Tuesday, October 16, 2012

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:

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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:

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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.

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

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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.

Saturday, October 13, 2012

Making RGS 0409 in HOn3, Part II

Today we had a Railroad Prototype Modelers meet here in the Seattle area, so of course I wanted to see how far I could get with my caboose kit. I made a mold for all four the the cupola walls and here is what I got:

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In other words, it turned out great. Well, sort of. It took me several tries to get tries to get a good shot. Most of the shots were either short or blew the mold apart, resulting in a blob of plastic. Here is what a short shot looks like (on the left) and a “blown” shot (on the right):

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As you see on the left, the plastic didn’t complete fill the end walls. Fortunately, at the RPM meet, I ran into Dave Hussey, who is the owner of Cannon & Company, and he gave me two suggestions, which together completely solved my problem. Here is what he suggested:

  • Cut an air vent into the mold to allow air to escape
  • Raise the melt temperature

I had been using a melt temperature of 460 F for most of my work so far. Dave mentioned that he goes up to 600 F for some of his molds. Since the polystyrene flows easier at higher temperatures, the theory is that I should be able to use a slightly lower injection pressure and fill the mold. I cut the vent slot, as you can see here:

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There is a vent slot that goes from one side to another. This slot is about 1/8” wide and about .015” deep. Next to each window of an end wall, I cut a 1/8” wide slot about .0005” deep (I’ve circled one). These vents are shallow enough to keep plastic from flowing into them, while still allowing air to escape.

I next increased the melt temperature a little at a time. At about 530 F, and with a slightly lower injection pressure, I was able to fill the mold every time with absolutely no flash!

Thanks to Dave and the wonderful sharing at RPM meets, I can declare success for this mold and move onto the next mold. I’ve decided to tackle the curved roof the the cupola next, which will present a new set of challenges. Once again, Dave had some suggestions that I’m going to try. The next RPM meet is on November 10 and is part of the Trains 2012 Model Railway Exhibition in Burnaby, BC, about a 3-hour drive away from here. We’ll see how far I get for that RPM meet.

Monday, October 1, 2012

Making RGS 0409 in HOn3, Part I

It’s been a long time since my last blog post.

I’ve made almost all my parts for Ragg’s…To Riches?, which is now winding down. As a result, I’m taking time to work on a project for myself. My plan is to model the RGS in HOn3. So far I’ve built a few models, including a short caboose. And for some time I’ve been wanting to build a model of the RGS caboose 0409. This caboose is a little shorter and wider than most short cabooses. As far as I know, there are no kits made for this caboose, so I had two choices: either scratch build one, or design an injection molded kit. So, of course, I decided to try making an injection molded kit (even there’s no market for one).

I’ve been working on the 3-D model for the kit, and here is what I have so far:

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The siding is tongue and groove with scale 1/2” wide gaps. I made these gaps triangular and I was curious to see how they would turn out when scaled down to HO scale (1:87). At this scale, the gaps would be just shy of .006”, which is pretty tiny. The sides of the cupola, which is shown in light green in this drawing (each part is shown in a different color), are very small, so they were a good candidate for a test mold. I made the test mold out of brass, and here is the first part out of the mold:

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As you can see, this is a very close blow-up. Some of the slots aren’t as deep as other slots because my Taig CNC mill isn’t as precise as I’d like (it doesn’t have a ball screw). As a result, some of the ridges in the mold (that represent the gaps in the molded part), were shaved off more than others. I’d really like to know why. Even so, the results look just fine!

My next step will be to make a mold for all four of the cupola sides and built up a cupola.