Design and Plan for Manufacturing/Assembly

Introduction: YoYo Design and Team Plan

Our yoyo design is inspired by sushi. The sushi yoyo will be comprised of two faces, one with regular sushi fillings and the other, a fish tail. Currently, the plan is to injection mold the yoyo base, featured below in the images in black, each half of the tail of the fish, and the sushi fillings. The sushi fillings are four separate pieces that come together in the center, creating a quad like structure. These will be injection molded all together in a single group mold, allowing for quick fabrication with a rotation of colors to allow variety. The rice component will be thermoformed. The thermoforming mold will likely be SLA printed to get more detail of the rice texture on the mold than would be possible (or easily manufactured) by machining.

We have divided each part amongst the team. Sherri and Fano will be in charge of the yoyo base. Billy and Shirley are building the sushi fillings. Rosa and Aya are creating the fishtail, and Jane is responsible for the thermoformed rice.

Below are our preliminary SolidWorks models of each component as well as a final full assembly. 

From left to right, top to bottom: half of the tail, one piece of the sushi fillings, the yoyo base, the thermoformed rice, the total assembly (sushi filling face and side view)

Manufacturing and Assembly of the YoYo

As mentioned in the introduction, the seaweed base, sushi fillings, and fish tail of our sushi yoyo will be injection molded while the rice will be thermoformed. The injection molded parts should be ready for assembly immediately after production, with little to no post-processing. Using a punch and die set, the center of the thermoformed rice will be punched out to create a hole for the posts of the fillings/tail to go into the base. The hole in the thermoformed rice will be just big enough for the posts to go through, but small enough such that the sushi fillings/tail pieces would hold the thermoformed rice to the base.

Here are the proposed assembly instructions:

(Assemble each half of the yoyo separately first) 

1. Fit the thermoformed rice into the seaweed base. 
2.  Choose four sushi fillings (ideally not identical pieces) and install the sushi fillings so that their posts go into the base and hold the thermoformed rice in.
3. Put the two fish tail pieces together using the posts inside each fish tail.
4. Repeat steps 1 and 2 with the fish tail to assemble the other half of the yoyo.
5. To put the yoyo halves together, install a nut into each half of the base.
6. Install a screw into one half and put the spacer over the screw.
7. Place the loop of the yoyo string onto the spacer and screw the other half of yoyo half on.

Below are some exploded views to help clarify the order of assembly:

Design for Manufacturing Considerations

Thermoformed Part:

When designing our thermoformed part, we made sure that we did not exceed the width to depth constraint of D=0.5W for straight vacuum forming with a female mold. Additionally, we added a 0.25 degree draft angle to each of the walls of the part so that we can remove the part from the mold. We also created a fillet with radius of 0.080 inches for each of our edges. This is at least two times the thickness of the part, which reduces the risk of thinning on these edges.

The material for the thermoformed part is intended to be 0.020 inches thick in order to get the greatest detail on the surface of the rice. We will also create our thermoforming mold so that the outward facing surface of the rice is sucked down into the female mold to give better definition to the rice than if the textured mold were touching the underside of the rice. The thermoforming mold will be 3D printed to make the rice texture even more distinct.

Injection Molded Parts:

The injection mold for the fish tail was designed such that we only needed one injection mold for both halves of the fish tail. To do this, we created our posts and holes in sets of two such that you can create two halves from the same mold and they will snap together to make the entire fish tail. If there were one post and one hole to snap the pieces together, we would have needed to create two separate molds for each half. Additionally, to reduce sinking on the surface of the part, the fish tail was hollowed out. Each half has two posts on the bottom in order to hold the thermoformed rice to the base of the yoyo.

The injection mold for the ¼ circle pieces of sushi were also designed such that all of the pieces can fit into any hole in the base, which means that we can cycle through various colors and end up with variety in our sushi pieces. We will create the sushi pieces from one multi-cavity mold, which increases the number of parts we can make per cycle. The part is 0.25” thick which should cause the surface to sink and make each piece of sushi unique. This should allow some variation in the appearance of each of the sushi pieces. Each sushi part has one post that is used to secure it to the base and to hold down the thermoformed rice. This design parallels the design on the other half of the yoyo, so that the base only needs to be one mold as well.

Each of the pieces has a radius that is slightly smaller than the inner diameter of the rice so that it is not a press fit. Their outer edges also have a fillet with the same radius as the thermoformed rice, which should ensure that there is no interference on this edge.

For each of the holes and posts that we created, we have a +0.001inch/-0.000inch tolerance on the posts and a +0.000inch/-0.001inch tolerance on the holes. These posts also have a 0.001inch interference incorporated in order to allow them to be a snap fit. In the worst case, the interference will be 0.003inches and these tolerances ensure that we never have a case of the post slipping out of a hole. These values and tolerance ranges were confirmed with Dave Dow in order to ensure that the tolerance and interference were realistic values.

The thickness of the walls of the base (which vary from about 1/16” to 3/16”) were designed to be thin enough that they would not sink on the surface and the part has a lot of open areas in it so that it does not require as much material for the base. The base has also been designed with the shims in mind. We designed the base such that the shims can be placed into the mold and will be held in place by a layer of plastic constraining its outer diameter. This keeps the shim securely in place. The shims were added to the design so that more of the mass would be concentrated toward the center of the yoyo. Another feature of the base was the four holes that the posts on the sushi pieces go into. After consulting Dave Dow, he mentioned that the thickness and the height of these holes were sufficient to be created without the use of ribs for structural support. The four posts were separated instead of being one big block so that the feature would not be thick.

Table of Specifications

The rotational speed of our yoyo was calculated as follows:

Gantt Chart

To stay organized and ensure that our design/manufacturing process follows the prescribed timeline, we have made a Gantt chart, which can be found at the following link: 2.008 Yoyomato Gantt Chart.

Here is a screenshot of what it looks like: