Session 2566

Introducing Plastic Product Design into the Machine Design Curriculum

Reginald G. Mitchiner, Ph.D., and John T. Tester Mechanical Engineering / Industrial and Systems Engineering Virginia Polytechnic Institute and State University Blacksburg, VA 24060

Abstract

We present our paradigm of plastic product design as a necessary part of the mechanical engineering design curriculum and how these concepts have been a historical part of the mechanical engineering educational agenda, though in other venues. We discuss the practical and accreditation problems associated with incorporating the "new" design features in an existing machine design course. A separate design course, dedicated to plastic product design, is also outlined. This last alternative is likely the best bridge from a machine design curriculum without plastics concepts to one with metallic/nonmetallic product design.

1 Introduction

Plastic products† are a dominant part of the manufacturing world. It is very likely that you the reader could, at this moment, reach out and touch a plastic product from where you sit. Yet, mechanical design curricula at universities, as a general rule, do not have plastic product design integral in their construction. The mechanical design curriculum has embedded within it the classical theory associated with metallic products; usually these products are assumed to be manufactured via metal-removal processes, if their manufacture is assumed in any manner at all.

As practicing mechanical design engineers, we (the authors) have frequently been required to design products with a plastic component as part of their basic structure. One of the authors' primary functions for several years was to design solely plastic products as part of electronic product packaging. The engineering knowledge required for such design tasks did not come from the traditional college curricula. Instead, such knowledge was obtained through many extension courses, plastic vendor training, and seminars. Indeed, the Virginia Tech Mechanical Engineering Department does not have plastic product design integral to any of the classes in the mechanical design course structure.

Plastic product design has its origins in a related field: Casting design. This is especially true we consider injection molding as the predominate process for plastics design. Both design disciplines have many similar design features: Draft angles, sprue locations, coring, cooling accountability and so on. The more seasoned of this article's authors recalls that the typical mechanical engineering curriculum of the past required casting processes and design as a fundamental requirement for graduating mechanical engineers.

Page 3.376.1 † In the context of this article, we consider only thermoplastic products. Thermoset products are not within the scope of this discussion. Some thirty to forty years ago, most mechanical engineering programs (indeed the many of the premier programs of the Midwest—Illinois, Ohio State, Purdue, Michigan State, among others) included significant metal castings and processing experiences in required coursework. These universities maintained extensive laboratories which provided practical as well as theoretical experiences for the student.

Over the intervening years, however, we have lost the provision to the mechanical design student of an integrated approach to mechanical structure design with the requisite considerations of formation. Many factors and forces have contributed to this shift in mechanical engineering undergraduate program direction. This paper is responsive to those forces tending to reassert integrated design and formation for the new millenium.

There are a few engineering programs which contain a solid basis in plastic product design. Most notable of the research-level universities is the University of Massachusetts, Lowell. Their engineering college has an entire department devoted to plastics engineering1. Though their curriculum shows a predominance of rheology-oriented coursework, there are at least two classes devoted to plastic product design at the undergraduate level.

There are several engineering technology-oriented programs with a healthy emphasis on plastics technology. Western Washington University's Plastic Engineering Technology program is a good example2. This course of study covers various plastic manufacturing technologies and design tools, in preparation for the students' bachelors degrees in Manufacturing Engineering Technology. Their laboratories are fully equipped in order to provide a thorough, hands-on educational experience for the Plastics Technology student.

The previous two programs do not address our concerns, however, because these programs are not mechanical engineering programs. They are instead accredited under their own genre of plastics engineering or plastics/manufacturing engineering technology. The question remains, then: How can a mechanical engineering department prepare a student for plastic product design, without devoting major resources towards such an effort?

2 Course Structure Alternatives

We see two alternatives for incorporating plastic product design in our curriculum: a) Integrate the material in one or more existing courses. b) Create a new, separate course, devoted to the topic. The first alternative appears to be the easiest to implement on the surface, but has several disadvantages. The second alternative may seem more difficult to integrate into an existing course offering, but will likely provide long-term benefits to both the students and the department.

2.1 Integration into Existing Courses

We use our own Department of Mechanical Engineering (ME) at Virginia Tech as a basis for exploration. There are four courses into which plastics design concepts might be introduced in Virginia Tech's ME curriculum: Introduction to CAD/CAM, Mechanical Design I, Mechanical Design II or Machine Design3. Before discussing these classes separately, there are some concerns Page 3.376.2 associated with changing the course structure. The first, and probably most important from a college point of view, is ABET accreditation. These courses, to various degrees, affect the accreditation of the Mechanical Engineering bachelors degree. Tampering with the course content significantly could be perceived as highly risky to university personnel dedicated to preserving such accreditation.

Of greater immediate impact to the instructing professors is the question: "What material is cut out of the course in order to make room for the plastics design topics?" This question is valid and relates to both the accreditation issue as well as the personal preferences of the professor. Being in a semester format, Virginia Tech's course content can provide a range of information for a given course. Nevertheless, most instructors would still contend that there is never enough time to impart all the topics they feel necessary to give to the students in a specific class timetable.

Noting the above problems, let's assume that they can be satisfactorily addressed. We now want to examine how well such concepts could fit into the existing course structures.

The first course, Introduction to CAD/CAM, gives the students an introduction to a computer- graphics design package which is integrated with a CNC computer package. This course is structured around the concept of taking a class project from design on CAD to manufacture on a CNC machine. Introduction of plastic product design concepts in this would require a complete overhaul of the course's concept. Furthermore, the department does not have the facilities (i.e., an injection molding machine) to finalize the final class product designs, whereas CNC machines are readily available for such tasks.

Two consecutive courses, Mechanical Design I and II, emphasize static and fatigue loading concepts in various components, including fasteners, springs, bearings, gears, shafts and brakes. Fatigue analysis instruction is considered a strength in Virginia Tech's Mechanical Engineering Department's approach towards mechanical design. Introduction of plastic product design concepts do not fit snugly into this course paradigm; plastics design (in our view) is more of a guidelines approach to component features. Such guidelines do not usually impact the fatigue analysis concept. Integration of this topic into the class would not be a smooth nor easy fit.

Machine Design is devoted to the analysis of machine components, such as bearings, gears, contact stresses, plates, rotating disks, press fits, torsion, springs and so on. This course would seem to be the best fit for a plastics design introduction. Some concepts of "design for manufacturing" (DFM) are brought to bear in this class; plastic product design is heavily oriented towards DFM. In addition, some aspects of plastic part assembly could be integrated here as well. The main drawback to integration into this course is the fact that this course is a graduate-level offering. Seniors with certain qualifications are allowed to take the course as an elective, but not all undergraduate engineering students are eligible or required to do so. If we believe that plastics design is important to the fundamental education of the student, then even modification of this course may be insufficient to answer our educational needs. Page 3.376.3 2.2 A Separate Course Offering

The above discussion leads us to believe that the instruction of plastic product design would be best served by introducing a new elective into the curriculum. This new course, arbitrarily entitled, "Plastic Product Design," will have the time to devote study in this field and emphasis those areas which are of importance to mechanical engineering design.

A proposed course structure outline (semester format) is summarized in the table below. Manufacturing processes are introduced due to the very close DFM requirements involved in plastic product design. Three processes will be covered: Extrusion, vacuum forming and injection molding. This order of presentation gives the student a concept of increased complexity for the processes: Extrusion is a linear process in the sense that the material is drawn through a static die. Vacuum forming (pressure forming is very similar) produces shapes that are more greatly impacted by design, in the sense that one surface of the mold will yield the primary, detailed features of the part. Injection molding requires the most complex design concepts, in that all sides of the mold cavity will contribute equally to the final part shape. Most of the design characteristics in the earlier processes are a factor in injection molding as well (draft, wall thickness limits), so that simpler process topics can help build the foundations for the later topics.

There can be an argument made for "assembly processes" to be an integrated part of all the process- oriented topics. For example, boss (standing post) design discussions for injection molding can be easily extended into related topics on boss design for cutting screws, metal inserts, and even snap- fit design features. However, since these assembly features are common to both injection molding and vacuum forming, we feel that the assembly design aspects would be better left as a separate topic, as a time-saver.

The course should be offered, at least initially, as a junior/senior level course, so that the students will have some stress analysis, mechanical design and manufacturing process coursework as prerequisites. It may benefit the course introduction to allow the plastic product design class as an accepted course for the ME graduate program as well. To gain an even broader base, it could be beneficial to allow Industrial Engineering graduate students in the manufacturing area to use the course as an elective in the manufacturing program as well. Of course, those IE students must satisfy similar prerequisite requirements.

Some concern may arise from the fact that stress/strain calculations for plastic products are, by definition, not linear in the strict sense. Undergraduate students might find such calculations intimidating since they are likely equipped with only the fundamental, linear stress/strain concepts of structural analysis. This factor can be alleviated by using plastic design guidelines, rather than strict structural analysis calculations, in the presentation of many of the design topics. The course instructor would emphasize that these guidelines vary from one material to another. It is likely that the course materials will draw heavily upon plastic vendor design guides, since plastic material properties can change from year to year. The largest distributor of injection molding plastics, General Electric, distributes illustrated design guides for its various materials; these guides can provide some useful tools to the class instructor4. Page 3.376.4 There are some plastic product design texts in publication, which would provide a solid, academic foundation for the course material. These texts provide similar design guidelines as those in vendor handbooks, as well as experimentally-based formulations5,6. Newer texts are available in publication that will undoubtedly include computer-aided design approaches to help teach design concepts.

Week Topic Comments 1 Introduction to plastic Common materials (polycarbonate, ABS, etc.) and their uses are introduced manufacturing processes here. Injection molding, vacuum/pressure forming, extrusion. 3 Extrusion/pultrusion Design concepts associated with these processes. 4 Vacuum forming A detailed description of vacuum/pressure forming and how the process impacts manufacturing design. 5 Vacuum forming design Design concepts for common-sized, vacuum formed products. 6–7 Injection molding A detailed description of injection molding processes. Will include manufacturing manufacturing impacts upon the design of product, such as cooling parameters. Discussion of mold tooling could be covered here as well, or during the next topic. 8–10 Injection molding design Design rules are covered here, for the standard range of common plastics. It would be desirable to include finite element studies of warpage impacts and other design/manufacturing issues. If time permits, more recent developments, such as foam molding and thin-wall molding design can be covered. 11–12 Plastic product assembly Covering plastic-oriented joining processes and related design requirements. processes Design for screws, inserts, and snap-together products. Ultrasonic, hotplate, spin welding are examples of permanent bonding techniques to be covered. 13 Finishes and tooling Finishing and other related parameters, common to most of the processes, are designations covered, to enable the designer to properly document designs. 14 Summary and A buffer for any unanticipated topics of interest miscellaneous

3 Summary and Conclusions

We have presented an outline for the introduction of plastic product design into an existing mechanical engineering curriculum. Though the concept of casting, or forming, product design education is not new, the proliferation of plastic products makes this education topic relevant today. A separate course offering, dedicated to the plastic product design topic, would best address the problems associated with accreditation and educational material coverage. A combination of academic texts and commercial vendor guidelines would help develop the course in its initial offering.

Bibliography

1 Plastics Engineering Undergraduate Plan of Study, James B. Francis College of Engineering, University of Massachusetts, Lowell, 1997; web location: http://www.eng.uml.edu/Dept/Plastics/programs/ index.html

2 Plastics Engineering Technology Program, Engineering Technology Department, College of Arts and Sciences, Western Washington University, Bellingham, WA, 1997; web location: http://www.wwu.edu/~techdept/pet.html

3 Mechanical Engineering Undergraduate Catalog, College of Engineering, Virginia Tech University, Blacksburg,

VA, 1997; web location: http://www.vt.edu/ugradCat/ucdME.html Page 3.376.5

4 General Electric Plastics Design Guide, GE Plastics, Pittsfield, MA, 1992.

5 Levy, S., and Dubois, H. J., Plastic Product Design Engineering Handbook, 2nd Edition, Chapman and Hall, New York, 1984.

6 Pye, R. G. W., Injection Mould Design: A Design Manual for the Thermoplastic Industry, 2nd Edition, The Plastic & Rubber Institute, 1978.

Biographical Information

REGINALD G. MITCHINER, Ph.D., is a professor in the Mechanical Engineering Department of Virginia Polytechnic Institute and State University. He has over two decades of experience in the practice and education of mechanical design.

JOHN T. TESTER is a Ph.D. candidate in the Industrial and Systems Engineering Department of Virginia Polytechnic Institute and State University. He has over a decade of experience in design and manufacturing processes, particularly in the area of injection molding and electronics packaging. Page 3.376.6