Figure 1: Fifty full assembled, essentially identical yo-yos!
The objective for this project was to manufacture a fun and creative yo-yo while learning about production for mass manufacturing. A significant amount of planning was required to optimize our design and production process. After considering the advantages and drawbacks of over fifteen potential designs, we concluded that making a “cartoon critter” yo-yo would be cute, playful, and appealing to a wide variety of audiences. We settled on designing a lion, which we believed was relatively unique and also geometrically appropriate for functionality.
Our goal was to manufacture fifty fully assembled yo-yos (Figure 1). Each yo-yo was comprised of two injection molded bases – each with an overmolded shim and hex nut, two injection molded face plates, two injection molded manes which would snap fit to the bases, two thermoformed facial feature plates, one turned shoulder bolt and spacer, and one string (Figure 2). Our yo-yos were approximately 2.732 inches at their largest diameter, 1.376 inches tall, and 46 grams.
Figure 2: The parts for one yo-yo.
After modeling our design on Fusion 360, we modeled and manufactured molds that could be used for the injection molded parts as well as a die for thermoforming (Figure 3A-3E). The molds for injection molding were manufactured out of aluminum with a CNC mill (Figure 4). When designing our molds, we avoided features that created un-machinable undercuts, and we also considered the sizes and capabilities of the end mills that were available. Since higher quality surface finish significantly increased machining time, we also determined which parts needed a smoother surface for cosmetic reasons, and left others with a rougher finish.
Figure 3A: CAD of one side of our yo-yo.
Figure 3B: CAD of the cross section of the inside of our yo-yo.
Figure 3C: CAD of the thermoformed facial features plate.
Figure 3D: CAD of the die for thermoforming.
Figure 3E: CAD of the molds for the injection molded parts.
Figure 4A: Molds were manufactured out of aluminum on a CNC mill.
Figure 4B: All final molds!
For the die, we included draft to accommodate the constraints of thermoformed parts while modeling it. We then tested 3D printing it quickly via FDM with a Dremel 3D40 to ensure it would produce the facial feature plate as desired (Figure 5A). Once produced, we decided to 3D print the die on a Form 3 using high temperature SLA resin so that it would be durable through many rounds of thermoforming (Figure 5B). For thermoforming, we also printed red and black splotches on the sheets of plastic so that the eyes, nose, and tongue wouldn’t need paint after thermoformed (Figure 6).
Figure 5A: This die was 3D printed out of PLA. We tested it and determined it fit well with the injection molded face plate.
Figure 5B: The die was then printed out of high temperature SLA resin.
Figure 6: Printed plastic sheets for thermoforming to avoid post process paint.
Figure 7: Our molds in the injection molding machine ready to produce some parts.
Our optimized design allowed us to assemble the parts with no post processing other than punching out the thermoformed facial features. After that, we inserted the thermoformed piece into the injection molded face plate, aligned the face plate within the mane using the small tab at the top center, and snapped the mane onto the base for each half of the yo-yo. These two halves were screwed together and a string was tied and wrapped around completing our yo-yo. Finally, we did a cost analysis for prototype manufacturing (Figure 8) and high-volume manufacturing (Figure 9), ranging from 1 to 100,000 parts. Prototype manufacturing was significantly cheaper per part until the 100,000 mark, where the costs for both became relatively even at $2 per yo-yo.
Figure 8: Cost analysis for prototype manufacturing.
Figure 9: Cost analysis for high-volume manufacturing.
Figure 10: Our final yo-yo!
Figure 11: My teammate and I after a long afternoon of milling and about half the chips we produced!