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This is a paid asset, but we’ll share Simple Mesh Combine Free for you. Enjoy!!!
Simple Mesh Combine v1.71
One click mesh combine tool
Easily combine meshes by attaching this script to a GameObject containing meshes with the same material.
Click the Combine button to merge meshes.
Optional Mesh Compression.
To decombine just click the Release button.
Easily decrease draw calls to improve game performance.
Export to OBJ
Compatible with Unity 5.1 up to new versions of Unity(2019+)
Support available for assistance if any problems should occur.