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I know there are some peeps out here with experience in mold-making using two part silicon rubber. I am making a large building and I want to put some detailed windows in rather than just cut holes in the foam core. So, I built a 'master' window that I intend to make a mold of and cast repeatedly in resin. I have Alumilite's silicon (quick-set RTV) rubber for the mold and Alumilite's resin for the casting. I created the master, mixed up the rubber, and poured in into my mold box.

So, my question is: Should I put mold release on the MASTER before pouring the rubber into the mold box? The master is made primarily of cardstock (matte board) with a little plasticard. The cardstock is pretty porous, as its just thick, dense paper.

I didn't put mold release on the master... I poured the rubber into the mold box and left it to cure overnight. This morning when I got home, I took apart the mold box and the master was very difficult to remove from the mold. I never actually got it out of the mold, but that was because the mold tore above the master (I didn't pour deep enough). I don't want to destroy the master trying to remove it, but I'm not sure if I can salvage it at this point.

So, what do you experienced mold-makers say? Should I put mold release on all masters from now on? Any other tips?



Ghidorah

can you upload a picture of your mould?

what release agent are you using or going to use?

Ghidorah wrote:I know there are some peeps out here with experience in mold-making using two part silicon rubber. I am making a large building and I want to put some detailed windows in rather than just cut holes in the foam core. So, I built a 'master' window that I intend to make a mold of and cast repeatedly in resin. I have Alumilite's silicon (quick-set RTV) rubber for the mold and Alumilite's resin for the casting. I created the master, mixed up the rubber, and poured in into my mold box.

So, my question is: Should I put mold release on the MASTER before pouring the rubber into the mold box? The master is made primarily of cardstock (matte board) with a little plasticard. The cardstock is pretty porous, as its just thick, dense paper.

I didn't put mold release on the master... I poured the rubber into the mold box and left it to cure overnight. This morning when I got home, I took apart the mold box and the master was very difficult to remove from the mold. I never actually got it out of the mold, but that was because the mold tore above the master (I didn't pour deep enough). I don't want to destroy the master trying to remove it, but I'm not sure if I can salvage it at this point.

So, what do you experienced mold-makers say? Should I put mold release on all masters from now on? Any other tips?



Ghidorah

Generally speaking:

Always put mold release on the master, or any time you pour rubber or resin for that matter. Chances are good even if you used mold release, your master would be destroyed by absorbing the rubber - cardstock's a bad material to use for this.

Likewise, the alumalite stuff I've seen in past expands like a bugger, making your molds and casts weird. I really recommend smooth-on silicone rubber instead.

The cover on the foam is paper. It is porous and even if you put the releas on the paper, it will absorb the liquid and still have the chance of sticking to the mold. Anytime you work with porous material other than cast metal you should cover the master with a sealant first. An inexpensive way to cover or seal your master is to cover it with diluted white glue. Let it set then reseal the master again. This should protect the master and now use the release agent on the master and your ready to mold again. Good luck.

You need to seal the master first.

The silicon is sticking to the surface of the cardstock (more accurately, into/through the cardstock).

There are some rtvs that really like PVA adhesives - as in will form a chemical bond that is impossible to separate without destroying both mold and master. Something to bear in mind. They're not common these days, though.

Does anyone here use a vacuum to remove bubbles. How much vacuum is necessary? (vacuum cleaner, aspirator, real vacuum pump).

I know some people who use a pressure pot (normally used for paint spraying/pressure rolling), but no-one who uses a vac chamber.

Unless you are doing large pieces, it generally isn't worth the efforts involved.

Vacuum Chamber:


If you are going to use a vacuum chamber you need a pump that will reach at minimum 27-28 inches of mercury, or 1 Bar on a vacuum guage. The problem with pressure casting is that the pressure must be maintained for the entire time the silicon is setting up or the air bubbles in the rubber will swell up again and ruin your mold. Most hobbyist pressure pots will have difficulty maintaining their seal for the 24 hour period that most silicons require to cure. You can use catalysts to accelerate the curing time but that will shorten the life of you mold. De-airing your molding material avoids both problems and makes the silicon very dense and more tear resistant.

With vacuuming, the closer you can get to 30", the better. Most commercial gauge won't get to 29" unless you have a serious compressor with a Venturi Generator or a super vacuum pump. 27" or 28" is sufficient however. As far as setups go the a good compressor and venturi pump will come out cheaper and be easier to acquire. A good refrigeration 5 CFM+ vacuum pump will run anywhere from $300-$700. But you can get them a rebuilt one on E-bay for around $180. For your chamber I recommend a vacuum desiccator, its a standard piece of laboratory equipment and is their for far cheaper than a purpose built "hobby" vacuum chamber.


Pressure Casting:


If you do decide to pressure cast 40 PSI is more that sufficient, just remember that the pressure cannot fall while the silicon is curing. So, set the pressure gauge on your compressor to 100 PSI, and the gauge on the pot to 50-60 PSI. Leave your pot connected and the value open, that way if you have a slow leak the compressor will kick in and slowly refill the tank before pressure gets to low.



Hope that helps!

Thanks Sersi.

Well I couldn't resist so I have a vacuum pump on order 27.6" Hg max vacuum. Still looking at desiccators. Does anyone have any recommendations on how big of a desiccator to get? I am looking at one now that is 9.5" ID and 10+" deep. It is very project dependent, I can't think of any need to get anything larger than that at the moment though.