RE: Silicon molding help needed

From: Deanda, Jerrold (
Date: Thu Dec 11 2003 - 00:46:36 EET

  We do a bit of silicon rubber molding and the only time we've had trouble
with surfaces not curing was when we used modeling clay to fill out the
parting line around the master. The toy store clay has something in it (I
can't remember the exact material) that inhibits the material from curing on
the surface against the clay. Then you remove the clay, clean it off, put
on the mold release in your normal way, pour the other side of the mold and
it glues itself together just as if you'd poured the whole thing at once...
really a great adhesion promoter, if that's what you want. :>) We got
away with it sometimes by using PTM&W spray mold release, in copious
quantities.... Say, 20 heavy coats, well dried between each coat. We went
to a regular clay for moldmaking from Silpak and the problems went away
  I too have hesitated to answer because I wasn't sure which materials you
were using. Sheba says, it may be that a barrier will give you
the quick fix... My choices to try would be the PTM&W mold release, (no
measurable buildup if you need an accurate part) or good ol' vaseline...
It's surprising how well this old stuff works. You can put it on real thin
and wipe it off until you can just barely see it on there. Works better
than some of the exotic (read: expensive) special mold releases sometimes.
  I hope this helps... Let us know, OK?

-----Original Message-----
From: [] On Behalf
Of Bathsheba Grossman
Sent: Wednesday, December 10, 2003 1:45 PM
Subject: Re: Silicon molding help needed

On Wed, 10 Dec 2003, Jon Broome wrote:
> We are new to silicon molding and are having trouble with the silicon
> not curing uniformly (and staying gooey) where the silicon contacts
> the plastic parts. Has anyone solved this? Any assistance or
> directions to assistance wold be greatly appreciated.

Well, I'm far from the most knowledgeable person here, but nobody else seems
to be speaking sounds like the material of your master is inhibiting
the cure of the mold.

Your options are pretty much: use a different master material, use a
different mold material, or put a barrier between them. Probably #1 is out.
For #2, you could try different rubbers. For #3, if tolerance is not too
tight you could try shellacking the part or something like that.

For more advice, you'd probably have to reveal the material of your master


Bathsheba Grossman                 phone (831)429-8224, fax (831)460-1242
Sculpting geometry                                
Solidscape prototyping                           
Protein crystals                             

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