For my Pennsy Branch layout I needed cut stone abutments for the two bridges on the layout. These should really be made out of plaster for the best painting and weathering effect. I proceeded to make a quick-n-dirty form to pour Woodland Scenics Lightweight Hydrocal into. After spending about two days building the form, the final results were very disappointing. First, I had made the form so sturdy that I couldn't get the cast out of it. Then, as I did manage to get a large chunk of the cast out, it was so very brittle and fragile, even after a day of drying time. I won't be using that kind of Hydrocal again. I finally decided that since I needed four of these abutments, it was going to be worth my time to build a true master and make a rubber mold of it. This page documents how I did that.For my Chartiers Branch layout I needed cut stone abutments for the two bridges on the layout. These should really be made out of plaster for the best painting and weathering effect. I proceeded to make a quick-n-dirty form to pour Woodland Scenics Lightweight Hydrocal into. After spending about two days building the form, the final results were very disappointing. First, I had made the form so sturdy that I couldn't get the cast out of it. Then, as I did manage to get a large chunk of the cast out, it was so very brittle and fragile, even after a day of drying time. I won't be using that kind of Hydrocal again. I finally decided that since I needed four of these abutments, it was going to be worth my time to build a true master and make a rubber mold of it. This page documents how I did that. You need a master (i.e., an original) to be able to make a mold. This can be made out of whatever material. Since I am a woodworker, I made mine out of wood. This is going to be an abutment for a single-track branch line. I worked out the dimensions of what I needed, and cut a piece of plywood to size. "Cut stone" abutments are made out of blocks of stone. I don't have any prototype measurements, so I decided to make my abutment based on individual stones that are 16 inches deep, 16 inches tall, and 32 inches long. This matches prototype photos I have analyzed. The dimensions make it easy to work with in S-scale, because that means each block is 1/4" by 1/4" by 1/2" in real-world dimensions. The ends of the abutment are therefore then staggered. I cut grooves in the plywood to the correct depth, as can be seen in the first photo. I used my table saw for this.
I then used a handsaw to cut off the individual grooves. An abutment has a ledge upon which the edge of the bridge rests. I used the table saw, set at the correct depth to cut 1/4" away from the thickness of the master near the top. I made it three stones deep, about 4 scale feet. I then sanded and filed the master smooth and filled some gaps created by minor mistakes while cutting.
Finally, I used a carving blade to mark the mortar lines between the stones. The idea is to do it once in the master, so that I don't have to do it for each casting I will make of the mold. The master has to be absolutely accurate, so it is well worth the time invested creating it. Now looking back, I should have also cut mortar lines on the top, horizonal surfaces. I had forgotten about those. Oh well, I can make those into each of the casts.
The next step was to make a box that will hold the liquid rubber that will become the mold. I decided to use 1/4" foam board with which to make the box. The foam board came from Hobby Lobby, but you should be able to find this inexpensive material at any crafts store. I cut the parts using a single-edge razor blade. I, then, used ordinary white glue to hold the box' parts together. As seen in the next photo, clamps were used to hold the side pieces to the long side panels. The master was glued to the bottom of the box using white glue also. The vertical pieces are almost 1/2" taller than the master.
The next photo shows the final box. I added diagonal pieces to reduce the amount of rubber mold material needed.
Years ago I had bought the mold-making kit by Micro-Mark. Unfortunately, when I opened it to use for this project, "part A" had completely hardened. I guess that stuff has a shelf life of only a few years. Be aware of that when you buy it. So, off to Hobby Lobby I went and bought a one-pound container of Alumilite's Quick-Set RTV Silicon Rubber (the product's name has been changed). This particular product (and they have several, depending on your needs) is great for simple one- or two-piece molds. Mine is a one-piece mold, because the back of the abutment is flat up against the bottom of the mold box. The backs of the abutment castings will be buried in scenery, so there is no need to model details in them. It wasn't until I got back home and went on their web site that I learned that a one-pound container makes about 21 cubic inches of mold. I was hoping that it was enough. My mold box wound up being 49 cubic inches, but that is not accounting for the angled pieces I added and the space taken up by the master. This one-pound package is $30, so it is fairly expensive.
This system works by measuring the weight of the two parts. You need ten parts to every one part of the catalyst (the smaller of the two containers). Since I needed a lot of this material, I decided to mix the catalyst directly into the larger container. I made sure to wear an apron and gloves. After I mixed the two parts, I carefully and slowly poured the contents into the mold box. The result is shown in the next photo. I probably needed a bit more material, but it should be enough.
I let the mold cure overnight. You only need 4 hours with this particular product, but I was in no hurry and I sure didn't want to ruin $30! Although hard to see because it is essentially cream-on-white, this next photo shows the sides of the mold box carefully broken off of the base. It then removed the base from the mold. All this takes some time and a bit of patience, because you don't want to tear the mold.
I then removed the master from the mold and this is the result. The mold has a bit of flash yet to be removed. This was because the liquid rubber managed to go under the master. However, the flash was very thin and easy to remove with a sharp blade.
To give you an idea of how amazingly accurate this is at mimicking the master, you can tell from this close-up shot the "stubbles" left from the individual layers of the plywood master. I actually wanted to use a piece of solid wood, but I didn't have one that was big enough for this abutment. I don't mind the roughness, though, because the castings will then have a bit of a rougher edge, rather than a perfectly smooth surface. That is probably more realistic anyway. You can easily see the mortar lines in the mold. Very cool! I was very happy with my first mold-making experience.
Based on my above-mentioned experience with Woodland Scenics Hydrocal, I decided to use my trusty "Plaster of Paris", which I had used for casting rocks for previous layouts. I mixed about 8oz of plaster with about 5-1/3oz of water and poured the whole kit-n-kaboodle into the mold. It was just enough. The first one (shown here) wound up with a bit too much plaster. The mold is actually a bit too deep for what I wanted (I wanted a master that was 1/2" thick, but I decided to take the easy way out and just used a piece of 3/4" plywood). For subsequent castings I simply didn't fill the mold to the top. What you are looking at in the photo is the back of the abutment, which will be hidden in the scenery. The bubbles that formed can be sanded off later on, if need be.
I waited for about 45 minutes before I removed the casting from the mold (the "Plaster of Paris" instruction say to wait about 35 to 40 minutes). I then set the casting aside overnight. The next day I put it in what will approximately be its final position. It may be the humidity here in Houston, but it was still not quite dry the next morning. But, as you can see from this photo, the mortar lines and all the other impressions from the master all transferred to the casting.
This was my first experience in making a mold from a master, and then several castings in the mold. It was a simple project to start with, because this was a one-piece mold. In the future I will have to tackle more advanced molds. All in all, it was easy to do. It just requires some patience. The hardest part is making sure the master is accurate. The rest of the time you are just waiting for things to set and dry. I really like the Alumilite product. Even though this mold cost $30 to make, I will be able to cast a good number of abutments from it. Here's a photo of the abutments in place.