Spring steel sheet bowing / flexing mid print  

RE: Spring steel sheet bowing / flexing mid print

Update: I forgot to mention if I print off centred, then I dont have this issue, its only in the middle of the bed on multiple prints

How old is the print sheet?

What happens if you use the other side?

Cheerio,

RE: Spring steel sheet bowing / flexing mid print

It is quite old about 6-8 years and same on both sides, I print very sparingly though maybe most would be 20-40 print jobs between 4-12 hours a year 

OK, the two obvious first checks are:

Place the print-sheet on a known flat surface, try both ways up, is it flat?  If not, replace.

Place a straightedge across the heatbed - is it flat?

Cheerio,

RE: Spring steel sheet bowing / flexing mid print

I put it onto a flat surface and its completely flat to my eye and touch, I cant feel any flex, I have a spare steel sheet and put this on to try an alt, as this is flat also, but I noticed once my bed got hot, during the auto bed leveling checks where it taps the bed, I can feel the spring steel sheet lifting off the bed, maybe the magnets in the head bed are deteriating and needs replacing ? Maybe the heat decreases the quality of the magnetism ?

Place a straightedge across the heatbed without the build sheet - is it flat cold?

...and hot?

Cheerio,

RE: Spring steel sheet bowing / flexing mid print

While cold, there was enough gap to slip a piece of paper but only a single sheet in the middle point where I had the ruler flat side across the bed in a diagonal direction. I will heat the bed in the morning to check to see if this changes, I did notice the flex while cold was little greater maybe 1/2 a sheet paper thickness but when I loosen the middle bed screw slightly the blowing lessened.

I will let you know how the head bed flex is when hot using a ruler in the morning.

RE: Spring steel sheet bowing / flexing mid print

Hey Diem, firstly thanks for your help with this. I should’ve acknowledged this so I apologise for that rudeness.

Ok so this is what I know so far

Removable steel printing surface (original from printer and spare Prusa original – same for both)

1. When the steel sheet is on a cold bed, it seems level to the eye using both sides of a metal ruler.
2. When steel sheet is on a bed heating it starts flexing at 80°C during heated bed warming prior to print and remains after.
3. Prior to 80°C the sheet is magnetically adhered perfectly.
4. Taking this hot sheet off the bed and placing it onto a flat bench surface I can see the flex is still applied. I can tip it as it balances on its middle point and also flip it over to de-press the flex spot in the middle of the sheet.

Machine RAW heated bed

1. When cold there is enough room to slip a single piece of paper in the middle point. This amount does (I think, as I have no real way to measure) change ever so slightly when you loosen or tighten the middle bolt, but still only a single sheet can fit in the gap.
2. When hot, this deviation of the sheet does not change – it still remains at a single sheet with the middle bed bolt tightened (finger tight) or loosened.

Conclusion

I feel the heated bed when testing raw had an extremely minor defect but I don’t feel it’s anywhere enough to represent the extent the steel sheet is flexing. Being the sheet is flexing after it comes to temperature and not prior, I assume the quality of the metal sheet has deteriorated? I say this as the symptoms occur on 2 steel sheets about the same age and about the same print hours – one of them possibly only half what I previously stated (20–40 print jobs between 4–12 hours a year). I’m not completely sure the steel sheet is to blame or if the heated bed is getting much hotter than reported and maybe the heated bed temp sensor I installed 4–5 weeks ago might be incorrectly reporting.

Note: the sensor was re-installed (original from Prusa) in the exact same position and following the guides on how to remove and reinstall using the correct adhesive tapes.

Hope this helps clarify the situation in detail, again thank you so much for your help.

RE: Spring steel sheet bowing / flexing mid print

Search through the forums for bed levelling modification.  Well worth the work.  I just used some silicon tubing and the screws that came with the printer.  Cut the tubing a little longer than the spacers that are on the printer.  Helps with z-cal as well.

Another test is to pre-heat the printer and watch the build plate.

Also check that the magnets are working in the central area of the bed.  Excessive heat will destroy magnets.  Not sure if you can get the bed hot enough to do that.

Taking this hot sheet off the bed and placing it onto a flat bench surface I can see the flex is still applied. I can tip it as it balances on its middle point and also flip it over to de-press the flex spot in the middle of the sheet.

This is probably the root of the problem.

I have heard of a sheet being spoiled by an impatient user always bending it the same way to dislodge prints whilst still hot - if you allow them to cool they usually pop off by themselves but you don't seem to be printing enough to cause a set to develop...  It *might* be possible to correct the bend by repeated flexing the other way - but that's just a guess.

However, print sheets are regarded as consumable so I suggest you buy a new one.  

I doubt you've ever got even half way to the Curie temperature of your magnets so they should still be ok.

Cheerio,

RE: Spring steel sheet bowing / flexing mid print

No problem and makes sense, as I do often flex the sheet while hot/warm rather then letting it cool then flex/remove the prints. ill order a new sheet, thanks again for the help.

RE: Spring steel sheet bowing / flexing mid print

Sorry didn't realise there were different options, is the basic (cheapest) Smooth PEI Print Sheet the one that comes with the printer by default / out of the box? Would you know by chance?

7 years ago when I got my Mk3 only the smooth sheet was available; these days I would suggest the satin sheet as the best general one for default use.

Cheerio,

RE:

Hey Diem, 

I got the new sheet and it is doing the same, but I wish I realised this earlier but I now noticed that my bed if divided in to 3 sections running left to right, so I would have a front, middle, back section when facing the machine, only the middle section is getting heat, front and back section of the bed is not heating up, and is cold to the touch when doing the bed levelling.

I replaced the thermostat on the unit about 6 weeks ago, would this be faulting and not reading the temp properly? being its in the middle of the bed and the middle section its hot I think this isnt the issue.

Maybe its the bed as a whole and I need to replace the heat bed (Magnetic heatbed MK52 24V (assembly) https://www.prusa3d.com/product/magnetic-heatbed-mk52-24v-assembly/)?

Are there any tests or checks I can do on the beat heating?

That's rather odd and it implies partial shorting in the heating circuit; Icbw.

The safest way to approach it is to contact Prusa support via chat.  They might ask you to take a reading with a multimeter, if you have one, have it ready.

Are there any tests or checks I can do on the beat heating?

If you have the equipment to do so, let the bed heat a few minutes and take temperature readings, 5 should be enough at corners and centre.

Cheerio,

RE: Spring steel sheet bowing / flexing mid print

Will do, thanks for that, if left for 5+ mins the heat spreads from the centre out but the font and back deffinitly doesnt get hot, I will contact Prusa to find what the warranty period on the printer us im sure its well out of warranty as purchased Jan 2019 also asking their help in testing and whats best to do to replace it, maybe the PCB or the PSU is failing not supplying enough current, maybe.

RE: Spring steel sheet bowing / flexing mid print

Diem is correct. From your description, the power supply is working, it sounds like there is an issue with the bed and it will need to be replaced.  

Quick search found these measurements for Prusa Printers.  MK3 series printers, heat bed resistance should be 2 to 4 ohms.

Prusa Printer Heater Cartridge and Thermistor Resistance Values, Plus 1.75 mm, MK2.5, MK2.5S, MK3, MK3S, MMU1, MMU2, MINI, MK3S+, MINI+

RE: Spring steel sheet bowing / flexing mid print

Awesome thanks.