Life Preserver Yo-Yos
Designing for Mass Manufacturing.
Mass Manufacture of 100 Life Preserver Yo-Yos
Design & Manufacturing II (MIT Course #: 2.008)
Team Lead (Team of 6), Led Overall Part Design (focused on mold for red part)
Sept - Dec 2017
Manufacturing (CNC Mill/Lathe, Thermoforming, Molds) Prototyping, Press Fit Tolerance
SolidWorks - CAD, MasterCAM - gcode
Working in a team of 6, we designed a yo-yo to mass manufacture 100 quantities. The design of the yo-yo is based off a life preserver; on both sides of the yo-yo is a life preserver design. There are 3 different injection molded components: a blue base piece (bottom), red stripe disc (middle), and white stripe disc (top). Each injection molded parts' molds were created and generated using SolidWorks & MasterCAM.
The small orange fish pieces and blue disk were both laser cut from a piece of acrylic, which were epoxied onto the center of the yo-yo on both sides.
As mentioned before, the life preserver yo-yo is made up of 3 injection molded pieces: a blue base piece (bottom), red stripe disc (middle), and white stripe disc (top). We also had detailing which was created from laser cut pieces of acrylic (3 small orange fish pieces and a blue disk) which were epoxied to each side of the yo-yo.
The blue injection molded piece had 12 pegs. On top of the blue piece would lie the red piece which had holes in the 12 o'clock, 3 o'clock, 6 o'clock, and 9 o'clock positions. These aligned with the corresponding positions on the blue piece. Lastly, the white piece would go on top and have holes in the remaining blue pegs.
Here are a few key characteristics done to the molds to increase the efficiency of making the injection molded parts:
Hammered dowels into mold because of difficulty in machining thin pegs
Drilled vent holes and scratches on white part’s core side to allow for air to escape to prevent air entrapment and burning of the plastic
Gate opened wider from the original mold to allow for more plastic to enter the mold initially since the white part contains a lot of volume of plastic
1 degree draft was added to these surfaces to make it easier for plastic to slip off the molds during the injection molding process
Inner raised portion creates a hole in the center of the part in order to create space for the yo-yo nut
Created CAD for Molds on SolidWorks and produced G-code on MasterCAM for CNC Mill & Lathe
White Part Mold
Red Part Mold
There were a variety of small things that were optimized through the injection molding process. The majority of problems occurred in the red and white parts of the yo-yo. Two problems were flash and shot size. Flash occurs when a thin layer of material is forced out of the mold cavity at the parting line or ejector pins location. This excess material remains attached to the molded article, and normally has to be manually removed. Shot size refers to the amount of plastic we choose to inject into the mold. In our case, we had too little shot size. Below, I have a short bullet point list of solutions that were tested to optimize our injection molding process.
The red part had flash near ejector pins and inner diameter of part (1)
The high pressure speed profile was reduced at the end of the barrel’s cycle
This allowed more time for the plastic to compact together before the barrel withdrew
The white part had problems with flash and burns (17 & 18 )
speed profile was lowered towards the end to allow more time for the heated air to escape into the vent
drilled vent holes and scratches on white part’s core side to allow for air to escape to prevent air entrapment and burning of the plastic
Issues with the plastic not filling up the mold (17 & 18)
Shot size was increased to completely fill the mold
The assembly of yo-yo was done in the style of an assembly line. We assigned tasks to each team member to increase our assembly time rate. By timing each step of the process, we were able to find the bottleneck of our assembly time which was screwing in the thread screw to connect the two sides of the yo-yo together.
Cost per yoyo for 100: 700 USD
Cost per yo-yo for 1,000,000: 0.16 USD
Cost breakdown for the case in which we are using “industrial-grade” equipment as if we were manufacturing in a large manufacturing plant (large volume production). Graph shows how tooling and equipment are the largest cost contributors until the costs start to converge at higher production volumes.