Heated bed - need to be turned on for entire print?

Does the bed need to be maintained at an elevated temperature for the entire duration of a print in order to prevent warping? Or can it return to normal / room temperature after the first few layers?

As far as I know, at least for ABS prints, if the bed drops back to room temp, the part can release from the bed. If it does so in a way that causes it to move, or if the printing action itself imparts momentum to the object, then you could end up with mis-registration of layers subsequent to the release.

It's a good question, and it's one that I'll have to try once I get my machine up and running. Makes sense in terms of reduced power draw, but I wouldn't want to screw up a 1/4 lb print to save a few cents of electricity (or any print in fact).

There *are settings that allows you to drop the bed temp after the first layer, but I suspect that's more to avoid sag by reducing it slightly, rather than literally dropping the temp back to ambient.

I was wondering this same thing. When I print in PLA and leave the bed at 60 degrees for the entire print, sometimes the bottom few layers become mis-shapen. The skeinforge chamber plug-in looks like it can gradually reduce the temperature as the print progresses - down to whatever minimum you specify. I agree with Xiando that having the print prematurely release from the bed is a concern, but I was thinking of experimenting with this plug-in and only reducing the temperature from 60 to maybe 40 to see it things improved.

If I remember rightly you want the temperature to be slightly higher for the first few layers (ensure the print sticks).
After those layers you drop the temperature slightly, but never down to room temperature until the print has set.
Otherwise as xiando said you run the risk of the print detaching from the printbed.

---

My Reprap blog

jds-reprap.blogspot.com

(Printing PLA) I leave the bed on for the full duration. I've a manually controlled heated bed. I have tried turning it down, and even off at various stages of printing so it would take lest time to get to 'cold' when the job finished, but it's not worth it as the job invariably separated from the printbed before the printing finished.
i leave it on (60Deg C aprox) for the duration of the print. When the job finishes I turn the bed off and switch a fan on to cool the bed quickly. The printed piece just lifts right off once everything has cooled down fully. I hope to automate this process eventually. My electronics doesn't control the heated bed so I'm planning to position a switch that will be triggered by one of the carriages moving to an end position on completion of the job, hitting the switch to turn off the bed and switch on the fan.

---

[numbersixreprap.blogspot.com]

Thanks guys. All useful info.

My query was due to the fact that my ATX power supply was struggling while powering the bed, rather than cooling down the bed to remove the printed object - but that's a good point also.

Anyway, the Gen7 electronics has a separate power connector for the heated bed - so I hooked a 12 V SMPS to that, and it seems to be working much better. Leaving the bed heat till the end hopefully won't be a problem now.

What Temp would you recomend for ABS part with a PLA raft? How are you guys regulating the temp? I have a MK2A, Rapman 3.2

What is room temperature for you guys. Mine is currently 30 degrees. I have a glass bed with kapton tape over it which i clean with acetone. If the bottom layer is big enough even at room temperature it won't come off, I ussually have to pry it off.

Mine is between 60 and 70 degrees, depending on how much cold I can tolerate this time of year.
(~ 15 -21 C)

You must either live in the southern hemisphere (where it's approaching summer) or be quite rich to be living in a home with 30 degree ambient.

You are quite correct, i live in Brisbane Australia, sadly with the 30 degree c temp we also have 60%+ humidity. The cold would be nice

Ah...well happy summer then! Today is the first day of summer down under. Lucky you. We're getting our first snow storm today... on the first day of Winter. bleh!

Anyway...back to topic.

One thing I'd point out regarding the heated bed is that once you've brought an object up to temperature (whatever that temp is), your primary concern is of thermal expansion, or perhaps better stated as the converse...which is why objects tend to self-release once the temp has dropped a certain amount.

The trick is finding out how much the temp can drop without causing "critical" contraction of the object. That's the big unknown, and I'd guess it's dependent on both the material and the temperature difference. For me, it's not worth pursuing, since the single largest cost of the extrusion process over time is plastic, that is, aside from my time invested (which in point of simple fact far outweighs any material cost)

Hey Guys, I have a RapMan 3.2 double head. I am experimenting with a MK2A attached to the factory acrylic bed with standoffs in the 4 corners. On top of the MK2A I have a 1/4" Fire rated piece of glass held down with small aluminum brackets secured to the acrylic bed in 4 places. Now here is the problem, almost immediately upon turning on the power the bed begins to rise as much as .040 at 130 C. I set a dial indicator in the center of the bed. (See attached pics) Anyone have any ideas on how to solve this problem???

joestephano,

Thermal expansion/contraction is a byproduct of temperature control acting on an object, and a standard issue in instrumentation which utilizes thermal augmentations (be that heating or cooling) I'd recommend that you calibrate only once you've reached operating temperature.