Shrink Mould Castings

Shrink Mould Castings

Postby Bernd » Sun Jul 31, 2016 3:48 pm

Here's something that might be applied to TT scale. I'm thinking of an HO scale shell and shrinking it down to TT scale. Go to the 3:00 minuet mark. The technique is applied to shrinking S scale cars to HO scale cars. Something I'm going to investigate down the line.



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Re: Shrink Mould Castings

Postby ctxmf74 » Sun Jul 31, 2016 4:18 pm

I've seen references to this technique for years but apparently it has never caught on with mainstream modelers . I recall seeing discussions about the problems involved with it but can't remember what they were. If someone can figure out how it could be used for new TT stuff that woud be great......DaveB
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Re: Shrink Mould Castings

Postby Bernd » Sun Jul 31, 2016 6:08 pm

ctxmf74 wrote:I've seen references to this technique for years but apparently it has never caught on with mainstream modelers . I recall seeing discussions about the problems involved with it but can't remember what they were. If someone can figure out how it could be used for new TT stuff that woud be great......DaveB


Probably the reason it hasn't caught on is many modelers don't want to experiment. I think it's an instant gratification thing with some. :P

Here's a link to Smooth-On as to what the materials are.

https://www.smooth-on.com/tutorials/shrinking-casting-novocs-silicone-solvent/

I might give this a try this winter.

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Re: Shrink Mould Castings

Postby areibel » Sun Jul 31, 2016 6:47 pm

Rob M. has some experience with shrinking castings, he might be able to give you a few clues.
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Re: Shrink Mould Castings

Postby Bernd » Sun Jul 31, 2016 9:26 pm

areibel wrote:Rob M. has some experience with shrinking castings, he might be able to give you a few clues.


Haven't seen Rob on at all. Is he still around?

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Re: Shrink Mould Castings

Postby Rob M » Mon Aug 01, 2016 1:17 am

Bernd wrote:Haven't seen Rob on at all. Is he still around?

Bernd

I still check in most days :wave:

Shrink casting is very doable but also very complicated.

To go from HO to TT you have to do it in two steps. If you try to do it in one step the silicone won't setup properly.

If you are doing a two piece mold both halves must be poured at the same time, in the same mold so they have the exact same mixture. You'll need something to separate the two halves, I used epoxy coated aluminum flashing.

And, the mold needs to be a sealed box so it can be put on a rotisserie while curing. If not, the thinning agent will settle (or rise?) and the two halves of the mold will shrink at different rates.

I used Smooth-on's Omooo 30 (?) and Coleman fuel for the thinning agent. Naptha can also be used but I don't know if the ratios would be the same. The ratio by volume in both steps needs to be:
1 part A
1 part B
1.5 part thinner

The initial curing time will be similar to regular mold making. You need to then let the mold dry for about a week. Rotate it frequently. It stinks and will be giving off flammable gases, dry it outside!


BTW, the step between HO & TT looks like a nice size for trains, about 104:1.
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Re: Shrink Mould Castings

Postby Bernd » Mon Aug 01, 2016 7:53 am

Hi Rob,

Glad to see you're still here.

Thanks for the tip on mixture. From what is written on the Smooth-On website about shrink casting says that 18 to 25 percent shrinkage is possible. So if I understand this correctly since TT is 27% smaller than HO that I would need to do two molds to get to TT scale?

I'll have to wait till it gets more toward the fall before I can do any experimenting. To many out door projects yet to do that need to be finished this year.

Thanks for the tips. They will be helpful in the experimentation.

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Re: Shrink Mould Castings

Postby Richard-B » Mon Aug 01, 2016 11:05 am

I used to follow a now dormant Yahoo! Group called 'HOslotcarcasting'...
which covered a lot of the techniques for shrinking. That led me to:
http://www.industrialpolymers.com/hydrospan/hydrospan-400

I haven't tried this yet... but it does have possibilities...
especially for non-rectilinear shapes (i.e. not boxcars) where any distortion would be visually amplified.
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Re: Shrink Mould Castings

Postby areibel » Mon Aug 01, 2016 11:52 am

I tried the Hydrospan with very poor results- They have two flavors, one you cast and then soak in water to expand, the other (the 400, IIRC) you mix water with and make your casting and it supposedly shrinks. I didn't try a two step, it might have worked better but for my experience the finished "unshrunk" piece is very fragile and will tear if you look at it sideways. It was about the consistency of an overcooked lasagna noodle so it wants to collapse. And unless it's pretty uniform thickness it won't shrink at a uniform rate- I tried doing an HO SD45 shell and it didn't work, I even tried doing a solid block (thinking I could hollow it out and fit a chassis) but it turned out somewhere between a banana and a boomerang. Plan #3 would have been to cut the shell into flat parts and try to shrink individual pieces, but the Norkin SD45 came along and I gave up.
I think Elmer McKay was trying the other "expanding" version, using N scale building parts but I don't think he had much luck either.
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Re: Shrink Mould Castings

Postby Richard-B » Mon Aug 01, 2016 12:19 pm

areibel wrote:I tried the Hydrospan with very poor results.
This does NOT surprise me...
As I mentioned... perhaps a possibility for 'non-rectilinear shapes' i.e. no straight lines... no large flat surfaces.

At this point in technology time; it is starting to be viable to place the larger part on one of the available 3D carousel laser scanners... e.g. https://store.makerbot.com/digitizer (possibly owned/operated by a service bureau...)
generate a point cloud, then massage that into a CAD drawing, and resize for TT-scale 3D printing.

Note that even the 3D laser scanned examples tend to be anthropomorphic... not rectilinear.
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