Most of you probably already know this. Recent sets have had an issue with curled/bend cards. Especially the foil ones. See the picture for reference. Here are the tips and tricks I have learned so far to avoid these issues for my cube.
There are two types of curves: Convex and Concave.
Convex: The face of the card is curved in a way where the center of the card is closer to your eyes than the borders of the card. A Convex curved cards needs higher humidity in order to return to normal flat.
Concave: The face of the card is curved in a way where the center of the card is further away from your eyes than the borders of the card. A Concave curved cards needs less humidity in order to return to normal flat.
There are several ways to remove humidity from your concave curved cards. One is an expensive super solution called a dehumidifier which is a machine that operates on electricity and removes humidity from the air in its surroundings. Another way are the cheap Silica bags that works excellent but needs to be changed every now and then.
When trading or buying cards from other players I ask for confirmation that the cards (especially the foil ones) are not curved and I ask for picture to evaluate the cards myself. Beforehand it was enough to check for that ‘Near mint’ or ‘NM’ condition but now we need an additional scale to evaluate cards from: Curved or not. If you ask for a picture before buying, you will know for certain if the cards are bend or not. In my experience the cards that are not bend when they are opened, will never bend.
I hope you can use my experiences to avoid getting curved cards yourself.