Bowed urethane castings - thoughts on a fix?

[sarge]

Dave, no worries here. I was taking a couple comments by others to task as being a bit wide of the mark, though being quiet whilst they continued until the evidence was in. The photo of the car with the weight pretty much proves the point.

Give whichever approach you choose a go. My only concern is that the casting will continue to shrink once relaxed. Depends on the material choice whether it will or not; I've seen it go all over the map with cold-resin casting as the material isn't consistent. As examples, I've never had a problem with Berkshire Valley resin castings, at least when they were owned in New England. Old Chooch on the other hand was infamous for severe shrinkage. Lots of large flat castings just don't have the stability in three dimensions using resin. I've found stiffening by other means pretty much my only option.

[Rufus T. Firefly]

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[DaveJfr0]

Glued in place?

Yes.

Hmmm......does look like the warpage occurs beyond that steel at both ends so adding a stiffener extending from that could work....unless you succeed in making a hinge point there....

With the steel plate in place I would not try any of the heating methods either.

I'm already about 80% dislodging it. The glue gave up pretty easily. The temperature difference between it in and the urethane is quite noticeable. I may looking to replace it with a sheet of styrene if I can salvage the build and weight it with a load. (or I could try to put the steel back in there, but not glued.)

Question is - should I bother trying to boil it without the steel or just go straight for Brian's plan of attack.

[sarge]

Boiling might be too brutal. Laid upside down on a piece of nice flat straight wood cut to size and a couple clamps on the ends to haul it flat, then a gentle hit with a heatgun might be the first approach. I think you'll have to sneak up on it a bit.

Once it cools, let up the clamps and see how you did. Repeat more friskily as required. Once it's straight, then stiffen with some sort of spine, be it a new weight or angle run behind the sidesills or whatever you might choose.

[Rufus T. Firefly]

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[DaveJfr0]

Two last photos for now.

It's more or less still straightening out on the wood. Only issue is I think i made need to clamp on all four sides as I see lateral warping across the ends. [EDIT - I just placed some basswood across underneath each clamp which should provide the pressure needed across the sides and center sill one it warms up]



Here's what it looks like from underneath. There's a lot of gussets and ribbing along with the bolsters that all will need modding to accomodate some brass infrastructure. I was originally thinking of using some brass angle on either side of the sill and maybe the sides of the car to still allow for full truck swing. I do also have 1/8" brass square rod that may be enough by itself if I run it along the car sides. There might be enough room to countersink 0-80 screws into it, but that'll require me sourcing some 0-80 flat head machine screws from microfasteners. I'll do that after I can reliably do the other modifications without throwing this car against the wall.



I'll try the near-boiling water later this week.

[sarge]

Did you try a heatgun when you had it clamped up? If not, you might give it a go. Just sweep back and forth and warm the casting to the touch (you don't want it to suddenly go off like a piece of shrink-tube. Grin!).

I've done this with the old Chooch flatkits with reasonable success. I've also been paranoid enough I get my fingers in between the casting and the heatgun as a crude thermometer to check how nicely I'm sweeping the gun; amazing how careful you get in the process when your fingers are in the mix.

[DaveJfr0]

Did you try a heatgun when you had it clamped up? If not, you might give it a go. Just sweep back and forth and warm the casting to the touch (you don't want it to suddenly go off like a piece of shrink-tube. Grin!).

I've done this with the old Chooch flatkits with reasonable success. I've also been paranoid enough I get my fingers in between the casting and the heatgun as a crude thermometer to check how nicely I'm sweeping the gun; amazing how careful you get in the process when your fingers are in the mix.

I didn't have it clamped as well as I do now which is why it probably started warping laterally when I took my heat gun to it. It also still had the decking and steel weight attached. As to sweeping heat, I also probably focused the heat mostly on the ends as some of the brake components were already folding to a few seconds of directed heat by the heat gun. I think a few minutes in near boiling water will be more even across the model. And I won't visually watch the plastic parts melt, so I won't remove the heat from the center of the car.

[sarge]

Yeah, sweeping even heat across the entire casting is the trick to it. Concentrated heat is too brutal; overall warmth rather than local hot. I was under the impression you had removed the weight and deck from your earlier comment, else I would have made comment of it. A big metal thing is not a help for even warming; a monolithic piece is what we want. Dissimilar materials are not your friend at this stage. Later, when you are stiffening it to remain straight is when metal stiffeners come good.

I'm curious to see what the water does for you, though. I've never tried it.

[Rufus T. Firefly]

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[DaveJfr0]

I'm curious to see what the water does for you, though. I've never tried it.

It works well but I've not applied it to a finished model. I've only done that prior to building to flatten sections.

Also, looking back at the David's photos, it appears to my lousy eyes that the greatest amount of bowing occurs beyond that metal insert where the car structure is more airy and open.

You are correct. Anywhere beyond the core of the car where the steel stiffener was is where it went haywire and parts of it were open to the wood decking, so not much there.

As to being finished, this car didn't have this problem years ago when I built it.

[E7]

JMHO, But if you have a heat gun, I'd clamp it flat and take that to it. I'd guess you would have better control with that than boiling or an oven.

Control may not be what you want. Everything I have read indicates even heating across the item, while clamping or weighting, allows stresses across the item to dissipate evenly.

Not sure point heat with a gun is optimal.

I kind of figured that common sense would indicate that you would start with low heat and move the gun across the affected area to gradually warm it. That is what I meant by control, not point and blast! I don't associate what you describe as control.

[ScaleCraft]

It is interesting to re-read this thread. The original data was to clamp it on a cookie sheet, place in a 200 degree oven to relieve stresses.

Then we end up with oh, no, cut the cast in frame ribs out, glue in brass strips....then we find there is a WEIGHT where the casting SHRUNK just a bit, hit the weight, and pushed the ends out and down.
What's gonna happen to the deck, sides, straightness when the ribs are cut away from the sides?

Now we're back to even heat, clamped, which is almost full circle, except heat is on one side (bottom?) of car, and may or may not be as effective in relieving stress throughout.

As long as the weight is out now...you may have some good result.

[sarge]

I don't believe that is the case, Dave.

One shrunk with no weight and bowed end for end. The casting warped because the cast in frame girder and side sills pulled against the flat and bowed evenly.

The second tried to do the same thing. The bit that kept it from bowing evenly was the metal weight. It's stiffness kept its area straight. The casting didn't warp because it pulled on the weight; the casting stayed straight where stiffened by the weight.

So, yeah. relieve it straight with nothing but the casting present, then add stiffening of metal full length to keep it straight rather than allow it to be compliant.

[E7]

I've looked through all my cook books and can find NO recipe for "Flat Car"!

Sorry guys, couldn't resist.