Extrusion of low density polyethylene tubular film

Melt capillary flow has been employed to characterise some low density polyethylene grades, differing in degree of an incorporated slip agent (Olemide Commercial). The same grades were processed into blown film under different extrusion conditions by varying screw speed, blow-up ratio and haul-off rate. The rheological properties affected by the slip agent were, namely, the melt viscosity and the critical shear-rate, after which fracture (turbulence) of the given extrudate occurred. Ease of draw-down property was found to be lower for the low slip grades and increased significantly with increasing degree of slip. (ASTM) and (B.S.) methods were used to test the mechanical properties of the films produced from the film-blowing process under the same above extrusion variables. The effect of these variables on tensile, tear and impact strengths of each grade were analysed. Orientation of the molecular structure, during its passage through the die and immediately after extrusion, as a result of longitudinal or transverse direction drawing strongly affected these properties. Increasing the extrusion variables, generally, resulted in a balanced orientation in both machine and transverse directions (balanced film) and an optimum blow-up ratio was found for a film of balanced strength properties. An attempt was made to correlate characteristics of flow of the different polyethylenes with the above extrusion variables and, hence, with the mechanical properties of the consequent films.