TEACHING MOLD DESIGN: FROM CONCEPT TO CREATION

Stephen Johnston & David Kazmer University of Massachusetts Lowell, Lowell, MA

Abstract

Traditionally mold design has been taught on a theoretical basis where student’s designs never leave the drawing board. Today’s computer aided manufacturing techniques allow for hands-on design projects. A junior level mold design engineering class was revised to include an intensive art-to-part project. Given constraints, students designed new plastic parts, manufactured the tooling using CAM software and CNC machining, and injection molded prototype parts. In addition to mold design, students learned machine tool practices and the difficulties associated with commissioning new tools. The project required considerable initial investment, but was met with an overwhelmingly positive response.

Introduction

The Plastics Engineering Department at UMass Lowell has been providing students with a solid foundation in the areas of materials, processing, and design since it was founded in 1954. Numerous advances in technology have changed the face of the plastics industry over the past fifty years, particularly in the area of design. Advances in 3D computer aided design (CAD), finite element analysis (FEA), rapid prototyping, and computer aided manufacturing (CAM) have all streamlined the engineering design process and reduced the time to market.

Plastics mold engineering is one of the most comprehensive courses students take during their undergraduate studies since it bridges the topics of design, materials, and processing. Until this past year, plastics mold engineering was taught on a theoretical basis. Traditional lectures on the theory of mold design were accompanied by a project that required students to design a mold for a plastic part using CAD. Unfortunately, time and facility constraints dictated that the projects were not developed beyond computer models.

The engineering college recently purchased a 3-axis vertical CNC milling machine and CAM software for programming complex 3D tool paths. These investments provided the capabilities necessary to take student’s designs into an academic manufacturing environment and generate real prototype parts. The time investment required to take student’s projects from initial concept through finished working prototypes presented considerable barriers to implementation. However, the perceived benefits were numerous and similar projects implemented by other plastics engineering programs have been very successful. [1-3]

This article will focus on: 1) how the art-to-part project was included into the structure of the conventional plastics

mold engineering course, 2) how the project was designed and implemented, 3) how the project was received by the students, and 4) what improvements are recommended for future years.

Course Format

Plastics Mold Engineering is a 3-credit junior level class taught in the spring semester. The course consists of a lecture that meets for an hour and 15 minutes twice a week, and a lab that meets for two hours once a week. In the spring of 2008, the course was revised to include the art-to-part project. The lecture portion of the class was taught using Injection Mold Design Engineering by Kazmer. [4] Table 1 shows the lecture topics that were covered during the class. The lectures were designed to provide the students with a sound understanding of the fundamentals of mold design, analysis, and pricing.

The grading scheme for the class was as follows: individual homework assignments – 40%, midterm exam – 10%, final exam – 10%, project #1 – 10%, project #2 – 10%, and project #3 – 20%. There were 10 individual homework assignments that required students to do all of the analysis work for designing a mold. The lecture and homework material was tested with an open-book midterm and an open-book final. Although the projects built directly on the information learned during lecture, the project required that the labs proceed at a slightly different pace. Therefore, the projects were coordinated independently of the lecture during lab time. The project will be discussed subsequently.

Project Implementation

Students worked on the semester long mold design projects in teams of three. The project was divided into three sub-sections so that teams could receive support and feedback through the development process. The three sub-projects focused on part design, mold design, and mold manufacturing.

Project #1 – Product Design, Analysis, & Prototype During the first project each team of students was

required to design a new plastic part, analyze the moldability of the part, and create rapid prototypes. The teams were encouraged to design a completely new plastic part, but were allowed to redesign an existing part. It was specified that the parts were to be manufactured in a 2-plate cold runner mold without side action, and that the parts must fit within the insert volume shown in Figure 1. It was also specified that parts were to be ejected using 4.78mm (3/16”) round ejector pins and a standard 1° draft angle was to be

incorporated into all designs. It was recommended that the parts be made from either polypropylene or polycarbonate.

Figure 2 shows the CAD model for one component of a sand timer, which was designed and built. This model undoubtedly had the greatest detail since the lettering was on the order of 1mm in width. As the part designs were being developed, additional considerations were introduced including the location of the parting line, the gating location, and the approximate ejection locations. Filling analyses were utilized to verify that the part cavity could be filled based on the specified injection location. Similarly, a draft analysis was performed to determine if appropriate draft was applied to the part. At the conclusion of the first project the teams submitted their final part files along with filling analysis results and a full sized rapid prototype of their design. Rapid prototypes are shown in Figure 3. It can be seen that the part designs varied greatly ranging from a soap dish with a hinged lid to an airplane model made from multiple components that were assembled after molding.

Project #2 –Insert Design & Tool Paths The second portion of the project focused specifically

on developing mold inserts that would manufacture the parts each team designed. An interchangeable mold base was specially designed and built for the course, which would accept two sets of inserts at a time. The cooling lines were located in the A-plate and the support plate so that cooling channels were not located in the part inserts. Although this did not provide optimal cooling, it greatly simplified the mold layout and prevented potential water leaks. Part ejection was accomplished using a grid of over 100 possible ejector pin locations. The CAD files for the mold base were made available to each team. Based on the provided mold geometry, teams generated CAD models of the mold inserts required to manufacture their parts. Runner systems were also designed and analyzed based on the final part layout. The runner system analysis for the family tool that manufactures the airplane model is shown in Figure 4.

Once the mold insert geometry was finalized, teams met with the lab instructor and generated the tool paths required to manufacture each insert. To simplify setup time, students were limited to a library of common machine tools. After each tool path was generated it was verified to check for collisions. Figure 5 shows the tool path verification for the soap dish cavity insert.

Project #3 – CNC Machining & Injection Molding The third portion of the project focused specifically on

manufacturing the mold inserts designed in project #2 and molding prototype parts. Each team worked with the instructor to machine their set of mold inserts using the vertical CNC machine. Figure 6 shows the machining of the A-insert for the spork. Teams were then responsible for hand polishing the inserts, cutting ejector pins to length, and fine tuning fits. The inserts were assembled into the interchangeable mold base and prototype parts were molded.

Figure 7 shows the fully assembled mold with the soap dish and the sand timer inserts installed in it. Mold

assembly and troubleshooting were very educational experiences since students needed to account for minor defects in the design or manufacturing of the inserts. By providing the students with hands on experience machining mold inserts and starting up a new mold they learned first hand how critical design flaws become in manufacturing. Additionally, they learned how to troubleshoot a wide variety of processing issues as they performed the start-up.

At the conclusion of the project each team made a 5-10 minute project presentation detailing their entire project. Figure 8 and Figure 9 show two of the teams with their mold inserts and finished parts.

Student Feedback and Faculty Perspective

At the conclusion of the project a survey was conducted to judge the students response to the project. The student feedback was very positive. The students identified the strengths of the project as follows:

“I liked the hands on part. Most classes are written and technical, but this brought a new experience.”

“Actually seeing how we can take an idea and put it through to a final part.”

“Its strength was having peer bonding.”

“Seeing my concept go from Solidworks to aluminum and become real.”

“I love the whole process from design to creating a part! Oh Wow! It just blew my mind.”

“Problem solving – designing parts that could be molded - making adjustments to produce a part”

Similarly, students identified the weaknesses of the project as follows:

“Maybe limit part designs more. Some of the real complex parts proved difficult to machine.”

“Really wonderful project. Only suggestion is 1 credit for lab credit. Seeing as how it perfectly took us the full semester.”

“I wish there had been more time with Mastercam. It wasn’t very difficult, but it was rushed”.

“More people who know Mastercam so more people can work on it at once to increase efficiency.”

Students indicated that they had the hardest time learning the CAM software. This was largely due to the fact that the University only had two licensed seats of the software at the time. Therefore the topic was covered with individual teams rather as a lab exercise. This increased the workload on the instructor while decreasing the time that students were exposed to the software. More seats have since been purchased for the upcoming semester.

From a faculty perspective this was a very fast paced labor intensive course, but the rewards were worth the effort. As indicated by the positive responses above, the students

were very excited when their projects went from concept to reality. This excitement was largely responsible for the success of the project. As teams began to see actual inserts and parts be manufactured, their effort dramatically increased. At the end of the semester each team had molded prototype parts and successfully completed the project.

Conclusions

The implementation of an art-to-part design project into the one semester long plastics mold design class was a success. The students were very pleased with the course and found the project to be rewarding. From the standpoint of the instructor, this class tied multiple disciplines together into a strong learning experience. The extensive time commitment for both the students and the faculty was a primary consideration with this project. In the future, teaching assistants will assist with the class to ease the instructor workload and provide additional student support.

Acknowledgements

This paper was funded in part by the Engineering Education Program of the National Science foundation, grant EEC-0530632.

References

[1] P. Koch, B. Young, B. Johnson, and M. Dropik, SPE-ANTEC Tech Papers, 53, 2894, (2007).

[2] J. Meckley, SPE-ANTEC Tech Papers, 53, 2899, (2007).

[3] W. Thielemann, SPE-ANTEC Tech Papers, 46, 294, (2000).

[4] D. Kazmer, Injection Mold Design Engineering, Hanser Publishers, 2007.

Figure 1: Insert volume provided to students. (Dimensions in inches)

Table 1: Plastics Mold Engineering Lecture Topics

Session Topics Reading 1 Mold functions & structures 1-9 2 Mold types & components 9-13 3 Product specifications 13-27 4 Design for molding 28-35 5 Mold quoting 39-43 6 Part cost estimation 60-66 7 Mold & insert layout design 67-77 8 Mold base selection 77-89 9 Cavity filling overview 91-101

10 Cavity filling design & analysis 102-112 11 Feed system overview 119-132 12 Feed system design & analysis 132-150 13 Gating overview 161-175 14 Gating design & analysis 175-183 15 Venting 185-197 16 Cooling system overview 199-219 17 Cooling system design & analysis 219-230 18 Shrinkage & warpage 233-250 19 Ejector system overview 259-269 20 Ejector system design & analysis 269-277 21 Ejector system design & analysis 278-297 22 Structural system overview 299-305 23 Structural design & analysis 306-317 24 Structural design & analysis 317-331 25 Advanced mold designs

Figure 2: 3D part model of sand timer.

Figure 3: Rapid prototypes of student projects.

Figure 4: Filling analysis of model airplane family mold.

Figure 5: CAM programming of tool paths for soap dish.

Figure 6: CNC machining of A-insert for spork mold.

Figure 7: CNC machining of A-insert for spork mold.

Figure 8: Students displaying their finished i-Pod hold.

Figure 9: Students displaying their finished combs.