So I usually heat my PB Metal Plus bed to between 70C and 80C

So I usually heat my PB Metal Plus bed to between 70C and 80C for normal PLA printing etc. Today decided to try out some HIPS dissolvable filament with ABS and heated the bed to 100C. I usually keep the bed around 80C for ABS but wasn’t sure about the dissolvable filament so figured it wouldn’t hurt. I never realized what the change in 20C on the bed would do to the Z offset. The combination of higher bed temp and higher hot end temp made me have to change my Z offset by almost .15mm which, in terms of tenth of a mm, is a big deal with printing.

I couldn’t figure out why, all of the sudden, my printer was printing so close to the bed and resulted in really rough first layer. The effects of heat on the metal never occurred to me.

That is how a mercury thermometer works.

Shit, @Mark_Rehorst you beat me to it. :slight_smile:

@Mark_Rehorst ​ anything special needed to activate or get that working. I’m familiar with calibrating the hot ends but never tried it on a bed.

I have a different solution for you. Preheat. I actually had the same problem but realized it is because of the thermal expansion of the metal bed AFTER the heater plate has already been at temperature. It takes 20-30 min for the bed surface temperature to get even close to the heater plate temperature in a PB Metal Plus. If you preheat for about 10 min prior to setting the print to go then your auto Z height calibration will work just fine and you will not need to adjust Z calibrations between material types.

@Nicolas_Duarte , preheating does nothing when you start adding thermal mass to the bed, moving it around, etc. Your temp won’t just stay stable with bangbang. It’ll fluctuate 10C or so. That’s enough to cause rough looking edges on a print.

That’s correct, I should rephrase my statement. To get the best result in temperature you will need to implement PID, however it will not help with the thermal expansion problem that Michael is running into. Specifically with a Printerbot Metal Plus, even though the bed heats to temperature before getting the Z level, it doesn’t heat the surface to temperature, only the heater plate (which is on the bottom of a large aluminum plate). There can be up to a 40C difference between the setpoint that the control has settled at and the bed surface temperature because of that thermal mass (when the bed think’s it’s at 100C it is actually at 60-70C on the surface and takes ~10 min to get up to 80-90C…these are all gross estimates since I have not done an exhaustive study on this). That temperature difference results in an expansion of the bed which causes both strain on the rails (which means you need to tighten the rails at the temperature you want to be printing at) and causes enough of a vertical expansion to throw off the z calibration (since it is head based with a proximity sensor).

I honestly haven’t checked if the Printrbot uses PID or bang bang for it’s bed control, but I would consider the PID as a secondary solution to Michael’s problem.

@Nicolas_Duarte ​ it’s bang bang. I did preheat for some time. This is just something I do anyway on the plus. It takes a while for the large hunk of metal to actually take on the heat. It’s not really a problem. I was just pointing out that I never really thought of how the heat can cause enough expansion to warrant recalibration the z offset to the point I had to.

I’m not having a problem keeping the bed at 100C. That wasn’t the purpose of my comment. It stays at a steady consistent temp. If it does dip it is to 99 then back up to 100. I was just commenting on the effect 20C has on the expansion of the metal bed and hot end nozzle.