Oh boy, not a single collector I know avoided warping of some raw ungraded banknotes. Mostly it is due to climate and occurs heavily in areas that have all four seasons. Most of us collectors dont have expensive aircon/dehumidifier or dessicator boxes with inert gas that make climate stable
I use Leuchtturm banknote pages and albums plus silica beads bags inside and i still see some warping on some banknotes. Why Leuchtturm? They use hard PET which doesnt bend easily, it is stiff like coke bottle and it is not chemically reactive. I avoid mylar because it is too expensive and not so rigid. PP and PVC album pages are the worst solution, especially for polymer banknotes
Usually my banknotes that have large security threads, e.g. Libya 50 dinars with Qaddafi or Fiji 10 dollars polymer banknote have some warping near security threads due to material properties difference at junction with banknote paper/fabric. When temperature of the environment changes there can be a lot of stress in thin material so bending happens. Sometimes i get a “tellers nick” on top of banknotes with large security threads but these banknotes are usually of poorer quality, e.g. Venezuelan bolivar of low denominations. I have no issues with dollar banknotes or any banknote with no security threads or small security threads. These albums and pages are perfect for small to medium size banknotes and almost perdect for large ones.
My advice is to get a banknotes album with pages that are made from hard nonPVC material, store it in a place with no humidity if possible, vertically, add couple of bags of silica beads inside album near ring binders. I use Leuchtturm grande so there is plenty of space inside.
