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Rubber Slipcast Mold Masters - Need Help

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Hi there, this is my first post on this very helpful site.

I can't seem to find much information anywhere about using rubber to make mold masters for recreating slipcast molds.


I have been experimenting with making rubber masters of my molds (mostly 2 to 3 piece molds). But I find that whenever I pour a new mold from the master, they don't quite fit together as well as the original.


I have experimented with a few different types of rubber, but am limited to products that don't need degassing.


Is there any one particular product that is recommended for use in this situation?


I've attached a couple of photos to show how I am constructing my masters.

The blue handle mold was made using Smooth-on Oomoo 25 (which is very soft), and the light colored one was made using Polytek Polygel 35.


Any little tips or tricks would be greatly appreciated.










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I was looking on their site and saw:


button_icon_msds.gif Soma Foama® is a soft two-component platinum silicone casting foam that is versatile and easy to use. Soma Foama® can be poured into a mold or over other surfaces. Vibrant colors can be achieved by adding Silc-Pig® silicone color pigments.Cured foam is high heat resistance (will resist up to 350°F / 176°C), water resistant, UV resistant and resists oxidation and ozone degradation. Applications; Soma Foama® can be used for a variety of industrial and special effects applications including making foam filled appliances, padding/seat cushioning, orthotics/orthopedics, potting and encapsulation of electrical circuits and vibration dampening.




Would this work better capturing detail in your mold to recast into another working mold

just a thought


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When I've read about using flexible moulds it's often recommended that the rubber is backed with a harder

substance (e.g. plaster, plaster-bandages, glass-fibre, ...) to prevent deformation when you use the mould.



Regards, Peter

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We had our masters made -the steps are case then masters-case molds are made from harder than normal plaster-(like hyrda cal etc)

Then the masters are taken from those. The rubber is very hard and not soft at all. Also not shipped in winter as freezing ruins it

You should be able top buy th you make a working rubber master which you can pour plaster directly into them as they have side walls that are very stiff-I think you are missing a step and using the wrong rubber

The master is set aside and the working masters are what you pour the plaster into. This will take a few steps to go from a positive form to a negative. We had a professional mold maker make ours years ago in a slip business as he was a friend-this priocess is an art in itself. I still have all the masters and cases.

You should be able to find out this info on the net nowdays.

Another way these are described is


model-your object to cast

Master mold-the one taken from model

Mother mold-the ones for making your working molds from


Its a three step process either way before the plaster working molds are made


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For simple moulds I would use ceramic products, because they would be cost effective.

I would use silicons for casting plastics, because you do not need a mold release.

For the money Look into urethane mold making materials, they may be just what you are looking for.

If durrability is what you want, You can beat the snot out of these and they will last forever

be aware that Urethanes have a short shelf life if you do not store them properly.


PMC® -746 Urethane Rubber Compound would work...but look around smooth on is generally over priced

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Your molds are too thin. I just poured some rubber molds using Polytek 74-55 and they allowed me to cast 50 molds so far. The walls of the molds are 2 inches thick. $600 worth of rubber but they'll last 20 years.

As far as terminolgy is concerned the original is considered the "model". The first mold, of the model, is considered the "master mold". 

The mold that you use to produce more molds is considered the "block and case mold". (The "block" being the mold face while the "case" is the box that surrounds it.) In the past the block mold was made of rubber while the case was made of wood or cement. Nowadays the whole mold, block and case, is made of urethane rubber or silicone.

Good Luck

Jeff Longtin


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