Process Optimization Activity

1.

Although unexpected, one of our most interesting parts is the seaweed base part of the yoyo. Although it looks fairly simple on the assembled yoyo, the different features of the mold and ways in which they were manufactured/machined actually make the part very cool.

A.

The cavity mold for the base is fairly simple in the sense that it doesn’t have too many features, but in fact a lot of aspects of the part and the mold had to be considered when designing the mold. Firstly, we wanted to ensure that our part had fairly uniform thickness throughout, but with the many different features of the base part, this became somewhat difficult. In the cavity mold, there is a boss (picture "Cavity Mold Boss") that creates a large groove in the base; this feature is used to remove some of the extra wall thickness in the outer area of the part and keep the wall thickness of the whole part relatively uniform. Moreover, this area is not visible on the assembled yoyo, so we were able to ignore the aesthetic implications of the feature. This boss, however, forced us to decrease the depth of the cavity because no tool on the lathe could get as deep into the cavity as we wanted without crashing into the boss at some point.

The core mold is the much more interesting half of the mold because it basically creates all the features that hold the assembled yoyo together. There are sixteen different through holes on the core mold for multiple reasons. Of course, there had to be some for the ejector pins, but we also added some for magnets that would hold a shim on the mold and for some shafts that we used to create post holes in some bosses. There is also a small trepan that will be used to allow a small amount of plastic to flow around the shim on the mold and hold it onto the part. Finally, we made sure to have draft angles on areas where there would be a lot of vertical wall engagement between the part and the mold.

B.

Process plans for the molds can be found here

The manufacturing of the cavity mold was quick and easy because all of the machining was done on the lathe. We first drilled out the hole (a through hole) for the string shaft at the center of the mold, and then we turned out the area within the boss with tool 1 as the area was large enough that tool 1 could do all the work. To complete the boss and outer diameter of the part, we used tool 9 to turn out a deep groove in the mold. Finally, we reamed the shaft hole, so that the shaft would be able to fit into the hole easily.

As one can tell by looking at the core mold, it was a lot more complicated to manufacture than the cavity mold. We had to start by drilling all twelve of the through holes on the mill. From there (also on the mill), we drilled out the pockets that would translate into bosses on the part. Once that was done, we moved to the lathe where we used tool 1 and tool 3 to rough and finish the main shape of the mold and then used tool 9 and tool 7 to turn some of the finer details like small grooves and trepans. Once the machining of the molds was finished, we still had to ream some of the through holes to allow the ejector pins and shafts to fit in their respective holes. We also press fit magnets into some of the through holes; these magnets would be used to hold the shims on the core mold while the mold was being filled with plastic. The shim is used to move weight away from the outside face(s) of the assembled yoyo and closer to the center (as in the string). We also had to press fit shafts into the through holes that were drilled within the pocket to create the post holes in the four bosses that stick out of the part in the center.

C. Pictures of finished molds

Cavity Mold

Cavity Mold Boss

Shim Magnets

Core Mold

Through Holes

Post Hole Feature

Trepan for Shim

D.

The final process parameters can be seen in the Figures 1 and 2 below.

Figure 1

Figure 2

When we first tested our injection molding, we experienced severe short shot as seen in "First Run - Short Shot" (below), so we increased the shot size accordingly, but then started experiencing some flash which would normally be a sign of overfilling, except for the fact that we were still experiencing some minor short shot as seen in "Intermediate Run - Flash". We knew from there that we had to change other parameters to get rid of the flash since it was obviously not caused by overfilling (seeing as we were experiencing short shot at the same time). We went through a series of parameter changes to try to mitigate the flash including changing injection pressure, injections speed, packing time, and even re-machining our cavity mold to be completely flat on the edges, but none of it seemed to have any significant effect on reducing the flash. Finally, with Dave’s help, we were able to create a profile for the injection velocity as seen in Figure 2 (above), which allowed us to better control how the plastic was filling the up the mold. This meant that all the plastic would immediately enter the mold, stopping short shot due to premature gate freezing, but would also make it so the plastic would fill the mold slow enough to stop the flash from generating. The optimized part can be seen in "Optimized Part".

First Run - Short Shot

Intermediate Run - Flash

Optimized Part

2.

During the process optimization activity, the seaweed base wasn’t the only part that had problems--in fact, the entire team experienced some obstacles with the different parts, but with changes to either the molds or the part itself, we were able to find solutions to our issues. For some of our other injection molded parts like the sushi pieces, Shirley and Billy ended up having to add more ejector pins and holes to their molds as they were having trouble getting the parts to be fully ejected off the molds when finished. Like the base, the fish pieces were experiencing short shot, but this was eventually solved by changing different injection parameters. Jane, who was in charge of the thermoform rice part, found that thermoformed parts have a threshold size that you can’t go below or else the entire part will just break. Finally, in one way or another, we all experienced the importance of doing a mini production run (about 10-15 parts) before considering the process optimized. Many times when trying to run things automatically or in quick succession, parameters that had worked for a single part before require a slight tweak to meet the needs of a larger production run.

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:

HELLO!

We are Yoyomato, the sushi yoyo team of 2.008, welcome to our team blog! We are all juniors in the Department of Mechanical Engineering here at MIT. Our team name was inspired by Yamato, the Japanese sushi restaurant in Boston. Using our amazing design and manufacturing skills and of course, our common love of sushi, we will produce the most unique and appetizing yoyos 2.008 has ever seen. Make sure to stay tuned to our future blog posts to check out how the process of designing and producing our sushi yoyos in bulk turns out!

From left to right: Fano Razafindrakoto, Sherri Green, Jane Fessenden, Shirley Lu, Aya Suzuki, Rosa Ruiz, Billy Ruschel