Design for Manufacturing
EGGIE Transitions to Remote Learning...
April 7, 2020
Things certainly took a turn in the past few weeks. Due to precautions for COVID-19, MIT sent us all home mid-March, just one week after our EGGIE Design Review. EGGIE was put on pause as our lives were suddenly uprooted. It has been several weeks since then, and we are all learning what it means to find meaning, hope, and maybe even some normalcy in a new time of online learning, shelter-in-place, and uncertainty. We are all grateful for the 2.008 staff's understanding and their hard work to carry on the learning objectives of this class, as we no longer have access to in-person meetings and of course, a machine shop.
A couple of weeks ago, we each received an interesting package in the mail from the 2.008 staff. In the midst of everything on campus changing as students, professors, and staff were instructed to move off, they had designed and manufactured simple yo-yo parts for everyone to be able to assemble three complete yo-yo's at home! To our great surprise, they had even taken the time and thought to design and manufacture custom thermoformed parts of our website's Gudetama logo! This really warmed our hearts. We truly have the BEST instructors ever <3
Holding our Gudetama parts!
Thank you 2.008 staff ❤️
Before we left campus, we had discussed design changes for our EGGIE based on the feedback we received at the Design Review. We met over Zoom to confirm our design, which we will be modeling and assembling in CAD this week. Next, we will be learning the Autodesk Moldflow software to simulate injection molding of our parts, which will allow us to analyze how well our design performs with shrinkage and other factors considered.
Eggie 2.0: Iteration after Design Review
We were given very helpful feedback during the design review in March, with the main points here:
24 pieces too complicated. Reduce part count however possible
Consider areal draw ratios for the thermoformed egg parts (long skinny legs)
Think about overmolding -> the individual pins on the floatie rings will require a lot of trials to fit well
Think about assembly -> how can we integrate the arms into the body?
Current size of the yo-yo is 3 inches in diameter -> very big to hold
Comparison of EGGIE 1.0 (left two images) and EGGIE 2.0 (right two images).
Our overarching design strategy when making changes was to minimize both part count and difficult fits. Now, we have a design with only 6 unique parts for a total of 9 parts, instead of 7 unique parts for a total of 24 parts. We're very happy with the updated design in terms of aesthetics, manufacturability, and assembly.
As shown in the exploded view, we have the following parts (all injection molded):
Egg top
Interlocking floatie stripe 1
Interlocking floatie stripe 2
Floatie base
Interlocking floatie stripe 2
Interlocking floatie stripe 1
Egg bottom
Below, we discuss each part in detail.
Egg Top
New and improved, injection molded egg body!
Side view showing organic shape of egg head
The arms and facial features are currently filled.
Design for Assembly:
The egg body was originally thermoformed to fit under an injection molded water piece, with the arms separate since we cannot thermoform overhanging parts sticking out of the body. We decided to remove the water piece to minimize part number and account for the smaller yo-yo size (2.5 inches diameter vs. 3 inches), so we changed to injection molding the egg body so we can integrate the arms into the body and reduce the number of parts that need to fit together. The bottom of the egg body will press-fit into the floatie parts discussed below, with an interference of 0.010".
Design for Manufacturing:
The parting line will likely be across the body, under the arms. Just as with our v.1 body, the facial features are pointing upwards, perpendicular to the parting line to make sure we don't have ejection issues. The arms, eyes, and mouth are currently filled in, because they are smaller than the wall thickness. We will pay attention to these solid fill areas while simulating injection molding in Autodesk Moldflow software to make sure we don't have problems from shrinkage. The facial features need to be painted black and white after the body is injection molded, so we need to think about efficiency for a high volume production process.
Design for Aesthetics:
This time, instead of extruding a uniform cylinder for the egg body, we learned and used Fusion 360's Freeform molding tool to create a more organic shape.
Egg bottom aka the "Gudebooty"
Design for Assembly:
The egg bottom piece will now be injection molded and press-fit into the floatie parts with an interference of 0.010 in.
Design for Manufacturing:
Our initial intent was to thermoform the egg bottom to fit under an injection molded water piece. However, we decided to nix the water piece in order to minimize number of parts total and account for the smaller 2.5 in. diameter of our new design.
The parting line will likely slice through the part to ensure the leg protrusions (that have a tangent filet and a 1 degree draft angle) can be made.
Design for Aesthetics:
Due to our design changes, the egg's entire body - head to legs (he doesn't have toes) - looks quite long. After making changes, this piece actually featured thicker diameter, longer legs. But, in order to make his body look less like a chunky worm, we decided to extrude thinner, nub-like features to act as his legs.
Interlocking Floatie Stripes
Green Floatie
White Floatie
Design for Assembly:
The original design called for 8 individually injection molded floatie pieces of alternating colors to all be connected with pins into the base of each yo-yo half. The next generation floatie design included using an over-molding technique which would create a singular injection molded part with the same color pattern; however, the over-molding design also proved to be a tad bit complicated. Our most updated design therefore neglects over-molding and simply includes two distinct halves of the each floatie ring. One green half has a unique circular center that press fits (0.010" interference) into a unique circular pocket of one white half. This center feature is large enough to keep the white half connected to the green half, while remaining small enough to not disturb the other unique pocket located radially just outside this center fit feature. This other unique pocket is where the egg body (top half of the yo-yo) and the egg bottom (bottom half) will press fit into the floatie. The final outer diameter of the floatie ring assembled is 2.500" and will press fit into the floatie base.
Design for Manufacturing:
Both the white floatie part and the green floatie part will have a very similar mold, as only the tiny center feature is different. The thickness of each part throughout their entirety is set to 0.080". We will monitor for shrinkage with our simulations, but we expect shrinkage to occur equally throughout the whole of each piece. The parting line for each part's mold will be located at the same location: right along the bottom edge of the outer diameter of the floatie pieces.
Design for Aesthetics:
One piece will be manufactured with green material, while the other will be manufactured with white material. We are unfamiliar with the level of color mixing/bleeding when applying over-molding techniques, which is why we decided to injection mold two separate pieces with their own colors. The center interlocking circular features of both parts will not be visible to the consumers.
Floatie Base
Old Design: Flat
Old Design: Round
New Design: Thin
Design for Assembly:
Three prevailing designs came about, all revolving around the aesthetics rather than function of the base. All our designs have a press fit feature at the edge of the base so the floatie pieces can press fit into it with 0.010" interference.
Design for Manufacturing:
All angles of this piece are set to 90 degrees in an effort to ensure no plastic while being injection-molded gets stuck in tight, small-angle degree corners. A small 0.250" through hole is located in the center along with a small raised feature so a nut can be over-molded into it. This is the main feature that will allow the two halves of the yo-yo to attach to one another. The nominal thickness
Design for Aesthetics:
The most updated design is the thinner base (far right), which was designed more simply to eliminate space between the two assembled halves of the yo-yo. The same green colored plastic will be used for this part.
Yo-Yo Stand
Pictures and description coming soon!
Injection molding Simulations
Using Autodesk's Moldflow software, we conducted simulations of injection molding each of our parts to analyze the manufacturability of our parts. We varied number and location of injection locations, adjusted mold and melt temperatures, and adjusted injection pressure and time. We analyzed fill quality, pack, warp, sink and weld line defects, gate analysis, temperature and pressure distribution, and more.
We are most concerned about our two most crucial fits, diagrammed below in red: floatie pieces to floatie base ring, and floatie pieces to egg top/bottom. We calculated the estimated shrinkage that will affect these parts. Check out the details on our simulations and shrinkage estimations at the presentation below.
We're feeling a little like EGGIE this week...so here's our EGGIE pose with imaginary floaties.
Go-To-Manufacturing Report
We put together this report that documents our final yo-yo design, tooling design, and instructions for manufacturing.
