Found this teardown of the MBI extruders via +Fabbaloo
. I wasn't surprised by much based on what I had already seen/read/reasoned-out, but there were a couple of interesting things. First, a minor surprise was that the encoder wheel is optical rather than magnetic as I had assumed.
Second, the nozzle actually moves up and down during retraction/recovery. This is an idea that I have been working on for a while. The idea is to get the benefits of Z lift without moving the Z axis by pulling the nozzle back slightly in its mounting. There are several types of actuators that could be used for this, but they're not necessary if you can get the nozzle pressure to push/pull the mechanism the way you want it. The normal back-pressure of the nozzle will push the nozzle down toward the platform when you are extruding, so you really just need counteract the weight of the nozzle to lift it (normal retraction will produce a negative pressure, but it may not be enough, and the weight will slowly pull it back down as the nozzle pressure equalizes after the retraction.
The obvious solution to this is to use a spring, but a spring's force increases as it compresses so the spring will want to reach an equilibrium with the nozzle pressure that may not result in the spring being fully-compressed when the nozzle is under pressure, and the spring's force (at least if it's a weak enough one to ensure that the nozzle pressure will completely overcome it even at low speeds) is likely to drop off completely when the nozzle is fully raised. You want it to hold strongly in the raised position and only pull weakly (just enough to lift with no nozzle pressure). You need a bistable mechanism that will hold strongly in the down position when the nozzle is under pressure, and hold strongly in the up position when it is not, but will transition easily between the two with the rapid movement of retraction/recovery.
The solution turns out to be a magnet, which pulls strongly when the assembly moves up and gets close to it, but the magnetic force drops off quickly as the metal moves away so that only a little nozzle pressure must be maintained to keep the assembly separated from the magnet. Apparently, $403 million in stock from Stratasys is enough for MBI to hire someone capable of figuring this out, rather than just staying two years behind everything the Reprap community has been doing.