Export to STL: open edges appear

Hello!I created a model for 3D printing in 3DsMax 2019. When I use STL check modifier it says there are no errors. When I export file from .max to .stl suddenly open edges appear.How to avoid that? Am I using wrong settings for export?Model was created using Boolean and then converted to Poly. Can that be an issue?
Attachment 11529
.max screenshot
Attachment 11530
.stl screenshot

Please help me finish my college project)

Yas! boolean is a very complicated operation. it is better to do everything in one object! and it would be better to do in editable poly (not mash)

If you need help, share some information about your printer and printing setup.


What printer are you using?
What material are you using?
What speed are you printing at?
What software are you using to slice the print and control the printer?