Can I make something spooky? #
This week we learned how to mold and cast! It was complicated process that involved machining a positive model, casting a negative mold, then casting the desired material from the molds. I wanted to try to make a small hollow pumpkin to put a small light in!
Group Assignment #
We used oomoo, a two part silicone material, to make our flexible molds. Oomoo is great to work with and non-toxic, but we still wore gloves to make the process less messy.
For our section, we only had access to one plastic material, which was a clear two part resin, with the liquid plastic and hardener with a 7 minute curing time. This was helpful as we did not have to wait too long to finish our final product, but it also meant that we had to work quickly before the product cured.
The fumes from the uncured plastic are very toxic, so we reviewed the safety datasheet to ensure proper usage. We used a ventilator to clear fumes and gloves for protection.
Isaac also made a cool sculpture pouring the uncured plastic and forming a spiral as it cured.
I also opted to wear a respirator to protect from the plastic fumes.
A mini pumpkin lantern #
To celebrate the spooky season, I wanted to make a clear pumpkin with a lid and a hollowed interior so I could place a small light in it, similar to a jack-o-lantern.
I decided to machine the pumpkin model from wax. I started the CAD from a full pumpkin made from different lofted layers around ribs to get the classic “pumpkin” shape.
The final pumpkin looks like this #
I then sliced the pumpkin from the top and hollowed out the bottom piece, then cadded the stock around it for my final model.
I had to scale the model a few times to get it in spec with the stock dimensions, but here’s the final model!
I then made the toolpaths using Fusion’s CAM. I initially used the classic rough pass with the 1/4" endmill and the finishing pass with the 1/8" ball endmill, but I realized that the 1/8" was doing a flat pass that was taking a lot of time, so I added an additional flat pass in between with the 1/4" to save time. Here are my final three toolpaths:
I used 1200 rpm spindle speed and a 150 in/min feedrate for wax for the 1/4" I also realized later into the machining that the vacuum intake was stuck with some wax chips, which is why there is a huge mess in the video. I unclogged the wax chips and cleaned up after, but I let the block mill to completion.
A Fail! #
On my first finishing pass, I forgot to switch the feedrate from 150 in/min to 80 in/min for the 1/8" ball endmill, so the wax chips got stuck and made a huge mess! I also realized that my pumpkin was too big and the clearance between the model wall and the actual pumpkin was too small.
After another finishing, here is my final model! I filled it with oomoo and the silicone mold came out quite nice. There were a few bubbles, but otherwise the finish was quite nice.
I mixed some orange epoxy pigments into the clear 325 smooth on plastic, then ran the liquid through the vaccuum to try and eliminate air bubbles. I also ran the lighter over the plastic to pop any remaining on the surface.
I also tried to place small magnets into the pumpkin so the lid would “close”. I placed the magnet in the top pumpkin at the right time, but the bottom was not fully cured yet, so the magnet sank. I am definitely going to try the bottom again by casting one layer first, placing the magnet on top of the cured layer, then pouring more liquid on top to prevent sinking.
The finish pumpkin looks nice! There are a few bubbles from the magnet sinking, but otherwise it makes a good spooky decoration.
Reflections #
This week was really fun and I was able to apply some of my previous skillset (CAD and machining) to a completely foreign fabrication process: casting. I had worked with epoxy before for the Solar Car’s body work and aero layups, but I had never casted anything as an object.
Notes for future:
- When adding magnets or solid objects to the cast, cas a first layer, let it solidify, then place the object on top. This way the solid object won’t sink to the bottom
- Remember color dye ratios for aforementioned step! This ensures even coloring throughout the object
- Always double check machining feeds and speeds, especially for tool changes!
- Mix oomoo really well, and discard any chunky parts of silicone! Tap out all the air bubbles as soon as possible!
- Slow, continuous beads of pour are very effective in avoiding air bubbles!