3d printing is probably less effective than manufacturing at scale, but it has much lower setup time and overhead cost. Not to mention, this escalated demand for masks and face shields is going to be very short - 1 or 2 years at most. Setting up a whole factory to fulfill only a year or two of demand is not effective. By comparison, you can use your 3d printer for other stuff after the pandemic is over.
There are factories that specialize in small batches. Just some quick searching seemed to suggest that you could be getting O(1000) parts a day at a cost of a few thousand dollars (for the initial setup) plus a few dollars a part [1]. I'd imagine that the lead time is a few weeks rather than days, but if there's already a stable design wouldn't this be the best bet?
