EQUIPMENT AND PROCESSING UPDATE FOR THE BLOWN-FILM EXTRUSION OF LINEAR LOW-DENSITY POLYETHYLENE

LLDPE molecule has no long-chain branching and only a few short side chains, primarily the result of copolymerizing polyethylene with certain alpha -olefins. These short side chains alter the processing characteristics chiefly by changing the molecular-weight distribution. This provides a film with superior mechanical properties, no entanglement, and good draw-down ability, but it reduces the ease of extrusion and bubble stability. There are arguments for both narrow and wide die gaps for processing LLDPE. Narrow gaps generally result in better machine-direction (MD) tear resistance, downgauging potential, and orientation balance in the film than wide gaps. However, the benefits are obtained by adding processing aids to the resin to prevent melt fracture. Wide die gaps offer reduced costs while providing higher extrusion rates, lower pressures, and better optics. Die-lip heating zones are sometimes added to provide additional heat to delay the onset of melt fracture. This eliminates the processing aids.