ausf is right on the money.
I read about the glue/knife tip method some years ago somewhere, and even though I didnt do any armour models, I gave it a go for giggles sake, and the result is quite impressive.
Just remember to break the tip off of your knife, use and old blade and snap off the very tip of it.
The place some cement on the bit you need welded, and after it has softened the plastic, gentle plunge the knife blade into the soft plastic.
Leave it for a couple of hours, and if the effect is not strong enough, go over it again following the marks you made before.
Practice on a scrappy styrene piece first, but it really is an easy method.
Sean