My quick fix would be to create a backdrop behind the object and align the camera to exclude any sky or ground, then shutdown any lighting you have - sun/moon shadows etc.

I'm not sure you can export as a png, but as a jpg you could use psp or ps to make the background transparent.

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All the Woes of a World by Jonathan Icknield aka The Bryster

And in my final hours - I would cling rather to the tattooed hand of kindness - than the unblemished hand of hate...