*What?* If the foil is not tight around the edge, it is often due to one of the following causes. It is very important that Mylar foil and paper can move very smoothly in the dies. Otherwise paper and foil rub too much causing to form air bubbles on the edge of the buttons. In extreme cases the foil or button even comes loose.
*1: no smooth dies* For perfect performance, the 3 dies (2 bottom, 1 top) must be very clean and completely free of unevenness or sticky dust. You can clean the dies by rubbing them clean with paper towels or a cotton cloth. Do this until they really shine again!
Especially the angled inner edge of die 2 is important! Lubricate these surfaces with a super-smooth pure silicon grease. Be economical with this and rub the dies again so that no residue remains on paper and foil. Your dies are now smooth again. Repeat each approx. 500 buttons.
*2: Too dark or heavy printing on edges* For some paper types and printing (usually laser prints with a dark or full color) to the edge of the cut paper, you get too much friction between paper and foil. This is on a microscopic scale, your prints may even feel super smooth. If you test a few blank papers without any printing, you will notice that the film closes tightly. (if item 1* above is ok)
The friction is mainly on the outer edge, where the paper disappears in the button back. The solution is to limit the printing to the visible zone of a finished button (to the edge where the paper disappears in between the shells). At the back of the completed button you will see a small white border as small as possible. But not visible in use.
The image above makes this clear. The outer circle is your thinly printed cutting circle. Then there is a white unprinted edge to avoid the friction. The central circle with cross is the front (button size) of your buttons. What lies beyond is the visible side of the buttons that extends to the white zone.
For all sizes, this white border is slightly different (and the small sizes have no problem):
38mm> total diameter visible zone to white edge: approx. 40.50mm
44mm> total diameter visible zone to white edge: approx. 46.50mm
56mm> total diameter visible zone to white edge: approx. 59.00mm
75mm> total diameter visible zone to white edge: approx. 79.90mm
Finally, limiting the printing to this visible zone is always recommended to obtain extra smooth edges (and to save ink).