Volume 20, Issue 4

On the basis of the method of representative values and the flow pattern, that is, the position of the isobars parallels to the die orifice (PIPDO), the design of dies for the sheet and annular parisons is improved in the present paper. Since this pattern includes the effects of the sloped manifold and the die design for the circular manifold are independent of flow rate and the polymer viscosity, the distribution system of a die gives better uniformity of flow. In the previous study [1] a defect is found, that is, the calculated position of the path of the island area lags behind actual position. This defect is eliminated, and the modified equation is obtained. Another flow pattern, the pressure loss on all flow path is equal (PLFPE), is introduced. The equivalency of the above two flow patterns is proved theoretically. Although the flow uniformity isn’t perfect in the simplified analytical design of dies proposed in the previous paper, this simplified design is still of advantages. Therefore, its design criterion is put forward in this paper.

A new process for elaborating continuously organic-inorganic hybrid materials by reactive extrusion is presented. This synthesis consists in formulating a copolymer of ethylene and vinyl acetate (EVA)/tetrapropoxysilane (TPOS or Si(OC 3 H 7 ) 4 )/catalyst blend by a corotating twin screw extruder followed by two successive reaction steps: on line microwave crosslinking reaction and hydrolysis-condensation reactions leading to a silica network cografted onto the organic matrix. Extrusion feed rate (Q), temperature, microwave oven power (P), and microwave curing time (t mw ) were studied. Crosslinking density was evaluated before and after hydrolysis-condensation reactions and compared with the previous results obtained in static conditions. These materials tailored by an on line process present a permanent elasticity at high temperature (up to 200 °C) and a certain optical transparency. This new approach offers the possibility to synthesize massive samples based on organic-inorganic hybrid material.

Water assisted injection molding technology has received extensive attention in recent years, due to its light weight, relatively lower resin cost per part, faster cycle time, and its flexibility in the design and manufacture of plastic parts. However, there are still some unsolved problems that confound the overall success of this technology. Unstable water penetration in molded symmetric channels is one of them. This report was to study the water penetration behavior in the water assisted injection molded symmetric cavity. Experiments were carried out on an 80-ton injection-molding machine equipped with a water injection unit. A plate cavity with symmetric water channels was used. The materials used were amorphous polystyrene and semi-crystalline polypropylene. Various processing variables were studied in terms of their influence on the water penetration behavior inside the symmetric parts: melt temperature, mold temperature, melt fill speed and fill pressure, short-shot size, water temperature and water pressure, and water injection delay time and water hold time. The effect of channel layout was also investigated. Tapered channels were found to mold parts with better penetration stability than flat channels. In addition, the penetration behaviors of water and gas in molded symmetric parts were also compared. The experimental results suggested that water penetration stability is more stable than that of gas in molded symmetric parts.

Polymeric thermoplastic materials can be subdivided in amorphous and semi-crystalline ones. In contrast to semi-crystalline polymers amorphous thermoplastic materials have no defined phase transition and hence no melt enthalpy. The glass transition region describes the change of the material behaviour from the state of a rubber like to an under-cooled fluid (glassy state). Setting the melt enthalpy in the Stefan-Neumann problem zero gives no valid results for the solid bed profile. Thus this condition is unsuitable for describing the plasticating process of amorphous polymers. In this paper a new model for the plastication of amorphous and semi-crystalline polymers in single screw extruders is presented. Therefore a new calculation method for the determination of the moving velocity between the glassy solid bed and the softened film region on the barrel surface is developed. The whole enthalpy to heat up the polymer on a critical flow temperature in the screw channel is considered.

The simultaneous biaxial stretching behavior of three Ziegler-Natta-based isotactic polypropylene resins of varying isotacticity was studied using a laboratory film stretcher that simulates closely stretching conditions encountered in the commercial process. The effect of isotacticity on the final morphology and end film properties was associated with the effect of isotacticity on the thermal properties of the resin. The results showed that the degree of undermelting, △T um = T m –T, determines the phase composition of the polymer before stretching and the morphology and orientation of the stretched films. The size and orientation of the crystallites in the stretching plane is mostly determined by the degree of undermelting. The elongation at break was correlated with the residual crystallinity of the film before stretching, and the elastic modulus was correlated with the final degree of crystallinity and the density of tie chains.

A software package for optimization of polymer extrusion dies is developed. Die geometry is optimized such that a uniform velocity distribution is obtained at the die exit without excessively increasing the pressure drop in the die. The software uses the BFGS optimization algorithm, and employs the adjoint method to obtain the design sensitivities. The optimization software is successfully used to optimize the geometry of a flat die for polymer sheet extrusion such that a uniform velocity distribution is obtained at the die exit, without increasing the pressure drop in the die.

The experimental study of mixing polymer (polypropylene, PP) with mineral filler (Talc) by both simultaneous and separate feed system in a twin screw extruder was described. In this study, melt flow rate (MFR) of PP, talc particle size and talc mixing ratio to PP were selected as parameters of interest. It was found that these parameters had influences on the maximum throughput rate of mixture. In the cases of constant talc particle size and mixing ratio while MFR of PP was varied from 1.8 to 33, it was observed that throughput rates of all mixtures with different MFR values increased as a function of screw speeds. Throughput rates of the mixture of lower MFR PP (MFR = 1.8) were slightly lower than those of the higher MFR mixture (MFR = 10, 33) for both simultaneous and separate feed systems. With regard to effects of talc particle size, compounds having fixed talc mixing ratio showed lower throughput rates when talc particle size was smaller. In the case where talc mixing ratio was varied, compounds with fixed MFR of PP and talc particle size showed lower throughput rates when talc mixing ratio was larger. The mixing state in the mixing zone was also observed. Based on the overall findings, the mechanisms of filler mixing were clearly understood. The observed mixing patterns were similar for both simultaneous and separate feed systems. It was found that the elimination of the air accompanying the filler when the filled polymer mixture passed down the extruder is important for mixing. Through the experimental observations, the schematic diagram, the model and the flow chart of the proper filler mixing process were proposed.

A full three-dimensional finite element analysis of the nonisothermal flow of generalized non-Newtonian fluids in counter-rotating tangential twin screw extruder is presented. Previous studies of the simulation of processing in tangential twin screw extruders have focused solely on the twin screw extruder, whereas here the coupled flow and heat transfer occurring in the integrated geometry of the extruder, connected to a die are considered. The FEM based numerical simulation of the coupled momentum-mass-energy conservation equations allowed the determination of the effects of some of the important system parameters, including the power law index and the staggering angle of the screws, on the pumping and pressurization capability of the extruder and the associated degree of fill in the extruder.

A numerical simulation of the heat transfer during laser transmission welding is presented. A finite difference approach was used to solve the one-dimensional unsteady-state heat conduction problem and to investigate the effect of welding conditions on the time-dependent temperature profiles for PA 6. For the needs of the simulation, the process was divided into heating and heat redistribution periods. The absorption coefficient of the laser-transparent part was measured experimentally and that of the laser-absorbing part was fitted using experimental data. The predicted temperature profiles were combined with experimental meltdown data to estimate the heat-affected zone thickness in the welded specimens. Good agreement was found between the estimated and measured heat-affected zone thickness values.

For the mathematical modeling of single screw extruders crucial simplifications are frequently made. In order to estimate the quality of these models it is necessary to evaluate the differences resulting from these simplifying presumptions. This study refers to the influence of two frequently used simplifications, the power-law model and the assumption of an isothermal flow. The differences due to the application of the power-law show an evident dependency on the descriptive material parameters. Depending on the Nahme-number the isothermal simplification proves partly clear differences from non-isothermal flows. These differences however are considerably reduced in the range of high Péclet-numbers.

Atactic polystyrene (PS) was used to study the effect of flow field (shear and/or elongational) on the intercalation of polymer/clay nanocomposites (PNC). Three grades of (PS), with different molecular weights, were compounded with an ammonium-modified montmorillonite (Cloisite 10A) in a twin-screw extruder (TSE). The compounds were subsequently fed to a single screw extruder, fitted with one of three specially designed torpedo-attachments. The attachments were designed to provide combinations of different levels of shear and elongational deformations. The resins, TSE compounds, and final PNC’s were characterized for the degree of intercalation, degradation, rheological behavior, and mechanical properties. The data showed that the thermal decomposition of the quaternary ammonium intercalant caused severe damage to both PNC components: a collapse of the organoclay interlayer spacing, and the thermo-oxidative degradation of PS. In spite of these detrimental effects, the attachment employing combined elongational and shear flow resulted in generally larger gallery spacing and more improvement of the mechanical properties than the other two attachments.

An experimental investigation was performed to characterize the flow field produced by a dual-lip air ring used in the blown-film manufacturing process for a solid model with a blow-up ratio of 3.5 similar to the shape of a typical LLDPE bubble. Distributions of the static and fluctuating pressure on the model and the flow field above the forming cone were measured for a range of settings on the dual-lip air ring. It was found that the distribution of the pressure on the bubble below the forming cone had many features similar to the measurements for a bubble with a blow-up ratio of 2.5 [1]. In the region above the forming cone, the upper jet appeared to merge with the lower jet rather than entraining the lower jet as was observed for a bubble with a blow-up ratio of 2.5 [1]. This resulted in a smaller local maximum in the normalized fluctuating pressure in the region above the forming cone and would likely result in less heat transfer in this region. It was also found that the initial angle of the upper jet exiting the air ring changed when the height of the bubble cone was adjusted for the bubble with the blow-up ratio of 3.5. This affected the static pressure in the region below the location where the upper jet attaches to the bubble and the pressure fluctuations in the region where the upper jet attaches to the surface. Finally, unlike the bubble with the blow-up-ratio of 2.5 [1], the presence of the bubble cone caused a region of negative gauge pressure in the region above the forming cone that should act to stabilize the bubble against the bubble cone. The pressure in this region and the region below the forming cone both decreased when the porosity of the bubble cone was reduced.

The micro-injection molding is used to fabricate ultra-thin polymeric components used in optoelectronics and biotechnology. Due to the extremely high flow resistance and fast heat transfer, ultra-thin parts is difficult to mold. In this study, a lab-made impact type micro-injection machine and a rapid heating/cooling system are used to perform the micro-injection molding. A mold with test-sheet cavities of thickness 100 lm and 200 lm are constructed. The molding capacity and the effects of processing conditions are systematically investigated. The processing conditions include the mold temperature, air pressure of accelerating cylinder, and air pressure of packing cylinder. The thicker parts have larger operation window. The high mold temperature reduces the area of short shot but induces more shrinkage and sink mark. The increase of air pressure in accelerating and packing cylinders can effectively reduce shrinkage.

Hot embossing has become a popular method for replicating precise micro-features onto large plastic plates. However, a long cycle time of the process due to conventional electric heating or hot oil heating is one of problems that confound the overall success of this technology. This study proposed a novel hot embossing method by using ultrasonic vibration as a heat generator. Experiments were carried out on a 2000-watt ultrasonic welding machine. A 2 mm thick polymethyl methyl acrylic (PMMA) plate was used for embossing microstructures onto its surface. The results in this study suggest that ultrasonic vibration hot embossing can provide an effective way of molding microstructures onto the surface of polymeric plates. Nevertheless, this novel process will need some improvements in the design of the ultrasonic vibration machine in order to make the process feasible.