The protrusions on the top layer of the part can be either open or closed. In fact, this is due to the sagging of the plastic, which does not have time to cool when printed in air without supports. Considering that there can be several such layers and all of them are of poor quality, we get this defect.How to fight:Provide Cooling - Your plastic should cool well on the top layer. According to the recommendations, at this stage the cooling should go to the maximum.Provide Adequate Layer and Wall Thickness - In addition to sufficient cooling, you must have the required number of layers. Again, according to the recommendations, your wall should consist of at least 6 layers of your thickness. Thus, when printing with a layer of 0.1 mm, you want your wall to be at least 0.6 mm. But remember - base layers that are too thin won't work either - they just won't hold themselves and other layers.Try to change the occupancy - some slicers change the occupancy configuration at a certain percentage of the occupancy, that is, the difference may even be between 24% and 25%. In addition, with higher occupancy, the distance that is printed in the air is less, which can also remove this defect.