For this project I attempted to cast a component based on Erwin Hauer‘s Design 4 that could be replicated infinitely to create a dimensional screen (check out this article about the Hauer screens in the new Foster wing at the Museum of Fine Arts).
Design 4, 1954
I began by testing different radii and different module scales. At the outset I tried the different radii to create varying porosity between multiple layers of the screen.
In the end I chose a radius that creates an inscribed circle when four modules are joined. I created a topography in the module itself to reinforce this notion. Ridges run along the center line in both directions with slight radii in the z axis to create a continuous spline with their neighbor. These ridges manifest themselves as compound curves as they move from their origin at the center of the piece to the perimeter of the module, creating a knife edge.
Based on the desired size of our casting molds I chose 4″ x 4″ for the visible portion of the module (the connection will protrude beyond this boundary).
Rendering of several modules connected and offset
After making this series of decisions, I began to mill the positive of the mold. I used blue foam on the TechnoCNC LC-4896 with these settings:
Horizontal roughing- 3/8″ ballmill with a 50% step over / 9000 rpms at 150 in/min for both plunge and travel speeds (6 minute run time)
Horizontal finishing- 1/4″ flatmill with a 25% step over/ 9000 rpms at 150 in/min for both plunge and travel speeds (1 hr 31 min run time)
Roughing Pass
Horizontal Finishing: 1/4" Endmill
Because I used different endmills I posted the files separately and re-Z’d the machine after the roughing pass. As the horizontal finishing pass ran I noticed that the first two-thirds of the job was wasted: the endmill travelled in the air above the material and did not remove any material. Additionally the endmill did not yield the resolution I was anticipating. I determined that the diameter of the endmill was probably too large and ran another horizontal finishing pass using an 1/8″ flatmill with a 25% step over (and the same rpm, travel and plunge speeds). This pass took approximately 3 hours and once again the first two-thirds of the job was wasted on “phantom cutting.”
Horizontal Finishing: 1/8" Endmill
After this series of passes the resolution of the part was still somewhat disappointing. I used 300 grit sandpaper to smooth the surface of the part and a 1/4″ endmill to carve out the concave registration marks (I did this manually—not using the machine—because the previous passes had rendered them rectangular).
Before the sanding
Next I painted the foam with Gesso to seal it. I was eager to cast the silicone mold, so after allowing the Gesso to dry for a half hour I tested soap as a cast release agent. I found that the soap removed the Gesso. I read the back of the Gesso and saw that the manufacturer recommends a 24 hour drying time. The next morning I bought vaseline to use as a cast release agent, fearing that the soap had some intrinsic gesso-removing property. I painted the vaseline on with a brush to get a thin-coat, attempting to maintain the fidelity of the mold.
After the Gesso
I poured the Oomoo silicone into the mold and tilted it back and forth to coat the high points of the mold. Ninety minutes later, when it had set, I removed it from the mold to loosen it and ensure that it had worked. Then I placed it back in the mold, mixed up a batch of Hydrostone (to the consistency of yogurt) and poured that over the back of the silicone. This created a rigid mold that I could clamp. After 25 minutes the Hydrostone had cured and I flipped the mold and removed both halves of my Hydrostone/Oomoo composite mold.
Oomoo Silicone Curing
Hydrostone Curing
The finished mold on the right
After wiping the vaseline off, I lined up the registration marks and clamped the molds together. Unfortunately, I had made the dowel part of my four-part mold incorrectly, so I in effect had three sprues.
The finished mold
Clamped and ready for casting
A leaky mold in action
On the first pour I attempted to plug them using paper-towels and duct tape. On the second pour I tried sandwiching the duct tape between the silicone at the perimeter. The first approach was slightly more effective, but both attempts yielded a cast with a partially hollow top portion. Here are the photos of the two attempts using the Smooth-Cast 305 Series Bright White Plastic:
Smooth-Cast 305 Liquid Plastic
Smooth-Cast 305 Liquid Plastic
Although the multiple sprues were incredibly problematic for casting plastic, they proved beneficial for casting foam. When working with an expandable foam, molds have a tendency to deform dramatically. The foam I used, the FOAM-IT 5 Series Rigid Urethane Foam, expands to 10 times its initial volume. The three sprues in the mold (which at this point served as pseudo-risers) allowed the foam to flow out the sides rather than displace the mold.
Foam oozing out of the sprue
Not enough material...
Still not enough...
A little too much, which is okay
I was a little disappointed with the casts that my mold yielded, so I decided to try another technique we had discussed in class: slip casting. To slip cast you make an enclosed negative mold for the object you want to cast, fill it with something like Smooth-Cast Roto Liquid Plastic, and rotate your mold. The rotation allows the viscous plastic to adhere to the walls of the mold, creating a hollow part.
I began by milling a new mold. This time I milled the negative and intentionally disregarded sprues and risers to create an enclosed shape. I also scaled the module by 150%. Once again I used the TechnoCNC LC-4896. After speaking with my classmates I changed the feed & speed settings and the endmill:
Horizontal roughing- 3/8″ ballmill with a 50% stepover / 9000 rpms at 300 in/min for travel speed and 150 in/min for plunge speed
Parallel finishing – 3/8″ ballmill with a 10% stepover / 9000 rpms at 300 in/min for travel speed and 150 in/min for plunge speed
This time I posted the job as one mopset and because it used the same endmill for both passes I did not have to re-Z the machine half way through. The entire job took 30 minutes, there was no “phantom-cutting” and it yielded a higher-resolution mold that did not require any sanding or further alteration. Check it out. Initially I tried to run the machine at 400 in/min for the travel speed, but I when it reached the first concave registration mark the machine stopped and gave me the error message, “no power to the z-controller.” I had to restart the job, and lowering the speed to 300 in/min resolved the problem.
Once again I painted the part with Gesso and vaseline.
Foam negative for roto-molding attempt
Next I poured Smooth-Cast 65D into the lower portion of the mold, joined the two sides of the mold and began rotating the module in an elliptical orbit. The smooth-cast worked its way through the slight gap between the faces of the mold, and because I had not applied gesso and vaseline to this area it bonded to the surface and fused the mold together. I was able to wrench the mold apart but it cannot be used again. If I were to do this again I would either gesso and vaseline the entire surface or, more likely make a positive mold in foam, create a silicone mold from that (with a Hydrostone backing) and use that for the rotomolding.
The cast after releasing the top of the mold
The extracted rotomolded piece
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COMPLETE FABRICATION 