I print directly on the build platform all the time. While it’s not difficult to do, it’s also not as foolproof as printing on a raft with supports. Some extra care and knowledge of how the printer behaves may go a ways to help you achieve success if you decide to try it.
- Less (or no) post-processing
- Very flat, clean surface on bottom of part (especially with a fresh BP)
- Quicker print times (depending on what you’re printing)
- Slightly more usable print volume
- Inaccurate dimensions within the first few mm of your part
- “Bleeding” around edges where your print contacts the base. This can be mitigated to some degree by adjusting Z-Offset.
- Difficult to remove parts if you haven’t built a release-chamfer into your model and/or don’t have a razor scraper (and quicker wear of your build platform if you’re using a razor)
The first few layers of your print are always overcured, in order to establish firm adhesion to the build platform. This means the laser passes over them multiple times, resulting in “bleed” around the outline. Simply put, the more times you pass the laser over a line, the “fatter” that line becomes - for one thing you get more curing where the laser actually travels, and you also get a cumulative secondary effect from the edges of the light which are ordinarily not strong enough to cause any curing on their own in a single pass (search the forums on “laser flare”). The new spatial filter on the Form 3 might help a bit with that.
Further, the build platform is “squished” into the PDMS for the first several layers, which produces compression. I illustrate both these effects (greatly exaggerated for the diagram) in my BabyForm2 post:
By placing your model on supports and elevating it above the build platform, Preform avoids this “unpredictable” zone.
Based on your description it sounds like you’re also asking why Preform orients your part at an angle instead of parallel to the build platform. The reasoning for this is slightly different. For one thing, you want to try and avoid overhangs (the design guide talks about this). That’s probably something you’re familiar with from FDM printing. Angling your part often helps achieve this.
Further, abrupt changes in cross-sectional surface area of your print causes similarly abrupt changes in peel forces, which can have an effect on the quality - especially if there’s inadequate support from beneath at the layers where the transition occurs. My understanding of this (which might be incomplete) is that during the peel the part will flex a little bit until it releases from the PDMS. When the layer takes more force to peel, it may not “snap” back to quite exactly where it was the last time around, which could cause a small shift. Usually it’s too subtle to notice, but I’ve seen this in the form of walls that are supposed to be vertically straight, coming out wavy, or striation lines that warp. I believe one of the things Preform does when it orients your part is seek to minimize cross-sectional changes. Generally going from more cross-sectional area to less isn’t as bad as the other way around (so printing pegs on the top of Lego bricks is fine, but printing the bricks upside down might show you some of these effects).
There are also some natural warping effects caused by shrinking when the layer cures (kind of analogous to the way thermoplastics behave when they cool). I saw this along the front bottom edge of my BabyForm2 lid when I tried to print it inverted, and only angled it in one direction instead of both (click the image for full-size album):
Due to “shrinkage” forces the wall layers “pulled up” near the edges, causing a very visible warping of the layer line striation patterns and ultimately a slightly bowed edge where it was supposed to be flat. I didn’t see it at all when I printed directly on the base, and you shouldn’t see it if you angle the parts along both axes instead of just along one. The final effect will be very dependent on your part geometry (big bulk parts probably won’t exhibit this to the same degree as those with thin, tall vertical features - and you won’t see it near the build platform, where the platform itself helps to keep your part from warping).
The software/firmware mitigate these effects to some degree, but it’s harder to do that when you have several layers stacked that are all leaking extra light (hence more curing) on the same sections that have already been cured. Angling the part tends to produce a cross-section that’s constantly shifting sideways, which helps to minimize the effect.
Finally, angling the part often helps to disguise the layer lines, resulting in a cleaner-looking finish.
Note some of the effects I’ve discussed are pretty subtle - I didn’t encounter them until I had some particularly challenging prints.
EDIT: If you’re curious, here’s a print that just came off my build platform yesterday, and as far as I’m concerned the results are splendid:
If you click the image to view the album, you can see the bleed I’m talking about around the edges in the second photo (it’s likely exacerbated by the fact Draft resin uses a 300um layer height). Raising my BP might help with that, the tradeoff being reduced adhesion (which incidentally might be handy in this project as the parts were a pain to get off the platform. Though there are other tricks I’ve heard folks use, like applying heat or freezing the BP). If you’re printing direct on the base, there are user-created calibration prints out there that can help you dial in the perfect Z-offset.
Hope this helps!