rectifying bowing plastic wagon sides

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jon price
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rectifying bowing plastic wagon sides

Postby jon price » Sat Jun 18, 2016 9:21 am

So, having recently assembled and painted and lettered a rare-as-hens-teeth plastic wagon kit the sides are bowing in. Obviously I can rectify this with a coal load, but it there anything to do at this stage that will restore the sides to the correct straightness whilst still empty?
Connah's Quay Workshop threads: viewforum.php?f=125

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Guy Rixon
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Re: rectifying bowing plastic wagon sides

Postby Guy Rixon » Sat Jun 18, 2016 9:36 am

I try to avoid the bowing by shimming out the floor with microstrip. Putting ~ 0.015 of strip in the centre of the side and tapering off towards the ends forces an outward bow at floor level which tends to cancel out the inward bow at the top of the side. The side ends up with a twist but this is not obvious to the eye. Of course, this is hard to do after assembly.

The other thing I've tried is to pack the sides with a chunk of wood so that they bow outward at the top and then heat the wagon with a hair-drier set on "warm". One needs to soften the plastic just enough to let it slightly reshape at the corners. Sometimes this works, once I melted the wagon, other times I bottled out before getting it hot enough to reshape. The block needs to be about 20mm wide so that the outward force is distributed and the heat needs to be concentrated near the mire joints in the corners. I don't know what would happen to paint, printing or transfers at the temperatures needed to soften the plastic.

Of course, if the mouldings didn't have stupid mitres we wouldn't have this problem. And the designers who put mitres between sides and end doors need a slap. </rant>

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jon price
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Re: rectifying bowing plastic wagon sides

Postby jon price » Sat Jun 18, 2016 9:44 am

Thanks Guy. I'll try the hairdryer and block process with an unfinished wagon to see what happens to paint and transfers. If they survive I'll use the method on the finished wagon, but the livery is too time consuming/expensive in transfer letters to dare risk having to do it again
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Tim V
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Re: rectifying bowing plastic wagon sides

Postby Tim V » Sat Jun 18, 2016 4:49 pm

Look very carefully at the chamfer on the body ends (if there are any!). Often as not, not 45°. Correcting this (carefully), will improve things. Then use a spacer between the sides to hold them apart, leave for about a week to dry and harden.

Back in the old days, I would use ABS cast wagons, just because they wouldn't bow, in fact you could put the characteristic outward bow of a well used coal wagon in them.
Tim V
Scalefour News Editor

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steve howe
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Re: rectifying bowing plastic wagon sides

Postby steve howe » Sat Jun 18, 2016 7:44 pm

Having had several plastic wagons develop an inward bow over the years and had to insert loads to get them back in line, I'm inclined to agree with Tim that whitemetal open wagons are the pragmatic solution for empties. Apart from stability, they generally don't need much in the way of additional weight. D & S were of course the bees knees in this vein, and ABS have (had?) a good range in the past, but I hear news that Adrian Swain has been poorly. I wonder if ABS kits will end up going the same way as D & S?

Steve

billbedford
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Re: rectifying bowing plastic wagon sides

Postby billbedford » Sun Jun 19, 2016 7:06 am

jon price wrote:So, having recently assembled and painted and lettered a rare-as-hens-teeth plastic wagon kit the sides are bowing in. Obviously I can rectify this with a coal load, but it there anything to do at this stage that will restore the sides to the correct straightness whilst still empty?


Plastics, especially the polystyrene used for kits, will warp if only painted on one side, So did you paint the underside of the floor?
Bill Bedford
Mousa Models
http://www.mousa.biz

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jon price
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Re: rectifying bowing plastic wagon sides

Postby jon price » Sun Jun 19, 2016 10:01 am

Didnt paint the underside of the floor Bill, but it is heavily braced by being attached to a couple of your suspension units, longitudinally by the solebars, and by a couple of bits of lead. The interior of the wagon sides are of course painted.
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billbedford
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Re: rectifying bowing plastic wagon sides

Postby billbedford » Sun Jun 19, 2016 10:20 am

Mmm, plastics can deform in ways that other materials don't, so that the bracing and attachments to the underside of the floor don't affect the top side of the floor. what happens seems to be that the solvents in the paints leach out components in the plastic and make the top of the floor shrink slightly. This shrinkage then pulls the the bottom of the side in by a very small amount, which by a lever effect shows a noticeable distortion at the top of the sides.

I suggest the answer is to either paint all the components before assembling, or at least paint the underside of the paint the underside of the floor, and leave it to dry hardl, before fitting the w-irons etchings etc.
Bill Bedford
Mousa Models
http://www.mousa.biz

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jon price
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Re: rectifying bowing plastic wagon sides

Postby jon price » Sun Jun 19, 2016 10:40 am

I thought there might be some kind of more chemical thing going on, rather than just a mechanical thing related to the fit of the pieces. I have started to undercoat the pieces before assembly, and to finish the livery on the sides before assembly. This and painting the bare planking flat rather than assembled is also easier. Unfortunately this one predates that practice.
Last edited by jon price on Sun Jun 19, 2016 6:03 pm, edited 1 time in total.
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Re6/6
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Re: rectifying bowing plastic wagon sides

Postby Re6/6 » Sun Jun 19, 2016 11:38 am

Thinking about to Bill's suggestion, when I build any opens (for empties) I always replace the plastic floors with roofing lead sheet (code 3 @ 1.32mm thick or code 4 @ 1.80mm) and haven't yet seen any inward bowing on my ancient Airfix minerals.

They have remained stable for some years and using lead obviously helps with weighting, so I guess that Bill could well be correct.
John

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jjnewitt
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Re: rectifying bowing plastic wagon sides

Postby jjnewitt » Sun Jun 19, 2016 11:43 am

I generally find that when a plastic wagon bows it is down to the fit of the parts. Small bows and streses that are introduced during assemebly aren't necessarily noticeable at first but gradually work their way through the body. The main culprit for this I've found is the floor which seems to rarely be exactly the right size. I take great care to pack out any undersize floors around the edges to make sure that everything fits well.

If your sides are bowing in and the floor is slightly under size then running a scapel down the joint between floor and sides could free the wagon of these stresses. This has worked for me in the past.

All this talk is academic if the sides weren't straight to begin with. If the parts were warped, to any extent (usually as a result of coming out of the moulding machine a tad too soon) then the only way to get them straight again is to heat them to a point where they can reform themselves.

Justin


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