For prop class, we spend a couple of days each semester building molds and casting. The product we have used most in our shop is OOMOO® Silicone Rubber. It is a two part molding medium and comes in two speeds. The one pictured above is the fast kind which sets up in 75 minutes but only has a fifteen minute pot time. The other product we used, which most of the images in this post will be of has a 6 hour demold time and a thirty minute pot life. They are, respectively, OOMOO® 25 and OOMOO® 30. They are a product of the Smooth-On Company.
For this demonstration, I asked Ray Versluys, our technical director to teach us some of the finer points of mold making.
Step #1: The Model
The first step of mold making is securing the piece you are going to mold in a frame. This is called the model.
For the OOMOO® to work properly, Ray said we needed to have at least one inch of clearance all the way around the model we were casting, and the sides of the frame should be one inch taller than the highest point of the relief.
We built our frame on a luan base with pine sides. First we took measurements, then we did math. We cut the lumber and assembled our frames with a pneumatic stapler.
|Measuring the pieces we were casting|
|Cutting the lumber|
|Assembling the frames|
|Attaching the bottom|
Once the frames were complete, we then used hot glue as a caulking around the inside joints of the frames. That was to keep the OOMOO® from leaking out. Even though it is very viscous, the structure of the OOMOO® is very fine so it will fill and find any space. The hot glue was intended to get rid of any of those types of spaces.
Then we used Super 77 Spray adhesive on both the model and the frame to create a semi-permanent bond. When using Super 77, I like to spray both surfaces, wait for at least a minute and then join the two together. You get a better bond that way.
|Spraying the model|
|Spraying the frame|
|Whistling the Final Jeopardy theme song|
|Placing the model in the frame|
|The model in the frame|
|Placing the other models in the other frames|
|The models in the frames, waiting|
Step #2: Measuring
Once the models are secure in the frame, it's time to estimate the amount of OOMOO® you will need to create the mold. OOMOO® isn't cheap, so it's best to mix just enough. If you mix too much it is wasted. If you mix too little it can be worse because you may have to scrap the whole batch and start over. It's tough to try to mix and pour two batches in one mold. Doesn't always work so well.
The way Ray estimates is to pour sand into the frame with the model in place, then pour the sand off into measuring vessels. When we know the amount of sand that filled the frame, we know how much OOMOO® to make.
|Adding sand to the model|
|Screeding the sand to get an accurate measure|
|Pouring the sand off into a 5 gallon bucket|
|More pouring of sand into a 5 gallon bucket|
|It can get messy|
|Pouring the sand into measuring vessels|
|A couple of molds ready to go|
Step #3: Mixing the OOMOO®
When the sand has been measured, we have to do more math. Since the OOMOO® is a 1 to 1 mix, we divide the amount of sand by two. Then we have the amount we need of Part A and Part B.
I think Part A is the silicone rubber and Part B is the activator. Each of the parts are inert until they come into contact with each other. Once they make contact, a chemical reaction takes place and the silicone rubber begins to form. I might call the chemical reaction an exothermic reaction because it produces heat. (The real reason I used that term is because it sounds just a little pretentious and I like sounding smarter than I am.)
In the OOMOO® 30, there is a pink part and a blue part. When mixed together they create a purple viscous liquid. It is critical to mix them completely. If you do not, you run the risk of the OOMOO® not setting up completely in places.
|Mixing with a really big spatula|
|Stirring OOMOO® 25|
|Stirring OOMOO® 30|
|Bigger batches call for drastic measures, here we're using a power drill to mix|
|And it turns purple|
Step #4: The Pour
Prior to pouring the OOMOO® it is essential to spray a mold release agent over your model so the silicone rubber doesn't adhere to it and stick to it. If you don't, you could destroy your mold.
Once the OOMOO® is mixed, it's time to pour it over the model. It's best to not go too fast and it's best to not go too slow. A nice steady pour is ideal. As you pour, it's good to direct the stream into the lowest parts of the relief. Once the entire frame has been filled, it's a good idea to tap the sides repeatedly with a hammer to ensure that all the air bubbles rise to the surface. If air bubbles get trapped in the relief, they will create negative shapes that will show up when you cast into the mold.
Once the investment is poured (that's what the silicone rubber is called when it's mixed) and the air bubbles have been driven to the surface, you need to put the frames aside and let the OOMOO® harden and cure. It's best to put them in a safe place and then go away. It's always tempting to mess with it if you stick around. The OOMOO® 30 takes six hours to cure. You shouldn't mess with it before that time.
|Pouring a large investment|
|Finishing it off|
|Set to the side|
|Now is a really good time to clean up|
Step #5: De-molding
This is the fun part. This is also the scary part because what if... What if the model shifted during the pour? What if there were trapped air bubbles? What if we didn't mix it well enough? What if I didn't spray a mold release agent? What if...?
We broke the wood apart with hammers and pry-bars. When that was done, we lifted the silicone rubber away from the model and we had our mold. At that point, any overhangs that don't belong are trimmed off with a razor knife.
Casting is essentially a positive image that is then converted to a negative image which is then converted to a positive image. The second positive image is a duplicate of the first positive image. The OOMOO® gives a very faithful duplicate image. I have seen body castings of some of the Smooth-On products that are so faithful, they capture pores in the skin and even the raised flesh where a tattoo is. The Smooth-On family of products are among the best in the industry.
|Breaking the frame|
|Peeling up the silicone rubber|
|That's what it looks like|
|More frame breaking|
|Cleaning up the edges with a razor knife|
|Some of the finished molds|
|The original and the faithful mirror image negative|
Step #6: Pete and Repeat--Measuring Sand
Now that the molds are completed, it's time to cast them. To do that you have to measure how much casting investment you need. Once again we turn to the sand. We fill the mold with sand, then pour it off into a measuring vessel. Same as before.
|Sand in the mold|
|More sand in the mold|
Step #7: Mixing the Resin
I am not sure what resin product we used, but I do know it is from the Smooth-On family of casting mediums.
The one we used was another two part mixture. This one is also mixed 1 to 1. Part A and Part B in equal amounts. Once again, it is imperative to completely mix the investment.
|Combining Part A and Part B|
Step #8: The Pour
Once the investment is thoroughly mixed, it's time to pour it into prepared molds. We prepare the molds by spraying them with a mold release agent. At this point, you can also brush bronzing powders into a mold. Bronzing powders are finely ground bits of colored metal which, when suspended in different mediums will give the appearance of that object being made of that metal. If you suspend gold bronzing powder into a clear paint, for example it will be like painting with gold leaf.
Like the first pour, you need to pour a steady stream into the mold. Not too fast and not too slow. As the mold fills up, you slow the pour to ensure you don't overfill. That's messy and wasteful.
|Molds prepared, the two in the center are coated with bronzing powder|
Step #9: The Hard Part, Waiting
Once the investment has been poured, you have to be patient and wait for it to harden. Even though it takes a little time, it's cool to watch. You watch for several minutes and nothing happens. Then after that eternity, the investment begins to turn white. Typically it begins where the mold is deepest. Has something to do with the exothermic reaction I think. Where there is more investment there is more heat and therefore the reaction is quicker. As it begins to harden the white color will spread visibly through the piece.
When the exothermic reaction is finally done, it's time to de-mold the duplicate and see how you did. That's the payoff with casting. That's the best part. The fun part. I love doing this with my class.
|Now we wait|
|Here's another one|
|The exothermic reaction has begun|
|Pretty much all done|
|The two on the left had bronzing powder brushed in the mold|
A good time had by all. We had some fun and we learned some stuff. That's always a great combination.