Sudden Change in Print Quality

I have been printing succcessfully with my Prusa Mendel on a glass cover over the heated print bed, using 1.75 mm PLA (various sources and colors) with a 0.35 mm nozzle on the hot end. I use Slic3R and Printrun, and have generally been getting good results. Suddenly, something changed- the PLAis having trouble sticking to the glass, and I wind up getting what I call "blisters" (several photos here). One day I get a good print, the next day it is garbage (same g-code file used for the good print). It appears that the filament is breaking lose from the glass at random points (varies between print attempts). The glass is cleaned before each print with a mixture of vinegar and water (same procedure used before the problem developed as after the problem developed). It appears that I may be laying down excess polymer, but the filament dimensions have not changed, and the problem persists with different filaments and colors. I suspected humidity might be a culprit, but I do not see the normal indicators of humidity (no en-trained bubbles in the extruded material, no steam puffs or bubbles from the hot end). To further eliminate the humidity problem, I printed with brand new filament, fresh from the bag it shipped in. The problem persists. I have evaluated temperatures, I have checked and double-checked all my settings, I have played around with lying to Slic3r about the filament diameter (to change the amount of filament being extruded), and have played around with layer width. I have tested with higher and lower temperatures. Nothing seems to change the situation. Every once in a while I return to the original *.ini and *.stl files that produced my last good print of this object (I had about 10 good prints of variations on the object before the problem began). I have been able, by really reducing the layer height, to get the material to once again stick to the glass, but this layer height results in filament jams in the extruder due to excess resistance from the print bed.
Has anyone encountered this sort of issue before? Any ideas what the cause might be? Any suggestions on what to do about it?

Is the end of your nozzle clean? Maybe a build up of burnt plastic could increase the drag on the first layer.

Have to tried cleaning the glass with alcohol or a solvent like acetone? I don't know whether vinegar removes everything. Some pigments seem to have a greasiness that reduces adhesion.

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Problem solved- a new piece of glass was the solution. Nophead- I had been using acetone, but, before I started using the BRIM solution in Slic3r, I had a different issue with prints warping and breaking loose. I noted that the acetone seemd to leave a residue on the glass, so I switched to vinegar and water. Worked well for a while, then the blisters started appearing. End of nozzle was cleaned when the problem first showed up. I haven't tried cleaning with alcohol yet, but one thing I observed- if I did not clean the old glass between prints (only removed the obvious trails), the blister problem seemed to be less. I wonder if PLA likes a little residue on the glass for adhesion? Now that I have the new glass installed, I am going to run a few more experiments- and try alcohol as a cleaning agent...

Who would think that one could wear out a piece of glass with too-vigorous cleaning???

How do I mark this thread "solved"?

Edited 1 time(s). Last edit at 07/18/2012 11:29PM by cwarner7_11.

I clean my glass with methylated spirit but I do it when the glass is hot, get no residue and PLA just sticks

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Experimenting in 3D in New Zealand