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3D printing gurus,

I've read on several forums that people have had issues with printing with PETG or even PLA on glass and it sticks so well to the glass that chunks of glass rip off due to thermal stresses from the glass/plastic bond thermally contracting at different amounts.

I'm looking for a way to actually increase the adhesion to that point (or beyond) so that I could get a watertight seal on a plastic to glass interface. A lot of the forums mention that these problems occur when the extruder head is too close to the glass build plate and thus "bonds" the plastic to the glass somehow. Is this the case or are there other methods (with just the filament and the glass) that could make this happen?

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    $\begingroup$ I see you are a new user around here, so I hope you don't mind if I bring to your attention that you should upvote any answer that helped you along the way, eventually accepting the one that was closer to solve your problem or - in case none of the answers made it for you - write your own answer and accept it (so that others will be able to benefit from it when searching). :) $\endgroup$
    – mac
    Jan 24 '18 at 23:49
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    $\begingroup$ I am having a similar problem- need to make sure that the print sticks to the glass with a water tight seal. Could you share what worked best for you in the end? $\endgroup$
    – Alex Boika
    Jun 9 '18 at 18:33
  • $\begingroup$ Have you found and fixed the problem? If so, has the answer below led you to the solution? Please vote to accept the answer so this question is not bumped up once in a while and can be removed from the unanswered question list. You may even add your own solution and accept that after 48 hours! If you have not been able to address the problem please update your question. $\endgroup$
    – 0scar
    Sep 7 '18 at 6:25
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There are seven factors that I can think of, that contribute to the adhesion of the first layer onto bare glass:

  • glass cleanliness: dish soap and hot water, dried by patting down with quality kitchen paper is what seems to work best for me. Beware of fingerprints.
  • glass roughness: a common way to increase adhesion to glass is to rough-up its surface by scratching with some fine sandpaper. This works best if you take care of sanding in multiple directions (so that you have grooves crossing each other). I suppose an experiment you could try is printing on frosted glass (the side that has been sandblasted or etched of course).
  • tuning of the bed: your bed should be super-flat and perfectly leveled. Use an indicator gauge if you can.
  • filament chemistry: that is not only to say that certain plastics, like PLA, stick better than others, but also to say that certain brands work better than others (as the additives each manufacturer adds to the plastic influence how the strength of the interface with glass).
  • printer settings: typically a slow speed, no active cooling and a bit of overextrusion work best.
  • area of the interface: the larger the area of the interface, the better. But also: for large uninterrupted interfaces, problems with curling begin to appear.
  • temperature: each filament has it's own sweet spot, but typically some heat make prints stick better. However this is true until the print is in progress, once the print is done and the printing bed cools, prints tend to separate by themselves, so in your case you may want to print with a cold bed (which is totally feasible with PLA).

All that said, this would not be a complete answer without a strong warning: any functional part which integrity depends on this kind of adhesion is bound to catastrophically fail very quickly, if working at all.

For one this "assembly" would be extremely susceptible to change in temperature. While I read too of broken glass due to too good bonding, I only recall this having happened to people using some sort of substance on the glass. Usually the differential in shrinkage is exactly what makes the print come off the glass (a common tip for stuck prints is to put them in a freezer).

Secondly, as soon as some air (or other fluid) will begin seeping between glass and plastic, it will take very little for it to separate the two completely. It's a bit like using a crowbar: as soon as there is a crack you can stick the tip in and use for leverage, it is very easy to pry things open.

Finally, be aware that managing to print watertight shapes with FDM printers is relatively difficult. You can help your luck by extruding at slightly higher temperature than required.

It's difficult to advise on alternatives solutions without knowing what you are trying to achieve, but I would be very surprised if there was not a better alternative. :)

EDIT: the "nozzle too close to the bed" is the wrong way to achieve what you factually achieve with setting the flow rate so as to overextrude during the first layer: you want to increase the pressure of the molten plastic beyond what you normally would do so as to really "squish" it on the glass.

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  • $\begingroup$ Those are very good points. My goal is to 3D print a microfluidic chamber on glass. Basically this, but 3D printed with a glass slide on one side: elveflow.com/wp-content/uploads/2014/12/… I considered 3D printing a mask, then spraying with hair spray, removing the mask, and printing on top of the unmasked portion, but that's just time consuming so I'd rather not. (Note: I know for a fact that this method works) To prevent the fluid from opening the chamber, I thought I'd add some nail polish or glue to the outside edges to compensate. $\endgroup$ Jan 13 '18 at 23:49
  • $\begingroup$ I'm not sure I understood the technology your are trying to emulate, but I would say that at that scale, you actually have some chance of succeeding as - relative to the contact surface - the entire print will be tiny. Again... not sure I understood how you want to build the chamber in its entirety, but from the pic it looks like you could get away with a single layer, which would greatly improve the chances of water tightness. $\endgroup$
    – mac
    Jan 14 '18 at 0:23
  • $\begingroup$ Exactly. Basically I want to print a single or maybe two layers onto glass. To seal the top I use kapton tape or cellulose tape with the back of an eraser and honestly, that part works great. If you do it correctly you can even print on top to seal the kapton tape inbetween layers of a print so that it becomes a monolithic chip. (It's basically just a small channel on a microscope slide that you can flow liquid through for scientific evaluation. Think glucose meter tests or pregnancy tests, that's essentially what this is. $\endgroup$ Jan 14 '18 at 0:43
  • $\begingroup$ Additional comments: A) Layers can be as high as ~70% of the nozzle diameter, you may therefore prefer switching to a larger nozzle rather than printing 2 layers. B) A technique used for finishing is heating a glass to 200°C and squishing it on the top of the print. Maybe it could work as a more durable replacement for kapton tape? C) Probably you know this already, but beware FDM prints are porous and a perfect breeding ground for micro-organisms. Also, you may consider using a steel nozzle to prevent lead from the brass leaking into the plastic, if toxicity is a thing in your application. $\endgroup$
    – mac
    Jan 14 '18 at 0:54
  • $\begingroup$ I'm going to use these as low pressure (syringe pump) disposable flow cells and as for microorganism growth, I actually want that, so yea... haha. The lead leaching is a good point. As for the top of the print, I'll keep that in mind, but for my application I actually need a plastic-like material for the top. A glass top actually won't work because it will reflect the acoustic signal that I plan on getting using this. (It's complicated but it involves lasers and sound and glass reflects a lot of sound, etc.) $\endgroup$ Jan 14 '18 at 1:22

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