Welding Flow of Low Density Polyethylene Melt

Abstract

Weld-lines degrade the mechanical and optical properties of products. This is because the polymer molecules near the weld-line highly orient owing to the elongational flow and the molecular orientation is fixed by solidification of the polymer melts before it returns to a random condition. Birefringence experiments were, therefore, carried out for welding flows of a low density polyethylene (LDPE) past a spider supporting a mandrel. The die used in the present study has a glass window to observe the birefringence pattern. The birefringence in the polymer melts was measured by the photoelastic method. The purpose of the present study is to elucidate the effect of the die temperature, spider shapes and a viscoelastic property of polymer melts on the anisotropy of the molecular orientation in the stress relaxation process downstream of the spider. The birefringence pattern in the welding flow of LDPE was measured at the die temperature T190, 205 °C and the polymer melts temperature 719O °C for flow rates of 0.7, 1.0, 2.2, 2.9, 4.6 cm3ls. The results showed that complete relaxation of birefringence depended on the flow rate. For high die temperature (205 °C), the distance necessary for the relaxation of molecular orientation was short. Moreover, the birefringence pattern in the welding flow was numerically calculated using the single-mode Giesekus model as a constitutive equation, and the results were compared with the experimental data. The numerical predictions agreed with the experimental results.

Weld lines in extrusion: Understanding the role of the flow conditions

“…The distribution of N1 ( Fig. 8) allows to conclude that when the spider leg is closer to the die exit the flow front interface is subjected to high average N1 values, which is known to difficult the molecular entanglement between different flow fronts [14][15][16][17]. This is in accordance with the experimental results obtained (Fig.…”

“…1a, the relative fatigue life was reduced cca 90%. Later, Mitsuhashi et al [15][16][17] presented a remarkable numerical study, extending the work developed by Huang and Prentice to viscoelastic fluids. They showed that by changing only the attack angle of the upstream spider leg region (see Fig.…”

“…Although Huang and Prentice mention "extensional stresses", their conclusions are based on the shear rate and shear stresses (also, they considered only inelastic fluids). Mitsuhashi et al [15][16][17], improved the work of Huang and Prentice [14] by considering viscoelasticity, and showed that a reduction of the normal and shear stresses at the spider leg rear end was possible with slight changes in the shape of the spider leg. The inclusion of normal stresses and normal stress differences in their study is crucial, since the mechanism believed to be behind the formation of weld lines is based on the presence of an extensional flow, as shown in Fig.…”

“…In this work, we want to study the influence of the spider leg location and flow rate on the impact of the formed of weld lines, and therefore, based on the work of Mitsuhashi et al [15][16][17], in our numerical simulations we measured the normal stress difference N1= τ xx -τ yy , from the spider rear end to the extrusion die exit (Fig. 7).…”

“…In the literature we can find a few works regarding the numerical simulation of polymer melt flow around spider legs of different shapes [14][15][16][17], in which the main conclusions obtained were that the type of flow promoted by the shape of the spider leg has a strong influence on the strength of the weld line it originates. Huang and Prentice [14] showed that for a geometry that does not promote high shear rates at the downstream region of the spider leg, the relative fatigue life of the polymer sample extracted from the weld line region was 90% of the fatigue life obtained for the remaining regions.…”