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TEBerg -> Silicone Mold and Plastic Casting (4/29/2008 12:24:51 AM)
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I was asked on the warbirds forum to post a tutorial on casting parts for my project. (Global Skyraider Build Thread: http://www.rcuniverse.com/forum/m_7276786/tm.htm ) Again, I’m no expert, but I’ll follow up on that request with the two types of casting I recently completed.. I do have some more photos of the “in between” steps if anyone is curious and wants to see a bit more painful detail..
For this casting project I’ll describe a “simple” cast with a flat bottom part that has minor undercuts. Just follow the discussion with the photos and you should be able to tell which each step looks like.
Project: I have clips for my Vortac bomb releases. However, since they are no longer made, I figured I’d cast what I want to use on my ordinance and If I start loosing parts, I can make more [smile]..
Simple Part Casting:
I collected my parts, a glass block (any solid non-stick surface), a container to surround the object(s) and glue/hot glue gun. Then the “flat bottom” object to be cast is super-glued to the base. I marked the depth of the objects in my container making sure I added at least ¼-inch coverage over the top. Approximately ½-inch is recommended for a more durable mold, but I cut that down to save the mold silicone because I’m not going to use the mold very often. With the container marked, I filled it with “yucky brown rice” [laugh] to estimate the volume of mold silicone required. You want to increase the amount of silicone by about 10% for the loss in mixing and pouring. Then the mold container is hot-glued to the base to hold it in place and seal the edges. I have also used the foam poster board that has a shiny side to make mold containers and even used a piece in this cast to reduce the amount of mold silicone required (that stuff is not too cheap)..
You are now ready to mix and pour the silicone rubber. Again, I like the 1 to 1 mix because you don’t need a scale, just equal parts. Also, I used to measure the parts separately and then pour them together. However, you loose a LOT of mold rubber that way [frown].. Now, I have found that careful measuring into one cup that is graduated equally will work fine for mixing the rubber. It is like epoxy, and if you are close, it will set up just fine. About 3 ounces (liquid) were needed to fill the casting. I measured 1.5 ounce of the first part then filled it up to 3 with the second and mixed with a tongue depressor for about 1 minute. You have 15 minutes of working time, so you don’t have to rush, but you don’t want it to set up and get lumpy either. You “stir” the mix until it is all one color, but don’t whip too much air into it. Next you pour the silicone into the corner of the cast, but not right over the object. To help pop the bubbles, you pour with a thin stream into the box. Pouring right on the object could also trap bubbles, so wait until you have coverage before you move the silicone around. The silicone will ‘flow’ over the object. If you have any undercuts (which I did) you can brush some of the silicone up onto the object directly.
Next you want to get rid of all the bubbles you can.. No, I’m not cutting the mold open on the jigsaw [laughing].. I’m using it to vibrate the bubbles out of the silicone. If you have a “big” setup, you can also vacuum them out in a chamber. After the silicone sets up (the instructions say 2 hours, but I wait for at least 3) you pry the mold off the base carefully. The hot glue comes off very easy and you can carefully “pop” the super-glue object(s) off the base also.
Now we have all the original parts and the new silicone mold. These molds are pretty “flexible” even with ½-inch of material around the edge, so if you have a heavy casting, you will want to use some way to support the mold when you fill it. OK, now why is the mold in the oven??? The plastic sets up like epoxy and the heat helps the little edges and corners to completely cure. Now, the silicone is rated up to something like 600 or 800 degrees so you can supposedly cast low-temp metals in these also. However, for heating them up, it is suggested not to exceed a couple of hundred degrees. So, I just pre-heat the oven for a few minutes and put the mold in for about 10 minutes. Hot enough to retain some heat, but not so much you can’t handle it (I do use a hot pad on the metal tray). I carry the mold outside wrapped in newspaper to help insulate it as I mix the plastic. I skipped the plastic mixing because that is pretty easy, you just measure equal amounts and pour them in a cup. For this small amount I use some old baby medicine teaspoons and pour it into a disposable plastic cup. Then carefully pour the plastic into the mold and tap the edges to get any bubbles out.
You can see from the picture that the plastic is turning white and setting up around the warm silicone mold first. You can also see that I wasn’t very successful at “carefully” pouring the plastic in the mold [laugh].. Not a problem, keep it over the newspaper and everything is fine. You have about 2 minutes to work the plastic so I usually mix for 20 or 30 seconds then pour. In 10 to 15 minutes, the plastic is completely set up. Now, it is kinda “flexible” for a little while, so I usually give it 30 minutes to harden completely. You clean the spilled plastic right off the mold and then pop out the cast..
Lastly I have 4 new sets of clips for use with the Vortac bomb releases.. I did end up with small bubbles in the corners of some clips.. I don’t think they will be a problem with the release, but if needed, I’ll just pour another set and work a little bit more to get the bubbles out. You don’t want to try that on the jigsaw, it is better to tilt or squeeze the mold a bit to get out casting bubbles.
OK, I hopes this helps some people.. Please feel free to write or post questions..
I’ll get on with my glove casting description next…
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