摘要:

AbstractThe viscoelastic responses of some molten polymers, and particularly of low density polyethylene (LDPE), are known to vary with processing history. Reasons for the variations include the effects of shear history on morphological states of the polymer, or on its molecular weight parameters. A typical low density polyethylene has been used to test the shear‐history dependence concept following a variety of processing steps. The polymer was sheared in single‐screw and twin‐screw extruders, and in a high speed melter / mixer (Gelimat). Samples also were precipitated from very dilute solutions in trichlorobenzene and in p‐xylene. GPC analyses showed that, in general, these procedures did not affect the various moments of molecular weight. An exception was the Gelimat‐mixed sample, for which mild reductions inMnandMwwere noted. In contrast, melt viscosity and elasticity readings, the former from low shear evaluations and the latter from extrudate swelling, were affected by the various procedures. A drop in melt viscosity and in elasticity was observed, being most pronounced for precipitated and twin‐screw extruded versions of the LDPE. Reductions also were observed in the specimen sheared in the Gelimat instrument. Following conditioning at the test extrusion temperature (170°C), viscous and elastic responses tended to revert to those of the unsheared control sample, the exception again being the sample sheared in the Gelimat melter / mixer. Of the various mechanisms proposed in the literature to account for transient property changes such as those reported, temporary changes in the degree of chain entanglement appear the most satisfactory explanation. Irreversible alterations in viscoelastic properties may be associated with changes in molecular weights due to processing a

ISSN:0032-3888    

DOI:10.1002/pen.760330702

出版商:Society of Plastics Engineers    

年代:1993

数据来源: WILEY

摘要:

AbstractResidual density distributions are determined in injection‐molded poly(methyl‐methacrylate) (PMMA) with the aid of a quantitative Schlieren optical technique. The gapwise distributions typically show a maximum beneath the surface. The height of the maximum as well as the level in the core vary with distance from the gate of the mold. The density distributions and the influence of the molding parameters are explained by the pressure course measured in the cavity and the process of vitrification of the sample during molding. The magnitudes of the variations of the gapwise distributions agree with the average density variations measured in a density gradient column. Residual stresses do not contribute significantly to the determined distributions. The density maximum is found closer to the surface than the maximum in birefringence that is induced by the shear flow during filling. The magnitude of the density variations is less than predicted by the pseudocompressibility, as determined in samples vitrified under constant press

ISSN:0032-3888    

DOI:10.1002/pen.760330703

出版商:Society of Plastics Engineers    

年代:1993

数据来源: WILEY

摘要:

AbstractThe univariant element,Q1P0, and the multivariant elements,Q 1+P0andR 2+P0, are compared for the numerical simulation of the flow in extrusion dies. The pressure distribution obtained by using theQ1P0element was found to be afflicted with the checkerboard pressure mode. On the other hand, the multivariant elements,Q 1+P0andR 2+P0, gave accurate and physically reasonable velocity and pressure distributions. The computed values of the pressure drop across extrusion dies matched well with the pressure drop determined experim

ISSN:0032-3888    

DOI:10.1002/pen.760330704

出版商:Society of Plastics Engineers    

年代:1993

数据来源: WILEY

摘要:

AbstractA model for the filling stage of injection molding of viscoelastic thermoplastics in cavities of complex shape is presented. The model considers two‐dimensional melt flow, with converging and diverging flow patterns induced by complex boundary shape and by the presence of an obstacle. The model is non‐isothermal (with the melt loosing heat to the mold walls as it travels into the cavity) and handles a viscoelastic (following the White‐Metzner model) material with properties that vary with temperature, shear rate, and pressure. The numerical method is based on finite differences, with boundary fitted curvilinear coordinates used in the mapping of the flow field (which has an arbitrary shape that evolves with time) into a time invariant rectangle. The numerical results reveal geometry‐induced asymmetries in the flow and thermal fields as well as the effect of various process parameters on the pressure and temperature profiles in the cavity. The model admits variable cavity thickness, thus allowing for a treatment of the cavity thickness as a process parameter in the simu

ISSN:0032-3888    

DOI:10.1002/pen.760330705

出版商:Society of Plastics Engineers    

年代:1993

数据来源: WILEY

摘要:

AbstractAn experimental program has been carried out on a reciprocating screw injection molding machine in order to establish the validity of a proposed mathematical model for the filling stage of injection molding. A cavity of complex shape with an insert and variable thickness was constructed and used in these experiments. Good agreement between predicted (through computer simulation) and observed (through short‐shot experiments and transducer response) results is obtained for free surface shapes and free surface locations with time. The theoretical pressure predictions are in fairly good agreement with experiments, with the maximum deviations occurring towards the end of filling and for longer filling times. This points towards the possibility that wall solidification during filling interacts with the flow processes across the gap of the cavity and makes necessary a more detailed characterization of the heat transfer at the melt/mold interfac

ISSN:0032-3888    

DOI:10.1002/pen.760330706

出版商:Society of Plastics Engineers    

年代:1993

数据来源: WILEY

摘要:

AbstractPolyethylene foam experiments were conducted on a twin screw foam extruder to study the process effects upon foam nucleation. The results indicate that, other than quantity of nucleators, throughput rate and die opening are important process parameters, underscoring the importance of shear in the die on nucleation. It is also found that shear effects increase as nucleator level increases. A modified cavity model shows at least qualitative agreement with experimental results. Dimensionless Capillary number, ratio of shear force to surface tension force, provides evidence that shear force rather than shear rate alone is the principal parameter affecting foam nucleation.

ISSN:0032-3888    

DOI:10.1002/pen.760330707

出版商:Society of Plastics Engineers    

年代:1993

数据来源: WILEY

摘要:

AbstractThe coefficient of dynamic friction is often the controlling factor for solids conveying, pressure generation, and thermal decomposition of a resin in the feed section of a single‐screw plasticating extruder. The coefficients of friction are, however, very poorly understood, and the interpretation of the measurements are complicated by the dissipation of frictional energy at the sliding interface. A new instrument was recently built to help understand dynamic friction, and a numerical technique was developed to estimate the interface temperature. The coefficients of dynamic friction for a low‐density polyethylene resin are presented in this paper as a function of the surface temperature, pressure, and veloc

ISSN:0032-3888    

DOI:10.1002/pen.760330708

出版商:Society of Plastics Engineers    

年代:1993

数据来源: WILEY

摘要:

AbstractThe fracture behavior of oriented poly (methyl methacrylate) (PMMA) has been investigated. The stress intensity factor KIC, characteristic distance a, and angle θ of switching propagation direction decrease with increasing draw ratio in the range of 1.25 and 3.25. The microstructures such as the plastic zone and the features of morphology are in good agreement with the variations of these three variables, i.e., KIC, a, and

ISSN:0032-3888    

DOI:10.1002/pen.760330709

出版商:Society of Plastics Engineers    

年代:1993

数据来源: WILEY

摘要:

AbstractThe elastic deformation of polymer macromolecules in a shear field is used as the basis for quantitative predictions of viscoelastic flow effects in a polymer melt. Non‐Newtonian viscosity, capillary end correction factor, maximum die swell, and die swell profile of a polymer melt are predicted by the model. All these effects can be reduced to generic master curves, which are independent of polymer type. Macromolecular deformation also influences the brittle failure strength of a processed polymer glass. The model gives simple and accurate estimates of practically important processing effects, and uses fitting parameters with the clear physical identity of viscoelastic constants, which follow well established trends with respect to changes in polymer composition or processing condition

ISSN:0032-3888    

DOI:10.1002/pen.760330710

出版商:Society of Plastics Engineers    

年代:1993

数据来源: WILEY

ISSN:0032-3888    

DOI:10.1002/pen.760330701

出版商:Society of Plastics Engineers    

年代:1993

数据来源: WILEY