Wenyang, I can verify there is a difference. In my quick test
of the K-Factor method with a simple box shape with four flanges
the difference in mass between the bent and flatpattern
configurations is less than 0.05% You'll get more variation than
that in the actual material density, so I wouldn't worry about it.
I would guess the difference is a function of SW using any of the
available sheet metal approximations (Bend Table, K-Factor, Bend
Allowance, or Bend Deduction) to calculate how much to
expand/contract the material. I'm not a sheet metal expert, but I
don't believe any of the methods are exact, ergo the inconsistency
in mass.

the k factor is .4. Now, I check different K factor, and find
the results of mass are totally different. I feel it is the reason
of solidworks, which calculate mass using volume multiplies
density. after bending or unbending, the volume is different basing
the calculating method of solidworks. Is it right.

OK, I see it know. If the K-factor is .5 the mass doesn't
change.

The mis-match happens because SW does not allow for stretching and
thinning of the material at the bends. If the flat pattern sizes
yield the correct actual formed shape, then the flat pattern mass
would be the correct value to use.

wenyang... what K-factor are you using?The mis-match happens because SW does not allow for stretching and thinning of the material at the bends. If the flat pattern sizes yield the correct actual formed shape, then the flat pattern mass would be the correct value to use.