Sealing surfaces occur when two components meet. E.g. nozzle and cylinder head or cylinder head to cylinder (see illustration). Sealing surfaces must be absolutely flat, even and undamaged so that they seal without gaps at their boundaries. If this is not the case, the molten plastic penetrates the gaps or damages and cracks there. The burnt plastic then builds up layer by layer in these places. Above a certain size, parts of the combustion residue are carried away by the melt flow and black dots/spots make the plastic product unusable.