Hey everyone, thought I'd help christen the new resin forum with a little how-to mold-making guide, or at least how we do it. There may be a few steps missing, but hopefully I can fill in the gaps with words. This first picture has the top half of Victim#2 embedded in sulfur-free kleen klay (sulfur causes silicone to only partially cure, so its bad). We typically make two part molds for our toys. That allows us some flexibility in this stage, and we can control the registration. It does take a little longer because you have to carefully clay up each piece. Cliff is smoothing out the clay to the parting line of the sculpture, trying to keep a 90 degree angle to prevent undercutting later on. Undercuts are your enemy. There are better guides out there explaining this, I'll link to them if I can find them. Basically, you want your final cast piece to be able to get free with no resistance from the mold. This also just takes practice and experience; you get a feel for it eventually. Next, spray with universal mold release. Its not as crucial at this point, because by nature silicone doesn't want to stick to anything but itself. We've built boxes with foamcore and hot glue. You want to make sure your boxes are airtight, because that wily silicone will find a way out. I don't even want to think about how much time and $$ we've wasted on escaping silicone. Okay, now its time to mix and pour your silicone. We've used a few from different companies, they're all pretty much the same. You want something thats not too hard and not too soft, because you want it flexible enough to pop your piece out, but sturdy enough so its not a floppy mess (there are ways around this though.) We're using Smooth-on Mold Max 30 for this mold. You probably want to mix more than you think you need. That may seem like a waste, but it's worth it in time saved. Just have a few little molds lying around to pour excess into. The more you pour, the better you get at estimating how much silicone you need to mix. It's a 10:1 ratio by weight, but we kind of fudge it and do it by volume. Basically you want to err on the side of using too much catalyst (the red stuff) otherwise your silicone may not cure, or may take a really, really long time. Temperature is also a factor, with silicone curing faster in a warmer environment (typically. Again, results can and probably will vary between silicones). Also of note is the type of silicone we're using here. The two kinds we've experienced are Tin-based (like this Mold Max 30) and Platinum-based. Platinum-based molds tend to last longer, but in our experience are much harder to mix up. They're also more expensive. This Mold Max 30 silicone, on the other hand, mixes like warm chocolate. Another note, and this is pretty important; the two may not be compatible. They make tin- and platinum-based resins to use in their respective silicones, but you can't use a Platinum-based resin in a tin-based mold. It won't fully cure, and you'll end up with a sticky mess. You can typically use tin-based resin in platinum molds, but it's probably not the best idea (especially when you get into clear resins, which are tricky enough as it is). Okay, next step: degassing. This is NOT a necessary step in the process, and requires semi-pricey equipment. But basically, air bubbles are your enemy. They can get into important details and ruin your mold, and you can't have them if you ever want to pressure cast (which we'll cover in a bit). So you want to degas your silicone with a vacuum chamber. You can spend hundreds of dollars to buy one ready made, our you can do it like us and buy a vacuum desiccator from ebay, vacuum pump off of craigslist, and a reinforced plastic hose from Home Depot. We spent maybe a little less than $200 on the setup? Much better than the ready-made kit for $1000 at Reynolds Advanced Materials. So now we're degassing. We made sure the seal was tight on the desiccator, and we let it run for a few minutes (longer or shorter depending on the working time of the silicone). I think it pulls about 30lbs of pressure, but we never bothered to get a vacuum meter to check it out. The silicone soup froths and bubbles as the air is sucked out. Make sure you use a big enough container, as it expands to about four times its original size. Eventually, the bloom with collapse, but if it's taking too long, you can shut the vacuum off and let a little air back in (just not too fast, or it could blast everywhere). Now this next step anyone can do. You want to pour the silicone from as high up as possible, and you want to create the thinnest stream. Your air bubbles probably still exist, so this stretches them out and helps them escape before the silicone hits your mold. Always have it hit away from your piece, just in case some air bubbles did survive. Let the soup fill the box up while you maintain a steady stream. Another note; you typically want a 1" barrier of silicone on all sides of your piece, it'll help with mold stability. This next step isn't necessary, but you may as well if you have a pressure pot (again, I'll go into pressure casting further down). Basically we're atomizing whatever air is left in the molds, just to be safe. Gotta be quick... Ok, first half of the mold is done. We have removed the box, but we're keeping the pieces for the other side. Note the ~1" thickness of clay and silicone. Take the kleen klay off and save for your next project. You don't want to disturb the sculpture at all, because you don't want new silicone creeping underneath. Carefully clean off all traces of klay (it'll show up on your finished cast piece if you don't), you can use alcohol and a brush to dissolve it. Water works too. Next we're keying in our registration/locks. Some people make them in the in the kleen klay itself, before the silicone is poured, but doing it this way removes a generation and provides a more secure lock. Now, the second half should fit perfectly. If you use these carving routers, take care not to cut deeper than the diameter of the blade to avoid undercutting. We're ready for the second half of the mold. Remake your box, making sure its watertight. Spray mold release generously on everything, and you can even brush petroluem jelly on the inside of the box just to make sure it pops out nicely. When you're done, you have this: We then cut our pour spouts with an exacto. Its always a good idea to know where you're going to put them before you even start claying up, as it'll inform everything from your parting line to your piece's orientation in the mold. We usually have to cut it out a few times after a couple test pulls, but generally we try to keep it as small as possible (less to cut off afterwards). Now we're ready to cast! We sometimes build a foamcore box to house the mold and secure it with mold straps to keep it registered. But we have good luck with lots of rubber bands too (sometimes better luck), so again, just something you're going to have to try out. Thats it! ON PRESSURE CASTING: Basically, if you want crystal clear castings, you want to pressure cast. The pressure pushes the air out of vents in your mold, giving you a clear casting. Even if you're not casting clear, it helps push and pull the resin into all the nooks and crannies of your mold (if these areas are properly vented), so you have less ruined castings. This is especially helpful on detailed pieces. IF you pressure cast in an UN-degassed mold, your cast piece will look like a pickle. The pressure is forcing the air pockets in the silicone mold to distort. This will also probably permanently distort the mold. This is our pressure pot. Its a Harbour Freight Paint Pot with brass fittings and a hose connected to an air compressor. We built it with schematics we found online; I'll link to them here when I find them again. We typically cast at no higher than 40 psi, but 30 usually does the trick. The pot was quite simple to make, you just need to make sure your fittings are the right size and are airtight. I think we paid about $150 for the whole set-up. ($60 for the paint pot on sale, $60 for the compressor at a Harbour Frieght sidewalk sale, and ~$30 for fittings and hose). \ Okay, thats it! I probably skipped/glossed over some things, so please ask questions, and if anyone wants to supplement this be my guest. Also, a disclaimer/word of caution; PLEASE be careful with this stuff. Resin and silicone is poisonous, wear gloves and a mask and work in a ventilated area. And if you choose to go the vacuum/pressure route, make sure you understand the forces involved. Do LOTS of research, and take it easy when you're starting out. I don't want to be blamed for anyone blowing off a hand. Thanks for reading, have fun casting!
Nicely done. Two things I thought I'd add: #1 - You can add your pouring cup (where you pour your resin when you're casting) and air vents to the sculpt during the mold making process so you don't have to cut them in later with an x-acto knife. Cut pieces of wire coat hanger should work for the air vents, and of course the pouring cup channel should be quite a bit larger - you can just make a cylinder of clay for the channel with a larger, squat cylinder at the top which should butt up to the wall of your mold box. #2 - After you've clayed up and before you pour the first side of silicone you can put plastic wrap into your mold box, and push it down around your sculpt as best you can, then fill it with water and measure how much water it holds. Mix a bit more silicone than this. Now you won't waste quite so much silicone! (You can do this for measuring how much resin to mix too.)
Great tut, Seanb, I had know idea about pressure casting. Its too late for my molds of my Shizen boodoo. As The silicone wasnt degassed at the beginning so I think pressure casting will distort the mold too much. But definately for my next ones! I need to get some degassing equip made up. I have a question about the pressure casting. In your experience, will the pressure casting help to force the resin into unproperly vented parts, displacing the trapped air i.e. forcing the air somewhere else if not out of another vent? And have you thought about using rotocasters at all. was thinking about making one (or getting one made for me as they look like a I FART A LOT to make) to get rid of air traps until I had seen this pressure casting method. cheers.
Thanks Jcrazy and uh oh! We started to wait until after the molds were made to vent because we'd still have random pockets of air that we would have to cut out anyway. But we do sometimes clay it in, especially if the part is going to be particularly complicated. This is an awesome idea! We usually try to estimate how much silicone we need by sticking our mixing cup into the mold and eyeballing it. But water displacement is the way to go, it seems! The pressure casting is really most effective when problem areas are properly vented, because the escaping air is pulling the resin along with it. Sometimes even pressure casting doesn't work, and you'll have to try something else to get the resin where you want it to go. Sometimes doing an open pour, where both halves are exposed, lets you make sure that the resin is in those tiny crevices and details. Then you just put the halves back together and fill it in with more resin. Also, whenever we pour a mold, we immediately pound it on a table and tap it a few times. This helps to jog out any trapped air, even air that pressure casting won't fix. Its just an extra insurance policy, and is pretty necessary. Rotocasting is definitely its own beast. We haven't tried it, as we don't really have any pieces big enough to warrant it (yet), but I've heard that its very time consuming and your reject rate will probably be high. I think Smooth-on makes a resin specifically for roto, which I think you can even do by hand, so maybe you want to check that out.
Awesome documentation of the process! You've got full set of tools there What I am doing is degassing twice. First time right after mixing the silicone and second time after pouring it into the box (I have extra space on top for bubbles to come up). And I am pouring silicone as low as possible, because, in my experience, it contains much more air when pouring your style. Thanks for the guide!
Don't forget protection! When working with silicone, make sure your gloves are "Nitrile" and not Latex because Latex can cause silicone not to fully cure... I think Vinyl gloves work too, but they usually suck. Also a respirator and good ventilation is a good idea. It'd be pretty lame if your toys gave you cancer.
#2 - After you've clayed up and before you pour the first side of silicone you can put plastic wrap into your mold box, and push it down around your sculpt as best you can, then fill it with water and measure how much water it holds. Mix a bit more silicone than this. Now you won't waste quite so much silicone! (You can do this for measuring how much resin to mix too.)[/quote] There's also formulas for figuring out amount of silicone and resin needed. I use: (Cubic inches of mold box - cubic inches of part X 21 = how many grams of silicone needed) and (cubic inches of part X 0.554 = how many grams of silicone needed to cast) I always use products that are 1:1 ratio for easier measuring. I "rotocast" manually not using the Roto Resin because it only comes in white. Thanks for posting this tutorial....I learned a lot which is awesome. I am also always happy to help anyone I can with casting resin. Just contact me.
Great tutorial! There should be a way to turn the pressure pot into a degasser too? Maybe by adding a vacuum pump outlet? So it does the work of both?
I definitely need to follow the logistics of your pour techniques for my next molds... I'm working with Tap Platinum, and it's amazing stuff, but my mold layouts are lousy, and make casting harder than it should be. I'll have to grab some Foamcore to make the next molds. The first batch I made, I poured silicone into a round container, and then suspended the part halfway on top of the silicone the claying stage, which always seemed like a pain. Suspending the parts halfway in the silicone works pretty good, and the rods used to suspend the part end up forming air vents in the top half, but all of my molds are tapered and round on the outside, so they are tough to rubber-band up nicely.
wow, after this explanation and that of paul kaiju, if I can not get a good job ... that means I'm stupid!!! I think I can use machines for pressure them if I bring home it... my wife ...
wonderful information on molding and casting. i am working on a project and was wondering is it necessary to use silicon as a mold? my friend says he can make me half inch to three quarter inch thick vacuum form molds using plexiglass and or lexan. he claims to be able to properly vent the molds for pressure casting. has anyone tried this? or does anyone know why i shouldnt use this method? thanks
I'd be curious to see the detail fidelity of such a thing, but it sounds badass if it works. Silicone is nice because it's really something the 'everyman' can use.
your very correct. im just looking for a more permanent mold without using metals. honestly i think for detailed sculpts half inch lex or plex would be way to thick to really pickup the detail. but with something thicker than say an RC car body maybe eighth inch thick i think its plausible the vacuum form could pick up the detail no.
I wouldn't have much idea... I've seen vacuum formed kits before, but they don't ever really have surface textures of any sort... not sure how much you can pick up by vac-forming.
