Plastikard Warping – Causes and Cures?

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Ex-Pat
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Plastikard Warping – Causes and Cures?

Postby Ex-Pat » Thu Jan 25, 2018 9:23 pm

In my 23rd October 2017 posting on page 7 of my layout thread, the more eagle-eyed may have noticed how the base of the building “lifts” at its extremities.

It’s got me thinking – what causes it to warp this way – is it temperature – is it the length of the base (mine is 15”) – or what?

How do you take the stresses out, and cure it? Sever the base at various points? Or what?

I finally did something I could have done almost 2 years ago – I have used drawing pins to straighten out the base on a piece of MDF – is this likely to have a permanent corrective effect? (At least it will enable me to add the remaining two sides of the water tower to a flat base and hopefully keep it square.)

P1070675.JPG


P1070676.JPG


What have been your own experiences in this respect, and how did you overcome the problem?

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Re: Plastikard Warping – Causes and Cures?

Postby Dad-1 » Thu Jan 25, 2018 9:54 pm

I can see a bow in the front wall as well ......
Plastikard is a reasonably stable material, but large constructions will often show some warping.
Corners tend to be a problem as the liquid solvent soaks into the plastic and it expands slightly.
Then as it dries out it shrinks and twin layer building like this and mine can end up with slightly
concave walls and ends unless it's restrained. This is why I always add a tight fitting first floor.
In your case a ceiling would have been better than a floor.

Restraining early on may have held it, but once fully dry I doubt anything other than fixing down
secure enough to fully anchor will work. For such a long footprint perhaps a ply core may have been
better. Just wish I could advise of a miracle fix, but life 'aint like that.

Geoff T.
Remember ... I know nothing about railways.
viewtopic.php?f=22&t=32187 and Another on viewtopic.php?f=22&t=28436&start=60&st=0&sk=t&sd=a

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Mountain
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Re: Plastikard Warping – Causes and Cures?

Postby Mountain » Thu Jan 25, 2018 9:57 pm

Just a thought. Now I know with resin, when one makes a boxed structure like a coach, it is possible that the fumes from glue can warp the structure as they have no where to get out. Yet, put a decent size hole in the base and it prevents this warping. Could this be a similar issue? Glues can heat up quite a bit while curing.
Another issue is the structure built onto the base could be slightly altering in size? Could the base be made in sections instead? This gives some room for expansion.
Ideally, if the structure is made to float on top of the base it maybe the solution?
Generally the initial thought was to ask if the base was made from a thick enough sheet of plasticard?

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Re: Plastikard Warping – Causes and Cures?

Postby TimberSurf » Thu Jan 25, 2018 10:11 pm

Styrene is a thermoplastic, it is formed when "hot", shaped, then cooled while in that shape and will stay that way (until warmed again). The fix is to use hot water (80c) and then mechanically bend straight and hold while cooling, a hair dry can be used but be warned, overheating will cause major problems and might impact other parts! The issue is the surface and the internal part are slightly different, either through a machining process, rolling process or the extrusion process, both allow heating/cooling differences between the two, which induce different stresses in the plastic. Over time, or with heat, one will expand/contract faster than the other and will cause a bend. I believe the experts who make industrial models always make large flat areas with a 'Cell' construction, a similar method is to make a laminate, preferable (like plywood) with opposing 'grain' for each layer. If the sheet has two different sides (shiny/matt), use opposite ways up. Yes, both paper and plastic sheets both have 'grain'!
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Re: Plastikard Warping – Causes and Cures?

Postby TimberSurf » Thu Jan 25, 2018 10:17 pm

Mountain wrote:Generally the initial thought was to ask if the base was made from a thick enough sheet of plasticard?


As explained above, the thicker the sheet, the more likely to warp!
Internals can be braced, or cell/box construction, a thick base is not advisable, better either laminate 2,3 or 4 thin sheets to make it, else use a plastic that does not warp, like acrylic or polycarbonate
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Re: Plastikard Warping – Causes and Cures?

Postby TimberSurf » Thu Jan 25, 2018 10:26 pm

Find a greenhouse supplier who does spare sheets of unbreakable plastic (perpex) like Countrywide store or online http://www.plasticsheets.com/greenhouse ... FAQAvD_BwE

2mm @ £1.50 per square foot
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Re: Plastikard Warping – Causes and Cures?

Postby Peterm » Thu Jan 25, 2018 10:40 pm

Laminating in odd numbers should stop warping.
Pete.

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Re: Plastikard Warping – Causes and Cures?

Postby allan » Thu Jan 25, 2018 11:38 pm

This is a complex issue, and a fit subject for an organic chemist. I'm no organic chemist!

The problem is in the interaction between the base material (polystyrene sheet) and the glues that are used - and a lot of the glues, the best of them, are organic solvents. These solvents dissolve the plastic, and then evaporate off, allowing the plastic to harden again. The more volatile (hotter, faster) solvents evaporate more quickly, so do not penetrate the plastic so far, and cause less distortion.

There are other factors in play, too. The environment is critical. I'm fairly temperature tolerant, so I may work in temperatures between 15 and 30C. In hot weather I'll glue with MEK, but find that it is too slow for cooler times, when I prefer to use tetrahydrofuran (we are talking organic chemistry...). Ventilation is important as well: chemicals with names like that cannot be good for you, and besides, the faster that they evaporate, the faster that you can work. And the faster the evaporation, the less is the penetration, and the distortion.

The wetted area is also important. Gluing 0.05mm sheet edge on is hardly a problem, but try to laminate it and you'll finish up with a gooey (?gluey) mess. The solvent is trapped between the two sheets, and unable to evaporate until it melts through. If lamination is necessary, use a non-solvent glue.

Consider the thickness of the maerial as well. For a small structure, the thinnest styrene that I'll use is 1.0mm: for bigger structures I'll use 1.5mm sheet for the base.

Finally, note that polystyrene is a bit of a blanket label - it comes in a variety of grades. The harder material is harder to work, but more stable (think injection moulded kits...). And not all white plastic is polystyrene. Some is ABS, which will not glue, readily, with some styrene glues.

Correcting distorted styrene is challenging. The safest way is to fix it to a stable backing. The best, but riskiest is to heat it, and allow it to de-stress. Not a process for the faint of heart! The chance of disaster is consideralble. In the case under discussion I'd remove the curling material, cut as close to the walls as you dare, and glue fresh material on to replace it. The fresh material will not carry great stress, so 0.75mm, even 0.5mm will do.

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Re: Plastikard Warping – Causes and Cures?

Postby Ex-Pat » Thu Jan 25, 2018 11:58 pm

Many thanks to all who have replied so far. I am really glad I asked the question as it has resulted in a real education, and given much food for thought.

Yes,Geoff, I know there is a bow in the wall but I'm not too concerned about that, as I already plan to put in a couple of cross-sections to bind it together.

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Re: Plastikard Warping – Causes and Cures?

Postby b308 » Fri Jan 26, 2018 8:37 am

Peterm wrote:Laminating in odd numbers should stop warping.


I didn't think Plasticard had a grain so I'm not sure how that would work... Also as Allan pointed out the glue would cause major issues if you glued the whole surface...

I tend to do the same as the kits which have a reasonably thick base (40/60 thou) but have the centre cut out so the walls are mounted on an "edge" about 1/2" wide.

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Re: Plastikard Warping – Causes and Cures?

Postby carnehan » Fri Jan 26, 2018 9:25 am

Ex-pat, I can add no words of wisdom on how to correct the warping, all I can do is add my two pennies worth on my own plasticard experiences.

As you know, I build using a card former and then apply the plasticard sheet to this surface. Prior to bonding, I coat the card in PVA and leave to dry overnight. The plasticard is then glued using Revell Contacta glue applied liberally over the entire surface of the coated card.

To date I have had no issues with card or plasticard warping. All buildings are sitting in a dry roof space that experiences extremes of temperature throughout the year.

As said, it doesn't help you with your own problems, but perhaps it can eliminate or highlight key areas in the process such as temperature or bonding agents being used.

Hope you get the issue resolved.

Paul

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Mountain
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Re: Plastikard Warping – Causes and Cures?

Postby Mountain » Fri Jan 26, 2018 9:37 am

b308 wrote:
Peterm wrote:Laminating in odd numbers should stop warping.


I didn't think Plasticard had a grain so I'm not sure how that would work... Also as Allan pointed out the glue would cause major issues if you glued the whole surface...

I tend to do the same as the kits which have a reasonably thick base (40/60 thou) but have the centre cut out so the walls are mounted on an "edge" about 1/2" wide.

I was going to suggest this, as many manufacturers of plastic kits adopt this method. Just have the base as a framework that follows the walls, rather then a whole sheet. The kits will use a floor next to an open doorway for visual effect. Every here and there you will see aa thin section of base going across to keep the structure solid. Hope this is of use. No doubt the manufactures have had similar issues with plastics in the past and had to find a solution.

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Re: Plastikard Warping – Causes and Cures?

Postby b308 » Fri Jan 26, 2018 12:02 pm

Just to add to the above, I use triangles of plasticard in the corners to make sure the corner is square and strengthen it.

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Re: Plastikard Warping – Causes and Cures?

Postby flying scotsman123 » Fri Jan 26, 2018 12:31 pm

b308 wrote:
Peterm wrote:Laminating in odd numbers should stop warping.


I didn't think Plasticard had a grain so I'm not sure how that would work... Also as Allan pointed out the glue would cause major issues if you glued the whole surface...


It seems quite a few people reckon odd numbers of laminate should stop warping plasticard. I've read a few accounts of people constructing carriages out of several layers of 10thou plasticard and they always use odd layers with seemingly little trouble.

Anyway, with buildings I always try and give it as much support as possible, triangles in corners, tight fitting interior floors, and sometimes really thick plasticard section glued horizontally but attacked to the inside of walls along its thin edge helps, if that makes sense.
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Re: Plastikard Warping – Causes and Cures?

Postby Dave » Fri Jan 26, 2018 1:30 pm

Are odd numbers a railway modelling myth :?:

Will four or six layers warp but not five?

Why do an odd number of wagons look better than an even number?

Why are five trees more realistic looking than six?


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