You think about it the wrong way. Do it progressively : 3d print the thin plastic form 10cm high, then pour the concrete, let it dry, print 10 more cm, pour again ... That way you need very few plastic.

That would be a lot of waste plastic.

I wouldn't think so. This may be one of the few cases where recycling plastic is actually practical. Recycling plastic is normally a big problem because it's a very dirty supply stream. Plastic products often have multiple types of plastic laminated together, often include plastic labeling, etc. Even if you make a plastic consumer good out of a reusable thermoplastic, recycling is still impractical. Plus the main contaminants - food remains, react chemically at the temperatures necessary to remelt the plastic.

But this? You would be printing out huge slabs of solid plastic. Imagine big blocks and slabs of plastic 3"-12" thick. They could be made entirely of a single type of plastic. You use a thermoplastic, one that can be melted and reused repeatedly. Make them with no dyes and a chemistry optimized for maximum recyclability.

The printer prints out big plastic forms. Then the forms are injection-filled with concrete. Workers then pry or chip off the plastic forms and collect the debris for recycling. At the recycling plant, the pieces are given a basic cleaning, but the contaminants are almost entirely bits of concrete. Plastic melts at a temperature far far below the melting or decomposition point of any of concrete's components. Concrete dust can simply be filtered out from the stream of molten plastic.

And that is a lot of plastic on one job. But because of that, builders would actually be incentivized to recycle it. You're talking potentially thousands of dollars worth of plastic sitting there.

This wouldn't be a practical replacement for regular rectangular concrete formwork. If you just want straight vertical walls and floors, since you have to tear-down the dorms afterwards, this isn't going to be competitive with regular formwork. But what if you want to live in a curvy house? What if you want a more flowing space with continuously varying arches, curves, and vaults? This kind of almost organic home form is not within the budget of the average person. The forms required are just too complex. For this type of organic modern architecture, I could see 3D printing plastic forms being a practical option.

You're right, it would require a lot of plastic. But existing concrete forms also require a lot of material. We're just able to reuse the forms many times.

That's kinda my point. It's really a materials and structure engineering problem. How do you automate house-building? Maybe you don't need concrete at all, or only in strategic places.