Volume 13, Issue 3

Twin blade internal mixers are the main devices used in rubber processing industry to mix natural and synthetic rubbers with carbon black and other compounding ingredients. To facilitate mixing, these internal mixers are always partially filled. Consequently, the flow regime that becomes established inside the mixer is a multiple random free surface flow with moving external boundaries. The stress field associated with this flow regime is usually very uneven. In addition, the flow stream is continuously divided and rejoined by the action of the counter rotating blades. The combination of these complex effects results in the break down, dispersion and distribution of carbon black agglomerates within the rubber matrix. The efficiency of the entire process mainly depends on the geometry of the mixer blades and the operating conditions. Predictive computer modelling offers a very convenient method for the quantitative analysis of mixing process and can be used to design more efficient internal mixers. However, the successful modelling of a complex process such as rubber mixing requires the development and use of sophisticated mathematical algorithms that can take into account its main characteristics. In this respect, a major difficulty is the imposition of the transient boundary conditions in a continuously varying flow domain. In the present paper we describe a robust method which can very effectively resolve this problem. This method is based on the combination of Lagrangian and Eulerian approaches for the modelling of moving boundary flows. We have used this scheme to simulate flow and mixing in the cross-sectional plane of the blades of a tangential rotor internal mixer.

This paper evaluates a method of measuring shear stress, in-situ, in polymer flows. The rupture response of two different dye containing polymeric microcapsules, CM511 and CM567, is examined in both an internal batch mixer and a lab scale single screw extruder (SSE), using ultraviolet/visible spectrophotometry (UV/VIS), colorimetric analysis (CA) and optical microscopy (OM). It is observed that both particles rupture over a wide range of shear stresses and times. In particular, both CM511 and CM567 begin to rupture at ∼ 100 kPa, while complete rupture is only detected for the CM567 at average stress levels greater than or equal to ∼ 230 kPa. Complete rupture for the CM511 is not observed over the average shear stress range investigated. The microcapsules further indicate the change in the melting mechanism in a SSE when the processing conditions are changed.

A method of modeling the two phase flow and fusion process for a blend of low viscosity and high viscosity polymers in a modular intermeshing co-rotating twin screw extruder is presented. The delayed melting/fusing process in a large viscosity ratio system is simulated mathematically. Calculations were made for specific modular screw configurations. The screw configurations included both screw elements and kneading disk blocks. The temperature rise in a suspension of low viscosity melted and unmelted solid pellets was calculated along the screw axis. The suspension-blend transition temperature to a blend system was then determined. Furthermore, we tested the usefulness of this model by comparing the simulation with experimental results.

The majority of LLDPE resins is processed via blown film extrusion techniques for various applications in the industrial and consumer packaging business. These LLDPE resins all have, independent of the molecular structure differences, the same intrinsic limitation in blown film extrusion processing. These limitations are: low output, high sensitivity to surface defects (stripes, sharkskin), high machine power requirement, need for large die gaps, lower bubble stability, and low melt strength. As a consequence, the more versatile—manufacturing and property wise—LLDPE resins are not used to their full potential. The industrial objective is therefore threefold: • develop a new LLDPE resin overcoming the above limitations, • develop a new flexible blown film extrusion line tailored to the new generation of LLDPE resins, • develop an efficient 3-D finite element simulation code to assist in the resin and blown film extrusion line development.

A problem of penetration of a viscous liquid into a porous medium with simultaneous squeezing along the surface of this medium is formulated and discussed. The movement of a liquid is initiated by compressing a viscous drop positioned between the upper plate and the porous medium. The system of equations determining the changing of pressure and evolution of an impregnation front was formulated and discussed in dimensionless variables. Typical solutions were obtained for different sets of characteristic parameters including the case of an anisotropic porous medium. Varying the determining parameters leads to different relationships between impregnation and surface squeezing. It was proven that the formulation of the problem under discussion and solutions obtained in dimensionless form are valid for a “rheokinetic” liquid (viscosity changing due to chemical reactions during flow), if an isothermal situation is considered. However flow of a rheokinetic liquid ceases due to unlimited growth of its viscosity and necessity to increase pressure if we want to maintain the preset velocity of the upper pressing plate.

The morphology of injection molded polyacetal was investigated using spiral-flow test mold and shear effect on crystallization phenomena is discussed. Morphology differed for skin, shear and core layers. The thickness of the shear layer was proportional to flow time.

Crystal orientation states in injection moldings of talc-filled polyolefins have been studied. Talc-filled polypropylene injection moldings show an unusual crystal orientation, in which the plate planes of talc particles are aligned parallel to the molding surface, the c- and a*-axes of polypropylene crystals are bimodally oriented to the flow direction, and the b-axes are oriented to the thickness direction. In injection moldings of talc-filled high density polyethylene and polybutene-1, although talc particles are aligned parallel to the molding surface, polymer crystals do not show any unusual orientation. Only talc-filled polypropylene injection moldings show the unusual crystal orientation.

A homo isotactic polypropylene (PP) and random and block PP copolymers with ethylene were added with 0 to 10 ppm of γ-quinacridone which is a β-crystal nucleator of PP. Flexural and Izod impact test specimens were injection-molded from the PPs at cylinder temperatures of 200 to 320°C and the structure and properties have been studied. 1) Although the β-crystal content of the block PP is nearly the same as that of the homo PP, the β-crystal content of the random PP is largely lower than that of the homo PP. 2) The Izod impact strength is increased by the addition of γ-quinacridone. The increase is high for the homo and block PPs and very low for the random PP. There are cases where the Izod impact strength is increased by more than four folds for the block PP. 3) The flexural modulus and strength are decreased by the addition of γ-quinacridone and the decreases of the random PP are lower than those of the homo and block PPs. From above, it may be said that the modification effect of PP by the addition of γ-quinacridone is prominent for the homo and block PPs in which the β-crystal is ease to form and the effect is weak for the random PP in which the β-crystal is difficult to form.

This paper mainly treats the thermal effects during the thermoforming process while most of the previous analyses consider an isothermal deformation. A non isothermal three dimensional finite element model of the thermoforming process is proposed. It couples the thermal equations in the thickness and mechanical equations on the mean surface of the sheet. The mechanical resolution is done by a finite element method using a membrane approximation. The deformation is driven by a pressure difference through the sheet. The thermal resolution uses a one dimension finite element method in the thickness with convection or conduction at the surface and dissipation of mechanical energy. The polymer cooling is very efficient during the contact with the tools. The coupling is done by the thermal dependent rheology. The respective contributions of friction and thermal effects in the thickness of the part during the process are discussed. The model also considers a possible multilayered material, with specific rheological parameters inside each layer. The rheology of a polystyrene was measured under elongation as a function of temperature, strain and strain-rate and described by a viscoplastic law. The predictions of the model were compared with measurements on an instrumented thermoforming machine and with the local thickness of axisymmetrical parts and with 3-D parts thermoformed with the same polystyrene.

Exact knowledge of anisotropy in thermal and mechanical properties of moulded articles is important since it is directly related to undesired phenomena like warpage. In this study careful measurements of the coefficients of thermal expansion, elastic moduli, Poisson constants and linear compressibilities showed that the anisotropy of these properties was about 10%, for injection moulded polypropylene plates. A simple composite model was proposed which could predict all thermal and mechanical properties mentioned above from data of the amorphous and crystalline phases. The degree of crystallization, skin thickness and average orientation of the fibrillar precursors are the main input parameters. It turned out that although individual values were slightly over or underpredicted, the amount of anisotropy was usually predicted correctly. The only prediction which disagreed with the measurements was that of the coefficient of thermal expansion, which was measured to be larger in flow direction than in width direction. The changes of shrinkage and shrinkage anisotropy along the flowpath were studied experimentally for different processing conditions. The most remarkable observations were that variations in injection velocity only affected width shrinkage, while melt and mould temperatures specifically acted on length shrinkage. Holding pressure, on the other hand, affected both length and width shrinkage with the same amount.

The system of gas penetrations through melt at symmetrical rib channels is found unstable. By varying cross-sectional area of gas channel along the flow length, this research is devoted to observing the effect of the gas penetrations system in tapering symmetrical ribs during gas-assisted injection molding. With decreasing the cross section along the flow length and decelerate the penetration of gas-tip fronts, the degree of uneven gas penetrations can be reduced and stability is improved.