Separate part from bed

Thoughts about Raise3D, 3D printing and making in general.
mark_3d
Posts: 72
Joined: Tue Feb 20, 2018 3:50 pm

Separate part from bed

Postby mark_3d » Wed Mar 14, 2018 11:33 am

I had trouble separating the print from the bed. The part was printed with abs on a built tank surface with dimafix. I broke the glass two times. The bed was cold and i tryed to separate the part with a spatula and a hammer. What was wrong with this?

Jetguy
Posts: 2697
Joined: Tue Mar 22, 2016 1:40 am
Location: In a van, down by the river

Re: Separate part from bed

Postby Jetguy » Wed Mar 14, 2018 12:11 pm

#1 print adhesion to the bed is based largely on first layer height (nozzle actual height, not some setting in the slicer). How much you squish that first layer and literally melt it into the bed surface is how much of a bear it's going to be to remove it later. Yes, opposite is nozzle too high and no stick at all and the part breaks free. It's fine balance here.

#2 Print removal of high shrink materials like ABS is achieved by exploiting this high shrinkage fact. You put down the first layer hot while the build plate was hot. This means the plastic is thermally expanded when you put the layer down and then a shearing stress is created on that layer to bed surface interface when the plastic cools and solidifies. The key to understand here is that the shrink rate is NOT linear to temp. So you extrude at say 230C or higher for ABS and the plastic is probably right at the expected 3% thermal expansion. Then as it cools to solidify, we maintain a higher than ambient bed temp and resulting surrounding air temp around the part (chamber temp), These prevent the plastic layer from making that full drop of exrusion temp to room temp (about a 200+C degree spread there), and so the idea is that this minor drop to where the plastic solidifies but does not cool fully means the non linear shrink rate- you might see only 0.5-1% shrinkage lets say. So the shearing stress in minimal the layer to layer stress is minimal, the part doesn't shear off the bed and and layer cracks do not form. Then after the print is finished, the heaters turn off and the entire part of all layers cools down slowly to room temp and as a unit, shrinks together. Glass on the other hand has extremely LOW rates of expansion and contraction based on temp. As such, depending on the total part size (bottom area in terms of max XY dimensions) develops a shearing stress trying to break this layer. So for most prints, if you get this first layer nozzle height correct, you will then be able to remove the part when the bed and part have cooled because of the sudden increase in shrinkage. 99% of the time, correct manual setup (because the first layer nozzle height is a combination of manual bed leveling and the endstop screw adjustment) results in reasonable prints that break free from the bed with minimal outside force and no damage of the plate.

So let's say you adjusted the nozzle waaaaay too close on first layer. The part is basically welded right into the bed surface.
Things one can try:
Heat cycle the part by heating the bed back up to 110C. Let it get there for at least 20 minutes at temp. Then remove the glass plate and carefully (it's hot) set on a suitable cooling surface (a cookie or baking cooling rack is ideal). Caution, setting hot glass on say a wooden or granite counter could cause shattering do to the rapid cooling caused by direct flat contact. You want the bed basically exposed to air to cool at a slow and relatively even rate.

Another method is put the entire glass plate from room temp into a freezer. Do not take from the bed while hot and then stick into a freezer. The idea being the greater the temperature differential from when the part was printed (100C or greater bed and 230C or greater extrusion) to negative degrees celcius (-18C typical) compared to room temp is even more stress that the part should break free.

A last tip to try is that alcohol dripped into the seam of the part to bed interface may precipitate inward and this action can separate the part from the bed.

And, these might be tried in series or parallel attempts.

Jetguy
Posts: 2697
Joined: Tue Mar 22, 2016 1:40 am
Location: In a van, down by the river

Re: Separate part from bed

Postby Jetguy » Wed Mar 14, 2018 12:56 pm

Also, maybe not use http://www.dimafix.com/ on top of the Buildtak and then wonder why the part sticks too well?


Return to “General”

Who is online

Users browsing this forum: No registered users and 3 guests