Post-Production: Final Thoughts

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Production Costs: 

Note: When plotted in a log-log scale we are able to perceive the radical difference in cost per part when making yo-yos to scale given that we will be using expensive stainless steel molds for each one of our parts. The cost per part with a low production are too high since the fixed capital costs are significant. However, when producing at large scales, the initial capital expenditure is divided over a large volume of parts.

Rationale: The cost of manufacturing our yo-yos includes material costs for the four sets of molds required, plastic pellets, associated hardware (acorn nuts and stainless steel roulette balls) and polystyrene covers, the labor costs of programming and manning the machines, depreciation costs of the machines during production runs, and instruction and overhead to produce our parts. We calculated that for the 50 Yo-Yos we manufactured, it would cost around $30.08 per Yo-Yo to manufacture. Twenty hours were required to design the Yo-Yo and, on average, seven hours were required to program, machine and set up the eight mold halves. We assumed that we used two hours to injection mold the retaining rings, four hours for the bodies, three hours for each of the black and red inserts and a total of four hours to assemble these parts and injection mold the faceplates. Finally, it was estimated that three hours were needed at the thermoforming machine to make the covers.

If we were to manufacture 1000 times as many Yo-Yos, we estimated that it would cost approximately $12.74 per yo-yo to manufacture. In going from 100 to 100,000 parts we would go from making our injection molded parts using aluminum molds to stainless steel SPI mold class 103. Given that these molds are designed for up to 500,000 parts under medium to high precision requirements, one set of molds per part are sufficient for this production. We used the Custompartnet.com cost estimator tool to have an idea of what our molds would cost (they range between $8,000 and $10,000).  Other than this, the other cost sources that would change in going from 100 to 100,000 parts are the plastic pellets and thermoform material used, and all of the hardware employed on a per yo-yo basis (hex nuts, axle sleeves, stainless steel balls and acorn nuts) which are all assumed to scale linearly by number of parts. This is not a reasonable assumption for this radical change in production, particularly concerning the stainless steel balls and acorn nuts, which were bought via McMaster. A graph of the cost per part vs. production volume is shown below.

Reflection on our Manufacturing Process: 

Body:

In going from a 50 Yo-Yo production run to mass production we would decrease the wall thickness of the body, given that the walls are well above the industry standard for IM parts of this kind. The current design's thick walls were beneficial to our performance (particularly in reaching 5,800RPM top speed), something that we could offset by using heavier nuts and other metallic subcomponents of the assembly or by adding metallic washers within the hollow regions between the faceplate and the body. Making these changes to the body mold would not require us to change any of the equipment in the 2.008 shop other than our mold, which we would machine out of steel or order from a mold manufacturer to increase our volumes effectively (not changing aluminum molds every ~10,000 parts). Another modification could be to add more bodies within this same mold, possibly going from 1 part per cycle to 2 or 4 parts per cycle. This would require us to change injection-molding machine to one that can make parts of the required volume and pressure.

Wheel Insert:

Another change that was necessary to make for our design was manual loading of the red and black inserts for the over molding. This was required because there is no automatic way to do it in the 2.008 shops. For a larger scale production, this process would be automated to reduce time and cost, as well as increase uniformity of parts produced. We were also unable to finely create the detail of tight thin ridges in our yoyo insert because the bit size was limited. A five-axis mill may be more useful for task such as this. 

Snap Fit:

The biggest challenge our Yo-Yo design ran into was the number of snap fits incorporated into the design. The retaining ring's snap fit was easily adjustable, and as such would not need to be changed for manufacturing (once optimized it should work for all production runs). However, the inner gold insert was not so easily adjustable: this part had both a snap fit on the outside and the inside: this means that its fit was only predictably adjustable by changing physical dimension of the mold, since changing cooling times and other injection parameters to alter shrinkage changes the parameters of both the inside and outside snap fits. If, say, the outside snap fit of the body was too small, and the shrinking reduced by having a longer cooling time, the central nut-sink feature will also shrink less and hence potentially not fit inside the inner hole of the inner ring, even if the outer diameters were now matching.

Suggestions for the class:

The biggest challenge in 2.008 was making sense of the concepts displayed in lecture and those taught in lab, particularly with regard to the Yo-Yo project. There is definitely a tradeoff in talking about the individual projects and commenting on setbacks throughout the Solidworks Design, Mastercam programming, manufacturing and assembly processes within lecture but there is a lot of potential in using examples from the lab component of the class for lecture materials, particularly for the sessions on statistical control. It might be interesting to consider adding short labs on some of the other processes described in lecture, particularly those on casting and sheet forming as they remain integral components of the manufacturing space, but this might conflict with the scale of the Yo-Yo project. Acquiring additional injection molding machines, or having more time availability, particularly for labs that have three groups within the same lab section, might aid to this end.

More portion of the class should be spent teaching finer detail quality of the processes. Although the big picture ideas were taught, we still had countless difficulties with our project because of small details such as plastic burning, obvious weld lines and gate marks, and various other defects that tarnish the quality of the final product. While these points were addressed in lectures, hands on labs dealing with fine detail could have been more beneficial to see what factors actually can be adjusted to mitigate these defects in a real manufacturing setting. 

For us, the most valuable learning was had in the shop. Lectures, while interesting, were at times quite abstract, while the difficulties and DFM realities encountered in the shop forced us to rethink methods and truly understand the most important concepts in manufacturing with injection molding techniques. Along this same line, the problem sets did not seem very relevant to our lab and though they related to the lecture material it often seemed that the questions probed to far into specifics (particularly picking out small, passing examples/details from lecture), which resulted in problem sets forcing students to research isolated specifics and not really contributing to overall understanding of what I felt the class aims to teach.

Also, we suggest a better way to organize the lab sessions. Of course there are limitations to scheduling, but the shop was only open for a relatively limited time compared to the day, and while the scheduling system was organized, sometimes it was inefficient and a little unfair, especially considering that some lab periods had only 1 group, while others had 3 (such as ours) which made it very difficult to schedule time.