So, three days on and here we are; time for the payoff.
Basic precautions and materials: Part 1
The Mold-making process: Part 2

As before, wear your safety gear; respirator, gloves, and disposable clothing. This stuff will give you chemical burns and make your skin insanely sensitive to even the most minor irritants. It doesn’t really wash out, either.
Use a waxed paper sheet, big enough to hold ALL the molds you’re casting in this run and your resin containers with at least 3″ of room for overspills around the edges on your surface (You can see some of the results of NOT doing that in the last couple posts..). For the love of pants, put something between you and the floor, especially if you have carpets. I like cardboard because it comes in big sheets for basically free behind any local business.

Mixing the resin is a delicate and fiddly process. That’s why there aren’t really any pics of me doing it; the detail required is too high, and the times so short once the curing agent hits (plus the chances of getting resin on a couple hundred in electronics..) that there simply wasn’t time.

Step 1: Prep your molds. Here’s our mold from last time. You want to make sure there’s no dirt, old resin, or clay clinging to the surface. You can use a thin layer of mold release to extend the life of the mold, but in my experience it only gives you about 5-10 more casts before the mold wears out; important for infantry or trees, not so much with bunkers and artillery, or fittings for books and costumes. Place the clean mold halves, opened, side by side on your dropcloth; try to arrange them like book pages, so you can quickly and accurately fold them together when the resin starts to set (see step 4, below).

EDIT: I have begun using talcum powder as a mold release. It is freaking amazing and you should do it too. Look for a scentless baby powder (scented if you can handle the stink) with “talc” or “talcum powder” as the only ingredient. Cornstarch fucks with the chemistry, but does make a fantastic filler putty (see: https://www.youtube.com/watch?v=NA2XJ7PTT3M ) . Wear a mask, because inhaling a bunch of it will fuck your evening up. Tap the talc lightly over the mold halves, then clap them together a few times to distribute the powder evenly. It shreds the resin’s surface tension, and I’ve managed to pull much cleaner and less-bubbly casts than ever before with it. Get dat shit.

You will probably want to cast multiple items simultaneously; it’s very difficult to measure the exact amount you need for a mold by eye (again, as we’ll see below..). It’s a good idea to have an additional mold on standby just in case, so as not to waste resin – one model’s worth per run adds up FAST.

At the same time, resin cures faster when there’s more of it; more than about 4 mold’s worth of a fast-curing resin will sieze up in the mixing cup before you can pour it. For the Smooth-cast 300 I use, I’ve found the sweet spot is 3 molds’ worth – about 2Tbsp each of the two parts of the resin. I’ve tested 325 Colormatch before, and found it a bit lacking for small-scale casting; it’s very sensitive to mix ratios, fills with bubbles if you don’t vac it, and cures unpredicatably.

Step 2: Mixing the resin

No pix here, but I’ve got some pointers.
• Stir the resin in each jar separately, using a smooth circular motion to avoid bubbles. Let the jar sit for a few minutes before pouring, which will also help.
• DO NOT allow ANY of either component of the resin to enter it’s opposite number’s jar. You can instantly ruin it. I store the jars on opposite sides of my workroom, in fact. Use separate, labelled stir sticks for Part A, Part B, AND the mixing cup. Pour the resin parts into opposite sides of the mixing cup.
• If you you accidentally pour too much of one part into the cup, MATCH IT. Bad proportions of the resin can keep it from curing, or cause it to attack the mold; both can ruin a day’s work slowly and painfully. It’s better to waste half a pour from curing too fast than wreck multiple molds.

Once it’s in the cup, you’ll notice that one part is smokier and a bit yellow, while the other’s clear. You can use a desklamp from the side to see the two liquids as you mix; stir with a smooth, circular motion until the two blend completely and the mixture looks clear. About 15 seconds is enough for a small pour.

Step 3: The Pour
This is where you make or break your piece.
You want to pour the resin evenly and smoothly, starting with the smallest and most detailed items and moving to the largest last. To minimize bubbles, aim for the lowest area on the mold and pour slowly. You can use the stir stick to guide the flow to a certain extent.
Remember, it’s better to overpour than get too little into the mold, but every dram that you overpour is contributing to the mess. Try to keep the resin from overflowing the mold cavity, but don’t freak out if it does.
If the resin starts flowing less like water and more like honey, it’s about to set; it will also heat up significantly.

Step 4: Pricking Bubbles (Optional)
So, you don’t have a vacuum table/viber? It’s cool.
Remember that pin/paperclip I mentioned a ways back? Here’s where it comes in; this step is optional, but will dramatically improve the quality of your final piece. You only have 30 seconds to a minute to do this, depending on how much resin you mixed and poured. Keep an eye on the other molds; if you see the “bloom” (see next step), or if the resin starts to “skin up” on the one you’re working, STOP.

Now, what you’re trying to do isn’t actually popping the bubbles, just pulling them off of the surface of the final piece.
This was a pretty clean pour; I’ve already pulled one bubble on the left, and I’m working another one out of the rivet hole on the right. Work delicately, you don’t want to tear the mold; it’s usually better to put the pin over the bubble and coax it up by stirring the resin than to dive in and crank it out.

Step 5: The Bloom

As the resin is about to cure, you’ll see a skin develop, then a white cloud appear in the middle of the cavity.

Once the resin blooms, you have to work fast. If you’ve overpoured and let it cure too long, you won’t be able to press the mold halves close enough together, leaving massive mold lines and distorting the piece. If it doesn’t cure long enough, the resin will run out of the major cavities inside and, again ruin the piece (see below, “Candling” for some more on this).

Take the more-cured half of the mold – and there will be one, it’s usually the one you poured second because of the weird-ass ways resin works. Using your gloved hands (you do have gloves on, right? This shit will squirt..) fold it quickly and firmly onto the other half, matching the keys and making absolutely SURE the sides are perfectly aligned. Press down gently but firmly, or squeeze it using both hands for even pressure; you want to make sure the halves are as close as possible to minimize flash (the extra stuff around the edges) without distorting the mold. At this stage any major distortions you put it through will show as wrinkles and bulges on the final pieces.
To avoid distortion on particularly delicate pieces, you can go back to the legos: build a frame just high enough to enclose the bottom half of the mold (if you enclose the top, it’ll prevent excess resin from escaping, and it will likely catch the top mold half up, further distorting the cast), then press the top half down on it.

Step 6: Waiting

Set the pressed molds down and walk away. If you’re worried about the molds slipping, you can place a thin board or sheet of plastic on top and a weight on top of that: the board will keep the weight distributed evenly, and protect whatever you’re weighting it with.
For a fast-cast resin, you’ll want to wait about 10 minutes, or a little longer: you can use the resin around the sides to judge when it’s fully cured.
This isn’t quite cured: the resin is shiny and flexible. You’ll want it to be a little more matte, and just losing its flexibility.

Step 6: The moment of Truth (Sort Of)

You can take the gloves off for now, since most of the reaction is done. Keep the mask, though. When you open the molds, the resin will still be warm to the touch and slightly flexible: the flex will help you extract the models. Now you get to see how well you did.

Well crap.

This is a common error: I underpoured the resin for the bunker half. You can try to salvage an underpour by adding more resin, but you’re more likely to just wind up with a piece that’s got weird raised and lowered areas all over it from the new resin partially flowing under the old.

Step 7: Candling (optional, but recommended)

Take your piece and hold it up to a light. As you can see, I’m using a CFL desklamp (hooray for ecological responsibility AND cheapness)
Any internal air bubbles – like the massive one here – will show up as much lighter areas. This is caused by closing the molds while the resin is undercured, and/or squeezing it together too hard.
For smaller holes, if you have modeler’s epoxy putty – I use Green Stuff and JB Weld Qwik Wood – you can carefully cut out a section from the least-visible part of the bubble, then force some putty inside to reinforce the part. With a larger bubble like this, I usually just scrap the piece. Small surface bubbles can also be patched with epoxy.
So what can you do with these trashed parts? Battle-damaged scrap, terrain, base embellishments.. You can also take correctly-cast parts and save them: this one had perfectly fine gun barrels. If one of the next passes winds up with messed-up barrels, I can just glue them in place, or save them for converting other pieces.

Step 8: Cleanup.
So, you’ve got a finished, non-screwed-up piece. All that extra crap around the edges is called “flash”. It’s usually best to wait until the resin fully hardens to do the fine detail work, but you can trim the flashing down immediately with your diagonal cutters. While the piece is still soft and pliable, you can straighten detail parts, and easily do some preparatory cutting work for any planned repositioning.

After the part hardens, mask up; resin dust is still a lung and eye irritant. It helps to wipe down your work surfaces with a wet rag or paper towel and vacuum the floor after any trimming or filing.

Te remove the flashing, it’s best to trim it close to the surface, then sand or scrape off the remainder. To scrape, hold your blade facing at a slight angle away from you, and drag it towards yourself slowly with very light pressure, while holding the piece in a clamp or your other hand.
For sanding, I use several jeweler’s files for the aggressive work, but I mostly resort to a very fine-grit 3M sanding sponge; you can wash it out after every batch and re-use it pretty much indefinitely.

The final product!

Okay, the caption’s slightly premature: the tiny bubbles you can see in the windows need to be popped off with the tip of a craft knife, and the barrels need a wee bit more sanding. Note how the barrels on the cast piece are straighter than the master, and the detail seems sharper: these are common benefits when you’re recasting from lower-quality plastics and rubbers.

I hope you’ve found this tutorial helpful, and wish you success with your own casting setup.

Okay, so you’ve gathered up the gear I listed in Part I, and are keeping in mind my warnings and tips. What now?

First off, you’re going to be working with some nasty, nasty shit. It won’t kill you, but it will damage your skin, make you itch like mad, and ruin everything it touches. Wear your damned gloves and clothes you won’t miss.

Step 1: Assemble Your Gear.

Here’s my starting setup. Note the horribly resin-stained sheet of waxed paper on my desk, and the damage from the resin on the surfaces outside it. The giant red lump is Van Aken oil clay. It’s non-hardening, and more importantly, non-reactive with all the things we’re about to expose it to. It also works easily. Knead it a little bit before you start building the mold frame, it’ll get more flexible. Also, it stains.

Step 2: Building the First Frame Layer, Testing Layouts

You’ll need to do both these steps at the same time. First, lay out your master on your baseplate, trying a couple of configurations. My master today is a generic artillery turret I got in a sack full of stuff from China. I need more arty and pillboxes, so into the silicone it goes.
I sprayed this master with mold release beforehand – it’s not usually necessary, but I wasn’t sure about the material, and the mold release helps shield it from what’s about to happen. There’s also a buildup on the bottom right corner of the bunker, which I didn’t notice until I was ready to pour. I keep around a shitty old watercolor brush to help smear the release around and pick up excesses. It ruins paint, so, yeah, make sure you keep it out of your regular paint caddy…

Anyway, what you’re doing here is working out a good layout for your pieces. You want to leave some room around the edges for “keys” – parts that force the mold to line up when you’re casting. For this style of mold, you also want at least 1/8″ of space around each edge to keep it from being damaged in a cast.
Once you find a good layout, build up the blocks to about half the height of whatever you’re casting.

Step 3: Placing and Prepping the Clay.

Once you’ve got a good layout chosen, tear off some clay and pack it into the block frame, then roll it flat. I used a wine cork because I’m cheap, but really almost anything will work. Make sure, if you’re doing a deep 2-part mold, that the clay is deep enough to sink the model half-way in, or you’re going to have to redo this step.

Step 4: Embed the Masters in the Clay. 

THIS STEP IS CRITICAL to the quality of your finished piece.
Do not fuck it up.
I’ve done something slightly naughty here because I was short on time: since resin cures faster the more of it there is in a piece, you usually want to make separate molds for parts of drastically different sizes like these two. In fact, the turret base could get away with being a one-sided mold, even though there is a lightening space on the other side of the master.

Place your master on the clay.  With long, thin parts (like the gun barrels on this one) push them down into the clay, then pull them out completely and set them gently back into the holes. You want them to lie as naturally as possible; otherwise, the part will be deformed in the final resin cast.

This clay is now exactly the shape the second mold will be.In fact, if you only want to cast one, one-sided part, you can skip the rest of this and carefully pull the master, then skip ahead to the next section (mixing and pouring the resin)
The three divots in the clay were all made with a pencil eraser: they will be the keys in the final cast, along with the 2×1 brick in the top left corner. You ideally want keys going into both sides of the mold to help the alignment; you can also press bricks into the clay and then pull them out. Make sure the keys align at least three corners and two sides, and put keys on either side of small or fiddly parts to keep them aligned.
AFTER you’ve made the impressions for the keys, carefully take your sculpting tool and bring the clay up to the edges of the material being cast. You want to avoid what are called “undercuts”. That’s places where the mold wraps around a part; they shorten its life, and make bubbles much more likely.
For round or cylindrical pieces, try to make the mold line come right up to the center of the part, along its long axis.
For a more pyramidal part, like the gun and the turret base here, you want to keep the mold line at the bottom.
If you’re doing more cubical parts, try to keep the mold lines running vertically on the “sides”; that’s the area and placement your eyes will be least likely to make out any distortion.

Also, you don’t want a really strongly raised or lowered area in your molds, so if you find yourself having to cut or build up a large area inside, try tipping the master at a diagonal. If you’re casting a pre-cast plastic master, follow the existing mold lines; plastic casts absolutely cannot have any undercuts, which makes your job easier.

Step 5: Raise the Mold Frame’s Sides
Now, you’ll take your blocks and build up the sides of the mold frame. You want the top of the frame to be at least 1/4″ higher than the highest point on the master inside the frame. After this photo was taken I added another level just to be sure.

Step 6: Mix and Pour the Rubber

Once the frame is high enough, mix and pour your rubber according to the instructions. If you have scrap rubber, don’t completely cover the piece; you want it to just barely submerge all the components.

This is the second most important part of the process; accidentally disturbing your master or getting surface bubbles will ruin the mold and you’ll have to start over. You can minimize bubbles by stirring the rubber inside the cup with slow, linear motions instead of a fast, circular one. If you have a vacuum table, great, but most people don’t. This is the most cheapest and most effective method I’ve found yet.

Step 7: Basic Degassing
Once you’ve poured, stir the rubber inside the frame in a slow, gentle back-and-forth motion at the top of the mold, as you see below.

It’ll take 3-5 minutes for the mix to stop actively bubbling and quiet down. You may have noticed the bits sticking out of the rubber there, and be asking yourself “why is it so thin?”
Here’s where recycling comes in.

Step 8 (Optional): Reinforcing and Recycling

If you’ve fucked up (or, in this case, worn out) a mold, you can use it to reinforce a new one; it’ll also stretch your available material considerably.

Here’s a trashed mold (I actually poured badly-mixed resin in and it destroyed the casting surface, but that’s for next post). You can also use the pucks of leftover rubber that will build up in the bottom of your mixing cup.

Slice off a chunk of the scrap with your craft knife, then dice it into smallish chunks. The small blue bits you can see inside there are actually from another salvaged mold; you can keep doing this pretty much indefinitely. Pick off any resin stuck to the diced scrap. You want to keep the scrap to about 30% of the total volume of the mold half; too much more and it starts to become unstable.

To insert the scrap, place it on the surface and gently push it in with the end of your stir-stick. This helps keep you from trapping large pockets of air under or between the bits, and also from disturbing the mold.

Once you’ve put in the scrap, you’re done with this half of the mold for the next 8 hours. Put it on a level surface where the cat/kids can’t get at it, and walk away.

Step 9: Extraction and Cleanup

(8 hours later).
You’re ready to trim and prep the bottom half of the mold. Bust open the frame (carefully), and peel the mold away from the master and clay.
See all those dangly bits?  They need to come off. This looks like a job for Mr. Craft knife!

Lay the piece on its side on a cutting mat (or not, if you don’t care all that much about your furniture..). Make a straight, slightly beveled cut on each side, rolling the mold as you go. You can also tear or trim off the areas where rubber snuck between the blocks now. Here’s a shot of the finished outside.Note how all that mixing still didn’t get all the bubbles out; the rubber gives off gas as it cures. But at least this way they’re inside the rubber, not on your casting surface.

Well.. mostly. After inspection, I’d normally trash this mold half – but the bubbles are small and not in immediately noticeable areas, and I’m low on rubber, working with a relatively unimportant piece. I’ll give it a bit of clemency.
Look over the inside of the piece for rubber “tags” – small bits of unsupported stuff hanging off. You can see a few places on the turret base where I had to cut them away. Also, you’ll want to clean any clinging clay from the mold, or it’ll interfere with casting the second half. Note also how I trimmed down the keys slightly to keep them from binding in the other half of the mold.

Step 10: Prepping for the Second Mold Half

Now we come to our second toxic friend – mold release.
This shit is nasty. It burns your lungs and gets slippery oily crap everywhere. It’s kind of what it’s supposed to do. Wear a mask, and do this outside.

Spray a thin coat of it on the mold half, without the master model inserted. Smear it around, making sure to get all the nooks and crannies; I use a shitty old Crayola watercolor brush.
Wait 5 minutes and do it again.

Step 11: Here We Go Again

For casting the second half of the mold, the procedure is almost the same, though there are a couple of pointers:
• Completely disassemble the mold frame, place the mold on the baseplate, and completely rebuild the frame around the first mold half. DO NOT shove the mold into the old frame; it will distort it and ruin the second half of the mold.
• Make absolutely, completely, 100% sure that the master parts are inserted firmly and held in place on the first mold half. If not, you’ll have to start over. By the same token, be especially gentle when outgassing and pouring the second half. If I’m working with a really, really fiddly part I use a thin layer of white glue to hold it into the first mold half. Be aware that you’ll have to wait about 6-10 hours for the white glue to dry so it doesn’t screw with the rubber’s chemical reaction.
• Once again, do not screw with the mold for at least 6, preferably 8 hours. I usually time it so I wind up pouring the second half right before I go to bed so I’ll be less tempted to do something stupid..

Click through to Part 3 here, where I’ll show you how to use your new mold with yet more toxic stuff!