How I Do It: My Molding and Resin Casting Process (2023 Edition)

Introduction

Last year I wrote an article that delved into my silicone mold making process and while the information in that article is still valid and the basic principles remain the same, I’ve picked up a few new methods since then that I think makes the whole molding and casting process that much more efficient and effective. Also, this new article will include my resin casting process and a basic rundown on pressure casting and I’ll be breaking the whole process down into different stages (which I really should have done the first time around).

With that out of the way, here’s the main object (or the master) that I’ll be molding and recasting. It’s a 28mm Survivor Witty miniature from Black Site Studios' Demon Ship along with the two head sculpts that came with it. The type of mold I’ll be discussing here will be a simple, one-piece cut-block mold since that’s about 90% of the types of molds I make, with the other 10% being even simpler, one-piece open face molds. Both of which I explain in that previous article. Before going any further, make sure the part that you intend to mold is clean and dry (I like to use some 91 to 100% isopropyl alcohol and let it air dry).

I’ll get to the specific materials and consumables as we go along, such as the type of silicone rubber and resin, but I will list the major equipment off the top.

Equipment Used:

• Ablaze 1.5 gallon vacuum chamber and pump kit
• Digital scale (optional depending on the silicone)
• Plastic or silicone measuring cups
•  California Air Tools 255C Pressure Pot for Casting (2.5 Gallon)
  
• California Air Tools CAT-1P1060S Light and Quiet Portable Air Compressor
• Monster Clay Medium (as well as a cheap crock pot to melt the Monster Clay)
•  Pair of hobby snips/side cutters
  
• Safety Equipment/PPE:
• Plastic gloves
• 3M half face respirator mask with 6006 respirator cartridges

Stage 1: Air Vents and Pour Spouts

The most important part of prepping a master for molding, at least to me, is properly venting your parts as well as adding pour spouts for when you pour in the resin. The more molds you make, the better you get at determining where to put the air vents and pour spouts. Just never lose sight of the fact that every “empty” mold is full of air and as you pour (or in my case, inject) resin into the mold, it fills up from the bottom of the mold’s cavity (the negative) and pushes that air up to the top. This air being pushed up and out by the resin needs channels at the top of the mold in which to escape and without them, the surface of your castings will be marred with air bubbles, pockmarks, and even entire areas missing due to pockets of trapped air that prevents the mold cavity from being properly filled. Robert Tolone illustrates this simply and eloquently with his “coke in a cup” youtube short.

There are, however, a few hard and fast rules of thumb that I always abide when planning out where to place my air vents:

• If a section of the part is kind of hanging out in space apart from the rest of the model (like the example miniature’s rifle or a figure with an outstretched arm), it needs an air vent.
•  If you’re molding a human or humanoid figure, always vent the hands. Unless the hands are posed on the body or, in the case of the Survivor Witty miniature, holding an object. In which case, see the rule above and vent that object attached to the hand.
• I typically like to place my vents along either side of the object (like I did with the head wearing a cowboy hat) because I almost always find it simplest to cut the molds along the sides. Also, if there are visible mold lines in the final castings, it’s better that they be on the sides than say, right down the middle of a figure’s face. For more complex objects, sometimes having to cut the mold in more than one direction is unavoidable, but you want to minimize that as much as you can by trying not to put your vents on the front or back of the object.
• But like I said, sometimes this is unavoidable like with the hatless head that (while it isn’t visible below), has a ponytail that’s sticking out the back and thus needed to be have an air vent placed behind the head connecting the ponytail to the ground.

I use cut up pieces of toothpicks and skewers for my air vents, which I glue onto the masters with superglue. For the record, I’m not calling it CA glue or Cyanoacrylate because you all know what I’m talking about when I say super glue. I also find the use of superglue accelerator to be necessary to secure the vents, but I’ve noticed that in certain circumstances (such as when you’re using the cheapest possible platinum-cure silicone), it may actually inhibit the silicone from curing and result in a wasted mold. So if your superglue accelerator comes in a spray or aerosol form and it gets on the master part, you might want to clean it with isopropyl alcohol again (and/or purified water) and let it dry.

Most of the pour spouts I use are little cone-shaped objects that I sculpt out of polymer clay (which I am able to reuse most of the time) in the exact sizes I need for that particular mold. For the two head sculpts here, I made the spouts out of Super Sculpey Premo though I primarily use Super Sculpey Firm for this. I’ve made pour spouts out of epoxy clays as well. For the main body of the Witty miniature, I opted for a generic 28mm base instead of my traditional cone-shaped pour spouts since I find that’s the easiest way to mold most miniatures. Resin goes in one of the foot holes and as the mold is filled up, it rises out of the other foot hole as well as the channels created by the air vents and then continues to rise until the entire circular negative in the finished silicone mold is filled up.

If this was a larger figure) that I was casting (one that’s say 2.5 to 5 inches or taller), I would eschew a base and instead put a cone-shaped sprue on the bottom of each foot (as well as an additional little piece of a vent under the front of each foot since again, they’re sticking out from the rest of the body) and add another sprue to the bottom of her kanabo club connecting it to the ground. Some might argue that I wouldn’t need to place pour spouts on the feet at all, but in my experience with pressure casting, the resin you fill the mold with tends to recede or shrink down as the pressure pots obliterate any remaining air pockets during the curing process and often left me with castings where the feet aren’t fully filled when I take the mold out of the pot. You can, of course, top up the feet before demolding and place it back into the pot because resin tends to stick to resin, but that I have zero time for that. So now, I place spouts and vents on the feet when I absolutely have to fill the negative of a model starting from its feet.

I suppose you could, in theory, also orient the figure so that it is upside down, placing a bigger pour spout on the neck stump, vents on each shoulder, and a vent on the gun barrel to connect it to the ground but that brings me to my final point (for now) when it comes to venting your masters: try to orient it in such a way that you require the fewest number of air vents. When it comes to one-piece cut block molds, the simpler sprue/vent configurations typically yield the cleanest results.

One final note on orienting the master part that you intend to mold is that you want to think of it in terms of gravity. Imagine the part as a negative cavity already within a mold and ask yourself this: what direction should this part be facing in order for the resin to do the least amount of work to fill up the cavity. Since the force of gravity pulls objects down (in case you needed to be reminded), it takes less time and effort for resin to fill up a mold cavity where the object was oriented vertically (like how the miniature is shown in the picture below) more so than if the object is positioned on its side.

Stage 1.5: Test the Material

Since I’m using a comparatively cheap platinum cure silicone that I got off of Amazon and not the more expensive (but considerably more versatile) Cast-A-Mold 25T tin-cure silicone from Specialty Resin and Chemical that I used in my previous article on mold making, I decided to first test it out on the sci-fi base that that came with the Witty miniature to see if the resin that the original miniature is made from causes any cure inhibition in the silicone rubber. This was a bit of a concern because miniatures produced by smaller companies sometimes produce them using 3D printer/UV resins that are not typically compatible with platinum-cure silicones and I didn’t know if that was the case with Black Site Studios.

Thankfully, my fears were unfounded. This was a rousing success and now I know that I can proceed with the molding process.

Stage 2: Prepare a Mold Surround

There’s no shortage of ways to create a mold surround (often called a mold box, but I use the term “surround” because my molds aren’t typically box-shaped), many of them I’ve used in articles on this very blog. Plastic cups and lego were my go-to for a while, and while I still use cup molds on occasion (in fact, I do so later on in this very article), I’ve happened upon a new and better way. In fact, that’s why I listed Monster Clay in the equipment section from earlier. One that eliminates the need for disposable cups, hot glue (or silicone caulk, which I used in previous articles), and acrylic sheets on which to secure said disposable cups. I can understand if this method isn’t for everyone, but I try to reduce waste wherever possible and that includes wasted silicone leaking from inferior mold surrounds that are improperly secured using hot glue (which I still hate).

First, I take a plastic lid and fill it with a shallow layer of Monster Clay that I melt in a cheap garage sale crockpot until it’s liquid. I give the Monster Clay a few minutes to cool down and start firming back up (but not too firm) before pressing the masters into the clay. I keep a lot of my plastic lids because they have that “lip” around the edge that helps contain the Monster Clay that’s just tall enough for the later step in this process. You could also use a bowl or a plastic carryout/meal prep container or whatever other kind of found object you want, as long as it has walls around the outside.

Again, before the Monster Clay is fully cured, I press the piece of a cardboard tube from an empty roll of paper towels or toilet paper over the parts. These are my preferred types of mold surrounds at the moment for miniatures. For larger figures and parts, I use different sized sheets of chipboard that I buy in large packs on Amazon or Ebay and bend those into tube shapes that I customize to the size of mold surround that I need. For really large parts, I’ll glue multiple sheets of chipboard together and then make a tube. If a tube is too big for a particular miniature part, I’ll cut an existing tube, roll it so it’s a smaller tube, and then re-glue the sides.

Oh, and I pretreat every cardboard/chipboard mold surround with a few coats of Shellac sealer just to be safe. I’ve never had an unsealed cardboard mold surround get soggy from the liquid silicone and cause problems, but it’s still best practice to seal any porous surface with a sealer before introducing it to silicone (especially platinum cure silicone, which is much fickler than tin cure silicone).

Yes, you want a decent amount of space between the sides of the master parts and the interior walls of the mold surrounds but not so much that you needlessly waste silicone. The rule of thumb you see in a lot of places is 0.25” between the part and the mold surround on all sizes and that’s a good rule, but sometimes I do cut it a little close and as long as the silicone isn’t too thin once its cured, you can probably get away with a little bit less. Just don’t push your luck and if any part of your master is touching the inside walls of the mold surround, then you clearly don’t have enough space around the master.

Once Monster Clay returns to its typical level of firmness, any master parts and mold surrounds that were pressed into should be fully secured. I often test this by holding the lid upside down and giving it a gentle shake to see if the objects fall off and have never had that happen to me (knock on wood). Once the Monster Clay you’ve already poured into the lid has cooled down and firmed up, pour another layer of melted Monster Clay on top of that. Fill the lid until the Monster Clay rises up over the bottom of the mold surrounds to seal off the bottom from any potential leaks. It might help to slosh the Monster Clay around by gently rocking the lid in different directions.

Now not only are the parts held firmly in place, the mold surrounds are also secured against any silicone leaking out from the bottom.

Stage 3: Mixing the Silicone

I mentioned in the list of equipment that I’m using a digital scale here, and that’s because the silicone I’m using here is a 1:1 mix ratio going by either volume or weight. I prefer weight but if the silicone you use is mixed strictly by volume, just use a measuring cup like this one or one of the smaller varieties of graduated measuring cups that are available. For big batches of silicone for bigger molds, I’ll mix the silicone in a 1 quart measuring bucket.

For this project, I guesstimated the amount of silicone I would need (150 grams by weight) but spoiler alert: I ended up with about 50g more than I needed. Thankfully, I had a “dump mold” ready to go to catch any excess silicone rubber but we’ll talk more about that later. What I should have done, like what I do with so many other molds, is use the handy volume calculator on the Alumilite website to help me more accurately determine how much silicone I would need. And while it’s more accurate than eyeballing it, it’s still not completely accurate so I still temper the calculated results with a fair bit of discretion. There are two volume formulas, one for rectangular shaped molds and one for cylindrical shaped molds. Those are listed on the website I linked, and I won’t delve too deep into the mathematics here but know that the volume calculations are basically the volume of the mold container minus the volume of the master part, and the difference translates to the amount of silicone you will need to make the mold. In theory. As I said before, the formulas are still not completely accurate because the formula for the cylindrical shape mold also assumes that the object you’re molding is also a cylinder (and likewise for the rectangular formula), which is almost never the case for me. But I digress.

If you’re mixing by weight and using a digital scale, the first thing you’re going to want to do is place the empty container on the scale until the weight registers and then zero out the display by pressing the tare button. That way, you’re just dealing with the weight of each amount of silicone and you don’t have to rack your brain trying to remember how the weight of the container factors in.

When you’re vacuum degassing silicone, you’ll want the cup or container of silicone to be half full or less for reasons that you’ll see when we get to the vacuum degassing phase. I actually didn’t do that here, which was another gaffe on my part and definitely NOT best practice, but I’ll show you a workaround when we get there.

The platinum cured silicone that I’m using here is probably the most common kind of silicone rubber available and often the cheapest by the gallon and I typically buy my silicone by the gallon. As I mentioned above, it’s a 1:1 mix ratio that comes in two equal sized jugs (labelled Part A and Part B) that you mix in equal amounts by weight or volume. The last mold making article I wrote, the silicone was a 10:1 ratio and mixed strictly by weight, so always be aware of the materials you’re using for the job and follow the instructions that come with the kit. I can’t stress that enough.

That leads me to my next silicone mixing tip: I always keep the Part A and Part B jugs in the box that they were shipped in. When I’m ready to mix them up, I take both jugs out of the box. Whatever Part I pour first (it can be A or B), I put it back in the box and leave the other one out. Then I pour the remaining Part and put that back in the box. That was, I don’t accidentally mix a Part A into a Part A that I already poured. This is an especially useful thing to do when the colors of the Part A and Part B aren’t too terribly dissimilar to one another.

Pictured below, I poured 75 grams of Part A since I want to mix 150 grams total. When I was pouring it, I saw that the material filled up more of the cup than I would have liked and I still have Part B to go. There is room enough in the cup for both Parts but not as much space to accommodate for how much the silicone rises during the vacuum degassing process. So don’t do what I just did and use a bigger mixing container than what you’ll need.

After pouring Part B to reach my total of 150 grams (75 grams of Part A and 75 grams of Part B), I mixed in a little bit of pearlescent red mica powder just to make things more visually interesting and help differentiate these molds from other ones I made using this particular gallon of silicone. This silicone is translucent in color and while it allows for pigmentation with mica powders, other brands and types of silicone rubber might not. So again, read the instructions that come with the kit beforehand.

Another tip I’d like to pass on: I find that wooden dowel rods make the best stirring sticks for silicone because their round shape makes it easier to scrape the sides and bottoms of the cup so that you get a nice even mix. Every youtube video about making silicone molds says to scrape the bottom and sides of the cup when you’re mixing and for good reason. If you don’t mix your silicone properly, it probably won’t cure properly either. That is true for resins as well.

Depending on the potlife/working time of your silicone (or working time, which is about 30-40 minutes for this stuff), you’ll want to mix it slowly and for several minutes depending on the amount. Sometimes, the instructions for silicones will give you a specific number of minutes to mix it while others will just say to mix it until it’s one even color and streak-free. I say once more: follow the manufacturer's directions. I think I mixed this batch for about a minute or two. I don’t really recommend using a stirring attachment for your power drill, though I do use one to mix resins with a short potlife.

Mixing slowly will reduce air bubbles and while vacuum degassing in the next stage with pretty much obliterate any bubbles that formed during the mixing process, this just means that it will take less time to fully degass. Which brings us to our next step…

Stage 4: Vacuum Degassing the Silicone

We’ve reached the penultimate step before pouring the silicone into the mold surrounds, but first we have to get the air out. If you’re pressure casting like I am (which I consider a necessity), then you’ll want to degass the silicone rubber after you mix it and before you pour it into the mold surround. If you don’t degass the silicone, all of the tiny invisible air bubbles and pockets in the finished mold will pop under pressure, creating equally tiny cavities in the mold. This leads to castings that are full of bumps and spikes all over the surface and a mold that’s completely compromised.

A couple things to keep in mind when using a vacuum chamber:

•  Line the bottom with foil or parchment paper in case of any spills or overflow once the silicone starts to bubble up the container when the vacuum ramps up.
• If your silicone is in a smaller container, like a plastic cup, place it Inside of a larger container. When you introduce air back into the vacuum chamber after degassing (which is necessary to remove the lid from the vacuum chamber), the air will blast in through a little nozzle that in my chamber is perpendicular to where the container is standing. This gust of air will knock over a cup this size (or smaller) and cause it to spill out. But by placing the cup in a larger container like this mixing bucket I use for larger batches, the bucket takes the brunt of the air and the cup remains upright.
• Note that larger batches in bigger containers like this mixing bucket aren’t at risk of being knocked over when the vacuum is released.
• Make sure the lid is adequately centered when you place it onto the vacuum chamber, making sure the rubber edging around the lid aligns with the rim of the chamber. A misaligned lid might cause the metal around the rim to bend or get deformed when placed under vacuum.

Everyone’s vacuum chamber is probably going to be different depending on the make and model, but I like to think that the basic operation is the more or less the same. Always check your manufacturer-supplied manuals and instructions to best identify where the various components (inlets, outlets, pressure gauge) are located. But for my vacuum chamber, the outlet valve is the blue one on the left and the inlet valve is the red one on the right. When in the open position, the outlet valve allows air into the chamber. Closing the outlet valve will cut off the flow of air into the chamber.

Conversely, the inlet valve is connected to the vacuum pump (via that black hose) and when the valve is in the open position, it allows the pump to suck the air out of the chamber. When the valve is closed, it blocks off the pump from acting on the chamber.

Pictured below is the vacuum chamber with the lid on and with the inlet and outlet valves in their CLOSED positions.

Check to see if the outlet valve is closed and the lid is secure? Good. Now turn the vacuum pump on and move the red inlet valve into the OPEN position as shown in the picture below. As the pump does its work and creates the air bubble-destroying vacuum, keep an eye on the gauge (indicated by the black arrow in the picture). My vacuum chamber only really goes to about the halfway point between -25 Hg and -30 Hg, so once it reaches that point, I’ll turn the inlet valve back to the CLOSED position.

Note that once the chamber has reached the desired vacuum, you can turn off the vacuum pump as long as the valves are all closed. No need to keep it running throughout the entire vacuum degassing process.

While the Hg is ramping up, you’ll also want to keep an eye on the silicone in the cup to make sure it doesn’t bubble up over the edge of the cup and spill out. That’s why I said earlier that you’ll want to mix the silicone in a container that is twice the size of the amount of silicone you need.

If you’re seeing this and still wondering if you need a vacuum chamber, might I direct your attention to the picture I’ve taken of the silicone under vacuum? Look at how much the material expands! All that would have ended up in your finished mold.

Also remember when I said I had a work around for accidentally mixing my desired quantity of silicone in too small of a cup? When the expanding silicone gets dangerously close to the lip of the cup, as seen in the picture above, slightly open the blue outlet valve but don’t fully open it yet. This will slowly bleed out some of the vacuum and cause the air bubbles to recede before spillage happens (if you’re quick enough). Once the air bubbles collapse, close the outlet valve and turn the vacuum chamber back on if you’ve turned it off. Reopen the red inlet valve and let the vacuum ramp back up, keeping an eye on the silicone all the while.

It will bubble up again but it shouldn’t expand quite as much as it did before since a good chunk of the air bubbles have been obliterated. But in case they haven’t, repeat the slow bleeding and re-vacuuming process. Once the bubbles have subsided enough to where they’re no longer a danger of foaming out of the cup, close the inlet valve and turn off the vacuum pump.

The duration in which I leave the silicone to degass in the vacuum chamber depends on a couple different factors. After taking into account type of silicone, the length of the working time/potlife (also referred to as “geltime” by some manufacturers) and the viscosity rating of the silicone rubber it’s usually between 10-25 minutes.

For high viscosity tin-cure silicone rubber with a one hour working time, I’ll give it about 20-25 minutes depending on the size of the batch before taking it out of the vacuum chamber. But for the purpose of the comparatively cheaper and more common platinum cure silicone with a very low viscosity and a 40 minute working time (which is about 90% of the silicone rubbers that you find on Amazon), 10-15 minutes is enough time to let a batch this small degass. Set a timer and if your silicone rubber is mostly bubble-free like what is pictured below, you’re almost ready for the next phase. If there are any bubbles remaining, they should be mostly tiny, surface-level bubbles that will be eliminated when the vacuum is released.

Turn the blue outlet valve to the FULLY OPEN position. You’ll hear a fairly loud sucking of air as its introduced back into the vacuum chamber and if you vacuum chamber is similar to mine with the placement of its air inlet, it will blast out in the direction indicated by the blue arrow in the picture below. If you placed your cup of silicone in a larger container like I did, the gust of air will push against the bucket and while the cup containing the silicone will also be pressed towards the back of the bucket, it will remain upright and not spill all over the place.

Once the vacuum is fully released, you’ll be able to take the lid off and remove the cup from the vacuum chamber.

Stage 5: Pouring the Silicone

Okay, there’s not really much to this step. You just pour the silicone into the mold surround. Though there are a couple things to keep in mind:

·         Pour from on high so that the silicone fills the mold surround in a long, thin stream to minimize any new air bubbles forming.

·         Try to pour in one spot in the mold surround and fill up the mold that way, keeping it away from the master part as best you can. I’m told that if you drape the master with silicone, there’s a chance it will trap air but I’ve never encountered any air pockets whenever I’ve accidentally draped the master part (knock on wood). I’d say avoiding draping the part is more of a best practice than it is a “do this or you’re sunk” type of scenario.

Stage 5.5: Bonus Mold

So I mentioned earlier that I had 50 grams of silicone left over from mixing too much. From past experience, I know that 50 grams of silicone is enough to fill up a little plastic beer pong cup and little plastic beer pong cups are just large enough to accommodate a de-based Heroclix miniature. All the makings for a dump mold to catch that excess silicone. Consider this a review of some of what we’ve been over before.

I had this random SHIELD agent Heroclix in my fodder drawer, who I selected largely because his look was generic enough for future kitbashing and his post made him quick and easy to vent. Behold:

Using much of the same technique as before, I poured some warm Monster Clay in the bottom of the cup and stuck the miniature in it base-first to secure it in place and ensure it won’t float away when the rest of the silicone gets poured in.

And then we dump the rest of the silicone into the dump mold, again taking care to pour in a thin stream and fill up the mold around the part and not pour directly on top of it. Now it’s time to let the molds cure (about 12-24 hours for this platinum cure silicone).

Stage 6: Demolding the Masters

Once the silicone rubber is fully cured, remove it from the Monster Clay and peel away the cardboard/chipboard mold surround. The great thing about using Monster Clay as a mold support is that you can peel it off the plastic lid, put it back in the crock pot, re-melt it and use it again. Silicone doesn’t bond to Monster Clay, so if you dribbled any onto the clay, just peel it off.

To remove the master parts from the molds, use an Xacto knife (with an adequately sharp blade) to cut the silicone along the sides doing your best to follow the air vents with your blade. Here’s some additional things to remember when demolding a master part from a one part cut mold:

• First and foremost, do not cut the mold into multiple pieces. In fact, try to avoid cutting clear to the bottom of the mold, only cut it enough to free the part and prevent future castings from getting stuck. The main advantage of a one piece cut mold is that it remains in one piece.
• Start at the top of the mold where you can see the vents and sprues protruding and cut along the sides in a jagged or zig-zag pattern so that the mold will “key” back together when the two halves of the mold are pressed back together. The cuts don’t have to be too wild, just as long as the cuts aren’t perfectly straight. Though unless you have an incredibly sharp blade or you’re as nimble as a neurosurgeon, I’d imagine you cuts aren’t going to be anywhere near perfectly straight even without consciously trying to cut in a jagged line.
• Make sure all the bits of air vents and sprues are removed from the rubber mold.

Witty’s kanabo club and one of her feet broke off when I was demolding her, so I superglued those back on, put the original miniature back in its blister package, and backed away slowly.

Here’s the bonus mold cut open with the master part extracted and the sides of the mold also sealed up with some packaging tape. I seal up all of my molds with packaging tape for reasons I’ll get into a bit later on.

Stage 7: Mixing and Optional Vacuum Degassing of the Resin

Ideally, the way to determine how much resin to use in a mold is to weigh the object, along with the sprues and vents (since those would factor in to the overall weight of the master), and use roughly that amount in resin by mixing Part A and Part B of said resin by weight like I did for the silicone. If you’re working with a very limited amount of resin and if you’re just starting out, I think this is your safest option if you’re really at a loss for how much resin to mix. Though I would ask you to keep in mind that the weight and of whatever material the original object (and your vents and sprues) will likely differ greatly from your casting resins, so I would still mix up a little more resin than what you think you’ll need

For me though, I can count on one hand the amount of times that I actually mixed resin by weight instead of volume because I’m so used to working with flexible polyurethane resins with a short three minute potlife where there simply isn’t time to weigh everything out before the resin starts to kick. I should probably note that I could weight part A and B in separate containers and then mix them after I take my measures, but I usually don’t because it’s more expedient for me just to mix the resins by volume. Now, the issue with the potlife isn’t a problem here since I’ll be using epoxy resin with a 40 minute potlife, but my point is, I still prefer to mix resin by volume 99.9% of the time in graduated measuring cups.

You could also use the aforementioned Alumilite volume calculator to get a general idea of the amount of resin to use, and I emphasize the general idea part given the limitations of the volume calculations that I discussed in the silicone mixing stage. 

Or, if you buy your materials in bulk, you could do what I do and mix a set amount of resin per each batch of castings. Say 60ml or 100ml or whatever, and have a bunch of molds in addition to the main ones that you want to cast gathered together for one big resin pour. For resins with short working times, I will typically only do one or two molds at once and rarely mix a batch of resin larger than 60-80ml and have a dump mold on hand to catch what little excess is left. But for resins with a long working time, I’ll make sure to have enough molds on hand to use up all or most of the room in my pressure pot and do them all at once provided they’re castings that I plan to paint in full and I’m not worried about them being any specific color. If the castings do have to be a certain color, I’ll use that one color for the main molds and all the dump molds on that that don’t need to be any particular color. If said color needs to be created by mixing different pigments and dyes, I’ll do that in a separate container well before I start mixing any resin. The containers where you mix the pigments needs a lid to keep them from drying out. I recommend those tiny plastic bead containers you find at the Dollar Tree.

Whenever I’m working with epoxy resin, I like to vacuum degass it in my vacuum chamber just like how I do my silicone. This step probably isn’t all that necessary, but since epoxy resin has such a long working time compared to other resins I use, it’s still something I take advantage of for about 10-15 minutes of the 40 minute potlife just to better ensure I get the best possible castings.

Whether you’re mixing your resin by weight or volume, regardless of whether you’re weighing everything out for one specific mold or simply just mixing a batch for multiple molds, if you choose or choose not to vacuum degass your resin, I really only have one golden rule here: make sure you mix Part A and Part B of your resin in equal amounts if it’s a 1:1 ratio so that your resin properly cures. Most of the resins you’re likely to find are a simple two-part 50/50 mix though, like liquid silicone rubbers, there are some resins out that that require a different mix ratio. I’ve seen some Deep Pour epoxy resins that have a 2:1 mix ratio and certain polyurethane casting resins that are strictly 100A:50B by weight, though I’ve personally never used them. And in those and all other situations, I advise you to be aware of the type of resins you’re purchasing for your projects and follow the manufacturers labels/instructions.

Stage 8: Filling the Mold(s) with Resin

Before mixing up your resin (or while it’s vacuum degassing per the previous optional stage), make both halves of your mold are properly aligned with one another and securely taped together. You heard that right, I seal my molds with packaging tape after I cut them and before I pour resin into them. I’m sure that nearly every youtube video you’ve seen or are liable to see on the subject of mold making says to use rubber bands to hold the mold together and on the surface, it makes perfect sense. Rubber bands are cheaper than packaging tape (which I buy in bulk like pretty much all of my materials) and provided they don’t get too crusty from resin spillage, they can be reused multiple times.

But after much trial and even more error, I’ve found that rubber bands don’t really do it for me. For one thing, I primarily cast in flexible polyurethane resins with very, very short working times. Like three minute working times wherein I have to mix the resin and pigment, adequately fill the mold, get it in the pressure pot, secure the pressure pot lid, and then get the pressure pot up to 50-55 psi (more on this later) all while the resin is still in a liquid state. I found that the only way to accomplish this is to use a 20ml or 60ml plastic syringe (depending on the size of the object I’m molding), which ensures I fill the mold completely and rapidly, giving me more time to do all that other stuff. I’ll get more into the weeds when we actually get to making the initial casting from this mold.

Due to the pneumatic force created by the syringe’s plunger action, resin is liable to squirt out of the sides of the mold if they’re not secured all around using tape. Every time I attempted to do that with molds where both halves were “secured” by rubber bands, I ended up regretting it.

Granted, I won’t be using flexible polyurethane resin for these molds, but rather, your bog standard epoxy resin that you can get anywhere from any number of different brands or sellers none of which really matter since pretty much every epoxy resin I’ve used has been the same stuff across the board. Unlike the FlexIt 90 polyurethane resin that I use to cast action figures with moveable joints or specific parts for custom action figures, epoxy resin is by far the most common, widely used, and comparatively inexpensive type of resin that you’re likely to come across. Epoxy resin is also my preferred medium when I’m recasting miniatures

I’ll still be injecting epoxy resin into the molds even though I have a considerably longer working time (40 minutes rather than 3 minutes) since miniatures are tiny, highly detailed, and my pour spouts on the molds are quite small and given how viscous epoxy resin is, I don’t trust a simple pour to really get down into the mold cavity and fill everything up without the resin getting in its own way. So yes, injecting the resin with a 20ml syringe (or smaller) is the way to go here.

Since epoxy resin has such a long working time, it might not be a bad idea to fill some of the molds up most of the way, rotate the them around in your hand as if you’re rotocasting so that the resin coats the walls of the cavity, and then reinsert the tip of the syringe and finish filling the molds. Most of the time, you don’t really need to do this since you’ll be using a pressure pot, but every once in a while, you’ll get a casting with a weird air bubble even though you added vents and pressure casted it (usually around the area where a pour spout was connected). You might also occasionally get a mold that you filled up all the way, only to take it out of the pressure pot once its cured to find that the top part of the mold actually didn’t fill up all the way. This is usually due to air bubbles in the resin when it was injected getting compressed down to nothing while it was curing in the pressure pot and thus, leaving a mold that isn’t all the way filled up like you initially thought. Filling, rotating (and lightly squeezing), and then filling some more alleviates this relatively rare occurrence. So file another one in the “best practices” list.

Once all the molds are filled with resin, place them in the pressure pot. Much like my vacuum chamber, I cover the bottom of the pressure pot with a layer of parchment paper as well as a circular silicone baking mat to protect the interior from any spilled resin.

Stage 8: Pressure Casting

Here's my pressure pot and air compressor set-up. Ideally, you'll want an air compressor with a tank capacity that matches that of the pressure pot, but it's not the end of the world if your air compressor is a gallon or so smaller than that of the pressure pot. Mine is. It just takes a little bit longer for the pot to pressurize.

Place the lid on the pressure pot and secure it by tightening the four wingnuts. Make note of the port indicated by the red arrow in the picture below. This port is where the air hose attached to the compressor gets connected and it also acts as both and inlet and an outlet for the pressurized air to enter and leave the system.

Before going any further, confirm the red valve handle is in the upward or OPEN position as shown in the picture below. This opens the port to allow air from the compressor to enter the system and pressurize the pot.

Shifting our attention over to the air compressor that will be supplying the pressure pot, I’ve indicated two gauges. The one being pointed at by the blue arrow is the gauge attached to the regulator as well as the outlet port where one end of the air hose is connected. The regulator dictates how much air is getting pumped from the compressor, into the hose, and subsequently into the pressure pot at one time once the valve on the pressure pot is opened up. Since my pressure pot’s operating psi (pressure per square inch) is about 60 psi, I have the air regulator on both my compressor and my pressure pot set to less than 60 psi. Whenever my pressure pot reaches my pre-set regulator pressure of 55-58psi (as pictured below), it won’t allow any more air to pressurize the pot any further even if the compressor is running and the pressure pot’s valve is fully open. Similarly, when set using the red knob, the air compressor’s regulator will not allow any air past its maximum psi setting to leave the compressor and enter the hose.

The gauge attached to the compressor’s tank that is indicated by the red arrow is the overall psi that is present in the compressor. This pressurized air builds up in the tank when you turn the compressor’s switch on (after plugging it in, of course). It will keep building up until the needle on this gauge reaches the red marked area of the gauge, after which point the compressor’s motor will momentarily shut off but the compressor’s tank will remain pressurized.

The point in which the compressor stops ramping up and its motor shuts down is the point where you should attach the air hose to the pressure pot and allow the air compressor to pressure the pot until it reaches whatever maximum psi you have preset via the pressure pot’s regulator. You’ll hear the air flooding into the pressure pot as you watch the needle on the pressure pot’s gauge (picture below) steadily climb until it reaches its regulated stopping point. At this point, attach the air hose to the pressure pot.

Once the pressure pot gauge reaches its desired stopping point (again, I cast everything between 55-58 psi), move the red valve handle to the downward or CLOSED position.

With the pot pressurized and the valve closed, disconnect the air hose and shut off the air compressor. Much the same as the vacuum pump, you don’t need to keep the air compressor running while the molds are curing in the pressure pot. Just don’t move the air compressor while the tank is still pressurized. The pressurized air will bleed off on its own until the overall gauge reaches 0 psi, at which point you move it.

The only thing left to do at this point is to wait 16-24 hours (or whatever your resin’s cure time might be) for the resin to fully cure.

Stage 9: Demolding the Castings

Unfortunately, I forgot to take any pics of the finished Witty miniature and bonus castings fresh out of the molds with the air vents and flashing still attached. So here are some different miniatures recasts as they appear right out of the mold.

Use a good pair of snips/side cutters to carefully snip off the air vents (and the pour spout if one was present in your particular casting) and trim off the flashing with a sufficiently sharp Xacto or hobby knife.

Here are the finished castings after I snipped off the air vents and cleaned off (most) of the flashing.

Here’s the pièce de résistance of my little demonstration, the Survivor Witty recast. I’ll let the result speak for itself.

And here’s a closer look at our bonus SHIELD agent.

As well as this random Heroclix Mercenary miniature that was a dump mold from a previous silicone mold pour, who I picked to mold and cast largely because he’s also generic enough to act as a decent enough starting point for an original miniature kitbash (and thus I wouldn’t mind having more than one of him).

And that’s my process! To recap, we prepped a part for molding, created an efficient and leak-free mold surround, mixed and poured some silicone rubber, vacuum degassed the silicone, poured a mold, and cast some resin copies in a pressure pot. It’s a long process for sure, but it’s an immensely fun and rewarding one when you get it right.