In a theoretical analysis of draft resonances observed during extrusion casting of plastic films, it was noted that cooling of the extruded polymer plays a major role in stabilising the casting process and that isothermal melt draw becomes unstable at draft ratios greater than 20. Draft resonance in power-law fluids is investigated theoretically with infinitesimal perturbations, and intermediate stability curves are derived, which depend on the power-law exponent and the draft ratio. Nonlinear stability is analysed for viscoelastic fluids with constant viscosity. Fluid elasticity extends the range of stable operation and a second stability region exists at very high draft ratios. Finite periodic solutions are obtained by solving the continuous momentum equation directly without relying on perturbations.

It is shown that the critical stretch ratio associated with the onset of instability increases with the power-law exponent (force). Different critical values of p exist for oscillatory variations in thickness and width values. At force values less than 1.2 oscillating changes in thickness occur, but the width remains uniform; at p > 1.2 the instability itself passes sequentially through the width of the film like a ripple. The draw resonance instability during film casting was investigated experimentally, and the experiments showed an interesting matching phenomenon between the thickness and width fluctuations. An accurate and unambiguous understanding of the mechanism of the fluctuations is not possible at present, due to the complexity of the two-dimensional stability problem and the fact that the matching phenomenon is again nonlinear between the two directions.



In order to reduce the draft resonance phenomenon during film casting, a draft resonance limiter was placed at the air gap between the extrusion die head and the quench roller. The limiter was used on a cast linear low density polyethylene (LLDPE) film. A method of stabilising the cast film, this is done by controlling the length and position of the air gap with a two-chamber vacuum box near the air gap.

The Key to Problems in the Film Industry

Film casting is the primary method of stretching a polymer melt extruded from a die, which later solidifies into film on a cooling roller. It is important to study the mechanism of this method from an industrial point of view. The subject of research has been focussed on the stability of melt cast films. The instability never increases in conventional cast production, which is mainly used in the film industry. However, casting is not always stable. In some cases it also becomes unstable, resulting in pronounced periodic and very large fluctuations in the thickness of the cast film. This phenomenon is called draft resonance and has been so called since this terminology. It occurs in the regular industrial production of cast films, which must be carried out under stable production conditions.



Draft resonance is an unstable state of melt flow, which is different from the phenomenon of melt rupture, a severe deformation of the extrudate surface ranging from simple surface roughness to the appearance of spiral indentations. The main reason for the occurrence of draft resonance is related to the cooling effect of the extruded film between the die head and the cooling roller. Draft resonance occurs when cooling is inadequate. Since the phenomenon of draft resonance reduces productivity and this is due to the fact that draft resonance makes the film thickness non-uniform, many researchers have analysed the detailed results of draft resonance obtained both theoretically and experimentally for melt pumping in terms of obtaining a stable processing range. Basically, the draft resonance behaviour is essentially the same in both melt extraction and film casting, with a few exceptions in film production, such as thickness/width matching.



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