First effort disaster

Sorry to hear your having problems. The hydrocal was too thick. It should pour into the mold and flow into the crevices. Drying time should not be a problem either. 45 minutes should be plenty of time to dry. Did you spray the mold with wet water before putting plaster in it? What type of hydrocal did you use and how old is it? I have used hydrocal, casting plaster, molding plaster, and plaster of paris. All had similar results. I made the exct mold you are using. I made sure the mold was level by using packing peanuts to surround it. Hope this helps.

Bob

Thanks for the advice.  I checked for reply's earlier. You hadn't chimed in yet.  I did

some general web research and found a site where they said to put water in the container and sprinkle handfuls of hydrocal into the water until the water stops absorbing the hydrocal. then wait 5 minutes for the water to soak up all the solids and mix slowly by hand to insure there are no lumps or clots stuck to the bottom or sides. Pour into the mold.

  Still need to know how many castings I can expect to get from this mold.

The integrity of the mold will depend on how many layers of latex you used and if you reinforced it with gauze, cheesecloth, or etc. First, I painted the foam rock with latex house paint. After it dried, I put the first three layers of latex mold compound on very thin with a cheap 2inch paint brush. I did this in winter and had to wait about 3days between coats amking sure it was dry. The 4th coat was put on heavier and the 5th had cheesecloth embedded in it. I put more cheesecloth in the 8th layer too. I ended up putting on 13 layers, but I wanted it to last. It is extemely durable and has held up well.

Using wet water as a release agent helps to keep the mold from tearing. I ended up making this brick wall mold because Bragdon Enterprises was out and it would many months before he could make another one. Their molds are very thick, thats why I made mine with so many layers. 

Another consideration is mixing the plaster slowly so air bubbles are not trapped and make small bubbles in the finsihed casting. You can tap lightly on the sides of the mold to remedy this issue.

Good Luck,

Bob

I use a few drops of Dawn dish soap and water in a spray bottle for wet water. if the results don't improve, I would get different Hydrocal. 

Bob

Taylorra

   Welcome to the world of hands on mistakes! CP Bob's info is correct. I have done the same only with just Plaster of Paris in molds. I know how quick it drys and how it takes pigments or paint. The other products I have no experience with.

   As to your molds, I have found that the thin latex layers are the only way to go. Latex seems to "ball up" when it is thick. The mesh will help keep its shape.

     The dovetails were a problem on the first mold but were not going to be used anyway as the pillasters were in the plan. This gives you the ability to control the length of a section. I used a small coping band saw to cut the dovetail or part of the wall off. Take your time it is brittle and don't throw any pieces away. They seem to find perfect spots on the layout.

     The attached picture shows rocks that were made of plaster and colored with acrylic spayed on from an old windex bottle. Hope this all helps.

Ray Marion

taylorra:

In order for a mold release agent (MRA) to work it has to create a non-wettable interface between the mold surface and the media, in this case, HydroCal. The use of so called "wet water" in this application is useless as a MRA because the water is simply absorbed into the HydroCal. The surface active agent (Dawn Liquid) plays no part in releasing the mold.

I would suggest using either Polyvinyl Alcohol(PVA) or even better PAM spray oil or or vegetable oil as your MRA. A thin film is all that is needed.

Don't be locked in to a fixed water/HydroCal ratio as water and ambient air temperature play a role in final viscosity. Add sufficient water to achieve the thin consistency you need. Lastly, the use of HydroCal is a waste of money in your application. The difference between HydroCal and Plaster of Paris is a distinction without a difference for your purposes.

Taylorra

    SkyHookDepot said it best. I forgot about the wetwater/pam trick.

Ray Marion

This is not good advice. Time is your enemy with these materials. Just as a point of fact, the water does not absorb the HydroCal, the HydroCal is hydrated by the water

and the faster you can mix the better. Using as small a quantity as I need for an application, I use a latex glove and force the Plaster and water between my fingers until all the lumps are gone, and then add any additional water to yield the desired consistency. This mechanical action force hydrates the plaster in the shortest period of time and the order of addition is unimportant. Then just drop the glove into the trash and you are good to go.

This casting is a tricky business.  I used up my hydrocal, and not finding any local source, I bought an 8lb container of Plaster of Paris (where did that name come from). and tried it tonight.  It produced a thinner casting then hydrocal.  I poured a thin mix into the mold, and, within a couple of minutes when I inspected my mixing pan I found a layer of thick plaster on the bottom of the pan. I had carefully mixed the batch so that it was very smooth before pouring.  Since the solids in the mix settle so quickly, I am going to try tilting the pan, scooping out settled solids and liquid to pour into the mold.  I am sure I can do this quickly enough to get good pour, and hopefully a better casting.  Will post what results I get.

I haven't used plaster of paris in eons, but if mixed right I don't think it should separate or settle like your sounds it did.....maybe try layers...it should bond to itself{anyone wish to comment on that theory}.

The same time frame I was messing with plaster eons ago I was resin casting as well - big pours were a hassle{but we were making G scale rolling stock and engine bodies-they aren't usually small items}, I had to do multiple pours due to a time crunch. That worked for resin....I hope it could work for your plaster.

Hey, speaking of plaster{of paris}, have you tried real wall plaster from a home improvement store?

I'm not sure if I've ever read this much sturm & drang over hydrocal casting,

I just mix it up in a Dixie cup with a tongue depressor or equivalent stick with just enough water to make a nice thick soup that will pour, and into the mold it goes with no delay, no mold release, and no drama.  

BTW, time crunch on resin casting should not be an issue.  You can get resin that has a 20 min working time instead of the 2-3 min rubbish.

Taylorra

     For the molds you want to make a dixie cup is too small. I used a drywall mud pan from Home Depot like this.

http://www.homedepot.com/b/Bui...l-Tools/N-5yc1vZaro4

It took about 2 to 3 cups of plaster mix to about one to one and a half water. Add the water to the powder. I used a 2 inch metal mud knife to mix. The consistency when correct is like melted ice cream. Don't add all the water at once and mix thoroughly. There always seemed to be a little leftover which was tossed.

Ray Marion

Margarine tubs are useful, too.

Well, it wasn't like that 25yrs ago...that would've made things alot easier!

I tried to post a detailed description of my latest casting efforts but the format wouldn't allow it.  Condensed version:  Can anything be added to the plaster of paris to make it less brittle?  To avoid breakage, I want to try mounting the castings in

eight foot sections to 4" wide flexible plastic strips then mounting the strips to the

layout frame with liquid nails.  What adhesive should I use to mount the castings to the plastic?