1 part or 2 part mold with flexible silicone mold material ??

[Jack]

Hi,

I have a smoothly shaped form I am polyurethane resin casting with one
curled edge to it and I really dont want to do a 2 part mold b/c of
the finishing hassle a seam will create...I wanted to generally ask
that if I am using a flexible mold like silicone, why cant I just
twist and slip the piece out ? This may stretch the silicone just a
bit, but I cant foresee a problem with that...it is flexible. Is the
rule of thumb that for any kind of undercut, you must use a 2 part
mold ??

Thanks!

Jack

[Sculptingman]

Your question is really impossible to answer because I have no idea
what you are molding.
I can tell you something as complex as a human hand CAN be molded with
a one piece "glove" mold that literally will turn inside out as you
peel it off the casting- however this would only work if the silicone
was thin enough to actually look like a glove- with individual fingers
shapes and gaps betwen.

The one condiftion that REQUIRES a 2 piece mold is when you have an
opening in the sculpture- like say one created by a figure with its
hand on its hip.
However- another factor is the fragility of the casting- asking very
thick sections of rubber to flex and de-mold from very thin, brittle
sections of casting is somtimes just asking too much.

In general- to ensure success- make your silcone mold as thin as is
practical, no more than 1/4 inch thick and use a silicone with a low
durometer, nearer 20.
This may require a casing (or mother mold) that has more separate
pieces to capture a more complex surface- but is well worth it if yu
want to avoid a flashing line.
christopher

jack_p...@yahoo.com (Jack) wrote in message news:<209b7e58.03051...@posting.google.com>...

[Simon]

"Jack" <jack_p...@yahoo.com> wrote in message
news:209b7e58.03051...@posting.google.com...

as long as the mould is either well supported in a jacket that is removable
with no undercuts ... or a self supporting piece of silicone ..... by the
sounds of it you should be fine ..... i have used silicone in 3 pieces
....... where a concern for undercuts would have had me using a 25 piece
mould if I had used GRP

Simon