Apparatus (1) for forming plastic preforms (10) into plastic containers (20). In a closed state of a blow mould (14), a contact area is formed between side parts (14a, 14b), in which the two side parts (14a, 14b) rest against each other, the side parts (14a, 14b) form a mould separation seam (5) in the contact area. In accordance with the invention, at least one section of at least one side part (14a, 14b) has a coating and/or an additional element (60) in a region of the mould separation seam (5), so that a heat transfer between the plastic preform (10) and the inner wall (52, 54) in the contact area can be delayed at least in places at these at least one section.