Volume 27, Issue 1

In this study, corrugated variations in part thickness designed to match the various mold temperature differences between the mold core and mold cavity sides, along with their effects on part warpage, were investigated. Part thickness was varied in a pitched manner and thickness reductions of 10%, 20%, 30%, and 40% of the original part thickness were implemented. The results show that when there is a mold temperature difference between core and cavity, using a corrugated thickness reduction design can greatly reduce or even eliminate part warpage. For a uniform thickness part molded under unbalanced cooling, the thermal neutral axis (where the maximum melt temperature is located) will deviate from the gap center toward the mold wall side with the higher temperature. Variation in thickness will pull the deviated thermal neutral axis back to the center line leading to a reduction in part warpage.

In this paper, we present a novel injection molding approach using water containing thermoplastic polyolefin (TPO) to reduce the cycle time and to improve the dimensional stability of parts without any equipment or mold modifications. A small amount of water was pressurized into TPO pellets pre-mixed with water carrier particles such as highly porous activated carbon or tubular-shaped nanoclay. Our results showed that the cycle time could be drastically reduced with substantially reduced surface sink mark and part warpage without any adverse effects on mechanical properties. Water evaporation during molding resulted in internal cooling, which led to a large reduction in cycle time. This phenomenon, however, could not be explained by the average water content in TPO alone. It is believed that the accumulation of water in the thickest section of the mold cavity, i.e., the base of the conical shaped sprue in the molds used in the study, during injection molding played a major role. The lower sink mark and part warpage were due to water induced internal cooling and foaming. Complete filling of thin parts could also be achieved at a lower packing pressure in the presence of water, perhaps due to steam (from water evaporation) induced mold cavity heating during filling. This was supported by the measured temperature rise at the mold surface in the early stage of the molding cycle.

In this study, the flow and rheological behaviors of low density polyethylene (LDPE) and single-phase LDPE/N 2 polymer/gas solutions at various gas contents were measured using a high-pressure slit-die rheometer. The resulting rheology data of LDPE and LDPE/N 2 were curve fit using the Cross–WLF model to obtain the seven material model constants, which were then used in the simulation of conventional and microcellular injection molding processes. The pressure effect on the shear viscosity of LDPE was also studied. The pressure dependency coefficient, D3, in the Cross–WLF model was determined by the use of simulation to best fit the experimental results. In addition, a three-dimensional plot of shear viscosity as a function of shear rate and pressure at various temperatures was constructed. Based on this three-dimensional viscosity–shear rate–temperature–pressure plot, the increase in viscosity due to the pressure effect was found to be more profound at high pressures, low temperatures, and low shear rates.

For better understanding of thin-wall injection molding processes, a visualization injection mold and high speed camera were used to observe the dynamic filling behavior of melt in cavities with different thicknesses during the injection molding process. The impact of the scale effect on the melt-filling rate and filling imbalance in the multi-cavity were also studied. Results indicated that the filling behavior of the melt during the multi-cavity molding process was related to the cavity thickness. The experiment results can be used to describe the filling imbalance phenomenon during the injection molding process and to provide experimental evidence for precise injection molding technology at the micro scale.

This paper studies the thermal response on the mold surface of a circular cavity in the preheating process by high-temperature gas via a numerical approach, where the heat and mass transfer inside the cavity and mold are predicted by a two-fluid model. The dimensionless parameter group is identified from a dimensional analysis of the gas preheating problem. The correlations among dimensionless parameters are established by a least square method applied to the numerical predictions. The upper limit of the temperature rise on the cavity surface at a given gas flow rate is found approximately proportional to the square root of Q ∗ and h ∗ , respectively. The lower limit of gas flow rate to achieve a given temperature rise on the cavity surface is inversely proportional to h ∗ but grows with (Q ∗ ) 1.5 . The first limiting line to describe the dependence of Q ∗ on t ∗ is independent of h ∗ and T ∗ , but the second one is governed by Q ∗ c . For reaching a specified temperature rise on the cavity surface, the growth in the thickness of thermal resistive layer reduces the required preheating time but increases the corresponding gas flow rate. The dependence of (t ∗ f , Q ∗ f ) on the parameter group (T ∗ , h ∗ ) is given in this study to obtain the reasonable combinations of (t ∗ f , Q ∗ f ) based on the balance between the allowable preheating time and heating capacity.

A multi-layer injection molding machine control system is proposed in this paper. The first layer, an advanced process parameter control layer, provides accurate and robust control of key process variables. The second layer consists of two modules, a profile setting layer determining the shapes of set-point profiles for the key process variables, to achieve an evenly distributed part quality along the flow directions; and a process fault detection and diagnoses system monitoring the operation of injection molding process. The third layer, a quality control layer, consists of a quantitative quality prediction model and a quality controller. This final layer provides a mechanism for closed-loop control of certain part quality. The related work that has been conducted on these three layers is discussed extensively in this paper, and the future work is introduced as well.

High strength and toughness are of importance for composite plastics used as mechanical structure parts. How to provide general and engineering plastics with high performances is one of the critical challenges in advanced manufacturing. In the past decade, the phase dispersion, interfacial interaction, injection molding simulation and fracture behaviors of composite plastics were investigated systematically in our research group in order to simultaneously reinforce and toughen polymer matrix. Here, we review the progress and advances which have been made in the above items.

The demolding of replicated parts can result in damage to both the replication tooling and finished parts and is a particular problem for the replication of smaller parts which can be quite fragile. Various techniques have been proposed in the literature to solve such problems by reducing the overall demolding force. This paper presents the challenge of demolding replicated parts and reviews the proven solutions from the literature which have been developed. A summary chart of these solutions is presented which may be used to implement plans to solve demolding problems with replicated parts. Such a rationalization of existing knowledge will enable replication tool developers to systematically select and apply proven solutions to solve, and ultimately prevent, demolding problems.

Processibility of LDPE is known to be sensitive to processing history. Significant changes in processibility and product properties can occur through repeated extrusion without detectable changes in molecular weight and molecular weight distribution. Such changes of LDPE rheology are simulated with DSM micro-compounder and characterized with the small angle oscillatory tests and the SER uni-axial extensional flow with a K-BKZ analysis. Both the linear and the nonlinear rheology data suggest the major mechanism in the extrusion with the DSM micro-compounder is the reduction of the effects of long chain branching. The linear rheology analysis, along with GPC and NMR characterization confirms that the changes in the extrusion are not due to chemical crosslinking, though an increase in backbone molecular weight at extreme conditions is perceivable through the minor increase in molecular weight distribution, the increase in zero-shear viscosity and the decrease of melt index. The extensional rheology results, on the other hand, reveal the depression of the strain hardening with prolonged extrusion at 190 degree C or higher extrusion temperature. The stress growth data were evaluated with a K-BKZ constitutive with exponential damping function. The depression of strain hardening is well characterized with the strain hardening factor and the non-linear exponent beta as function of the extrusion conditions. The damping factor β is observed to be especially effective as the parameter to follow the depression of the strain hardening due to microcompounder extrusion. The microcompounder extrusion and the SER analysis with the K-BKZ analysis prove to be a new useful tool for simulating the shear modification of the LDPE.

Besides comfort and hygiene of textile materials sufficient barrier properties, such as protection against ultraviolet (UV) irradiation, are also often required. For the evaluation of barrier properties of textile materials against UV irradiation, a quantitative expression, “ultraviolet protection factor (UPF)”, is used. This factor correlates with the time period of human skin protection against UV irradiation in the open air. One of the experimental methods for quantitative UPF evaluation involves a spectrophotometric method of fabric testing. This paper describes a novel modification approach to transfer this method to non-woven and non-knitted fabrics. The aim of the transformation was to modify the used method to determine barrier properties of polymer composite fibres so that the measurement could be simplified while keeping the reliability of the obtained results. As a testing material samples of polypropylene fibres with dispersed nanoTiO 2 filler reeled on the sheet-metal frame were used. The results show that this modified method gives reliable results and is suitable for development of new kinds of fibres with higher barrier protection against UV irradiation.

This research aims to develop a new dry nano powder inverse imprint technology, and probe into transferring and replication of micro patterns. First, a polydimethylsiloxane (PDMS) mould surface, with micro patterns of grating structures, undergo activation treatment with oxygen plasma in order to improve mould surface energy, and thus, spin coat photoresist onto the surface more easily. Then, the surface is heated to restore its hydrophobicity, and by means of nano powder, micro patterns are transfer printed onto the polymethylmethacrylate (PMMA). Research results show that dry nano powder soft coat transferring and forming technology can accurately reproduce the required patterns. In the experiment, gas assisted nano powder imprint technology is used to apply even pressure, coupled with even temperature rise for solidification, then, patterns can be effectively transfer printed and completely formed. Further, these patterns are in perfect contact with the PMMA surface, thus, greatly increasing the valid imprint area and enhancing transferring reproductiveness. The technology integrates technologies, such as, soft lithography, solidifying photoresist, and gas assisted pressure application, and is utilized to transfer print micro patterns in a grating structure, and thus, complete the transferring process of dry micro structure patterns.

In this work the numerical modeling of the flow inside co-rotating twin-screw extruders is performed and solutions are analyzed to determine the mixing behavior of two screw elements: conveying and mixing elements. The flow around intermeshing screws is computed using an immersed boundary finite element method capable of dealing with complex moving solid boundaries. The flow is considered isothermal and the material behaves as a generalized non-Newtonian fluid. Because the viscosity depends on the shear rate, solutions will be shown for various rotation velocities of the screw. The 3D solutions are then analyzed in order to determine various parameters characterizing the flow mixing such as the residence time and the linear stretch. Residence time distribution inside the twin-screw extruder is first computed by using a particle tracking algorithm based on a fourth order Runge-Kutta method. A large number of particles are tracked inside the extruder and the resulting particle data is used to determine the distribution of the residence time and of the linear stretch. The spatial distribution of the residence time is also computed by solving a transport equation tracking the injection time of the polymer melt. The methodology shows important differences in the mixing behavior of the screw elements considered.

The rheological and mechanical properties of PVC-lignin blends were studied, and their alterations were investigated after lignin was treated with poly(ethyl acrylate-co-maleic anhydride). The addition of lignin changes the early rheological properties of the blend comparing with PVC control because the rigidity of lignin particles leads to more intensive shearing in the system, but has little effect on its rheology during the equilibrium period. The processing stability slightly decreases with the increase of lignin content in the blend. The mechanical properties of the blends are worse than that of PVC control. The rheological properties of the blends are little influenced after lignin is treated with polyacrylate, but the mechanical properties, especially toughness, are improved, and the maximum impact strength is 37.7% higher than that of PVC control.

The performance of gravure coating is strongly dependent on the break-up of the liquid bridge connecting the surfaces of the moving roller and gravure cell. The mechanism of the movement and subsequent break-up of the liquid bridge were studied experimentally using a three-dimensional flow visualization technique. Three stages of liquid motion were observed, and two characteristic lengths were defined to quantify the liquid pick-out and bridge break-up. The pick-out and break-up mechanisms were influenced by the capillary number, coating gap and the geometry of the gravure cell. A universal correlation between the pick-out and these variables was found.

The increasing consumption and resistance to natural decomposition of plastic materials has caused an ecological problem that is waiting for solutions, including economical recycling technologies. In this work, the properties of compression molded plastic boards made of commingled pellets of Polyethylene (PE) and Polyvinyl chloride (PVC), two incompatible widely used polymers, are investigated. The commingled pellets were mechanically mixed and flat boards with different ratios of PE and PVC were produced by compression molding, one set using radio frequency (RF) electromagnetic heating and the second using conventional conductive heating (CH). The main advantage of the RF system over the CH system is the much shorter heating time required to reach processing temperatures, making it an economically attractive alternative. However, the speed and intensity of the heat acting on the PVC in the RF process caused a reduction of the mechanical properties of the boards as a consequence of the degradation of the PVC. This problem was solved by the addition of process-specific additives, organic and inorganic, placed at the pellet's interface. The results of this work demonstrate that commingled plastics can be efficiently compression molded using radio frequency, providing an alternative to conventional compression molding using conductive heating.