My First Custom Mold Experience: Disaster Ensues

I am aware that not everyone who reads this blog likes to make terrain as much as I do. I am also aware that many (most?) of you don’t even own Hirst Arts molds. That’s cool. I try and keep the arts & crafts stuff easy and attainable because that’s how I like my projects. Plus I’m hoping someone tries one of my easy projects, and discovers how much fun, and how much awesome, terrain is.

So that’s usually. Usually the projects I talk about are not complicated or “advanced.” However, today I am going to talk about an advanced project I did, and where I screwed up. I apologize to those of you who are not interested in the subject; perhaps some sense of Schadenfreude will keep you reading. I promise to try and  make it as entertaining as possible.

Whenever I start a new arts and crafts project, there’s usually a bit of excitement or brainstorming, and always a shopping trip. There is rarely a touch of dread and procrastination. This time was an exception to the rule. The source of the dread? The cost involved in doing this project, and the potential cost of messing up.

The shopping trip this time around wasn’t to the craft store as usual, but online. It was in response to an article over at Ben’s RPG Pile describing how to make custom molds out of Hirst Arts pieces. I clicked the link to find out how much molding silicone is. $100 a gallon? You’ve got to be kidding me. But… think of the time it could save. Think of the things you could do with it. (I’m not sure if those words were from the little devil on my shoulder, or the little angel.) So there I sat, credit card in hand, tapping it on the keyboard, trying to decide whether or not to pull the trigger. Then, like a bungee jumper falling forward (at least, that’s how it worked in my imagination) I hit ENTER, $100 out the door for a gallon of molding silicone. I’m pretty sure even Starbucks coffee isn’t $100 a gallon…

When the package arrived, it was promptly opened, but the bottles of the precious liquid were put on a shelf for a time when everything was “ready” and “perfect.” In truth, I think I was waiting to feel ready.

All feelings aside, I knew what my first project was going to be: floor tiles. Anyone who has cast using Hirst Arts molds know that floor tiles are tedious. The mold makes 7 squares at a time. And then you have to glue them together. To put that in perspective, the “Secret” (final) GenCon Fourthcore Deathmatch terrain I did last year required about three HUNDRED floor pieces. And it was big, but not huge; it felt like a small dungeon. So 43 castings for that piece alone, not to mention the other two terrains I did, and not to mention the other parts of the models (like… walls). Since floors are so tedious, and since you almost always make them in pretty standard sizes, they seemed like a good candidate.

So, I spent a couple of week’s worth of free minutes here and there casting floor tiles. Oh, and there was a shopping trip for extra supplies (Yes! To the craft store!). The article recommended a 12″x12″ surface, and to make the casting “box” out of foamcore, so that’s what I did – I was able to fit 100 floor tiles into that space. Four 4×4’s, two 4×2’s, and five 2×2’s. One hundred. No more casting 7 tiles at a time…I was getting excited.

But how much of the precious liquid to pour? Well, a 12×12 box that needs to be poured 1/2″ deep converts into almost exactly 40 ounces. So that’s what I shot for. Oy, $35 in one pour. But what could go wrong?

With all my papercraft experience, cutting up some foam core for a molding box was a piece of cake

Before I get into the ensuing disaster (that we all see coming), I will note a few ways that I deviated from the instructions presented in Ben’s article.

  • Instead of using plastic measuring cups, I bought disposable “tupperware” bowls with a 6 cup capacity. They were far cheaper than the measuring cups that were suggested, and held more material. They did require that I measure out water into the bowl ahead of time, and mark the required volume on the side with a marker.
  • While I did use masking tape to seal up my box, I also used some leftover bathroom caulk (instead of the recommended glue). If you have it, I highly recommend it; after all, it’s made to seal up the space between two joints.
The box is ready to go – taped and caulked the seams
The tiles get glued to the box to keep the silicone from seeping under them too much

Ok, so I took my marked bowl (marked at 40 oz!), put it on a scale, and filled it to the line with silicone. My heart was racing. Not because I was in new arts and crafts territory, but because there was freaking 35 dollars sitting in the bowl. “Don’t spill it Benoit.” I got the weight on the Silicone, and figured 10% of that, which is how much catalyst you need. The catalyst triggers the hardening of the silicone, and on a completely unrelated note, is Lich Purple (bonus points if you know what that is) and smells fruity. Dumped the catalyst in, and I was committed. Stir stir stir for 10 minutes, and let me tell you, molding silicone is THICK stuff. Marshmallow fluff thick. My arm was sore when I was done, and the mixture was no longer white, but a lovely shade of pink.

I actually paused to take a picture before I poured the silicone in

Then, the moment of truth, when all the preparations came together to the final product. I poured the thick mixture over my floor tiles and… the box filled up, but I had a whole lot of extra silicone left over.

Not pictured: A bowl with too much molding material left over

This is where things started to go downhill. Remember how expensive that stuff is? I sure did. I’m looking at the leftover silicone in the bowl and thinking, “Wow, there’s a lot left over here… I’ll bet there’s enough here for another small mold.” Here’s the problem. I wasn’t prepared to cast another mold. I had a couple of pieces ready, but I didn’t have a box ready. And this bowl of molding material has a fuse on it. A ticking timer. Crap. So, as the silicone is slowly curing in the bowl, I try to think of the fastest way to construct a box to cast something in. I could use more foamcore, sure, but I remember seeing Bruce Hirst’s tutorial on casting where he uses Legos. I have Legos…Legos are probably faster… so I take my container of Legos that I use to square pieces when I’m assembling models, and start constructing a box. As I’m constructing, a feeling creeps up on me… there aren’t enough. There aren’t enough Legos. I try to push the feeling away… of course there are enough Legos. There has to be enough Legos. Except, there aren’t enough Legos. Crap. Crapcrapcrap.

Back to plan A (plan A, if you’re wondering, is a hastily constructed foam core box). Of course, that makes it sound like I already tried plan A… at any rate, I’m feeling the pressure of the silicone curing, I can practically hear it, and I need to get this box made NOW.

“Honey,” my wife calls down the stairs to me, “are you ready to take the kids to the park for the community Easter egg hunt?”  ………… “Honey?”

Crapcrapcrapcrap. This just went from bad to worse. I explain to my wife (very calmly, and without a bit of stress in my voice, and I most certainly did not snap at her) that there’s a clock ticking, and to go ahead. I would catch up.

I get my box formed. No caulk this time, no time for that. Just tape. I place my piece in the box, and it fits perfectly, with just enough space all around it. Things are looking up. I pour the leftover molding silicone over the piece. Pour, pour, pour…it only covers the bottom half of the piece. I stare in disbelief. I frantically scrape the bowl out, and get about an eighth of an inch more.

“You have got to be kidding me.” Now this piece (which took me no small amount of time to construct) is basically ruined, coated in thick pink goop. I look around. There is no way this silicone is getting the best of me; I am going to find something to cast. Aha, another piece, this one much shorter and smaller… no time to construct a new box, so I sloppily pour the material from the box back into the bowl (well, most of it made it into the bowl) and wipe out the bottom of the box with a paper towel. Place the new piece in and pour… the box is too big. The silicone doesn’t cover the new piece either. Great, another piece ruined.

Knowing that discretion is the better part of valor, I decided at this point to admit defeat. I began angrily cleaning up the product of my haste: the various puddles of goopy, slimy silicone that now dotted my workbench, my casting table, and the floor. Behold, the trashcan post-Silipocalypse (as I have come to call it): 

If you look closely, you will see a box, an overturned bowl, a mixing spoon, and lots of wasted silicone. Well, you don’t really have to look too hard for the silicone.

In retrospect, the extra silicone wasn’t a huge deal. I mean, yes it was money down the drain, but I had kind of gone into the project with the mindset that there may be some money “spent” on “the learning curve.” I just forgot that in the middle somewhere.

What did I do wrong?

Quite simply, I had not accounted for the volume of the pieces being cast, and neglected to subtract that volume from the amount of silicone I needed to pour. Yes, I knew that tiles took up space, but I didn’t account for them because it didn’t seem like they took up that much space. After the fact, I calculated that the tiles actually took up 13 oz worth of space. Almost two cups. Ouch.

What did I learn?

Well, next time, I’m going to be more prepared in a few ways:

  • I will have a few small things ready to be cast in case I overestimate the amount of silicone I need. I stressed myself out when I decided at the last minute to try and use the leftover silicone on something else, and I had to scramble to put another molding box together. Had I been prepared ahead of time “just in case,” this project would have been a breeze.
  • I will try and slightly underestimate the amount of silicone I need, because if it turns out I don’t have enough, mixing up a little more only takes 10 minutes. Let’s face it, this stuff takes 8 hours to fully cure; it’s not going to set up in the 10 minutes it takes to mix up a little more.
  • And, on a related note, I will be more accurate in my measurements of the amount of silicone I need. While I had a good estimate of the volume of the molding box, I neglected to subtract the volume of the tiles. They seemed like they would be negligible, and thus didn’t need to be factored in, but in reality they took up quite a bit of volume.


So the next day, the mold was fully cured, and I cut the box open. The mold was quite a bit harder to separate from the tiles than I thought it would be, but with gentle, steady pressure, it did come free with no problems. (The same kind of slow steady pulling it takes to peel the label off something). Almost immediately, I cast my first batch of floor tiles from the mold. Below is the product:

The first batch cast from the new mold. Click for (really) big.

If you click on the picture, you get the big version; you’ll see upon close inspection a cluster of bubbles, and a few less-than-distinct lines between tiles. I’m not sure of the source of the bubbles, so those aren’t really a concern, and the lines I can live with. Bottom line? I’m very happy with how it turned out. Even if some silicone had to get thrown away.

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15 Responses to My First Custom Mold Experience: Disaster Ensues

  1. Amanda says:

    That was so cool of you not to snap at or yell at your wife in any way. After all I’m sure she had no idea that all that mayhem was going on. After all it was Easter weekend and I’m sure she wouldn’t dream that you would attempt such a feat on a holiday weekend 😉

  2. Toby says:

    You can get rid of the bubbles by vibrating the bottom of the mold after you’ve poured in your silicon. The process of pouring in the silicon from one vessel into another introduces bubbles which do not always filter to the top. If you have a hand sanders (like the craftsman Mouse) you can take off the sander pad and use that to vibrate your table space from below your mold. This will help the bubbles rise to the top and give you a cleaner mold.

    • Benoit says:

      When I said I wasn’t sure of the source of the bubbles, I meant that it may have been casting error. If the bubbles are in the silicone, it’s no big deal, but I don’t know how I could have stirred it any more gently – the silicone is so thick, it’s virtually immune to vigorous stirring.

  3. Adam Page says:

    when Mike Mearls offered me the chance to cast the Forge of Dawn Titan in 3d, I thought it would be simple. In the end, that was over 700 floor tiles, and god knows how many wall pieces. I ended up buying 4 floor moulds to get it done in time, and it was still nearly 4 week nights of castings. To do the walls and the circular platforms I too made my own custom moulds. The silicon in the UK wasn’t anywhere near $100 but I think I got a smaller amount for about £20/$30. I used a smaller amount of catalyst to give myself much longer to fix anything as I went along, but at the same time, it took about 3 days to set fully. I did one frame, for 7x 2 inch wall pieces, in lego and did nothing to limit the tiny cracks between bricks and lost about 5 to 10% of the silicon through them. The second one for the circular platform (taking 4 casts of the original moulds to get enough bits) I did in a Games Workshopminis blister, but totally misjudged the depth and wasted a load of silicon.
    One thing I found, was their was a tiny rim for the gap between the pieces being cast and the actual base, about half a milimeter, but enough to alter the size of the blocks when used in bulk.

  4. Ben says:

    God, I loved this article. Several times I was in stitches with laughter (only in a supportive way of course). You did great. The first of anything in terrain never goes right, A hobbyist is always learning.

    Time has a price. I know you want to be economical (who doesn’t) but some stuff is so worth it. You ultimately got more game time and family time by making that mold. Pack a work lunch for a couple weeks and you’ll nearly be even steven.

    Bravo. You took the leap. A new RPG medal is on your terrain jacket:)

    • Benoit says:

      Thanks. Yes, I do feel more accomplished now, and certainly less afraid of custom molding. The cost is no big deal, I rationilize it by telling myself that I don’t spend as much on minis as some gamers do. 🙂

  5. Tourq says:

    My problem is that I always try to go too big. My first attempt at simply gluing the pieces together resulting in a traumatic heaping mess that I probably have yet to recover from.

    Oh well, someday!

  6. Enygma says:

    Hey buddy I am glad you got to do this project. A few things to note for you to help you out.

    1. The work time for the silicone is 45 mins to pour. that means that if you dont apply it within 45 mins it will not set to the peices properly to get the ridges and indentions you need.

    2. I reccomended the cups for the same problem you had in the post. It made a great read btw. But if you use cups then you can pour 2 cups per wall mold or 1.5 cups per floor tile mold and have no waiste. But I also cast 6 molds at a time to avoid that issue too. But you did great for the first time.

    3. If you use the mold release suggested before pouring the mold on the tiles you are duplicating, then they seperate from the mold easier. Usually what keeps them stuck is some of the mold agent gets underneath the casted peices making it abit hard to peel off.

    4. The air bubbles if not from the original peice are caused from pouring to fast. Pour slow from each corner then in the middle. If you take some time to brush on the silicone to your casting before you pour, the air bubbles will be gone. The air bubbles wont appear from stirring, but will trap from the pour thats why the pour is so important.

    5. the lines are usually caused by not putting enough glue in the gaps of your original peices. However you can fix this problem by taking finger nail clippers to your new molds on the peices sticking up above the normal height of the seam where you glued the tiles together and trimming them down.

    6. the silicone is usually easier to clean once it dries as it peels right off of where ever your working. So the bowls should be reuseable. I know my cups are.

    all and all for your first time the mold turned out great. I hope my tutorial helped you and I hoped you learned aot from it, as well had a great time. It was a great read and a great post thanks for taking the time out to share. If you have any more questions or would like anymore tips feel free to get ahold of me. Ben has my number and your welcome to shoot me a text or a call if you need some help!

    Your friend

    • Benoit says:

      Thanks for the input, as well as the How To at Ben’s RPG Pile. It was *very* helpful as I was going through the process.
      I did use mold release agent, and didn’t really have *too* much trouble pulling the mold off, it was just harder than I thought it would be. As a matter of fact, this weekend I cast another piece, and forgot the release; it was *much* harder to separate. I also found that it is indeed easy to remove the dried silicone from the bowl and spoon; the first got thrown away in frustration more than anything.

      Good advice on the pour, I’ll try that next time. With brushing the material on – what do you use for that? I’m guessing that it ruins the brush…

      • Enygma says:

        Ya I buy the cheap 1 dollar pack for like 30 different size brushes at the dollar store or walmart. I know you have seen it before the bag of brushes. You can actually peal the latex out after it dries but sometimes you lose brush hairs. But at the same time you can use one brush for all your casts that day. So I found the 10 cents or less per brush was way worth it.

        Wait till you start mass producing walls too. The custom molds have made Bens @ and my life so much easier. Thanks for the plug to btw. Anytime you want to chat about terrain tips or what have you let me know. We should do a 3 way skype one day. I think that would be awesome!

        Until then let all your NPC hits be a nat 20!!!

  7. Ben says:

    Loved the epilogue. It looked like the final product turned out great. Did it set evenly? Sometimes you have to be careful on the mold so that your bottoms stay flat.

    • Benoit says:

      Yup, it set flat. I did have to round off the corners because the silicone “crept” up the sides of the form a little bit.

  8. Chris says:

    Silison rubber sticks to itself, you could have made smaller batches, and poured them in over a few days, pouring liquid silicon over the already hardened silicon levels. Good try though, and you’ve educated a lot of people, so it wasn’t a total waste.

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