laminated roof problem

[Ealing]

I recently built an elliptical roof using the laminated technique as recommended by the South Western group in MRJ 150. I stuck strips of plasticard together with superglue and scrapped back to form the profile. Then I covered with 5 thou plasticard to create the skin - but stuck this with butanone. I was really pleased with the result and it fitted the body well. However when I returned to add the vents etc a couple of days later, the ends of the roof had started to curl upwards banana-like by a few mm.

Perhaps I should have used superglue on the final layer and this is what is exerting the stress. Can anyone recommend a cure? I've thought about running some T section stuck underneath, or perhaps another layer of 5 thou stuck underneath would counter the skin on the top?

Any suggestions would be gratefully received. Thanks.

Andy

[Russ Elliott]

Most plasticard laminates misbehave. Be brutal with them:

big-purlins.png (2.42 KiB) Viewed 432 times

[martin goodall]

I just touched a button on the side of my mouse and lost the reply I was writing. Let's try again.

I make curved roofs (if they have a circular curve) by heat-forming a single layer of styrene sheet (about 15-thou if I remember rightly). Find a can (drink can or similar) of the right size, or slightly under-size if you can't get exactly the radius you want. Tape the styrene sheet to the can with 'hot tape', then wrap it tightly with crepe bandage to hold everything in place. Pour boiling water into the can and leave it for a minute or two. Then pour out the hot water and pour in cold water to cool everything down. When cool enough to handle, unwrap the crepe bandage to reveal a perfectly formed roof, which will keep its shape with no further help, and certainly without any bracing. If the radius is slightly under-size, stick it down firmly to the ends of the coach/van to hold it in place.

If you still prefer laminated assembly (e.g. for an elliptical roof), remember to observe the 'law of laminations', which says that any laminated assembly should consist of an odd number of at least three layers (and that the corresponding inner and outer layers should be the same material and of the same thickness). Laminates made of styrene sheet should not misbehave if you observe this rule.

I would not recommend 'brute force' as the sheet will sooner or later overcome the restraint and decide what shape it wants to be after all!

[James Moorhouse]

You could try placing the warped roof in a warm oven on a flat surface. After time and sufficient heating the bottom of the roof will take the form of that surface. I would use a piece of MDF or tufnol sheet I know to be flat rather than a metal baking tray as the metal would become hot too quickly and melt the plastic.
It is very important to constantly watch the oven, as the plastic deforms quite quickly once a certain temperature is reached. I have used this method on thin plastic roofs, such as those supplied in Ratio coach kits, to good effect, but not on a solid lump of plastic with layers of superglue in amongst.
I have produced solid plastic roofs as per the Southwark Bridge method, which I think is the same as the method you have used:
http://www.sbmodels.org/sbm_toptips.html
I used the method for roofs with domed ends where the plastic laminations are scraped to form and the plastic skin is dispensed with. I have found these roofs to be very stable.

James Moorhouse

[Ealing]

Thanks for all the information and tips. Very interesting. I added another lower layer the same thickness as the top skin to try and pull the roof back - no luck yet. I think I may try a layer of thin paxolin superglued to the base.

[martin goodall]

James - Regrettably, your warped roof may be beyond redemption, and it might be better to start again. I have certainly had to abandon a vehicle roof in the past when it went wrong. It is probably quicker to do this than to spend time trying to remedy the fault in the existing roof.

[Will L]

James - Regrettably, your warped roof may be beyond redemption, and it might be better to start again. I have certainly had to abandon a vehicle roof in the past when it went wrong. It is probably quicker to do this than to spend time trying to remedy the fault in the existing roof.

No, I'm also pretty sure adding the third laminate after the event will do no good.

For small roofs(i.e. wagons) I've always found one layer of heat moulded plasticard a la James Moorhouse is all that is necessary.

For longer roofs (coaches) I find a single layer moulded pasticard roof with a flat ceiling and purlins a la Russ Elliot produces a rigid and shape stable roof. See cross section diagram. Use the cantrail to hide the joint with the body. I don't stick them on, the're quite rigid enough to bolt down and I prefer a removable roof on a coach.

coach roof.jpg (12.14 KiB) Viewed 254 times

You can use profile pieces at the ends (only) to ensure you're got the roof profile/purlin hights right but remember not to produce a fully enclosed box section when solvent welding plasticard. I've done a roof like this in brass, once, but that was overkill!

[James Moorhouse]

For small roofs(i.e. wagons) I've always found one layer of heat moulded plasticard a la James Moorhouse is all that is necessary.

To clarify, the method I suggested to Ealing (placing a plastic roof on a flat surface in a warm oven) is a fix for a warped roof.
When I make roofs out of a single layer of plasticard sheet I do this by heat-forming the plastic in boiling water. For this I have an off-cut of metal pipe (part of a boiler flue pipe which happened to be the right diameter), around which I wrap the plasticard sheet and hold in place with some Tubigrip support bandage. Meanwhile some water in a deep-sided roasting tin has been brought to boiling point on the cooker top. Once the water is bubbling vigorously, the pipe is immersed ensuring all the plasticard is brought into contact with the boiling water. The pipe is then removed and immediately immersed in a sink full of cold water, after which the plasticard can be removed from the form. This method ensures the whole plasticard sheet is heated to form in one go and avoids the risk of any inconsistencies in the formed plasticard due to sporadic heating and cooling.
I currently favour making coach roofs from laminations of plasticard as per the Southward Bridge method. This is because of the demanding profile of the coach roofs I am currently modelling though.

[allanferguson]

I've never been happy with plastic roofs -- they don't seem to be stable unless firmly glued on. But I hate being unable to access the inside, so have always made my roofs from brass sheet, with spring clips fitted to secure them inside the ends. They can easily be removed and will reliably maintain their shape. It's also easy to solder on lamp tops, ventilators, etc. To put the bend in I use a rolling pin (as in baking) in its curved holder as a former. However all my roofs are of simple arc form! I try not to model complex things!

Allan Ferguson