Initial Mold Design
Organic shapes and adventures in CAM
Following Spring Break, our group worked to produce as many mold and CAM files as possible. A significant difficulty that we faced was in designing the mold for the petals, where the parting line did not follow an easily defined curvature. While Annie tackled the petal molds, the rest of the team worked on the molds and CAM files for the remaining parts.
Read on for more details on our processes and strategies.
Petal Molds
Parting Line Challenge:
A cross section analysis reveals that there are actually two directions of curvature for the petals. Each petal not only curves upwards, but also curves in on each side to form the characteristic spoon shaped petal. This presents us with the challenge of finding a parting line that does not leave any undercuts in the mold because the curvature must be considered in all directions, not just in the mold pull-apart direction.
Parting Line Solution:
To find a parting line that does not leave any undercuts in the mold, we need to find the local maxima along the edge of the petals. If we look at the part in the pull-apart direction, the local maxima is defined by the silhouette of the part. Using the "Silhouette Split" function we can draw a line around the edge of the part at the local maximum all around the edge of the part.
Creating the Cutting Surface:
The petals do not form a continuous surface that the mold can separate along, so an additional surface needs to be created for the molds to separate along. This non-planar surface was created by lofting arcs between each petal.
Ejector Pins and Runner:
We will need to add a circular runner around the outside of the petals in order to eject the part. The ejector pins will push on the runner instead of on the part. The size of the runner is still to be determined.
Stigma Molds
Top texture of the stigma
In our previous design, we showed the bulls-eye stigma design that is shown on the far right. After receiving feedback encouraging us to incorporate a more natural stigma top, we designed a raised spots design that is shown in the middle photo. We 3D printed the design to physically compare it to the bulls-eye and decided that we preferred the natural stigma top with raised spots. We continued producing molds with the raised spots.
Cavity Core
Designing for manufacturing
Riley fought valiantly against Fusion 360 to produce the molds and CAM for our stigmas. We decided to attempt to fit as many stigmas as possible onto one mold, a challenge that was made more difficult when considering runners and how the molds would individually fill.
Our team considered how to physically drill the raised dots on the top of the stigma. To assist with manufacturability, we removed the large fillets from the top bumps and decided to rely on the drill bit tips themselves to produce an attractive top texture. We are planning to begin the holes with a center drill then further round the divots using a ball end mill.
We also decided to make the base of each stigma in the mold an ejector pin hole, for easy removal. Currently both the stigma shaft and ejector pin hole are the same diameter (0.125in), so we will need to ream out the stigma shaft section after trying making some test pieces in order to get the snap fit with the body the way we would like.
Runners layout
Riley investigated multiple options for runner paths that varied in amount of stigmas and their orientations. Our focuses were on establishing symmetry (to ensure equal filling) and maximizing number of stigmas filled. Five of the possible paths are shown to the left and below.
After discussing runner paths with Joe, we were reminded that backtracking toward the sprue should be avoided. This disqualified many of our prospective paths (for example, see the first three paths below) and led us to prefer the design to the left. The middle stigma is placed outside of the regular arc to properly align the stigma with an ejector pin.
CAM for Stigma Molds
The tool paths used to manufacture the core and cavity sides of the stigma molds can be seen at left. In both cases we start with a center drill to prep all hole locations, before drilling, milling, facing, and finally using a contour tool path to create the runners and gates for the plastic.
We use a scallop path to create the triangle shape and drafted upper portion of each stigma (this potentially could be made into two tool paths, one with the same ball nose end mill to make the triangle slot and then a drafted tool to drill out the upper portion in one pass). After drilling all holes, the core side is faced and then runners are drilled.
For the cavity, we tried to keep the tooling as simple as possible since there are so many holes that need to be drilled for the top of the stigma. As mentioned earlier, we removed the extreme beveling from our design so we could simplify to two machining passes - a centering drill and then a ball nose 1/16" end mill pass. The last pass on the cavity side is a facing pass.
1/16" Ball Nose End Mill on Cavity Side
Peck Drilling Ejector Pin/Stigma Shaft Holes
Adaptive Pass for Clearing Triangle and Drafted Features
Facing Pass of Cavity Side
Other Progress
Mold production for remaining parts
We produced molds for the yo-yo body and snap ring, as well as the beginning of a CAM file for producing the snap ring.
Lab schedule
After obtaining approval to manufacture one of the molds, we plan to manufacture the stigma molds during the remainder of lab.
Blog post written on April 2nd, 2019