10. Molding and Casting¶
Parallelepiped Sketches¶
I want to make a hard platic piece. So first I need to make a hard mold, use that to make a soft mold, and then cast the hard plastic resin into that.
What’s a Parallelepiped?¶
(all images are mine and original!)
3D Models of Molds - downloadable files¶
I made a few changes from the sketch. Seems simpler now. I first made the final product, then worked backwards to create the negative and the negative of the negative - the postive.
The blue parts are to be milled out of wax. The red part will be the rubber that comes from the blue molds.
I also modeled the milling head to make sure there would be no accidental collisions.
The rubber molds nest together and will receive the hard plastic resin, which will form the actual parallelepiped block, seen in green.
One problem to resolve was how to make the air/pour holes. I’ve decided to not mill out the pour hole or the air hole. I will use a hand drill to simply drill the hole in the rubber mold and therefore not have to worry about milling the cylinder in the wax, which would have introduced additional problems.
Rhino Mold Bottom File Download Rhino Mold Top File Download
Milling the Wax - Shopbot CNC¶
Here is the VCarve setup for milling in 3d. I used a rough pass and finish pass to get the slope steps as smooth as possilbe. I didn’t have much choice in determining the direction of the cut path, so an odd pattern emerged on the surface as the cut path spirals out from the center. I also adjusted the step over to 1/64”.
I affixed the wax to the CNC bed with inudstrial double stick tape.
Lesson learned!¶
Don’t use the downcut tool when milling wax! It just remelts the chips right in place…I had to start over! I used a 1/4” square bottom upcut tool.
Here you can see the stepped cuts from the rough cut and the finish cuts on the sloped face.
A funny artifact was created - a kind of soul patch - likely from the sloped angle plus the somewhat strange auto toolpath generated by VCarve.
A detail showing the remains of the VCarve toolpath. Ideally it would have criss-crossed itself, as opposed to a box spiraling out.
Here are the two molds for the parallelepiped cast!
Pouring the soft rubber¶
I used OOMOO 25 for the soft rubber. It does cure fast, so be ready to pour quickly!
I stirred for 3+ minutes, poured very slowly, perhaps too slow for the first mold - it got a little thick towards then end of the pour
I also used a vacuum chamber to suck out the air…we’ll see how they turn out!
Here’s the results.
Side one looks pretty good - no bubbles!
Modifying the mold¶
I drilled holes for the pour and air.
Pouring the hard plastic¶
Here’s the setup, including the data/safety sheets. I used the Smooth-Cast 326 for the hard plastic. I chose this because I could add color tint. I used Smooth-On So Strong color sampler.
I really paid attention to the timing for this pour - I think that is critical. I also practiced the routine and made sure everthing I had was at hand - like paper towels, extra gloves, sticks, etc.
I painted release agent - Ease Release 205 - on inner walls. This seemed to help.
Lessons learned¶
Here’s the result, with the pour and air holes stil attached. It was ok. All the step-over patterning definitely translated. But the seam at the middle between the 2 molds didn’t really do very well. I think I could design a better connection next time. I think my seam still allowed for misalignment.
The final block after sanding:
Overall I was happy with the end result. It has some character, and some nice coloration and glow. The block sanded well and I could achieve a very smooth surface. However, the bubbles seemed to become more apparent as I sanded. They’re especially visible under harsh light! Next time I think I’ll try 3d printing the mold!