FDM printer first layer infill swell upward My Kossel Mini printer was working well. Recently I made some changes including replacing the nozzle and throat, adding a silicone heater cover. Now it has the problem of first layer adhesion. 




The perimeter sticks well but the infill line swells upward in the middle, while start and end has no problem.

In my experience the adhesion problem only occurs at corner but never in the middle. So I don't know what configuration needs to be changed to fix it.

Delta calibration has been done before the print. PVP glue was used on the bed.
Had tried below and didn't work:


Clean the bed
Use a brand new glue stick
Verify delta configuration and make sure z-plane is flat and parallel to bed 
Adjust z height to minimize the distance between nozzle and bed when z=0
Increase first layer extrude width


Printed again and watched carefully, then found that it was pull up because the printed PLA slice shrank. I realized that the problem was related to my change on the fan duct. Before, the fan blowed directly to the tip of nozzle; after changed it blows around.   

Tried to turn off the fan on first layer, the problem was solved. 

I suspect that the fan duct change made the extruded material not cooling down immediately. It shrank when the air blew on it. I need further test to verify it.