摘要:

AbstractThe effect of the screw configuration of a closely intermeshing co‐rotating twin‐screw extruder on residence time and mixing efficiency was studied for an uncompatibilized immiscible PA6/PP (80:20) bend. Alternative screw configurations were investigated systematically. The residence time distribution (RTD) was found to be a poor indicator of the total mixing efficiency, whereas the mixing intensity function yielded considerably better information. High shear stress, sufficient residence time, and high fill ratio in the melting section of the screw were the most important factors in achieving good dispersion of the minor phase. The evolution of morphology along the screw axis depended strongly on the screw configuration. The downstream flow characteristics after the screw end determined the final morphology of the bl

ISSN:0032-3888    

DOI:10.1002/pen.760350302

出版商:Society of Plastics Engineers    

年代:1995

数据来源: WILEY

摘要:

AbstractAn investigation was carried out into the effect ofa microwave cure on an epoxy prepolymer with a cycloaliphatic diamine mixture, as compared to a standard thermal cure. The microwave waveguide and process (propagation mode TE01) were adjusted to obtain large homogeneous samples. The extent of reaction,x, was measured during the microwave processing by size exclusion chromatography and differential scanning calorimetry. A good estimate ofxwas found using a modified DiBenedetto equation correlatingxand the glass transition temperatureTg. The homogeneity of the samples was checked during the last steps of cure, showing the efficiency of the microwave processing and waveguide. The influence of the nature of the mold (metallic or dielectric) on the reaction kinetic was also investigated. Samples cured by both thermal and microwave processing were characterized by dynamic and static mechanical properties and then compared with those of fully crosslinked networks, i.e., postcured at a high temperature.

ISSN:0032-3888    

DOI:10.1002/pen.760350303

出版商:Society of Plastics Engineers    

年代:1995

数据来源: WILEY

摘要:

AbstractFlashing in pure polyphenylene sulfide (PPS) and a blend containing PPS and liquid crystalline polymer (LCP) during injection molding was investigated by differential scanning calorimetry and scanning electron microscopy. The shape of the flash was observed by use of a projector. Flashing was detected in pure PPS and 90/10 PPS/LCP blend but was not found in other compositions, including pure LCP. The DSC thermograms of the flash revealed both exothermic and endothermic peaks at around 120° and 285°C. The first peak, known as crystallization temperature on heating, occurred as a result of early crystallization of PPS. The observed double peaks indicated that the degree of crystallinity was lower in the flash than in the molded part. The morphological studies revealed the presence of LCP fibrils in the skin region and droplets in the core region of 90/10 PPS blend. The absence of flash was attributed to the diameters of the fibrils and droplets, which were found to increase with increasing LCP componen

ISSN:0032-3888    

DOI:10.1002/pen.760350304

出版商:Society of Plastics Engineers    

年代:1995

数据来源: WILEY

摘要:

AbstractInjection molds often contain blocks of dissimilar material for improved cooling; they may also contain blocks of movable metal as a means of ejecting large parts from the mold. In this case, the blocks of metal are made of the same material, but the resistance at the interface between them has a marked influence on the cooling in the local area near the interface. In many other cases, inserts may be required because of wear in a particular mold section, or because efficient mold design is needed to produce similar parts. Hence, any mathematical model for analysis of heat transfer in injection molds must be general enough to apply to interfaces with and without gaps (i.e., with and without resistance to the flow of heat at the interface) for similar, as well as dissimilar, materials. A new and accurate model for prediction of heat transfer inheterogeneous (zoned) moldsis presented in this paper. Through the solution of real problems with this model, the effects of differing material properties andinterfacial thermal resistanceare studied and the results are reported. It is observed that inserts have both local and global effects on the injection molding process; the overall ejection time for a part may be shortened, and the surface appearance of a part may be improved by correct placement of inserts.

ISSN:0032-3888    

DOI:10.1002/pen.760350305

出版商:Society of Plastics Engineers    

年代:1995

数据来源: WILEY

摘要:

AbstractThe dynamics of a large number of bubbles separated by distances of the order of their radii in highly viscous fluids with specific application to foams is investigated. The growth of bubbles is due to diffusion of gas from the fluid and the momentum transfer between the fluid and the bubbles. Equations governing the growth of a single bubble in a shell of fluid containing limited dissolved agas are coupled with the transport equations for the fluid under non‐isothermal conditions. The resulting set of equations are solved numerically for a system of bubbles growing along the axial direction in a mold. The results predict a bubble size distribution along the axial direction with large bubbles close to the melt front and smaller bubbles close to the gate, which results in a density distribution in the molded article. Experimental studies on structural foam under nonisothermal conditions are performed. The transient bulk foam density is measured by monitoring the melt front as the foam expands. The predicted values of the foam density are compared with the experimental results and the sources of error are discusse

ISSN:0032-3888    

DOI:10.1002/pen.760350306

出版商:Society of Plastics Engineers    

年代:1995

数据来源: WILEY

摘要:

AbstractIn a plasticating screw extruder, a polymer melt forms in the melting zone of the extruder. Pressurization of the molten polymer takes place in the melting and the metering sections so that the melt can flow through the restricted passage of the die and assume a desired shape. In a melt fed extruder, the throughput is governed by the pressure rise over the entire length of the extruder. The pressure developed in the screw channel may also be employed in rapid filling of molds, such as those in injection molding. When the geometry of the screw, the barrel temperature, and the die are selected, a unique set of operating parameters arise for a particular flow rate or screw speed. In the present study, numerical and analytical methods are used to calculate the transport in the extruder and the pressure drop in the die. An iterative numerical method based on solving the equations of motion and energy in the screw channel and a correction scheme to couple the die with the screw channel is discussed. The numerical algorithm is capable of handling an arbitrary variation of the viscosity of the polymeric fluid with the shear rate and temperature. The results obtained by simulating the fluid flow in the screw channel are compared with available numerical and experimental results in the literature, indicating good agreement. The performance characteristics of the extruder, for chosen thermal boundary conditions and screw geometry, are presented for different die geometries and different fluids. The important considerations that arise in the numerical simulation of the extrusion process are also discussed.

ISSN:0032-3888    

DOI:10.1002/pen.760350307

出版商:Society of Plastics Engineers    

年代:1995

数据来源: WILEY

摘要:

AbstractIt is of great importance in designing extruders to predict the screw characteristics, that is, the throughput in terms of the screw speed and the pressure gradient along the down‐channel direction. The screw characteristics depend on the extruder geometry, the operating conditions, and material properties of non‐Newtonian fluids. This paper suggests a simple approach to determining the screw characteristics for a three‐dimensional flow in a channel with a finite aspect ratio (ratio of a width to a depth,W/H) by introducing a Total Shape Factor (Ft) to correct a two‐dimensional flow analysis for a channel with an infinite aspect ratio. In the present study, the Total Shape Factor (Ft) was defined as a ratio of a net flow rate obtained by the three‐dimensional analysis to that by the two‐dimensional analysis. In the proposed approach, the quantity, ∂Ft/∂(H/W), which turns out to be almost constant, offers important information for understanding the effects of the flights. Threfore, ∂Ft/∂(H/W) is extensively reported in this paper in terms of several dimensionless parameters. This simple approach with such a database will be very useful for extruder designers to predict sc

ISSN:0032-3888    

DOI:10.1002/pen.760350308

出版商:Society of Plastics Engineers    

年代:1995

数据来源: WILEY

摘要:

AbstractThe dielectric relaxation of unsaturated polyester resin during cure was modeled using a modified Williams–Watts decay function. The dielectric response was studied using dielectric measurements coupled with two dynamic mechanical measurement methods. It was found that the variation of the relaxation time during cure is a WLF process usingTg(α) (α‐conversion) as the varied temperature. It was shown that this process fits the Williams‐Watts decay function Φ(t)=exp(‐(t/τ)β) where τ‐relaxation time and β‐empirical parameter are time dependent. It was found that τ obeys a time dependent power law rule and β depends linearly on log(time). Using this modified decay function, it was shown that the experimental dielectric response measured during cure agrees well with the computed response. Relaxation times above and below the dielectric measurement system capability were computed thus, demonstrating the capability of yielding the entire relaxation times spectrum during cure, out of a single limited frequency di

ISSN:0032-3888    

DOI:10.1002/pen.760350309

出版商:Society of Plastics Engineers    

年代:1995

数据来源: WILEY

ISSN:0032-3888    

DOI:10.1002/pen.760350301

出版商:Society of Plastics Engineers    

年代:1995

数据来源: WILEY