Thanks I am printing a model directly on the build platform with no supports. Yes, I know it’s bad practice, but I have designed the model for this.
Is there any more information available about how PreForm handles the first few layers? The only thing I have found is docs that say to expect some compression of the first few layers.
The printer motion control systems are all open loop and the mechanical systems like the build plate and resin tray attachment points are designed so that everything doesn’t need to be machined to tight tolerances. As a result, there can be a fair amount (relative of course, when you’re talking about a 0.05mm layer, 0.1mm is a 100% error) of variation in the Z height from platform to platform or resin tank to resin tank.
When the printer starts printing, it drives the build plate down a little past the “Z=0” point (the point where the platform is supposed to be in contact with the PDMS). If you watch you can see the resin tank being pushed down slightly as the build plate reaches the bottom of the resin tank. How far the resin tank gets pushed depends on the specific tolerance stackup of all the various parts. So there’s no way to know exactly how many of the initial first layers will be compressed, or how compressed they will be.
If you need accuracy in the Z dimension and you want to print directly on the build plate, the only way to know how much compression you’re getting is to print some stuff and measure it. And you need to keep in mind that when you change out the resin tank to a new one, the calibration is going to change and need to be measured again.
This is why printing on the build plate is a bad idea. Some of the guys doing Dental stuff have had success with this, but only because they don’t care about the absolute Z height of the print. The bottom few layers will be compressed and dimensionally inaccurate but that part of the print doesn’t matter anyway.
If you’re printing something that needs accurate Z dimensions, and especially if you have features near the Z=0 reference of the print that have to be accurate, printing directly on the build plate is going to require a lot of trial-and-error to characterize and compensate for the distortion in the initial, compressed layer region of the print…
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