Jason (Tse-Kang) Wang
How to make (almost) anything!

Week 09

Molding and Casting

This week: design a 3D mold, machine it, and cast parts from it.

I decide to design and cast a coin with Fablab and Fablab Taipei logos on it. The procedure is as follow: draw the coin in Rhino, CNC the positive mold onto machinable wax, cast the negative with the wax positive, and use the negative mold to cast out the final coin.

Instead of casting the coin out of resins, I would like to try a low melting metal pewter alloy called "Cerrotru" (Tin (Sn) 42%, Bismuth (Bi) 58%), which has a melting temperature of 281°F/138°C. To be able to cast pewter, I will use "Smooth-Sil 950", with Useful Temperature Range from -53°C to 232°C, to make my mold.

"""FabLab Taipei Coin Files"""

Test Mold and Drawing

First, I need to make sure the mold material and casting material can work together, so I grab a random 3D print (a creepy teeth in this case because the no overhang feeature of it) and start the test process. "Smooth-Sil 950" mold with the 3D printed teeth in it. I use a stainless soup ladel to melt the pewter metal on stove top, and pour the molten metal into the mold after removing the 3D printed teeth. Taadaa! A set of metal teeth. The mold and metal combination works! And is able to capture all the detail from the 3D print, even the 0.1 layers are clearly transfered.
Know the mold and metal can work with very fine detail, I start drawing the coin. The coin I draw is 40mm in diameter, and 4mm in thickness. I design the two part mold with a sprue and a riser on top. Also some half circles for allignment. Originally I design the mold with walls on each side, which turned out to be really hard to CNC correctly because the V shaped end mill I use to carve out the details.

Making of the molds

Using a Shapeoko 2 CNC, I start machine out the positive molds out of MachinableWax. The whole process uses four passes to finish with two end mills, one flat end 1/8" end mill, and one 0.2mm V shape end mill. After first two passes with 1/8" flat end end mill. After last two passes with 0.2mm V shape end mill. Left: Successed mold. (After a bit clean up with soft tooth brush) / Middle: failed mold due to bad tool path. / Right: Semi failed mold due to the high wall which break the 0.2 end mill. (The really smooth finish is done with a hot air solder as a test to see different ways to clean the mold after machining.)
Using foam core panel to build the walls and casting the negative mold with "Smooth-Sil 950". Removing the walls after the mold cures. Mold seperated with some cleaning on the edges. Here comes the problem, bubbles!!! Although the test mold turns out bubbleless without degassing, this one is not that lucky. I will try making the mold again after I finish building/hacking a bicycle pump into a vacuum pump.

Casting the coin with pewter metal

Because of the bubble issue, I only have one side of the mold ready for casting. Eagerly to try casting the coin, I use a piece of aluminum plate as other side of the mold. The pewter alloy I use can be melt easily on stove top. Pouring the molten metal into the half mold.
Will, the cast is a fail/success. The sprue and riser work beautifully. The molten metal was able to travel all the way done to the buttom of the mold. However, The aluminum plate I use as the otherside of the mold also functioned as a giant heat sink, which cool the metal way too fast before the excess metal rise from the riser to the same hight as the sprue. Also because the pressure from the clamp , the thin middle part is closed from the metal. Other than the bubbles and middle thin part, the detail of the coin is clearly visible. I will come back to update the result after remaking the mold and casting with both molds properly.

What's next?

I learned a lot with the casting molding process, and definitely going to do another attempt with what I learned above. First, remaking the molds with proper degassing (also document my diy 20$ vacuum pump if it success.) Secondly, using back plate when clamping the molds together so it doesn't change shape due to the uneven pressure. Lastly, I want to redraw the coin with more 3D detail and see how smalll of the details I can produce with this technic. An attempt to add some detail to the wax with hot air solder gun. It produce a nice effect on the surface. Make the flat part looks ancient, and give chinese charactors a nice 3D edge detail. Will try this on the next attempt. Updated(see below)!

Update 4/13 magical Talc powder = finer detail

After a few more attempts (shaking, rotating, viberating the nold while pouring) to find a way to produce better detail, I read a blacksmith post about using talc powder to "release the surface tension of molten metal in the mold". Eagerly to try it out, I get an ordinary baby powder (contains only talc and perfume) from farmacy store, and dust a light coat of it onto the mold before pouring. It WORKS! Almost like magic! All the details appears on the cast, and no more imperfection on the perpendicular surfaces.
The coin on the top left is the one without talc powder, and the one on the right is the one with talc powder on the mold. The different is tremendous. The only problem now is the textured surface due to the talc powder. After some more experiment, I find out only a really thin coat of talc powder is needed to do the job, which will reduce the grainy textrure to minimum. In fact, I use compressed air to blow on the mold after coated the mold with talc powder. The result is the almost perfect cast on the right. (Left is wih too much talc powder left on the mold.) Now I just need to produce a mold without bubbles to have a perfect coin cast! Cheers.

Update 4/20 Lost cast with 3D printed moldlay filament

Update 4/26: I learned from the awesome people in Wellington that, If you apply dish washing liquid to the plaster mold and let it dries out before pouring the metal, it will become much easier to remove the plaster after the metal cures. I definitely will give that a try when I get the chance!

Laser cut Mold

Here is another method of producing really quick but detailed 2.5D mold for molding with Cerrotru.

On the left are the new batch of coins I cast with a new bubble free mold, which turns out nearly perfect.

On the right is my first attempt to cast on a laser engraved MDF. It turns out the metal was too hot when I pour. This generated a lot of bubbles that disturb the cooling and give the face a ancient rustic look.

Here is the result of the workshop I host within Fan2. One laser cutted 6mm MDF acts as boundry, and one engraveds 3mm MDF provides the face mold of the coin.

Two pieces mdf clamped together to provede a mold for a one sided coin.

Freshly opened mold.

It turns out, if I pour the metal right after it melts, the temperature won't burn MDF at all, and the result cast have a nive texture with very fine detail. All done without talc powder of dish wasing soap!

OOMOO test

After the success of using smooth sil as mold material, I decide to give both OOMOO 25 and OOMOO 30 a try. To see how well they holds up to the metal's temperature.

In fact, OOMOO 30 holds up to the temperature really well! Now I can just use OOMOO for soft mold instead of having to use smooth sill and doing all the degassing.

OOMOO 25 holds equally well compare to OOMOO 30 and smooth sil. Although a bit soft compare to the others, I still managed to get more than 7 casts out of a single mold before the mold shows any sign of tear. Still, not demage due to the heat, awesome!