What should I use to fill in the mold? Plaster or hydrocoal?
I used Plaster of Paris, but many have used (and prefer) Hydrocal. Just make sure that if you are using a commercial rock mold, you follow the directions to make sure the plaster will release from the mold.
I was cheap made my own molds using Aluminum foil. I coated the inside with a cooking oil spray so the plaster would release. It worked fine for me.
Same here. I always use Plaster of Paris.
By the way, when I put them together to make up a rock face, I use Artex which is that stuff you use on ceilings. It’s very flexible, holds when wet and dries slow enough for you to manage it and it doesn’t crack.
Hope that helps.
Whichever material you use, Hydrocal or regular plaster, be sure to use the same material to fill in the voids between castings so they will take paint or stain evenly. Here is a link to learn more about what plaster is and the different types of plaster http://www.plastermaster.com/plasterpower/plasterpower2.htm and http://www.plastermaster.com/usg/
Here is a sorce for molds and mold making supplies and castings http://www.bragdonent.com/catalog.htm
I wouldn’t recommend using any kind of mold release because that may interfere with paint / stain absorption.
Have fun but don’t wash plaster down the drain, otherwise that’ll put a stop to your fun big time!
G.
Without mold release, how does the casting release from the mold? (That sounded kind of redundant).
Rob
Hi Rob
The molds are made of either silicon or latex rubber which plaster won’t stick to. You just simply and carefully peal off the mold from the casting.
Casting plastic on the other hand may require a mold release because the resin is sticky. You can wa***he mold release off plastic but plaster will only absorb the release agent and won’t wash off.
G.
I have several Woodland Scenics molds. Though hydrocal is much lighter I use plaster of paris as it is much cheaper and sets in about 20 minutes.
I spray the molds with a soapy water solution (dishwater detergent) which I find helps the plaster come free from the mold with no problems.
I use the system described in an old MR article by Malcom Furlow. He made rock castings using canned foam, the spray stuff from Home Depot. The full detail of the method was explained in the article, part of a series on building the San Juan Central I believe. If you have an interest in this, you can get a copy of the article downloaded from MR. By the way, this method is fast, neat, clean and gives very good results.
Tom
Could someone tell me what the proportions of water to hydrocal are for rock molds?
Thanks.
It’s a little complicated with doing it by weight and all, but here’s a link to the scientific way: Water-to-plaster ratio page. (click on highlited words)
Remember to add the Hydrocal to the water and not the other way around.
Since I’m not shooting rock castings into space, I just mix the Hydrocal with plain cool tap water to the consistency of thin pancake batter. I also pre-wet the mold with plain water.
The water with detergent spray on the mold helps my plaster release very well when dry. I use the light hydrocal and have been happy with it. As per the instructions from woodland scenics, the mix is 1 part water to 3 parts hydrocal for a mix. It does set up and dry in about 30 minutes. When finished with painting etc, I spray the rocks with the scenics cement at this seals the plaster from dirt etc.
Can you also use Plaster Of Paris for finishing off and filling in gaps and seams in your mountains and hills after the plaster cloth base dries?
As gsetter mentioned, Bragdon Enterprises is one source of rock molds:
They have a different system. The castings are made with a chemical foam product. It produces a lightweight, very fine-scale casting. It’s considerably more effort than simply pouring Hydrocal into a rubber mold, but I think the results are outstanding. One nice thing about Bragdon castings is that they can be re-heated with a hair dryer, and they will become flexible if you need to bend them around a curve.
The Bragdon molds, by the way, can be used with plaster or Hydrocal. They are more expensive than Woodland Scenics molds, but the detail is better.
I have had good success with Hydrocal I use a little heavier mix of wet water then is recommended for ballasting track. Use a good squirt rather then a few drops of dis washing detergent and make sure your molds are stable when you pour the hydrocal in. If your mix is a little soupy thats ok it will just take longer to dry but it still yields a nice rock. By supporting it if it is soupy it won’t pour out allover the floor. I set them in a box lid full of those pesky packing peanuts. Supports them perfectly.
Hi from Belgium,
Because I can find hydrocal easily here in Europe I use molding plaster for my rocks molds (homemade and Woodland).
It’s cheap; something like 6$ for a bag of 15kg, beleive me you got a lot of molds with one bag!
I mix just enough plaster to fill the mold with a consistency of a Yoghourt.
The mold is just sprayed lightly with a spray of water and a few drops of detergent as a mold release.
This kind of plaster dry fast, you have no more than 5 to 8 minutes to work whit, but because it’s a molding plaster it keeps the details very well and it’s also very strong.
Below some examples of the already placed molds on my Maclau River in Nscale
Marc
I, too, have used the Bragdon latex molds with the geodesic foam. Because they are flexible and you have working time, you can use the rock molds for tunnel portals as well as rock faces.
Sue