Step 60 : Mold Making
Time to Complete : 35 hours.
Total Time to Date : 378 hours.
The best way to look at my first attempt at making rubber RTV molds is to say that tuition for my self taught class, with me as the only student, cost me $225, which is the cost of 2 gal of Smooth Sil 920 RTV. I wasted a ton of material trying to get the "art" of mold making figured out as well as many hours "in class".
Those of you familiar with Kevin McLeod's Oscar II build will instantly recognize the following techniques.....I stole them directly from him. This post is meant to try and further clarify the process and describe some of the things I screwed up on. It is only with the help of Kevin and Steve Neil that I was even able to attempt this process. I tried to do as much research as I possibly could before attempting to make the molds as the materials involved are VERY expensive and the time to actually do it being quite considerable. Having said that, it was only until I got up to my elbows in RTV and got into it, that the real learning began.....much like anything else.
The materials that I used are the Smooth Sil products from Canadian Sculpture Supply. I have been very happy with these products as well as the "deliver to my door" phone in ordering from Sculpture Supply. Unfortunately, the Smooth Sil 920 was initially backordered, causing a delay. Walter at Canadian Sculpture Supply has been most helpful. The nice thing about Smooth Sil 920 is it's 1:1 mix ratio. Pour equal amounts of product from each bucket, stir, pour, ....that's it. Very easy to actually use the material....if only the same could be said for prepping.
The following are the tools/supplies that I used to make my molds.
The building of the mold boxes is where I made my first mistake. I failed to realized the importance of quality built boxes, and instead, I kinda' just rushed through this process, quickly throwing the boxes together. This mistake was realized when I began pouring RTV into the mold box...and the box began to leak. There is NOTHING you can do to stop an RTV leak. Once that stuff starts seeping out, ....she's game over. In my failing attempt to stop the leak (you will notice the duct tape), my crap built box fell apart. ....I'll let the picture tell the rest of the story. Another thing I did learn from this is that the best way to clean up RTV is to let it set, and then peel it up. If you try to wipe it up while it's "wet", you have a hell of a mess on your hands.
After this little experience, I spent much more time and care constructing my mold boxes.
The process is to fill the mold box 1/2 way with clay (I use Klean Clay) and then press your masters 1/2 way into the clay. A very handy tool to have is a rolling pin. If you don't have one, you "might" have a similar tool that I used.
I found that I was not able to press some of the parts with large surface areas (rudders, dive planes) into the clay because it was too stiff. What I did was place a little ball of clay into the box, and then press the parts into this, squishing the ball until the part was the proper depth in the box. I then just filled the clay in around the part up to the part's centre line. Once the clay was in place, I put in brass rod where I would be injecting the casting material into the mold as well as where the air trapped in the mold would be vented out. Then divits were pushed into the clay with the end of a paint brush to create the registration "bumps" in the final mold. These bumps ensure that the mold is properly lined up when the two rubber halves are put together.
It can be seen that I put some vents out the bottom of the mold. DON'T DO THIS. Make your vents come all the way back up to the top to the same side that you will pour. The vents at the bottom caused me all kinds of grief with the casting material seeping, resulting in a partially casted final part. I had to redo all my molds to have proper vents. In addition to this, make sure you build your mold boxes big enough to allow for proper venting channels. Some may argue the need for venting channels. I feel that when the molds are being constructed, it takes very little extra effort to include the vents. The vents are extra insurance that your final casted part is not going to have air bubbles. You decide if you need that insurance.
Initially, I clamped the lids on. I later discovered that I could get more mold boxes into the pot if I taped the lids on, and taped the boxes together. MAKE SURE your mold box is completely sealed when the lid is on. If there are small crack openings, seal them up with clay, and tape over. Do whatever it takes to makes sure that box is not going to leak. As mentioned, once it starts....you're hooped. The box is now ready for the first half of the mold. Pour the Smooth Sil 920 into the mold box (being careful to pour slow enough to allow the material to actually get into the opening. If you pour too fast, the RTV will get all over the place...except in the opening.....yep...learned that one too). Once the RTV is poured, place the box into the pressure pot. I set the pressure on my pot to 75 psi and let it set for 8 hours. Ensure that your pot lid is tightly clamped so that the pressure doesn't go down. The pressure pot ensures no bubbles in your final mold. The pot works very well. The largest mold box I could get in the pot is very close to 9" x 9" x 2 1/2 ".
After the 8 hour setting period, the lid is removed. This may take a little prying, but it will peel off nicely once started. The Smooth Sil is slightly transparent so you can see the masters through the rubber. Then, this half of the rubber mold is peeled back and removed. The RTV removes easily without the need for a release agent. With the first half removed, the masters are removed from the clay, and then the clay from the box.
With the box now empty, the first half of the rubber mold is placed in. Then, I sprayed release agent over the entire rubber mold. I really like this spray on release agent for the convenience alone. A can of this cost me $10.00
The masters were then placed back into their respective places with the OTHER side now being molded. Make sure you mark the masters so that it is known which side has been molded. The reason that I put the release agent on the mold before putting the masters back in is because IF some RTV seeps under the master in molding the second half, it can be peeled out later. YOU NEED RELEASE AGENT when you mold the second half of the mold. There is a misconception that RTV will not stick to ANYTHING. It WILL stick to itself. If you do not apply release agent, you will have a block of rubber with masters inside. Ask how I know this. The second half of the mold is now done in the same manner as the first. There you go, your mold.
I had quite a few parts that required molding. You will want to group your parts as per the depth of the mold mox you will require. If your mold box isn't deep enough, your final mold will be too thin. I initially tried to shave and cut as many corners as I could to make my mold boxes as small as possible, which would mean the least amount of RTV. This is ANOTHER mistake. The boxes need to be big enough....don't shave TOO many corners or you'll end up just redoing, which doesn't save a whole lot of that very pricey RTV.
This photo does not reflect my final molds. I ended up molding fewer parts per box.
In summation, some key points :
- Build good mold boxes that are big enough, and deep enough.
- Pour your RTV slowly into the opening.
- Use release agent when pouring your SECOND half of your mold. Be quite generous with the stuff to avoid having a rubber block instead of two nice rubber halves.
- Let the RTV set before cleaning it up.
- Use a sulphur free clay, such as Klean Clay. RTV will stick to sulphur clays.
- Run air vent channels to the same side as the pour channels.
This should get anyone new to making molds a good start. Jump in, do it, and learn ....as anyone who has done it has had to. This will hopefully prevent those new to it from making the same COSTLY and TIME CONSUMING mistakes that I made.
"I don't have anything else planned for this afternoon." - Lt. Col J.O.E. Vandeleur
A Bridge Too Far (1977)