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Debubblizing RTV mould compound


#1

I ordered some RTV silicone from Contenti to make molds of some
objects in white metal. I was told that RTV compounds are prone to
form bubbles and vacuum debubblizing similar to what is used for
investment is needed. Any suggestions as to time and technique?

Russell Trenholme


#2

Russell, mix the two parts in a container with a volume 3to 5 times
larger than the volume of the mixture. Place it under the vacuum
dome and vacuum until the volume collapses fully after expansion.
Then carefully pour into the mold frame until the model is covered.
Again vacuum until the expansion collapses fully then add another 1/2
to 1" of material to top off the frame.

You can use brushed on contact coat(use a soft bristle brush) on the
model and eliminate the vacuum if you do not have one. The body of
the mold will have tiny bubbles throughout and it will loose some of
its strength but the brushed on contact coat should eliminate bubbles
on the surface of the model. Brush on a thin coat of mixed mold
material to completely cover the model and work out any air bubbles
that might be on the surface or trapped in model detail. Then fill
your mold frame with the rest of your material. The big thing with
RTV silicone is to make sure you have NO oils on the surface of the
model or mold frame…and make sure your mixture is measure
accurately…that is the basics.

Luck
Frank Goss


#3

Russell,

mixing silicone components in a vacuum mixing bowl will avoid
formation of air bubbles in the liquid RTV. Getting rid of trapped
air bubbles sticking to the object requires some practice. Evacuation
of the mold with slight agitation, e.g. vibrating plate after pouring
is a good remedy in most cases. Tricky situations may be treated
successfully with curing under pressure (after evacuation) in a
suitable pot.

Sandor Cser,
Ti-Research


#4

Hi; you will need to make your frame as tall as possible because when
you mix it you will introduce alot of air that will need to be
"de-aired" and it will raise at that point you will have to release
the vacuum pressure so it doesn’t overflow like you do with
investment. but if you have a taller mold frame it will require less
up and down pressure releasing, usually after about 12 to 15 minutes
is enough for most jewelry items in standard frames. Don’t try to get
every bubble out becuse there will always be some that will be there,
however they will be super tiny once you take the vac pressure off
that they won’t even be noticeable. so let it go up and down with
pressure releases until it won’t over flow any more the let it go an
additional ten minutes and you will be fine, Also it is possible to
over vacuum, so long that the RTV starts to set up while under
pressure, and that is not good, remember once you mix in the catalyst
the clock is ticking and you want to get it into the vac as soon as
you can because the RTV will be the thinnest and easiest to de-air at
that point. you be fine…


#5

Russell, I have been using the Castaldo liquid pink RTV for most of
this year, and couldn’t be happier! I use the gallon size kit and
re-order it every three months or so, and learned the hard
way…follow the manufacturer’s directions!! Measure to within a
tenth of a gram or better, and vacuum debubble according to the
directions…less time in the vacuum and the bubbles hang on, and
more time results in the rubber breaking down (with resulting lousy
waxes!). I haven’t used Contenti, but tried a couple of other brands
and wasn’t too happy. I get no shrinkage from the liquid pink, and
the molds are an easy cut with great detail. I have no experience
with how long they last, but I have one rubber that has blown me
well over 250 waxes and did another 20 today with no apparent loss of
detail. So that’s my two cents…follow the directions! the
manufacturer spends a lot of time and money to help you get the most
optimal results from the product and it’s their loss if they steer
you wrong.

Tim Dwornick


#6

twic - we used a clear rtv from akron rubber did some experimentation
(of course) found that being frugal and carefull with the hardener
was to our advantage. we only vaccumed the compound once after that
we would set the mold frame up next to a desk lamp and the heat from
the bulb was just right, the bubbles would all rise to the top and
the stuf would harden up perfect every time some of you may scoff at
this account but i can assure all that it is true

best regards goo


#7

I thought I would add a little note on mixing RTV, this might have
already been written about but I have never seen it. When I mix RTV I
use a plastic sandwich bag, once the both materials are added push
out the air and seal the bag. Start squishing the bag to mix, I then
set it in a bowl to hold it up while I vacumn. It does not expand
much because there is not much air to begin with. Then I cut a corner
out of the bag and push the stuff out into the mold frame, (like a
cake decorating bag) when you are done throw the bag away no messy
cleaning.


#8
I have been using the Castaldo liquid pink RTV for most of this
year, and couldn't be happier! 

Tim and all using the Castaldo pink. Yes it makes great rather
inexpensive RTV molds, but don’t count on the molds made with it to
be around more than 2-3 years. It’s a urethane derived product and
seems to be very susceptable to post cure humidity. Your molds will
revert to a liquid state over time and there’s nothing you can do,
except keep them sealed away from humidity, to stave off this
change. This material seems to be a close relative of Smooth-on’s 724
and I’ve thrown out many, many molds over the years which were made
before discovering the downside of using this particular material.
There are other urethanes elastomers that are a little more costly,
but have much greater library lives. Also, you want to make sure you
have good ventilation if you ever do any burning to modify a mold.
724 has mercury in the formulation and I’m pretty sure the Pink does
too. Another tip about using it, and probably all other RTV’s, is
thoroughly, thoroughly mix both parts of the system before CAREFULLY
weighing out required amounts.

Frank


#9
we used a clear rtv from akron rubber did some experimentation (of
course) found that being frugal and carefull with the hardener was
to our advantage. we only vaccumed the compound once after that we
would set the mold frame up next to a desk lamp and the heat from
the bulb was just right, the bubbles would all rise to the top and
the stuf would harden up perfect every time some of you may scoff
at this account but i can assure all that it is true

Yes this is absolutely true you can accelerate the cure with heat as
a matter of fact you can cure an RTV mold faster than a vulcanized
mold if your model can withstand the tenp. I have fully cured RTV
molds in 1 hour, but most of the time we set them in a warm place and
cut them in the morning.

frankenstein


#10

I have been using RTV rubbers for mold making since 1974 and I have
never heard of this technique. Thanks Bill I HAVE to give this a
try.

Frank Goss


#11
I mix RTV I use a plastic sandwich bag...push out the air and seal
the bag... squish bag to mix.. set it in bowl to vacumn...does not
expand much... not much air...cut a corner out of the bag push the
stuff out into the mold frame...when done throw the bag away no
messy cleaning. 

Thanks Bill, that was such a great trick to share. I had used the
RTV only when forced to because it was such a sticky mess to clean
up. I felt that I was wasting a ton of time with the clean up. Great
idea!

Mark


#12

twic - we used a clear rtv from akron rubber the bubbles would all
rise to the top and Good morning Goo:

I highly value input from customers, so was pleased to see your
comments relative to bubbles and lamp.

Although we often recommend using light bulbs to accelerate cure, it
never occurred to me that the bubbles might boil out before the resin
has a chance to cure. The timing must be a bit tricky. We’ve not
tried this.

Can you tell me more about this method?

Are you still doing this?

Warm regards,
Bill Mull
www.zerodproducts.com


#13

I have put them on 300 degree vulcanizers and cured them in minutes,
with aluminum frames and glass sides, but I think it is better to not
do that. but sometimes you get a model in and you have to get copies
on trees and into the ovens that night so you can cast hand finish
samples and get them red labled out the next day. I used a shinitsu
platinum cure clear rtv.

Frankenstein