Quick-and-Dirty Mold-Making and Resin Casting For Newbs, Part 2 (The Mold)

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.

Starting materialsHere’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

test layout
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.

rolled clayOnce 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. 

Clay pre-sculptTHIS 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
building up the sidesNow, 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.

stir 0 stir 1 stir 2 stir 3It’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.

Scrap moldHere’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.
Diced mold scrap
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.

inserting scrap 1inserting scrap 2inserting scrap 3To 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.
Opening the finished moldSee all those dangly bits?  They need to come off. This looks like a job for Mr. Craft knife!

Trimming 1Lay 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.Trimming 2Note 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.

Finished insideWell.. 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.
mold release
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!

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  1. Quick-and-Dirty Mold-Making and Resin Casting For Newbs, Part 3 (Resin Casting and The Final Product) | Doc Schott's Lab

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