The Big Think

Warning… long and maybe boring post with lots of detail. I decided to document my procedures and what I’ve learned over the past 3 weeks, so skip this post if you’re not interested in that kind of stuff.

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I’ve always enjoyed model building and came across a site a few weeks ago that discussed mold making and casting. I thought it might be interesting to get some experience making my own blocks for modeling (like making your own Legos), so I embarked on some self-education about room temperature vulcanizing rubber (RTV), sculptural clay, and casting replicas in different kinds of plaster (Plaster of Paris, Dental plaster, Hydrostone, etc). It’s ostensibly so that I can add another Maker Ability to my library of skills, but it’s mainly because it looked like a lot of fun. Maker Faire had a booth dedicated to creating your own prototypes, and the idea of making something and casting it in plaster, plastic, or even pewter kind of blows my mind (the guy at Maker Faire had made a prototype pizza cutter out of polyethylene). There’s a whole dedicated community online that casts their own building blocks for amazing Medieval castle and LOTR constructions, so I’m using that as a template for making my own. In fact, I’m trying to replicate this mold:

Once the initial mold is made you can use it to pour dirt-cheap plaster (100lbs for $30!) to make as many blocks as you want. I used the mold pictured above as a template but added a few extra pieces and subtracted a few others. Bruce Hirst, the owner of the site sells these molds for about $30 each, but they’re VERY worth it if you don’t want to spend hours making your own perfect master blocks. I’ve got to hand it to him. These things are hard to make. Making one block that’s approximately 1″ x 1/2″ x 1/2″ is fairly easy. Make it accurate to a thousandth of an inch is demanding. Now make a couple dozen of them to those tolerances and I can almost guarantee you a case of eyestrain. And if you don’t have access to a few thousand dollars’ worth of tools, you’re in for a frustrating experience. But I do, so I decided to spend three weeks and quite a bit more than $30 to make my own mold. Hey, it’s the journey, right? I spent days trying to figure out the best way to make the blocks accurate and after much trail and error, I had my basic blanks. I had to devise a few jigs from wood, cram Sculpey clay into them, bake the whole thing in the oven at low temp (good thing the wooden jigs didn’t burst into flames!), then cut on my band saw and sand on a belt sander with a custom jig I built and measure with calipers until they were accurate to 1000th of an inch. After the sanding was done I spent several hours with a magnifying headset and sculpting tools carefully texturing and detailing the bricks. They have to look detailed and interesting, but also interchangeable and kind of “bland” so that you don’t have one specific pattern on one brick that repeats itself. The idea is to cast hundreds of bricks from a few originals (9 on my final mold), and you don’t want one specific brick to stick out. Anyway, I’ve made 30 or so different bricks (and floor tiles, and cobblestone road bits, and brick wall sections) and have picked the very best ones to go in my mold. If I have leftover RTV I’ll cast the rest in a second mold.

Here’s a few pictures of the process:

Here’s me detailing the pieces. You can see one of my makeshift jigs in front of me (the Home Depot paint stirrer sticks)

I didn’t take any pictures of the building process, but it was a long one. Probably 15 hours total to make all the bricks. Here’s a sample of the flat floor tiles I made, all accurate to a thousandth of an inch:

Here’s a brick wall section after detailing. I ended up not casting this one (yet) because it was a first try. Looks like a tiny section of brick wall, though.

After a lot of research I decided to use an RTV called Oomoo 30 from Smooth-On corporation. They make all kinds of industrial and commercial rubber and plastics. Their website is amazing. I had no idea what you could do with this stuff. Oomoo 30 is a very viscous 2 part epoxy-like substance that hardens into a smooth shell over your originals. Technically, it’s a tin-cured silicone but it’s supposed to be pretty beginner friendly. The more expensive platinum cured silicones are stronger (to avoid tearing) and last longer before the finished mold dries up and cracks, but the platinum-cured products are harder to use. The platinum cured RTV also requires you to mix much more accurately and benefits from a vacuum degassing process. With the Oomoo 30 you don’t have to build a bulky and expensive degassing system to get the bubbles out as long you use a few tricks. So I ordered the smallest amount of Oomoo 30 I could (2 lbs) and waited. I always seem to order things at the worst possible time, and this time Christmas got in the way. My order took an extra week! Oh well. That just gave me time to prep the Master blocks. I actually dropped a few of the master blocks and chipped them very slightly, so I had to remake a few. A couple also didn’t make the final cut to go in the mold because they had odd irregularities in them that would have been annoying if they had been replicated 100 times on a project.

Once the masters were done and detailed and the Oomoo arrived I was ready to pour. I was pretty nervous about it since I didn’t want to make any mistakes. Not only is the RTV very expensive ($22 for 2 lbs, or approximately 1 quart!), but if I really screwed something up I’d end up destroying my original masters and that would be a tragedy. At this point I’ve got too many hours of learning and creation in those little blocks. Bruce Hirst makes them just like this but gets to sell his molds over and over. I was only doing it once just for me. If I want to build anything different in the future I’ll spring the $32 for one of Bruce’s molds. He uses the platinum-cured RTV so his molds are higher quality. Some of the platinum stuff retails for a couple hundred dollars per gallon. Yikes! If I were doing this commercially like Bruce does I would figure out a way to have the master blocks cast in some kind of metal. Even if it cost more, pouring many master molds from the master blocks would probably eventually destroy the master blocks themselves.

So anyway, tonight was pouring night. I got out all my pieces, did a test layout, and cut the MDF base for the mold box. Next, I blew off the pieces with compressed air then dunked them in Future floor wax for a few seconds. This step seals the baked ceramic and acts as a lubricant so the final RTV will release without shattering the masters. I took the pieces out of the floor wax and dabbed them with a paper towel. Once they were dry I superglued them into place on the base with exactly 1/4″ between them all, checking to make sure that the bottoms made a very smooth seal with the MDF. If you let the RTV seep under the pieces it can entomb them in the mold. This Is Very Bad. Don’t do this. Then I cut some foam core and made a wall all the way around the 7 1/2″ x 4″ mdf base to contain the RTV. I taped the foam core to the bottom of the MDF, and ran a bead of Sculpey along the inside of the box to keep the silicone from seeping out.

I then measured 1/2″ above the highest block and made a mark on the inner surface of the foamcore wall so that I’ll have a nice thick “floor” of silicone under the block casts. Then I filled the cavity with sushi rice (hey, it’s what I had around) up to the mark.

Wait… say what?!?

Well, the silicone has to be measured in a 1:1 ratio of its two parts, and it has to be accurate. I also wanted to make sure that I used enough, but not too much (remember the $ part?), so I used the rice to estimate the total amount of material I’d need. I transferred the rice to a measuring cup (350ml of rice total), then divided the rice evenly between 2 disposable plastic cups and made a mark on the cups. This will be my fill mark for the part 1 and part 2 of the RTV. Are we having fun yet?

Next, I took the advice of the guy at Smooth-On and gave my masters a couple of light coats of Krylon Crystal Clear acrylic to seal the Sculpey pieces so they don’t stick to the RTV. I waited a couple hours for the Krylon to dry. Then just to be extra sure I gave the pieces a few very light coats of the release agent I bought from Smooth-On (Ease Release 200). Once it’s all dry I plan on using the 2 container mixing method followed by the bombs-away pouring method (explained below) I’ll let it sit out in the garage on a very level surface for five or six hours (around 70 degrees) and then bring it in where it’s warmer overnight. Hopefully when I wake up in the morning I’ll be able to demold everything and have a nice shiny mold. Wish me luck…

I tend to be very slow and methodical learning new things. The downside is that everything takes me three times as long, but the good side is that I rarely make mistakes. Instead, I read about everybody else’s mistakes and try to learn from them. Hope that applies here.

Here are a few pictures of the process so far.

The Oomoo 30 and release spray, next to my mold box:

Mold box closeup. These are the main building block bricks. You can also see a pair of double brick on the bottom row for faster building:

After letting the second coat of Krylon Crystal Clear dry I did a couple of quick coats of the Ease Release 200. Yes, I’m paranoid about my masters sticking to the mold. After the second coat of Ease Release was on, I noticed that the detail on the blocks was somewhat softer than before. I had cut details to look exactly like Bruce’s rough-cut “normal” stone, seen here:

Unfortunately, all the coats of sealer I had applied started to soften the sharp details slightly. Now when I compare my pieces to Bruce’s I have fewer valleys and mountains. If you’ve been keeping track, I have no less than FIVE thin coats of sealer (a Future dip, 2 coats of Krylon, and 2 quick coats of Ease Release). In retrospect, I should have only done the Future and the Ease Release. Live and learn. The details are still very visible, but I wonder how much they’ll stand out when I dry brush the paint on.

So then I laid out all my RTV mixing supplies, poured part A and B into two different cups, and started my stopwatch. Oomoo 30 has a 30 minute “pot time” until it becomes unpourable and I wanted to be pouring within 10 minutes to give myself plenty of working time.

Here’s the pouring setup:

You can see the plumb bob and leveled MDF on the floor. When the time comes I’ll put the mixed RTV in the cup in the edge of the table and pull the tape off the side (it covers a 1/4″ hole). Then the RTV should fall right down the string line (I’ll move the string) and hit the mold in an empty spot to avoid bubbles. The falling RTV should form a very thin string and pop any air bubbles as it falls.

Next lesson I learned: The oomoo comes in 2 parts, a thin blue part (part B), and a HIGHLY viscous bright pink part (part A). What I learned was not to pour the thick pink stuff into a cup and then transfer it to a mixing vessel. The part A was so thick it was very difficult to get out of the measuring cup. I should have used a bigger initial cup for the part A and then just poured the thinner stuff into it. I got most of the pink stuff out of the initial measuring cup I used, but I got a higher percentage of the thinner part B out of its cup. So even though I carefully measured it to be equal parts A and B, due to the fact that I left a little bit more of the part A behind I think I was “B” heavy. Again, it’s not a big deal since I imagine that there’s room for a tiny amount of slop in the mix ratio, and it wasn’t more than probably 2-3% difference, but next time I’ll add the blue stuff to the pink stuff and not transfer the thick pink stuff an extra time.

So, mixing. This stuff is thick. I can’t believe that it’s considered one of the LEAST viscous RTV’s! It took about 3-4 minutes to completely mix, and even then I was paranoid that I wasn’t mixing it thoroughly, so I kept on for a minute or so longer. If you don’t mix it very thoroughly you’ll have gooey unset spots in your final mold. Again: bad. I used a cut off pencil to stir the mix together- round shapes don’t cause cavitation and introduce air bubbles like scooped spoon shapes do. Once it was pretty well mixed, I transferred it to a second container (which I had cut the small hole in and taped over) and scraped out the first container as well as I could. Then I stirred it a bit more until it was really well mixed.

At this point I was only about 7 minutes into the mix but thought that I could detect it starting to thicken up, so I used a small brush to paint a layer of RTV onto all of my pieces. This first layer will hopefully keep air bubbles from the final mold. Next, I put the mold on the ground on the piece of leveled MDF, put the cup on the edge of my workbench, and pulled the tape from the hole in the cup. Bombs away! I had set up a string with a plumb bob so that I was sure that the first stream of RTV would hit a blank spot on the mold. It worked! The 35″ that the RTV had to fall made the bottom of the stream VERY thin, and I could see air bubbles popping in the stream all the way down. I estimate that the stream thickness was slightly thicker than dental floss by the time it hit the mold, so I feel very good about not having any air bubbles. Who needs a vacuum degasser when you can let gravity do it for you? Say it with me: SCIENCE!

I had to sit and babysit the falling RTV for the ten minutes or so it took to empty out of the cup (my hole in the cup was only about 1/4″ in diameter). I gradually tipped the cup up as the RTV oozed out and used the pencil stub to push leftover material toward the hole. Once it was all done I had a very neat rectangular puddle of RTV in my mold box, perfectly covering my 21 mold pieces 1/2″ over their tops.

I let it sit out in the garage for a couple of hours and firm up a bit, but it started getting down into the mid 60′s and the instructions on the RTV say to not do any molding under 65 degrees, so I made a level spot on the kitchen counter and moved the mold box inside to harden over night. There’s a 6 hour waiting time to demold, which is technically around midnight tonight, but I think I’ll play it safe and leave it until tomorrow to take the mold apart. If colder temperatures affect the vulcanizing time then I’ll give it more time to make sure. At this point I’m willing to be patient.

Hopefully then I’ll finally get to do the fun part- cast plaster and build!

I have to say this whole process of making the masters in Sculpey clay to 1/1000th of an inch tolerance, carving and detailing with sculpting tools, making a mold box, sealing, and pouring RTV has been a real adventure. I’ve learned a lot of new skills and it’s been fun. But now the silicone is sealing and it’s time for bed…

*LATER*

It’s done! I got up this morning and successfully demolded the pieces after they had been curing for 12 hours.

Cured RTV

Removing the box:

Moment of Truth (drumroll):

Success!

I very carefully broke the seal at each of the corners and worked off the edges, then popped off the middle. The RTV came off very easily (thanks to those 5 coats of different sealer/release) and I’m very happy about it. The white line around the mold is the Sculpey sealer that I lined the box with. It peeled off the RTV just fine. You can see some slight raggedness on a few holes where some of the RTV crept under a couple of pieces.

The cavities took some careful trimming with a very small, very sharp pair of scissors, but other than that they’re perfect. My master is also in good shape. I’ll wrap it up and store it someplace safe for when this first mold breaks down and I have to re-cast a new one. Three years is the estimated “library time” for Oomoo 30 RTV.

There was also a small bit of tearing on a few undercuts. I had asked Erin to make the gargoyle piece and it ended up having a lot of detail and deep holes. There were also some pretty sharp undercuts on the piece, but I decided to try it anyway. Most of it came out okay, but there’s a small bit of RTV left on the master sculpt in a few of the deepest spots. Not sure how it’ll affect the final cast pieces yet. I’ll make my first cast later on today.

After this experience I now have the skills needed to replicate whatever I can imagine using silicone RTV molds. It’s possible to get high-temp RTV and use it to cast pewter or even brass (hmm… focus wheel for the telescope?) It’s a surprisingly easy process. Well, the molding part is. The actual creation of those tiny pieces was a pain. But I could easily copy and cast any small flat object. It’s also possible to make 2 part molds to replicate a 3d object (like if you wanted to copy a chess piece or star wars figure), and I’ve even seen molds that are elephant sized… literally, of elephant statues. It’s pretty amazing what you can do with this stuff and I’m glad I took the time to learn. I don’t know what I’ll use the skill for, but it’s good to know I have it. Thanks for reading along!

Oh, and if you came to this page via Google sometime in the future and this 3500 word article was helpful, please drop me a line or leave a comment. You can email me at jasony3131 and the domain is yahoo dot com.