Quoted from Freeplay40:
the actual forming is the easiest part! Vacuum forming is not rocket science...however expect a great deal of trial and error. Early on I had one hell of a time figuring out how best to do the ramps for Genesis. I lost count of how many trials I threw away, but it was close to 100.
I think a lot of people don't realize what goes into making a part, ESPECIALLY low volume parts. When I make a 3d printed mod, the customer has no idea how many hours went into modeling the part, having to tweak features because something doesn't print well, and even if the model is optimized there are still the typical 3d printing issues (nozzle gets clogged, bed isn't calibrated, prints won't stick), and you're tossing away partial or ruined finished prints (if you're printing overnight), and that doesn't cover all the other wear items on the printer (extruder teeth, worn belts, worn PEI sheet on the bed).
Even in manufacturing there is fallout (injection molding machines have bad fill, too much heat burns the plastic, bad ejections cause marks, tools wear out after so many cycles).