"Cold spot" on my heated bed?

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. :eek:

Old Man Emu

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Originally Posted by old man emu

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?

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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 :D

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.

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.

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Originally Posted by printbus

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.

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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

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

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...

How essential is it to have a fan on the extruder? I'm coming into winter now and room temperature is down to about 15C.

There are plenty of fan shroud stls on thingiverse I could print.

OME

That´s depending on the hotend you´re using and what material you´re printing with - PLA or ABS?

In most cases cheaper hotends needs a cooling anyway. The reason is that the insulator and/or the sleeve can´t withstand temperatures above 200 or 210 degrees C. On top of that there is no thermic insulation between the parts of the hotend and they are usually to short to do so.

There are some hotends out there which needs not be cooled at all, I know at least one which is J-head compatible. Costs about 70,- EUR + shipping, but believe me, that solved a lot of my issues with wraped prints.

Again, it would be good to seee a photo of your printer or at least an link to your extruder (incl. the hotend).

Here is a photo of my printer:
Attachment 1666

I have a Magma hot end and the recommendation is to run a fan with it.

OME

OME, I also have an all metal hotend (hexagon) that has a recommendation to run the fan at all times. Not running the fan accidently caused my printer to fail after 30 mins of printing ABS. The problem is that without the fan, it heats up the aluminum mounting plate that holds the hot end in place, and the ABS extruder parts begin to melt which caused a really bad jam for me.

OME, looking at your picture makes clear what the problem is: This seems to be a huge fan at your extruder, approx. 80mm in diameter? Maybe it would help to have a smaller one... I´ve used a small 30mm fan for my old extruder and it was more than enough. With a smaller fan it will be a lot easier to control the flow of the cool air, with your big one it is simply impossible.

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

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Originally Posted by old man emu

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

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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.

Quote:

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Originally Posted by old man emu

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

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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.

Quote:

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Originally Posted by old man emu

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

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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.

If you have a laser thermometer you can measure a bunch of spots and see if you to have a large cold spot. The thermometers are pretty cheap at some tool stores. We have Harbor Freight here in the US. I'm not sure what you guys have OME.

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 :o

OME

Some random thoughts...

Have you tried repositioning the clips for the glass? The MK1 I got with my i3v kit wasn't very flat, which I assume is typical. It could be that you've got a spot on the heat bed that bows away from the glass as mounted, and repositioning clips might help. Or add a few more?

Another thread mentions your use of kitchen foil between the heat bed and the glass. I'm curious on the reason for that. It'll add more required soak time before printing, won't it? I remember a picture somewhere showing ripples in the foil. Couldn't inconsistent ripples in the foil lead to variations on how heat is being conducted to the glass?

You mentioned the room temperature being 15 degrees C. Is that adding a challenge to ABS printing? I haven't done anything in ABS, but printing in a cool room seems contrary to reading how some people use a printer enclosure to keep the print area warm for ABS.

You raise some good points, Printbus. I've put aluminium cooking foil between my heat bad and glass, and cork sheeting underneath the heated pad. This was in response to thoughts in a thread. I think that the aluminium foil would distribute the heat across the glass.

If I remember correctly, my heater pad was bowed in the middle a little. I did try to take the bow out by putting some 1/4 x 1/4 stick under the centre of the bed, but then I took it away because I figured that the glass was flat. I don't think the stick helped much.

The more I think about it, the more I approach the conclusion that the best way to stop this lifting problem is to:
1. Give the heated bed some time to stabilise its temperature. Perhaps it would be wise to let it heat for about 10 minutes before starting a print.
2. Lay down at least 2 layers at 0.2mm at a speed of about 30mm/second
3. Put a raft under the print, and a skirt around it.

OME

4. Print big parts in PLA.

Incidently... When you get your Auto_Bed_Leveling going... And we add the enhanced G29 code to produce the Topographical map of your bed... You are going to be amazed at how not flat your glass is. There will be parts of your bed that you avoid on certain types of prints.

OME, I think your new approach will work better but if you can get things to stick without the raft your life will be easier. Rafts are hard to remove cleanly.

I don't know if you have 3d modeling capabilities but you might try something I've been doing. I put long 2.5mm thick tabs on corners. It's holding my parts down and keeping them from warping. I use borosilicate glass with 3 coats of Aquanet hairspray. Spray the Aquanet let it dry and repeat twice more for a thick coat. Pet and ABS are sticking very well to it. I posted last night in this thread http://3dprintboard.com/showthread.p...e-to-ABS/page8 which shows the tabs on a part. They are easier to cut away than a raft and don't waste much material.

We have the same printer, if I'm not mistaken, so I think that your new process of using your #1 and #2 above will begin to give you better results. Adding tabs like I have done might be helpful too.

I also agree with 'printbus', I'm dubious about the aluminum foil. I'd remove that. Finally, if you do decide to pick up a laser thermometer, you can measure the glass to see what "dialed in" temperature actually reaches the glass. Mine is about 2 degrees C different. So if I want a bed of 120C I dial in 122C.