Subject: Rubber Mold Directions

From: [email-masked] (Cori Stewart)
Date: 11 Apr 1995 14:15:26 GMT

By popular demand (from email I've been sent) I will post the complete 
directions I was given for making rubber molds for casting pewter or 
other metals which melt at a similar temperature:

Materials needed:

  Permatex RTV "copper" high temperature caulking (comes in a tube @ $8)
  vaseline
  mineral spirits
  normal indoor/outdoor clear caulking
  plaster of paris
  plasticine

Proviso: PLease understand that I havent completed a full process yet, 
and at a point I will be simply quoting the man that gave me the 
instructions.  Also, this method is from my perspective, casting pendants 
which are flat backed.  this method can be easily adapted for small 
scultures etc...

Method:

1. Find a vessel suitable for casting.  The vessel should have straight 
sides so that you can remove one half of the mold, turn it upside down 
and put it back into the vessel to cast the other half.  I have cut the 
bottom off of a plastic bottle, so I have a round vessel, whose 
cross-section looks roughly like this:
                                       |               |
                                       |               |
                                       |               |
                                       |               |
                                       |               |
                                       \_______________/

If you can find a square vessel (shouldnt be hard, i  just didnt have the 
patience to look beyond home) all the better, because then you can stand 
your mold up when casting with no need to prop it.

2. EMbed about 2mm of the flat back of your original (my originals are made 
of a polymer clay ... brandname: FIMO) into plasticine.  Remember to also 
embed a spru and funnel at the top touching the side of the vessel

     |               |               ----------
     |               |             /     \/     \
     |               |            /      ||      \
     |               |           |      -  -      |
     |___--^-~--_____|           |    /      \    |
     |   -------     |           |   |        |   |
     \_______________/            \   \      /   /
                                   \    ----    /
                                     ----------
3. Using an old paint brush, mix a small portion of vaseline and mineral 
spirits in a 50/50 ration, to get a kind of mucous-like (yuck) 
consistency.  Paint this, thinly on all surfaces you will apply the 
rubber: on the original (in all nooks and crannies) and over the 
plasticine and sprue.  You dont need t opaint the sides of the vessel, 
but it may help if you wish to be diligent.

4. Again, using an old paint brush, brush the high temperature caulking onto 
your original and the plasticine and the sprue in a THIN coat.  You 
should still be able to see some detail on your original, even though 
much of it will be masked.  Let this cure for four hours.

5. Brush on a thick coat of the high temperature caulking.  You should at 
this point be able to see no detail, just a blob covered with caulking.  
Let this cure overnight.

6. Brush on another thick coat if you think it necessary and let this 
cure throughout the day (or overnight whatever).

7. Using a caulking gun, pump in a load of indoor/outdoor caulking, 
filling the vessel all around the original and the sprue, until you are 
satisfied.  Let this cure overnight.
     |               |
     |               |
     |---------------|
     |___--^-~--_____|
     |   -------     |
     \_______________/

8. Take the whole concoction out of the vessel and remove the pasticine.  
Be careful not to remove the original from the rubber. Fill the bottom of 
the vessel with plaster of paris and let set overnight.

9. Put a small amount of indoor/outdoor caulking into the vessel, 
covering the dried paster of paris so there will be a bond between the 
dried silicon and the paster.  Turn your silicon rubber & original mass 
upside down and place it into the vessel as shown below.
     |               |
     |               |
     |____-------____|
     |    --=-v--    |
     |_______________|
     \_______________/

10. Repeat the steps 3-7 on the underside of the original.  Make sure 
that your sprue and funnel are still in place.  You may have to do some 
build up with plasticine or other found-things to make this (I should 
havementioned this before, but since you are diligent and reading this 
whole thing before you try it, things should work out just fine right?)

11. Let the whole thing cure for FOUR DAYS.

12. Take it apart and inspect your mold.  You may wish to build up the 
funnel more with the indoor/outdoor caulking.  It doesnt matter if the 
hot metal touches the indoor/outdoor caulking, its just that if you used 
it for the mold itself, you would lose detail very quickly casting after 
casting, because it is a lower grade of caulking.

You may also cut any of the silicon (indoor/outdoor and high temperature 
alike) with Xacto tools if you need to augment your sprues or funnels, or 
if you need to make air ducts in your mold in difficult places.

Note About Plaster:  This is to help the rubber hold it's shape.  If you 
really want, you could devise a way to cover the whole thing with a 
plaster shell.  I dont know how necessary this is, I've only theorized.  
You see, I really havent got past step 8.

This really is all I know.  You'll have to post questions about this here 
to the experts.

T. Shawn Johnson
(using Cori's account)