Continuing the discussion from Uneven base thickness, base doesn't stick to platform, with a nod to several other threads of this sort or at least going in the same direction:
Testfile didn’t print succesfully - Base plate too thin
Adjustable Build Platform
Parts sticking too much on the build platform
Injuring yourself on the build platform because of sticky builds!
I have 4 build platforms, and after lots of printing, a mirror cleaning session and so on, plus the odd scratch or scrape from straining to get a part off the build platform (both on build platform and me), I had finally had enough of the fact that getting the models off one of the platforms is mostly pretty easy (spatula under a tab, slow and careful turning, move on to next tab, model pops off sooner or later), while getting models off the other 3 is from ‘difficult’ to ‘have-to-use-a-hammer-and-chisel near-impossible’. In addition to this frustration, with a new resin tank I suddenly, for the first time, with a reliable platform, had a total BIG ‘rafter’ (model floating in the tank) from a model I had already successfully printed several times in the previous resin tank.
While I’ve read (but admittedly not tried out) the tip of sanding down a build platform between prints (support said this was not necessary), as well as the ‘use a razor blade’ (my test prints below were removed using a scalpel), I can only say that all of my build platforms are approximately the same with regards to scratches… at least none are new, have been used many times and correspondingly have lots of scratches. So I figured, even if the sanding helps, and the razor blade is a successful (but somewhat dangerous) way forward, particularly with the ‘rafter’ problem appearing, there’s got to be more to it than that…
So, I started some measurements/tests and discovered a few things. The first was that all 4 (rev. 2) platforms exhibit, from hinge-to-peel, approximately 0.2mm bow (= model base approx. 0.2-0.25mm thicker in the centre ‘column’); this bow is roughly the same whether along the centre, or the rear or the front of the platform; from front to rear there is no significant bow, either in the centre, or at the hinge or peel end:
This bow is not due to mounting stresses, i.e. not coupled to the 8 x 2mm hex fixing screws attaching the base plate to the plastic top - here is the disassembled base plate with the same visible bow:
It may be due to one-sided milling to obtain a flat print surface…? At least, I have had the same kind of bow phenomonen due to one sided milling.
In addition to this, there is a general tendency, at least with my printer, for all of the platforms to slope upwards, from rear to front; all platforms will, I guess, also have some degree of lateral slope, either minimal (~0.1mm) or more, in one direction or the other… we’re living in the real world - there are a lot of tolerances from resin tank to build platform, and FormLabs have a pretty neat way of compensating for this, it’s just not quite perfect.
These 3 factors lead to wide variance in the nominal thickness of a model base, tested with 9 x 3mm cubes like this - I believe it to be, for this purpose, a better suited test file than the butterflies, since it is more likely to uncover non-adhesion tolerances/limits (i.e. first-layers big enough to cause ‘rafters’) without using lots of resin or taking much time (9 cubes in 30 minutes in white at 100µ):
I made the first test print with my ‘Officer Friendly’ platform - so dubbed, because I usually get the parts off relatively easily… naming instead of numbering thanks to FormLabs… the tanks got numbers 'cos I went there first 
! I had, on some level, realised that tabs near to the edges (read: hinge and peel edges) were more likely to be brittle; I measured 0.94mm at the thinnest point (nominal 2.00, no z-axis correction) (position: rear-hinge model back left corner) to 2.18 at the thickest point (front-centre front edge). It should be noted that the base thicknesses are not even; for the figures below I took a rough average having measured all 4 sides a couple of times (with no claim to +/-0.01mm measurement accuracy 
!) ; in addition, signs of over-compression (excessive bottom layer ‘spread’ coupled with brittleness) are marked using bold type… this is a subjective and not very reliable classification!
Note: I made a resin-saving 3mm cube, with 9 copies in the extreme positions with standard 2mm base and 5mm clearance, 4 points on each corners set by hand - nothing ground-breaking, but in case anyone else wants to compare/use, here it is (PreForm 1.8.2 file):
2017_07_22-Cube_3mm-x9.form (95.4 KB)
This test size also allows comparing x and y dimensions in all 9 positions - granted, the dimension is small, so measuring accuracy is low, but the differences are still marked, not many values measured as yet.
I then ran a second print - didn’t change a thing, the only difference was approx. 45 minutes between start times; the build platform was the same. Result: 3 of the cubes didn’t stick - here’s a comparison of the 2 print jobs:
This time the thinnest point on any base was a meagre 0.79mm (again rear-hinge or position ‘1’ like on a telephone number pad).
I drew a few conclusions from this:
- Due to the compression effect, parts will stick better where the measured base is thinner… at the same time, the thinner bases (since the first layer is exposed 9 or 10 times!) will tend to be more likely over-cured, and will therefore tend to be more brittle.
- Repeatibility of print jobs is limited… kind of a no-brainer, I guess - we are living in the real world :)! - but nevertheless worth saying, I think… the not even 0.2mm difference I measured is really pretty small for all of the parts involved. This non-repeatability can lead to statistical variance between print jobs, even if ‘nothing’ is changed (from one job to another there are always changes, even if it’s only removing and reinserting the build platform, or stirring the resin…).
- In the case of ‘tough’ resin, which sounds like it in general tends to stick not-so-well, these build-platform-to-resin-tank tolerances will probably have a more significant effect, and will most likely mean that a) a small part may stick in one area of a build platform, but not in another, and b) a larger part may well only partially stick, until the ‘right area’ is found.
More test runs follow with other build platform/tank combinations:
More were ready to be measured, plus a few further runs were planned, but the tendency was clear, as I had seen in other people’s posts: small models will tend to not stick at the front, where the build platform is simply too far from the resin tank bottom for the first layer to be properly cured and adhere well (despite 10 laser passes); move that same model to the back, and hey presto, it should be ok… assuming your build platform is slanting in the same direction as mine (otherwise reverse the effect), and the values are not too far out. To test that, run the little 9 x 3mm cube file - you probably won’t even have to go to the trouble of measuring base thickness if you are having problems, because the difference between ~1mm and ~2mm is easily visible… even more visible if the parts don’t even stick !
Ok, so, problem found and recognised, I thought… but what to do? Somehow the tilt has to be counteracted. I took the build platform to pieces - not difficult, others have done it too… 8 x 2mm hex screws and you’re in; then I loosened the 2 large cross screws, so I could add a paper shim before re-assembling/tightening everything (DON’T OVERTIGHTEN, in case you’re wanting to try this too!!). Easy job, less than 10 minutes:
With a corrective factor of Z = -0.8mm I then got this - a reasonable success or at least a significant step forwards:
It’s not perfect, but the biggest difference has shrunk from about >0.9mm to ~0.35mm… and the models are all easy to get off.
One other such modified build platform finished printing a couple of hours ago… I shimmed THAT one with 0.4mm, results are better but not good enough, at least it’s on the way.
The last test for today was printing a proper part, with an 8 x 7cm base, which I have printed about 10 times now, twice on this build platform… and I have NEVER gotten the part off so easily as today: all tabs elastic, tweaked with the spatula in both directions, none even cracked much less broke, and the part was off the platform in less than 20 seconds with nothing left on the aluminium. Result!
Anyone care to comment? Corrections, as always w.r.t my posts, are more than welcome… it’s late on Friday, and maybe I’m neglecting something somewhere…
!
