kjd
Go make something!
In another thread it was pointed out a C channel would be easier to mold and cast than an I beam. It is true that a C channel could be cast in an open face mold but a lot of things are not flat on one side.
One of the best parts of model railroading is all the skills and knowledge about seemingly unrelated subjects we can learn if we want. Molding and casting is one I happened to have learned.
Below is a photo of a mold of a dynamic brake hatch and a casting from the mold. The casting has since been modified to match the one on a particular SD40-2 rebuild.
To prepare the master, I made sure it was clean and put scotch tape over the inside of the fan holes. This just makes the separation between the front and back easier when cutting the mold. Next I made a sprue from the end of a syringe. They don't have a needle, just a opening like an eyedropper. I drilled the end out to fit a piece of 3/16 aluminum armature wire. That is just what it is called and is used for making doll armatures or holding things up. I flattened one end of the wire to a little less than the thickness of the edge of the master and filed it flat. After bending the wire into a U shape, I hot glued one end of the wire into the syringe piece and super glued the other end to the part. It needs to be oriented to not trap air, just look at how it will sit when being filled with resin and make sure there aren't dead end spots and all the air can be pushed out. In a more complex part, more vents can be added.
Next, I glued the syringe part to a scrap of foam core. At this point it is upside down from how it will be cast. At the opposite end of where the sprue attaches, I glued a small ball to create a reservoir for the resin so when it shrinks back as it cools it won't deform the edge of the part. The ball is also glued to the scrap of foam core and completes the path for the resin to flow, into the syringe, through the sprue, fills the part and ball and any extra flows out the top of the mold. I built a foam core box around the master and sprue leaving 3/8 to half an inch all around.
Next, I filled the box with tin based silicon rubber and let it cure over night. The next day, I pulled the foam core off and had a block of blue rubber with two things trying to poke out one side.
I used a scalpel with a #12 blade to cut on three sides of the mold. This is the initial separation line. I made sure to cut through the center of the circles made by the syringe and ball reservoir. After the initial straight cut, I used a jeweler's cut to create a texture that will key the mold back together. Instead of cutting straight, the hook of the #12 blade is swiveled back and forth making the peaks shown in the photo. Subsequent cuts follow the pattern of the first. As you pull the rubber apart, a white line shows where it is stressed and you try to cut on the line. It takes a little practice to not cut the peaks off or leave them hanging. A surgical retractor is helpful for keeping the mold held open. I got mine on ebay for a couple dollars each. I don't think they were used. O ring pliers are also useful, the ones that open when you squeeze the handles.
I followed the sprue down to my master and when got close made a straight cut to the part and continued the jeweler cut away from the part. Looking at the photo shows the cuts. I cut until I could get the sprue and master out. This mold is a few years old and broke the rest of the way when I opened it to take the photo. It is still useable, just in two parts now.
To cast the part, I held the mold closed with two scraps of wood the size of the mold and two large rubber bands. The wood keeps the rubber bands from deforming the rubber which would deform the casting. Because I used the end of a syringe to hold the sprue, a complete syringe with the plunger removed makes a perfect funnel to pour in resin. I mixed up a little resin, about 2oz and poured it in until it just started to come out the other hole. I use a paint pot as a pressure pot so dropped mold with uncured resin into the pressure pot, attached the lid and put about 40lbs of air in it. This squishes any bubbles to negligible size. Wait the required time for the resin to cure, in my case about 15 minutes, and carefully open the mold revealing the casting.
Baby powder sprinkled in the mold and then brushed or blown out with compressed air will help the resin flow into small details like the grillwork. A little mold release will prolong the life of the mold but too much can leave a texture on the castings. Depending on the complexity, I probably wouldn't use it on a simple part like this.
One of the best parts of model railroading is all the skills and knowledge about seemingly unrelated subjects we can learn if we want. Molding and casting is one I happened to have learned.
Below is a photo of a mold of a dynamic brake hatch and a casting from the mold. The casting has since been modified to match the one on a particular SD40-2 rebuild.
To prepare the master, I made sure it was clean and put scotch tape over the inside of the fan holes. This just makes the separation between the front and back easier when cutting the mold. Next I made a sprue from the end of a syringe. They don't have a needle, just a opening like an eyedropper. I drilled the end out to fit a piece of 3/16 aluminum armature wire. That is just what it is called and is used for making doll armatures or holding things up. I flattened one end of the wire to a little less than the thickness of the edge of the master and filed it flat. After bending the wire into a U shape, I hot glued one end of the wire into the syringe piece and super glued the other end to the part. It needs to be oriented to not trap air, just look at how it will sit when being filled with resin and make sure there aren't dead end spots and all the air can be pushed out. In a more complex part, more vents can be added.
Next, I glued the syringe part to a scrap of foam core. At this point it is upside down from how it will be cast. At the opposite end of where the sprue attaches, I glued a small ball to create a reservoir for the resin so when it shrinks back as it cools it won't deform the edge of the part. The ball is also glued to the scrap of foam core and completes the path for the resin to flow, into the syringe, through the sprue, fills the part and ball and any extra flows out the top of the mold. I built a foam core box around the master and sprue leaving 3/8 to half an inch all around.
Next, I filled the box with tin based silicon rubber and let it cure over night. The next day, I pulled the foam core off and had a block of blue rubber with two things trying to poke out one side.
I used a scalpel with a #12 blade to cut on three sides of the mold. This is the initial separation line. I made sure to cut through the center of the circles made by the syringe and ball reservoir. After the initial straight cut, I used a jeweler's cut to create a texture that will key the mold back together. Instead of cutting straight, the hook of the #12 blade is swiveled back and forth making the peaks shown in the photo. Subsequent cuts follow the pattern of the first. As you pull the rubber apart, a white line shows where it is stressed and you try to cut on the line. It takes a little practice to not cut the peaks off or leave them hanging. A surgical retractor is helpful for keeping the mold held open. I got mine on ebay for a couple dollars each. I don't think they were used. O ring pliers are also useful, the ones that open when you squeeze the handles.
I followed the sprue down to my master and when got close made a straight cut to the part and continued the jeweler cut away from the part. Looking at the photo shows the cuts. I cut until I could get the sprue and master out. This mold is a few years old and broke the rest of the way when I opened it to take the photo. It is still useable, just in two parts now.
To cast the part, I held the mold closed with two scraps of wood the size of the mold and two large rubber bands. The wood keeps the rubber bands from deforming the rubber which would deform the casting. Because I used the end of a syringe to hold the sprue, a complete syringe with the plunger removed makes a perfect funnel to pour in resin. I mixed up a little resin, about 2oz and poured it in until it just started to come out the other hole. I use a paint pot as a pressure pot so dropped mold with uncured resin into the pressure pot, attached the lid and put about 40lbs of air in it. This squishes any bubbles to negligible size. Wait the required time for the resin to cure, in my case about 15 minutes, and carefully open the mold revealing the casting.
Baby powder sprinkled in the mold and then brushed or blown out with compressed air will help the resin flow into small details like the grillwork. A little mold release will prolong the life of the mold but too much can leave a texture on the castings. Depending on the complexity, I probably wouldn't use it on a simple part like this.
