"Burnt" looking ABS printing

[dorseymp 0]

I haven't been able to find information on this issue. With some prints, I wind up with brown spots that seem to be burnt areas. These spots have occurred not only on the layers near the base plate, but also higher up on prints as far as a couple inches. The brown areas are also not consistently occurring on certain areas of the print bed. I'd figure it has something to do with the temperature but am unsure.

Print temp = 255

Print bed temp = 70

Edited September 11, 2015 by Guest

[ataraxis 51]

have you tried to clean the nozzle (atomic method), maybe thats some old PLA?

[gr5 2,237]

255 is a little bit hot for ABS - that is hot enough to burn bits of it and also hot enough to cause a nasty clog if you print too slow. I suppose if you are at 3 cubic mm/sec it might be okay (to check printing volume speed hover mouse over the print speed - it's a combination of nozzle diameter, print speed, and layer height).

Also the variation on temp sensors can be + or - 10C. So consider printing at 250C instead. Or even 245C. I can't go below 245C or I don't get good layer bonding.

Also 70C bed temp is not hot enough to prevent warping corners off the bed for ABS. I recommend 100C bed temp and enclose the front of the machine if not also the top. Although if your part is sticking to the bed then I suppose you don't need to worry about the temperature thing.

[ataraxis 51]

Also the variation on temp sensors can be + or - 10C.  So consider printing at 250C instead.  Or even 245C.  I can't go below 245C or I don't get good layer bonding.

That's exaggerated, right? Even if the Pt100 is a class B type, it should be around +-1,6°C.

[+/- (0,30 + 0,005 x t)]

But I like the way you answer - it's always comprehensive

[Greet 2]

When printing, some filament sticks to the nozzle and get burnt. When there is enough burnt filament on the nozzle this will get off and leave the brown marks. Try to heat your nozzle and rub off the burnt filament.

[gr5 2,237]

I like greet's theory. Sounds very likely. Alternatively sometimes chunks sit inside the nozzle.

Regarding temp sensors. I sell them and I test every one I sell at 260C. About 1 in 10 are off by 8C or more. In both directions! Which I don't understand at all. To be off in the postivie direction (read higher temp) they have to be higher resistance which is very possible - you caqn have higher resistance where wire connects to pt100 for example inside the crimped cylinder. Or a bad wire that is only 1 strand. 10C error is (very roughly by memory) 1 ohm.

But how the hell are they off in the other direction? Maybe there is a partial short between the 2 leads inside the cylinder? Seems unlikely. My impression was that the resistance was controlled by the amount of platinum and if it works at 20C it should work at 260C so either they are not tested at 20C (very unlikely) or something strange is going on.

The temp sensors I use -- I'm not sure if they are the same ones Ultimaker uses but I've tested some of those and quality seems similar - about 1 in 10 off by more than 7C. Maybe my test rig isn't good enough. But the ones that are out by > 7C I usually let them cool and test them one more time and have always been consistent.

[ataraxis 51]

Regarding temp sensors.  I sell them and I test every one I sell at 260C. About 1 in 10 are off by 8C or more. In both directions!

Wow, okay - that's really much, too much! Can you tell us how you test them? It would be interesting for some of us I guess, if the method is easy enough to do it without too much equipment.

Edited September 14, 2015 by Guest

[gr5 2,237]

I have a chunk of aluminum with 3 holes drilled in it. 1 at 4mm for a heater, 2 at 3mm for temp sensors. I put a random temp sensor in there and hooked them up to my UM2. Then I put the temp sensor to be tested in the empty hole. Run things at 260C and measure resistance of the sensor being tested when the temp is stabilized at 160C according to UM2. Use a PT100 table to convert resistance to temp. I tested 20 temp sensors and 80% agree with each other within 3C. After discarding outliers and taking average, I was a little lucky that the average was 260C meaning the random sensor I had picked was at the average. So I designated that one as the "reference" temp sensor.

Also my chunk of aluminum is wrapped in a rag for insulation.

Maybe I should have taken one at the low end after discarding outliers.