> The warping one is strange, warping is caused by thermal expansion/contraction and resin doesn't really change temperature during the printing process.In my experience with resin, you can get misshapen parts for a bunch of reasons:
1. Resin will change size a bit during and after curing. Not by a large amount, and of course it depends on the type of resin you use etc - but enough. And of course if your design has some thick bits and some thin bits, one might shrink more than the other.
2. Every time the machine exposes the resin, it sticks to the previous layer and to the projector screen/LCD. The machine then lifts the model, to unstick it from the projector screen. This applies force to the model, and can cause it to bend by a tiny amount - which can gradually build up over several layers.
3. Resin has to be somewhat UV transparent, for each layer to cure all the way through in a short exposure time. Because of that, UV curing later layers can pass through the part and cure resin that shouldn't be cured, on earlier layers. This is particularly the case if the shape has areas resin can pool up and won't run off.
> PLA is brittle as well, about the same as standard resin. Just like with FDM, there are other formulations, but the materials are different, and perhaps there are no good analogs for FDM materials.
For many printers, resin has to be brittle because of the way the printing process works.
You cure a layer, it sticks to the previous layer and the projector screen/LCD, then you raise it to unstick it from the screen.
If the resin is flexible, the unsticking process would cause the layer to bend, giving poor dimensional accuracy. So resin is formulated to be stiff and unyielding.
FDM can print flexible materials like TPU - common 'print upside-down' designs of resin printers simply can't. Although you undoubtedly could engineer a printer that could, such as a resin equivalent of an SLS printer, which doesn't face the unsticking issue.