Arc XV: 100 Part Production Run

Manufactured Parts

Update: During testing, we were able to get the yo-yo to sleep at up to 4100 RPM!

Each yo-yo is consisted of both manufactured components as well as COTS components. The manufactured components of each yo-yo half are:

Thermoformed Window
This clear, thermoformed part protects the circuit components from the outside world. The key feature of this component is the raised islands. These islands fit into the negative spaces of the ring design. Consequently, the height of these islands determine how well the ring is able to snap into place.

While we were eventually able to get the desired island resolution, the defined parameters made production slower than optimal. One opportunity to improve the production of this part would be to redesign the mold in a way that would give us the desired resolution without having increase the heat time as much.

Circuit Assembly

This clear, thermoformed part allows us to create a sub-assembly from the electronic components and snap this sub-assembly into the body. This feature makes for a much cleaner design and assembly process. The critical dimension in this part is the height of post. This holds the battery and presses up against the top of the yo-yo half assembly, keeping the circuit sub-assembly in place while the yo-yo is tossed around

The design of this part was relatively simple, making the thermoform process very fast, with relatively few errors. However, improvements in the design that include wire-routing channels would increase the speed of circuit assembly.

Injection Molded Ring

You could almost say that this is the, "one ring to rule them all." The injection molded ring serves as the main contributor to the aesthetic design as well as the one piece that keeps all the yo-yo components together. One of the critical dimensions is the outer diameter because this ring snaps into body, holding the entire assembly together. The other critical dimension is the thickness of the ring.

Despite our concerns about dishing and how well the design would come out, the ring had a very consistent thickness. If given more time, we would explore redesigning the mold to more accurately account for shrinkage as well minimize the visible weld lines that occurred due to the number of negative spaces that we had in the design.

Injection Molded Body

The body consists of all the cavities in which the other parts fit into. The body also features a shim in order to increase the weight and improve the performance of the yo-yo. It has three critical dimensions, the most important of which is the inner diameter into which the ring snaps. The inner step is the area on which the ring is press-fitted and the post height is the part on which the circuit is pressed onto. The height of it determines how snugly the circuit sub-assembly is held in place.

We accurately predicted the shrinkage rate of body, meaning that almost all parts were within the defined specifications. While the quality of the yo-yos were all acceptable, an opportunity for improvement would be to redesign the mold to minimize the dishing on the string gap face of the yo-yo.

Figure 1: This breakdown shows all the components required to assemble a yo-yo including COTS parts.

Figure 2: All the manufactured parts in a yo-yo half. From left to right: 1) Injection Molded Body, 2) Thermoformed Circuit Assembly, 3) Injection Molded Ring, 4)Thermoformed Window

It works!

Design Specifications vs. Measured Specifications

The following charts show the measured critical dimensions of each part, comparing it to the design specifications.

Dimension	Design Specifications			Measured Averages
Dimension	Lower Limit	Nominal	Upper Limit	Measured Averages
Body Inner Diameter	2.245	2.250	2.255	2.2405
Body Step Height	0.120	0.130	0.140	0.1342
Body Mid Post Height	0.215	0.225	0.235	0.2315
Ring Outer Diameter	2.255	2.260	2.265	2.2595
Ring Thickness	0.090	0.100	0.110	0.1023
Window Feature Height	0.090	0.100	0.110	0.1053
Thermo-Circuit Post Height	0.325	0.330	0.335	0.3286

As can be seen above, almost all our parts came out according to spec which means we did a good job of setting our parameters and environmental conditions were also favorable for us. Perhaps the only part that fell below our design specifications was the body diameter which resulted in a much tighter snap fit than we originally planned to have. But as long as it fit, it only adds to the structural integrity of the yoyo as well us making sure the numerous components inside the body cavity stay intact where they are.

The Production of the Ring

Manufacturing the ring was not as dramatic as machining the molds for it (see previous blog posts) but we still had to make a few adjustments from our optimization runs. We first tried to make silver colored yoyos as we had originally planned when we designed the completed assembly. Unfortunately molten metal particles propagate to the weld front so our weld lines were very visible. This was a defect that we couldn't avoid without having to do without the silver. So that's exactly what we did - we replaced the original silver color with dark gray plastic pallets. Those turned out looking better with a red body than the silver anyway! We were able to decrease the visibility of the weld lines significantly but we couldn't remove them completely because of the many islands that exist on the ring hence the many weld fronts that would need to form.

The next issue that we run into was noticeable dishing early on. This was a bit confusing since we didn't change any parameters from our optimization run and we were able to minimize dishing then. The first thing we tried was increase the material shot size because we thought perhaps we were not packing enough plastic into the mold before it cooled and that was what caused the dishing. Upon several runs of trial and error were able to eliminate the dishing almost all together.

Once we were comfortable with all our parameters and the parts we were getting, we switched the injection molding machine into fully automatic mode and cranked out about 2 parts every minute for a total of about 1.25 hours (1 hour and 15 mins).

You can find out more detailed charts of our production run here.