Hope to get some input on printing issue (similar, I think, to that reported here: [
forums.reprap.org] ; without clear solution...)
Big problems getting repeatable success with PLA printing on my Mendel RepRap. Can print ABS + HIPS with excellent results, and great repeatability. With PLA, some runs print fine and others fail mid-print due to filament-extrusion issues. None seem to fail at the beginning of print, so it's not an issue with adhesion to bed, or initial gap between hotend and bed. Also, upon failure, hobbed-bolt chews gouge into filament, so culprit is likely in hotend section, not extruder / motor / gear / bolt section.
In all cases, I'm printing same GCode (sliced from ~1in "wedding cake" calibration STL supplied by "Make:" printer shootout), with same hardware, same US-made PLA filament, and same hotend/bed settings. Hardware is fairly beefy: all metal hotend and direct-drive (non- Bowden) extruder, with all-metal gears. Temp control of hotend, after manual PID loop tuning, is good to accuracy of thermistor (1%). Print-speed settings are all well under standard defaults of slicer (say, ~40mm/sec). Filament spool is mounted on bearings to prevent binding as it is fed into extruder (and not, no problem with this for ABS / HIPS).
Please see attached pics, showing successful print, and various different failed prints. Also, see picture of tip of PLA filament, pulled out of hotend after failed print.
Some initial thoughts (and attempts to minimize such issues):
(1) Extruding PLA filament while hotend is not hot enough, resulting in eventual back-pressure up thermal break and forcing jam... To prevent this, I systematically tried successive prints with higher temperatures until prints started working (some / most of time). Right now, I'm at 225C hotend, and I still get many failed prints (but not nearly as many as when extruding at 195C or 205C or 215C). Given filament supplier's recommendation to extrude their PLA at ~210C, haven't wanted to go higher than 225C, without first getting input here. My all-metal hotend could easily go higher.
(2) Heat-/thermal-break could be getting too hot... However, I have a dedicated fan (with high CFM air-flow) blowing directly over the heat-break's fins, and set it to run 100% of the time during prints, so I'd be a bit surprised if that were the cause.
(3) Filament might have adsorbed too much water due to hygroscopic nature of PLA. My environment is reasonably dry, but I have dessicants on order to counter this (have *not* yet received them, so, for now, it is relatively uncontrolled). However, I have tried other brand-new PLA filaments (not shown in pics) right out of properly sealed shipping bag from filament supplier, and seen similar issues. That would suggest it's not really a moisture issue.
(4) Filament might not have specified diameter (or ovality) as nominally expected (1.75mm). To handle, averaged multiple filament measurements with calipers (got 1.78mm with slightly greater than expected deviation, but still within tolerance); used as input to slicer to ensure proper metering of extruded plastic.
Thoughts / ideas / input much appreciated...