Making RTV silicone molds.

[Galane]

Here's how to make bubble free RTV silicone molds and resin castings, without using a vacuum pump.

MAKING A TWO-PART RTV SILICONE MOLD

I've been making silicone molds and resin castings since 2004 and am willing to share what I've learned so others can skip the error part of trial and error.

Any questions, just ask. (Except for asking me to make molds or castings for you. Too busy to take on more work!)

 

[robn1]

Nice tutorial thanks, and welcome aboard!

 

[Galane]

I like to share what I know because I'm tired of seeing so many kits and bits that are full of bubbles because either the person making them didn't use a vacuum pump, or did and it didn't work well, or just did the mold and casting without doing anything to try and get the bubbles out.

Two things are big problems when doing RTV molds. Sharp inside corners are very tough to get silicone into, and for some reason on vertical or near vertical surfaces the silicone will hold small bubbles just beneath the surface of the mold cavity. When you pressure cast in a mold that hasn't been pressure or vacuum cast, you get bumps every place there's a subsurface bubble. If the resin breaks through the bubble you get little balls on the surface and they tear the mold when you pull the casting.

Vacuum casting the mold can work as well as pressure casting, but you need an extra tall mold form to contain the silicone as it foams up, and have to be 100% on sealing the master to the base for the first half, sealing the surface if the master is porous, and you need an expensive vacuum pump. Some tiny leaks don't bother much when pressure casting silicone. Of course it's always best to not have leaks.

I lucked out on getting a very expensive rotary vane vacuum pump used for only $100. Haven't used it for anything but air conditioning work a couple of times after giving up on trying to make good molds by degassing silicone but still getting bubbles.

 

[IEDBOUNTYHUNTER]

Vacuum casting the mold can work as well as pressure casting, but you need an extra tall mold form to contain the silicone as it foams up, and have to be 100% on sealing the master to the base for the first half, sealing the surface if the master is porous, and you need an expensive vacuum pump. Some tiny leaks don't bother much when pressure casting silicone. Of course it's always best to not have leaks.

if you degas the silicone before you pour into the mold you dont have that problem.

 

[Galane]

Tried degassing the silicone, still got bubbles in tight inside corners and deep details with sharp corners and subsurface bubbles.

Pressure casting the silicone guarantees absolutely no bubbles in the mold. Don't need extra tall mixing containers to keep the silicone from making a mess and you get more working time with it because there's no waiting several minutes for the stuff to degass. When I was trying that I had a big propane tank I'd pull a vacuum in. That was connected with a T to the pump and a valve between the T and the pressure pot I was using for the silicone. The propane tank was bigger than the pot so once the valve was opened the pressure in the pot should have quickly dropped to at least 1/2 ATM, plus I'd leave the pump running. Never worked worth a bleep. Always still had bubbles in the silicone even after 10 minutes in the vacuum, and after 10 minutes the silicone I was using was getting close to starting to set. That was General Electric 862 which is now 100% un-get-able. I've switched to Silicones Inc. P-60. Slightly stiffer but maintains a better mold cavity finish longer than the GE stuff.

I dunno when I got the idea to pressure cast the silicone, but since the first mold I did that way I've never given a thought to trying the vacuum again.

I have a variety of pressure tanks. One is 30" x 14" diameter and another is 13" by 14" diameter. The big one has a data plate from 1980, rated to 110 psi at 650F and the shorter one is from 1953, rated to 135 psi at 650F. 60 psi at room temperature is no problem for those tanks. I have a stand to put the tall tank on horizontally for molds and castings that won't work standing on end. For some big molds I've had to lay the tank down to make the mold then stand it up to cast. One such is a mold for 1963 and 1964 Buick Rivera park light lenses. That's the biggest casting I've done yet, over 300 grams of Crystal Clear 202.

The Harbor Freight tank and a similar one I have are rated to 80 psi. All but one of the others are rated to 60 psi and one is rated to 50 psi.

I use 60 psi or just under the rated working pressure, whichever is lower. I might turn all the small tanks down to 50 psi because I've had no bubbles in castings in that smallest pot.

I have a stand to put the tall tank on horizontally for molds and castings that won't work standing on end. For some big molds I've had to lay the tank down to make the mold then stand it up to cast. One such is a mold for 1963 and 1964 Buick Rivera park light lenses. That's the biggest casting I've done yet, over 300 grams of Crystal Clear 202.

 

[Ray22]

I thought degassing the silicone and puttting it in a pressure chamber was the same thing?

 

[IEDBOUNTYHUNTER]

Degassing the silicone means drawing the air out. pressure casting or applying pressure to curing silicone shrinks any remaining air while it cures.

 

[opal1970]

I am pretty new to casting/molding, but I have found that pouring the silicone from about 4 to 5 feet above the item and keeping the flow slow and very thin (thus most bubbles burst on the way down) tends to produce very good molds. I sometimes "pre-silicone" troublesome areas with a toothpick and make sure that silicone is worked in, although, in doing so you have to be careful not to create any bubbles.
This has made to-date very good molds, without degassing and/or vaccuming.

hope that helps,
Jason

 

[falcondesigns]

I am pretty new to casting/molding, but I have found that pouring the silicone from about 4 to 5 feet above the item and keeping the flow slow and very thin (thus most bubbles burst on the way down) tends to produce very good molds. I sometimes "pre-silicone" troublesome areas with a toothpick and make sure that silicone is worked in, although, in doing so you have to be careful not to create any bubbles.
This has made to-date very good molds, without degassing and/or vaccuming.

hope that helps,
Jason

It's called the "Bombs Away" method,been used for years.....

 

[tictoc]

I am pretty new to casting/molding, but I have found that pouring the silicone from about 4 to 5 feet above the item and keeping the flow slow and very thin (thus most bubbles burst on the way down) tends to produce very good molds. I sometimes "pre-silicone" troublesome areas with a toothpick and make sure that silicone is worked in, although, in doing so you have to be careful not to create any bubbles.
This has made to-date very good molds, without degassing and/or vaccuming.

hope that helps,
Jason

It is a good tip for those new to RTV molds.

 

[opal1970]

Thanks TicToc,

I have done quite a bit of trial and error, and have been lucky on most occasions. Aside from pouring a bubble-free silicone, (which decides in the end if the time invested was worth it or if it is time to go back to the drawing board), I think the most important part is having a decent plan to make your mold.

As far as materials go I use Lego building blocks, they come in a variety of sizes, making it easy to make a damn to fit any object and do not let the silicone leak out (even using the single row blocks).

In the begining I used wax paper to provide a bottom barrier, legos on top (back then I was using Duplo blocks... works but definatly not as good as normal legos), and a thin layer of Plastilina molding clay on top of the wax paper to make an air tight connection to the piece I am trying to copy. The configuration worked pretty good, but the Plastilina that I have tends to retain bits and pieces of silicone in it and is at times smeary. VERY important, when choosing a molding clay make sure it is sulfur-free, most of your "normal" clays have sulfer in them and this does not react good with silicone.

Since then I have replaced wax paper with "TetraPak" cardboard... not sure if it is called that in the US, but here in Germany most liquids i.e. milk, juice, right down to your tomato sauce is in a thin cardboard packages that have a plastic coating on the inside. I know it is not as common in the US as it is here, but I do remember seeing some items in the local grocery store being sold in these when I was in DC last year. Anyway, this works excellent... silicone does not stick to it, Plastilina does not stick to it... really a dream. I just cut up enough TetraPaks until I have a large enough work area for a given project, but I believe you can buy sheets of this if you want something without the fold marks.

For demolding I use Talcum Powder,,, to be honest I bought a spray can of release agent just in case, and have never used it. Talcum powder is awsome, it works great and is very cheap... doesnt get any better than that. If you do decide to use a spray, make sure to apply it very lightly, if applied to heavily you will loose some of your fine detail and you may get what is called "orange-skin" which will make the surface of your casts rough and bumpy like an orange. Anyway back to talcum, when applying, use a normal dark-haired painting brush, the kind you get in the watercolor sets... dip it into the Talcum bottle and apply very lightly to the higher areas first (as the powder falls from the higher points it will automatically coat the lower points and you won't end up with a ton of talcum powder at the base). I do not expicitly coat my legos but I do coat any molding clay that might come into contact with silicon. Before doing this, make absolutly certain that your mold, legos, and what have you not is completly dry. Talcum is a powder and will suck up any moisture and create big ugly clumps... my first mold the plastic was out in the cold stairwell and when I brought it in to mold... well you get the picture, beware of moisture. Lastly you have to get rid of the access talcum powder, if you applied it sparingly there should not be too much, nevertheless here is where I advise caution, I do not think that talcum is dangerous if you get it in your lungs, eyes, or any other openings, but play it safe...

1. if at all possible, use an air compresser to "lightly" blow away any access
2. Do this in a well ventelated room, or even better outside
3. Do not do this in a room with other people, childeren, animals.
4. Leave the room after your work is done to let any dust settle.

unfortunatly, just tipping it upside-down doesnt work.

all that being said and done, you are ready to start pouring silicone. Any experts out there, please feel free to correct me... as mentioned I am far from an expert on this topic, this is only what I have learned through trial and error.

Hope that helps,
Jason

 

[trooper]

to make sure you dont get bubbles i always paint on a small skin of silicone, when that sets up then i do a full pour. i never get bubbles in my molds. i do the same thing when pouring resins
but now that i have a vacuum pump and pressure chamber, i wont need to do this, it will speed the process up a bit.

 

[darthviper107]

So if you really wanted to make sure you don't get bubbles you can do it this way?

-Use vacuum chamber to degas the silicone
-then when you've poured your mold you can use a pressure chamber to further make sure you don't have bubbles

-then use pressure chamber when casting

Anyone know of a good place to get a vacuum chamber and pressure chamber?

 

[avianoguitarist]

Thanks for sharing that great info--it's sure to help many!

 

[Galane]

With a pressure chamber you don't need the vacuum. Pressure forces the air bubbles into solution in the silicone and resin, much like nitrogen in a diver's blood. It is very important to keep the pressure on long enough to ensure the RTV and resin is cured enough to keep the air bound up. I keep molds in at least 12 hours and resin at least 24, longer for slower setting resins.

The same tank can be used for pressure and vacuum by adding a tee and second ball valve between the tank and regulator and using quick connects on a vacuum and pressure gauge on the tank or just put a valve between the pressure gauge and tank to close when vacuuming. Close off the regulator valve too then open the vacuum valve and connect the hose to it.

Harbor Freight, Northern Tool and other importers of Chinese tools sell pain pressure pots that work well for this use. Unscrew the paint tube from the bottom of the lid and any fitting from the top paint outlet. Screw a pressure gauge into the paint outlet. Put a pipe plug into the spray gun air outlet on the regulator and a 1/4 turn ball valve into the air inlet. Put the gauge that comes with the tank into the third hole on the regulator if it's not already there. Use teflon tape or liquid teflon sealant on all this, don't get any into the regulator or gauges.

There are two main types of regulator. One is constant bleed where air comes out a small hole all the time. To use this type for pressure casting your tank needs to be able to hold air pressure at least 24 hours. Put the ball valve between the regulator and tank. If the tank won't hold pressure with the valve shut, you'll have to leave the valve open and air connected and a constant bleed regulator will run your compressor constantly.

The other type doesn't bleed off air so the valve can go on the inlet or between the regulator and tank. However this type can be harder to adjust because if you turn it up too high you have to bleed off air with the safety valve, turn the regulator down then adjust it back up as air is put back into the tank.
Some of these also have an air loss protection feature where when the hose is disconnected and the valve is opened, a little air wil come out then it shuts off. The safety valve has to be opened untill the pressure drops enough to open the regulator's internal shutoff.

When pressure casting I discovered I works best to crack the valve open and wait until it gets to at least 10 psi before fully opening the valve. Just popping it full open can force air through mold seams and through small holes in mold form joints, causing large bubbles.

Why you have a gauge on the regulator and one on the tank lid is the one on the regulator is to set the pressure and the one on the tank directly measures the tank pressure. Watch the tank guage while pressurizing! Keep a hand on the inlet valve and if the gauge goes any over the level you expect, shut the valve and check the regulator.

The Harbor Freight tank is rated to 80 psi. Possibly it could, but I bet the lid gasket wouldn't hold. 60 or even as low as 50 psi is enough, though I use 60 for silicone.

Finally, always always ensure your tank is fully depressurized before loosening the lid clamps! It's especially important with the pressure loss preventing regulators like come with the Harbor Freight pot.

 

[nick daring]

This thread answered some questions I had. Good info.

 

[robn1]

Since reading this thread I've been using pressure on my molds and it works great. I've never used vacuum, just pressure. I mix the silicone, pour it, and place in the pressure pot at 60psi. Every mold I've made this way has been perfect.

I use a Grizzly paint pot, I described the modifications here http://www.therpf.com/f9/pressure-casting-grizzly-paint-pot-conversion-184777/