Making a two part mold in one part???

JupiterStation · Post by **JupiterStation** » Mon Feb 26, 2007 2:39 pm

Hey

I see Micromark has a "translucent" silicone rubber, for making two part molds in one pour. After it cures, you can just cut the mold in two parts, and away ya go! Anyone tried this? Does it work??

Thanks for your input.

Vince Hoffmann · Post by **Vince Hoffmann** » Mon Feb 26, 2007 4:07 pm

That method works for any kind of molding rubber that is stiff enough to support its own weight.

The big problem you'll encounter if you cut your one-piece mold in half will be re-alignment. To do a 2-piece mold properly, you need to create interlocking keys.

Simply cutting a zig-zag will not do enough in most cases to properly align the two mold halves.

If you want to just make one pour, then your best bet is to create a "butterfly" mold by cutting only halfway (or less), thereby keeping the mold intact with the ability to align and remove the casting once cured.

Arklan · Post by **Arklan** » Mon Feb 26, 2007 4:22 pm

i've seen the butterfly thing done ALOT with jewelry molds, rings and the like. one pour, one mold, but flexs open sorta like a sandwich to allow the peice to be taken out.

JupiterStation · Post by **JupiterStation** » Mon Feb 26, 2007 9:09 pm

Well, doesn't sound too promising...maybe I'll just stick with the tried and true methods. Thanks for the info!

Wug · Post by **Wug** » Mon Feb 26, 2007 11:35 pm

I haven't tried it. However, in Brick Price's Model-Building Handbook he uses a single piece mold to cast a rocket nozzle. He tears the mold apart at the base of the nozzle producing separate molds for the inside and outside of the part with a jagged edge to key them. This puts the parting line on the flat base of the nozzle where it's easy to sand. On complex shapes like figures, he uses two-piece molds.

If you try a one-piece mold, let us know how it goes. Good luck.

davehal9000 · Post by **davehal9000** » Mon Feb 26, 2007 11:55 pm

I've been using them in my tiny viper MK II project. I made a halfway decent fuselage and didn't want to risk screwing it up when I added the intakes. Instead I copied it and can experiment with it at will.

My Viper is about 1 1/4 inches long. I glued a piece of Evergreen tube to the tail, then glued that to a board. I formed the usual Lego dam around it, then poured in the rubber.

Once it was cured I scored down one side of the rubber block to free the master. As small as it is I really have to work to keep it free of bubbles, but I've yet to have any form of parting line on my casts.

the DOCTOR · Post by **the DOCTOR** » Tue Feb 27, 2007 6:12 pm

You can always glue rod or tube to the base board, pour the mould as usual, and use the resulting 'holes' in the mould as locators to silde the rods into.

unsc shipworks · Post by **unsc shipworks** » Sun Jun 24, 2007 9:17 pm

Vince Hoffmann wrote: If you want to just make one pour, then your best bet is to create a "butterfly" mold by cutting only halfway (or less), thereby keeping the mold intact with the ability to align and remove the casting once cured.

I like this lazy 1 piece method for relatively small parts. I find it helps a great deal to use a small disposable plastic cup for the mold wall. I cut the cup off when the RTV is cured, and then whenever I am using the mold I put it into a new cup of the same size before I pour the resin.

Lonnie Sweet · Post by **Lonnie Sweet** » Wed Sep 19, 2007 9:46 pm

Would this work?

Make a slightly larger box to make the mold a little larger on the edges. Prior to cutting the mold in half, drill 3 or 4 small holes all the way through the outside rubber. Cut the mold, and when you go to reuse it, put wooden dowels or brass rods through the holes to align the halves.

Chas · Post by **Chas** » Sun Sep 23, 2007 10:39 pm

I used to work in a rapid prototype shop and we did all of our tow part silly-cone molds as single pours, It's pretty easy once you get the hang of it - just like anything else I guess.
What we did was to build a box for the part that was open on the top. across the top we placed copper welding rods. These were then glued down where they came in contact with the frame of the box. We then glued more rods to the pattern where we wanted the vent holes to be. the patter was then lowered into the box so that it was suspended by the rods for the vent holes which were glued to the rods that had been attached across the open end of the box. then we pumped in the Silly-cone and through the whole thing in a vacuum chamber to get the air out of it and then through it in walk in oven to cure.
Now the trick to makinng the cutting open of the mould easy is to mark where you want the partting line to be with scotch tape and then colour the edge of the tape with black marker so that it is easily seen once the mould has cured.
For the moulds that used keys we would use larger diameter rods glued vertically to the bottom of the mould box and once the mould was finished when it came time to shoot the part we assembled the mould and placed these larger rods back in the mould to ensure correct alignment.