Wednesday, December 11, 2013

Blog 5- Reflections & Cost Analysis


Our design was adapted to meet the constraints of the manufacturing equipment in a couple different examples. During the design process, we designed the Bugs Bunny shape such that the smallest diameter tool for the mill would be able to trace out the details of the design. During the manufacturing process, we noticed that the red ring did not work in our yoyo assembly. We did not correctly account for the shrinkages of the ring and thus could not fit our middle red ring inside the orange ring. Therefore, we had to redesign and machine the red ring mold to allow for the 0.01" interference that was necessary for a tight snap fit.

In order to change our design for mass production, we would need to automate the assembly process using different machines to transport parts, remove runners/excess material, and snap the pieces together. To make the automation process easier, we might need to change the design of our pieces slightly to allow for quick and reliable alignment of the parts by the machine. We also would need to make our molds out of steel or another more durable metal to ensure that the molds would not deform over time. These new molds would have more than one part on the same mold, allowing for the production of multiple parts at the same time.

In terms of specific changes to the parts, we would also change the Bugs Bunny design to allow for a larger flow channel of the plastic. This would remove the need to spray the mold with mold release every 10 parts, reducing the part production time. It would also allow for better part quality.
Deformation of the disk around the tail feature.
By removing the leaves  on the carrot, more plastic could flow through this channel.
This would reduce the deformation of the part. 

Additionally, we would have embedded magnets inside our body cavity mold to allow us to mold the metal ring inside out body part. This would have saved us a second on assembly time.

The metal ring sits inside the body along the flat face. It could have been molded inside of the body to reduce the number of steps in the assembly and the assembly time. 
Lastly, we would have spent more time machining the finishes on our molds to allow for better finishes on some of our plastic parts. Because the molds would be used on a high volume of parts, the additional time needed to machine the molds would result in a large number of good looking parts.

The bottom of the body was sanded to remove burs, but the unevenness of the sanding resulted in groves that translated over to our molded part.

Cost Analysis:

In order to accurately analyze the cost of production, we needed to use an accurate model, so we picked Ashby's cost model. This means that we also included the cost of buying an injection molding machine and a thermoforming machine. By entering the appropriate values (such as scrap fraction, machine uptime fraction, tooling costs, etc.) into the Excel spreadsheet shown below, we were able to come up with the unit cost of production of each part. The graph on the right is the Unit Cost of the part vs the log-scale of Quantity Produced.

Ashby's Cost Model

To analyze the production cost of a yo-yo, we must combine the individual costs of production of the parts, as well as the costs of the off-the-shelf parts. We were also careful to include two parts for each side of the yo-yo, and one part for the axle and the axle sleeves.

By combining values from the individual Ashby's cost models

 By summing the cost of the parts, we were able to come up with the unit costs of 100 yo-yos and 100,000 yo-yos:

Including costs of machines:
Unit Cost of Production Run of 100 Yo-Yos:          $ 881.25
Unit Cost of Production Run of 100,000 Yo-Yos:   $ 8.57

Excluding costs of machines:
Unit Cost of Production Run of 100 Yo-Yos:          $ 81.25
Unit Cost of Production Run of 100,000 Yo-Yos:   $ 7.77

As expected, the unit cost greatly decreases as the quantity produced increases, and especially so when there are large fixed costs (cost of machines) involved.