PLA doesn't stick to glass plate


I did just try to make the frist print with PLA. But it didn’t work at all. To improve flatness of the bed, I did use a 4mm glass plate on the heatbed. The top surface is ground with carbo 320, and it is degreased with Acetone. The extruder distance is adjusted to 0.25mm in all corners. Extruder temperature was 190°C, heatbed temperature was 50°C and 60°C, but I assume it’s less at the top surface of the glass. There are two problems:

  1. The first PLA layer doesn’t stick to the glass plate at all. The extruder easily breaks off the already printed structures and moves them around, as can be seen in this picture:
    I tested this 5 times and always got the same bad result.
  2. At the beginning, a big blob is extruded at the home position. When I stop the job after about one minute, the extruder is moved back to the home position and collides with the cold blob of PLA! Why isn’t the extruder moved up when I stop the job?


There can be more than one reason to your problem :

Glass plate too cold : a thick plate takes a long time heating. To improve things I’ve used a mirror (metallic coating helps) with thermal grease to ensure a better heath transmission.
The stock cooler is cooling the bed and the PLA before it can stick --> use a fan nozzle.
The extrusion width is too low : the amount of plastic extruded for the first layer must be sufficient to be squeezed against the bed in order to stick. 0.4 to 0.5 mm is needed to achieve this (eg layer: 0.35 and 1st layer extrusion width about 145-150% or 0.5 mm).

I did already try to heat up the glass plate from the top side with a heat gun. But it didn’t help.

The fan was off.

The extrusion width is too low : the amount of plastic extruded for the first layer must be sufficient to be squeezed against the bed in order to stick. 0.4 to 0.5 mm is needed to achieve this (eg layer: 0.35 and 1st layer extrusion width about 145-150% or 0.5 mm).[/quote]

I did use the parameters for Repetier 0.90 from the download page: …
Are there any changes required to these parameters? Which parameters must be changed?

I did set the extruder to the home position and then I checked that the distance to the glass plate is 0.25mm in all corners and also in the center. I did use a thickness gauge, 0.25mm just fits under the extruder, and 0.30mm doesn’t fit. Is this correct or is a smaller distance required?


To check if the height is right you can simply turn the z motor axle by hand when it is printing! I learned this from the german computer technik magazin who did have an item about the k8200 and it is very simple to do during the outline or easier when it is printing the bottom layer. During turning by hand you can suddenly see if it starts behaving like you want it to.


I did now get a good print without the glass plate. It seems the temperature was too low with the glass plate.

But there is still one thing I don’t understand. I have seen in the G code that the first layer is printed at Z=0.30mm. If the home position is 0.25mm above the plate, then this means the extruder is 0.55mm above the plate when printing the first layer. Isn’t this too high?


Hi Fieldmouse,

it may be, but there are (as often) many parameters affecting the height of the first layer, e.g. the flow of the material through the nozzle, the nozzle itself, the print speed, etc.
Have you measured the height of a single layer print? Not the skirt, but a layer with (at least partially finished) infill. If it’s not 0.3 mm high as set in Slic3r, you can adjust the Z endstop screw accordingly. Just be careful, you may end up with your nozzle very close to the bed in the home position, so the bed has to be very flat for such a setting to prevent collisions between nozzle and bed.

While 4 mm does work, a glass plate (or mirror) with only 3 or 2 mm thickness might work better. Two questions: did you mount an additional thermal shield below the heatbed PCB when the glass plate was on top? And did you take out the screws that usually fix the heatbed PCB at the same time, or alternatively, did you take care that the screw heads do not stick out of the PCB surface (i.e., did you make the counterbores larger)?

Why should it move up? All printed parts are necessarily lower than the nozzle, so there is by definition nothing else on the heatbed that could collide with it. The printer does not even know where its movable components actually are (apart from the home position), it just assumes that they are where they’re supposed to be. What you want is some kind of intelligence that this machine simply can’t offer.
What you (since you have this intelligence!) can do is:
a) raise the nozzle via manual commands before stopping the print in case it is too low for passing over the home blob,
b) remove the “home X and Y” command from the commands executed when the printer is stopped,
c) lower the prime extrusion length so the blob will be smaller, or, and this is my personal recommendation,
d) remove the blob by hand approx. 10 to 30 seconds into the print.

For the latter option: at that time the blob is cool enough so it won’t burn your fingers, but not yet fully hardened, so it usually detaches quite easily. Plus, while printing the skirt, there’s no real risk of “disturbing” the print. Of course, keeping one’s fingers away from the nozzle is mandatory, but that should be common sense.



I did measure about 0.3mm. But I don’t understand why. It should be 0.55mm, because the home position is 0.25mm and the first layer is printed with Z=0.3mm.

No, I didn’t. What type of additional thermal shield do you recommend?

Yes, I did make the counterbores larger.

Where are these commands defined?


Hi Michael,

well, I reckon you’d have to get quite deep into the kinetics of molten plastic flowing through a small nozzle to really understand what’s happening. The behaviour may differ one day when you use another sort of filament, had to replace the nozzle or some other part of the hotend, or the extruder, or …
In such a case, I’d say - as long as your printer does what Slic3r told it to do, everything’s fine.

I got some cork tiles from a local DIY store (actually had to buy a pack of ten, so still enough left over for nine more printers) and cut out a piece as large as necessary. The plain stuff without any kind of coating is obviously best suited. Another glass plate or some kind of ceramic should work, too, but those tend to be heavier. And less easy to drill, which you would have to do if you want to keep the four bolts (I didn’t).


Where are these commands defined?[/quote]
At least at two locations. Slic3r’s settings, Custom End G-code usually has some kind of homing command, but that would only apply if the print finishes on its own. Additionally, there is the option “Go to Park Position after Job/Kill” in Repetier Host’s Printer Settings, which, as the name suggests, should be responsible for said action in case of an early, manual stop of the printer.


to make PLA stick to glass, just apply blue painter tape:

If you do that, it will stick really well, even if the plate is barely above room temperature, and the bottom will still be quite smooth.

You mentioned that that you measured the gap at the corners, but did not mention that you measured the gap in the center. Looking at the image you uploaded, I saw that you attached the glass with only two clips. I think the glass could be bowed enough to lessen the gap in the center. Check the center and try four clips, if needed.