Modeling the effect of cooling system on the shrinkage and temperature of the polymer by injection molding

Injection molding is one of the most exploited industrial processes in the production of plastic parts. Shrinkage behavior of a molded plastic part plays an important role in determining final dimensions of the part. In this paper, the effect of the cooling system on the shrinkage rate of a polystyrene product during injection molding is carried out. A compressible fluid model for the physical system is presented. A finite volume numerical solution is used for the solution of the physical model. A validation of the numerical model in case of square cavity filled with polymer material is presented. The compressibility behavior of polymer material observed by the pressure, volume, and temperature for the state equation (P–v–T equation) is represented by Tait's equation. A Cross type rheological model depending on the temperature and pressure is assumed for the polymer material. The studied configurations consist of a mold having T-shaped plastic part and four cooling channels. Different cooling channels' positions are assumed and the effect of their positions on the cooling process is studied. The results indicate a good agreement between the numerical solution and those of the literatures. They also, show that the position of the cooling channels has a great effect on final product temperature and the shrinkage rate distribution throughout the product.     

Influence of using pulsed cooling for mold temperature control on microgroove duplication accuracy and warpage of the Blu-ray Disc

The usage of pulsed cooling for mold temperature to improve the molding process has been gaining increasing attention. In this study, Blu-ray Disc substrates, adapted for the commercial format media, were injection molded by combining pulsed-cooling technology. When applying pulsed cooling, coolant circulation is usually stopped during the melt-filling process and the mold opening and closing period. This leads to the additional cavity surface temperature increase and may vary part qualities. The correlations of mold temperature, cycle time, and pulse cooling duration with microgroove duplication accuracy and substrate qualities such as warpage in different directions were investigated in details. Measured results were also compared with those of injection-compression molded Blu-ray Disc using conventional cooling. The experiments showed that by using pulsed cooling in a proper manner one may manufacture Blu-ray Discs with lower warpage, higher accuracy in microgroove replication meanwhile reducing coolant temperature by 8 °C and shortening the cycle time by10% as compared to the conventional-cooling process. The usefulness of pulsed cooling and its potential in improving part quality and reducing cycle time during injection molding have been successfully demonstrated.     

Investigation of cooling effect at corners in injection molding

Many parameters influence the warpage developing at the corners of injection molded plastic parts. One of the main causes of this deformation is the asymmetrical cooling of the injection mold. This study presents an injection molding analysis of the heat flow developing in injection molds. The analysis shows that significant temperature difference appeared between the two sides of the mold after the hot polymer melt had filled the cavity. It was highlighted that the unevenness of the cooling should be considered during the mold design in order to avoid the warpage of the parts.     

Effect of asymmetric cooling system on in-mold roller injection molded part warpage

In-mold decoration (IMD) injection molding has been the most promising surface decoration technique in recent years, with the in-mold roller (IMR) injection molding being the most automated production process. During the IMR process, heat transfer in the cavity surface is significantly retarded because of the low thermal conductivity of film. As a result of the asymmetric melt and mold temperature, thermal-induced part warpage easily occurs. To understand the variation in the temperature field of the core and cavity caused by the plastic film, this research uses simulation and experiments to investigate the influence of the mold's (core-and-cavity) asymmetric cooling system temperature on product warpage, and examines the impact of the film's heat retardation effect on the crystallinity, tensile strength, and surface roughness of the treated products. Our results show that the film causes a higher contact temperature between the hot melt and mold during the molding process, resulting in asymmetric temperature in the mold (core and cavity), increasing the crystallinity of the cavity and consequently increasing product warpage. In plastic, the warpage increase is from 0.03 mm to 0.62 mm when the film thickness is 0.175 mm and the temperatures of the mold and hot melt are 50 °C and 230 °C, respectively, a great increase than with steel (P20). With the asymmetric cooling system design, in which the cavity temperature is 50 °C and the core temperature is 65 °C, the warpage can be reduced by 53%. For crystallinity and crystalline size, the film heat retardation effect of the IMR process increases the crystallinity of the cavity by 16%, and the crystallite size by 12%, along with some increase in tensile strength. In addition, the IMR process can also increase the smoothness of the product surface, reducing the surface roughness by 50%.     

Effect of cooling system on the polymer temperature and solidification during injection molding

Cooling system design is of great importance for plastic products industry by injection molding because it is crucial not only to reduce molding cycle time but also it significantly affects the productivity and quality of the final product. A numerical modeling for a T-mold plastic part having four cooling channels is performed. A cyclic transient cooling analysis using a finite volume approach is carried out. The objective of the mold cooling study is to determine the temperature profile along the cavity wall to improve the cooling system design. The effect of cooling channels form and the effect their location on the temperature distribution of the mold and the solidification degree of polymer are studied. To improve the productivity of the process, the cooling time should be minimized and at the same time a homogeneous cooling should be necessary for the quality of the product. The results indicate that the cooling system which leads to minimum cooling time is not achieving uniform cooling throughout the mould.     

Transient mold cooling analysis using BEM with the time-dependent fundamental solution

Cooling system design is of great importance for plastic injection molding because it significantly affects the productivity and quality of the final products. The purpose of mold cooling analysis is to determine the temperature profile along the cavity wall to improve the cooling system design. In this study, a fully transient mold cooling analysis formulation is developed using the boundary element method (BEM) based on the time-dependent fundamental solution. Then the cyclic transient cooling analysis is performed using this approach for a T-shape plastic part. Compared with the previous work done using finite element method or dual reciprocity boundary element method, domain discretization or use of points internal to the domain can be avoided.     

连铸机结晶器不同冷却方式的数值模拟

建立了方坯连铸机结晶器冷却过程的二维非稳态传热的数学模型，对结晶器在喷淋冷却与循环冷却下的冷却过程进行了模拟，对不同冷却方式的数值结果进行了对比，并得最喷淋冷却结晶器上的最佳热流分布。     

Simulation of injection-compression mold-filling process

A numerical algorithm is developed to simulate the filling state of injection-compression molding (ICM) process. Hele-Shaw fluid flow model combined with the control-volume/finite-element (CV/FEM) method is implemented to predict the melt front advancement and the distributions of pressure, temperature and flow velocity dynamically during the melt-filling process. Processing characteristics were understood by changing processing parameters including compression speed, switch time from injection to compression, compression stroke as well as initial cavity thickness using disk parts. The simulated molding pressures were also compared with those required by conventional injection molding (CIM) assuming the same entrance flow rate. It was found that the compression speed and compression stroke are the two factors affecting the molding pressure most significantly. Switch time also shows apparent effect on the pressure profiles. Using higher switch time, lower compression speed and higher compression stroke will result in lower cavity pressures. The melt velocity far from the gate was found to be higher than that near the gate during the compression stage contrary to that of CIM resulting in different part residual stress and melt temperature distribution. The simulated pressures for both ICM and CIM show good coincidence with those obtained from cavity pressure measurements.     

Flow and heat transfer characteristics in fuel cooling channels of a recooling cycle

Developing fuel with higher heat sink is widely carried out to meet the cooling requirement for an air breathing hypersonic vehicle. Especially, a recooling cycle has been newly proposed for an actively cooled scramjet to reduce the fuel flow for cooling. Fuel heat sink (cooling capacity) is repeatedly used to indirectly increase the fuel heat sink in a recooling cycle. The variation of fuel thermal property related with heat transfer and flow as temperature and pressure is added to the one-dimensional analytical fin-type model for rectangular ducts. Heat transfer performance comparison between recooling cycle and regenerative cooling is carried out, and detailed discussion about the variation and influence of heat transfer and flow characteristics caused by the introduction of the recooling process are discussed. Performance comparison between a recooling cycle at supercritical pressure and it at subcritical condition is also investigated.     

Modelling of a pin-fin heat converter with fluid cooling for power semiconductor modules

This paper presents a way to design a finite-element computer model of cooling system with a complicated geometry. The computer model is developed on the basis of a commercial software package ABAQUS. The steady state forced-convective fluid cooling of a pin-fin heat converter for power (∼1 kW heat power) semiconductor module has been investigated on the basis of computer simulation. A phenomenological equation has been used for calculation of the local value of the heat transfer coefficient for the liquid-solid interface. The impacts of the thermal conductivity of the pin-fin sink material, volume flow rate of the cooling liquid and geometrical design of the pin-fin sink on the thermal resistance of the converter are shown. Copyright © 2003 John Wiley & Sons, Ltd.     

Modelling and optimization of two-phase ejectors for cooling systems

A one-dimensional compressible flow model, which is based on the control volume approach, has been formulated to model and optimize one and two-phase ejectors in steady-state operation with particular reference to their deployment in a jet cooling system. The working fluid can be both single-component (NH₃) and two-component (NH₃–H₂O). The developed model takes into account the duct effectiveness, wall friction, momentum loss, ejector geometry, shock waves as well as the acceleration of the induced flow in the conical part of the mixing section. Neither the usual assumption of mixing at constant pressure over the mixing chamber cross section nor that of a constant mixing chamber cross section were made. A comparison with other computation methods as well as with available experimental data from the literature is presented. The performance is significantly influenced by the ejector geometry.     

Optimization of microchannel-based cooling systems

Abstract

In this article, microchannel systems for cooling applications such as in the thermal management of electronic equipment are investigated and optimized. Numerical simulations are carried out to study the conjugate heat transfer and flow behavior. The numerical model has been validated by comparing with analytical results. Two sets of design variables are evaluated: (Set I) Incoming flow rate and the number of channels and (Set II) Incoming flow rate and heat flux input. Response surfaces are used to represent the thermal and fluid behavior in the microchannel systems. Based on the polynomial response surface (PRS) modeling results, a multi-objective optimization problem is formulated to reduce both pumping power and thermal resistance. Two major practical concerns, hot-spot temperature and pressure difference, serve as optimization constraints. With varying weights on the two conflicting objectives, Pareto frontiers are obtained. It is also shown that an optimal configuration exists under pressure and temperature constraints. This study provides a feasible design domain and optimal solutions for microchannel-based cooling systems. The optimization process can also be applied to different applications of similar thermal systems.     

Comparative performance analysis of cogeneration gas turbine cycle for different blade cooling means

The paper compares the thermodynamic performance of MS9001 gas turbine based cogeneration cycle having a two-pressure heat recovery steam generator (HRSG) for different blade cooling means. The HRSG has a steam drum generating steam to meet coolant requirement, and a second steam drum generates steam for process heating. Gas turbine stage cooling uses open loop cooling or closed loop cooling schemes. Internal convection cooling, film cooling and transpiration cooling techniques employing steam or air as coolants are considered for the performance evaluation of the cycle. Cogeneration cycle performance is evaluated using coolant flow requirements, plant specific work, fuel utilisation efficiency, power-to-heat-ratio, which are function of compressor pressure ratio and turbine inlet temperature, and process steam drum pressure. The maximum and minimum values of power-to-heat ratio are found with steam internal convection cooling and air internal convection cooling respectively whereas maximum and minimum values of fuel utilisation efficiency are found with steam internal convection cooling and closed loop steam cooling. The analysis is useful for power plant designers to select the optimum compressor pressure ratio, turbine inlet temperature, fuel utilisation efficiency, power-to-heat ratio, and appropriate cooling means for a specified value of plant specific work and process heating requirement.     

Effects of coolant flow rates on cooling performance of the intermediate pressure stages for an ultra-supercritical steam turbine

The effects of the coolant flow rates on the thermal and flow fields of the first two intermediate pressure (IP) stages of an ultra-supercritical steam turbine was numerically investigated using the Conjugate Heat Transfer (CHT) method, and the un-cooled case was also simulated for comparison. The three-dimensional Reynolds-Averaged Navier–Stokes (RANS) solution was utilized to analyze the flow field and temperature distributions of the first two IP stages. The numerical results show that the steam cooling system has proved its ability to cool down the high temperature components, even in the case with the low coolant flow rate. The cooling performance can be improved with increased coolant flow rate, however, great flow losses would be induced and make the stage efficiency reduce by 1.23%. The different coolant flow rates also lead to great differences in the flow field in the front cavity of the first stage: ingress dominated flow condition with the low coolant flow rate and egress dominated flow condition with the high coolant flow rate. The negative influence of the ingested hot main steam on the cooling effect was also described.     

Effective Modeling of Conjugate Heat Transfer and Hydraulics for the Regenerative Cooling Design of Kerosene Rocket Engines

The present work is motivated to develop a unified framework to simulate multi-physical processes which are crucial for trade-off design of liquid rocket thrust chambers among propulsive performance, regenerative cooling, and pressure budget. In this paper, an effective modeling of conjugate heat transfer and hydraulics through the regenerative cooling passage has been performed to quantitatively evaluate detailed cooling designs, including spirally twisted channels and bidirectionally branched circuit, as well as to provide the wall heat flux to a compressible reacting flow solver in an interactively coupled manner. The kerosene fuel used as coolant is modeled by a three-component physical surrogate, and the fluid properties required for calculation of a Nusselt number correlation and empirical resistance coefficients are computed over the entire thermodynamic states from compressed liquid to supercritical fluid using the NIST SUPERTRAPP. The present method has been applied to an actual regeneratively cooled thrust chamber and validated against measurement of hot-firing tests in terms of temperature increase and pressure drop of the coolant through the cooling passages. Based on the numerical results, supplementary effects of peripheral fuel cooling injection and thermal barrier coating are addressed.     

An experimental and computational approach to the heat transfer mechanism in the cooling channel of a texturing process

An experimental and computational investigation of the cooling process during the heat set previous to the texturing has been presented. Different yarn speed values between 600 and 1400 m/min and inlet temperature values with two different channel material (austenite and ferrite steel) and fiber type (polyester and polyamide) have been compared. The convective heat transfer coefficients have been computed iteratively based on the measured values. To reach a reasonable relationship between the geometrical parameters, materials type and the heat transfer values, three-dimensional numerical simulations of the convective flow along the cooling channel were performed.     

A comparison of temperature distribution in PEMFC with single-phase water cooling and two-phase HFE-7100 cooling methods by numerical study

Thermal management has been considered as one of the critical issues in proton exchange membrane fuel cell (PEMFC). Key roles of thermal management system are maintaining optimal operating temperature of PEMFC and diminishing temperature difference over a single fuel cell and stack. Severe temperature difference causes degradation of performance and deterioration of durability, so understanding temperature distribution inside a single fuel cell and stack is crucial. In this paper, two-phase HFE-7100 cooling method is suggested for PEMFC thermal management and investigated regarding temperature change inside a fuel cell. Also, the results are compared to single-phase water cooling method. Numerical study of temperature distribution inside a single PEMFC is conducted under various conditions for the two different cooling methods. Fuel cell model considering mass transfer, electrochemical reaction and heat transfer is developed.The result indicates that two-phase HFE-7100 cooling method has an advantage in temperature maintenance and temperature uniformity than single-phase water cooling method, especially in high current density region. It is also revealed that the cell temperature is less dependent on system load change with two-phase cooling method. It indicates that the fuel cell system with two-phase cooling method has high thermal stability. In addition, the effect of coolant flow rate and coolant inlet pressure in two-phase HFE-7100 cooling method are discussed. As a result, two-phase cooling method showed reliable cooling performance even with low coolant flow rate and the system temperature increased as coolant pressure rose.     

Numerical modeling of a zeolite/water adsorption cooling system with non-constant condensing pressure

A new transient two-dimensional model with non-constant condensing pressure for a zeolite/water adsorption cooling cycle is proposed in this paper. This numerical model focuses on the heat and mass transfer behaviors in the adsorber and is solved by the control volume method. Due to the heat transfer limitation in the condenser, the simulated pressure during the isobaric generation phase of the cycle is not constant and will decrease with time. Compared with the model for constant condensing pressure, the cycle duration and cycled adsorbate for the base case are increased. Furthermore, the effect of mass flow rate of condenser cooling water on system performance is also investigated. It is found that both COP and SCP increase with an increase in the mass flow rate of cooling water in the condenser.     

Application of CFD to closed-wet cooling towers

Computational fluid dynamics (CFD) is applied to predicting the performance of closed-wet cooling towers (CWCTs) for chilled ceilings according to the cooling capacity and pressure loss. The prediction involves the two-phase flow of gas and water droplets. The predicted thermal performance is compared with experimental measurement for a large industrial CWCT and a small prototype cooling tower. CFD is then applied to the design of a new cooling tower for field testing. The accuracy of CFD modelling of the pressure loss for fluid flow over the heat exchanger is assessed for a range of flow velocities applied in CWCTs. The predicted pressure loss for single-phase flow of air over the heat exchanger is in good agreement with the empirical equation for tube bundles. CFD can be used to assess the effect of flow interference on the fluid distribution and pressure loss of single- and multi-phase flow over the heat exchanger.     

Mist film cooling simulation at gas turbine operating conditions

Air film cooling has been successfully used to cool gas turbine hot sections for the last half century. A promising technology is proposed to enhance air film cooling with water mist injection. Numerical simulations have shown that injecting a small amount of water droplets into the cooling air improves film-cooling performance significantly. However, previous studies were conducted at conditions of low Reynolds number, temperature, and pressure to allow comparisons with experimental data. As a continuous effort to develop a realistic mist film cooling scheme, this paper focuses on simulating mist film cooling under typical gas turbine operating conditions of high temperature and pressure. The mainstream flow is at 15 atm with a temperature of 1561 K. Both 2D and 3D cases are considered with different hole geometries on a flat surface, including a 2D slot, a simple round hole, a compound-angle hole, and fan-shaped holes. The results show that 10–20% mist (based on the coolant mass flow rate) achieves 5–10% cooling enhancement and provides an additional 30–68 K adiabatic wall temperature reduction. Uniform droplets of 5–20 μm are used. The droplet trajectories indicate the droplets tend to move away from the wall, which results in a lower cooling enhancement than under low pressure and temperature conditions. The commercial software Fluent is adopted in this study, and the standard k–ε model with enhanced wall treatment is adopted as the turbulence model.