I have a customer who uses negative material mass in Inventor to calculate buoyancy for submerged constructions. After upgrading to 2013, we have a problem: Inventor 2013 does not accept negative material mass
The current Inventor 2013 behavior is correct. Inventor uses Protein Library from Inventor 2013 and it doesn’t accept a negative density. That’s why negative density doesn’t work.
As for workaround, you can "Disable" protein library temporarily. You can do it in the following way:
Rename the folder name of “C:\Users\Public\Documents\Autodesk\Inventor 2013\Design Data\Materials”
Rename the folder name of “C:\Program Files (x86)\Common Files\Autodesk Shared\Materials\2013”
If you need to have a “negative mass”, you can also override the mass with negative value for your calculation in this case. This is possble both via UI (iproperties dialog box) and API(MassProperties.Mass).