Under construction at this time
Using flat castings.
I built several commercial resin kits that used flat castings and it wasn’t as bad as I thought it would be so I decided to try building a gon from flat castings.
I studied the P&R gon diagrams and there were 3 gons that used the same stake spacing on the sides, and the same ends. Two used the same underframe. Class GMa had 1 board high sides with 8 stakes, class GHa had 4 board high sides with 8 stakes and a flat bottom, class HS-a had 4 board sides with 8 stakes and a single hopper bottom.
Making the molds
I made masters for the three board sides, flat bottom and three board ends (plus made a new mold for a resin underframe to fit MDC 36 ft house cars). The master for the sides and ends is shown below. The master was made from basswood and styrene castings. It was glued to a scrap piece of polycarbonate to keep it flat and a form was built around it with cardboard hot glued at the seam to make it “rubber tight”. Once I had the mold dimensions I used that to calculate the cubic inches of the mold which I used to estimate how much RTV I needed to make.
I used Smooth-On OOMOO-30 RTV. Normally I have been using Dow HS II or HS III, but the OOMOO I could get on sale. Its mixed 50-50 by volume. It says it doesn’t need vacuum de-airing, but it had a lot of bubbles in it so I did anyway. I used a home-made vacuum chamber. Its an 8″ PVC pipe coupling with a 1/2 ply base and lid. The lid has a 1 1/2 in hole in the center and the top an bottom are covered with craft foam sheets. It will hold the quart mixing bucket and I put the shop vac on the top. It gets enough vacuum to make the bubbles come to the surface.
Casting cars
After making the molds, I waited a day to allow the RTV to fully cure before I tried casting
The casting supplies I used were Alumilite RC-3 Resin. It is available in some Hobby Lobby’s. I have used it before and have had good luck with it. All my P&R 15000 class resin boxcars are cast from Alumilite. The RC3 has a 3 min cure time which allows a fair amount of time to work with the resin before it gels. It is a two part resin, A and , that are mixed in equal parts by weight or volume. I also use a spray mold release, Stoner was the brand I have used. Also I have black and brown dyes made for casting that can tint the resin. While I did not use them in this example I have used them on other projects to give the casting a color in case paint chips or wears away.
Some of the tools I used are shown below. I am old school and use a triple beam balance to weigh out my resin amounts. you could also use a digital scale, but I prefer the analog one because I find it easier to match an indicator than hit a number. I use either vinyl or food service gloves. The resins are chemicals and can irritate your skin. I pour the resin into 1 oz Solo “Souffle” cups. I have toothpicks to get bubbles out of small spaces. I use rail nippers to cut one end of a popsicle stick square to use as a mixing tool.
One slightly odd tool is a warming oven I made from two turkey roasting pans. On the top pan I cut the rolled rim of so it would set on the bottom pan and then cut a hole in the “top” so I could shine an incandescent light into the pan to warm it. I lined the bottom with aluminum foil to catch drips.
To start casting I measure out the resin parts separately. I normally estimate the number of grams of resin I need then divide that by two. If I end up with extra resin, I weigh it and know how much less I need to mix. For the sides/ends I use about 4.75-5 gm of each part and for an underframe casting I use about 6 gm. One thing I like about Alumilite is they pack their resin in great bottles with really handy pour spouts. Normally I don’t label the cups, but I did for the demo.
Next I pour an equal amount of part B. Since the amounts are so small, getting them equal is a little harder and a little more critical. If you were doing a big casting, being a gram or two off wouldn’t be a big deal.
If you did want to tint your resin, now would be the time. Put a couple drops of dye into the part A and stir it in well.
Once the resins are set, its time to mix them up pour the resins together. For the smaller castings I can mix them in the measuring cups. For larger amounts I pour each part into a larger plastic cup. I have tried paper cups but the resin can soak through and eat into the wax coating on the cups. There probably is a difference in mixing A into B or B into A but I’ve never had a problem either way (yet).
I use a popsicle stick to stir the resins together. With Alumilite the resins get a little cloudy, then as they become well mixed they become clear again. I mix it gently to minimize bubbles. As soon as you combine the two parts a chemical reaction begins and you have about 3 minutes before the resin gels and starts to harden. Time is important, but 3 minutes is a lot longer than it sounds.
With the resin mixed its time to pour it into the mold. I generally spray the mold every 5-10 shots with mold release. I pour the resin in in a thin steady stream, trying to pour it into the lowest points and letting it fill UP. I slightly overfill the mold (although in this case I overfilled a bit more than normal because I was trying to take pictures and cast at the same time).
Once the molds are filled with resin, I use a toothpick to work in all the recesses and edges of the mold to chase out bubbles and work the resin in every nook and cranny. This is why I like a longer (3 min.) gel time. Once again, because I was fiddling around with the camera, I didn’t do a very good job with the toothpick, you’ll see more on that later.
If you have any small detail piece molds, keep them handy and if you have any resin left over from a pour, you can use it to cast a smaller detail piece.
One of the aspects of a “flat” casting is that it normally needs to be flat. For example the sides of the gon need to be a certain thickness. Even on an enclosed car, like a boxcar, if the sides are too thick the car won’t look right and the parts won’t fit right. When I made the molds I use a thick piece of plastic as the bottom of the master, that gave me a very flat smooth surface on the top of the mold. By making the mold 1/2 inch thick it gave me a sold block of rubber. In order to get a flat casting, I cut pieces a of thin (.080 inch) clear plastic, available as storm door glazing in a home improvement store, into pieces just smaller than the size of the mold. I liberally coated both sides with mold release and then starting at one end of the mold, pressed the clear plastic down onto the mold, pressing it into the rubber as I went. The object was to get it as tight against the mold as possible.
With the cover in place I then put the mold into the warming oven. I normally have the light on, but with the reflections off the aluminum foil I turned it off for the photo.
You may have noticed the piece of hardboard under the mold. I cut some scraps of hardboard as “carriers” to support the mold when I picked it up and moved it into the oven. originally I didn’t have those, and then in mid-carry, the mold flexed slightly and caused a void in the casting under the clear plastic cover. To eliminate that problem, I made the carriers. Its an optional thing, a solution to a particular problem I had.
And now we wait. If I have multiple molds I can use this time to prepare and cast the next mold.
You don’t necessarily need to use an oven to warm the castings, but I have found it gives better results. The resin cures with an exothermic process, it gives off heat as it cures and it needs heat to cure. With thin castings, there might no be enough resin to generate enough heat to cure as thoroughly as a thick casting. By using an oven to warm the casting while they cure, it makes sure that the casting will cure completely regardless of the cross-section thickness. Here is a picture of a detail casting for another project that I cast at the same time as the side casting, and the picture was taken right before the side casting was ready to come out of the warming oven. Note how the thin sections and edges are still the brownish color of the resin. The warming oven speeds the process. One caveat is that the molds become warmer. The next casting I make using a warmed mold will have about 15-30 seconds faster gel time because of the warm mold.
The casting has warmed for about 10 minutes and is now ready to be be demolded.
First the clear cover is removed.
And then the casting is removed from the mold. Depending on how much time you allow for the casting to cool in the mold will determine how stiff the casting is. Since I removed the casting less than a minute after removing it from the warming oven the resin was still pretty soft and the casting peeled out of the mold.
Normally when I am “production” casting, I am trying to turn the molds fairly quickly so I commonly pull the castings from the molds while warm. To insure they end up flat I put them on a piece of glass to cool.
If I have done it right and evenly pressed on the cover sheet, I will end up with a minimum thickness flash around the castings that will require minimal cleaning and will trim easily. In the picture above, the casting isn’t that bad, but once again I was concentrating on the camera and not as much on the casting and didn’t get as thin a flash as I could. The picture below shows a better result.
I mentioned I was bit negligent in chasing bubbles and working the toothpick around the mold. This was the results. On the back of the casting was one fairly large bubble. But it could be repaired with filler putty (Squadron or Tamiya).
On the front of the casting were several voids. if I wanted to save the casting I could also fill those with body putty . This illustrates how a few moments of work making sure the mold is filled can pay off later.
One other think I will point out in the above picture. Remember my saying that Smooth-On said the RTV didn’t need to be de-aired. Well, maybe, maybe not. Note on the bottom of the car end, the small rectangle near my thumb, there is bubble on the bottom. Then on the bottom of the casting at the 3rd stake from the left there is a big bubble. That didn’t appear until about the 5th casting with the mold. It was actually hidden in the RTV and then the wall weakened and ripped out after several castings. Since moth of those bubble are next to the castings and not on them, they are easily trimmed off.
Here is a full set of castings for one car. When I have stuff humming, I can make about one set of three castings every 20 minutes. After a one hour session on each of four nights over a week, I had enough castings to make a dozen cars.
How much does this cost? The resin and RTV both retail for about $30-35, but I bought both with discount coupons so actually paid less. The RTV gave me enough for about 3-5 molds, depending on how big they were (I made 4 molds). A mold should last for 30-50 pulls depending on how many undercuts. The Alumilite resin has been enough to make about 20-24 equivalent cars so far (and there is enough left to make another half dozen or more). If you figure 20-30 cars from the set of resin, its about a buck a car for RTV and a buck a car for resin. The only expensive tool I’ve used on this project was the triple beam balance, and that was about $40. A cheaper scale could work as well. Even at that amortized out over the equivalent 30 or 40 cars I’ve made, its still pretty cheap. All in, on a per car basis, the resin castings probably cost as much as the trucks and couplers or paint and decals on the finished model.
I hope this narrative helps you with your own casting efforts or encourages you to try it yourself.
The castings assembled
The castings assembled for a test fit. Still have all the details to be applied.
