Two piece mold for rotocasting, suggestions?

[Thoramyre]

Hi guys,
I have limited experience with making two piece molds for rotocasting objects of much size.
I am working on getting my foam build ups ready to cast with smooth-on 65D.
I have searched the forum and looked online for tutorials but few actually talk about making the mold itself, specifically what is the best method for creating the dividers for the two piece mold and which is the preferred way to divide the two halves.
Any suggestions from some of the experienced members here?
Any help is greatly appreciated!
Thanks, James

 

[Jaitea]

As you've said, you've probably seen this video....this guy is doing Iron Man parts:

I've made 2 piece moulds, but for model parts,....lay the part down in a container and build up modelling clay up to its mid so that only one half is exposed....place on some Keys i.e. Hex bolts so that when the whole project is done the two silicone halves have somewhere to lock together,....fill the exposed half of the model in the container with silicone
When the silicone has cured, remove the entire silicone, model & clay from the container,.....place the silicone back into the container,...put the model part back into the silicone mould leaving the un-moulded part of the model exposed,....give the interior of the container (the model & silicone) a blast of silicone release
Pour new silicone into the container,...when the silicone is cured remove from container,...you should have a two piece mould.....you will likely need to trim some of the silicone to allow for a filling point for the resin to go in & likely make an exterior container to hold the two halves together whilst casting

J

 

[NormanF]

There are a couple ways to skin this cat. I am surprised at how few videos demonstrate the way I would do it.
I would do a two piece brush on mold. A block mold for something that size is going to use a lot of silicone. You still do most of the process as in the above video for making a two part block mold, but you will brush on several layers instead of just pouring it on. You can get a general idea here: http://www.therpf.com/showthread.php?t=135886&p=2341797&viewfull=1#post2341797 Ignore the way I did my keys, they suck.

Instead of using 20 pounds of clay for the base I used foam core and small amount of clay. Here is the most important thing: Make sure the silicone is thick enough. If you use Rebound 25 it needs to be at least 3/8 of an inch or 1cm thick. Otherwise it can flop around while you are rotocasting. This video demonstrates how do do a brush on mold, but they do a one piece mold and cut it, which is also and option. I would also add a few keys for the support shell which is not demonstrated in the video.

You will also need a support shell that I mentioned above for that mold. Fiberglass, plaster, Plasti-paste, whatever.

 

[Thoramyre]

Great help!!
i like the idea of using a piece of foam backed with the clay!
The seam is the part I'm struggling with, on a cylinder shaped piece of foam it's harder than it looks using clay alone.
Block molds waste too much expensive materials and weighs too much.
And yes, a rigid mother mold will be used.
The feedback is appreciated, thanks guys for the help!!

 

[DonS]

I've made several 2 piece molds for roto parts and the process is largely the same as making any 2 piece mold...the ones I've done were made as shown in jaitea's post. I've done some large parts using block molds because I'm familiar with doing it that way. It works, but gets expensive and my molds weigh up to 40 pounds. In future, I'd like to try NormanF's method just to save weight and money. Been hestitant to try brushing as I'm afraid to miss spots and leave voids. Pouring a block fills in everything....but I'd still like to try another way just to learn something.

Do a search for rotocasting multiple layers and you should find some great techniques for making parts as laminates as opposed to doing them in a single pour.