David Martin Hershey on mon 25 aug 08
Hi Debbie,
I think the short life of silicones is just part of the deal- a trade
off with working properties. The softer the silicone, it seems the
shorter the life. Mine have been kept in a cool dark dampish place, and
they still start to weep and change. Maybe hot & dry is better.
Catalyzing is definitely a factor- you can really see the performance
difference in resins that have been over-catalyzed.
As far as urethanes go, I think you would need to look into the
different hardnesses, and how they perform. I've got to believe that
there is a soft one that would perform comparably to silicone, but I
don't know about life span. The one I'm using is fairly hard, as the
pieces are simple, and I don't have big undercuts or anything.
Joani makes her whole case molds out of fairly hard urethane. She starts
with a casting of the horse in urethane, and then pours the urethane
case around the urethane casting. She then casts the plaster molds from
the urethane case mold and master. It's economical for her, because the
horses are fairly small.
As far as the French molds distorting, all I can think of other than
keying, is maybe differences in thickness of the rubber are imparting
subtle stresses that eventually cause distortion? Probably if you could
talk to a chemical engineer who makes these things, he could tell you.
Try calling Dow-Corning, didn't they invent silicone? I called them
years ago about another product, and the engineer was very helpful.
Best, DMH
David Martin Hershey
DMH Studio + Design
2629 Manhattan Ave #137
Hermosa Beach CA USA
90254-2447 310.379.6890
http://www.dmhstudio.com/
D. L. Engle wrote:
> Hi David,
>
> Thanks for the link. Great DVD for $11.71! I'll send people there to buy it
> next time I'm asked how to make molds. So now I know what Polytek 1512x is,
> thank you very much.
>
> What does shorten library life of silicon molds? That is the million dollar
> question!
> No one seems to be able to say with certainty other than "many variables!".
> It's only a gut feeling on my part, but wax at 175F (it's only that temp for
> such a short time, a minute or less), isn't one of the big factors.
> Experimenting, I've kept scrap pieces of RTV in a molten wax pot for several
> days of pouring and noticed no discernable difference in quality. And then
> some molds loose quality during a long inactive storage where hot wax is not
> a factor. I wonder about ambient room temperature and air quality as
> factors, others have mentioned these but don't know for sure. I'd like to
> find someone living in a hotter and drier climate than me that uses them.
>
> I've not used urethanes, yet. It seems their longevity is offset by an
> inability to take a good deal of manipulation such as what my molds need to
> do. If you can imagine skinning an animal, that is what many are like. I get
> the impression that urethanes are best for molds with no severe undercuts
> and simple or straight parting lines so that they will not be stretched or
> bent. Is that your experience with it? (I know your board strap is a rather
> extreme example)
>
> I'm really beginning to think about urethanes for master cast of the molds
> as I can see a lot of room for improving over plaster (which I have been
> using for this). I think this is what Joani does, I will ask her.
>
> My molds are stored in casings (some sealed, some not) of plaster, ceramical
> or FGR and a sealed wax inside so that there is no air contact or slumping
> inside. Casings are tight and secure fits. One thing I forgot to mention is
> the fast catalyst will absolutely contribute to a short lived mold whereas
> the slower one is much more stable. In fact, I've had the short catalyst
> molds begin shrinking and become brittle and easily tear in only 3-4 years
> while the slow catalyst ones out last them by many, many more years. The
> manufacture makes the slow catalyst for use in French pours but I more often
> now brush them up with a thickening VRM-65 and it seems to work better.
>
> So I'm still trying to figure out what the factor is with the thicker,
> French poured RTV molds distorting. Still it takes many years for this to
> happen and even then not in all cases, or casings .
>
> I really enjoy comparing notes with you about these materials, Debbie
>
> D. L. Engle Sculpture
> www.home.earthlink.net/~dlenglesculpture/
>
>
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