Filament jamming in extruder

Subject as above. I have checked all of the usual suspects - but with no luck. I had done about 40 hours of printing; Yoda-Lite, Screwing Jewlery Boxes etc., without problem before I got a jam and now can't get more than a few layers without it fouling up. The 1.75mm ABS filament is knotting up directly after the hobbed bolt. The temperature in this region is not above 35 degrees C and the bolt is not dragging the filament out of line. Pushing filament through the hot end by hand feels good and results in a good production of 0.35mm hot filament. I have even filmed a run to see if there were any kinks in the filament, but without seeing a problem

I would attach a drawing, but by my CAD package (Turbocad) is sulking at the moment, so I will describe what I have.

The extruder is a standard Wades's design with a bolt hobbed with a 2mm thread to drive 1.75mm ABS filament. The hot end is my own design with a steel tube lined about half way down with 4mm OD 2mm ID PTFE tube. At the cold end is an aluminium heat sink with a small fan. Themperature profile is: Inner tip of extruder 230 degrees, Top of heating area - where the PTFE liner reaches is 160 degrees, Heatsink temperature 37 degrees, Inside PTFE halfway through heatsink is about 50 degrees, top of PTFE tube 40 degrees. Above the PTFE tube is a PTFE block with a tapered mouth accepting the ABS feed - temperature here is 30 to 35 degrees.

Any information or advice would be greatfully received.

I think that I may have found the problem and have described what seems to be happening in case it helps anybody else.

After I had this problem I replaced the hobbed bolt with a somewhat different design and seemed to have no further problems for a while. I have attached photos of the original hobbed bolt and a new one with teeth on the shoulders only . The new hobbed bolt worked fine with one type of ABS, in black, but was not much good with orange ABS: It didn't kink the filament, just got clogged and stopped driving the filament. Returning to the first bolt worked well with orange ABS.

Totay I was making some bits with black ABS when it stopped working - kinked filament as before . Careful examination of the teeth marks in the filament which I withdrew from the extruder showed that there was a mangled patch every approx 22mm. . There was one just above the kink and examination of the bolt showed that a group of teeth had become clogged.

I think that the mechanism of the failure is that the teeth digging into the filament cause the filament to bend towards the idler bearing. This is normally very slight, perhaps about 200mm radius of bend. However, when the teeth become clogged it can cause the filament to bend much more sharply. Simple trials have not duplicated the kinking, but have shown that if there is any other weakness in the filament - such as local impurities or bubbles, thatere may be enough bend for the filament to kink and stop feeding.

Wrong!! I had several more repeats, but this time I think I have run the miscreant to ground.

The hobbed bolt (pretty standard Wades extruder) was sticking about one millimeter below it's final resting position when I lined it up with the filament. Because my extruder is not the most visable I did not notice that when the nuts on the hobbed bolt were tightned it moved further down.

The moral is: make sure that the hex recess on your big gear does not have a ledge which stops the bolt head going fully down.

Mike