a polyurethane question
Dan S
One side of the PU containers always hardens too soon, or at least forms a
skin, and this seems to turn to a very tough clear rubber. Is this the
"real" PU, and the other stuff makes it "less real?"
Is the clear-yellow stuff pretty much always the same and the other, what
modifies it?
And back to sleep.
--
Dan Spector
archica...@earthlink.net
http://www.archicast.com
Gary Waller
"Dan S" <archica...@earthlink.net> wrote in message
news:i8Mxb.17805$n56....@newsread1.news.pas.earthlink.net...
Uncured resin is basically a staged chemical reaction, waiting for the
trigger. Many chemical triggers are an oxidation process - most typical for
example in polyester resins, triggered by a superoxidizer. Another
phenomenom is carbonation, where a material absorbs carbon from the air -
why quarrystone, for example gets harder and harder on the surface (also how
lime 'cures'). So the PU resin is reacting to either the moisture, oxygen or
carbon dioxide in the air. We do know however that nitrogen, the most
abundant gas in air, is completely inert, and a 'blanket' of uncured
nitrogen over your uncured resin, especially the 'B' part, will inhibit any
reaction by separating the material from 'air'.
As I gain a greater understanding of the polyurethanes I realise just how
poisonous they really are, and potentially are (the untested compounds and
reactions). It is disturbing that as the USA, Europe and Canada become
increasingly aware of this, industry has been able to delay things with the
argument that there is no acceptable substitute -their use (abuse) just gets
pushed into countries like Mexico or China. This really amounts to a form of
'environmental piracy' as far as I am concerned. I have had great success in
'gelatin' molds, and would encourage all molders to explore this technique.
I was originally holding back, hoping to package and sell my own formulation
(which I still might do) but the 'public good' is too persuasive - here is a
link to get you started http://www.make-stuff.com/formulas/flexmold.html
Then lets get a dialog going, leading to a F.A.Q. Many of the secrets of
gelatin are found in the old books - remember too that most of the
ornamental plaster you see in the world was created well before the
invention of PU and silicone rubbers. Five to twenty castings are easily
possible, even a 'disaster' like a too hot resin/plaster/concrete melting
your mold is no problem once you realise the material is infinitely reusable
and that poured blanket mold technique takes hardly any time at all. A set
of new molds could easily be made as the last job at night, starting the
next day with fresh, perfect molds. As a tip of my hat to Chris, who has
always advocated the use of silicone over polyurethanes, I must say this now
makes much more sense from an environmental perspective as well - make sure
you are very careful with the catalysts however.
Tryptophan poisining huh - this time of year of the leftover turkeys come on
sale in Canada, will have to go hunt one down.
Simon
own way depending on your perception of reality in a tryptophan-induced
world...... ;-)
The only problem I see with gelatin as a moulding compound is that most
casting materials display varying amounts of exotherm, and with gelatin at
such a low melt (although I have heard that certain additives increase it's
heat resistance) you run the risk of the gelatin surface melting and
sticking, even with a wax layer and although you might be able to get a few
pulls out ..... I think the finer details would be lost rather quickly
....... most of the old time moulds were made with an incredibly complex
fibrous plaster jigsaw puzzle with inter-locking pieces ... Tussauds still
work this way .. heads, alone, often take days to mould and can contain 20
pieces or more.
Simon ......
(who isn't "little" ... and I tried to reply to your e mail dan .. but it
kept bouncing)
Dan S
sentient sculptors would remove that bit.
Once those people sent me gelatin and I bought some alum but I never got
around to it. I hear it's pretty smelly, and I do value the constant size of
my urethane molds which I make pretty thin. I am fully able to make the
interlocking plaster backups, in fact they are fun.
But my question was more about PU and how the 2 components work.
--
Dan Spector
http://www.archicast.com/Lifecast-index1.html
----------
In article <1B%xb.6593$VF6....@news-binary.blueyonder.co.uk>, "Simon"
Simon
> The reason it bounced is prob the NoSpam I added to the address knowing
that
> sentient sculptors would remove that bit.
:-P ...... lol ..... I DID try that .. although it caught me out the first
time because I have a normal reply to e mail address for my e mails ... and
a newsgroup address with the usual so i wasn't expecting it.
> Once those people sent me gelatin and I bought some alum but I never got
> around to it. I hear it's pretty smelly, and I do value the constant size
of
> my urethane molds which I make pretty thin. I am fully able to make the
> interlocking plaster backups, in fact they are fun.
> But my question was more about PU and how the 2 components work.
Sorbitol is supposed to make the gelatin a little more heat resistant, or so
it is said... I am sure there are other additives that might work ... and i
am sure Gary will be playing in his kitchen ;-)
The interlocking plaster moulds that Tussauds use aren't case moulds .....
that's all it is ..... I have never seen anything like it before ...
seriously about 20 pieces per head and they are like a chinese puzzle ...
real old school stuff..
As to PU ..(I assume you mean 2 part). it's a chemical reaction between
polyol and diisocyanates. ... but the reason it forms a skin in it's
separate component form is that it is moisture sensitive and moisture causes
the chemical process to begin
for PU foam ..... the primary foaming agent is water.
that's about the limit of my poor knowledge ..... but there is a book called
Polyurethane Handbook by Günter Oertel ISBN 3-446-17198-3. This should
have everything you want to know
Gary Waller
"Simon" <sycofx...@blueyonder.co.uk> wrote in message
news:1B%xb.6593$VF6....@news-binary.blueyonder.co.uk...
>>
> The only problem I see with gelatin as a moulding compound is that most
> casting materials display varying amounts of exotherm, and with gelatin at
> such a low melt (although I have heard that certain additives increase
it's
> heat resistance) you run the risk of the gelatin surface melting and
> sticking, even with a wax layer and although you might be able to get a
few
> pulls out .....
Also RE: http://www.make-stuff.com/formulas/flexmold.html
Yes, an alum solution toughens the surface, and the sugars increase the heat
resistance. But this is also a different way of thinking - if the mix is so
hot that it melts the mold - so what - the exotherm usually happens AFTER
the shape has already been taken, all you are losing is the shape of the
gelatin, which is easily thrown back into the pot for remelting. This
becomes so easy if you have your master model setup properly that it
ispossible to think in terms of casting one mold per one cast if you had to.
I have enough experience trying to cast production molds in rubber to
knowthat this will never be an economical solution given Canada, USA and
Europe wage rates - yes the rubber lasts, but the time to strip, clean and
recase
the molds eats up that advantage very quickly. If then, molding and casting
is an activity for small scale or specialty production, lets put it back in
the hands of the weekend do-it-yourselfer, and the technophobic artists,
with a safe, low cost material that has a marginal environmental footprint.
Learn this gelatin technique, and pass it on to others - even, and maybe
especially, children and teens.
The only way to get the price down on this material is to buy the gelatin in
ton bags, and the glycerine in barrels - I should be able to then offer a
compound at a less price than running around buying all the ingredients
retail. Does anyone no how to obtain the 'non toxic' stamp on art materials?
Does anyone know how to buy or build a large quanity low temperature double
boiler safe enough around kids (the gelatine is around 110 degrees F 40 c or
a bit higher) - maybe a 'drum' heater band setup - insulated 55 gallon steel
drum with water/antifreeze, and a shelf for five gallon plastic drums.
By the way, I should mention that gelatin is not the perfect material for
the vegans - it's primary source is pork skins - but there is a kosher
gelatin around somewhere. There are no plant sources of gelatin, but
agarfrom seaweed and guar gum from (?) are gelatin like. There are also some
chemicals which are used, for example, in the paint industry which quickly
turn water into a jelly.
Gary Waller
> The reason it bounced is prob the NoSpam I added to the address knowing
that
> sentient sculptors would remove that bit.
> Once those people sent me gelatin and I bought some alum but I never got
> around to it. I hear it's pretty smelly, and I do value the constant size
of
> my urethane molds which I make pretty thin. I am fully able to make the
> interlocking plaster backups, in fact they are fun.
> But my question was more about PU and how the 2 components work.
>
> --
> Dan Spector
> http://www.archicast.com/Lifecast-index1.html
>
gelatin, at a throwaway price, or a batch that had started to decompose
(hence the need for alcohol or a chemical bactricide). The manufacturers
today would not produce 'rotten' batches - they are just slightly off the
food or chemical process grade, for a variety of reasons, and this grade is
only available in bulk quanitities to the glue manufacturers. The batch I
was offered was damaged by the fairly recent New York/Toronto power outage.
Note that since you want to concentrate on PU - I started a new thread on
gelatin.
Why do you need to know how it works - just know that the reaction is toxic
and to stop the moisture getting to uncured resin, use a nitrogen blanket.
You can buy small aerosol cans of this now - it also stops skinning of
varnish and paint. Nitrogen can also be used the reduce the ' diver bends'
from decompressing too quickly - so it is always handy to have around : =).
The right term for the interlocking molds is 'piece' molds, and the key is
to seal everything with shellac and use lots of plasterer's grease.
Andrew Werby
"Gary Waller" <moz...@telus.net> wrote in message
news:tf5yb.75738$oN2.62597@edtnps84...
>
> The only way to get the price down on this material is to buy the gelatin
in
> ton bags, and the glycerine in barrels - I should be able to then offer a
> compound at a less price than running around buying all the ingredients
> retail. Does anyone no how to obtain the 'non toxic' stamp on art
materials?
[Try these folks for the US: http://www.astm.org/ - I'm not sure who does it
for Canada.]
> Does anyone know how to buy or build a large quanity low temperature
double
> boiler safe enough around kids (the gelatine is around 110 degrees F 40 c
or
> a bit higher) - maybe a 'drum' heater band setup - insulated 55 gallon
steel
> drum with water/antifreeze, and a shelf for five gallon plastic drums.
[You're dreaming, Gary. Get together with your liablity insurance provider
for a wake-up call...]
>
> By the way, I should mention that gelatin is not the perfect material for
> the vegans - it's primary source is pork skins - but there is a kosher
> gelatin around somewhere. There are no plant sources of gelatin, but
> agarfrom seaweed and guar gum from (?) are gelatin like.
[This is the basis of alginate mold materials, like moulage, which works
similarly to the gelatin compound you describe. What advantage would that
pork-based material have over these?]
There are also some
> chemicals which are used, for example, in the paint industry which quickly
> turn water into a jelly.
[What are they? Is this gel hard enough to be useful as a mold?]
Andrew Werby
www.unitedartworks.com
>
>
Gary Waller
Will do. I think I already looked here for the carving blocks too.
>
>
> [You're dreaming, Gary. Get together with your liablity insurance provider
> for a wake-up call...]
Do they still teach 'home economics' in high school? This gelatin heating
should be no more dangerous than heating a pot of water - its not even
boiling or skin burning temperature. I think microwave is tricky because
just a few too many seconds can overheat the gel. Maybe there is a cooking
pan/double boiler out there - 'bain de marie' something like that, for
poaching whole salmon? I thought a premade gadget that would ensure safety
because the temperature cannot go above a certain point. I would let the
manufacturer carry the product liability - are you saying that I am
potentially liable because I recommend a certain piece of equipment? I still
think teens would have a lot of fun with this gelatin, when I say kids I
don't mean real young - I'm out of touch with that side of the world.
> [This is the basis of alginate mold materials, like moulage, which works
> similarly to the gelatin compound you describe. What advantage would that
> pork-based material have over these?]
I guess cheap - bulk gelatin is under $5 lb Cdn. Sort of like weiners
instead of steak. Moulage can be reused too, I recall.
Isn't it great to be discussing an artist's material on the same terms as a
food product, instead of just another health risk.
>
> [What are they? Is this gel hard enough to be useful as a mold?]
>
There is a big, long name for the chemical - but I recall seeing artist
quanities at www.kremer-pigmente.com, New York.
Its use to me was get lime wash or silicate paint to behave like latex on
the brush - especially for ceilings.
Lauri Levanto
Gary Waller wrote:
> > [>
> > [What are they? Is this gel hard enough to be useful as a mold?]
> >
> There is a big, long name for the chemical - but I recall seeing artist
> quanities at www.kremer-pigmente.com, New York.
> Its use to me was get lime wash or silicate paint to behave like latex on
> the brush - especially for ceilings.
bentonite clay works somewhat like that
-lauri
Simon
>
> Do they still teach 'home economics' in high school? This gelatin heating
> should be no more dangerous than heating a pot of water - its not even
> boiling or skin burning temperature. I think microwave is tricky because
> just a few too many seconds can overheat the gel. Maybe there is a cooking
> pan/double boiler out there - 'bain de marie' something like that, for
> poaching whole salmon? I thought a premade gadget that would ensure safety
> because the temperature cannot go above a certain point. I would let the
> manufacturer carry the product liability - are you saying that I am
> potentially liable because I recommend a certain piece of equipment? I
still
> think teens would have a lot of fun with this gelatin, when I say kids I
> don't mean real young - I'm out of touch with that side of the world.
I use something called a slow cooker for melting plastalene ..... would work
well with gelatin too ....... they're available at most cookware stores
......
http://www.johnlewis.com/stores/product.asp?shelfid=419&str=13&sku=230134894
I agree about microwaves ..... bad idea .. a bain marie set up is ideal for
gelatin and i have used it for years. ... there's no reason why a larger one
couldn't be constructed reasonably safely, in fact I am sure that catering
suppliers could come up with a suitable cook pot.
Gary Waller
Here is the exact link from kremer
http://www.kremer-pigmente.com/englisch/63812e.htm
the material is carbopol and there are a wide variety of variations from
http://www.carbopol.com.
I use the slow cooker too, I modified mine by drilling a tiny hole and
putting in a metal screw so it is not possible to get the heat higher than
200 degrees F - so plastilena and or wax does not burn or smoke (I use
jmac's 'classic clay' which is a wax base modeling clay and can be modified
by melting in micro waxes). It would be quite easy for a manufacturer to
retrofit, or cast a special replacement knob, to limit the maxinum heat to
120 degrees. I also found out that what you are supposed to do is to put
motor oil in the basin underneath the food/wax basin to distribute the heat
better, like a double boiler. I do know that if the fire inspector comes
around, he/she will gravitate to this pot like a bee to honey - there has to
be a way to idiot proof it. I am also thinking of five gallon and above
quantities - think of what you could try with a cheap/free five gallon and
above pour - as an experiment or prototype for example - doesn't matter if
the experiment isn't right - just remelt the jelly and try again. When you
have got it right, and maybe have already presold the piece of art or
casting, then go out and buy the five gallons of silicone rubber for $500 so
you can cast hundreds more.
Will have to get some sorbitol to try that in the jelly.
It is interesting too that this gelatin compound is available all over the
world too.
Gary Waller
the equivalent of a 'brush on' mold rubber. Gauze could be used in
subsequent layers, and then a plaster shell/mother mold. I will order some
next week, probably the liquid one.
Another thing I like about jelly in a cool climate, I can put the mold
and/or cast outdoors to 'accelerate' the cure for free, versus paying to
heat an area to accelerate PU or silicone.
You should be able to cast in the transparent gelatin, and also there should
be something you can add so the cast gelatin will shrink over time, with
careful drying, like you can do with PU, Silicone and Reducit.
I am having a lot of fun with this. Since I often think I have the mental
age of a twelve year old - I wonder what a REAL twelve year old could do
with this material?
Where is our art teacher in training - Elijah? If you can't teach tack
welding, at least you can teach gelatine and basic mold making.
Sculptingman
If you are using an elastomer- then switch to silicone- it is a far
less toxic material (keep the catalyst off your skin) and makes for
longer lasting molds- it is also less liable to go bad in the can;
staying usable for a year.
Stay as far away from vinyl as you can, it is the most toxic of all.
If you are using PU resins for casting- then there really aren't much
in the way of alternatives. Polyesters and epoxies are just as toxic-
and smell far worse. Realize that the two components undergo a
reaction to generate a new molecule, unlike silicones and polyesters
that will form polymers on their own eventually, even without
catalysis.
A byproduct of this reaction is Isocyanate in various forms- they
outgass from the mixture for about 15 minutes after A meets B. If you
must work with them- try and set up an exhaust hood that pulls air
past you, over the mixing area and mold, and out of the shop.
As to Gelatin:
Oh, I remember the days of gelatine molding- what a pain- heat is not
the biggest enemy of a gelatin mold- water is. (of course, we would
never cast urethanes into a gelatin mold) We used to treat the cavity
surface with a substance called TANOL to make it more water
resistant. In the theatre we made "gels" for the lamps- now, unlike
mold gelatin, this form of gelatin had no water content when it set,
and it could resist hundreds of degrees without even changing color-
but try cleaning one with a damp cloth and it will melt away like
cotton candy.
But oh how happy we were to pour silicone into one of those gelatin
molds and find out we could get 500 flawless plaster castings rather
than the rapidly deteriorating 20 or so we got with gelatin.
While gelatin IS reusable- it does get crudded up over time- my
expericence was that we had to keep cleaning it and lost some each
time- eventually replacing the entire volume over about 8 to 10
meltings.
christopher
Gary Waller
> As to Gelatin:
> Oh, I remember the days of gelatine molding- what a pain- heat is not
> the biggest enemy of a gelatin mold- water is. (of course, we would
> never cast urethanes into a gelatin mold) We used to treat the cavity
> surface with a substance called TANOL to make it more water
> resistant. In the theatre we made "gels" for the lamps- now, unlike
> mold gelatin, this form of gelatin had no water content when it set,
> and it could resist hundreds of degrees without even changing color-
> but try cleaning one with a damp cloth and it will melt away like
> cotton candy.
Tanol is apparently aluminum sulphate - alum. The water is no problem when
casting cements or plaster or even gelatin on gelatin - you have to use an
oil or wax. The fact that it is so stable when the water evaporates makes me
think a cheap dessication device, like that used with Reducit, would give
uniform dimension, shrinkable castings, which could be manipulated by the
percentage of water content. There are a wide variety of chemicals, such as
carbopol, fairly new to the market which could be used to 'tweak' the
gelatine in 100 different directions. I even think casting PU would be
possible in gelatin molds with the right barrier coat/mold release - not
that I will ever try this experiment I am thoroughly satisfied with sorel
cement rigids and foams. I have now developed a fast curing foam, castable
and sprayable, safe and absolutely fireproof, UV and exterior stable - but
it is not flexible and the equivalent of a 4 lbs a cubic foot PU foam weighs
about 20 lbs in Sorel foam. This still has tremendous potential in the
building and insulation market. Here is a weaker version
http://www.airkrete.com/ - look at the video where they melt a penny on it
then touch the foam right after.
>
> But oh how happy we were to pour silicone into one of those gelatin
> molds and find out we could get 500 flawless plaster castings rather
> than the rapidly deteriorating 20 or so we got with gelatin.
Bet the accountant wasn't happy paying $200 for one gallon of rubber though!
Why not make 100 molds for $10 a gallon and use each one five times - one
man could produce 500 flawless castings in one day that way, or 2,500 in one
week.
>
> While gelatin IS reusable- it does get crudded up over time- my
> expericence was that we had to keep cleaning it and lost some each
> time- eventually replacing the entire volume over about 8 to 10
> meltings.
> christopher
You are right on this one, but the debris accumulates on the bottom of the
pot, and if you get lazy and just skim all the time, you will eventually
develop hot spots in the pot.
Dan S
mold shrinks as it ages, so if you are trying to get a repeating molding to
fit a given space, cast some parts at the end of the day, as they will be
shorter than those cast in the morning.
Its essence is not long mold life but easy remaking.
A real 12-yr-old would pour it on other smaller lifeforms.
Dan
----------
In article <3dayb.210644$jy.145043@clgrps13>, "Gary Waller"
Gary Waller
> A real 12-yr-old would pour it on other smaller lifeforms.
> Dan
> ----------
celebrations of the human form'!
Sculptingman
> Why not make 100 molds for $10 a gallon and use each one five times - one
> man could produce 500 flawless castings in one day that way, or 2,500 in one
> week.
You are forgeting the cost of labor to pour the molds is almost as
much as the material itself ( with gelatin)- and the problems of
SCALE-Moldmaking is TOOLING for production, not production. In a
production facility you simply can not afford to spend as much time
tooling as you do producing. You'll go under.
I have run detailed pilot projects on every material available, Gary,
and the economics always favor silicone. (we routinely tested every
new silicone just to determine if it could give us more castings per
mold or less cost per mold or both- we ended up switching brands about
3 times over five years to gain about 1.2 cents in lower cost )
here's how it breaks down:
the price back then was about $120 per gallon- our average production
mold took about one and a half pounds of silicone- making about 6 and
a third molds to the gallon- that's around $19 per mold material cost,
plus an average man hour time of about 15 minutes per mold (production
casings- poured molds- not including one time per design tooling time
to generate the production tooling --this is just maintenance costs
I'm running)
A moldmaker was paid about 8 bucks an hour- so figure 2 bucks labor
cost for a total of $21 per production mold.
At around 500 castings per mold- that's about 4.2 CENTS per part mold
cost.
Contrasted to gelatin-
mold grade gelatin, tho less expensive, is not really cheap- It is
partially reusable at the cost of additional labor to clean it, but
has the additional cost of the gas to melt it up and the longer labor
cost associated with mixing and keeping track of the stove. Back in
the day we figured the average gelatin mold cost us about 7 dollars
per. Divide that by the average 10 castings we got before retiring it
(twice your five) and you come up with a per part mold cost of 70
CENTS per part.
Gary- that's SIXTEEN TIMES the cost of silicone mold production. ( and
this figure doesn't factor the additional casting labor required to
apply a waterproofing agent to the gelatin between each casting- a
cost silicone avoids)
Now let's talk scale:
Years before silicone we were using Urethane Elastomers because they
were about twice as long lived as gelatin and even with this longer
lasting material the production floor required about 230 REPLACEMENT
molds per day to keep up with losses due to wear (not including
tooling and producing new molds on new products)
After switching to silicone we were casting 15% more parts of higher
quality and replacing only 5 molds per day.
This meant that instead of having a mold shop staff of 10 we could run
even better with only 4.
Due to the strength of silicone we could design molds with fewer
parting lines- most products popping out of "glove" molds that are
impossible with gelatin. The coolest thing about silicone production
molds is that they hold up with virtually NO wear of even the finest
details until they fail catastrophically. Every other material except
latex shows appreciable wear with each successive casting until you
are forced to retire the mold for reasons of product quality.
Overall, Gary, silicone has been proven to be the highest quality
lowest cost choice for any production environment.
Of, course, most artist's are not doing production- so the major
factor becomes not how many how fast, but how durable the mold is in
storage. Several silicones now available produce molds with storage
lives of 15 years or so, remaining flexible and strong for a looong
time.
christopher
Gary Waller
"Sculptingman" <scul...@tfb.com> wrote in message
news:59d744d1.0312...@posting.google.com...
>> Overall, Gary, silicone has been proven to be the highest quality
> lowest cost choice for any production environment.
> Of, course, most artist's are not doing production- so the major
> factor becomes not how many how fast, but how durable the mold is in
> storage. Several silicones now available produce molds with storage
> lives of 15 years or so, remaining flexible and strong for a looong
> time.
>
> christopher
You make some excellent points Christopher, but just to clear something up
for the kids out there - there is simply no market for cast plaster items,
and cast concrete is a problematic backbreaker (abrasive on molds and
requiring at least three day wet cure in the mold for example). So most
casters (ceramics excepted) rely on urethanes and more typically, heavily
filled polyester resin - both of which eat up silicone and urethane molds (I
think the Polytek site explains how to get up 20 castings per mold, if you
wash and barrier coat them each cast. This reason is why the urethane
rubbers predominate in the typical casting plant - you might as well throw
away something which was one third the material price. No one is getting 500
casts per mold in polyester or urethane resins. I think I can get these
results with sorel, but every once and while you get a runaway reaction,
with heat as high as 300 F (?), so for production casting it has to be in an
ISO 9001 quality setting, with either silicone molds (high temp) or
'acceptable losses' in urethane. But hot urethane rubber must be emitting
something nasty?
I would agree that gelatin is unlikely to find its way back into the
mainstream casting systems supplying Cosco, Walmart and Target with their
decorative treasures, but it is shame that it is currently overlooked by
people wishing to cast a few items of 'cool stuff', for fun, or as 'what
ifs' by serious artists to show what something might look like in cast metal
or stone, or as components for a larger piece. Also, as I mentioned before,
most of the truly wonderful decorative plasterwork of Europe (and I'm sure
with variations in Asia), were made by hand modeling, 'stamp' molds, piece
molds and gelatin or wax type molds. An understanding of these techniques -
which have been completely lost within fifty years of 'progress' - brings
about a greater appreciation of the art form. These things tend to move in
hundred year cycles, and it is interesting that arts and crafts, art nouveau
movements are one hundred years old, we could very well be at a stage where
the public might desire the unique, handcrafted items once again.
Dan S
plaster casting company in Cleveland. They still make gelatin molds. Bob
there says a mold usually lasts a season- but summer is tough, heat makes
them shrink faster than cooler seasons.
Apparently they shrink a lot slower than I was told !
Dan
----------
In article <jcJyb.23486$sb4....@newsread2.news.pas.earthlink.net>, "Dan S"
Sculptingman
I just got back from China where every producer of filled polyresin
sculptures uses silicone exclusively. (This in a place where the
toxicity of urethanes is not even a thought- and they would rather
hire an extra twenty guys if it meant they could use a material that
is a little cheaper))
Average parts per mold with polyresin catalyzed very hot (six minutes
to gel) is about 40 to 70 castings. They use a very specific type of
silicone- very low durometer- but the same essential formula I have
seen used in Italy for poly-resin casting.
Not all silicones are ideal for resin casting- you must experiment
around to find the ideal rubber for the material you will be casting
in- ask suppliers to recommend their best materials and then test them
in a controlled run.
Back in the day we often opted for Platinum cured system- which are
even more expensive, but also offer better chemical resistance and
less swelling (hence more accurate castings) as the rubber heats up
with repeated casting.
Urethane molds, used to cast polyresin, will be lucky to render 15
usable castings. (based upon detail wearing out)
Used to cast urethanes- they will fail even faster as the slightest
misapplication of expensive separation agent will result in a nice
bond between the mold and the casting.
Cheaper urethane elastomers are currently only about 40% the price of
silicone, so do not yield as low a cost per casting as do silicones.
In a production environment, with semi-skilled labor- the low tear
strength of urethanes also results in much higher mold losses due to
manhandling of the molds in de-molding. (When we used urethane molds
nearly half of our high mold losses were due to tearing during
demolding)
Be aware, Gary, that at the factories I work with we routinely test
any new product coming down the pipe, be it urethane or silicone, to
ensure we are using the lowest cost production method available. If we
could save even one cent per casting we would switch materials in a
heartbeat.
Again- as you point out, many of these considerations are not very
relevant to the cost of a small art related studio- however, I also
maintain that silicone is far less toxic, far easier to use and far
more versatile than any other alternative for the hobbyist or fine
artist. With one can of silicone base and three small bottles of
additives I can make virtually any kind of mold I will ever need and
not have to bother with toxic release agents to cast essentially ANY
material I may choose.
Somethings are well worth the extra money you pay for them.
christopher
PS- one thing gelatin molds do have over other materials, tho- when
one's art does not sell, one can actually EAT gelatin molds.