This is a thick rectlinear infill pattern.

Since current infill modules generate single thin threaded pattern, they act like a support for top layers unless you set high infill ratio.

There should be several discussions about more rigid infill patterns but no such infill pattern is found in current Slic3r release. And I do not want to waste my time for waiting those modules.

For more infill pattern ideas, check to see https://garyhodgson.com/reprap/2012/01/thoughts-on-fill-algorithms/

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