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Mold Making & RTV


#1

Hi John & Molders, We already know there are several “Karens” on
the list, but please be advised there is more than one "John"
also!.. Silicon RTV’s are expensive (Silastic) and sometimes
difficult to use. I prefer the polyurethane based RTV’s. A lot of
the molds sold as “Silicon” are actually a poly based RTV. I use
two brands. One is Castaldo’s and is a ten-to-one ratio mix, white
in color and works very well. (Come see me at the Castaldo booth
at the Providence MJSA EXPO, May 16th-19th. I cut molds for them
during the show. Bring something to mold!) The other RTV I use is
from an outfit called Synair out of Chattanooga TN. Theirs’ mixes
one to one and has a long working time. Both are much cheaper than
Silicon. All of the RTVs work best with a vac pump to get the
bubbles off the pattern. If you don’t have a vac pump use a paint
brush with short firm bristles to “paint” some of the mixed RTV
onto the pattern before pouring the rest in. This will cut cut
down on the number of bubbles on your piece. J.A.


#2

Hi Folks, I suggested that people visit me at the Castaldo booth
at the MJSA Providence EXPO and bring something to mold. I want to
clarify… I won’t be able to do RTV molds at the show, but I can
make a rubber mold at no charge. Hope to see some Orchid members
there! J.A.


#3

John A.,

Gee I wish Providence weren’t so far away. I’d love to come and
watch, listen and talk. I used a french base materials silicon
from a “local” mold supply house in San Francisco. They have a
number of hardnesses and temperature tolerances available. I have
used other manufactures silicon materials too (GE and Dupont) and
they all have similar short comings. One, they all will have a
material settle out of the A part (the 10 in the 10:1) that has to
be mixed back in for the material to work, and it is a royal pain
to get this mixed back in. Two, the activators/hardners for all of
these products are short lived (6 mo to 1 year) and you don’t know
they have gone bad until you have a major mess! Three, none of the
silicons I have used release surface bubbles well, and they all
will have internal small bubbles unless the material is vacumed.
Four, They all tear pretty easily.

Good points though are a GREAT wax finish, really
beautiful. They also release very well. The new products are
supposed to have a very long shelf life as a mold (and they do not
have the problem of cold creep that some other mold materials
have). They are pretty easy to mix (you can see fairly well if it
is mixed or not). Excess mixed material can be “held” in the
freezer for days and used in the next mold mix.

I am currently using Poly Sulfide mold rubber, that black stuff
that has a jumping range of about 20 feet when you walk into the
mold room. Reasons I like this product (Smooth-on) is it doesn’t
go bad, not in years and years (the pre-thickened P.S. will start
to harden after a year or two, getting a rubber scum on the surface
that can be a bit of a pain). It is pretty bubble free (vacuuming
is not necessary)and surface bubbles can be released pretty easily
with a shot of compressed air (not so with the silicon). There are
3 hardners slow, reg and fast that can be mixed to get what ever
speed is needed in setup (the fast sets in 15 -+ min at about
75F). Premixed thickened material can be combined with the thin
material to get what ever consistancy is needed. It release from
wax pretty well and I know of 20 year old molds that are still in
use. It is pretty strong/tear resistant.

A bit difficult to see is the activator is mixed in (it is all
pretty dark in color) and the activator does settle (i put it in
plastic mayo/mustard bottles with a couple of big ball bearings for
mixer balls and just shake it up. Easy to dispense too.). It does
have a problem with cold creep so it is imparitive that stored
molds have a wax in them.

So this is some of my experience. There are so ==many== different
mold materials available that it is impossible to try them all to
find out what will work for a particular need. I am always
interested in hearing other peoples thoughts and ideas on this
subject.

John

John and Cynthia/MidLife Crisis Enterprises
Maiden Metals/C. T. Designs/ Bloomin’ Wax Works. etc.

PO Bx 44, Philo
CA 95466
Ph 707-895-2635 FAX 707-895-9332

The playfulness of the Universe
is reflected in the dance of the stars!


#4

Hi John @ Midlife, Have you tried Castaldos’ “Liquacast” ? It is a
10/1 mix, whie in color with a high tear strength. Because of the
color it is easier to see when it is mixed. It works well with
"clean clay" for a two part mold set-up also. Does that black
stuff have an odor? I recall some dark brown or black stuff that
had a memorable smell. J.A.


#5

John,

Haven’t tried Liquacast. I generally buy the stuff in 5 gal units
and I havn’t found a local supplier. The P.S. I use has a bit of
an odor, but NOTHING like the P.S. from Swest or Rio. I have molds
curing in the room while I work and others who come by to visit or
work never make any comment about it. Thanks for the info…

John

John and Cynthia/MidLife Crisis Enterprises
Maiden Metals/C. T. Designs/ Bloomin’ Wax Works. etc.

PO Bx 44, Philo
CA 95466
Ph 707-895-2635 FAX 707-895-9332

The playfulness of the Universe
is reflected in the dance of the stars!