Help! I just purchased the MPI classic Tenor recorder in Matte black from Groth music (http://www.grothmusic.com/p-24419-mpi-classic-tenor-recorder-outfit-baroque.aspx?utm_source=googlepepla&utm_medium=adwords&id=51320962143&gclid=Cj0KCQjw-uzVBRDkARIsALkZAdmYpDMmK2l2M7WTVxYKnRD56c8S99vx-VbQwtcvZ3wbOoHROn_-AXEaAmODEALw_wcB). I was thrilled to find a plastic tenor with a double key for such a low price! When I opened it and started assembling, I noticed the joints weren't fitting together smoothly, so I decided to use some of the grease that came in the packaging. It seemed to work, but the next day when I tried to take it apart again, I couldn't get the head joint off from the middle barrel! The foot joint came off fine, but no matter how hard I pull/twist, I can't get the head joint unstuck. I'm worried about damaging my instrument. I wonder if the type of grease they provided degraded the plastic in some way? Any advice on how to unstick it and prevent this in the future would be appreciated!
As Tim commented, warm water may loosen the grease, but for stuck woodwind joints we usually use a refrigerator or freezer. Chilling the plastic and/or grease can both shrink the joint and make the grease less adhesive.
Recorders that don't have cork or thread wrapping in the joints shouldn't be greased to try to make tight joints fit. Greasing the plastic should only be used to make joints that already fit well slide together smoother.
If a plastic joint is too tight, there may be a mechanical problem with the joint. Occasionally there is some flashing residue along the casting lines (a raised line of plastic where it seeped between the mold edges). Sometimes super fine sandpaper or steel wool can smooth it out enough to fit better.
Sometimes instrument manufacturers include grease when it isn't appropriate for the instrument. Some of our rental flute stock came with cork grease and I had a number of them come in for head joint un-sticking.
You shouldn't need to start sanding the joints down or struggling to assemble / disassemble it.
If you've just purchased it I'd not try to fix shoddy manufacturing but send it back as not fit for purpose.
Send it back, get your money back, and spend not a great deal more on buying something fit for purpose like an Aulos or Yamaha.
I suspect you've just found out why the MPI classic is so cheap!
I'm afraid this is to some extent a promblem inherent in any plastic recorder. Since plastic against plastic is pretty hard, unlike the cork or thread joints on wooden instrument, the joint has to fit very precisely to be comfortably between not falling apart and not locking up, which means that there's only a certain temperature range where it works. And if the head shrinks a tiny bit more than the body, because of whatever manufacturing reason, then you have your situation.
I agree with Steve M: send it back. Bad design.