Theoretical and visual study of bubble dynamics in foam injection molding

This article presents an experimental observation and a theoretical prediction of bubble dynamics in foam injection molding process with a main focus on the cell collapse phenomenon under pressure. Using a visualizing setup, cell growth behavior under a nonisothermal condition was monitored. In conjunction with the growth behavior, dynamics of cell collapse under different pressures and the effect of growing time on collapse behavior and final cell size were studied. Theoretical simulation of bubble behavior included power law model, which predicted bubble dynamics during foaming process. The results show that collapse phenomenon strongly depends on both exerted holding pressure and growth time. The presented model can also give a reasonable prediction of growth and collapse of cells and could give insight to control of cell size in injection foaming process. POLYM. ENG. SCI., 2010. © 2009 Society of Plastics Engineers     

Visualization of bubble dynamics in foam injection molding

This article presents a visualization study on nonisothermal bubble growth and collapse in the foam injection molding process (FIM). Observation study can give more insight to the bubble growth in foaming process, especially in the challenging injection foaming process. In this study, besides the growth of bubbles, collapse of the bubbles was also observed which could provide knowledge to the final foam morphology. Cell growth vs. time was recorded and analyzed using a software‐equipped high speed camera. To investigate the cell collapse, various holding pressure was exerted on the gas‐charged molten polymer. The amount of holding pressure had noticeable effect on the rate of bubble collapse. © 2010 Wiley Periodicals, Inc. J Appl Polym Sci, 2010     

The dynamics of peak cavity pressure in injection molding

The modification of the injection-molding machine, incorporating additional filters, an accumulator, and a servovalve, was shown to reduce the peak cavity pressure variations, in as much as the variations using the microprocessor system were random in comparison to the variations using the normal mode of operation, which exhibited nonstationarity and autoregressive behavior. If the peak hydraulic pressure could be randomized, then the peak cavity pressure variations also would be random. No closed-loop control would be needed to control the peak cavity pressure.     

A basic experimental study of sandwich injection molding with sequential injection

An experimental study of sandwich injection molding is reported which involves sequential injection of polymer melts with differing melt viscosity into a mold. In isothermal injection molding the relative viscosity of the two melts is the primary variable determining the phase distribution in the mold. Generally the most uniform skin-core structure occurs when the second melt entering the mold has a slightly higher viscosity than the first melt injected. Large viscosity inequalities lead to nonuniform skin thicknesses. The influence of blowing agents and non-uniform temperature fields on the extent of encapsulation is described. Temperature fields are very important especially if the first polymer melt injected has a greater activation energy of viscous flow (or a greater temperature dependence of the viscosity function).     

An injection molding study. Part I: Melt and barrel temperature dynamics

The dynamics of the key variables in the plastication phase of injection molding were studied using a microcomputer controlled laboratory scale injection molding machine. Interfaces for the measurement and manipulation of suitable variables were developed. An approximate theoretical analysis provided a preliminary understanding of the effect of a step input in heating power on melt and barrel temperatures. Deterministic and stochastic models were derived from experimental data for the melt and the front and rear zone barrel temperatures with heater power manipulation. Experiments were performed for evaluating the effects of back pressure manipulation, heating zone interaction, and process disturbances on the melt and barrel temperatures. Experiments showed that it is desirable to directly control the melt temperature rather than the barrel temperatures.     

A study of rib geometry for gas-assisted injection molding

Large, thin, plate-shaped parts usually are strengthened with structural ribs. Ribs also serve as gas channels with gas-assisted technology. The layout and geometry of these gas-channel ribs are critical to the gas-assisted injection molding (GAIM) process. In this study, the effects of rib geometry, including aspect ratio and fillet geometry, on the GAIM process are investigated. Experimental results indicate that increasing the rib width widens the allowable operation range and thus improves the moldability. Adding fillets to the rib corner significantly enhances the moldability. Adding fillets also reduces the loss of rigidity due to void formation in the rib. A curved fillet improves moldability and rigidity more than one that is straight.     

A visual barrel system for study of reciprocating screw injection molding

A “visual” barrel system has been developed to promote the understanding of polymer behavior in a reciprocating screw injection molding machine. This system allows the dynamic status of the polymer inside the injection barrel to be conveniently visualized though photography and video recording, and the corresponding material conditions can be measured via a computerized data acquisition system. High density polyethylene (HDPE), a commonly used thermoplastic, is chosen to demonstrate the polymer melting behavior in injection molding. Effects of processing conditions on the melting behavior are analyzed, suggesting that the modeling of the plastication phase should not be treated simply as an extrusion process. The system, therefore, can not only assist in the understanding of the process but also can be used to verify quantitatively the modeling and optimization of an injection molding system.     

MuCell微发泡注塑成型技术应用

MuCell微发泡注塑成型技术的使用日趋普及,其制品主要集中在品质要求较高、材料较贵的产品上。近年来,选用微发泡注塑成型技术的中国企业数目快速增长,其应用领域也正在扩大。     

Studies on structural foam processing II. Bubble dynamics in foam injection molding

An experimental study was carried out to gain a better understanding of the dynamic behavior of gas bubbles during the structural foam injection molding operation. For the study, a rectangular mold cavity with glass windows on both sides was constructed, which permitted us to record on a movie film the dynamic behavior of gas bubbles in the mold cavity as a molten polymer containing inert gas was injected into it. The mold was designed so that either isothermal or nonisothermal injection molding could be carried out. Materials used were polystyrene, high-density polyethylene, and polycarbonate. As chemical blowing agents, sodium bicarbonate (which generates carbon dioxide), a proprietary hydrazide and 5-phenyl tetrazole, both generating nitrogen, were used. Injection pressure, injection melt temperature, and mold temperature were varied to investigate the kinetics of bubble growth (and collapse) during the foam injection molding operation. It was found that the processing variables (e.g., the mold temperature, the injection pressure, the concentration of blowing agent) have a profound influence on the nucleation and growth rates of gas bubbles during mold filling. Some specific observations made from the present study are as follows: an increase in melt temperature, blowing agent concentration, and mold temperature brings about an increase in bubble growth but more non-uniform cell size and its distribution, whereas an increase in injection pressure (and hence injection speed) brings about a decrease in bubble growth but more uniform cell size and its distribution. Whereas almost all the theoretical studies published in the literature deal with the growth (or collapse) of a stationary single spherical gas bubble under isothermal conditions, in structural foam injection molding the shape of the bubble is not spherical because the fluid is in motion during mold filling. Moreover, a temperature gradient exists in the mold cavity and the cooling subsequent to mold filling influences bubble growth significantly. It is suggested that theoretical study be carried out on bubble growth in an imposed shear field under nonisothermal conditions.     

Thermoset injection molding dynamics and the distribution of cure in thermoset molded parts

A detailed characterization of a commercial-filled unsaturated polyester molding compound has been carried out to determine the kinetics of cure and the rheological behavior of the material at various temperatures and shear rates. Molding experiments were conducted in a 2 1/3 oz, 68 ton reciprocating screw injection molding machine, in conjunction with a simple rectangular cavity. The cavity and nozzle were equipped with pressure transducers to determine, the variation of pressure with position throughout the injection molding cycle. The injection speed was determined with the help of a position transducer. Finally, the moldings were analyzed to determine the distribution of cure states and tensile properties in the molding at various cure times. Significant differences have been observed. It is expected that studies of this type should be helpful in obtaining a better understanding of the thermoset injection molding process and the development of mathematical models to simulate this process.     

Mixing study of L shape mixheads in reaction injection molding

The effect of mixhead geometry on the impingement mixing in the reaction injection molding (RIM) process is largely unknown. In this study, high speed photographs are used to show the flow patterns produced by L shape mixheads. The mixing quality of the conventional I and various L shape mixheads is quantified by an emulsion test. The adiabatic temperature rise of a polyurethane/urea system is followed to further characterize the mixing produced by I and L shape mixheads. The results show that L shape mixheads give a better mixing quality than the I shape mixhead, especially at lower Reynolds numbers. In addition, the L shape mixheads can provide a more laminar flow from the mixhead, which is important for mold filling.     

Screw design in injection molding

The performance of screws of advanced design in injection molding has been investigated with respect to four different objectives: (1) improvement of distributive mixing; (2) improvement of dispersive mixing; (3) increase of plasticating capacity; and (4) reduction of inhomogeneity of melt temperature. The screws used are three zone screws with different compression ratios, screws with pineapple or Maddock/Egan mixing elements, with one or two channel barrier sections, with static mixers mounted in the valve or in the nozzle, or with combinations of these different elements. The best mixing quality is obtained with multi-channel Maddock sections. The highest plasticating capacity and, consequently, the shortest cycle times are achieved with the barrier screws. Temperature measurements show that these screws improve melt homogeneity considerably with a relatively small loss of plasticating time. In all cases, increasing the back pressure gives inferior results compared with improvement of the screw design.     

Pressure-Induced densification in injection molding

The effect of pressure on the densification of amorphous polymers in the Injection-molding process is examined. Density distributions in molded polystyrene slabs were measured for several well-defined molding histories. In all cases the density of the molded part was spatially inhomogeneous, and its distribution in the slab was closely related to the pressure and temperature histories that prevailed in the molding cavity during the process cycle. The density profile in the gapwise direction followed a characteristic “parabolic” pattern with a minimum at the midplane of the slab. A simple phenomenological model, based on the pressure-induced densification effect, was constructed to explain the observed density profiles, and close agreement with experimental data was found. Annealing of the molded article at a high temperature (<T_g) caused the density to decrease overall and become more uniform across the part. This is generally consistent with volume-recovery data for the densified material, which were generated independently in a controlled pressure-densification experiment.     

Molecular orientation in injection molding

A semiquantitative model is proposed to explain the complex molecular orientation distribution, observed in injection moldings of amorphous polymers. The model incorporates flow and heat transfer mechanisms coupled with molecular theories. The orientation in the surface skin is related to steady elongational flow in the advancing front, whereas the orientation in the core is related to the shear flow, behind the front, between two solidyfying layers. Coupled with the elongational and shear-induced orientations, a molecular relaxation process takes place which is determined by the rate of heat transfer. The bead-and-spring macromolecular theory was used to calculate root mean end-to-end distances of macromolecules in the various flow fields, as well as the relaxation process.     

Reaction injection molding of polyureas I. Kinetics study

Summarized in this paper are the experimental results and mathematical modeling of processing of polyureas. The experimental part includes the applications of solution polymerization to study the reaction kinetics of polyureas. The theoretical part includes a kinetic and heat transfer model that can apply solution polymerization data to predict the bulk polymerization of polyurea in the reaction injection molding process.     

橡胶注射成型机注射装置结构分析研究

主要介绍了橡胶注射成型机及其注射装置的发展情况,阐述了橡胶注射成型的特点以及注射装置设计要求,并对三种常用的注射装置的结构特点及应用作系统的分析比较.     

新型注射成型机

该文重点介绍了一种新型注射成型机,其创新之处在于可控制温度,能除去注射筒内的滞留胶。     

橡胶注压成型机

介绍了宁波千普公司设计开发的橡胶注压成型机的合模机构、注压机构、液压传动系统、电控系统.对橡胶注压成型机所需要做的进一步发展和提高进行了简要的分析.     

一步法注射成型机

简述了注射成型法与模压成型法相比所具有的诸多优点，同时指出了传统注射成型方法所存在的问题。着重介绍了一步法注射成型技术及其设备的结构特点、工作原理及工作特性。