Plaster Molds

Discussion in 'FAQs' started by ATSF-Loco, Dec 1, 2005.

  1. ATSF-Loco

    ATSF-Loco New Member

    Has anyone made molds for plaster before? For example I want to make concrete tunnel portals and Bridge abutments, but I don't like any of the commercial stuff. I would like to make a mold so I can create of my own with hydrocal.........or has anyone tried to simulate concrete walls out of foam? I could make the portals and abutments out of blue foam and......

    Thanks Everyone,
  2. ezdays

    ezdays Out AZ way


    I'm sure most everyone here has done something like this at one time or another, made plaster molds and used foam for concrete walls. There are numerous discussion threads and "how-to's" in the archives, so I would do a search and find a few.

    I've seen some amazing stuff where people have carved plaster to make brick and rock walls. If you want to make your own molded portal, you would have to start with a master be it from wood, plaster or plastic, then using molding compound, create your negative mold where you can cast duplicates.

    Since I model N scale, I have been able to make some small brick and rock walls using epoxy in rubber molds that I created. I don't know if I'd try that with a bigger piece, but we find out the limitations by doing and learning.

    I've not done anything in plaster, but you should be able to find a few threads here where others have done so sucessfully.
  3. doctorwayne

    doctorwayne Active Member

    If you're making "concrete" abutments and tunnel portals, styrene can be used to either represent concrete or as a material to make moulds. I use .060" styrene, bought as 4'x8' sheets, for either. If you use it to represent concrete, you need to layer it to get the effect you want (buttresses, curbs, or whatever relief you require) and then paint it to suit. If you use it to make moulds, you should first sketch out on paper the shapes you require. Unless the shape is very simple, the mould should be designed in two or more parts to facilitate removal of the finished part. I usually cast bridge abutments in place on the layout, whereas piers are easier to do at the workbench. If you go to the Photo Forum, I have several posts that show various "concrete" structures. Probably the best one to view, as far as variety goes, is entitled "A mysterious move...".
    The first bridge that the train crosses has poured-in-place abutments and workbench-poured piers, the second bridge has workbench-poured abutments, (not visible) and piers. The tunnel portal is sheet styrene, as is the long retaining wall by the coal dealer's yard near the large station. There is also a bridge, not shown, that has high concrete piers that are installed across the uneven bottom of a wide ravine. To make these, I made one styrene mould, in two pieces. The piers are tapered on all four sides and are of varying heights. The mould consists of one piece encompassing two adjoining sides of the pier and the second piece the other two sides and the top. There are a few aligning tabs glued onto the outside of the mould so that the two halves don't become misaligned during the pour. I use a paper towel to lightly coat the inside of the mould (before assembly) with vegetable oil. After assembly, the halves are secured with elastic bands and the mould stood upside down on a convenient work surface. I made this mould tall enough for the tallest pier required. For the shorter piers, I made a pencil mark inside the mould and filled only to the line, allowing a little extra height for carving to suit the site. Because the piers are shortened at the bottom, the profile at the top remains constant across the length of the bridge. Rather than Hydrocal, I used a patching plaster called Durabond 90. It sets firm in 90 minutes, and over the course of a day or two, depending on the thickness, becomes harder and less brittle than Hydrocal. If you want to carve stone detail on this stuff, do it before it gets really hard.
    I hope that this is of some assistance.

  4. zedob

    zedob Member

    I can relate. I started making molds for my own abutments for the same reason, the commercially available ones just don't look like any of the ones I see on a daily basis. So, I made my own cut stones, built up my master patterns and created a bunch of molds.

    The pattern making is the hard part. It just takes a bunch of time, so that the casting comes out right, detail wise. I enjoy it, but I also enjoy drawing pen and ink drawings (yeah, I'm a glutton for punishment). Lots of tedium, but quite satisfying when that first casting comes out of the mold.

    Just remember, you can pour crappy models out of crappy molds just as fast as you can create quality castings. The key is to make your masters as flawless as possible. You don't want to keep repeating a flaw. Of course, as usual, one doesn't notice any flaws until one has made a mold and poured the first casting.

    Alot of the times the mold flaws are due to improper de-gassing. You don't want to have a mold with air bubbles. The bubbles keep getting cast in place. However, this is not much of a problem if you are making a few castings for your layout, but if you plan on selling the extra that you make, you don't want customers complaining about crater filled castings. I'm not going to name names, but I've seen some extremely poor models being sold that I wouldn't put on any layout. It's as though the companies have no regard for quality.

    Alot of people say that you need to follow the 3ft rule meaning that if you can't see the detail from 3 ft then it doesn't need to be there. Chances are, those people don't take close-up pics of their layout. A camera CAN see those details if you are not careful. Nothing like having a well executed scene, only to have a retainig wall with a big smooth bubble hole in it that looks as though someone pried a bowling ball out of it.

    Check out for molding materials.

    i've never simulated concrete walls out of foam. I would (I plan on it) build up a master wall out of some scribed wood sheet. Scribe some vertical lines in each board row to simulate varying lengths of boards.

    Good luck and if you need any more pointers, post here or send me a message.
  5. XavierJ123

    XavierJ123 Member

    Making rubber latex molds for tunnel portals, etc is my thing. I have been copying John Allen's portals and viaducts by making a HO size "form" out of wood and styrofoam. The styrofoam is the curved shape of the tunnel entrance surrounded by the wood frame about 3/4 inch thick. Cover a piece of wood with wax paper and nail the styrofoam and wood to it. I use plaster-of-paris because it is easier to carve than hydrocal and allows more carving time. After I pour the plaster into the "form" I let it dry for 30 minutes and then remove the wood and styrofoam. Now the fun begins. I have a blank tunnel portal on which I carve brick or stone with cheap wood chisels. It takes me about four hours to carve the damp plaster. Practiice makes perfect. Mine are usually one of a kind, but I have made rubber latex molds to duplicate my work for friends.
  6. zedob

    zedob Member


    How's your viaduct coming along? I tried emailing you a few weeks ago, but wasn't quite sure if it had gone through.

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