Polyresin vs. Hydrocal

[Big_Boy_4005]

QUOTE: Originally posted by plasticlizard

The only thing I don't like about Hydrocal is that it tends to break easily. I was wondering if anyone has used 2 part resin for making rock molds, walls, etc?

Any thoughts?

It sounds like you might be making your castings too thin. Are you using any release agent before filling the molds? When you fill the molds, try to fill them above the rim a little, making the middle thicker.

The nice thing about gypsum based rock castings is they take color nicely, and are inexpensive. Resin will give you neither of those features, and unless you buy some really expensive stuff, it will smell bad too.

[Anonymous]

I'm not sure why you might be having problems with Hydrocal for such things as rock molds. I've been using the stuff for years (as have countless thousands of other model railroaders) and have never found it to be too brittle or to break easily. If anything, it's one of the stonger plaster products, in addition to being somewhat lighter than some other types. Are you sure you're not using too much water in your mix?

[eZAK]

As an alternative to Hydrocal you can use Plaster, i.e. 'Plaster of Paris', Casting, Modeling, or Dental plaster.

You can get #5 bags at home centers for about $8.

[daan]

You can reinforce plaster molding easily, so they don't brake apart that quick.
If you make your molding, add a piece of chicken wire (mazed metal fencework) in the plaster before it hardens. It will do the same trick as concrete iron in buildings. Make sure you use rusttreatened stuff, since plaster is causing metal to rust quickly.
Also make sure your mixture is right. If it isn't properly mixed, with too much plaster or water or if the hardening already began when you start pouring it into the mold the plaster will not set as strong as it should.
Joint compound or wall filler is, as far as I know, not suited for molding, since it only hardens out if used in thin layers of only half an inch deep at max.
I use the chicken wire reinforcement trick a lot in molds and landscaping and so far, didn't have any complaints about it.

[Anonymous]

urethane plastic is easy to use. it does cost more than plaster. you can get small amounts from companies like smooth on or polytex. they also both make odor free formulas.

[Roger Bielen]

If you try using joint compound for molding it is a bit too thick to fill a mold properly and tends to crack if it is a thick slab as it dries. If you thin it with water it make a brittle casting. I think there are other casting plaster than Hydrocal available at craft stores but I haven't looked.

Another thing I've done for adding color and thinning joint compound is to use latex paint. I've also added sand for texture.

[Anonymous]

has anyone used joint compound for making things like molds for rocks, walls and things?

Maybe this is the best way to go, instead of hydrocal.

[Roger Bielen]

Haven't used it but its working time is probably shorter and fumes could be a problem. Also most resin are costly compared to Hydrocal. The only thing I used Hydrocal for was casting rocks from molds, for attaching and sculpting I used pain old joint compound. I think I've at least three 5 gal. pails on my layout.

[Anonymous]

The only thing I don't like about Hydrocal is that it tends to break easily. I was wondering if anyone has used 2 part resin for making rock molds, walls, etc?

Any thoughts?