Thanks for a great semester: Thoughts on 2.008

Overall I thought this class was pretty enjoyable and interesting.  However more office hours for pset and understanding deliverables would be nice, the office hours this semester all conflicted with my schedule. I also think it would have been cool to get experience with other manufacturing tools beside the lath, mill, injection mold and thermometer machine. Like possibly doing some welding, 3D printing or composite laying.

I enjoyed the yoyo project and the class overall. However, I think the psets were a bit too long given all of the deliverables that were due for the yoyo project. I also agree that office hours should be held during different hours (maybe after 5pm). This would probably be beneficial to a lot more students. I thought the notes for the class were sometimes difficult to follow because they did not always explain enough. For example, sometimes equations were given and the variables in these equations were not labeled. I enjoyed experiencing injection molding and therrmoforming, but I would’ve also liked to experience other processes if possible like different types of welding. Lastly, I enjoyed attending the factory tour and found it very useful to see some of the material we had learned in class implemented in a real world setting.

Thank you for an incredible semester!

Introducing...50 completed yo-yos

The completed yo-yo:

Our yo-yo has four individual pieces that come together to create each side of the yo-yo. There is the base, the ring, the cover, and the gears. During the first few attempts at production, we ran into many problems with fit. The base was too large and the ring was too small for a press fit. Additionally, the internal hole for the gears were too large and the pegs were too small for a press fit. However, after a few iterations we came to a final mold pattern that consistently created parts that were able to press fit to create a completed part. At the end, we were successful with the press fit, specifically between the gear and pegs. There is room for improvement with the press fit between the base and ring. While we were able to join them all, it required some effort and a hammer.

Base Ring Cover

Gears

50 yo-yos:

Specifications

Part	Design specification of Critical Dimension	Realized Average of Critical Dimension	Standard Devation	Percentage of Parts In specification Limits	Comments
Large Gear	.219	0.2189	0.001159	99.99%	Press fit worked perfectly every time, though design specification limits might have been too wide
Medium Gear	.219	0.2186	0.001145	99.99%	Press fit worked perfectly every time, though design specification limits might have been too wide
Small Gear	.219	0.2190	0.001214	99.99%	Press fit worked perfectly every time, though design specification limits might have been too wide
Cover	1.92	1.9387	0.01268	23.45%	While the cover ended up being too large and exceeded the specification limits, it ended up still working in the assembly because the ring still fit around it. We designed the cover such that there was significant clearance between the ring and the cover for the assembly to still fit. Because the only constraint of the cover’s inner diameter was to be bigger than the overall diameter of the gear system, and the cover’s inner diameter was bigger than specified, the cover still fit the gears. Our design specification of mean +/- .01 was probably too strict.
Ring	2.325	2.3212	0.003377	63.42%	The ring ended up shrinking slightly more than expected causing decent amount of the parts to be below the design specification limits. However the base also shrunk more than expected, in the end none of the rings were actually too small.
Base	2.315	2.2909	0.004092	0.00016%	The base shrunk much more than expected causing pretty much all the the base parts to be below the the lower specification limit which was the specified mean -0.005. As mentioned before the the ring also was slightly small. In the end the two parts were still able to press fit together abiet with considerable effort. The design specification for this part where also probably too strict considering the parts still assembled even though none of the parts were within spec

Gears - Process capability and run charts

Part	Process Capability
Large Gear	1.437
Medium Gear	1.452
Small Gear	1.367

Overall, the gears holes had similar process capabilities (Large = 1.437, Medium = 1.452, and Small = 1.367). All dimensions were close and fit within the upper and lower control limits.

Team Mike Presents:

Mike, the Yoyo - A Stacy Mo production

Costs and Analysis

Total cost table:

				Gears				Ring		Cover			Base	Total
Total cost of 100 parts				$920.84				$593.65		$854.12			$973.50	$3,397.11
Fix Costs				$868.62				$543.62		$678.12			$748.77	$2,839.13
Marginal Cost per 100				$52.22				$50.03		$176.00			$224.73	$557.98
Cost per part				$9.21				$5.94		$8.54			$9.74	$33.42
Marginal Cost per part				$0.52				$0.50		$1.76			$2.25	$5.03

Costs graph:

Cost analysis for prototyping versus manufacturing

The prototyping cost per unit part is relatively high. For the first 100 yoyos, the cost per part is $33.42. The is due to the following reasons:

• The hours invested working in CAD for the drawing of the parts and their assembly are labor intensive and expensive
• High labor costs. The initial stages involve the development of a concept and design each of the components making the yoyo.
• The molds of each part need to also be drawn with the appropriate dimensions to account for any shrinkage or press fits required.
•  A large amount of hours is also invested in Mastercam, to be able to fabricate the molds.
•  Prototyping requires the iteration of molds and the running of several trials to ensure the quality and matching assembly of the yoyo.
• The cost of labor per hour is one of the highest of the entire process (as seen in the cost chart) This results in a very high cost for the prototyping stage

The manufacturing cost per unit part is relatively low. The marginal cost per unit is only $5.03. In addition, the total cost per unit is only a few cents above this prices, as for a batch size of 100,000 parts, the fixed costs of $2,839.13 becomes almost negligible.

-       The manufacturing costs of large batch sizes are mostly dominated by the material costs, the machines run time (costs of running the injection molding and thermoforming machine) and the overhead costs.

-       The manufacturing costs is relatively low for a profit to be made

Assumptions on the cost estimation:

•  No prototyping or modification of molds required after the first 100 parts (after finalizing the first batch of yoyos)
• The costs of the machinery and tools has not been included as part of the fixed cost (since it was not given as a variable in the available spreadsheet)

Limitations

Our team decided to make three injection-molded parts (the gears, ring, and base) and one thermoformed part (the clear cover) instead of two injection-molded parts and two thermoformed parts because there was only one thermoformed machine available in lab. We knew this would be the bottleneck since this machine is not quite as automatic as the injection molding machines. Thus, we saved time and decreased our cost.

Due to time limitations (fast approaching deadlines) our team had to make decisions quickly. Thus, we focused on one design instead of multiple like it is usually done in an ideation process.

We were limited to certain ejection pin diameter configurations. Therefore, we considered creating an outer ring concentric with our ring with runners to the ring so that we could eject our part properly. Fortunately, we were able to find an ejector pin diameter configuration and there was no need for this extra ring.

Changes for Mass Production

Cover:

We would add an assist to our thermoformed part for mass production. During our production run for 100 runs, the covers were produced with small deformations on its apex due to air. This was difficult to address, especially due to time constraints. The assist would therefore help eliminate these deformations.

We observed that the ring displayed sink marks characteristic of shrinkage at the middle section of the thickest cross section. We would also consider decreasing the thickness of the ring to minimize the amount of shrinkage.

Gears:

The use very little material and have a low cooling time which is ideal for mass production. The limitations that we encountered was that the gears, due to shape and mold design, often stuck to the cavity. In mass production this could be a large problem because it would require human intervention. The problem could be mitigated by creating a taper along the gears holes. Another consideration is during assembly. Due to small size and shape, assembling the gears on a peg becomes a problem of specific placement. The taper along the holes could also help with less precision during placement for gear assembly.

Base:

We had limited range of ejector pin placement, where in a factory we could generate our own ejector pin framework. This significantly impacted our design, forcing us to re-machine our molds to comply with existing framework. Besides the ejector pin constraints, the design gave no issues in low-scale manufacturing.

Optimization (Improvisation?)

Last time we posted, we had just completed our molds. Since then, we have actually had the opportunity to test, optimize, and do a final production run on the gears molds. Here we will walk you through the journey that started as optimistic young machinists and lead to (somewhat) experienced injection molders.

Our first day of injection molding optimization we showed up ready to run a successful first round. We set fairly arbitrary initial parameters and put the machine on semi-automatic. During the first run we eagerly anticipated the first tiny gears to come out and....nothing!

All the gears got stuck in the cavity! We spent a few long minutes prying each individual gear out of the cavity and generously sprayed some release agent into the mold. Take 2:

This time our little gear poped out, there is still hope! We set to work filing away at the gates so that there would be more resistance as they pull apart and once again spray some release agent. Take 3:

Medium and small gear! Improvement and hope! We then started looking at the parameters and increased cooling time. Take 4:

After take 4 we completed our gears successfully and all popped out! We then took the gears out to our base piece to test the press fit. Not even close! Our gears floated around the pegs. This meant that we were back to the mill machine. 

Since the holes were too small all it took was a single extra pass around the core and our mold was ready to test again. 

Second Optimization Test:

We tried our mold with smaller gear holes. This time they press fit perfectly! 

Snapped onto the new base piece, they looked great and there was enough room for the cover and ring to fit around them! Looking closer at the gears we noticed a few problems that we set out to fix in the following tests. Firstly, the gears were off center. While this did not change the machine parameters, it definitely did not fit into the aesthetic appeal of our yo-yo. This turned out to be an easy fix. We simply put brass pins that had a smaller diameter in to replace the regular pins. As we attached the core, we would push it all the way down and to the right before attaching it. Now we had centered gears. On to more optimization. 

Flash: The unfortunate case when extra plastic escapes the mold and creates a flare of plastic around the parts. To solve this issue we reduced our shot size so that it would correctly follow our pressure profile.

Short shot: We actually reduced the shot size to the point that there was not enough plastic injected to fully fill the mold. We proceeded to change the shot size until we came to a happy medium at 5.2 inches.

Dimples and weld lines: Some of the final problems that we encountered were dimples and weld lines in our part. We realized that overall, it was inevitable that we would have some dimples and weld lines since our part has different thicknesses and the plastic entered from only one side. However, in the spirit of optimization we set out to minimize the visual effects of each defect.

Overall we changed a number of parameters:

• Clamping force: Placed at max to stop flash
• Shot size: decreases and increased to 5.2 inches
• Gate walls: Files down so that more plastic could pass
• Brass Pins: Added to center the gears
• Cooling time: Decreased to 15 seconds so that the gears did not get stuck
• Release agent: Generously applied

After three optimization runs and our final production run we ended up with a whole stack of three different colors of gears. 

Here is an image of the final parameters used in our production run:

Stay tune for our first assembled yo-yo!