Making a mold

daltman1967 · Post by **daltman1967** » Fri Aug 19, 2011 5:16 pm

Ok, I know someone out there has done this. I've scratchbuilt my secondary hull, and kitbashed my primary hull. Everything is done.

What I'd like to do is to make four two-part molds: two for each hull.

However: I think a brush on molding compound would be better (and easier) than pouring, because I don't know how I'd suspend the parts.

Once I have the bottom part done, I need to fill the mold with SOMETHING to create the thickness of the model walls. Whatever this SOMETHING is, it can't damage the mold material or obscure the details. Once the mold is filled with this SOMETHING, then I can do the second part of that mold. I now have a complete 2-part mold for the bottom half.

This process would be repeated 3 more times, for a total of four 2-part molds.

The top half and bottom half sections should be about 1/8" thick. The reason I'm doing this is that I want to light the finished model.

My biggest piece is approx. 18" x 24" x 4". It has a deep undercut on the bottom side, as well as many carved and stand out details.

I'm looking for suggestions on the mold material, and the material to fill the mold with to make the thickness. I'm presuming it's best to use resin to pour the actual mold.

Joseph Osborn · Post by **Joseph Osborn** » Fri Aug 19, 2011 5:47 pm

I suggest you go to Smooth-on.com and watch every video they have that pertains to mold-making. You will probably find your solution there. You can also go to YouTube and watch all kinds of different mold-making videos. There's no better investment than time spent watching something being done.

Blappy · Post by **Blappy** » Sat Aug 27, 2011 8:59 am

What you said makes no sense. Suspend the parts???

Watching all of the video about two part molds will help. Smooth-on has a youtube channel as do others.

Joseph C. Brown · Post by **Joseph C. Brown** » Sat Aug 27, 2011 9:41 am

Perhaps a link to some pictures of what you have already built?

And some simple sketches of what you are planning to do with the molds? Because it would really help the rest of us in trying to advise you!

Rocketeer · Post by **Rocketeer** » Sat Aug 27, 2011 10:27 am

What you're trying to make is a hard-shell mold, so you might try googling a bit using that term. I believe I've heard of the shell part being made from plaster-of-paris or fiberglass.

Ziz · Post by **Ziz** » Sat Aug 27, 2011 11:50 am

Did you design the parts with molding/casting in mind or was this an afterthought? Depending on the shapes and angles, it may mean tearing some of your work apart or re-engineering it to fit together differently to make the molding/casting process easier.

And as Joe said - pictures. We're a visual crowd, we can't figure out where you want to go without seeing what you have to start with.

MillenniumFalsehood · Fri Sep 16, 2011 11:47 am

Blappy wrote:What you said makes no sense. Suspend the parts???

I think he means he doesn't know how to keep the part from submerging itself. It's a common misconception with people who have no experience or knowledge of casting to think that the part has to be suspended in the liquid rubber at the halfway mark, then when the rubber is set they'll pour the other half of the mold box.

At least, I know I did that when I first experimented with casting resin.

I have since learned that you "suspend" the part in clay first and pour the first half of the mold, then when it's set you flip it over, remove the clay, add the runners, and finally pour the second half.

ajmadison · Post by **ajmadison** » Tue Sep 20, 2011 9:00 pm

Thought that there are two problems. One, how to create a plug so the hull moldings won't be solid, and how to suspend the plug so it doesn't just sink to the bottom of the hull mold during casting, or just float up and out of the hull mold during casting.

Here are my suggestions, albeit my casting experience is limited

For the plug, just carve it out of some relatively strong foam. It doesn't have to be perfect, just roughly a 1/4" away from the hold surface. If its made out of foam, and a couple of test shots show a problem, you can always refine the plug without heartbreak.

Though an alternative technique I've read about, is to take your hull mold, use a high viscosity resin, paint it onto your mold, then embedded a fiberglass or similar cloth in it.

For suspending the plug, I would recommend engineering the entire molding "system" so you can attach both exterior mold and plug to a piece of plywood. Cut one or more holes in the plywood, and pour your resin through them.