Its function in polymer matrix is to reduce the friction between PVC molecules and lowers the viscosity during processing which improves the flow of PVC compound. Internal lubricants help to reduce the mechanical torque of the extruder during the gelation stage and offer good compatibility with resin & balance the fusion properties. External Lubricants have a  higher molecular weight and less compatibility with resin. It helps to reduce the friction and sticking between PVC & metal surfaces of the machinery. They help to improve surface gloss & offer release properties.