Mahmoud Ansari

Entry Flow of Polyethylene Melts in Tapered Dies

Abstract The excess pressure losses due to end effects (mainly entrance) in the capillary flow of several types of polyethylenes were studied both experimentally and numerically under slip and no-slip conditions. These losses were first measured as a function of the contraction angle ranging from 15° to 90°. It was found that the excess pressure loss attains a local minimum at a contraction angle of about 30° for all types of polyethylenes examined. This was found to be independent of the apparent shear rate. This minimum becomes more dominant under slip conditions that were imposed by adding a significant amount of fluoropolymer into the polymer. Numerical simulations using a multimode K-BKZ viscoelastic model have shown that the entrance pressure drops can be predicted fairly well for all cases either under slip or no-slip boundary conditions. The clear experimental minimum at about 30° can only slightly be seen in numerical simulations, and at this point its origin is unknown. Further simulations with a viscous (Cross) model have shown that they severely under-predict the entrance pressure by an order of magnitude for the more elastic melts. Thus, the viscoelastic spectrum together with the extensional viscosity play a significant role in predicting the pressure drop in contraction flows, as no viscous model could. The larger the average relaxation time and the extensional viscosity are, the higher the differences in the predictions between the K-KBZ and Cross models are.

Wall slip of HDPEs: Molecular weight and molecular weight distribution effects

The slip behavior of several high-density polyethylenes (HDPEs) is studied as a function of molecular weight (Mw) and its distribution for broad molecular weight distribution metallocene and Ziegler–Natta catalyst resins. It is found that slip depends strongly on Mw and its distribution. First, the slip velocity increases with decrease of molecular weight, which is expected to decay to zero as the Mw approaches a value with characteristic molecular dimension similar to surface asperities. For HDPEs that exhibit stick–slip transition, the slip velocity has been found to increase with increase of polydispersity. The opposite dependence is shown for HDPEs of wider molecular weight distribution that do not exhibit stick–slip transition. A criterion is also discussed as to the occurrence or not of the stick–slip transition which is found to depend strongly on Mw and its distribution.

Surface fractionation effects on slip of polydisperse polymer melts

The slip behavior of several high-density polyethylenes with broad range of molecular weight (MW) including bimodals is studied as a function of molecular weight (MW) and its distribution. A formulation similar to the double reptation theory is used to predict the slip velocity of the studied polymers as a function of MWD coupled with a model of surface molecular weight fractionation. While surface fractionation has a minor effect on slip of narrow to moderate MWD polymers (particularly unimodal), its role is significant for broad bimodal MWD polymers. The entropy driven migration of short chains toward the die wall has a profound effect and should be considered in order to calculate the effective MWD on the boundary layer and thus the correct magnitude of wall slip.

Extrudate Swell of High Density Polyethylenes in Slit (Flat) Dies

Abstract Extrudate swell of industrial-grade high-molecular mass high-density polyethylenes (HDPEs) in flat/slit dies is studied using both experiments and simulations. The experimental set-up consists of an optical micrometer to measure the extrudate dimensions and a pair of radiation heaters to control the extrudate temperature outside the die attached to the capillary rheometer. The simulation of extrudate swell phenomenon is carried out by using a well-known integral K-BKZ model. The effects of several rheological characteristics, die characteristics, and processing conditions on swell measurements are studied systematically, and the corresponding two-dimensional, steady-state numerical predictions are presented in this paper. This study includes the effects of polymer molecular characteristics, apparent shear rate, die geometrical characteristics (length to die gap (L/H) and width to die gap (W/H)), and distance from the die exit. It is found that the integral K-BKZ model predicts well both the width and thickness extrudate swells. Extrudate swell measurements demonstrate that the thickness swell is predominant in comparison with width swell.

Paste Extrusion and Mechanical Properties of PTFE

Abstract The paste extrusion process of two types of PTFE has been studied in capillary extrusion using dies having different reduction ratio (RR) and die entrance angles. The extrusion pressure shows a weak increase with shear rate over a wide range of flow rates and a more significant increase with reduction ratio. Moreover, the extrusion pressure exhibits a minimum for entrance angle at around 30°. A simple analytical model based on the radial flow hypothesis (previously developed) has been found to represent the extrusion pressure adequately as a function of flow rate (shear rate) and geometrical characteristics of the capillary dies. The extrudates collected at different processing conditions were dried and tested in uniaxial extension to assess their effect on mechanical properties. The tensile modulus, yield stress and ultimate tensile strength of the obtained extrudates were found to be increasing functions of reduction ratio, although the opposite effect was found for the ultimate elongational strain. These mechanical properties are also found to be insensitive to changes in the die entrance angle although the ultimate tensile strength has shown a maximum at the entrance angle of about 60°. The PTFE paste extrudates show a Poissons ratio equal to zero in tensile experiments, thus exhibiting expansion (significant density reduction with stretching). Finally, a simple model was derived for the density change in tensile deformation by taking into the account the Poissons ratio and the strain recovery (recovery of the elastic energy stored upon removal of the tensile stress).

Flow behaviour of rubber in capillary and injection moulding dies

ABSTRACT This study is concerned with the flow behaviour of a rubber compound in capillary and injection moulding dies in the temperature range of 80–120°C. The injection moulding die designs had a tapered angle ranging from 40° up to 150°. The rheological characterisation of the rubber compound in the capillary dies showed that rubber slips at the wall, and this was modelled with an appropriate slip law. The pressure drops in the system were measured for all tapered dies. Numerical simulations were then carried out with a purely viscous (Carreau) model and a multimode viscoelastic (K-BKZ) model. The results showed a good agreement with the experiments for both the capillary and the injection moulding dies, provided that slip is included in the simulations as determined experimentally.

The extrudate swell of HDPE: Rheological effects

The extrudate swell of an industrial grade high molecular weight high-density polyethylene (HDPE) in capillary dies is studied experimentally and numerically using the integral K-BKZ constitutive model. The non-linear viscoelastic flow properties of the polymer resin are studied for a broad range of large step shear strains and high shear rates using the cone partitioned plate (CPP) geometry of the stress/strain controlled rotational rheometer. This allowed the determination of the rheological parameters accurately, in particular the damping function, which is proven to be the most important in simulating transient flows such as extrudate swell. A series of simulations performed using the integral K-BKZ Wagner model with different values of the Wagner exponent n, ranging from n=0.15 to 0.5, demonstrates that the extrudate swell predictions are extremely sensitive to the Wagner damping function exponent. Using the correct n-value resulted in extrudate swell predictions that are in excellent agreement with ...

Wall slip of linear polymers (HDPEs)

The slip behavior of several high-density polyethylenes (HDPEs) is studied as a function of molecular weight (Mw) and its distribution for moderate to broad molecular weight distribution resins. It is found that slip depends strongly on Mw and its distribution. The slip velocity increases with decrease of molecular weight, which is expected to decay to zero as the Mw approaches a value with characteristic molecular dimension similar to surface asperities. For HDPEs that exhibit stick-slip transition, the slip velocity (before the stick-slip) has been found to increase with increase of polydispersity. The strong slip observed above the stick-slip flow regime is found to be independent of the molecular weight characteristics. A criterion is also discussed as to the occurrence or not of the stick-slip transition which is found to depend strongly on Mw and its distribution.