Molding and casting very a very large rock.

[Random]

This is a rock which I found about 35 years ago in the middle of a dried up lake northwest of Austin, Texas.  I have used it over the years to make rock molds by layering latex rubber and then casting in plaster. 



I have had the (possibly harebrained) idea to copy this rock... twice, once in a vertical orientation and once in a horizontal orientation, for the basis for a large rock standing above a small port.  I was thinking that expanding foam would be the best way to cast this to keep the weight to a reasonable level for a portable module.  Do you think it would be practical to create a "full body" mold with latex and then fill that mold with expanding foam to copy it?  Is there a better approach?   If anyone has done something like this I would appreciate some your feedback.

Thanks

[bparrish]

That sir............ is one cool rock ! ! !

thanx
Bob

[ReadingBob]

Neat rock Roger. 

My only experience at trying something like this came a long, long time ago and it was an absolute disaster.     However don't base anything off my experience.  I (apparently) didn't know what I was doing at the time.  Remember "Mountains in Minutes"?  Is that stuff still around?  I tried using in a really neat rubber mold for a stone wall that I bought from Blue Ribbon Models.  I guess I didn't use the right mold release agent because the foam stuck to the rubber mold and pretty much ruined it.     But it certainly seems like it could be done if done properly. 

[cuse]

If you build industrial-strength benchwork like John S., you could just pile up real rocks and boulders...tricky part is getting the forklift into your basement





John M.

[BrianM]

You might try chatting with Joel Bragdon to see if he has any suggestions using his methods and materials.  More expensive than plaster but extremely light weight.


BrianM

[S&S RR]

The latex mold idea will work fine.  I have some very large latex molds that I have made from similar rocks. The only issue is the cost of the mold.  And the cost of the foam duplicate in full size. I have a piece of petrified wood that my Grandmother found in the field of our family farm that has been used to make many different latex molds for the rock work on my layout (see S&S RR layout thread).


By the way, Brian mentioned Joel - he has great molds that work fine with either plaster or foam.  The only issue is the size of your wallet vs. the size of your forklift, as John mentioned.

[JimMooney]

these guys have the stuff to do it...
https://www.smooth-on.com/

[rpdylan]

I'm wondering about the cost also,,,, I got some molds from Sterling Models and they absolutely blow away any rock molds that I have ever seen or used-I don't think I could make my own molds as good as the ones that I got from them. If you wanted something really big, those foam rocks from Cripplebush are really beautiful. But if you do make a mold from that monster rock that you have, you gotta post the process!

[bparrish]

I don't know where I got the idea but long ago when I made a bunch of cut stone molds I used the Woodland Scenics product.  I buried a layer of cheese cloth about half way through the stacking of the repeated building of the rubber stuff.


The molds lasted forever.

see ya
Bob

[darrylhuffman]

I personally find large molds are both hard to handle and tend to be "flat".

Unless you are modeling a cut through a hillside your rock surface will not be basically flat.

I like to build my support for the rock face using blue or pink foam sheets and then applying individual molds.

Your rock is great looking.  I wish I had it myself.

But if I had a large one piece mold of that rock I would end up cutting it into smaller pieces.

In fact, I usually end up cutting up my Bragdon Enterprises molds to make smaller ones.

[GPdemayo]

Neat looking rock face Darryl.....

[deemery]

...
In fact, I usually end up cutting up my Bragdon Enterprises molds to make smaller ones.

You'd rather cut the mold than the castings? 


dave

[Random]

I have two molds made from faces of the rock.  These are made by layering latex.  I made them ages ago and didn't know about imbedding cheese cloth to reinforce them but they have held up well with limited use. 




I settle a mold into a peanut tray so that I can create some contour and lesson the flatness.  I try to keep a mold ready when I pour any plaster castings and use the excess in the rock mold.




I also tend to only pour only small areas of the mold.  This lets me build up a selection of smaller rock outcroppings.




I need to make some more smaller molds.  There are many interesting areas on it. 

Back to job hunting. 

Cheers!