Help With Mold Making

[ramius-ii]

I need some advise on how to solve a problem making a mold. Each time I mix the rubber and activator and pour in into the mold form, there are tiny bubbles that form both in the rubber and around the part! I have tried different techniques from a slower cure time to CO2 to using my shop vac to create a vacuum. None of this works! Any ideas? Thanks in advance.

Ed

[JWLaRue]

Ed,

Are you seeing bubbles next

Ed,

Are you seeing bubbles next to your part once the RTV cures? I've not (yet) used a pressure pot or vacuum chamber for my RTV casting and have been able to avoid any bubbles.

My process is to only pour the RTV into the mold away from the part to be cast. This allows the RTV to flow over and on top of the part, which does not produce bubbles. Once I have the entire pour done. I then gently tap the entire form against the work table to help release any trapped bubbles, causing them to float to the top.

-Jeff

[scott t]

Saw a man pouring molds

Saw a man pouring molds up on a how we made the props for a movie.
He poured it from more than two feet above the mold form and said that doing so kept out air bubbles.
The RTV strung out pretty thin before going into the form.

Don't know if it will work or not.

Scott

[ramius-ii]

Bubbles

Hi guy,
Thanks for quick answers and suggestions! Since the mold is less than 1" in diameter, I did try using the shop vac as a vacuum source. The bubbles are very tiny and are both in the RTV and some form around the part as well. I cut some of the molds with an exacto knife and sure enough the bubbles are all over the place! I will try again using the taping method. If it is not successful, if I went to a pressure system, how much pressure (PSI) is normally used?

Best, Ed

[KevinMC]

G'morning Ed,

I cast all my

G'morning Ed,

I cast all my RTV molds in 60-70PSI of pressure. This may be overkill, but the pot could handle it so I figured I had nothing to lose.

KMc

[ramius-ii]

Thanks

Thanks Kevin,
I will have to go out and buy a pot!

Best, Ed

[Scale Shipyard / WLU]

Molds

Hi Ed:

How big of parts do you need to mold?
I have a couple of chambers I use here and I am about 1/2 hour from you.

E-Mail me
scalshpyrd@aol.com

[tmsmalley]

Hi Ed -
I usually use

Hi Ed -
I usually use the RTV from Micromark. I pour it low and slow and never have a bubble problem.
Best,
Tim

[ramius-ii]

Thanks Guys!

Thanks Guys!
You are all most wonderful! The parts are the nose cone and tail cone for the 1/100 scale torpedo so size wise, the nose is about .280" x .5". The tail cone is .280" x 1".
Lee, we met before several years ago, you have the very impressive shop and if I remember correctly, we did part of a television program in you shop!
I am waiting for one more part to arrive, then it would be great to do both at the same time. It would still be great if I could learn to do this myself, and there are some things I do not understand. For example, what is the difference between using a pressure pot verses a vacuum chamber? I have heard of the Mirco Mark RTV and which one?
Pouring the RTV from about 2' for some reason did not do very well and was certainly worth the try. If you use a pressure pot, and I only ask about this since I have a compressor and not a vacuum pump, would you put everything in the pot and apply pressure or only the RTV, remove it and pour it into the mold or does it matter?
Again, I wish to acknologe and thank all. You are all a great group!

Best, Ed

[KevinMC]

Hi Ed,

FWIW, I went the

Hi Ed,

FWIW, I went the pressure pot route because like you I already had the compressor, so to buy just a pressure pot represented a small investment.

My process has always been to mix the RTV, pour into the mold box, then put the whole deal into the pressure pot and hold under pressure for the full cure time of the RTV.

[pdoman]

Tny Bubbles...

This works for me:

1. Mix the RTV slowly - so no extra air is introduced.

2. After pouring the RTV into the mold box, poke around the master in the RTV with a toothpick or piece of wire to dislodge any bubbles. Then rubber band or tape it to something that vibrates for at least 10 minutes. Longer is better. I use the table of a Delta benchtop bandsaw or sander. The vibration causes the bubbles to rise up and out.

This method seems to work fine on small or infrequent casting jobs.

[mylo]

I can support Kevin's claims

I can support Kevin's claims about the pot.

I've had no bubbles in molds using the pressure pot set at 60 psi. The thing cost me $80. Already had the compressor, which is just one of those little 2 gal ones, works great. You are restricted to a mold box size that will fit in the pot though, which max is about 9" x 9" x 3". I would say that if you plan on frequently doing molds, the pot is worth the investment. I've mixed the RTV quite aggresively and created many, many bubbles, and poured into the mold, still no bubbles in the final cure. Very happy with the pot. ....plus.....if I ever need it for it's intended purpose, which is to pressurize paint for spraying, I've got it.

Mylo

[fx models]

Pressure pots are valuable but

Pressure pots are valuable but if you DONT have one ... here is how you can pour very nice molds. We do this here because sometimes our 12" diameter, 36" long torpedo tube pressure pot cannot handle the size of the mold required. We have a degasser too, which uses vacuum to pull the bubbles out of silicone and casting resins as well and when our compressor pump blew up, showering oil all over the shop, we couldnt use either the pressure pot OR the degasser and had to use the following method as well.

Here is what we do. We mix a small batch of silicone first and use a disposable brush and brush the first coat onto the master part. We do this until the part is completely and utterly covered with a very thin 'detail layer' of silicone that is bubble free. Inspection allows us to make sure absolutely NO bubbles make their way to the surface of the part. In this way we build up the layer little by little until its about 1/8" thick. Then after it begins to gel up [you can kick the cure a little if you want but that reduces the life of the mold a bit], we mix another large batch, and pour it in... nice mold ... Cure with heat for faster cure such as over a small space heater. No ovens.

Just another offering of assistance!

Marc

[ramius-ii]

Thanks Guys!

Thanks Guys!
These are are all wonderful ideas and suggestions! I actually bought a 2.5 gal pressurized can for paint, which was not expensive. A few fitting, modifications, and the compressor method works great!

Thanks to all, Ed