TEBerg

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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…

TEBerg

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On to the second part of this experiment

Glove Mold:

If you have an object that does not have any big undercuts and you can glue down, then the simple part casting works great. I also did this with my 1/10th scale toilet (see the Skyraider build thread [smile]). However, I learned a valuable lesson trying to cast the display propeller blades – It is difficult to work modeling clay around a fragile part!!! And, if you use a two-part mold, it tends to pour out the bottom when you fill it with plastic [laughing].. However, that was not funny on my painted garage floor..

So, for the 1/10th scale M-117 bomb, I decided to TRY a glove mold.. Again, I say try, because I am no expert [laughing]..

OK, first of all this takes a LOT of silicone mold rubber… I wanted to go with ½-inch of silicone around the part but that would have taken about 46 liquid ounces of silicone… So, I went to OSH (I love those guys) and bought a 1-foot length of 2.5-inch PVC pipe.. That worked out to leave me with needing 18 ounces of silicone. I had previously measured the scale bomb in water as 8 fluid ounces in volume..

So, I put “air vents” on the bomb fins by gluing ½ of a toothpick on the back corner of each fin. I cut off the PVC pipe to be about 1-1/2 inch longer than the bomb and sawed the side open with a hand jigsaw. I taped the inside of the PVC pipe with regular blue masking tape. Then, I put a spacer in the bottom of the pipe to center the bomb and give me at least ½-inch in front. After that I put ¼-inch spacers on the bomb fins and built a frame/jig to hold the tail of the bomb in place..

Now I calculated that I needed 18 ounces of silicone so I measured out 22 ounces of the mix.. I had to use 2 pours for that volume.. Again, measuring twice and pouring in one big batch may be best, but you loose so much sticking to the sides that it is a real pain. So, I measured 5.5 ounces in one measuring cup and marked each mixing cup with a pen. I added the “blue ½” to each cup and got everything ready. Then I added the green to the 1st mix and started stirring. This stuff is pretty think and you need to work it for at least a minute to get a good solid color. I poured that cup in the mold down the side and noticed the bomb would push off one direction. Therefore I had to carefully move the pour around the mold to keep the bomb centered. I poured the green side in the second batch and started mixing.. Now, things started to get interesting!!!

As I mixed the second batch I noticed the bomb had “floated up inside the PVC Pipe!!!! Ok, now I’m just running out of hands here [laughing]… I finished mixing the second cup, pushed the bomb back in the silicone and started the next ½ the pour. With everything poured in, I probably ended with about 1.5 ounces more silicone then I needed for the project, but that was fine with me.. I DID NOT put this thing on the vibration table to try and shake any bubbles out of it.. With the delicate setup, it was just not worth it.. Also, notice I used the top of my good old Pana-Vise to weight the bomb down into the silicone!!

After about 4 hours I pried the pvc casing open and got my brand new “bombcicle”!!! [laugh]. From here I cut the silicone open from the inside of two opposite toothpicks to the center post. Then I started the “split glove” portion of the project.. I CAREFULLY started to cut down the side with a brand new exacto blade. You pull the silicone open a bit and just cut deep enough to split the mold. Try not to dig into your master bland (bomb) or pull so hard you tear the silicone.

Removing the bomb, the only problem was the fuse piece popped off the front. I got that out and will glue it back on the original. Then you have a glove mold ready to pour your casting resin or foam into.. I’m using the expanding foam and, as you can see from the first mold photo, you may need to test your volumes and the pour technique. I found that putting the a/b parts into paper cups first then into the plastic mixing cup gave me the best opportunity to pour and stir in 30 seconds or less. Because the foam sets off in about 45 seconds!!! My second try was much closer with 3 full fins and only a little rounded edge missing off the 4th fin..

The last photo shows a foam bomb with a #1 lead sinker in the nose of it.. The bomb felt so light that I thought it would not fall very well or accurate, so I added the lead sinker to the mold before I dump the foam in.. The tailhook I’m working on for the Skyraider is also in the photo…

Ok, that’s good for now.. More fun to follow..

TEBerg

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Two more things..

I forgot to mention - one of the big reasons I used the PVC pipe and the glove technique was to support the casting!!! I had so many problems with holding the two-piece mold together for the propeller blades (see below), that I wanted to be able to "squeeze" the mold while I poured the casting resin or foam.. So, with the glove mold for the bomb, I open the PVC again and put the mold back in the pipe.. That holds it all tight and straight for each cast!! Photos below..

And, from the Skyraider build thread, here are a few of the "basics" I learned...

What I have found out so far (besides the fact that my "paintable" release agent leaves fish-eye spots in the paint - sanding and re-painting today) is the following about casting plastic..

- I liked using the 1 to 1 Silicon Rubber mix for the mold. I did not want to purchase a small scale accurate enough to measure what I needed by weight (usually 10 to 1), so I went with the blue/green 1 to 1 that you can measure by volume for the correct mix.

- The easiest molds were where I could fasten a basic part on to glass and use a tin can or plastic bottle for the mold. You just super glue a flat side of the object down and then hot glue the mold sides to the glass. Pour in the mold mix and let it set for the correct time, which is about 4 hours for the 1 to 1 mix. The mold will pop right off the glass with the master piece and you can easily release it from the sides with some alcohol (rubbing, not drinking)..

- 2-part molds are easier to use, but much harder to make.. I tried to make a base under the master parts with modelers clay for the two prop parts. It is very dificult and took more time to try and build up a 1/2-inch thick base than I cared for.. I actually lost some of the twist I had built into my master propeller blade because of 'flexing' it when I worked the putty around and tried to seal it at the edges.

- I used a "medium" hardness plastic for the parts.. I have see on-line that Alumilite seems to have the highest numbers for the shear strenght and heat resistance. Micro mark also makes a "heavy duty" pourable plastic. The compounds I bought came from AeroMarine Products through John's E-bay store. The statistics were a bit lower than Alumilite and the thin propeller blades seem to be a bit flexable. Kinda like how the older white Top Flite blades used to feel.. But, John's prices were much better than the other 'name brand' mold products.

- I have "over mixed" more plastic than I care to think about .. You can measure the empty mold with rice and go with that volume, but I ended up short on a couple of casts. That also left little pieces of rice stuck on the silicone mold that ended up in the next parts.. The best way to estimate plastic volume was to submirge the master part in a measuring cup of water and read the difference in volume..

- If you need air vents or blank areas in a mold, USE SMOOTH Fillers such as tooth picks, bamboo scewers or straws. For example, we needed screw holes in my sons climbing grips, we just poked holes through the green styrofoam master with a drill and figured they would be good screw holes.. Well, we should have used straws, because the rough screw holes continue to grab the mold rubber and break the rubber off inside the cast piece.. We now just have nubs where the screws go and have to drill out the screw holes. But - That works great also

I like the following links the best for information about casting parts: http://www.micromark.com/html_pages/...rrresinart.pdf, http://www.info-central.org/construc...ldmaking.shtml , http://www.alumilite.com/HowTos.cfm.

What I really learned from the casting is that: unmixed silicon mold compound will stain your workbench The plastic compound Will come off a painted garage floor, if you work very hard and are careful scraping it up[X(]... YOU NEED MORE Silicone Rubber and Plastic than you think you will need... And, My son had a blast carving his own climbing grips and casting them in plastic..

TEBerg

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Well, some ideas just don't seem to work [>:]...

With the foam bombs, they were a little too light so I added the lead sinker – That worked, but left a mark in the nose area of the M117. There was also the issue of getting the foam into all the fins because they are pretty thin – That worked out with pouring faster and tilting/rotating the foam into the back, but you have to be quick...

I wanted a “better” bomb [smile], so I made up a way to solve both those issues and still have a nice clean finish. I had cut extra fins when I made the master, thinking that I may need to use them if the foam was too brittle (which I think they may break when dropped).

So, I put those two ideas together and suspended the sinker from the set of plastic fins (first photo). It looked like it would work... I set up the first cast, the fins went in very well and everything looked pretty good... I mixed the foam and boy was it fun trying to pour past the opening with the plastic fins in the way!!!

I did not get all the foam in before it started to activate, so I gave it a slosh/spin and let it set up... OK, I end up with an “open” looking bomb... Too bad bombs don’t have vent openings…

Well, I went ahead and set up the second one figuring that I had better mix faster than 30 seconds. I use about 10 seconds to pour out both Dixie cups about 15 seconds to mix and start to pour (takes about 5 seconds to juggle everything)..

So, off to the races... 9 seconds of pouring, 12 seconds of mixing and then off to pour only dropping one thing on the newspaper... BTW, we LOVE newspaper doing this stuff!!! Well, the “faster” pour did not go any better and I did not get as much foam into the mold as the first attempt… I realize from this test that TWO things were working against me the plastic fins were plugging the fin air openings I had made and too small of an opening was left where the fins crossed in the center fill spout... OK, lessons learned!!!

Now, I did accomplish the two things I wanted – The plastic fins got glued into the foam very nice and the lead sinker is suspended in the foam so you don’t even know it’s there... Too bad everything in the middle didn’t work out as well (second photo)...

I think I’ll glue some of the plastic clips I made on them, paint them a nice bright orange color and keep them on hand for some practice bombing. I don’t think they will last long at all, But – I do want to try “Loft Bombing” and won’t mind if these fly off somewhere I can’t find them…

These things would be a heck of a lot easier to just cast out of plastic.. Plastic would make them a lot heavier also.. That would be more "scale"...[)]

TEBerg

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I haven’t posted here for almost a year, but I’m still occasionally trying to mould things.

I was going to make a mould of the Skyraider tail hook so I can cast plastic replacement. Just in case I drag the tail down the runway [laugh]…

I have modified the technique of measuring two equal amounts in different cups and pouring them together. Instead, I measure the volume for the mould. The first photo shows the things I used – pipe, measuring cup and then a mixing cup.

The next photo shows the amount of water it took to fill my mould. Then you add about 10 percent to the volume and…. Divide that in half. You then measure “1/2” the amounts of water you need into the mixing cup and mark it with a line. Add the second half and mark the top line. Then you fill Part A up to the first line and Part B up to the second and mix.

The last photo shows the setup I tried out of ¾-inch PVC pipe and a coupler on the end to provide a larger area for the hood did not work.

Unfortunately this setup did not work well with the long PVC holding the silicone rubber. When I worked the coupler off, the end of the tail hook broke off the original part. Then the rubber tore getting it out of the PVC…

So, I’m off to make a new tail hook (at least this one will be a little better scale) and then back to making 2-part moulds…

Ram Jet

I haven't tried it yet but want to. I'm just waiting for a real need to. I walked by a kit that Hobby Lobby sells and nearly bought it. I have read a-little about the process and I think the silicone mold material is usually poured into an easily destroyed container like a milk carton or a flimsey plastic container that is easily cut with sissors or hobby knife. The part that blows my mind is cutting the silicone to get at the origial item that makes the silicone mold. There must be some good how to articles on the web or You Tube.

Bill

TEBerg

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Bill,

There are bunches of videos.. Sorry that the links I posted above are too old that they don't work.. If you search for silicone molda or 2-part molds you will find a lot of different brand advertisements that show you how to make molds.

I will tell you that starting off with a part that has a nice flat base is the easiest. You can just tack glue that down on a piece of glass, build a box around it and pour the silicone over the top.. As long as your part doesn't have any larg undercut it makes great casts...

Good Luck, and buy extra - It's just cheaper in the long run... Ask how I know

Ram Jet

Thanks TE. I nearly bought the mold kit to replace a 1" cross on a snow village church for a girlfriend but made one out of basswod and painted gold. Now, when I get a really deserving project . . .

Thanks again,
Bill

jonastecknare

Thank you for a very informative thread!
Ive been thinking about trying to mould landinggear parts in carbonfibre and epoxi!
Do you think this method would work?