A process for for making a cross-linked polyethylene article includes: (A) feeding a blend comprising a cross-linkable silane-group containing polyethylene copolymer and at least one additive to an extruder, (B) feeding a liquid comprising a silanol condensation catalyst to said extruder to form a mixture with said blend, (C) extruding said mixture to form an article and (D) cross-linking said article. The article may be a pipe or cable.