I have an ender3 v2 with a glass bed. Levelled my bed and running a bed level print. Edges fine, corners are okay but the middle is a mess. Print won’t adhere to the bed. I cleaned the glass bed with hot water and then applied some rubbing alcohol on it then applied glue stick to the bed when it was warmed up.
What could I be doing wrong or should do??
I am printing with value PLA nozzle 200 degrees and bed is 70 degrees
Is this a new printer that has always done this or is it one you have had for a while and it just started acting up?
the easiest suggestion I could make off the top of my head is to try and flip the glass. If it’s low in the middle in theory it should make it high after flipping.
Do you have any pics of the issue, It May be easier to see whats going on.
I have had this printer for over 2 years now and I upgraded the glass bed probably about a year now. And this problem started after I replaced the thermistor.
Will try and get some when I am home later tonight from work.
Melissa
A couple of points
Is the glass bed the “Official” Creality bed with the black bumps on one side. If so turn it over and use the flat side. I had no luck getting things to stick to the pebbles either.
The problems started with the new thermistor. Thermistors are not that accurate. They can be off as much as 10c, high or low and even when you set the bed temp to 200c it might only be as low as 190c which is on the low side. I can’t get PLA to stick at that temp. Try setting the bed to 210 and see if that helps. I generally do my first layer at 70 on the bed and 220 at the nozzle with the initial line width at 125%. After that it goes down to 50c on the bed and 210 at the nozzle. Nothing to say that those are the accurate temp settings, depends on the thermistor. I rarely have any first layer problems with those settings. Sometimes getting the prints off is fun.
Did you calibrate the PID . That will give a better temp. control.
I’ve never seen a thermistor off by anything close to 10 degrees C - that doesn’t mean that it’s impossible, but I would consider it to be highly unlikely.
It is possible that the thermistor has different characteristics than what is required by the Ender - where did you get it?
Also, how did you mount the thermistor to the heated bed? Personally, I use thermal grease (used for attaching PC processors to heat sinks - you can get a syringe at Canada Computers for $5) and cover it with aluminum muffler tape.
I agree that the thermistor is somewhere you should look, I just don’t think you have a bad one.
Possibly the thermister referred to is the one on the hot end?
Your right about 10c being the extreme but I believe that someone on this forum had one that far off a long time ago. It is unlikely but possible. Even a 5c error can throw things off. The theremister being a problem is only a guess, something worth looking at.
@mbura could you help explain the thermistor change? Ideally with photographs.
After a bit of thought, I’m wondering if the replacement thermistor isn’t in the centre of the print bed so you get a pretty significant temperature difference from the edges to the centre of the bed.
I find your first point interesting. I bought the official glass bed and I absolutely adore it.
I too had issues with adhesion at first. I noticed, though, that the temperature of the surface (taken with IR thermometer) was about seven degrees lower that what it thought it was.
This made sense, since the heater and sensor are under the bed, and you’ll lose some going through the metal and glass.
So, I turned it up to 67, which got me 60-61 on the surface.
That helped a lot.
I also use brim (occasionally Raft), and clean the bed with 99% IPA, and that’s usually all I need to do.
For nylon, and for stuff that does give me issues (shape, maybe), I’ll put some kapton tape on top of the glass, and that works too.
When you look at the parts off the glass, the pattern of dots on the bottom can be seen in the right light. That lets me easily see if there were any spots that weren’t sticking.
In fact, if it’s quiet as it’s cooling, just below 40C, you can hear little “ticks” as things release! Between 40&50, the parts are still stuck fairly well, but below 40, you can blow the part off the bed.
Is my good experience with the glass bed unusual,? Now I’m curious.
It is a toss up on the pebbled side of the glass beds. For some they work great and people love them. Others like me have no use for it, I can’t get anything to stick to it. I always use a brim but never a raft. Glue. I always use glue. Lots of people berate glue as a cheat, so what and yes it is old school but it really works. Now that I have my machine modded the way I like it and using the first layer settings I mentioned I almost never have any trouble with adhesion.
One last thing, maintenance. Do it. I was getting some wavering lines in my print, didn’t know why but while fixing my broken belt I noticed the wheels on the gantry were slack. I tightened them up and the prints look great again. There is a lot of wear and tear on these machines if they get used a lot, maintenance will stop problems before they start.
I was having a hell of time with what I thought was bed warp/ headed bed zone issues. I ended up replacing the whole headed bed. I am really note sure if that did it, (But the warp seems to be gone), or if it was the regimented re-“leveling” of the bed, including locking in the Z-offset starting point (right at the bed). She is working like a pro star now… (ENDER 5 PLUS).
I recently had a jammed hot end I had to clear. While I was at that, I went around doing maintenance. Nothing terribly unusual found, but, prints are better after. 
I have seen thermistor (not in 3d printing) off far more than 10ºC many cheap ones have a hysteresis of 5º naturally but if the leads are too long or short it changes the reading. If they get damaged they can change as well.
IR thermometers don’t measure glass well, reflective surfaces generally are unreliable. Emissivity issues.
I would suggest washing the glass with laundry detergent. It is designed to remove protine deposits that soap will leave. I found it works far better than soap to revitilise PEI surfaces I would extrapolate that to glass.