@Jensa: Yeah, there are a lot more variables in resin printing than FDM printers. Resin has for more variables and the printing process if far more opaque (literally!) than FDM.
If you want to do more research, here’s where I started:
Resins seem to have 5 major components:
- Polymer - Makes the liquid into a solid and is the "backbone" of resin. For low/non-VOC resins, which is pretty much all we want to use, this is usually some type of acrylate, like methyl acrylate. I learned a lot by reading through the storage/handling procedures for methyl acrylate which will give you an idea all the things this material can do and the factors relating to polymerization since the most unsafe aspect of acrylate storage is the risk of uncontrolled polymerization, which is the reaction we're most interested in.
- Photoinitiator - Takes energy from light and initiates a chemical reaction that tips the equilibrium in favor of polymerization
- Inhibitor - Consumes oxygen and tips the eqilibrium away from polymerization (the safe handling sheets cover inhibitors heavily since they are what make acrylics "safe")
- Filler - Polymerization results in a volume change, the filler affects the way this occurs somehow, and there are a lot of complicated effects here. There appears to be more written about how fillers apply to epoxy and polyurethane resins but many similar principles seem to apply to all polymers. For instance one area to look at is nucleating agents, like talc, which are commonly used to "improve" characteristics of epoxy and polyurethane resins, as well as PLA filament (the mystery ingredient to make "high temperature" PLA with higher heat deflection temperatures. It's fillers that seem to be used to affect shrinkage properties.
- Pigment - Lots of discussion in the forums about this, especially from @Vince. Besides affecting overall light transmission, it also affects selective wavelength transmission and may also have filler properties (kind of like how different colors of filament in FDM result in different hardness and material properties).
The printing process is also way more opaque that FDM printing, so you have to observe more deductively since you can’t see what’s going on in the vat under the surface of the resin (both because the resin may be opaque and because when the “action” is going on, it’s just too bright to look at).
The things I’ve learned to observe are:
- Looking at the vat from underneath after a layer has been exposed, especially during separation - you can see shadows of your structures, especially larger surface areas, and see those shadows change as they separate. You can often see if parts get stuck to the vat or if support structures are breaking based on this. I've sometimes been surprised by what I've seen here, like breaking support structures kind of look like flashes of light or shooting stars appearing across my part during separation. Not sure why this is.
- Looking at the vat flex, especially in the shape of flex at the points where the part is attached. In particular, getting a feel for the difference in the way flex looks between what is caused by liquid suction (between build plate and resin early in the print when the plate is agitating the resin) and by the part sticking to the vat. For instance, if I have a small part at the flexible end of the vat, I'll see an S-shaped flex pattern early in the build, where there is first an upward curve caused by the liquid suction, then when the curve reaches the part, the slope of the curve is reduced by the sticking of the part
- Sound - Lots of data here. The biggest thing missing from all those time-lapse Youtube videos. I wish there were more boring multi-hour close-camera videos on YouTube of various different printers just so I could listen to the different lifting sounds at different parts of the print on different types of vats. We may be in a better position to use sound with a single-motor system like the Titan1 compared to printers with motorized tilt (Form1) or sliders (B9Creator) since we don't have additional motors generating noise during separation. You can hear the sound of parts separating from the vat, different than the sound of parts separating from the build plate. You can hear when supports are separating versus larger part surfaces. I can't yet figure out the sound of a mid-part failure and haven't caught any "in the act".
- Metrology/Forensics - I keep calipers by my printer and measure all my successful and failed prints. Unexpectedly large XY dimensions can mean overexposure. The Z height of some failure feature (delamination, warp, weird flaps hanging off mid-print, anything that seems wrong) can give hints as to the cause (does the height coincide with a change in layer settings, a transition between layers of base supports, some other feature)
There are a lot of material properties in resin printing, but the experimentation process has been somewhat similar to when I started with FDM printing, especially when I was trying to figure out bridging, overhangs, and fitting parts (press fit dowl pin-in-hole type calibration). I had to print a lot of calibration parts to understand what I could print at what speed with what materials, temperatures, cooling, size, and finish quality. FDM did have some additional variables since there are more mechanical components, so you need to figure out whether the results you’re seeing are due to material properties (and thus need to be solved through settings) or because of mechanical problems (belt tightening, resonance, loose screws, etc…).
However, I do find that the difficulty in directly observing the chemistry and the points of curing make it overall more challenging to learn SLA printing.