"Cold spot" on my heated bed?

[old man emu]

I wonder if it is possible to get a "cold spot" on the heated bed? On a number of occasions I have been trying to print something and I get a corner lift at the left hand read quadrant of my heated bed. (Point 0,0,0 is at the right hand read corner of my bed, so I'm getting lifting near (-50, -50, 0).

I've tried slurry, hairspray, painter's tape or nothing at all, and I can't stop lifting in that area. The rest of the print sits down quite well. I've tried bed temperatures from 110 to 90 (printing ABS at 220).

Could the problem be caused by air coming off the extruder fan? Also, should the air pass over the extruder fan from the front of the printer to the rear, or vice versa?

I desperately need to resolve this as I have to print a minion for my daughter-in-law's birthday present.

Old Man Emu

[ciutateivissa]

Could the problem be caused by air coming off the extruder fan? Also, should the air pass over the extruder fan from the front of the printer to the rear, or vice versa?

Yeah, the extruder fan is ONE possibility. No mater from which direction the air comes, you should in any case prevent the print from any cool air. This was the reason I changed my hotend to a different type which needs no fan

Another reason could be, that a fan from the electronics (e.g. RAMPS or PSU) get sidetracked by the printer chassis and hit your print in this area. This was the case in my I3 as well, so I made a special box for my electronics where the fan is blowing in a direction away from the printer.

[old man emu]

The fan is the same size as the one supplied with the printer.

Maybe it is the heat bed temperatures I'm using. When printing with ABS, do we use 90 or higher for the heated bed?

OME

[Roxy]

Maybe it is the heat bed temperatures I'm using. When printing with ABS, do we use 90 or higher for the heated bed?

I print ABS at 110 C. But remember, that temperature is going to vary. In my case the thermistor is taped to the bottom side of the heater and the sides are sealed off. By the time the heat gets up to the surface of the glass, I suspect it is significantly cooler.

[ciutateivissa]

Maybe it is the heat bed temperatures I'm using. When printing with ABS, do we use 90 or higher for the heated bed?
OME

The heat temperature could even be higher, at last for the first three or four layers. I´m using 105° C for the first three layers, then reducing temperature to 96° C. It is quite important to get a consitant temperature over the whole heat bed so let the heat bed work at least for 10 to 15 min before your printing on it.

The fan is the same size as the one supplied with the printer.
OME

Ok, I understand. But maybe the new cooler is more effective (more rotations per min). I´m pretty sure that a smaller one would work much better, just give it a try... Disable the big one and use a 25 o 30 mm fan just for testing, you could just fix it with a tape, just to see what´s happening.

The fan schould just cool the tube of your hotend, not the nozzle itself.

[printbus]

For the heater to be at fault, the "cold spot" would have to be a pretty large area. There are multiple circuit paths snaking back and forth on the heat bed - I think it was four. They go from left to right, so if a trace opens up, that entire 1/4 of the heat bed wouldn't be heated. ie, the back 1/4, etc. Shorting out an area of the board would leave unheated areas, but tougher to do. You have foil under the glass, right? Is the heat bed mounted logo down? That would put the heater traces up against the foil and perhaps allow the foil to rub through the covering on the board and short an area. If the heat bed is mounted logo up, look underneath where your binder clips go. Have any worn through the board covering so that traces are exposed? That could short out an area, but this would still be across the entire width of the heat bed. Now thinking of that, adding a layer of kapton along the heat bed edges for protection might not be a bad idea.

I remounted my extruder fan so that heated air exhausts the front of the printer. That's kind of annoying if you're down close trying to look at the print, but I read elsewhere this did a better job of cooling the hexagon hot end with bottom of the X carriage being so boxed in.

[old man emu]

I remounted my extruder fan so that heated air exhausts the front of the printer. That's kind of annoying if you're down close trying to look at the print, but I read elsewhere this did a better job of cooling the hexagon hot end with bottom of the X carriage being so boxed in.

I did turn my fan around so that it pulls air to the front of the X carriage box. My heat bed is "logo up". I will have another look today to see if there is a short circuit on the heating pad.

Thanks for your replies.

OME

[old man emu]

I turned the fan around. I resliced the object. I put slurry over printer's tape. This time the print lifted in the (+X, -Y) quadrant.

OME

[ciutateivissa]

Ok...

Now we now that your problem is connected to the fan on your hotend. Any chance to redesign the fan mount so that the air don´t hit the heatbed? Maybe a photo would help us to understand...

[old man emu]

I did a print today with a 0.2mm first layer and slow speed for the first layer. The print was almost OK, but lifted after about 10 layers. I slopped some slurry under the lifted part and held it down intl it stuck. After that the print was OK.

I think I will increase the bed temperatures and also give it a good heating period before I hit the print button. My thermistor is under the heat bed, so it is not indicating the glass temperature. I think that skipping the heat up period is a sign of too much enthusiasm to get printing

OME