Quick-and-Dirty Mold-Making and Resin Casting For Newbs, Part 3 (Resin Casting and The Final Product)

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. Finished insideHere’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.
Pricking bubblesThis 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.
Resin Bloom
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.
Resin leaksThis 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.

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)
Candling the pieceAny 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.Resin pre-flash
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!

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.

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  1. Quick-and-Dirty Mold-Making and Resin Casting For Newbs, Part 2 (The Mold) | Doc Schott's Lab

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