D'oh!

So now that I made the .060 corner posts, I realize I can’t use them.  “Why?” you might say? Well, remember that part of the plan for this car is to make a set of one-piece push molds, or flat-back castings, for a side and end, then make many copies to create a series of similar cars. The problem is with these corner posts the parts won’t fit in the mold box properly. 

Here’s the thing: The car side will be made from .040 scribed styrene sheet. The windows I made will be mounted into holes cut in the sheet, but mounted flush with the bottom, so when the mold is made the cutouts in the window frames will be pressed against the glass press plate when the castings are made. After hardening, all I’ll need to do is a couple of light swipes with a sharp hobby knife and the windows are open, with nice, thin muntins and mullions. 

To make this process work, the back of the car side must be in contact with the bottom of the mold box when the rubber for the mold is poured. If I were to glue the .060 radius quarter-round corner posts to the car side, either the post would stick out noticeably from the end, or the back of the car side would be elevated from touching the bottom of the mold box. If I added a .040 shim around the edges (.020 for the extra width of the quarter-round and .020 for the bead strip on the bottom), rubber would run under the part through the open window frames. If I dammed them up with styrene from behind to stop this, there would be an .040 thick wall of polyester resin behind what is supposed to be a window with glass in it. That’s not acceptable. No matter how you cut it, the .060 corner posts won’t work out, though they were fun to make. 

I got some .040 quarter round strip and remade the corner posts. And this time, the corner post and bead will not be completely pre-assembled – the side wall will get a bead and the quarter-round strip, the end wall will get a bead only, and that’s where the joint for the corner will be. If both bead strips were attached to the quarter-round, I’d be back in the same boat having to shim .020 strips around the edge of the side wall. So we’ll split the corner post to get flat edges on both the End and Side pieces, and all will be happy again.

Please use the Back button in your browser to return to the Caboose article.