Gear Mold Cavity:

The gear mold cavity is composed of all three gears on a single part. The cavity shows the side of the mold that will be exposed on the final yo-yo. There will be a runner going from the injection molding plastic point of entry to each of the gears (not pictured here). 

Gear Mold Core:

The gear mold core is the back face of each of the gears. The mold core has three pegs (two of length 0.15" and one of length 0.1") that correspond to the gear placement on the gear mold cavity. The pegs are uniform in size throughout their length due to the small size of the gears. The gears will shrink around the center pegs and no tapper is needed. Around each peg is two holes for ejector pins. The ejector pin placement was aligned such that there is one pin on either side of the gear.  

Dimensions:

The shrinkage for the gears was derived from measuring a small circular plastic part of similar dimensions as the gears. The circle had a 1.45 in diameter and the mold for it was roughly 2.5 % larger. The shrinkage of the gears was then determined using this formula ( Mold Increase %) = 2.5%* (Gear Diameter) / ( 1.45 in). For the big gear the mold was 1.6% larger, for the middle gear was 1.0 % larger, and the small gear was 0.8% larger. The inner gear holes are sized to have 0.248" diameter to create a press fit on the base which will have pegs of 0.25" diameter.  

Process Plan:

Gear Core Process Plan

Gear Cavity Process Plan

Estimations of total machine time

Part	Machine time on Mill (minutes)	Machine time on Lathe (minutes)	Total Machining Time (min)
Base Core	35	2.68	37.68
Base Cavity	0	1.47	1.47
Gears Core	33.17	0	33.17
Gears Cavity	31.47	0	31.47
Cover Die	1	2.35	3.35
Ring Core	0	4.97	4.97
Ring Cavity	1.27	2.12	3.39
TOTAL	101.91	13.59	115.5

Variable time cost of making one Yoyo:

Part	Process	Rate determining step	RDS time (s)	Total cycle time estimate (s)
Base	Injection molding	Cooling of part	15	30
Gears	Injection molding	Cooling of part	10	25
Cover	Thermoforming	Heating of plastic	5	20
Ring	Injection molding	Cooling of part	5	15
TOTAL				90

As seen from the values above, the greater time cost involves the machining of the molds. Having a total of 7 mold components requires a total machining time of almost 2 hrs. However, the process has been optimized to keep this value to a minimum. The three gears are being made via the same mold, which decreases both machining time and injection molding time by almost a factor of three. Additionally, the thickness of the case and ring have been minimized in order to decrease the time taken for the cooling of these parts, which is the rate limiting step for injection molding. Thus, none of the components have excessive thicknesses. By using both, thermoforming and injection molding, for different components, we will be able to fabricate them simultaneously, improving the total process time of producing 50 Yoyos.

Our estimated cycle times for the different components are based on our knowledge of the rate determining step, the critical dimensions of the parts, and previous time estimates discussed in lectures. Typical cycle times from lecture were taken and scaled down to fit our parts. It is important to note the amount of time it will take to start up machines for both the making of the molds, and the actual injection molding process is relatively significant compared to other parts of the process, since it is not automated. The cycle time estimate considers this component. The addition of all these times sets an estimate of producing a hundred yoyo sides of about 4.5 hours. (This does not consider the need for multiple trial runs, or mold modification.) The assembly time is estimated to be 1 minute per Yoyo. This may seem as an overestimate, but we are accounting for possible fitting problems, coordination of different teams, possibility of errors, and the time it takes to familiarize oneself with the assembly process. (Assembly time can vary depending on the people taking part in the process.) Therefore, total assembly time for all Yoyos is estimated to be 50min.

Manufactured Molds:

The mold has three posts on the core side which the plastic will contract and stick to after cooling. The ejector pins will push out the gears from either side of the post. The cavity side has the gear shapes and includes a runner for the plastic to flow through. These are the second iterations of the gear mold machining. The initial test resulted in parts that did not line up due to an error in mirroring. However, each side of this mold now fits together very well and is ready for an initial injection molding test. 

Introducing the Design

Our yo-yo was inspired by something that seems to last forever but always ends, lets see if you can figure it out:

This thing all things devours:

Birds, beasts, trees, flowers;

Gnaws iron, bites steel;

Grinds hard stone to meal;

Slays king, ruins town 

And beats high mountain down

You probably guessed it, we were inspired by time! Our yo-yo takes the form of an unconventional clock. The three gears in the center represent the hours, minutes, and seconds hands on a conventional clock. See our design below:

Overall one side of the yo-yo has four different parts and six individual pieces. The base serves as external support to hold all of the parts. It includes three pegs for each of the gears and aesthetic design. The three gears press fit onto the base pegs and portray time. The clear cover goes over the entire assembly and the orange ring creates a press fit between the cover and the base. Below is an exploded view of the entire assembly. 

Base: Injection molding

The injection molded base will connect to the other side of the yo-yo. The central solid circle will securely contain the nut and each peg will press fit with the gears. 

Gear Assembly: Injection molding

The gears are all separate parts but will be injection molded with a single mold. The mold will include runners to each gear to decrease production time. The gear will be assembled according to the correct pitch diameter, however they will not be able to move because of the inner peg press fit. 

Cover: Thermoforming 

The thermoformed cover will be clear such that it can protect the yo-yo interior but maintain the overall aesthetic. 

Ring: Injection molding

The injection molded ring will be purely for structural support. It creates a solid surface for a press fit between the cover and the yo-yo base. 

Design for manufacturing:

The class requires that by the end of the semester we build a total of 50 yo-yos. That means a total of 4 molds, 50 yo-yos, 100 yo-yo sides, and in our case, 300 gears! Below we will explain some considerations that we took to finish on time. 

Rate:

Most of our parts are made through injection molding which is dominated by cooling time. To cut down on cooling time, we used thin walls on our parts. The design of the base, which has the thickest components, was creatively made such that the overall part is thinner.
The gears, while small in size, will be manufactured in the largest quantity. Therefore, we will design a mold that can hold all three and reduce number of injection molding cycles.

Press fit:

Another step in yo-yo creation which will not be automated is the assembly. The design was made for easy press fit between parts such that the yo-yo can be assembled by hand. 

Table of Specifications:

Component	Diameter (in)	Thickness (in)	Tolerance (in)	Mass (g)	Tolerance (g)	Measurement technique
Base	2.5	1.27in	±0.005	17.04	±0.05	Calipers for inner and outer diameter and attempt press fit with gears for posts
Gear Large	1.12	0.15	±0.005	1.66	±0.05	Calipers for inner and outer diameter and attempt press fit with base part for the internal hole
Gear Medium	0.83	0.15	±0.005	0.86	±0.05	Calipers for inner and outer diameter and attempt press fit with base part for the internal hole
Gear Small	0.64	0.15	±0.005	0.32	±0.05	Calipers for inner and outer diameter and attempt press fit with base part for the internal hole
Thermoformed cover	1.98	0.03	(+0.005/-0.000)	3.34	±0.1	Calipers for inner and outer diameter and attempt press fit with base part for the internal hole
Ring	1	0.13	(+0.005/-0.000)	3.73	±0.1	Calipers for inner and outer diameter and attempt press fit with base part for the internal hole

Gantt Chart:

Here is a google doc where we keep track of our progress: Gantt Chart

Meet Team Mike!

Hi, we are team Mike! We are MIT mechanical engineering students taking 2.008, Design and Manufacturing. This blog will follow our journey as we worked together to design, build and assemble 50 yo-yos! Our team name, Mike, may seem deceptive, so stay tuned for more information on our yo-yo design.

From left to right:
Kyle Saleeby, Stacy Mo, Stephanie Diaz, Carla Troyas, Jorlyn Le Garrec, and Ty Ingram