Randall . on sat 19 feb 05
I have more or less finished a little pictorial/tutorial of the
rubber mold making process and placed it on my web site here, enjoy!
http://www.lostnewyorkcity.com/mold-tutor.html
Randall
Kate Johnson on sat 19 feb 05
>I have more or less finished a little pictorial/tutorial of the
> rubber mold making process and placed it on my web site here, enjoy!
>
> http://www.lostnewyorkcity.com/mold-tutor.html
Randall, thanks for the step-by-step on the process...the outer supporting
mold of plaster is brilliant. Bravo!
On smaller rubber molds, I've incorporated cheesecloth into the layers for
additional strength--this works well and offers some support, but molded
items still want to sag and become slightly mis-shapen on occasion. The
plaster support would solve that (but add to storage problems.)
As I say, I've only made very small molds--I had an antique artist's
porcelain slant well palette from the late 18th or early 19th C. that I was
thinking of reproducing for sale. Too much work, so I have a few I use at
demonstrations. I wonder if the plaster support would help the process?
You don't mention what you do next. Are your molds intended for plaster
forms, or clay that will be fired? I found that clay in the rubber mold was
VERY resistant to drying sufficiently to remove cleanly.
Best--
Kate
Robert Seele on sat 19 feb 05
Another item that I have incorporated with rubber molds to give
additional strength is panty hose, strong & flexable, like the
rubber.....
On Saturday, February 19, 2005, at 09:14 AM, Kate Johnson wrote:
> On smaller rubber molds, I've incorporated cheesecloth into the layers
> for
> additional strength-
Bob Seele
Nicely outside of Chicago
Things should be made as simple as possible, but not any simpler :
Albert Einstein
carol ross on sat 19 feb 05
Randall, thanks for the photos showing the rubber mold process. Very
thorough - I understood every step but the last and have a couple of
questions.
I've made latex molds, using sawdust in the later layers to thicken it.
A lot like the Cab-O-Sil that you used. Once I had the first few
coats and knew that I'd picked up the detail, I started making the
applications thicker until the exterior of the mold had no undercuts.
Just a big mound of latex and sawdust. Instead of making your 3 or 4
part mold to accommodate the undercuts, I was able to make a
single-part plaster mother mold.
I wonder which way is more efficient? From the last photo, I'm
wondering if the 4 part mold now stays as one piece or does it still
come apart? Sort of like making a grandmother mold for your plaster
mother mold! Also, the rubber degrades more slowly than latex? Is
that why you used it?
Thanks again!
Carol Ross
http://homepage.mac.com/portfolio.carolross/Menu4.html
On Feb 19, 2005, at 9:14 AM, Kate Johnson wrote:
> I have more or less finished a little pictorial/tutorial of the
> rubber mold making process and placed it on my web site here, enjoy!
>
> http://www.lostnewyorkcity.com/mold-tutor.html
Randall . on sat 19 feb 05
At 9:14 AM -0600 2/19/05, Kate Johnson wrote:
>>I have more or less finished a little pictorial/tutorial of the
>>rubber mold making process and placed it on my web site here, enjoy!
>>
>>http://www.lostnewyorkcity.com/mold-tutor.html
>
>Randall, thanks for the step-by-step on the process...the outer supporting
>mold of plaster is brilliant. Bravo!
Thanks Kate!
>
>On smaller rubber molds, I've incorporated cheesecloth into the layers for
>additional strength--this works well and offers some support, but molded
>items still want to sag and become slightly mis-shapen on occasion. The
I've never added anything in the rubber, some rubbers may not "soak"
some fabrics or the like and the result is you have a rubber mold
that comes apart in sandwich like layers because the adhesion between
the coats and fabric wasn't good. Some use chopped fiberglass strand,
some use open weave fiberglass cloth, even burlap has been used.
On such molds you might try the Polytek 74-40 pourable rubber to make
a self supporting block, I have two of these, they are ideal for
small medallions, plaques and flat items. You need to make a little
form around the item to be molded and pour the rubber in. For larger
pieces it takes a LOT of rubber to do that and that's $$
I thought, or took it for granted that everyone knew about the
plaster - mother mold supporting shells, it's a very old method and
I'm assuming it's really a take from the much older plaster piece
molds for pressing clay or pouring slip processes. The plaster shell
in many larger studios is these days replaced by lightweight
fiberglass or resin shells that are usually sprayed on but I don't
plan to make the change.
>
>You don't mention what you do next. Are your molds intended for plaster
>forms, or clay that will be fired? I found that clay in the rubber mold was
>VERY resistant to drying sufficiently to remove cleanly.
>
The rubber molds are used to cast concrete and hydrocal, however, the
rubber molds can ALSO be used to pour the Polytek pourable rubber
into so as to make a positive RUBBER casting, with that you can do
several things, but one use would be as an intermediate (when you
don't want to damage an original and there are some undercuts)
towards making plaster piece molds for pressing or pouring clay. I
have done that in one form or another with a Manatee sculpture and
standing dog sculptures, it takes basically making a mold of the
original sculpture, making a rubber positive cast, taking that and
making a plaster piece mold of it, then taking each of those plaster
pieces and making rubber molds of those.
Some of these steps may not be needed if the original is "disposable"
anyway like plasticene or wax, but if it was a delicate antique with
undercuts, you need to do the full job- basically making a copy of it
in rubber and using/abusing that.
Basically those commercial plaster molds you buy for clip casting are
made in rubber molds, but it's a bit of a process to get to that
point, once there though making new replacement plaster molds for
slip casting or pressing clay is as simple and fast as pouring the
plaster into the molds.
It needs to be done accurately and with a rubber that doesn't shrink,
especially if it's a multiple piece mold- otherwise the pieces won't
fit well and leak.
At some point this summer when I make a mold to press clay into with
all those above steps, I'll add to the tutorial page, it'll be a
while before I do it but it's on the agenda with two pieces I have
that have minimal undercuts and will work well.
Randall
Randall
Randall . on sat 19 feb 05
At 4:26 PM -0600 2/19/05, carol ross wrote:
>Randall, thanks for the photos showing the rubber mold process. Very
>thorough - I understood every step but the last and have a couple of
>questions.
Welcome Carol,
>I've made latex molds, using sawdust in the later layers to thicken it.
> A lot like the Cab-O-Sil that you used. Once I had the first few
>coats and knew that I'd picked up the detail, I started making the
>applications thicker until the exterior of the mold had no undercuts.
>Just a big mound of latex and sawdust. Instead of making your 3 or 4
>part mold to accommodate the undercuts, I was able to make a
single-part plaster mother mold.
>I wonder which way is more efficient?
Carol, it depends on both the piece and the rubber, some rubbers are
very stiff, some are spongier but easier to tear. Soft and spongy
rubber = less tear strength.
Latex is extremely strong, you can't tear it by hand, but there are
many drawbacks, one being it doesn't "build up" by itself unless you
add a filler- in your case sawdust, I've used Cementex Latex rubber
and some of their filler for it that was a powdered ground up pink
rubber powder. The problem with latex and filler is it dries and
cures by air circulation not chemical curing as the 2 part rubber
does. Years ago when I used latex and did what you do to fill in all
those nasty deep undercuts, the draw backs were it took forever and a
day for the rubber to cure, and some times it still remained
marshmallowy in those thick sections because the surface cured and
the liquid and filler inside was trapped. The other issue was it was
very inflexible there due to the thickness.
If your rubber is thick enough to counteract the weight of the
casting material filling the mold, you can put plasticene into those
deep undercuts, make the plaster shell and there will just be a
cavity there where the plaster shell is not supporting that small
area of the undercut. Of course you have to know your rubber and end
use WELL to determine just how thick the rubber does have to be so it
won't BULGE when the liquid hydrocal etc is filling the mold. Some
pieces like lifesized standing statues this can't be done because the
weight of the casting media will bulge the rubber anywhere there is a
gap between the plaster shell and the rubber, so it all depends on
the design and your rubber.
From having done both, in my opinion it's better to have the multiple
piece plaster shell than chance problems. The plaster shell takes no
time to make, you just mix the plaster thick and start trowelling it
on with a tool, working on it till it sets. I did the 4 pieces of
that mold and the 4 pieces for the other mold together. The way I did
that was mixed up a batch, apply the first section quickly and found
I had enough left over to make one of the small sections on the other
mold so I made that too.
Same with the second section and third section. The last section (the
base) was made individually.
Both of the plaster shells probably took at most 2 hours start to
finish and that includes clean up.
Soon as the plaster sets and gets warm, I coat it with Murphy's soap
and start on the next section.
> From the last photo, I'm
>wondering if the 4 part mold now stays as one piece or does it still
>come apart? Sort of like making a grandmother mold for your plaster
>mother mold! Also, the rubber degrades more slowly than latex? Is
>that why you used it?
>Thanks again!
The 4 parts all come apart every time a cast is made, the "base"
section rests pretty permanently on the bench and is levelled up with
wedges and shims so when the mold is poured the hydrocal comes to the
top of the mold nice and level all around. It's made so when banded
with a nylon mold strap, I can pick up the cast, rubber and 3 of the
sections all together in one bundle leaving the base on the bench,
and then take it apart on another table and demold the cast.
Latex deteriorates fairly fast, and wickedly so if copper or brass is
molded with it, but more than that it shrinks badly and this starts
the day it's removed from the original model. I had molds that were
24-36" long that had shrunk 1/2" or more, I believe the shrinkage
rate is about 1/4" per lineal foot. That can be significant because
it means the rubber no longer fits snugly in it's supporting shell
(be it plaster, resin or fiberglass) which does NOT shrink. The
results are the casts come out warped looking, the formerly straight
sides and all are now bowed outward.
When I see casts like this on Ebay, I can always tell it was latex
mold rubber that was used!
Latex also takes coat after coat after coat of it and long dry times
between them, I used to have to apply 20-25 coats to get a mold
(fillers by the way increase shrinkage) it would take 2 weeks to make
a mold depending on the temperature and how fast the stuff dried.
Trying to force dry the latex with a little warm air created gas
bubbles or trapped moisture that lifted the rubber clear off the
model in pockets. I have a large wood section that was coated with
latex, it started to shrink before the mold was even done, enough in
fact that the rubber peeled off the varnished smooth wood carving, so
it was useless.
The 2 part rubbers don't shrink, and I can make a mold in one
afternoon, cure over night and use it the next morning.
I'm still working on the layout for that page, but it's working.
Randall
Randall . on sun 20 feb 05
> This remains light and spongy so it doesn't hinder removing
>the castings (in my case of fragile wax) and offers good support.
Hi Debbie,
Yes, in your case liquid wax is not likely to distort the mold
rubber, but if it was a 2' tall standing statue mold you were filling
with concrete or hydrocal you would have some issues, like I pointed
out it all depends on the model and the rubber what you can do or get
away with.
My very first plaster mold when I was 16 was a big flop, doesn't take
much of an undercut to really mess you up!
>Large light weight low volume mother molds:
>For mother molds on large pieces were weight and storage volume is a
>problem, I use Forton MG. This special gypsum product is formulated to use
>with fiberglass matt and will make a mother mold stronger than plaster at a
>fraction of it's thickness! (1/2" - 3/4" for very large pieces) Takes a bit
>more time to make (laying up the layers) but it's worth it for me on pieces
>2' and larger. And it's not toxic! I also use Forton MG for large plaster
>press molds for ceramic work.
Oh boy, I wouldn't have fiberglass anything in my house, not even the
fiberglass insulation, the stuff is in my opinion the next asbestos
class action lawsuit in the wings;
http://www.sustainableenterprises.com/fin/victims.htm
The Markel family suffered greatly because their air conditioner,
immersed in an attic full of fiberglass, spread the tiny fibers all
over their home. The family's two boys, ages 4 and 6, appear to have
permanent respiratory disease.
# The Branstetter family of Ft. Myers, Florida, had an air
conditioning system that was so leaky, the air handler unit became
literally packed full of fiberglass. The daughter, now 15, is
diagnosed with respiratory disease.
# The Branstetters settled their fiberglass injury case. Although the
settlement was for $125,000, much of that will go to the attorney and
the family still has a sick daughter and a contaminated house. "How
do you figure I won?" asks Cheryl Branstetter.
Red Randolph worked for 30 years installing fiberglass insulation.
His attorney calls him a "pulmonary cripple."
http://www.sustainableenterprises.com/fin/News/uneasy.htm
A few of the guys started talking about "pinklung," likening it to
coal miners' black lung or cotton-mill workers' brown lung. But some
of the other guys said that was a lot of crap, that maybe the dust
was bad for you but nobody knew for sure. The company had been
providing face masks and respirators for years, though most people
didn't use them. Especially the ones who already had trouble
breathing.
'
Fiberglass dust had caused cancer in laboratory rats and hamsters,
and there was some evidence that it might be responsible for higher
cancer deaths among workers in factories like the Owens-Corning plant
in Newark. The World Health Organization had declared fiberglass to
be "possibly" carcinogenic in 1988. Based on that finding, American
insulation manufacturers were already labeling their product as a
"possible" carcinogen. So to some of the scientists, the decision
seemed annoyingly obvious.
This time, there was intense discussion, but no more delay. When the
question was called to a vote, the six voting members of the panel
made it unanimous: Yes, fiberglass was "reasonably anticipated " to
cause cancer, and the United States government should say so.
Like asbestos, fibrous glass is potentially lethal only when its most
microscopic fibers become airborne and are inhaled. Scientists
believe that only the tiniest of them pose any danger -- fibers less
than about 3 microns in diameter but more than about 5 microns in
length (the diameter of a human hair is about 70 microns). Only those
fibers -- considerably less than 5 percent of the total -- are
sufficiently aerodynamic to be inhaled deeply enough into the lungs
to cause cancer.
====
>
>Randell, excellent web tutorial! Thank you.
>I'd like to ask you about your experience with long term storage on the
>polytex RTV products. How long do these hold up on average w/o loss of
>quality? I use a different type of RTV and am looking for longevity of 10 or
>more years in storage with light use. Thanks!
>
>Best Wishes, Debbie
>
Glad you enjoyed it, I have been absent from casting hydrocal and all
that for maybe 12 years and only recently started up again. Back then
I used Polytek 74-40 pourable to make molds of my standing dogs for
bronze, I still have them, good as they were when new. Seal the
plaster shell where it contacts the rubber and store them in the dark.
I did my waxes a little differently, the models were plasticene 8"
tall standing on all four legs on a textured base, lots of details
and undercuts. I made a multiple piece plaster mold- usually took
about 8-12 pieces, then made Polytek molds of each of those pieces,
and then I could cast waste molds out of 50/50 sand/plaster, assemble
the little pieces together which were well soaped and pour the wax
in. I used 50/50 sand so the plaster would be weak, then it was a
fairly simple process to break apart the plaster pieces from the wax
casting inside. I always had them cast completely solid not hollow.
Here's a photo of one in bronze, 1993, recently re-acquired from the
customer who lost his job and had it for bid on Ebay;
http://www.lostnewyorkcity.com/forums/uploads/post-3-1108946464.jpg
And a developing Gt Dane model in plasticene, 1993;
http://www.lostnewyorkcity.com/forums/uploads/post-3-1108946504.jpg
The plaster piece molds worked pretty well for wax casting, even the
Dane with it's fragile tail came out perfect and I never had to
attach separate pieces of wax for the tails, ears etc, they were all
cast in wax in one piece.
For those who like ceramic dogs, I have a small collection of Beswick
china (England) and a few photos of some;
http://www.lostnewyorkcity.com/forums/index.php?showtopic=95
--
Note: This email address is set to delete all email to it unread as
spam. To contact me you can mail
lostnewyorkcity at aol.com
Debbie on sun 20 feb 05
Enjoying this thread that is more in line what I do. I'm a bronze sculptor
and make lots of rubber molds!
I'd like to share a couple of tricks I've found helpful.
Undercuts on outer surface of the rubber:
One way that I use to get rid of undercuts on the out side of the rubber (so
that the plaster mother mold can have less or no sections) is to fill the
undercuts with foam. This foam, the type used for upholstery, is torn up
into tiny pinch sized bits, mixed with just enough rubber to cover them then
trowled into the undercut. A smooth layer of rubber can be put over it's
surface later. This remains light and spongy so it doesn't hinder removing
the castings (in my case of fragile wax) and offers good support.
Large light weight low volume mother molds:
For mother molds on large pieces were weight and storage volume is a
problem, I use Forton MG. This special gypsum product is formulated to use
with fiberglass matt and will make a mother mold stronger than plaster at a
fraction of it's thickness! (1/2" - 3/4" for very large pieces) Takes a bit
more time to make (laying up the layers) but it's worth it for me on pieces
2' and larger. And it's not toxic! I also use Forton MG for large plaster
press molds for ceramic work.
Randell, excellent web tutorial! Thank you.
I'd like to ask you about your experience with long term storage on the
polytex RTV products. How long do these hold up on average w/o loss of
quality? I use a different type of RTV and am looking for longevity of 10 or
more years in storage with light use. Thanks!
Best Wishes, Debbie
> >I've made latex molds, using sawdust in the later layers to thicken it.
> > A lot like the Cab-O-Sil that you used. Once I had the first few
> >coats and knew that I'd picked up the detail, I started making the
> >applications thicker until the exterior of the mold had no undercuts.
> >Just a big mound of latex and sawdust. Instead of making your 3 or 4
> >part mold to accommodate the undercuts, I was able to make a
> single-part plaster mother mold.
> >I wonder which way is more efficient?
>
>
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