A mold stack comprises: a split mold insert split into parts, each having a mating face with an inner face region, an offset, and an outer face region, the insert further having a male projecting portion with a shutoff face and an outer surface, at least a part of the inner face region terminating at the shutoff face, the offset terminating at the outer surface; and an adjacent mold stack component having an associated a female receptacle, the insert having a cleaning configuration wherein: the shutoff face acts as a molding surface; the male projecting portion cooperates with the female receptacle to define a melt barrier; the complementary inner face regions are spaced apart to form an extension of the mold cavity terminating, at least in part, at the shutoff face; and the complementary offsets cooperate to prevent melt from passing therebetween and to guide melt towards the melt barrier.