<p>I am extending the bed of my TronXY X3 FDM RepRap printer.</p>

<p>I am extending the bed from 220&nbsp;mm x 220&nbsp;mm to 220&nbsp;mm x 300&nbsp;mm.  For now, I will keep the existing bed and add and aluminum sheet on top.  That leaves 40mm on front and back of the original bed.</p>

<p>Right now I only plan on running PLA; but, I do plan on heating the bed.</p>

<p>How thick does the aluminum sheet need to be?</p>

Main factors that control the process of the print bed selection are
weight: too thick plate increases inertial force, limiting maximum acceleration/jerk (decreased print speed)
stiffness: too thin plate will warp when heated or bend during calibration (decreased print quality/printer reliability)
For table sizes around 400x400mm I would think of 4mm plate, but it still can warp if heated unevenly.
Sometimes it makes sense to use a sandwich-type table: lower level is MDF, cork panel for heat insulation and thin (1.5-2mm) aluminum heated bed on top.