Hot End Tips

Sometimes it happen that the flow rate of the plastic into the hot end is not stable, Consequently you can get trouble with your printed parts. 

During a normal printing session, after a few hours we get these cracks coming out on the sides of the pieces, 
Although all the other layers were well placed from time to time some parts of the flow was missing.
Being sure about the filament quality, we checked the temperature of the hot end in order to verify that it was correct.
Once we realized that there was not temperature problem, we decided to change the tip of the hot end and replace it with a spare one.

Luckily On 3D LabteK printer changing the hot end is a matter of minute, not hours

As a result, we get again smooth printing without problems.
So, we thought about impurity inside the hole of the hot end that was periodically clogging the nozzle.
We cleaned the nozzle very carefully, but we did not find any impurity due to filament. 
Investigating inside the nozzle we found particles that were not due to impurity but to the material of the filament itself.
Vitreous particles were formed inside  the nozzle tip.
Being impossible to take a picture inside the nozzle, we have taken a shot of a piece of filament extracted by the nozzle with the same formation. 

The Dark part around the filament it is not melting at the working temperature of the ABS.Unfortunately, ABS is a thermohardening material that once it is getting melted it is becoming harder ( this is why, the  filament producer should not use recycled/dirty ABS  to produce new filaments for 3D Printer).
The solution of the problem is very simple, Once you preheat your nozzle, if you have to cool it down just flow out some plastic. In this way the life of your hot end will be much longer than before.