Print head jam

My printhead jams up like clockwork after about 45 minutes.

What it seems to be doing is in the gap around the filament in the hot end fills up around the filament. As the liquid filament gets to the top it solidifies and locks up the print head.

Can anyone shed some light on the theory of adjustments to get rid of this problem.

It does not seem to happen as badly with ABS as ot does with PLA.

Some of the things I've tried:

More heat / less heat. From 180-230 C
More pressure on fillament / less pressure.
Different ways of applying pressure. The original set screw on the qubd extruder is no good. I replaced it with the two different variations of essentially Barts Mod, one uses a small bearing the other uses a 608 bearing ( this one is had the advantage of letting me see the bearing and I marked it with lines to see when it stopped turning)
The hight of the tube above the cooling block closer to the hobbed gear seems better.
Ive switched from a resistor to a heater cartrige to see if I could stabilise the heat block temp more. It can fluctuate by 10'C
I've run a 3mm tap over the hobbed gear to improve the grip (did this early on and it made the biggest difference to print quality of anything I've tried
Current from driver to stepper up and down (Down resulted in 24 hours of agony a few days ago I posted in another post about that)

Firmly in the category of grasping at straws:
I have not been able to locate a different hobbed gear locally. (ZA)
Ive tried print a geared extruder but with the print jobs failing its been had to complete sucsesfully.

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Schrödinger's Print: when you go to bed leaving your printer printing, when you wake up you may have the perfect print or your printer may have blown up. Both realities are true till you get up and look.

About my Printer:
Built a Repstap inspired printer from a clean sheet.
It has a theoretical build volume of 310x450x350 (the biggest thing I've managed to print half decent is about 30x50x40)
Electronics are Arduino Mega with Ramps1.4 Pololu drivers
The printer is constucted form 25mm Aluminum tube with connectit connectors
Most parts where improvised and modified as needed.
Linear bearing are based on what I found on google and is 2 angle peices with 608z bearings
I have been printing parts mainly to upgrade the printer things like motor mounts etc.
No heated bed, but my

Sifting through posts Ive seen one thing I've not checked is the actual temp of my hotend. I've trusted that I have the thermistor running correctly but how can I be sure it is? I'm fairly fortunate in that I have a laser thermometer, but readings from there are very irratic so im not convinced im getting the correct temp reading from there and it does seem to be within 10% of where it says it should be.

I would suggest getting a thermocouple. I bought a really cheap one since checking my hot end temperature was my only use for it. It reads within a few degrees of what my hot end is set at. The below link is the one I purchased.

[www.ebay.com]

Edited 1 time(s). Last edit at 08/15/2013 01:20PM by new to3Dbob.

Just a suggestion if you have not looked here yet- check out some common causes of that issue on that at PTPG [www.reprap.org]
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You probably need to put a fan on the top part of the print head to keep heat from creaping up. This varies with the type of print head design.