Bent Foil Cards

[Carl333]

All my foil cards are automatically bending, for some reason. Just wondering if anyone else has this problem or knows how to fix it.

[dmaz]

This always happens for some reason. They curve upwards and inwards.

I noticed that the foils that I had around when I was in Brazil seemed to do it more often than my home in the USA.
The humidity where I was at in Brazil was much higher on average, so I think that the level of humidity affects how much they curve, though they all have the tendency to do this.

I find that if I lay them flat underneath a heavy surface for a while, I can press some of them back into relative normality.

My recommendation would be to keep them in sleeves (hard plastic if you can) in a room without much humidity. Air conditioner probably helps.

[Air Power]

The paper and foil material both expand/contract as temperature and humidity change, but at different rates.  If the paper grows by 1 mm and the foil only grows by 0.1 mm, then the card will bend to keep the bonded points bonded but allow this expansion (the slight difference in radius between the inside of the curve and the outside is enough to compensate for the different card/foil lengths).  This same effect is used in bimetal switches in some thermostats and thermometers.  One solution would be to find the temperature and humidity at which the cards are straight; however, in practice using a hard case or binder page is much easier.

tl;dr: it's magic, and dmaz' solution works

[Carl333]

Thanks everyone for the answers. It does work.