
- S. K. Gupta and G. Fowler. A step towards integrated
product/process development of molded multi-material structures. In Tools
and Methods
of Competitive Engineering, Lausanne, Switzerland, April 2004.
- X. Li and S.K. Gupta. A step towards automated design of
index-plate multi-shot molds. In Tools and Methods of Competitive
Engineering Conference, Lausanne, Switzerland, April 2004.
- X. Li and S.K. Gupta. Manufacturability analysis of
multi-material objects molded by rotary platen multi-shot molding
process. In ASME International Mechanical Engineering Congress and
Exposition, Washington, DC, November 2003.
- M. Kumar and S.K. Gupta. Automated design of multi-stage molds
for manufacturing multi-material objects. Journal of Mechanical
Design, 124(3):399--407, 2002.
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
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/