注射成型过程中非牛顿塑料熔体的粘度模型

本文在探讨塑料熔体粘度模型基础上,重点讨论了一种适用范围更广的Cross粘度模型,并根据塑料熔体在注射成型过程的流动特点,采用Arrhenius方程建立了适用于注射充填过程的五参数Cross粘度模型和利用WLF方程建立起适用于注射保压过程的七参数Cross粘度模型。具体讨论模型的特征和适用范围,为注射模设计和成型模拟提供了理论依据。     

Injection molding of glass fiber reinforced phenolic composites. 2: Study of the injection molding process

This study of injection molding of glass fiber reinforced phenolic molding compounds examines fiber breakage and fiber orientation with key material and processing variables, such as injection speed, fiber volume fraction, and the extent of resin pre-cure. The fiber orientation, forming discrete skin-core arrangements, is related to the divergent gate to mold geometrical transition, the extent of pre-cure and injection speed functions of the melt viscosity. Transient modifications to the melt viscosity during mold filling produce variations in skin/core structure along the flow path, which are correlated to the mechanical properties of injection moldings. The melting characteristics of the phenolic resin during plasticization impose a severe environment of mechanical attrition on the glass fibers, which is sequentially monitored along the screw, and during subsequent flow through runners and gates of various sizes. Differences found between the processing characteristics of thermosets and thermoplastics raise questions concerning the applicability of thermoplastic injection molding concepts for thermosets.     

Rheology studies of foam flow during injection mold filling

This work studies the flow behavior of a developing two‐phase gas‐polymer suspension during injection into the instrumented mold cavity of an injection molding machine. In the experiments, blowing agent type and concentration were varied along with processing conditions, to generate controlled cell structures in two different polymers, low density polyethylene and thermoplastic polyolefin. Experimental results indicate that the rheological properties of two phase gas‐polymer suspensions were sensitive to shear rate, blowing agent concentration, melt temperature, and mold temperature. The viscosity of all gas‐polymer suspensions revealed a reduction compared with neat polymer melt in the presence of gas bubbles, because of the reduced volume fraction of polymer matrix. A two‐phase rheological model has been used for fitting with our experimental results for estimating the shear viscosity of two‐phase flow in the mold cavity of the injection molding machine. POLYM. ENG. SCI., 47:522–529, 2007. © 2007 Society of Plastics Engineers.     

In situ flow state characterization of molten resin at the inner mold in injection molding

Online viscosity information on processing lines can reflect the material flow resistance and offer valuable guidance for manufacturing across various industries. Considering the accuracy, devices, and processes involved in injection molding, characterizing the melt's flow state during material processing poses a significant challenge. To reduce investment in viscometers, avoid influencing the components' surface aesthetics due to the installation of sensors, and make the flow state detect online in mold, this study designs a rheometric mold with cylindrical runners for identifying the in situ viscosity of molten resin during injection molding. The detection conditions of injection speed and cavity pressure variations, the entrance effect, and the viscous dissipation for Polycarbonate are analyzed under various conditions. The in situ viscosity is identified and compared with the standard cross-WLF model. The result shows that the melt velocity and cavity pressure variations during the filling process create a stable environment for in situ rheological characterization and the detected viscosity is related to the shear rate, melt temperature, and channel dimension in injection molding. The designed mold with cylindrical runners for determining the in situ thermal-rheological behavior of polymer is distinguished successfully and exhibits prospects for the development of low-cost, nondestructive, and inner-mold measurement in manufacturing applications.     

A method for determining the flow front velocity of a plastic melt in an injection molding process

During the filling phase of an injection molding process, the flow front velocity of the plastics melt has a decisive influence on the form part quality. It has been believed that a constant flow front velocity of the melt leads to distortion‐free and residual stress‐free form parts. A process control strategy based on a constant flow front velocity of the melt, however, requires the full understanding of the flow front position as a function of the screw position of the injection molding machine. With current methods, this can only be achieved by direct measurements using a number of sensors inside the mold, which leads to complicated structure, great efforts, and high cost for the tooling equipment. This article proposes, designs, and develops an innovative method for determining the flow front velocity of a plastic melt in an injection molding using only one pressure sensor at the front of the screw and based on the idea of mapping a simulated filling process to a real injection molding process. The mapping ensues that the characteristic event points are identified and matched for both the simulated and real filling process. The results of the simulation analysis and experimental evaluation show that the proposed method can be used to determine the flow front position and the resulting flow front velocity of the melt within the cavity of the mold and provide evidence that the new method offers great potential to process control strategies based on machine independent parameters. POLYM. ENG. SCI., 59:1132–1145 2019. © 2019 Society of Plastics Engineers     

Control of polymer properties by melt vibration technology: A review

This paper reviews the technology of melt vibration (more specifically at low frequency) to reduce viscosity during processing of plastics and to enhance mechanical performance of the solidified parts. The effect of vibration frequency and amplitude on melt viscosity is explained in terms of shear-thinning criteria. The effect of pressure and temperature on shear thinning is also reviewed to predict how these variables interfere with melt vibration. Practical applications of the principles of melt vibration are provided in injection molding, extrusion and compression molding/thermoforming, from reduction of viscosity to lowering processing temperature and pressure to the elimination of melt defects and weld lines, to the enhancement of mechanical properties, stiffness and strength, by modification of the amorphous and semicrystalline texture and orientational state. Commercially available equipments are reviewed. Results showing the effect of melt vibration during processing for two classical polymers, polystyrene and polypropylene, are discussed. The paper concludes on the remaining challenges to bring the benefits of the new technology to full commercialization.     

Effect of dissolved gas on the viscosity of HIPS in the manufacture of microcellular plastics

The microcellular plastics (MCPs) process is a foaming process that has been developed to reduce the weight of a product without significant changes to the mechanical properties. To apply microcellular plastics to mass production systems such as extrusion, injection molding, and blow molding, research must be done on material properties, such as viscosity, glass transition temperature, and melt index of polymer resin. Among the properties, it is critical to predict the change in viscosity with the amount of inert gas, which can be an index of the injection molding working condition of polymer resin. The purpose of this paper is to study the relationship between the amount of dissolved gas and the viscosity of high impact polystyrene (HIPS) resin in extrusion. The experiment was carried out with newly designed gas supply equipment and with a screw and die modified for MCPs. In addition, a pressure gauge was set up on the end of a barrel for measuring the pressure change. The experiment has shown that the viscosity of polymer decreases with increasing amounts of inert gas. A new model was applied to estimate the viscosity change as a function of the amount of dissolved gas.     

Recycling of Poly(phenylene ether) Blends

The effects of postindustry recycling of polymer blends composed of poly(phenylene ether) (PPE) on the properties of the PPE blends were investigated by simulated recycling with multiple molding cycles. Two compositions with different concentrations of PPE were reprocessed with an injection‐molding machine. Mechanical, thermal, rheological, and morphological characterizations were carried out on as‐produced and reprocessed samples to examine the influence of the number of molding cycles on the two specific PPE blends. Efforts were made to determine the effect of each molding cycle on the specific properties of the two PPE blends, including the Elastic (E), modulus, stress at break, strain at break, multiaxial impact, and melt viscosity. The results are discussed in detail. The retention of the properties correlated well with the unperturbed morphology of the compositions before and after recycling, as observed by transmission electron microscopy analyses on fractured tensile samples. However, more in‐depth microanalyses are required to identify the effect of recycling on the individual components present in the studied compositions. In this study, we aimed to establish structure–property relations upon recycling using several characterization techniques. © 2011 Wiley Periodicals, Inc. J Appl Polym Sci, 2011     

Effects of acid dyes on crystallization and mechanical properties of melt‐reprocessed nylon 66

Nylon fibers dyed with different types of acid dyes were melt reprocessed using a compression‐molding machine. The crystalline structure and mechanical properties of the melt‐reprocessed nylon were experimentally evaluated. It was found that metal complex acid dyes showed much more distinct effects on the structure and mechanical properties of melt‐reprocessed nylon than nonmetallized acid dyes. They decreased the crystallization rate of the molten nylon and reduced its crystallinity. They also reduced the imperfect form I structure in the crystalline region. Compression‐molded nylon samples showed inferior mechanical properties in the presence of metal complex acid dyes. The coordinate bonds between the Cr³⁺ ions and amide groups are possibly formed in melt‐reprocessed nylon, which could be the reason for the changes in the structure and properties of melt‐reprocessed nylon. © 1999 John Wiley & Sons, Inc. J Appl Polym Sci 74: 2386–2396, 1999     

The occurrence of surface roughness in gas assist injection molded nylon composites

Gas assist injection molding has increasingly become an important industrial process because of its tremendous flexibility in the design and manufacture of plastic parts. However, there are some unsolved problems that limit the overall success of this technique. The purpose of this report was to study the surface roughness phenomenon occurring in gas assist injection molded thermoplastic composities. The materials used were 15 % and 35% glass‐fiber filled nylon‐6 composites. Experiments were carried out on an 80‐ton injection molding machine equipped with a high‐pressure nitrogen‐gas injection unit. Two “float‐shape” axisymmetric cavities were used. After molding, the surface quality of molded parts was measured by a roughness meter. Various processing variables were studied in terms of their influence on formation of surface roughness: melt temperature, mold temperature, melt filling speed, short‐shot size, gas pressure, and gas injection delay time. Scanning electronic microscopy was also employed to characterize the composites. It was found that the surface roughness results mainly from the exposure of glass fiber in the matrix. The jetting and irregular flows of the polymer melt during the filling process might be factors causing the fiber exposure.     

气体辅助注射成型充填过程的数值模拟

描述了气体辅助注射成型的工艺过程及熔体充填和气体穿入的数学模型,采用有限元/有限差分/控制体积法计算充填阶段的压力场和温度场,确定熔体前沿和熔体/气体界面两类移动边界,并对典型制件充模过程进行了模拟.     

Determination of the heat transfer coefficient from short‐shots studies and precise simulation of microinjection molding

The filling process of a micro‐cavity was analyzed by modeling the compressible filling stage by using pressure‐dependent viscosity and adjusted heat transfer coefficients. Experimental filling studies were carried out at the same time on an accurately controlled microinjection molding machine. On the basis of the relationship between the injection pressure and the filling degree, essential factors for the quality of the simulation can be identified. It can be shown that the flow behavior of the melt in a micro‐cavity with a high aspect ratio is extremely dependent on the melt compressibility in the injection cylinder. This phenomenon needs to be considered in the simulation to predict an accurate flow rate. The heat transfer coefficient between the melt and the mold wall that was determined by the reverse engineering varies significantly even during the filling stage. With increasing injection speed and increasing cavity thickness, the heat transfer coefficient decreases. It is believed that the level of the cavity pressure is responsible for the resulting heat transfer between the polymer and the mold. A pressure‐dependent model for the heat transfer coefficient would be able to significantly improve the quality of the process simulation. POLYM. ENG. SCI., 2010. © 2009 Society of Plastics Engineers     

Computer simulation of stress‐induced crystallization in injection molded thermoplastics

Injection molding of semicrystalline plastics was simulated with the proposed stress‐induced crystallization model. A pseudo‐concentration method was used to track the melt front advancement. Stress relaxation was considered using the WFL model. Simulations were carried out under different processing conditions to investigate the effect of processing parameters on the crystallinity of the final part. The simulation results reproduced most of the experimental results in the literature. Comparison is made between the slow‐crystallizing polymer (PET) and fast‐crystallizing polymer (PP) to demonstrate the effect of stress on the crystallization kinetics during the injection molding process for materials with different crystallization properties. The results show that for fast‐crystallizing plastics, stress has little effect on the final crystallinity in the injection molded parts.     

Reactive mold filling in resin transfer molding processes with edge effects

Reactive mold filling is one of the important stages in resin transfer molding processes, in which resin curing and edge effects are important characteristics. On the basis of previous work, volume‐averaging momentum equations involving viscous and inertia terms were adopted to describe the resin flow in fiber preform, and modified governing equations derived from the Navier–Stokes equations are introduced to describe the resin flow in the edge channel. A dual‐Arrhenius viscosity model is newly introduced to describe the chemorheological behavior of a modified bismaleimide resin. The influence of the curing reaction and processing parameters on the resin flow patterns was investigated. The results indicate that, under constant‐flow velocity conditions, the curing reaction caused an obvious increase in the injection pressure and its influencing degree was greater with increasing resin temperature or preform permeability. Both a small change in the resin viscosity and the alteration of the injection flow velocity hardly affected the resin flow front. However, the variation of the preform permeability caused an obvious shape change in the resin flow front. The simulated results were in agreement with the experimental results. This study was helpful for optimizing the reactive mold‐filling conditions. © 2009 Wiley Periodicals, Inc. J Appl Polym Sci, 2009     

Effect of dynamic cross‐linking on melt rheological properties of polypropylene/ethylene‐propylene‐diene rubber blends

Study of melts rheological properties of unvulcanized and dynamically vulcanized polypropylene (PP)/ethylene‐propylene‐diene rubber (EPDM) blends, at blending ratios 10–40 wt %, EPDM, are reported. Blends were prepared by melt mixing in an internal mixer at 190°C and rheological parameters have been evaluated at 220°C by single screw capillary rheometer. Vulcanization was performed with dimethylol phenolic resin. The effects of (i) blend composition; (ii) shear rate or shear stress on melt viscosity; (iii) shear sensitivity and flow characteristics at processing shear; (iv) melt elasticity of the extrudate; and (v) dynamic cross‐linking effect on the processing characteristics of the blends were studied. The melt viscosity increases with increasing EPDM concentration and decreased with increasing intensity of the shear mixing for all compositions. In comparison to the unvulcanized blends, dynamically vulcanized blends display highly pseudoplastic behavior provides unique processing characteristics that enable to perform well in both injection molding and extusion. The high viscosity at low shear rate provides the integrity of the extrudate during extrusion, and the low viscosity at high shear rate enables low injection pressure and less injection time. The low die‐swell characteristics of vulcanizate blends also give high precision for dimensional control during extrusion. The property differences for vulcanizate blends have also been explained in the light of differences in the morphology developed. © 2000 John Wiley & Sons, Inc. J Appl Polym Sci 77: 1488–1505, 2000     

Online simulation‐based process control for injection molding

An online numerical simulation is presented that is capable of predicting state variables such as flow rate, melt temperature, shear rate, and melt viscosity by using real time data from a nozzle pressure sensor. The simulation solves the non‐Newtonian nonisothermal polymer flow into multicavity tools while executing rapidly enough for real time process control. Numerical accuracy and stability were first validated offline by comparing the online simulation to results obtained from a commercial mold filling simulation. Simulation‐based process control was then demonstrated by transferring a molding machine from fill to pack‐based on the predicted flow front position. The simulation‐based controller dynamically determined the appropriate transfer position for each part and transferred the machine at the correct time, thereby eliminating flash. The simulation, however, did increase process variability slightly due to delay times associated with the controller‐machine interface. A full factorial design of experiments (DOE) was performed varying injection velocity, mold temperature, and melt temperature. Results show that while the simulation dynamically adjusted the process on a part‐by‐part basis, it did not fully account for the process changes. Accuracy could potentially be improved by incorporating data from additional process sensors, by developing adaptive viscosity models, and by accounting for the melt compressibility. POLYM. ENG. SCI., 2009. © 2009 Society of Plastics Engineers     

Microstructure and crystallography in microcellular injection‐molded polyamide‐6 nanocomposite and neat resin

The effects of adding nanoclay to polyamide‐6 (PA‐6) neat resin, and the effects of processing parameters on cell density and size in microcellular injection‐molded components were investigated. In addition, the crystal sizes, structures, and orientation were analyzed with the use of x‐ray diffraction (XRD) and a polarized optical microscope. The standard ASTM D 638‐02 tensile bars for the analyses were molded according to a fractional four‐factor, three‐level, L9 Taguchi design of experiment (DOE) with varying melt temperature, injection speed, supercritical fluid (SCF) concentration, and shot size. It was found that the presence of montmorillonite (MMT) nanoclay greatly reduced the size of the cells and crystals, but increased their density in comparison with neat resin processed under identical molding conditions. In addition, at the sprue section downstream of the machine nozzle, cell size gradually decreased from the part center toward the skin for both the neat resin and the nanocomposite. It was also found that shot size was the most important processing parameter for both the neat resin and nanocomposite in affecting cell density and size in microcellular injection molding components. Weakly preferred crystal orientations were observed on the surface of microcellular injection‐molded PA‐6/MMT tensile bars. Finally, the addition of nanoclay in PA‐6 neat resin facilitated the formation of γ‐phase crystals in the molded components. Polym. Eng. Sci. 45:52–61, 2005. © 2004 Society of Plastics Engineers.     

Thin‐wall injection molding of polypropylene using molds with different laser‐induced periodic surface structures

In injection molding, high pressure is required to completely replicate the mold geometry, due to the viscosity of thermoplastic polymers, the reduced thickness of the cavity, and the low mold temperature. The reduction of the drag required to fill a thin‐wall injection molding cavity can be promoted by inducing the strong slip of the polymer melt over the mold surface, which occurs within the first monolayer of macromolecules adsorbed at the wall. In this work, the effects of different laser‐induced periodic surface structures (LIPSS) topographies on the reduction of the melt flow resistance of polypropylene were characterized. Ultrafast laser processing of the mold surface was used to manufacture nano‐scale ripples with different orientation and morphology. Moreover, the effects of those injection molding parameters that mostly affect the interaction between the mold surface and the molten polymer were evaluated. The effect of LIPSS on the slip of the polymer melt was modeled to understand the effect of the different treatments on the pressure required to fill the thin‐wall cavity. The results show that LIPPS can be used to treat injection mold surfaces to promote the onset of wall slip, thus reducing the injection pressure up to 13%. POLYM. ENG. SCI., 59:1889–1896, 2019. © 2019 Society of Plastics Engineers     

Model‐based melt flow virtual sensors for filling process of injection molding

Based on the model‐based virtual sensing approach, previously proposed by the authors, two melt flow virtual sensors are developed. The virtual sensors estimate the melt front position during the filling process by using easily obtained machine variables, specifically, the injection hydraulic pressure and the screw position and nozzle pressure. Both virtual sensors were theoretically analyzed and experimentally evaluated on a commercial injection molding machine. The theoretical analysis provides step‐by‐step design guidelines for the virtual sensors. Performance evaluation with a twin thin‐plate mold validates the feasibility of the proposed model‐based melt‐flow virtual sensors. POLYM. ENG. SCI., 2008. © 2008 Society of Plastics Engineers     

Computer simulation of the filling process in gas‐assisted injection molding based on gas‐penetration modeling

Gas‐assisted injection molding can effectively produce parts free of sink marks in thick sections and free of warpage in long plates. This article concerns the numerical simulation of melt flow and gas penetration during the filling stage in gas‐assisted injection molding. By taking the influence of gas penetration on the melt flow as boundary conditions of the melt‐filling region, a hybrid finite‐element/finite‐difference method similar to conventional‐injection molding simulation was used in the gas‐assisted injection molding‐filling simulation. For gas penetration within the gas channel, an analytical formulation of the gas‐penetration thickness ratio was deduced based on the matching asymptotic expansion method. Finally, an experiment was employed to verify this proposed simulation scheme and gas‐penetration model, by comparing the results of the experiment with the simulation. © 2003 Wiley Periodicals, Inc. J Appl Polym Sci 90: 2377–2384, 2003