@deckingman thanks for your time and dedication on the repply. I am planning to add some cork sheets on the underside and probably a heater for the chamber in the future for more complex materials. my mirror bed was 4mm as it was only supposed to "flatten" the bed (I was unable to find a good aluminium tooling plate on my country, ended up using a regular aluminim plate) and was actually reused from my home. this printer is only for large projects so I dont mind giving a full hour of cooling down after a 3 or 5 days print. for smaller projects I use a 200X200 printer.

@oliof thanks a lot, I have tested and works perfect for my use case.

I wonder if someone has a solution to gradually decrease heated bed temperature over a given lapse of time without creating a new ending script for each material and taking into account maybe the ambient temp when the print started as target temp.

Some background on the problem. I am running a large Custom printer (500 X 500 X 500) with a 220V AC heated bed on a SSR and a mirror laid over a 6mm aluminum bed as a printer surface to grant a complete even and level surface. printed parts stick impresively well, but due to the large format of the bed (and so the mirror) the heat expansion and cold temperature outside the enclosed chamber, my mirror has cracked twice this winter. (this used not to happen on summer with 30 C° ambient temp, but now at 0 C° at night, the cool down happens too fast and the part puts so much stress on the mirror that it cracks as it seems to cool faster than the plastic does. for reference, to avoid issues when I start a new big print, i set the bed temp to 75° and wait 30 minutes till I expect no further expansion of the bed and the chamber has also absorved some of the heat)

I can edit the ending script on simplify reducing the temp on steps (reduce 5 degrees, wait till temperature is reached, wait 5 minutes and repeat) but i think there should be a better way to reduce temp on a more linear way (maybe running a custom macro on end using Programming constructs. also, I should consider the season to set the final temp I want to reach, becuase for summer there is actually no way to command the printer to "heat" to 0 degrees as the ambiente temperature is higher.

my overall idea is so declare a variable when the printer starts with the ambient temp and last bed temp (to account for diffent material settings) , and then create a loop command reducing the temp by 1 degree from the "last target temp" each minute until the saved initial temp is reached (this is due to the lack of an existing gcode command to support relative temp changes). should this work? Is it actually feasible or there is any other recomendation?

this should also help people experiencing part warping on extreme harsh environments apart from saving some mirrors.