Hello,
I am new to model railroading, have only been doing it for about six months. I am a civil engineer by trade and lately I have been asked to give presentations to students about bridge building. The students range any where from 6th grade on up to 12th grade. I have been trying to make precast concrete beams using foam board molds and plaster of paris. When I get the mold perfected I want to use anchor bolt cement for the beams, deck, abutment, etc. I am having a lot of trouble with the foam board, because it is difficult to cut all the small angles for the beams. And when I made my last mold, the form sort of bowed out not making the beam even on each side. I am wondering if anyone has any better ideas about what to create a mold from, I am open to about any suggestions right now so any help would be appreciated. I am ultimately going to have an interactive model where I can show how a bridge goes together and how the forces work on it. Thank you!
Hi, and [#welcome] to the forums.
I use .060" sheet styrene to create simple moulds for casting bridge piers and abutments. It’s easy to work with (score-and-snap using a utility knife, cement together with a solvent-type cement - I use lacquer thinner) and relatively inexpensive. A 4’x8’ sheet costs between $20.00-$30.00 or, for smaller items, it can be bought at a hobby shop in smaller sheets.
I use Durabond patching plaster for the casting process - it sets quickly (various setting times are available, denoted by the number following the name - Durabond 90 sets in approximately 90 minutes) and is hard and durable.
For large surface areas, you can cut strips of styrene, then cement them, on-edge, to the outside of the mould to increase dimensional stability as you fill the mould.
A couple of photos to illustrate the idea:
I use a paper towel to apply vegetable oil to the mould before assembly. This aids in mould removal and doesn’t seem to affect the plaster.
Wayne
I had seen this in another place, I think I have the piers and abutments ready to go. The only real trouble I am having is getting the angles to come out right in the beam molds. Are you suggesting that I make my molds from styrene? Thank you for the help.
There is a product called Gatorfoam, which is basically foam core with a thin veneer of hardwood on it. This may work better than foam core, as it’s a little more rigid and easier to cut precisely because it doesn’t smash like foam core.
You might try visiting Micromark’s web site (www.micromark.com). They are a great source of all kinds of modelling supplies.
Have you considered buying pre-cast styrene parts? There are a number of suppliers of these as well.
Yeah, the styrene is inexpensive and easy to work with, and it should stand-up well to repeated use. You can add as much exterior bracing to the mould as is necessary to keep it from deforming when filling it.
I’m guessing that you’re thinking of an I-beam type of cross section, so the mould would need to be in multiple pieces. Two pieces, perhaps, with one side and the top as a unit, and the other side with the bottom included. It may be even easier to use if you make it as a four-piece mould: one piece for each side and a separate top and bottom. I’d do this as a vertical mould, so you’d need a fifth piece to act as the bottom. This should allow easy removal of the casting without stressing anything that has a thinner cross section.
If any of the faces are curved (concave or convex) you would probably need to use thinner material. Simply cut the exterior bracing to match the curve you want, then assemble the parts using solvent cement and clamps. Otherwise, .060" sheet should work well, unless you need a really large casting.
While the photo below isn’t of a mould (it’s the bracing on the back of a building flat) it does illustrate the use of strips cut from sheet stock to provide support. The wider the strip, the greater the support and you can shape the edge which contacts the exterior of the mould to impart curves.
Wayne
What size precast beam are you trying to recreate? If it’s a small section, then styrene is probably a pretty good choice. If you’re looking to make something a bit larger, then I’d think something like extruded foam insulation might work. The narrowoer web could be formed by gluing a trapezoidal-shaped piece to the inside of each vertical side form. And you definitely want some sort of form release agent, like cooking spray or the like, to keep the casting from bonding to the form.
I’m a structural engineer by profession, so when I read phrases like “precast concrete beam,” I get a little geeked up. [(-D]
Jim