How to increase control over extrudate flow?

I'm having some problems controlling the flow of plastic. This obviously causes some stringing etc which I don't like.

The problem is simple enough; just because the extruder motor stops feeding, that doesn't stop the plastic from flowing from the nozzle. It will keep flowing for a few seconds, slowly decreasing the flow rate.
This will cause the following problems:

1. Stringing obviously. This can be slightly remedied by higher transport speeds, but not entirely.
2. When going from fast work(e.g bulk infill) to slow work(small perimeters) it will flood the small perimeters at first, because the plastic doesn't start to flow slower right away. Right now I have to print the entire print at the lowest speed I'm gonna use to prevent this, the whole print has to be small perimeter speed. That takes a lot of time...

The traditional way to solve these problems is to use the retract function, which pulls back the filament x mm's... The problem is, I don't see that it's really helping. I've tried pulling it back a lot, or a little, fast and slow. It just doesn't seem to really matter. If anything it makes things look worse, because it will freeze in the position when it's retracting, oozing out plastic while it retracts. Maybe the stringing gets slightly better, but strings are easier to remove than ooze blobs... It also doesn't solve the problem of going from a fast region to a small region.

Also, people say that high temperatures can cause oozing. I don't quite understand why though. Is it because the plastic flows more easily at higher temperatures? Anyway, I've tried 190-240 degrees without noticing much of a difference. Maybe I should go even lower?

Anyway, can anyone give me some pointers here? I don't really know what steps to take, and this problem is really reducing the quality of my prints.

I'm using PLA with a 0.4 mm nozzle btw. Also, I have the type of extruder that hangs on the side of the printer, and feeds through a tube. I imagine this could make the situation worse, but it should be improvable anyway...

TL;DR:

How do I increase my control over the plastic flow? I want the plastic to stop flowing when the extruder motor stops, and start when it starts. Retracting isnt helping much.

Edited 2 time(s). Last edit at 10/04/2012 06:56AM by Speedsoda.

That sounds like a bowden style extruder and if so they are known for that. Reason being is that the filament will compress inside the feed tube. So when you use the retract method due to the compression of the filament it might not be retracting enough. This has a double edge sword to it. If the retract is set too high and it does do what you are trying to ask of it there is a chance for a delay in extrusion while the filament compresses again. I am working with this setup right now and that is what I have seen so far. I make no claims to be a pro (more of a n00b) really) but even n00b's have findings and discoveries.

Mike