cross - linkable polymers are prepared by the introduction of the cross - linking agent to the main body of the polymer to be cross - linked in an extruder . this method reduces the &# 34 ; heat history &# 34 ; of the cross - linkable polymer and allows greater pre - extruder processing flexibility . thus , mixing times can be reduced and the shelf life of the pre - extruded polymeric composition can be extended . after extrusion , the product may be cross - linked by heating .