Making silicon molds for plaster molds (for slip casting)

[Polyhedron]

I want to  make a silicon mold to create plaster molds that I can use for slip casting. I was wondering if anyone had experience with the method and had some questions:

1. I will create a plastic (3D printed) mold for the silicon mold. Can I just pour it in or do I need a mold release for the silicon/plastic?

2. What kind of Shore do people recommend? Is Shore 40 okay for this application?

3. Do I need mold release for pouring in the plaster in the silicon mold? I saw some video where they sprayed it with something but I have no idea what? Is it vaseline spray? 

4. Does silicon shrink much?

5. Any other things I need to pay attention to?

Thanks in advance! 

(PS: Thanks for all the help in the previous discussion, I decide to make a silicon mold to make it easier.)

[PeterH]

A flexible block mold seems a  good way to go, especially if you are going to need a lot of molds.

Just for your notebook, have you seen
3D Printing a Mold for a Slipcasting Mold
https://www.instructables.com/3D-Printing-a-Mold-for-a-Mold/

Where they use a [usually sacrificial] printed molds to make the pieces for a multi-part plaster mold.

[Jeff Longtin]

While I've not used silicone rubber, to make molds, I've used polyurethane rubber, instead,  I get the same basic qualities but with lower costs and less air bubbles.

I would apply silicone release to all surfaces coming into contact with the liquid rubber.  Once cured I don't apply anything when pouring plaster.

I make block and case molds with Polytek 74-55. I find it has a nice stiffness to it. I experimented with 74-30, on a small form, and find it to be too soft.  Shore 40 might be a happy medium? (74-55 does flex but still retains its shape nicely.)

Shrinkage - if I'm not mistaken rubber mediums don't shrink at all or only minimally.  

 

 

 

[Hyn Patty]

This might be helpful to you as well: 

 https://www.youtube.com/watch?v=mtvMW9qeaJ4

https://www.youtube.com/watch?v=8zWouEISprE

https://www.youtube.com/watch?v=X2JS8uCPVWw

This fella has a long list of YouTube videos that are pretty handy, especially the ones about mold making silicone production molds.  There are more of them than these three if you go looking.  I agree with Jeff that unless you need the production molds to be super high quality and very long lasting, there are cheaper rubbers you can use.  I vacuum chamber my rubber but if you don't have that option or don't want to mess with it, do make the effort to use rubbers that bubble up the least.  You'll be glad you did. 

Also these 'Tiki Technical Tuesday' vids have super cool suggestions on how to cut up and reuse old production molds to save on the amount of casting rubber you need to make new molds.  I need to find his index of all his videos again.  It's buried in my bookmarks somewhere.  I'm a fan of using silicone myself for a lot of reasons but it isn't cheap.

I agree, do use mold release.  I use a spray silicone but there are lots of options.

[Harold Roberts]

I also use Polytek 74-55. I find when pouring rubber molds with plaster without cleaning thoroughly the rubber molds between castings and using some form of release agent you will begin to get a film buildup of plaster on your rubber mold. I use green soap and very sparingly.