Making RGS 0409 in HOn3, Part III

My next challenge was to make the curved roof on the cupola. Again, Dave Hussey provided some suggestions. In order to get hard corners on the end of the roof, he suggested I make the cavity half of the mold out of three pieces that would be bolted together. I gave serious consideration to his suggestion. In the end, I decided on what I thought would be a simpler approach.

I cut some slots into the mold about .170” wide and them milled out the top part of the roof. Next I took some 3/16” square brass bar and milled it down to be slightly over .170” wide (I think I made them about .001” wider). I slightly tapered the bottom of each bar to make it easier to get them into the slots, and then I used a hammer to pound them in place, as you can see here:


At this point the bars are sticking up .020” inches. I then milled these down so there were nearly flush with the rest of the mold. I actually went about .0005” too deep, so I then sanded the molds flat, as you can see here:


Finally, I milled the runners and gates into the mold, and also the slightly higher curved ends (near the top and bottom of photo) that mate with the curved surface on the other side of the mold to create a “shut-off” for the plastic.


And here is the final result, blown up about 3 times larger than real life:


Not bad—the corners are nice and square! I see some imperfections that I’ll probably work on.

Next up the main roof and body. I think I’m going to start with the roof since I’ll then be able to build the roof with a finished cupola on top. Then I’ll move onto the molds for the body.


  1. This is great work. I'd like to do the same, but in N scale. What injection molding machine you have?


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