Little Brass extruder jamming

I am still finalizing my build of a DIYTechShop i3a, and was able to print a couple of calibration objects, and some brakets for auto-level probe. Next I was going to make some new endstop holders, but am now unable to print as the extruder is jamming. For example yesterday I soaked everything in acetone, and flossed out the end with some .47mm guitar string. I then heated it up to 235 for ABS, and was able to extrude about 30mm of filament until it jammed again. The melted filament is about 1/3 of the way up the teflon tube when I pull it apart. So why was I able to print, and now am not?

http://reprap.org/wiki/Little_Brass_Hotend

Thanks,
CP

235 degrees is a bit high maybe, but ABS should technically not be prone to jamming!

Once I had the experience with a bad filament vendor where the filament would constantly jam, form a 'plug' in the PEEK part and jam. This was because of the combination of bad filament and a cheap hotend. So I bought different filament and another hotend and the problems were over.

However, your hotend looks good and I don't think the hotend can be the problem.

The filament is black abs from toy builder labs, which has gotten good reviews. I can try a lower temp.

Did the reviews specifically cover their black filament, or all of their filaments in general? Black filament seems to be the industry's dumping ground for recycled plastic, regrinds, floor sweepings, and anything else that they can throw in as filler that will ultimately jam your hotend. Try a lighter color filament, and see if the jamming problem goes away.....

A few possibilities come to mind:

- Is it possible your hotend is getting tiny plugs of PTFE from the liner stuck in the tip (or if you're using a bowden tube or teflon guide tube, from that)? I've seen a number of cases where little flecks can get left where teflon tubing is cut, and these can end up clogging nozzles. Have also seen cases where cleaning can temporarily move the PTFE flecks out of the way, so things work for a bit, but then they can re-settle and again block flow.
- as mentioned, 235 may be high, especially with a hotend with a PTFE liner that goes down near the nozzle -- especially as most thermistors used nowadays are 3-5% tolerance, the standard Arduino boards used for driving generally have 5% resistors on the analog input… and PTFE starts to degrade around 250. The error range -- especially considering overshoot -- would get you past 250. Is your liner in good shape? That you've got melted plastic 1/3 way up the liner is somewhat concerning. May want to consider a fan on the hotend if you've room, or printing cooler. Do make sure your liner is in good shape though, and replace it if it seems like it may have degraded a bit.
- Make sure your thermistor is making good thermal contact; if not you may be running even hotter than you think (also increasing the likelihood of damage from overheating PTFE)
- In some cases, tightening a hotend too much in assembly can cause the liner to buckle or become mis-aligned, causing blockage… sometimes the blockage won't be much of an issue until you run plastic through & the heated ABS plug expands, pushing upwards on the liner.