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Silastic j


#1

Has anyone had experience w/ Dow Corning’s Silastic J RTV? I’ve
used it many times over the years and it’s been a great, although
spendy mold compound. Howver, the last couple of times, the cured
mold has been filled w/ bubbles-even the mold chamber itself-- to the
point where it is unusable.

I’m vacuuming the poured mold as I have in the past. Can anyone
help?

Andy Cooperman


#2
    Has anyone had experience w/ Dow Corning's Silastic J RTV? 
I've used it many times over the years and it's been a great,
although spendy mold compound.  Howver, the last couple of times,
the cured mold has been filled w/ bubbles-even the mold chamber
itself-- to the point where it is unusable. 

Hi Andy, When I use Silastic J, I vacuum the well mixed compound for
4 minutes. Then I pour it into my mold frame over the model and
vacuum this for another 4 minutes. During this second vacuuming, I
periodically release the vacuum to break the air bubbles and keep the
goo from overflowing the frame. I then leave the mold to cure for at
least 20 hours. I haven’t had any problems.

Are your “models” organic or made of some kind of material that
releases gases or reacts with the silicone mold compound?

HTH, Donna P.S. Even though the manufacturer says that Silastic J
should be used within a year, I’ve used up a gallon of the stuff over
a span of 7, yes 7 years. It was stored in a dark cupboard in an
air-conditioned room.


#3
Has anyone had experience w/ Dow Corning's Silastic J RTV?  I've
used it many times over the years and it's been a great, although
spendy mold compound . . .

Hi Andy; I’ve used Silastic, and while it’s a good product, it is
expensive, messy to work with, and you have to vacuum the daylights
out of it to get rid of the bubbles. I recently purchased a 10 pound
container of Castaldo’s new LiquaCast at about one fourth the cost of
Silastic. I love the stuff and won’t go back to the other products.
It’s easy to pour, easy to mix, de-bubbles easy, cuts easy and it’s
soft and pliable. I know, this sounds like a plug, but why not? It
works great and costs a lot less. I love it and you can publicly
flog me with a wet noodle if I’m not telling you the truth. I also
like the Castaldo Econosil for vulcanized molds. Much easier to pack
than conventional rubber, and no cutting, just pull the two halves
apart. I’ve liked the silicone stuff since it came out, but it used
to be a little pricey. Again, Castaldo came up with an affordable
version. Don’t know about the shelf life of a mold made with
LiquaCast, but the RTV molds I’ve seen usually outlive the vulcanized
molds. I know from experience that the silicone vulcanized molds
last longer than conventional rubber molds. There will be reasons why
people prefer one product over another, but I find that the Castaldo
products do what they’re supposed to and their prices are very
competitive.

David L. Huffman


#4

Andrew: I have been using Silastic for over 30 years and have never
seen or even heard of the problem you are having. You say you are
vacuuming as in the past. Have you checked your vac. guage to see if
you are truly pulling the correct inches of mercury?? Second, what
material are you moding could it be reacting with the silastic. I
doubt this seriously as almost nothing other than oil will effect the
stuff. Last is a suggestion to solve the problem. Pull the vacuume
and leave it sealed as the Silastic cures for about 8hrs… good luck
let me know if you get this problem solved and what the cause was, I
use a lot of Silastic RTV molds myself and have never seen this.
Frank Goss


#5

Andrew, here’s a couple thoughts. Perhaps your batch of Silastic is
past it’s shelf life. If it’s too old, or was stored at too high a
temp, it can become a bit more viscous, as well as not setting up as
quickly. that can make it hard to vaccuum. The other thought is
that we recently had some very hot weather. If you were experiencing
these problems during warmer than normal temps, be aware that silasti
set times are considerably reduced at elevated temps. I’m not so
familier with the silastic J, but have used a good deal of the
Silastic E. If you vacuum it too long, such that it starts to
thicken up a bit, then it becomes more and more likely that it will
become too stiff for the bubbles to fully be eliminated, and
vaccuuming only then manages to vaporize solvent componants, i think.
In any case, you then end up with worse bubbles than you’d have had
with no vacuuming at all. If your batch is still doing this, try
reducing the total time you’re vacuuming the stuff, and see if that
helps.

Peter


#6

Dear Andrew,

I suspect you are not doing something right. The Silastic products
line are the most consistent mold products I have ever used. I
started with the E RTV in 1977. Now I am using the L RTV for most
ring molds and the J RTV and M RTV for stiffer molds for class rings.
If you haven’t tried the L RTV it is slightly less rigid than the J
RTV. Great for rings with under cuts and hard to reach places. I do
love Castaldo also but not for liquid silicones.

Make sure you see the compound drop in the vacuum chamber. This is a
visual process check. Once it drops for small batches wait two to
three minutes before the next step. With larger batches give it five
minutes in the vacuum chamber. After pouring revacuum and bring the
vacuum up and then release it at least ten times.

Here may be your answer: Was it a whole batch or just one mold? Many
times when I make a batch of ten molds I will scrape the last of the
silicone out of the mixing bowl. This last mold many times has
bubbles in it. If you scrape the silicone excessively when you pour
you can add bubbles to the mold. That last scraped mold sometimes I
will vacuum twenty of thirty times (breaking the vacuum seal quickly)
and still have some bubbles. Not usually in the whole mold.

Bubbles in the entire mold would tell me you did not vacuum it right
the first time. When was the last time you serviced your vacuum pump?
I see molds like this in my mold class and it is most of the time
from not getting the first batch to drop.

Best Regards,

Todd Hawkinson
TR the Teacher
T.R. Hawkinson, Ltd.
http://www.trhawkinson.com


#7
   ...the last couple of times, the cured mold has been filled w/
bubbles... 

Is your vacuum still pulling the same as it was before?

The RTV we use sometimes has to be vacuumed for several minutes
longer in the mold frame than the instructions say. Silastic J has
around the same pot time. You might try an extra 3-8 minutes of
vacuum. You may be able to coax more bubbles to the surface by
turning the vacuum off and on several times during the vacuuming of
the mold frame.

Dana Carlson


#8
    Andrew: I have been using Silastic for over 30 years and have
never seen or even heard of the problem you are having. Frank Goss 

Hello Frank and Andrew; I remember once using silicone mold release
spray on a mold frame when making a Silastic mold and having the
problems of bubbles. It was obvious that the mold release was
creating the bubbles as you could see a concentration of bubbling
where the compound met the surfaces of the mold frame, especially in
the little seam where sprue former attached to the frame. I’ve never
tried the spray with an RTV mold again.

David L. Huffman


#9

I’m not sure that leaving the vacuum sealed is such a good idea, as
when I have done this in the past the oil of the vacuum pump
eventually pulled up into my chamber, creating a big mess. I’m not
sure that all pumps will do this, but you should be aware that it can
happen.

John


#10
 Don't know about the shelf life of a mold made with LiquaCast, but
the RTV molds I've seen usually outlive the vulcanized molds. 

Hi David, the following info on LiquaCast RTV molds was copied off
of Castaldo’s website
http://www.castaldo.com/~english/usinprod/liquacast.html

"Do not allow molds of one type of rubber to remain in contact with
those o f other rubbers as migration of oils or plasticizers from one
to another can cause swelling, shrinkage, or distortion.

If the mold is left in a distorted shape for any length of time it
can take a permanent set and may never recover to its original shape.

Cured LiquaCastAE molds will slowly soften with age due to ambient
moisture in the air. Proper mixing of components in the correct mix
ratio contribute s to longer mold life. Storage of molds in a dark,
cool, dry area will prolon g life, as will reducing contact with moist
air by enclosing stored molds in tightly closed plastic bags. Wax or
metal patterns should be stored in the molds to maintain their shape.
With proper care, molds should not soften appreciably for 2 - 4
years."

It’s important to heed these storage directions for LiquaCast molds!
If stored with gum rubber molds you can end up with unusable
LiquaCast molds and a huge gooey mess.

I use urethane molds (such as LiquaCast) when I want an affordable
mold of a wax model. I almost always take a second mold (vulcanized
rubber or silicon e mold) off the finished cast master. I don’t count
on the urethane mold bein g usable in 4 years. They get soft and gummy
over time. For a mold that will last me years longer I choose gum
rubber or silicone. HTH Kate Wolf in Portland, Maine - check out our
fall workshops! http://www.katewolfdesigns.com


#11

Hi Kate;

Thanks for that very important on storing molds. I keep
my molds in plastic cabinets, each mold having it’s own little
plastic drawer. I put a couple injected waxes in with each mold for
quick access to a copy of what the mold is for. About 1/2 my molds
are made from vulcanized sheet rubber, the other half vulcanized
silicone. I only have a few RTV molds, since up until now, I’ve
usually only used RTV to take molds directly from wax models. I like
working with the LiquaCast enough that I’ll probably use it more in
the future, so I think I’ll take your advice and store them in little
ziplock plastic bags. By the way, the regular vulcanized rubber
molds tend to get stiff and a little dry after a couple years.
Would it help to store these in bags too?

David L. Huffman


#12

Hi David, Just a warning note. I did an experimental mold with
LiquaCast, and found it to be a very nice RTV, but when I stored the
mold on my shelf in one of those black plastic trays, it reacted with
the plastic and firmly welded itself to the bottom of the tray. Be
careful where you store them!

Jack Reisland