Resin & RTV: MicroMark, SmoothOn, Polytek or Alumilite?

[bobbyfett]

They do indeed. However you still have to seal the bottom edge and sometimes rubber can leak through the cracks of the Legos. I know it sounds weird, but trust me, the foamboard method really works great!

[macfrank]

They do indeed. However you still have to seal the bottom edge and sometimes rubber can leak through the cracks of the Legos. I know it sounds weird, but trust me, the foamboard method really works great! :)

I use legos for mold boxes all the time. I use a Legos base, then build up the first layer of bricks. I fill in the space with some sulfur-free plasticine, flatten it, press the parts in, build up around the parts with more plasticine (to prevent RTV from seeping in under the parts) punch in some registration marks, build up the next layer of Legos and I'm done. I get a little seepage of RTV along some of the Legos joints, but it's never been an issue - it just forms a thin RTV "flash". When the RTV is cured, I remove the box from the Legos base, flip it over, press it back onto the base, scoop out the plasticene, add mold release to the exposed RTV and pour in the other side.

[Sparky]

Foam boards is quick and easy, but I use legos because I got square true molds.

The problem with a quick and dirty foam board mold has been illustrated recently to me:

First of all if you do a two sided mold, rebuilding the box to fit the first half will be a little more time consuming. With legos you just peal off the base, scoop out the clay and stack more legos on the bottom of the box. Build it down so to speak. Like macfrank described.

The second more devious problem comes when making simple one sided, open face mold. I got some parts from a fellow in the club whose RTV ran out in the middle of his project. The mold boxes were built already. They looked square to me. But after casting in the finished molds I see that the mold is not level. I had placed the molds on the floor to help make sure the rubber cured on a level surface.

But the problem is that the box walls were tilted just ever so slightly. Now when I fill the cavity for the part I can never get it to fill all the way, resin begins to over flow one edge. If you use this method mount the part you are molding up off the floor of the box. I use the thicker double stick tape. You'll get a large 'pour stub'. It depends on what you prefer, fill a lip/loose some of the lip detail, or belt sand a pour stub away. I prefer to remove extra materail rather than add it (a little epoxy putty is all you'd need though).

Just a heads up on the our foam core experiences.

[davehal9000]

BIG necro post here; what's the equivalent of the molding material sold by Micromark? This stuff is getting expensive...

[Blappy]

BIG necro post here; what's the equivalent of the molding material sold by Micromark? This stuff is getting expensive...

Micromark Resin is SMoth-On stuff repackaged.