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UM2+: Bed doesn't move correctly


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Posted · UM2+: Bed doesn't move correctly


I have a new out-of-the-box UM2+, so no HBK upgrade performed. When I print something small, it runs like a charm. Now I tried the first taller prints, and after ca. 2-4 cm of height, the print head touches the print. This happened with pillar-like structures, who were ripped from the bed. Obviously, that ruined the print, and it also was reproducible.

Is there any way to change the movement of the bed? As I understand, the layer height setting will affect both the amount of material used and the movement on the z-axis, therefore not solving the issue.



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    Posted · UM2+: Bed doesn't move correctly

    I don't think it's the bed. It could be the bed - if it's the bed you will see very thick overextruding layers occasionally where the bed didn't move as far down as it should have.

    More likely you are getting "raised edges" where the nozzle hits the pillars. Raised edges are caused because PLA acts like a liquid rubber band - like snot or mucus - as it comes out of the nozzle. In the first few milliseconds it cools a bit and shrinks but is still liquid and it pulls on itself. Like a liquie rubber band. So when printing pillars it is pulling inward. If the pillars are not perfectly vertical the problem is much worse on the overhang edge and the problem is cumulative.

    For pillars that are only 10X taller than they are wide the solution is to get the parts to stick so well the bed that they will never ever. Never! knock over. You should be able to pick the entire machine up by a 1cm diameter pillar if you do things right. Can you supply a photo of these pillars? If they are extra small - say 3mm diameter and 100mm tall then I don't think there is much you can do. Maybe print these as a separate part? Again a pillar would help.

    anyway - how do you get your parts to stick like hell to the glass? Below is my standard answer but the main issue is getting the nozzle closer to the glass (#6 below). The leveling procedure levels the nozzle too far from the glass so just rotate the 3 leveling screws half rotation counter clockwise to raise the be .25mm.

    lifting corners, curling corners, part sticking to glass

    1) Make sure the glass is clean if you haven't cleaned it for a few weeks. You want a very thin coat of PVA glue which is found in hairspray, glue stick, wood glue. If you use glue stick or wood glue you need to dilute it with water - about 5 to 10 parts water to 1 part glue. So for example if you use glue stick, apply only to the outer edge of your model outline then add a tablespoon of water and spread with a tissue such that you thin it so much you can't see it anymore. wood glue is better. hairspray doesn't need to be diluted. When it dries it should be invisible. This glue works well for most plastics.

    2) Heat the bed. This helps the plastic fill in completely (no air pockets) so you have better contact with the glass. For PLA any temp above 40C is safe. I often print at 60C bed.

    3) heat the bed (didn't I already say that?). Keeping the bottom layers above the glass temp of the material makes it so the bottom layers can flex a bit (very very tiny amount) and relieve the tension/stress. For PLA 60C is better than 50C. 70C is even better but then you get other "warping" like issues at the corners where they move inward but if you are desperate it's worth it. For ABS you want 110C (100C is good enough).

    4) rounded corners - having square corners puts all the lifting force on a tiny spot. Rounding the corner spreads the force out more. This is optional if you use brim.

    5) Brim - this is the most important of all. Turn on the brim feature in cura and do 10 passes of brim. This is awesome.

    6) Squish - make sure the bottom layer is squishing onto the glass with no gaps in the brim. The first trace going down should be flat like a pancake, not rounded like string. don't run the leveling procedure if it is off, just turn the 3 screws the same amount while it is printing the skirt or brim. Counter clockwise from below gets the bed closer to the nozzle. Don't panic, take a breath, think about which way to move the glass, think about how the screw works, then twist. This may take 30 seconds but it's worth it to not rush it. You can always restart the print.

    If you do all this you will then ask me "how the hell do I get my part off the glass?". Well first let it cool completely. Or even put it in the freezer. Then use a sharp putty knife under a corner and it should pop off.

    again please provide a photo.

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    Posted · UM2+: Bed doesn't move correctly


    thanks for the exhaustive answer! I guess I'll try to glue it to the plate like never before (some glue was applied, but seems like I need more power).

    My bed is at default setting, I think around 45°, so I'll increase that as well.

    I already added a brim in cura, as you can see in the picture it's twisted up behind the central pillar - that is due to the pillar ripping of from the plate, it was smooth until it was torn away.

    Thanks for the fast answer, I'll try it during the weekend!



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