Hydrocal rock molds

Discussion in 'FAQs' started by warren danzenba, Apr 7, 2003.

  1. warren danzenba

    warren danzenba New Member

    Using lightweight Hydrocal and Woodland Scenics rock molds, I can't seem to make rocks without them breaking apart when I try to remove them. Rock molds (such as the enbankment mold) are shallow rocks so there isn't a huge amount of plaster being cast. Have tried to follow mixing proportions exactly, and have tried to remove them after 30-min, an hour later or even overnight. Same result----they break apart. Anyone having "good" success with rocks that I might follow?
  2. Bill Pontin

    Bill Pontin Member

    Warren are you spraying the molds with some "wet water" before pouring the hydrocal or plaster? Mix some liquid soap detergent, good size cap full quantity to an average sized spray bottle. Spray your molds just prior to pouring and I think you should have no more problems. The only breakage I've experienced is with some of the deeper molds. The deeper molds I let set several days and then tease them out. Anyhoo!, if they break -piece them together as you adhere the pieces onto your layout, gives a better variety.
  3. philip

    philip Guest

    toothpicks or pipe cleaner

    Warren : As soon as you pour the hydrocal into the mold have a toothpicks ready and sink it into the hydrocal. This will serve as a reinforcement for your casting similar to Re-bar used in concrete. Pipe cleaners will also work. How are you mixing your hydrocal? Always add the hydrocal to the water. If you add water on top of hydrocal the mixture will have massive amounts of air bubbles and cause the casting to be weak. If this doesn't help you may have a bad batch of hydrocal which is very unlikely. Keep trying ! Mixing that stuff is a real artform....may take several attempts to get it right. Use the wetted water to.

    philip :)
  4. farmer ron

    farmer ron Member

    Warren, It sound like what Bill and Phil have mentioned. Your molds are "dry" they do not have any release material between the rubber mold and the plaster. I also use pam, the house hold spray that you use for cooking as well as wd40. Remember that after each use of the mold you put it in warm water and clean it well. Ensure that you get out all those little pieces of plaster that were left behind when taking out your mold. That is a great idea that Phil has in using tooth pics, I use strips of gauze that I cut up before mixing the hydrocal. Ron
  5. 60103

    60103 Pooh Bah


    or maybe those strips of drywall tape, the ones matthyro uses for windows?
  6. Vic

    Vic Active Member

    Hi Warren...Just some thoughts on the problem....I've used a lot of the WS molds and other brands and have even made some with RTV and haven't experienced a breaking problem when trying to remove the finished piece.

    Also I'm not familar with "light Hydrocal" although I have heard of it....assume its a mixture of Hydrocal and some kind of filler to cut down on the weight of the scenery. Regular 'ol Hydrocal is very dense and heavy once its set up.

    My first thought is that the "light" Hydrocal just may not be dense enough to make casting that have undercuts in the molds such as rocks. You might try using some regular Hydrocal if you can find it in your aera.

    My second thought is that possibly you are not allowing sufficent time for the "light" Hydrocal to fully set and harden. While it may appear to be hard it is still soft on the inside and consequently the casting breaks when you try to remove it from the mold. Its easy to get in a hurry when making these casting because its so much fun to do it!!!.....try doubling you set up/hardening time and see what happens.

    My third thought is that while you may be mixing your plaster in the right porportions it still may not be "heavy" enough. You want it to be the consistency of thick cream. Also mix it well to get rid of any trapped air in the mix. Tapping the mixing container on the table top a few times will help too as that will drive any air bubbles to the surface.

    Last but not least....always add water to the plaster when mixing it....doing it the other way around....plaster to the water....will vastly affect its strength. Also always add just a little water and mix and then add some more....depending on the temperature and the humidity you will find that you may achieve that "thick cream" consistency with less water than called for in the instructions.

    I've heard of wetting the molds with water helps but I have never done that...the use of a release agent is not necessary as Hydrocal will not adhere to rubber or silicon. It is very important to clean the mold thourghly between pours as any harden plaster remaining in the mold will affect the strength of the next pour.

    Hope this helped :)
  7. rsn48

    rsn48 Member

    Also, when mixing, stick your hand in the glop and run all the hydrocal through your fingers, squishing it. You will find there are many lumps in the mess and this is the best way to find them and get rid of them.

    By the way, hydrocal can really plug up a sink quickly. So dump your water from bucket outside.
  8. warren danzenba

    warren danzenba New Member

    Reply to rock molds

    I really appreciate all the suggestions that have come in....thanks guys! I forgot to add that I have been wetting the molds with wet water....but shaking most of it out so as not to leave a puddle in the bottom of the mold. The toothpick idea is one I will try! I said that I was using "light Hydrocal"...I should have added that this is the big yellow box of powder sold by Woodland Scenics. I will also try the Pam trick. I'm beginning to think I am adding too much water and the mix is too wet. Will experiment a little! I now have the biggest pile of "rock parts"!!!
  9. RailRon

    RailRon Active Member

    Warren, by all means don't throw these "rock parts" away. So you already have quite a lot of rubble which collects at the base of almost every rock wall.
    Dump these small rock pieces at the foot of any rock and fix them with the usual water/white glue mix (or your preferred method).

    This is a detail which immensely adds to the realism of the scenery. (Unfortunately this is often omitted. :( )

  10. Russ Bellinis

    Russ Bellinis Active Member

    Another place for your rock pieces is in making "rip-rap." Here in Southern California, the Santa Fe used a lot of "rip rap" where the tracks run along the beach. Sand tends to move around and wash out from under the ties and ballast, so the Santa Fe put rock fill in under the balast and then laid the ties and rail on top. I think that is a common railroad practice anytime the tracks pass close to a beach, either ocean or lake.
  11. Blake

    Blake Member

    2 things drove me nuts the first couple of times i used Lightweight Hydrocal. First, was the mix ration. A quick glance at the box and I saw 100 parts to 65 parts. I figured a little better than 50:50. WRONG, that was for measuring by weight. The next time I tried it, I noticed it's reluctance to mix with the water. To help this along add some Lysol or other type of soap to the water before pouring it into the Hydrocal. (Lysol is best as it also kills little biologicals that can grow form the plaster). Actually, any time you are mixing dry powder and water (plaster, hydrocal, sculptamold, celuclay) you should always do this. The soad breaks the surface tension of the water allowing it to mix easier with the powder.
  12. warren danzenba

    warren danzenba New Member

    rock molds

    Blake....good advice...thanks...did you wind up finding that the 100 to 65 ratio was right after all? Just curious....how long did you leave the plaster in the mold...before crossing your fingers!....and prying it out?
  13. Blake

    Blake Member

    Nope, that ratio is by weight. The proper volume ratio is 1-1/4 cups hydrocal to 1/2 cup of water. Leave it in the mold over night.

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