Manufacturing them requires a resin printer and a vacuforming setup, but that's still the easy part. It's a whole system with a dental 3D scanner, software for rearranging your mouth, and attachment points that have to be epoxied onto (and later removed from) your teeth by a dentist.

They used a specialized sort of 3D camera on a stick to get an incredibly accurate model of my mouth, any open source solution would need an equivalent. And you’d also need open source code from somewhere to work out which teeth need to move where and at what stage in the treatment.

1. A treatment plan: simulated movement of teeth at every step, taking into account all forces. That’s specialized software or external lab service.

2. Precision. You put too much pressure at the wrong angle and you will need a surgery to fix the damage, because the tooth root moved in wrong direction.

3. Plastic. You cannot use ordinary 3D printer ink. You need a plastic that can survive the chemical environment in your mouth, maintain the pressure, and you probably want it to look good (no discoloration etc).

4. Finish: Align Tech, Straumann etc do not stop after 3D printing, there are few other steps involved to make sure there’s no sharp edges etc.

5. Maybe you will need attachments (to focus pressure in the right direction on certain teeth) or wires.

Align Tech is Apple of clear aligners, but now competition exists, producing aligners at scale is commercially more efficient, considering all the risks and required qualifications, and of course the best materials for aligners are patented and not sold OTC to everyone.

Disclosure: I worked at Align 10 years ago and later was CTO of European DTC competitor.

The difficult part is not the manufacturing, but knowing how to do it properly so you don’t harm the patient.

But it’s also something that’s not responsible to shortcut. Shifting teeth around too fast can result in permanent root damage and even loss of teeth. There was a whole cottage industry in the US for a while focused on under cutting Invisalign with a reverse-engineered product, but they often moved on accelerated treatment timelines that caused a not-insignificant amount of harm to patients, and cut corners on intake (DIY at home mold kits) that also contributed to problems. Pretty sure all of the companies doing this are basically dead now.

If you keep them on soft food young, the jaw muscles don't get stimulated enough to grow so that the teeth that are coming out sit properly. Misalignment and opportunity for decay ensues.

If you give them decently hard food early on in life you solve most problems.

After that it can still happen but there's less chance.