Optimization

The adventure continues with 2.00yoyo. Last time, we talked about the Odyssey with the Yoyo body part. Now that we resolved the almost disaster, we were able to start optimizing injection molding parameters.

Below are the injection molding parameters decided for yoyo body.

Injection Molding Parameters

Part: Body Mold

Injection Mold

Injection Mold Pressure Profile: P7-P16
600	700	800	800	800
700	650	650	600	500
Injection Hold Time			Z2= 8 Seconds
Cooling Time			Z4= 20 Seconds
Set Screw Feed Stroke			C1= 2.75”

Injection Boost

Injection Speed Profile: V12-V21
1.5	2.0	2.5	3.0	2.0
1.5	1.0	0.7	0.5	0.2
Injection Boost Pressure			P6= 1400 psi

Screw Feeding

Screw Feed Delay time

Z3= 20 Seconds

Ejector

Ejector Counter

AZ= 2

Ejector Pin Length: 5.647”

Total Shim Thickness: ∅

These parameters were chosen to produce the best looking yoyo body. The pressure of 1400psi was optimized to prevent flash. The cooling time of 20s prevents the yoyo body from over shrinking and warping. 
At first, the snap ring did not fit into the yoyo body snugly. This was due to difference in shrinkage rates between the yoyo body and snap ring. To compensate for the differences, the Yoyo body cooling time was 5s more than the cooling time for the yoyo snap ring. This way the snap ring shrinks less than the yoyo body, allowing tighter tolerances and fit between the yoyo body and the snap ring. The ejector pin lengths were chosen to only protrude when the springs compress to eject the yoyo body.

Besides the design change explained in last week's blog, no real design change has been made thus far.
We were able to do optimization runs for all our parts (thermoforming snap ring, laser cutting compass face/needle, and injection molding the yoyo body, camp and snap ring). 
The only difficulty we faced in the optimization process was for the thermoformed part. At first, the plastic did not draw towards the mold. We initially thought this was due to lack of proper air pressure. However, we decided to raise the oven temperature (from 650F to 675F) and increase heating time to make the plastic more soft and easy to draw. This fixed our problems and allowed the snap ring to fit comfortably over the thermoform dome.
Our preliminary yoyo looks like this.

Dave was able to practice some tricks with the yoyo. The yoyo needle is able to spin when in use! 

After the optimization run, production runs were done for the snap ring, yoyo body, and the needle cap. Production runs to be done are the thermoformed dome and the laser cut compass face/needle. 

Lucky for us, this week, work went smoothly and we already conquered the biggest design change (explained in last week's blog), we were able to work productively as a team! 

Below are pictures of our team bonding and working! 

Connor testing out our new yoyo! 

Production run of snap ring

Connor and Kate meticulously keeping track of snap rings quality 

Beckett preparing the needle pins for the yoyo body

Production run of the Yoyo bodies

Team work!!!!!! (Production run of the yoyo body) 

Mens et manus: From Ideas to Tangible Creations

Hello World! In the past few weeks our team has been preparing the yoyo molds. After much brainstorming, we were able to start writing Mastercam code and process plans. Below is our ambitious estimate for manufacture time. 

Manufacturing Time

Thermoform Mold Machining (Lathe) Time	Front: 1m3.34s Back: 14.58s Time~(thickness)^2/alpha ~0.000762^2/(0.096*10^-6) ~6.048seconds
Lathe Machining Time for Body Mold:	Lathe Cavity of Body Mold: 10m30.57s Lathe Core of Body Mold: 10m25.58s
Mill Machining Time for Body Mold:	Mill Cavity of Body Mold: 47.19s Mill Core of Body Mold: 3m38.22s
Body Mold time:	Time~ (thickness)^2/alpha       ~(0.014224^2)/(0.096*10^-6)       ~35mins.
Snap Ring Time:	Lathe Cavity: 1min Lathe Core: 1min Mill Cavity: 4min Mill Core: 1min Time ~ (thickness)^2/alpha      ~ (.003m)^2/(0.096 × 10−6)  Cooling ~order of 1.5min, let’s say 5min TOTAL TIME: 12 min
Cap Mold	Cavity: 8m32.61s Core: 15m15.16s Time~ (Thickness)^2/alpha ~(0.00255016^2)/(0.096*10^-6) ~1.13 mins, let’s say 5 mins
Washer Mold	Cavity:8m46..41s Core:17m19.91s Time ~(Thickness)^2/alpha ~(0.006477^2)/(0.096*10^-6) ~7.28 mins, let’s go with 15 mins
Laser Cutting:	Estimate ~5 minutes for compass and needle

For the machines, we assumed the lathe time is accurate, and the core time may have up to a two minute delay for each tool change. For laser cutting, we assume to make 10 parts per operation, which will take around 5 minutes each. For the cooling, we assumed cooling time can be derived using the thickest part of the mold, and insert into the function ((Thickness)^2)/(0.096*10^-6) to derive the time. 
All our manufacturing process planning (Please click here to see detailed process plan) was finished prior to spring break. We had to wait an agonizing week to start machining!

Over spring break, some of us could not stay away from the excitement of 2.00yoyo and started machining the molds. The molds were finished as of Friday 4/3. We even started making some injection molded parts! 
Below are some pictures of our molds. 

Yoyo Cap Mold 

Yoyo Body Mold

Yoyo Thermoforming Mold

Yoyo Snap Ring Mold

We tested some of the snap ring and yoyo body parts. Below are pictures of success and failures. The failure was do to short-shot, the injection pressures and temperature was not high enough to push the melted plastic throughout the mold. 

Successful Yoyo Body with Snap Ring! 

Yoyo Snap Ring

Unsuccessful Short Shot Yoyo body :(

After an initial short shot yoyo body test, we adjusted the amount of plastic ejected into our mold.  Our next two tests were much more successful, and we were able to create our very first yoyo (minus the bells and whistles) shown above.

As typical in building processes, not everything goes smoothly or always according to plan.  Due to some miscalculations, our original Yoyo design underwent urgent revival. Here begins the odyssey of the Yoyo body. 
Below is the design of our original Yoyo body. 

This design included the following parts
1. Yoyo Body 
2. Compass Face and Needle (for aesthetic purposes)
3. Thermoform Cover
4. Cap (to hold down compass needle) 
5. Snap Ring (To hold down compass face and thermoform cover)
6. Dowel rods for Yoyo Axle 
7. Washer (To allow the compass needle to spin) 

When transferring the Solidworks files to the Mastercam files, we were cognizant of shrinkage. We took shrinkage rates from previous injection molded parts of the same volume and applied those to calculating the needed mold size. In our case we used a 2% shrinkage rate. 

After what we created what we thought were flawless G-codes, we went on to machining. A few simple mistakes were made. Tool 17 was not preset to have a pecking motion. As a result the tool got damaged and the ejector pin holes didn't go all the way through. But that mistake was easily fixed with a drill press. When we finished all our molds, we had a new sinking revelation: the yoyo body cavity was cut too deep! "There goes hours of our machine work!" we lamented. 

However, Kate, with her positive attitude (and desire to optimize lab time) came to save the day! "We can change our Yoyo design! No need to remachine everything!" Instead of creating a new cavity mold (time is precious and we cannot waste it if we hope to make 50 yoyos by May), we faced the rest of the core mold to have the same offset as the miscalculations. This change worked with all our existing parts except the washer. Luckily, the washer was the only mold we did not make yet. Perhaps this mistake was not a coincidence or a mistake. After all, it spared us have making another mold, and having to manage another part in our assembly. Our new manufacturing time should now be shorter by at least 3 hours (1 for manufacturing the body mold, 1 for optimizing injection molding parameters, 1 injection molding 50 washers, not to mention assembly time that accumulates). Below is a revised list of our yoyo parts: 
1. Yoyo Body
2. Compass Face and Needle (for aesthetic purposes)
3. Thermoform Cover
4. Cap (to hold down compass needle) 
5. Snap Ring (To hold down compass face and thermoform cover)
6. Dowel rods for Yoyo Axle 

Below is a Solidworks model of our revised design. 

The Revised Yoyo does not have a washer. The compass face inner diameter must also be made larger to fit in the yoyo. 

The best part of the odyssey was the team alliance formed and reaffirmed. After struggling through this ordeal and redesign, we managed to still tolerate each other and dare I say call each other friends!? Team 2.00yoyo still remains strong. Stay tuned for next week!

Below are some fun pictures of our team hard at work. SPECIAL THANKS TO DAVE AND DAVE for guiding ups through the machining and Mastercaming.

Connor intent on making sure the milling is executed perfectly

Kate and Beckett listening carefully to wise Dave

Clare focusing on her snap ring mold

Kate and Beckett Showing off their wonderful creation 

Clare Demonstrating our functional Yoyo!

James and Emma working on the cap core mold