Enhancement of induction heating efficiency on injection mold surface using a novel magnetic shielding method

Mold temperature is a major factor in the quality of injection molding process. A high mold temperature setting is feasible to enhance the molding quality but prolongs the cooling time. Induction heating is the method currently used to heat the mold surface without increasing the molding cycle. However, one unresolved problem of induction heating is the proximity effect resulting from two adjacent coils with different current directions. The proximity effect substantially decreases heating efficiency, which then causes non-uniform heating. This effect is difficult to avoid in a single-layer coil. The most common solution, which is to use magnetic concentrators to reduce the proximity effect, does not obtain satisfactory results. In the novel magnetic shielding induction heating method developed in this study, heating efficiency and temperature uniformity are enhanced by using ferrite materials to separate the conflicting magnetic fields caused by the repulsive proximity effect. Three typical single-layer coils are investigated in this study, including a reciprocated single-layer coil, a single-layer spiral coil, and a rectangular frame coil. Appropriate placement of ferrite materials on these induction coils successfully eliminated the proximity effect, increased the heating rate, and improved temperature uniformity.     

Induction heating with the ring effect for injection molding plates

Induction heating in injection molding has the advantages of rapid heating, reduced cycle time, and improved product quality. In this research, using both experiment and simulation, externally wrapped coil induction heating was applied to verify the heating capacity of a pair of mold plates. By applying different coil designs and mold gap, the effect of the externally wrapped coil induction heating was evaluated. Results showed that when a serial coil was used as an inductor, the heating rate reached 8.0 °C/s. From an initial mold temperature of 40 °C, after 15 s heating, the mold surface temperature reached 159.9 °C with the serial coil. The parallel coil shows a better heating uniformity but its heating rate is far lower than the serial coil. For the serial coil, the temperature distribution between the core and cavity plate are almost the same. The heating rate increases from 4.9 °C/s to 10.6 °C/s when the inductor design is changed from 5 turns to 7 turns. After 15 s heating, the temperature at point T2 increases from 40 °C to 166.7 °C and 106.1 °C with a mold gap of 1 mm, and 6 mm, respectively.     

Comparison between single- and multiple-zone induction heating of largely curved mold surfaces

This study applied three-dimensional steady-state finite-element numerical simulations of electromagnetic fields and temperature distributions to evaluate the effects of various coil geometries, regional depositions, and magnetic shielding materials on the induction heating of a curved mold plate surface used for fabricating automotive spoilers. Conventionally, the induction heating of large mold surfaces by using a set of long inductive coils entails employing a costly, high-power induction heating device. This study proposes a multizone induction heating approach that entails dividing a target surface into several zones and then applying numerous sets of short inductive coils that require only low-power induction heating devices to the individual regional zones for heating. In this approach, the coil design is relatively simple for efficiently heating these small-area zones. The simulation results are described as follows: (1) the geometry of the inductive coils with respect to the processed workpiece demonstrated a considerable effect on the electromagnetic field distribution and the heating efficiency of the system. (2) Magnetic shielding materials facilitated eliminating the proximity effect, which produces a nonuniform heating pattern along the workpiece wall. (3) Compared with single-zone induction heating, the multiple-zone induction heating of a largely curved mold surface enhanced the heating rate and uniformity performance.     

Feasibility evaluation of gas-assisted heating for mold surface temperature control during injection molding process

Dynamic mold surface temperature control has the advantage of improving molded part qualities without significant increases in cycle time. In this study, a gas-assisted heating system combined with water cooling and different mold designs to achieve dynamic mold surface temperature control was established. The feasibility of using gas-assisted heating for mold surface temperature control during the injection molding process was then evaluated from experimental results. The effect of mold design as well as heating conditions including hot gas temperature, gas flow capacity, and heating time on the heating efficiency and the distribution uniformity of mold surface temperature were also studied. Results showed that as hot gas temperature and gas flow capacity increased, as well as increasing heating times from 2 s to 4 s, mold surface temperature increased significantly. Fan shaped gas channel design exhibits better mold surface temperature distribution uniformity than tube shaped gas channel design. During gas-assisted heating/cooling, it takes 2 s to increase mold surface temperature from 60 °C to 120 °C and 34 s for mold surface to return to 60 °C. In addition, under specified heating conditions and using the best composite mold designs, the heating rate can reach up to 30 °C/s, a rate well-suited to industrial applications.     

感应加热温度均匀性影响因素的实验研究

镍基高温合金FGH4096是高性能发动机涡轮盘、环形件及其他热端部件的关键材料。以其为研究对象,搭建感应加热实验平台,设计不同工况,通过实验方法研究感应加热过程中影响合金试棒温度分布均匀性的因素。实验结果表明:在试棒两端安装陶瓷片对其温度分布均匀性影响不大;用保温棉包裹试棒,适当降低加热速率可以提高合金试棒温度场分布均匀性;感应线圈尺寸对合金试棒温度分布均匀性影响较大。     

钢板横向感应加热磁-热耦合的有限元数值模拟

采用电磁-热耦合二维有限元法模拟钢板感应加热,得到了加热频率和电流密度对加热时间、加热速度和温度均匀性的影响。结果表明,采用U型线圈加热钢板,可以形成一个完整的磁回路,符合感应加热钢板的要求;频率增加能够缩短加热时间和扩大钢板温度的温差;而电流密度增加,加热时间缩短,其对温度均匀性的影响则随电流增加温差增大。     

Mold temperature control using high-frequency proximity effect induced heating

A rapid heating in an injection molding cycle has the advantage of improving product quality without significant increase in cycle time. In this study, high-frequency proximity effect induced heating (HFPEIH) was developed and combined with water cooling to achieve dynamic mold surface temperature control. By applying the HFPEIH system on a pair of mold plates separated with a small gap, the relevant influence of HFPEIH design was evaluated under various parameters including different mold plate material, inductor designs, and inductor channel depths beneath mold surface as well as mold separations. Simulation was also conducted and verified with experiments. Results show that all the heating rates range within 2 °C/s to 4 °C/s for the mold plate size of 100 mm by 100 mm. For the inductor design with three channels of circular cross section, the heating rate is fastest whereas one inductor design of rectangular shape exhibits the best the uniformity of temperature distribution. When the channel depth is reduced from 12 mm to 4 mm, the heating rate is increased significantly. The heating rate is also sensitive to mold plate surface area. When stainless steel N700 was used as the plate materials in a smaller plate of 60 mm by 60 mm, the heating rate can reach 7.6 °C/s using one channel inductor design. The mold separation exhibits that it is less sensitive to the heating rate within 1 mm to 5 mm range and when it is greater than 5 mm, the heating rate starts to decrease slightly. All the simulated results show good coincidence with experimental measurements.     

Rapid mold temperature control in injection molding by using steam heating

The rapid heating cycle has the advantage of improving product quality in injection molding. In this study, steam heating was combined with cool water on the same mold design to achieve dynamic mold surface temperature to establish control. By applying the steam system on a TV housing mold, the advantage of using steam heating for injection molding was then evaluated and compared with water heating by experiment and simulation. The effect of steam on the quality of the part was also studied. Results showed that as steam was used, the heating time of the simple mold plate can be reduced from 18 s to 8 s with the heating rate of 9 °C/s, and the cooling time is reduced over water heating. When the target temperature is changed from 70 °C to 110 °C, the heating time of the TV housing mold plate varies from 7 s to 19 s. For the product quality, steam heating showed an improvement in both the gloss and hardness of the TV housing.     

Nonplanar mold surface heating using external inductive coil and magnetic shielding materials

A set of three-dimensional steady-state finite element numerical simulations of temperature distributions was performed for various coil geometries with and without magnetic shielding materials on a nonplanar mold plate surface subjected to induction heating. The surface was used for fabricating a miniature automotive spoiler. A comparison between the simulated and experimental results showed that the location of the inductive coils relative to the processed workpiece remarkably influenced the temperature distribution and heating efficiency. In particular, the proximity effect, which leads to nonuniform heating along the workpiece wall, was ameliorated by introducing magnetic shielding materials on the coil design.     

Cooling an array of multiple heat sources by a row of slot air jets

An experimental study of cooling an array of multiple heat sources simulating electronic equipment by a single row of slot air jets positioned above a critical row (row having maximum heat dissipation rate) of the array was conducted. The other low power rows of the array were cooled by the spent air flow from the air jets. The experimental work was carried out in two phases. In the first phase, each block of the array was heated at a time and the other blocks of the array were kept unheated. The Nusselt number of each heated block and the thermal wake effect on downstream blocks were investigated and correlated for different values of jet Reynolds number, position of the block with respect to the jet impingement point and the separation distance between the orifice plate and the impingement surface. A superposition technique was implemented to demonstrate the practical importance of the present correlations in predicting the operating temperature of any block in an array with multiple heated blocks. In the second phase, the experiments were carried out with heating all the blocks at the same time. This phase was carried out to verify the superposition technique used to predict the operating temperature of the blocks of the array in the case of the multiple heating.     

Analysis of thermal performance of electromagnetic induction based molten salt heating system

将电磁感应加热技术应用到低谷电加热熔盐储热供暖领域,搭建熔盐电磁感应加热实验系统,以感应加热器为研究对象,探究熔盐以及线圈冷却水在不同熔盐流速和线圈电流工况下的温度变化规律,计算加热效率和冷却水热损失率。结果表明：电磁感应加热器可以快速加热熔盐,熔盐温升主要集中在开始加热80～240 s之间,温升速率在100 s时最大;改变线圈电流或熔盐流速,可以产生不同终温的熔盐,流速0.177 m/s时,熔盐在不同电流下出口温度分别为201.452 ℃、203.891 ℃、207.599 ℃、212.975 ℃和221.454 ℃;熔盐流速一定,熔盐和线圈冷却水吸热量随线圈电流的增加而升高;线圈电流不变,熔盐吸热量随流速的增加而升高、线圈冷却水吸热量随流速增加而降低;熔盐流速0.296 m/s、线圈电流600 A时,熔盐加热效率为69.28%,线圈冷却水热损失率为16.45%。     

An Investigation of Optimized Length Scales of the Heat Treatment of Metallic Plates

A moving metallic plate subject to heating and cooling boundary conditions is considered in this work. The plate is heated by an imposed heat flux, and cooled down by an array of impinging jets through convection and radiation. The objective of the present work is determination of operating conditions for controlling the temperature distribution at the end of both heating and cooling sections. The results show that the temperature distribution becomes more uniform across the heating section with an increase in the heating length. An increase in the distance from the impinging jet to the plate causes an increase in the temperature values across the cooling section, and a decrease in the diameter of the impinging jet causes a decrease in the temperature values across the cooling section. It is also shown that an increase in cooling length and the addition of another impinging jet help to reduce the temperature values and increase the uniformity of the structure across the cooling section. Optimized values of the pertinent parameters for both hardening and tempering heat treatments were investigated.     

Numerical simulation of mixed convective heat transfer of a molten salt in a square tube with a thermally conductive wall

熔盐因具有传热能力强、工作温度高、使用温度广、系统压力低、经济适用等优点,成为太阳能热发电系统蓄热工质的理想选择。熔盐在实际应用中会因加热过程的非均匀性产生存在于固体表面和流体间的温差,造成流体工质中的密度梯度,因此出现重力导致的浮升力效应,其叠加到主流流动方向上即形成混合对流。管壁导热会对熔盐混合对流传热过程产生一定的影响。本文对熔盐在水平方管内非均匀加热条件下的单面加热的混合对流过程进行了数值模拟研究,在考虑壁厚的情况下研究了方管单面加热熔盐混合对流传热特性,分析了无量纲参数间的变化关系,并将结果与流型判定图和经典关联式进行对比。结果表明,非均匀加热时,浮升力效应会造成随流动距离增加主流核心区域的形状发生改变,且更加靠近加热壁面。Nu数随Re数、Ri数的增大而增大,局部Nu_x数随流动距离的深入先减小后增大。与忽略管壁导热数值模拟结果相比,主流核心区形状更加均匀,局部Nu_x更高且回升位置更加提前,流动特性和传热特性基本保持一致。     

EVALUATION OF THE DYNAMIC PERFORMANCE OF A HOT WATER TANK WITH BUILT-IN HEATING COIL

Hot water tanks with a built-in water-heating coil are commonly used in district heating house stations in Denmark for domestic hot water (DHW) production and storage. In this study, an evaluation of the dynamic performance of a hot water tank with built-in heating coil is carried out by applying a dynamic simulation programme which has been made previously, based on a simple dynamic model developed by the authors. System evaluation of the way in which system parameters, such as control valve size, heat loss coefficient of the DHW circulation pipe, position of the temperature sensor (for DHW temperature control) and fouling of the heating coil, affect the domestic hot water capacity and the average district heating water cooling for a given hot water tank is presented and discussed in this paper. The evaluation results show the importance of the correct design of the control valve size, the reduction of heat loss from DHW circulation pipes, the careful adjustment of temperature sensor position and temperature sensor set-point, and the reduction of the heat coil fouling growth rate in order to operate the hot water tank in an efficient way and to achieve significant cooling of the district heating water. © 1997 by John Wiley & Sons, Ltd.     

不同加热功率下充液率对无芯环路热管性能的影响

设计了以铝为管材、丙酮为传热工质的无芯环路热管。其蒸发段采用加热带加热,冷凝段用风冷降温。热管依靠蒸发压头使工质循环,并依靠重力作用,使冷凝液回流到蒸发段。搭建试验台并研究了不同加热功率下充液率对无芯环路热管的传热温差、传热量、热效率、热阻和当量导热系数的影响。结果表明:加热功率为150.00 W、充液率为30%时,无芯环路热管的均温性最好;传热温差和热阻均最小,分别为6.75℃、0.045 K/W。传热量132.00 W、热效率0.88、当量导热系数168 125 W/(m·K),均达到最大值。所以,该无芯环路热管在本实验研究范围内的最佳工作条件为加热功率150.00 W、充液率30%。     

The heat transfer and fluid flow of a partially heated microchannel heat sink

Numerical modeling of the conjugate heat transfer in microchannel heat sink is presented. As the most of the cooling applications deals with the partial heated sections, the influence of the heating position on the thermal and hydrodynamic behavior is analyzed. The laminar fluid flow regime and the water as a working fluid are considered. It is observed that partial heating together with variable viscosity has a strong influence on thermal and hydrodynamic characteristics of the micro-heat sink.     

Visualization of film wavelike characteristics and measurement of film thickness in spray cooling

An experimental investigation was performed to study the heat transfer in an eight-nozzle spray cooling system with de-ionized water as the working fluid. Visualization of the liquid-solid contact area and the flow near the heated surface was made using a microscopic lens system in conjunction with an advanced high-speed camera. The film thickness and film wavelike characteristics under liquid volume flow rates ranged from 2.78×10 -6 m 3 /s to 1.39×10 -5 m 3 /s and surface temperatures between 22℃ and 78.2℃ were examined respectively. The development process of the liquid film on the heated surface was observed. The local mean film thickness, the film wavelike characteristics and the behavior of the bubbles appeared in the liquid film were captured using an image processing technique. It is discovered that there exists a climax of local mean film thickness during the starting process of spray cooling. When the liquid film reaches the dynamic stable state, the dimensionless mean film thickness decreases with the increase of the liquid volume flow rate, and increases with the increase of surface temperature generally. Besides, the volume flow rate has a more significant impact on the wavelength and amplitude of the liquid film compared to the surface temperature.     

Cooling characteristics of an impinging spray jet which forms an ellipsoidal liquid film

The cooling characteristics of an impinging spray jet which forms an ellipsoidal liquid film were experimentally investigated in order to estimate the cooling performance of a rotating roll in a hot mill system. The following four conclusions were reached in the study. (1) In the case of a single spray jet, the local heat transfer coefficient at the center position depends on the forced convective heat transfer by the impinging jet. However, the average heat transfer coefficient is proportional to the flow rate density of the cooling water, and it does not depend on the distance between the nozzle and heated surface. (2) In the case of a double spray jet, liquid film interference occurs. The local heat transfer coefficient at the center position is greater, and the cooling performance increases with the increasing flow rate density of the cooling water. (3) The cooling performance of a multispray jet is proportional to the flow rate density of the cooling water. It does not depend on the nozzle construction, distance, or specifications. Also, there is no relation to the liquid film interference. (4) When the optimum specifications of the spray nozzle are used, thermal analysis of a rotating roll shows that the temperature at a depth of 1.3 mm from the surface is below 130 °C. © 2000 Scripta Technica, Heat Trans Asian Res, 29(4): 280–299, 2000     

Experimental study of impingement spray cooling for high power devices

An experimental study of a closed-loop impingement spray cooling system to cool a 1 kW 6U electronic test card has been conducted. The system uses R134a as working fluid in a modified refrigeration cycle. The spray from four vapor assisted nozzles is arranged to cover a large ratio of the heated area of the card. Investigations are currently focused on effects of mass flow rate, nozzle inlet pressure and spray chamber pressure. Experimental results are promising with a stable average temperature of around 23 °C being maintained at the heated surface, and maximum temperature variation of about 2 °C under suitable operating conditions. Heat transfer coefficients up to 5596 W/m² K can be achieved with heat flux input around 50,000 W/m² in this study. It is found that cooling performance improved with increasing mass flow rate, nozzle inlet pressure and spray chamber pressure, whereas uniformity of the heated surface temperature can only be improved with higher mass flow rate and nozzle inlet pressure. The mechanisms for the enhanced performance are also presented.     

高强度鱼尾螺栓锻造加热用感应器的设计

介绍了高强度鱼尾螺栓锻造加热时所用感应器的设计。本感应器为缝隙式，使用频率２５００Ｈｚ，功率２００ｋＷ、９匝。进行感应器的电计算时，采用了一种近似方法。试验结果表明，计算与实际调试结果基本符合，满足了螺栓联合自动机对加热节拍的要求，大幅度提高了生产效率。