Molding and Casting

This week, we use the desktop ShopBot to mill an object for a molding and casting application. The general premise is to start with a "postive" or the original object you wish to make many replicas of, pour a rubbery mold around this object to make the "negative", and then cast your replicas into the mold with a variety of materials.

In keeping with the space theme, I thought it would be funny, if a bit corny, to make a cast of Han Solo trapped in carbonite...as the premise of the plotline in Star Wars Episode V finds the unfortunate Han cast into this material as punishment from infamous gangster Jabba the Hutt. So I set out to make a meta-cast of Han's cast :)

Below, you can see the milling tool path confirmed in PartWorks 3D, and the milling working on the finishing path for Han. I selected two rounds of toolpaths (a rougher one with less overlap for each pass) and a finer one for a smoother finish. The milling turned out quite well, except for a lack of adequate wall thickness on two sides. These walls are needed as barriers for the liquid goo that will fill this area when forming the "negative" mold. Fortunately, we can improvize and form raised walls from cardboard or thin wood scraps.

Unfortunately, when I went to pour in the material for Han's mold, I chose the wrong brand! I mixed the Liquid Rubber, rather than Oomoo, and this forms a relatively hard, clear rubber layer. Because my milled positive was also rigid, I could not separate the too, leaving Han, perhaps as destiny would have it, permanently encased in yet another layer of "carbonite".

Still hoping to successfully finish a casting project, I began work on a new positive, this time for a snowflake. My first attempt did not use a fine enough finishing path, and the object depth was too shallow to allow enough space for sufficient mold backing. I show my first attempt and second, corrected snowflake, below:

With a new object in hand, I correctly mixed the oomoo (thanks to Viirj for the tutorial!) and pulled a vacuum on the mixture to remove bubbles. This is an important step, lest you wish to find large bubbles ruining the details of your object. Below, you can see the filled mold (reinforced with wood on the sides) and the rubbery, silicone end result:

Now, for the casting! I mixed the Hydro Stone X cement powder with water as directed on the material bucket, though experimented with the water to powder proportions. I found the stated ratio of 22 parts water to 100 parts powder too firm--better for mortar than pouring into a mold. After reducing the fraction of powder to create a less thick, but still viscous slurry, I poured just enough of the solution to fill the snowflake depression in the silicone mold. You can see the fresh pour and end result below:

The best part about molding and casting is the ease of making many copies! I have now made three snowflakes and look forward to decorating them as Christmas ornament gifts!

For future efforts in molding and casting (also known as...if I had had more time this week), I would love to explore surround-casting, where both sides of a 360 degree object are molded and poured together, usually using registration marks and air vent holes. It would also be interesting to cast with other materials, like metal powders or clear resins.