Need some help with achieving something in blender

[werto165]

I know this is probably not an easy question to answer but I have one regarding blender. 

 

If you don't use blender please just ignore this. 

 

I have a scene with two animated lines that are for an intro and I was wondering if I could do something like an alpha pass that follows a parented object (probably assign it to the tip of the nozzle) with a plane for example to cut the geometry (i.e left side of the plane it's rendered right side of the plane it isn't) and have it such that on the first frame only a small part of the object is visible and then the visibility increases over the frames as more of the geometry is viewable. 

 

I've made an accompanying paint drawing to illustrate what I mean:  

where these lines are the lines you see below in blender.

 

[werto165]

The build modifier almost does what I want it to do however it would be nice to have some control over which faces appear without having just a seed which randomises it. 

[werto165]

Nevermind, I've found this: https://blender.stackexchange.com/questions/5113/how-to-make-one-object-gradually-disappear-as-it-passes-through-another-object 

I think that the boolean modifier will work out just fine.