In extrusion coating a thin molten polymer film is coated on some kind of substrate. At high extrusion coating speed, even a minor disturbance on the melt web causes major quality problems, which very rapidly lead to large quantities of waste. Therefore, higher coating speeds require polymers with high and even quality in order to avoid waste due to polymer edge instability and web break.

The rheology-related phenomena that may cause problems in extrusion coating are neck-in (NI) and draw-down (DD). The neck-in is the reduction of the film width and may cause uncoated areas on the substrate. The neck-in is less if the melt elasticity is high. The draw-down is the ability of the melt to be drawn to thin films without breaking. Draw-down is favored by a melt that is more viscous than elastic.