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RTV Molds

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  • RTV Molds

    I've been pouring hydrocal into molds made with Woodland Scenics liquid latex and have noticed tiny bits or the mold coming off with the casting. I have not been using any type of mold release. Is there something I should be using as a release to prevent this?



  • #2
    First off, Woodland Scenics Latex is not RTV, it is latex as its name states. RTV stands for Room Temperature Vulcanizing. You make a 2 part mixture and pour it all at once over your master and it vulcanizes very quickly. With the latex you apply thin layers over many days allowing each layer to dry.

    Now, to your question. If the latex has fully dried, you shouldn't have any problem removing a casting. I have made a lot of latex molds and have never used a mold release. I do spray the mold with a 'wet water' mist to help get the plaster into all the nooks and crannies. I also tap the mold support a few times to help any air escape. I have no idea why latex is pilling off.



    • #3
      Hey Otto, I also spray the molds first. never have had a problem!
      Andy Kramer -- Modeling the Milwaukee Road in Wisconsin


      • #4
        Hi, Otto,

        Occasionally over the years, I have had the same problem with molds and castings. What I finally discovered was that it was a combination of two things. First, the molds had too many undercuts that made a clean removal of the casting difficult. Second, I sometimes left the casting in the mold too long. The combination of those two things would cause small bits of the mold material to tear and and remain embeded with the casting.

        The cure was to spray the mold with "wet" water as the others have suggested and not leave the casting in the mold for longer than 30-40 minutes.

        I have never used WS's latex, but have used other "craft" brands over the years.


        • #5
          Thanks for the replies, guys. Think I'll try the wet water.


          • #6
            I have some latex molds dating back to 1974, and find that they become "sticky" in some instances, and leave bits adhering to the casting upon removal. I have used a product called "Armor-All", obtainable in auto supply stores, (it purports to protect painted surfaces) with success in avoiding leaving bits of the mold on the casting. It does not interfere with acrylic washes used to color the castings.


            • #7
              Wallace....sounds like a great tip. I have plenty of it in the garage. Will try it.


              • #8
                Hi Otto,

                I use Dow Corning HS II RTV High Strength Rubber Silicone. It come in a small plastic jar with a catalyst with a mixing ratio of of 20:1. It is excellent for producing single piece molds. Most of the casting I do is in resin, but hyrocal works very well as well. Mikes comment using "wet water" is a good one. I also sometimes use mold release for resins, but usually only spray it about every 12-15 castings. You should get good life out of this product if you can find it in your local market. If you have a lot of undercuts you might want to consider making a two piece mold instead of one to prevent breakage or chipping. I have molds where I have pulled 100 odd castings and it is still functioning pretty good (resin will eventually dry out the mold due to the intense heating when the resin cures). It is always fun making your own parts and details.

                By the way, Lego and clear plastic blister packs work great as your container for making molds. Either rob your kid's Legos or the garbage can - both work great!

                Good luck! Let us know how you make out.

                Karl F.


                • #9
                  When trying to duplicate small parts how do you secure them to the bottom of the container?

                  Also when carving/creating a master wall what do you guys use most? Someone told me about balsa foam(Dick Blick)anyone use this item with succes when trying to make an RTV mold of it?


                  • #10
                    I use CA glue. Works very well. I use anything that works well, but typically styrene, wood, bits and pieces of household items, parts from 1/35 armor models - even carvin bars of soap has worked. Thanks, Karl F.