thanks

Dave

Question - my main need would be making covers for (p2k) 52' 6" steel service gondolas carrying coiled steel (in HO).  Would this be do-able?  I'm thinking I'd need to make a pretty complicated form due to the hollow nature of the cover, but it should work.  Should I try to compensate for shrinkage?  If so, how much should I expect?  Here's a link to what I'd like to reproduce:

http://trainweb.org/algoma/Images/Freight/ac920.jpg    

http://trainweb.org/algoma/Images/Freight/ac920end.jpg

The prototype is a three-piece set, obviously, but I'd settle for a single piece, rather than struggling with alignment and stability issues of a set.  

Oh, and if anyone is aware of a commercially available cover of similar nature for the Proto2000 line of 52' 6" drop end mill gondolas, I'm all ears...

Thanks

Blair Smith

Nice SBS explination

Dave,

How come you don't use Smooth-On's resin casting. I believe they have one that has a pot life of 7min.?

Bernd

3D castings

A casting like that hood would require a 2 piece mold, an exterior mold and then something to "hollow out" the interior.  As long as you didn't want to stack them the thickness isn't critical.  Doing a 2 part mold is doable, its just a more complicated.  I have cast one piece gon shells using a "squish" mold.  The female portion is the exterior, you pour resin in the cavity and then insert the male half, squishing the resin between the two molds. The other way is to have a 2 piece mold, then pour the resin into the mold to fill the cavity. 

The critical part with either mold is aligning the two mold parts and venting to make sure bubbles don't get trapped.

RTV shrinkage (OOMOO-30) is .0025 in/in per Smooth-on and resin shrinkage is .008  inch/inch according to the Alumite. So making it maybe .010 wider than scale and maybe .040 longer than scale would probably accommodate any shrinkage.  I don't worry about it too much since my castings are "free standing", they don't have to match anything, so .010 shrinkage won't be noticeable.

Here is a pic of my first squish molds and the castings.  The pick mold on the left is the male portion and mold on the right is the female portion.

My second version used a plastic frame, I found that the mold bowed out a bit when squished.  I could have either made a thicker mold or built a frame to keep the mold "rigid".  the frame also provides more consistent alignment of the mold halves, importance for a gon where the all thickness is both thin and visible.  For a cover its not that important, since it won't be visible.  The plastic pins align and retain the molds in the frames, the holes are asymmetrical so the molds fit in the frames one unique way.

@Dave,

Thank you very much.  That's quite illustrative, and makes me think the project would work, with perhaps a bit of experimentation.  Not, however, something I would take on first-up.  The bigger issue might be making the male and female mold sections such that the final thickness is consistent, and reasonable.  I'll have to come up with a more realistic first project to get my feet wet; maybe something like a steel mill ladle, as that's a smaller volume and simpler shape.

Regards

Blair

Smooth On Resin

I have used Alumilite before and if has worked well, plus it was on sale. 

I tried Smooth On resin once before and I can only assume it was an old batch, because no matter how much I shook it or mixed it the silicone oil kept separating out of the resin and the castings literally oozed oil over time. I called Smooth On and their advise was to shake it up before I use it.  Didn't work.  While I realize it was probably a problem with old stock, and lots of people use Smooth On, I haven't been very motivated to try it again when Alumilite is easier to find, cheaper, has an easier to pour bottle and I haven't had problems with it. 

I have also had good results with Hobby Silicone RTV and resin.

Progress

I have cast parts for a dozen gons and have started test assembling one.  I say test because I have to figure out all the brake rigging and how to secure the truss rods, etc.

Here is the first one, showing the underside and then the brake rigging from the side.  Still have to do grab irons, sill steps and handbrakes, plus add weight and an interior floor.

Nice looking model, looks

Nice looking model, looks like the art of building the models you want is not dead after all.

The art of building models

Thanks!  There are a lot more people building or kitbashing models than you would imagine.

For modelers in niche scales and gauges, its a necessity.  One of the other forums posted a copy of MDC ads from 1969, they are advertising the same "old time" cars Roundhouse produces today.  There haven't been any new cars added to that line in over 50 years.  You pretty much have to scratchbuild and kitbash.

Here's a P&R 29 ft class XM arched roof boxcar from the 1880's I'm kitbashing from the hulk of an MDC caboose.  It fits the dimensions almost perfectly.  Cut off the platforms, sand the ends and roof smooth, remove the sides, sheath it in scribed styrene car siding (.020 on the sides, .040 on the ends), add brake rigging and truss rods to the underside (figure out how to add weights to the underside because you sealed up the body before adding weights inside, Oopsies).

One of the local modelers does Gn15 (HO gauge, 1/24 scale), radio control (dead rail) and makes all his cars and buildings from balsa wood and cereal box cardboard.  Waaaaaay cool to operate.

Dave, it seems the common

Dave, it seems the common response to building kits or scratch building that I most often hear is the rtr business. I am glad to read about efforts of builders more than buyers. That is one thing I enjoy about the hobby more than anything, except running trains, building models.

Looks Good!

Though you didn't intend to share a tip, thanks for the tip about using an MDC caboose to yield a 29' boxcar!

And you are SO right: Modeling TOC19 (especially in the 19th century) REQUIRES creativity and willingness to do some kit bashing, some HEAVY kit-bashing, some semi-scratch building, and outright scratch building (by some).

That's why I'm trying to offset the time factor by using RTR and/or "good 'enuf" rolling stock as much as possible for my 1960s era for my dual era layout I'm about to start.

Andre

Great inspiration to try casting

I've cast several car ends and doors but always wanted to try passenger car sides. Great step by step and advice- inspires me to give it a try on a larger scale.

Great description

A very well done description of the casting process.

The "oven" idea is quite interesting. How does it compare with warming the mold before?

For my very fine window frame (0.5 mm thick) I simply put the small mold in hot water, the time to prepare everything.

Pierre-Yann

Heating the mold

I would be really leery about heating in water.  Water drops trapped in the mold would cause voids and some resins don't react well with water (won't cure.)

It is warming the mold "before" for every casting but the first.

Here's the cast gon with the details applied.  In the background is the 30 ft box car I'm kitbashing from an MDC caboose.

Vacuum & pressure

I'm also getting into resin casting...............Last summer I found a vacuum chamber online and built a pressure chamber.

Painted car

An update on the gon project, first car, test bed, finished to paint, but not lettered.

The homemade car is coupled to a commercial kit of a similar type car just for comparison.

Dave

Your resin car project is turning out very nice.

Glad they're too "new" for the late 1880s, or it would be tempting to twist your arm to purchase a fleet of them!

Reality check:

What am I thinking? I have PLENTY of rolling stock projects for my TOC19 era!! Silly me.

Still, though...

Andre

Outstanding tutorial

Thanks so much for the tutorial. Resin casting a couple of projects are in the pipeline for me. This has helped me get past the "can I do this, really?" stage.

Outstanding.

I don't know how you guys do

I don't know how you guys do it. I cant even build a wood kit structure without it warping shrinking or glue fail...let alone cast my own resin for rolling stock...hats off to you. nice post.