Opacity on Volumetric material?

Hello,
I’m trying to generate some volumetric Smoke in UE.
My problem is, that the material is complitely dense, even if I push up the Opacitay.
This is what I want, a nice soft edge:

What do I have to do, too get such edges, and not hard ones like I have now?

This is my material so far:

Thanks for your help!

Volumetrics is discussed in this forum thread.
Also check out this blog.