PLA on an unheated bed

I'm starting to print with PLA on an unheated bed (printing onto blue tape over garolite). In general, when using an unheated bed with PLA, do you need to print more slowly or do you want to print faster? Is it better to use a higher or lower hot end temp? More cooling or less? In particular, I am getting a lot of curling up (as opposed to sagging) on the edges of overhangs. This seems to happen whether or not I've got the fan going for cooling. Any pointers would be a big help.

Here's a summary of my current print settings:
1.75mm PLA, 200 deg hot end, 40mm/s perimeters, 60mm/s infill, 0.26mm layer height, 0.4mm nozzle orifice

Edited 2 time(s). Last edit at 09/26/2013 09:15PM by LoboCNC.

PLA on an unheated bed will work just fine. If at some point you find that your print is peeling from the tape, you might have to wipe the tape with alcohol first - and then it won't ever release.

If you're curling up, it could be temps or flow. You should always be running a fan on PLA - except maybe on the first layer.

What layer heights are you running?

Peeling up is not a problem - if anything, everything sticks too well.

I'm using a 0.26mm layer height.

The fan on or off doesn't seem to affect the curling. With the fan on, it still curls up but then hardens more quickly.

When you say "could be the temps or flow", for my curling up issue, should I increase the temp or decrease it? Increase the flow rate or decrease it?

BTW when printing things with vertical sides or parts that taper (underhang, if that's the term?), everything seems to print fine.

Aside from the curling issue I mentioned, in general, I'm looking for suggestions on how printing parameters would change when using an unheated bed vs. a heated bed when printing PLA. When using an unheated bed, do you want to print slower or faster? Hotter or cooler? More or less fan cooling? In other words, what do you have to adjust when going between an unheated bed and a heated bed? Many thanks for any tips.

If the PLA is good, you won't have to change any setting. That is, if the PLA has a high melt flow and comes out velvety smooth rather than brittle and glassy.

Hmm, maybe I'm missing something. What's the advantage of using a heated bed with PLA? Or is there any? I'm getting excellent adhesion (maybe too good) on the bed printing on blue tape.

You can't print on bare glass without a heated bed. That's all I can think of.

Is your part warping because it's pulling up the tape, or because it's coming off the tape?

Thanks fro the feedback. I'm not having any problems with the parts pulling up off of the tape, or with the tape itself lifting off. The only issue I am having currently is that overhangs want to curl up. (Note: the curling is *just* on the overhangs. The first layer stays nicely adhered to the bed.)

What I'm really asking is more general - what are the advantages/differences of using a heated bed (v. non-heated bed) with PLA? Is the only difference is that you can print on bare glass rather than needing the blue tape? I've seen references to a heated bed "saving a lot of time" when printing with PLA. Is the time savings just from not having to re-apply blue tape periodically?

The only thing I can think of now is that a heated bed - and even better, a heated chamber will reduce the stresses your part will have due to temperature differences. When the entire part is kept at (or slightly below) glass transition temperature during the print and slowly and evenly cooled afterwards, it won't have thermal stresses.
This is supposed to be a problem when working with acrylic glass / perspex : if you machine a part, you heat a certain area around the tool while the rest stays cool. As soon as the part gets in contact with solvent (e.g. in glue), it tends to crack due to the thermal stress. That is why acrylics have to be heat-treated after machining.
PLA doesn't exhibit this extreme behavior (or at least I didn't encounter it) and for most applications, these thermal stresses may not play a role at all.

pla objects less than 100mm in size should work ok. If you have issues with it sticking then enable brim or raft features in slicer program.

A tip I use if pla is peeling up is add another layer of tape down. Likely the bed height is slightly too high. Not enough to mess with the settings, but enough that another layer of blue painters tape adjusts the thickness so proper contact to surface is formed.

I think a heated bed is better with PLA. I've had some parts (mostly bigger parts with sharp corners) that I could not keep from warping. For most smaller stuff and round cornered objects, I didn't have a problem over a range of speed and hotend temp settings. If you're facing warping issues, I'd recommend not using a fan for the first 10 layers or so, so that it cools more slowly.

Thanks for the tips. My printer is pretty small (150x150) and I haven't printed anything over 75mm so far. Sounds like the advantages of the heated bed come into play with larger parts.

If you ever run into the situation that PLA doesn't stick to tape (due to size or whatever), definitely try UHU glue stick. I had to chisel parts off the platform, once even breaking the glass in the process. Also tried this with a cold build platform (just glue stick on glass) with excellent results.
Other glue sticks may behave differently, e.g. PLA likes to stick to Tombow glue stick on Kapton tape at >=60°C, but pops off easily at <=40°C, leaving the glue stick layer intact. I managed to get about a month or so of print time before having to reapply a little bit of glue.

We use Kapton tape on glass, on a heated bed, when printing PLA parts, for the following reasons:
Durability - 18 hours a day printing, 5 days a week, Kapton tape lasts at least 3 months, so long as you are careful not to damage it when removing parts. Blue painters tape on an unheated bed tends to need replacing every print!
Reliability - Cleaned once a week with Acetone on kitchen paper roll. Mainly to get rid of dust and oil from fingerprints
Quality - Reduced thermal stresses in the plastic, not as strongly trying to pull itself off the bed, consistent layers.

Unheated beds work fine for smaller parts, where the shrinking forces aren't as strong. The weak point is not the adhesion of the PLA to the tape, but the shrinking of the plastic pulling on the tape enough to pull the tape from the surface. This is particularly a problem with blue painters tape, which is designed to have a weak grip, so it can be removed.

Ian
RepRapPro tech support