i'm trying to understand (and solve) the following issue in qgis (2.16.3): i have a shapefile with hexagons which i want to merge. doing so works, but certain "borders" (not sure about the correct term, sorry) inside the polygon remain, and i cannot figure out how to remove them (see below).

input:

[![enter image description here][1]][1]

output:

[![enter image description here][2]][2]


  [1]: https://i.sstatic.net/1IZBM.jpg
  [2]: https://i.sstatic.net/zQles.jpg