I worked as a professional model maker in the rapid-protyping industry for 8 years.

Following your tutorial – you are only a little short of a really good two part mold.

I would recommend taking a hex key – 3/8″ making about 50-60 impressions in clay – roughly 1/8″ to 1/4″ deep. Then you spray release and then pour urethane into the depressions. When cured – sand the bottoms smooth and then mount them to the bottom of your mold box – these will provide the registration for the other half of the mold. These can be reused over and over again.

Flip the mold and demold only the registration pegs per your tutorial. We then used drywall/sheetrock joining tape wrapped and then taped around the now bottom half of the mold to provide the “pour box” for the other half. Spray the prepped bottom half with pattern release and pour the other half of the silicone rubber.

Once cured – demold all of the masters.

When pouring the urethane pour enough into both mold sides paying attention to not underpour either side – then in essence “slap” the two halves together matching registration pegs.

Hope this helps!
Art

I have tried garage two sided molding in the past, but as I said in the tutorial usually find the resulting casts a little disappointing. Mainly because of problems with registering the two mold halves and pouring with resin/plaster.

I imagine using a more rigid high-temp RTV product and pouring with white metal you could probably get reasonable results. I may try this in the future.

This is the best tutorial I have seen online for silicone rubber moudling.

Just want to confirm – this tutorial for an open faced – single piece mold?

Have some ideas for you on two piece molds – if you are interested.

Art

I use that ‘overflow’ technique for actual resin and plaster pours but prefer to estimate mold volumes for RTV. That also means I can make sure the RTV will cover the masters to a satisfactory depth.

I don’t use any ‘mechanical assistance’ in my mold pours and casts but I can imagine what you’re talking about.

Another tip to ensure your RTV is thouroghly mixed; once you’ve mixed it, pour the rubber into a fresh container and mix again. Any unmixed mix wil be left in the first pot. Pouring from a good height here will also get a few bubbles out.

Rather than estimating the amount to mix, I just make a very rough guess at the volume of the box then measure out a fixed volume of rubber and hardener. This makes the mix more accurate, and hopefully leads to longer lasting molds. Like you said,it is better to add extra hardener than not enough, but too much will make for stiffer molds with shorter lives. To avoid any waste I always have more boxes ready than I expect to be able to fill, so any excess will always have a box to be poured into. As you say, rubber will bond totally to itself, so it’s no problem to top up a half finished mold later.

I’ll go into the operation of the Viblomatic 3000 later.

One where I undermixed the catalyst and it stubbornly refused to set, even after a week. That was a mess to clean off my masters for sure.

Another where I peeled the mold too early and the interior was still gummy. It came away from the masters fine but totally ruined the mold.

Definitely try it again!

Your articles have made me think about retrying making a mould. Thanks for the advice.

Again – excellent work. Clear, concise, and helpful. Now I just have to wait a week to go home and TRY this stuff.

In the meantime, hola from Seattle!