• If you can't buy, multiply! (Replicating Retaining Walls)

    Some time ago I found a pair of Pre Size concrete retaining walls (to go outside my tunnel portals) at a LHS for about $3.60. Turns out it was an NOS product that never moved since they stocked it ages ago, because the product currently costs about $10 today!

    Anyway, I needed more retaining walls for my 4 other tunnel portals. Most of the ones on the market are the "stairstep" type design, (e.g. Woodland Scenics). I wanted the tapered end one like the Pre Size retaining wall but hardly anyone else makes those anymore.

    So I decided to put the retaining walls I have into a mold and cast duplicates out of Hydrocal!

    I was thinking of doing a silicone mold, but after researching it, it seemed like too much trouble for me (measuring, weighing, chemical exposure, etc) so I tried this product I found at a local art supply store:

    It's called InstaMold, made by Activa, and the 12-oz box sells for just $10. I thought I'd give it a try.

    The stuff comes in a cream-colored powder called alginate, the stuff used to cast dentures. When mixed with water, it sets as a flexible, gelatinous substance. It's a natural byproduct (an extract of seaweed) and totally non-toxic.

    So I made a mold out of thick basswood strips caulked to a sheet of styrene, and caulked my original retaining walls (themselves castings made out of resin) onto it:


    Then, after the caulk had dried, I poured in some uncooked rice grains inside to fill up the mold frame, and after it had leveled off, I dumped the rice into a cup and measured the amount on the cup.

    I used that amount to measure the equivalent amount of InstaMold powder, and mixed that with an equivalent amount of water.

    My first attempt at making a mold was a failure, there were too many lumps and air and water bubbles. It looked almost like a block of swiss cheese. I ended up throwing it away.

    The next attempt was much better. I had slightly more water into the mix, stirred it vigorously with a fork and slowly poured the mixture into the mold. I even poked around with a plastic fork to kill off any air bubbles.

    The second mold was much better, there were only 1 or 2 noticeable bubbles in the mold, and those imperfections could easily be sanded off. Here was my mold of the retaining walls:


    I later mixed in 2 1/2 heaping teaspoons of Hydrocal with water, then poured it into my mold. After about half an hour, I simply picked up the mold, bent it back and the casting easily pushed itself out.

    Here's three of the Hydrocal castings:


    As you can see, the board form detail came out pretty well. Should look even better after staining and weathering! I got me a bunch of retaining walls now! Woo-hoo!

    One caveat when making an InstaMold/alginate mold: It relies on water to keep its shape and flexibility. If exposed to air for a prolonged period, the entire mold will shrink and deform. If you plan to make castings at a later time, I strongly recommend storing your mold in a Ziploc bag (remember to push out as much air as you can before sealing) when not in use. If you want to make several castings, you're better off using silicone. I'm only going to make 4-6 pairs of retaining walls and then will no longer need the mold.
    This article was originally published in forum thread: If you can't buy, multiply! (Replicating Retaining Walls) started by MetroRedLn View original post
    Comments 6 Comments
    1. nmanic's Avatar
      nmanic -
      This works for me and you may want to giv e it a try,When I pour my molds I hold the cup up really high and let it pour in a thin stream this should help get rid of your air bubbles.

    1. Newbie51's Avatar
      Newbie51 -
      Is consideration being thought of for putting this in to the tutorial section?
    1. BryanC (RIP)'s Avatar
      BryanC (RIP) -
      Quote Originally Posted by Newbie51 View Post
      Is consideration being thought of for putting this in to the tutorial section?
      It has already been promoted to an article (which you may have noticed). The Tutorial forum also crossed my mind when I first read the OP. I don't think a second copy in another forum is really necessary (duplicate posts never go over very well) but a thread in that forum referencing this thread would certainly not not be inappropriate!

      I certainly understand why Metro chose to post it here. Sometimes posts such as this can cause a bit of a dilemma! This is why a link in the Tutorials forum would be perfectly acceptable.
    1. 10-7's Avatar
      10-7 -
      good post, I used to use a similar material at work.never fooled around with using silcone to mold things.the mix would be worth looking into for the layout ! thanks for the post

      10-7 aka Ken
    1. MetroRedLn's Avatar
      MetroRedLn -
      One major warning - if you need to store the InstaMold mold, keep it in a ziploc bag. But do not store it for more than a week or two; ironically, the mold will grow...mold! (ewww) I learned this after finding the bag with the InstaMold mold inside it, covered with lots of dark spots. Fortunately, I already made the 4 pairs of retaining walls I needed to make, so discarding the moldy mold wasn't a real loss.
    1. Scaper's Avatar
      Scaper -
      Awesome post. I need to learn how to do this...I could make so many duplicate parts and save so much money.