Re: An informal poll


destorzek@...
 




---In STMFC@..., <pullmanboss@...> wrote :



John Barry wrote:

> I vote for one piece bodies with separate floors.  If there are end variations, false ends.

That's a different matter than making the ends separate for ease of painting. With one piece body resin kits, where you are presumably looking for more accuracy than with even high-end injection molded kits, the practice is to make a separate body pattern for each variation. (If there are end variations, there are probably other variations that the plastic manufacturer will ignore but the resin manufacturer will include.)
=============

The same way that injection molding tooling can be set up with different changeable end inserts, so you get one piece bodies... and other one piece bodies with different ends.

People keep talking about those "high precision" injection molded kits, but in reality, differential shrinkage has to be dealt with there, too. The shrinkage of injection molded styrene is expressed as a range, typically 0.4% - 0.6%. That 0.2% difference is .002" per inch, or .010"- .012" over the length of the typical 40' boxcar in HO. Doesn't sound like much, but it needs to be taken in account, or problems will result. I've gone through this twice over the years, using different solutions for different situations.

This discussion started asking about refrigerator car models, but no one ever defined if we were talking wood or steel ends. Separate wood ends (or sides, in the case of the Accurail wood reefer) are really hard to do. The color separation is supposed to be right at the outside corner. There is no overlap to provide thickness to either part. The logical conclusion would be to do mitered corners, which the resin guys will just hate. The problem is, if the separate part is .010" short, that's going to be one ugly gap, with no easy way to close it. The Accurail car looks like it has mitered corners, and they are, for most of their thickness. The mitered portion of the side, however, is designed to be .010" shorter than the recess it fits in. The very surface of the part becomes a .010" thick rectangular bead, .010" wide, to cover this gap. That puts the color separation one scale inch around the corner on the end, and if the side is slightly too long, it will just appear that the last siding board is pushed a bit past the corner. Whether the side is a bit too long or too short, it doesn't show a gap.

The steel reefer we did a couple years ago had some different concerns. Steel ends that model W corner posts have enough thickness to be separate parts, and since a lot of steel reefers had aluminum color roofs, we decided to make the sides AND ends separate parts. The roof is molded with the sub sides and ends to yield a rigid structure. The same shrinkage concerns still apply. In this case, the sides are designed to be .010" longer than the sub-sides, so the ends will always go tight against the visible side. The ends, being only 1.3" wide, will only vary .002" or so, not really visible, BUT, the overhanging roof has to be long enough to look correct even if the shortest roof is used with the longest sides.

The differential shrinkage of resin parts is likely 5X as great, which becomes a really significant amount. While the shrinkage of the actual curing resin is relatively consistent, the rubber molds shrink throughout their lives. I can see why resin casters would really rather do one piece bodies.

Dennis Storzek

Join main@RealSTMFC.groups.io to automatically receive all group messages.