Volume 23, Issue 3

The mathematical model of polymer plastication in a reciprocating screw injection molding machine is presented. It takes into account all characteristic features of working of a real injector, such as periodical action of the three zones screw, to-and-from screw motion with controlled stroke, static and dynamic melting, etc. Methods for description of time changes of the solid bed profile in the screw channel during injection cycle as a function of operating parameters were developed. Ways of determination of pressure and temperature profiles in the screw channel were also discussed. Methods of caculation of the most important flow characteristics such as plasticizing capacity, power requirements, screw torque, energy consumption were analyzed. Results were presented in a form, which is most convenient for numerical applications in computer simulation programs.

A computer program for the simulation of polymer plastication in screw injection molding machine created on basis of the theoretical model presented in part 1 of this paper is discussed. As simulation examples various process characteristics, e. g. solid bed, pressure and temperature profiles, are computed using the material parameters for high density polyethylene and three-zones-screw of diameter 20 mm. Effects of different geometric and operating parameters on plastication course are also demonstrated. Simulation results with respect to the screw rotation time are compared with experimental data measured for low density polyethylene, polypropylene, polystyrene, polyoxymethylene and polyamide 6. It was found that the computer program reflects correctly all features of a true plasticating system in injection molding machine. It provides also good agreement of model predictions with experimental data.

This paper uses a finite element analysis to investigate the morphological changes of nano-cells in a polystyrene (PS) – CO 2 foaming system. The system was composed of a finite polymer melt with a central cell and eight surrounding cells. The computational domain was discretized using linear triangular elements. The growth and shrinkage of nano-sized cells were tracked using the moving mesh method. The effects of the initial bulk gas concentration, cell size, intercellular distance, and system temperature on cell ripening were examined. The results show that smaller nano-sized cell(s) are doomed to collapse very quickly once they have interacted with larger cell(s), making it difficult to survive. Efforts were made to improve the general understanding of the challenges posed to the formation of nano-cells in foaming processes.

In the past, the capillaries of fiber-spinneret were made by micro drilling or by micro electro-discharge-machining (M-EDM). However, the productivity and uniformity of M-EDM are restricted. Hence, a new approach related to LIGA technique was developed. The micro mold of the capillaries was made by LIGA. Micro plastic cores were then duplicated by micro injection molding. With consequent treatments and electroforming, the spinneret die can be mass produced. Better process conditions for micro molding will be determined by Taguchi's experiment design method. Through a single injection molding cycle, a 48-capillary structure was formed at once. The diameters of each capillary were inspected at three locations along the longitudinal direction carefully. The results show that the accuracy of the diameters of capillaries can be controlled within 200 ± 1 μm. Capillaries die to spin ∗ and w-shaped micro-fibers were also developed in this work. Thanks to LIGA process, spinnerets with higher aspect ratios (even up to 30) can be achieved more easily. By applying micro injection molding, consequent treatments and electro-forming processes, the spinnerets can be produced in a batch and mass production way. The technology developed here can also be used in the fabrication of Micro-Electro-Mechanical (MEMS) components.

A new type of polymer processing machine, a centrifugal extruder, is introduced and its performance studied experimentally and analyzed theoretically. Calculations show that by means of appropriate choice of the geometrical parameters of the rotor drum and the rotor speed, the pressure of the solid polymer in the rotor drum can reach a value of several MPa. Theoretical analysis also shows that the most important factors influencing the solid conveying pressure in the rotor drum are the radius and slope angle of the rotor drum, the rotation speed of the rotor drum, the friction factor between the solid polymer and the drum surface, and the density of the polymer. An experimental rotor drum was designed in order to validate the mathematical model used for calculation of the solid conveying pressure in the rotor drum. The results of the experiments show good agreement with the theoretical predictions.

The increasing utilization of polymers in sliding members has triggered much research on the enhancement of surface quality and tribological performance. In this regard, no work has considered the application of roller burnishing as a new surface treatment method for polymeric surfaces. The present work aims to investigate the influence of burnishing feed and roller contact width on the sliding friction and wear performance of acetal homopolymer and polyurethane. Dry and lubricated sliding conditions for both parallel and crossed burnished orientations were considered. The results revealed that low burnishing feed rate and smaller roller contact width led to a decrease in coefficient of friction for both polymers while improvement in the wear resistance was observed. Hence, it can be suggested that the roller burnishing process has high potential as a valuable polymer surface treatment technique.

In this paper, both experimental trials and numerical simulations are presented to improve our understanding of the heat transfer within granular polymer beds for systems with and without shear. Both amorphous polymer, polystyrene (PS), and a semi-crystalline polymer, high-density polyethylene (HDPE) of varying particle size from 6 mm to 25 mm were examined. The estimated effective thermal conductivity data from the experimental trials was found by fit to two simple semi-empirical models allowing readers to readily approximate the property based on porosity of the bed and solid thermal properties. Discrete particle simulations, i. e. DEM, were used to recognize the importance of heat transport through the stagnant interstitial gas phase in comparison to particle-particle conduction. The results underscore the importance of including an effective thermal conductivity for granular solids in an extruder when using continuum models.

This paper reports a new process for the fabrication of polymer optical waveguides using gas-assisted UV embossing. A polydimethylsiloxane (PDMS) mold with a multimode waveguide array was made by casting a pre-polymer of PDMS against a plastic master of a waveguide array. The plastic master was prepared using gas-assisted hot embossing of polycarbonate (PC) film over a Si mold with the waveguide array. In this study, well-defined patterns with fine roughness for the multi-mode waveguides (125 μm × 80 μm) were successfully fabricated. The residual layer was thin and uniform. The replication quality, surface roughness and optical property of the replicated waveguides were measured and proved satisfactory. The average propagation loss of the waveguides was 0.88 dB/cm at a 1310 nm wavelength. The results indicate that the proposed method is likely to be a promising technique for the fabrication of optical waveguides.

PS/PMMA blends were prepared using a twin screw extruder and then extruded using an electromagnetic dynamic plasticating extruder equipped with a specially designed split die. During dynamic plasticating extrusion, the dynamic rheological behavior of PS/PMMA blends was investigated. The extrudates were cooled immediately and collected for morphology measurements using a scanning electron microscope (SEM). Results show that the average shear stress decreases with the introduction of vibration force field into the whole extrusion process, and reaches a minimum at the vibration amplitude of 0.15 mm and the vibration frequency of 5 Hz. At the same shear stress, the melt viscosity of the blend under dynamic plasticating extrusion is lower than that under static extrusion and decreases with vibration intensity. With increasing vibration intensity, the power-law index of the blend melt has a tendency to increase. The drop size of the dispersed phase in 25/75 PS/PMMA blend decreases at a faster rate with an increase of shear stress, compared with that in 75/25 PS/PMMA blends, correspondingly, the melt viscosity of 25/75 PS/PMMA blend has a stronger shear rate dependence. During dynamic plasticating extrusion, a negative deviation from the additivity rule is observed for the studied blend compositions.

This paper proposes a rapid fabrication technique of polymeric micro-block arrays using an IR hot embossing process. A pneumatically actuated embossing facility with conventional IR heating was designed, constructed and tested in our laboratory. To mold micro-blocks onto the plastic plates, a silicon mold with an array of micro-block cavities was fabricated by photolithography and deep reactive ion etching. During the embossing process, a PMMA plate was first placed above the mold. A ceramic IR heater was placed above the PMMA plate to heat the mold. The IR passed through the transparent PMMA to reach and heat up the mold surface. After retrieval of the IR heater, the pneumatically actuated embossing facility pressed the plastic plate against the hot master mold. A micro-block array was obtained. The effect of various processing parameters on the replicability of the micro-blocks was investigated. Under the proper processing conditions, the arrays of polymeric micro-blocks, with a dimension of 100 m × 80 μm × 40 μm, could be successfully fabricated. The experimental results in this study suggest that IR heated embossing could provide an effective way of fabricating large-area microstructures onto plastic plates. This would provide significant advantages in terms of a shorter cycle time as well as improved product quality.

A computer code, that couples numerical modelling and optimization routines, is described and used to automatically optimize the performance of a multi-step calibration stage of a plastic profile extrusion line. The objective of the optimization process is to minimize two conflicting criteria: final plastic profile average temperature and corresponding distribution standard deviation, both computed at the end of the calibration/cooling stage. The optimization routine uses Multi-Objective Evolutionary Algorithms (MOEA). Having in mind the well-known difficulties in characterizing the interface heat transfer coefficient, the system optimization was performed using different constant values for this parameter.

Polybutylene terephthalate (PBT) and highy density polyethylene (HDPE) were blended and stabilized with four different stabilizers obtained from the organic derivatives of five-valent phosphorus. These stabilizers were cyclohexylphosphonic acid, its metallic salts of cyclohexyl phosponates of bivalent nickel, bivalent iron and trivalent iron. The stabilization improved the thermal stability of the PBT/HDPE blends and increased their mass loss temperature. The blend melts were thermally aged inside a viscometer barrel and the Melt flow Index (MFIs) was measured. All stabilizers increased the melt stability of the blends when used in low rates (< 0.2 % in mass). At higher stabilizer rates, some stabilizers further enhanced the thermal stability, while others decreased it. A melt parameter was introduced to determine the melt stability of the PBT/HDPE blends during extrusion. The best stabilization was obtained with cyclohexylphosphonic acid based on the metallic salts of bivalent nickel. The mechanisms that lead to the inhibition of thermal destruction of the blends were explained in terms of the stabilizers.

This paper presents a non-Isothermal numerical model developed at the KTP for calculating the drive power of single-screw plasticating units. It includes, firstly, a description of the mathematical and physical modeling process and, secondly, an explanation of the systematic approach, based on statistical methods, that was adopted for compiling an approximation equation with the aid of non-isothermal, three-dimensional FEA flow simulation results. In addition, the generated model is verified on the basis of the computation results obtained with a simulation program for screw layout, in which the model was implemented. The simulation results are compared with experimentally recorded power data for selected extrusion and injection molding processes and the quality of the model is subsequently assessed.