The film is in color and is probably from the late 1970s or the early 1980s. I could not see the ff. scene in Battle Beyond the Stars or Star Crash.
A woman is strapped on to a vertical plane, where another with tiny needles attached to transparent tubes is moved to her. Blood is slowly siphoned from her body. There is a brief close up showing her nipple as one of the tubes is attached to her breast.
I'm not sure, but I think the aliens (who are humanoids) need blood to stay alive, and plan to invade earth.