timgrocott Posted March 13, 2014 Report Share Posted March 13, 2014 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. Thanks Link to comment Share on other sites More sharing options...
Wyndham Posted March 13, 2014 Report Share Posted March 13, 2014 I was looking on their site and saw: 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 Wyndham Link to comment Share on other sites More sharing options...
schmism Posted March 13, 2014 Report Share Posted March 13, 2014 Is there a shrinkage factor to the products your useing that would be affecting the overall fitment once you have used those to create a new mold? Link to comment Share on other sites More sharing options...
PeterH Posted March 14, 2014 Report Share Posted March 14, 2014 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 Link to comment Share on other sites More sharing options...
timgrocott Posted March 14, 2014 Author Report Share Posted March 14, 2014 Good point Schmism, I hadn't really considered the shrinkage before. Although it says on the information sheets that shrinkage is negligible. But it could still be an issue. Link to comment Share on other sites More sharing options...
Mark C. Posted March 14, 2014 Report Share Posted March 14, 2014 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 Mark Link to comment Share on other sites More sharing options...
Mug Posted March 14, 2014 Report Share Posted March 14, 2014 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 Link to comment Share on other sites More sharing options...
Jeff Longtin Posted March 15, 2014 Report Share Posted March 15, 2014 Tim, 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 Minneapolis Link to comment Share on other sites More sharing options...
Mark C. Posted March 15, 2014 Report Share Posted March 15, 2014 Jeff has the terms right above-thanks Laguna clay/Axner sells all the materials you will need as well. The shelf life of those rubbers is short so they need to be used once cast they last for many years. Mark Link to comment Share on other sites More sharing options...
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