Hi guys! Need some help. I have a Wanhao i3 that i had used for numerous times but one day all of a sudden my printer can’t seem to print the rafts properly. It would print the outer part fine, but the moment it starts making linear passes it would seem to have problem extruding or adhering to the printing surface? After that it would just under-extrude and just produce very thin stringy extrusion. I checked the extruding motor and it works just fine. I was printing PLA at 215 degrees and bed at 50 degrees so i do not think i was printing cold. i noticed a slight dip in the middle of the printing bed where the heating element is causing the bed to not be level, could that be the cause? Thanks in advance 
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Leveling sounds like the issue. Have you re-leveled the bed or made sure the two X ends are parallel to the Y? Try watching the outer part as it is printing and adjust the leveling as it is printing until it looks slightly squashed.
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I think some kind of overwrite of g code instruction for infill. Run slicer with g code console on look at any M code instruct after initial warm up. I think if this code gets corrupted for certain materials it has the effect of cooling the extruder too much and literally “chocking” the filament so that it ceases to extrude. Conversly ALL bed level tests are best done by printing large circle band of plastic. Watch while printing you can adjust whilke still printing, easy peasy. 
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Perry_1
4
This sounds like an adhesion issue. Since you say you did not have this problem before, I would assume you do not need to add anything to the mix such as hairspray, etc.
Likely, your build plate is not level, and is not close enough to the build plate. You really need to squish that first layer.
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say a slight dip in the plt do you mean, you need to watch the print through its raft cycle, it wont take too long, the best way of picking up experience of 3d printing is to watch the printing cycle (including the gcode console) if you then notice that filament is not coming out of the nozzle its a heating,speed,retract prob, if it is coming out but not adhering, its a bed level, stick material, etc. get to know your machine by watching what it does, its the only way to diagnose faults, GUESSING can only lead to bad decicions.
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the nozzle is within the paper thickness clearance from the bed at the 4 edges during levelling, just that at the middle of the build plate there is a huge gap between the nozzle and the bed when it starts printing. The issue seemed to be more pronounced at the middle-back of the build plate compared to the front.
Also, the extruder is barely extruding any material for the raft layer and seems to extrude fine only when i manually push the filament into the extruder, but the moment i stop exerting pressure, the filament extrudes very thin stringy material that does not seem to adhere to the initial layer 
hey there! I had that issue where the filament is coming out in stringy consistency. during the initial pre-heating and replacing of filament the extruder was extruding fine! but when i start printing it just comes back to this stringy texture T.T
also make sure your extruder fan working properly, I had a snagged wire connection once and the 2 wires where making point contact. this made the fan operate on full all the time making it extremely hard to reach and maintain extrusion temp caising excavtly what you describe. But major issue is level bed get some glass or borosilate to give level plane.
