Large Scale Central

Taylor Tin Works Challenge build 2016

The MIK - Annual Build Challenge
#24

Well, This is about to turn into a mold making and casting tutorial. Those that already know/do may find this boring, but you just might find a tip or two.

I use AeroMarine out of San Diego for my casting supplies, Great company, great service. They sell professional products, not consumer grade, but the good stuff.

I will need to make 48 windows for the Taylor Tin Works building, Thats a lot, and the molds will have to stand up to repeated usage, so I flip for the pro stuff, there does seem to be a difference. Making the molds also has to done correctly if they are to stand up to re-peated usage. Since these windows are being cast in a two part resin, I can use the “Softer” Silicone RTV rubber. Great for details, and very flexable for part removal.

You need to design the mold with about 1/2" of rubber all around the part for flex and also strength to prevent tearing of the mold. For this I will design my Mold Box with enough clearance to make this mold last. 3/16" fiberboard sides will do nicely.

I used the pin nailer with 1/2" pins to hold the corners together. Same for the window divider box.

These were pined onto another bottom board to complete the mold box. The joints need to be tight so there is no escaping of the rubber.

Mold boxes can be made of most anything, as the silicone generally won’t stick to non-porus stuff. You can use card stock, cardboard, clay, and even Legos for the box.

These will be a single part mold, The resin will be poured into the cavity left by the masters, and basically just filled to the top of the mold. As simple as it gets. To keep the rubber from seeping under the pattern I will glue down the master with rubber cement, also it keeps it in the right place.

Glued the window frame down also. Next up it to fill the molds with Silicone rubber.

The Silicone is expensive and you don’t want to waste any, But once mixed that is it. But how do you know how much to mix? Here is the secret tip!

Yup. Not just rice but Sushi Rice. It’s hard and small grained, Just what we need. To find out how much silicone we need to mix up, all we need to do is fill up the mold boxes with the rice, and measure the amount of rice.

Now you know how much silicone to mix up. Neat trick huh…?

Tomorrow we mix some rubber.

#25

Cool tip for the right amt of silicone. I still have nt got this casting thing down yet…

#26

well, i am all eyes now.

(is it tomorrow yet?)

#27

Thanks for the molding class, Dave. Patiently waiting for the next session.

#28

Someday I may try my hand at this casting stuff. Looks easy enough if you know all the tricks (http://www.largescalecentral.com/externals/tinymce/plugins/emoticons/img/smiley-smile.gif)

#29

Today we make the mold. The RTV ( Room Temperature Vulcanizing) Silicone I use is a Professional product from AeroMarine. This stuff is a two part system. The Silicone ( the rubber stuff), and the Catalyst ( think of it as the hardener). The ratio for this product is 10:1, Silicone to Catalyst by WEIGHT. To properly mix to the right ratio you MUST have a Scale that weighs in Grams. Other manufactures have products that have a mix ratio by volume, 1:1 or 2:1 etc., and you don’t need a scale just measuring cups. We will weigh out what we need.

To get an accurate weight of the silicone, we place the plastic mixing container, with the mark of how much we need ( rice measurement ), on the scale and then zero out the scale with the mixing container on the scale.

Now we add the Silicone to the mix container, up to the line we drew from the rice level, and note the weight in grams of 176.

We now repeat the process for the Catalyst, after vigorously shaking the bottle for 1 minute. We add 17 grams to the plastic cup.

To aid in the release of the mold from the box, I now coat with mold release agent. A little goes a long way. You want a light coat, never a runny puddle.

Add the Catalyst to the Silicone, and mix together until you get a uniform color mix, no streaks. This Silicone has a “Pot Life” ( the amount of time that you have to work with it before it sets up to be to hard to be useful ) of 45 minutes, Plenty of time to get in the mold box properly. You want to be sure and stir together well, scraping the sides and the bottom as you mix. Now here is the clinker… Try not to stir in any air bubbles, or as few as practical.

Now is the time if you have a vacuum chamber to de-gas the mix, I sometime do, but mostly I don’t. BUT, I do let it set, and I bang the mix cup on the floor and work bench to assist with getting the air bubbles to the top. Not a really big deal because with this products 45 minute Pot Life the air rises to the top, and the top is away from the master part. You don’t want air bubbles trapped against your master.

Now we fill the mold box. There is a right way and a wrong way to fill the box.

WRONG WAY: Just dump it in on top of the master. Most likely lots of trapped air.

RIGHT WAY: You need to start the fill in the lowest part of the mold, away from the master, and then have the mix FLOW over the master. To help brake up any air bubbles in the mix, as you pour, slow down the rate, and rise the container up and away from the box, so that you have a small continuous narrow stream hitting the pile in the box. The stretching of the stream tends to brake up the bubbles on the way down.

Thats the hardest part. Now full, shake, jiggle, bang, tap, the mold box to assist the air bubbles to rise to the top. Set aside for a minute and tap and bang, drop, juggle again, and again, and again, until you feel confidant that none of the air bubbles are against the master. You will continual to see little air bubbles rise to the top. At the top is OK, on the master is not.

Now sit back and wait, 16-24 hours at room temp. Don’t try heating to get it to set faster, As it will change the consistency of the Silicone, Just go on to something else and get back to it tomorrow.

Tomorrow we de-mold and cast.

Dave's 2024 Mik Challenge
#30

Dave have you tried vibrating the mix before you pour? Helps the bubbles rise.

I’d suggest pouring in a circle than allow it to fold over in your mold, looks like bubbles form there. I had similar concerns pouring casting flasks. My vacuum table is spring mounted and a cheap battery operated motor with an out of balance flywheel stuck to it helped a lot.

Just a thought. Feel free to tell me to butt out, if’n I’m butttin’ in.(http://www.largescalecentral.com/externals/tinymce/plugins/emoticons/img/smiley-smile.gif)

John

#31

You make it look easy, Dave.

#32

i read in a german forum, that a guy made a three feet table for casting. two feet made of rubber blocks, as third an old electric shaver.

he wrote, that this vibration brings practically every bubble to the surface.

#33

Woah Dave, An ambitious build, but I guess It need to be so since you’ve had to supply a lot of tin to Challenge Town. I really like the lines of the building. It will look good with all those windows lit up. All those many, many windows…

#34

Back to the window making. 24 hrs later the molds have set and cured (the cured part is the important part). You want to remove the mold box from around the molded rubber, piece by piece, and then carefully and gently extract your original from the mold. Care here is the key, you don’t know how stuck the master is, and just pulling it out may tear the mold, gently pull the rubber away from the master and your good to go.

You will have some “Flash” to clean up around the edges and sides. I use tiny snip sizers for most work. A slicing motion with a #11 blade also works.

For the windows, I use a 2 part resin, that has a mix ratio of 1:1. 1 part “A” and 1 part “B” by volume. Equal parts of each. I highly recommend that you get prepared well before you start.

Here is what you need. Resin, two plastic cups to measure into the 2 parts of resin, marked with the same amount on each cup, label them “A” and “B”. A small mixing container, best to have flat bottom and straight sides.

Arrange your mold(s) so that it is easily worked and pored from above. Use a mold release to lightly coat the cavities. And I always have a spare mold that I can fill with any leftover resin so it doesn’t go to waste.

Oh yah, A stopwatch. I use my iPhone. Pot life is 3 minutes on this stuff. You really need to be organized and prepared.

Measure out the equal parts of A and B into the cups. Pour the A part into the mixing cup, Now prepare for the 3 minutes of stress. Do a Mental picture of whats going to happen in what sequence.

NOW PAY ATTENTION: As soon as you add the part B to the mix pot, the clock starts ticking, and the magic begins. This stuff goes from thin syrup to thick molasses in 3 minutes. You have to be quick if you are pouring into multiple molds as I am. Get your part B ready to pour.

As soon as you pour the B part, hit the start on your timer. No more pictures for a bit, as I’m way to busy to stop and shoot. Using a PLASTIC ( no wood) stirrer and mix for the first 30 seconds until you can’t see any separation of colors, And then start pouring into your mold(s) try not to get any air bubbles into the molds. Work quickly as the clock is ticking.

Here are my filled molds.

After a couple of minutes, you will see the magic start to happen, as the resin starts to set up.

Depending on the product that you use, and the temp, the set up and de-mold time will run from 10 minutes to over an hour.

Leave them alone, and just let the magic work.

When ready, gently remove from the molds. As easy as that.

Use a hobby knife, and some sanding to carefully remove the flash, and smile big. You just made your first resin casting. It really is easy.

Three down… 45 more to go… Who designed this building anyway?

#35

Who designed this building anyway?

I asked the same question when I was building windows for the train station (the North Conway station) that I never finished.

#36

Very nice.

#37

Cool tip about the rice. I will have to remember that next time. I’ve always has extra molds to make to pour the rubber into just as you did with the resin. Do you recycle your molds when they get worn out? I’ve cut up my old molds, and then used the old rubber to fill in the larger gaps in the new mold, thus cutting down the amount of waste.

#38

Good looking windows. I know it’s harder than you made it look, but I believe I may give it a go. Thanks for sharing the process.

#39

yes. i second that.

thanks for this well documented tutorial.

#40

I cast aircraft engines in 1:48 scale years ago. And its really that easy, and its difficult. The problem I had is I chose the wrong casting resin, and it didn’t set up in the really small detail areas, so I had to bake them in the oven before I demolded them. That had the added benefit of further vulcanizing my mold.

#41

These look as good or better than the polystyrene windows I bought a few years ago. Nice work!

#42

@Dan and Korm, Yes it is really that easy. A little attention to detail goes a long way.

Good point, David. I guess that I failed to mention,

A 2 part resin is exothermic. That means that when the two parts are mixed together they generate heat when they react to each other. It is this “Heat” that has to be there to get them to “Set Up” and get hard. If the part that is being cast is so small or thin that there is very little resin present, and with the mold absorbing part of the generated heat thru transfer, then there is not enough heat for the chemical process to occur properly. When this happens, you do need to heat up the mold to get the process to work. As to how hot? Well, that varies with the product you’re using, but 100 deg. F is generally enough to get it done.

Confession: Since, that it was brought up, I have been heating the molds for the window battens… I left it out of the “Tutorial” so as to Keep it simple, and not make it seem as it is way complex, and scare away someone from learning to mold their own stuff. Most common molds do not have the need to heat, only the ones that use very little resin, to create fine/small/thin parts. The batten mold did set on its own, but it took an hour + to set. With heating in the oven with just the pilot light for heat @ 95F, I could de-mold after about 25 min. Having to make 48+ of these that’s a BIG difference in time. One could achieve the same thing using a 150W light bulb in a small box, or even a child’s “E-Z-Bake Oven”. KISS principal comes into play, no need to make it more complex then it needs to be. Tip: If you have a mold that you know will need the touch of heating, Get it in the oven ASAP after pouring to get the reaction going from the get go. You can also pre-heat the mold to give it a kick start.

#43

Oh, is that all. With the resin I chose, I did the hit and miss method to determine that 20 to 25 min in a 200 degree oven did the trick. I threw out a few goopy failures before I hit upon the magic numbers. Different resins set up differently, and your mileage may vary.

Also with the resin I chose, I did the baking on a summer evening when I could have the windows open, because there was definitely an odor. And oh yea, my mom wasn’t home at the time.