Model Forum / General / Railroads / April 2006

Newyorkcentralfan@bigfoot.com spake thus:

I don't think you can do that after the part is cast. Think about it.

Actually that's not as far-fetched as it sounds since this is a clear
plastic resin which allows you to embed objects by doing exactly that.

What I have tried is to overfill the mold, leaving a thin layer of liquid
resin on the surface.  I haven't demolded it yet, but either that surface
layer evaporated or it really shrank because there are pieces in the mold
that are still not full.  I have to conclude that this product isn't
suitable for castings this thin.

   Norm

I was using two identical disposable pipettes -- one for each of the two
components -- and they were filled to the same level to within a small
fraction of an inch -- by eye probably only a few hundreds of an inch
difference which was easy to see since the liquid level was just above the
bottom of the bulb.     I don't think that was the problem here.

HOWEVER ---  each pipette full is approximately 1/8 oz and two possibilities
remain:
   1.  While they were filled equally, they might have not emptied equally
because of the differences in viscosity of the two components
   2.  The actual casting uses only a minute amount of even the 1/4 oz that
I prepared and it's possible that incomplete mixing might have left some of
the resin with different proportions.  Since it's a 5-minute from
liquid-to-solid resin, you don't have much time to do the mixing before you
start to loose it.

   Norm

Let me summarize by saying that I was having serious problems casing some
very small parts.  I've partially isolated the problem to a chemical
incompatibility between the resins I was using
   Castin'craft's Clear Liquid Plastic Casting Resin
   Alumilite Super Plastic (white)
and the mold release spray
   Mann Release Technologies "Easy Release 2000"
wihch seems to be preventing the resin from completely curing.

I thoroughly cleaned the mold with both Acetone and Alcohol and then I
repeated the casing with the Alumilite (5-minute) stuff with no release
spray.  Curing of the unused slug left in the cup seems to be completely
uniform and that resin is very hard.  Parts de-molded seemed completely
cured but were much more flexible than corresponding parts from yesterday
that were left in a closed box on the desk overnight.

SO ... I think there's definitely an incompatibility with the release spray
and the casting resins I'm using.  Since the primary purpose of a release
agent is to prevent curing at the surface junction between the RTV mold and
the part being cast, it's possible that the extreme thinness of these
parts -- some of which are only .025" thick -- may just be preventing the
part from curing at all.

BUT ... the difference between the hardness of the slug in the cup and the
parts I demolded leads me to believe that the RTV is partially at fault as
well.  I hate to get caught in the "You can only use my resin with my RTV!"
shtick that manufacturers sometimes pull by creating minor incompatibilities
between their product and their competitors' stuff, but there may be some of
that going on here.  For the record, the RTV I'm using is MicroMark's 4-hour
One-To-One which seems to me to create a rather poor mold anyway -- corners
aren't as sharp as they might be and the rubber is very fragile.

But the adventure continues.   More later.

   Norm