<p>I'm looking for a post processing method for increasing the functional strength of a 3d printed part originally made by FDM. I've tried printing my part with solid infill but the layer separation is still the primary failure point. I'm looking for a way to get something closer to a cast or injection molded part. Obviously less strength but there is a pretty big gap in material properties.</p>

<p>The only method I've thought of that might work is drilling a small hole, or series of holes in my part to inject an epoxy into the part. Haven't tried it.</p>

<p>I'm open to any possible ideas or advice if someone has tried something like this. Not sure if this is necessarily the best place but thought it's a good place to start.</p>

Layer separation will always be the primary failure point of FDM. So your best action would be to design parts in such a way that forces are applied across the Z axis, not along it.
If layer bonding is too weak, this is a problem that should be solved during printing. Possible reasons are:
Nozzle temperature too low
Filament contains moisture from the air
Filament covered with dust
Inappropriate ratio of nozzle diameter/layer height. We usually use 2 (0.4 nozzle for 0.2 layer) while theory suggests it should be more than 3.
Inappropriate cooling of part during printing, drafts.
If all this things are set right, no post-process treatment will improve the part. If not, you could reduce inner tensions by blasting the part with hot air gun, but it is better to solve the problem, not the consequences.