Effects of the Water-Cement Ratio and the Molding Temperature on the Hydration Heat of Cement

The effects of the water-cement ratio and the molding temperature on the hydration heat of cement were investigated with semi-adiabatic calorimetry.The specimens were prepared with water-cement ratios of 0.31,0.38,and 0.45,and the molding temperature was specified at 10 and 20℃.The experimental results show that,as the water-binder ratio increases,the value of the second temperature peak on the temperature curve of the cement paste decreases,and the age at which the peak appears is delayed.The higher the water-cement ratio,the higher the hydration heat release in the early period of cement hydration,but this trend reverses in the late period.There are intersection points of the total hydration heat curve of the cement pastes under the influence of the water-cement ratio,and this law can be observed at both molding temperatures.With the increase in the molding temperature,the age of the second temperature peak on the temperature curve of the cement paste will advance,but the temperature peak will decrease.The higher the molding temperature,the earlier the acceleration period of the cement hydration began,and the larger the hydration heat of the cement in the early stage,but the smaller the total heat in the late period.A subsection function calculation model of the hydration heat,which was based on the existing models,was proposed in order to predict the heat of the hydration of the concrete.

Dependence of Lower Molding Temperature Limit and Molding Time on Molding Mechanism in Dental Thermoforming

Effectiveness and safety of a sports mouthguard depend on its thickness and material, and the thermoforming process affects these. The purpose of this study was to clarify the effects of differences in molding mechanisms on the lower molding temperature limit and molding time in dental thermoforming. Ethylene vinyl acetate resin mouthguard sheet and two thermoforming machines;vacuum blower molding machine and vacuum ejector/pressure molding machine were used. The molding pressures for suction molding were −0.018 MPa for vacuum blower molding and −0.090 MPa for vacuum ejector molding, and for pressure molding was set to 0.090 MPa or 0.450 MPa. Based on the manufacturer’s standard molding temperature of 95˚C, the molding temperature was lowered in 2.5˚C increments to determine the lower molding temperature limit at which no molding defects occurred. In order to investigate the difference in molding time depending on the molding mechanism, the duration of molding pressure was adjusted in each molding machine, and the molding time required to obtain a sample without molding defects was measured. The molding time of each molding machine were compared using one-way analysis of variance. The lower molding temperature limit was 90.0˚C for the vacuum blower machine, 77.5˚C for the vacuum ejector machine, 77.5˚C for the pressure molding machine at 0.090 MPa, and 67.5˚C for the pressure molding machine at 0.45 MPa. The lower molding temperature limit was higher for lower absolute values of molding pressure. The molding time was shorter for pressure molding than for suction molding. Significant differences were observed between all conditions except between the pressure molding machine at 0.090 MPa and 0.45 MPa (P < 0.01). A comparison of the differences in lower molding temperature limit and molding time due to molding mechanisms in dental thermoforming revealed that the lower molding temperature limit depends on the molding pressure and that the molding time is longer for suction molding than for pressure molding.

Effect of Pre-heating Temperature on Structural and Optical Properties of Sol-gel Derived Zn_(0.8)Cd_(0.2)O Thin Films

Zn_（0.8）Cd_（0.2）O thin films prepared using the spin-coating method were investigated. X-ray diffraction, scanning electron microscopy, and UV-Vis spectrophotometry were employed to illustrate the effects of the pre-heating temperature on the crystalline structure, surface morphology and transmission spectra of Zn_（0.8）Cd_（0.2）O thin films. When the thin films were pre-heated at 150 ℃, polycrystalline Zn O thin films were obtained. When the thin films were pre-heated at temperatures of 200 ℃ or higher, preferential growth of Zn O nanocrystals along the c-axis was observed. Transmission spectra showed that thin films with high transmission in the visible light range were prepared and effective bandgap energies of these thin films decreased from 3.19 e V to 3.08 e V when the pre-heating temperature increased from 150 ℃ to 300 ℃.

Effect of pouring and mold temperatures on hot tearing susceptibility of AZ91D and Mg-3Nd-0.2Zn-Zr Mg alloys

Pouring and mold temperatures are two important parameters during casting magnesium components. The present study examined their influence on hot tearing susceptibility （HTS） of commercial AZ91D and newly developed Mg-3Nd-0.2Zn-Zr （mass fraction, %; NZ30K） magnesium alloys in gravity permanent mold casting condition. The results indicate that mold temperature shows much more significant influence on the HTS of both alloys than pouring temperature whose influence only can be distinguished at low mold temperature （341 K for AZ91D alloy and 423 K for NZ30K alloy）. Hot tearing susceptibility prediction model concerning feeding parameters, grain size and solidification range, is more suitable to estimate the HTS of different magnesium alloys than the model only concerning feeding parameters. In order to achieve better hot tearing resistance, the ranges of pouring and mold temperatures are suggested to be 961-991 K and≥641 K for AZ91D alloy, 1003-1033 K and≥623 K for NZ30K alloy, respectively.

Crystallization Temperature and Crystallization Ratio of Mold Flux

The factors influencing the crystallization ratio of mold flux were researched by rapid cooling technolo gy, and the factors affecting crystallization temperature were studied by single thermocouple technique. The results showed that the crystallization ratio of mold flux increases with the basicity and the content of Na2O, CaF2, Li2O and NaF, and decreases with the increase of the content of Al2O3, MgO, BaO, MnO and B2O3. However, the crystallization temperature of mold flux rises with the basicity and the content of NaF, Na2O and CaF2, and reduces with the increase of the content of Al2O3, MgO, BaO, MnO and B2O3. But for Li2O, crystallization temperature decreases firstly to a minimum value at 2%, and then increases gradually with the increase of Li2O.

EXPERIMENTAL RESEARCH AND NUMERICAL SIMULATION OF MOLD TEMPERATURE FIELD IN CONTINUOUS CASTING OF STEEL

Mold is the heart of the continuous casting machine. Heat transfer and solidification in a water- cooled mold are the most important factors during the continuous casting of steel. For studying the temperature distribution of a mold wall, a simulated apparatus of mold was designed and experiments were performed by it. The measured results indicated that the mold wall temperature approaches the temperature of cooling-water. An equivalent thermal-conductivity coefficient was proposed and deduced on the basis of the conclusion of the experiments. This coefficient was applied to solve the heat transfer between the melt and cooling water, and to characterize the heat transfer capacity of the mold. By this equivalent thermal-conductivity coefficient, it is very easy and convenient to numerically simulate the solidification process of continuous casting. And the calculation results are in agreement with the experiments. The effects of custing speed and water flow rate on the mold temperature field were also discussed.

Experiments on rockburst proneness of pre-heated granite at different temperatures: Insights from energy storage, dissipation and surplus

Many underground engineering projects show that rockburst can occur in rocks at great depth and high temperature, and temperature is a critical factor affecting the intensity of rockburst. In general, temperature can affect the energy storage, dissipation, and surplus in rock. To explore the influence of temperature on the energy storage and dissipation characteristics and rockburst proneness, the present study has carried out a range of the uniaxial compression(UC) and single-cyclic loading-unloading uniaxial compression(SCLUC) tests on pre-heated granite specimens at 20℃-700℃. The results demonstrate that the rockburst proneness of pre-heated granite initially increases and subsequently decreases with the increase of temperature. The temperature of 300℃ has been found to be the threshold for rockburst proneness. Meanwhile, it is found that the elastic strain energy density increases linearly with the total input strain energy density for the pre-heated granites, confirming that the linear energy property of granite has not been altered by temperature. According to this inherent property, the peak elastic strain energy of pre-heated granites can be calculated accurately. On this basis, utilising the residual elastic energy index, the rockburst proneness of pre-heated granite can be determined quantitatively. The obtained results from high to low are: 317.9 k J/m^(3)(300℃), 264.1 k J/m^(3)(100℃), 260.6 k J/m^(3)(20℃), 235.5 k J/m^(3)(500℃), 158.9 k J/m^(3)(700℃), which are consistent with the intensity of actual rockburst for specimens. In addition, the relationship between temperature and energy storage capacity(ESC) of granite was discussed, revealing that high temperature impairs ESC of rocks, which is essential for reducing the rockburst proneness. This study provides some new insights into the rockburst proneness evaluation in high-temperature rock engineering.

Improvement of machining quality of copper-plated roll mold by controlling temperature variation

Micro prism film used in LCD industry can be manufactured by roll to roll method with copper-plated roll mold. As copper-plated roll mold is getting larger, pitch error is getting severer. The pitch error drops the quality of micro prism film. The main cause of the pitch error was investigated during machining large roll mold whose machined length was 1 200 mm. The temperature of machining system was elevated during machining roll mold, and this elevation induced thermal expansion of the system. The temperature variation around the roll mold also made thermal expansion of the roll mold. The amount of thermal expansion had strong relationship to the amount of pitch error. Therefore, the roll mold was machined after warming-up of machining system and precise temperature controller around copper-plated roll mold was installed, which minimized the temperature variation. Finally, precise micro prism patterns without pitch error were machined on the large roll mold.

Theoretical modeling of cutting temperature in high-speed end milling process for die/mold machining

A parametric model of cutting temperature generated in end milling process is developed according to the thermal mechanism of end milling as an intermittent operation, which periodically repeats the cycle of heating under cutting and cooling under non-cutting. It shows that cutting speed and the tool-workpiece engagement condition are determinative for tool temperature in the operation. The suggested model was investigated by tests of AlTiN coated endmill machining hardened die steel JIS SKD61, where cutting temperature on the flank face of tool was measured with an optical fiber type radiation thermometer. Experimental results show that the tendency of cutting temperature to increase with cutting speed and engagement angle is intensified with the progressing tool wear.

Neural Network Approach to Predict Melt Temperature in Injection Molding Processes

Among the processing conditions of injection molding, temperature of the melt entering the mold plays a significant role in determining the quality of molded parts. In our previous research, a neural network was developed to predict the melt temperature in the barrel during the plastication phase. In this paper, a neural network is proposed to predict the melt temperature at the nozzle exit during the injection phase. A typical two-layer neural network with back propagation learning rules is used to model the relationship between input and output in the injection phase. The preliminary results show that the network works well and may be used for on-line optimization and control of injection molding processes.

Flow and Temperature Fields in Slab Continuous Casting Molds

In order to develop super-board and super-thick slabs, the flow and temperatur fields were studied in slab continuous casting molds under different practical conditions, such as slab dimensions, with-drawing slab speed, design of nozzles, and superheat tempera-ture. The results showed that it is preferred to incline nozzle bores downwards and the submerged depth of the nozzles is best kept be-tween 250-300 mm. In addition, the solidified shell is thicker at the wide face than that at the narrow face, while the thin points alongthe wide face ekist both in the center and in the some area toward each respective end.

Effect of yttrium on microstructure and mold filling capacity of a near-α high temperature titanium alloy

The addition of rare earth yttrium(Y) can improve the performances of high temperature titanium alloys,such as the tensile ductility,thermal stability and creep property,etc.However,few studies on the effect of Y on the castability of titanium alloys have been carried out,which is significant to fabrication of thin-walled complex titanium castings by investment casting.In this study,the microstructure and mold filling capacity of a Ti-1100 alloy with different Y additions(0,0.1wt.%,0.3wt.%,0.5wt.% and 1.0wt.%) were investigated systematically through investment casting experiments,and the casting experiments were carried out in a centrifugal titanium casting machine.The microstructures of the alloy were observed via the optical microscopy,scanning electron microscopy and transmission electron microscopy.The mold filling capacity was tested by using of a grid pattern and was evaluated by the number of segments completely filled by the cast alloy.The results indicate that the grain size is decreased and the mold filling capacity is improved significantly with increasing the addition of Y from 0 to 1.0wt.%.The average primary grain size of Ti-1100 alloy is reduced from 250 μm to 50 μm and the mold filling capacity is increased from 61.5% to 100%.Considering the potential harmful effect on tensile properties of titanium alloys due to high concentrations of Y,it is suggested that Y addition should be about 0.3wt.%.

Numerical simulation of mold-temperature-control solidification

A finite element method(FEM) for the numerical simulation of the columnar part of the mould-temperature-control solidification(MTCS) process was presented. The latent heat was taken into account and 3D transient heat transfer analysis was carried out by using the developed FEM software. The relative errors between the numerical and experimental data are less than 6%. Three MTCS cases were computed with this method. The first case only opens the cooling channels in the bottom of the mold. The second case individually controls the separate 7 groups of cooling channels by giving 7 control points. When the temperature of a control point reaches the preset value of 400℃, the corresponding channel will be opened. The third case opens all the cooling channels at the same time. The results indicate that in the second case, the solid-liquid interface keeps near-planar. The growth velocity of the solid-liquid interface is 0.3-0.4 mm/s, which is greater than 0.1-0.3 mm/s of the first case, performing better than the others. Thus the forming quality and efficiency part interior can be improved by mold-temperature-control and the numerical model is validated. The numerical simulation of MTCS can provide an available tool for the advanced investigation on the defect improvement and the crystal’s quality.

Study on Meniscus Temperature Fluctuation during Mold Oscillation in Continuous Casting by Modeling Experiments

The temperature of the meniscus in continuous casting was measured during mold oscillation by modeling experiments. It is found that the temperature of meniscus varied periodically along with mold oscillation. Based on the above phenomenon, the mechanism of some techniques, such as, hot top mold, high frequency and low amplitude mold oscillation, soft contact mold electromagnetic continuous casting and so on, which can improve the quality of continuous casting billets, was analyzed. The results show that the decrease of the temperature fluctuation of early solidification shell is their common mechanism.

Numerical simulation of temperature and strength distributions of mold(core) on heating

By using Visual C++, a model with post processing was carried out to simulate the temperature and strength distributions of the mold(core). The results are shown in 256 color graphic mode. With this model, the temperature and strength distributions of the mold(core) both in case of heating process for core in the furnace and solidification process for a thin wall aluminum alloy casting in the mold(core) are numerically simulated. The results show that the temperature and strength distributions of the mold(core) were uneven because the thermal conductivity of the resin sand was much small. This study laid a basis for the optimum design of the mold(core) properties. [

Study on the mechanism and law of temperature,humidity and moisture content on the mechanical properties of molded fiber products

The change of temperature, humidity and moisture content (MC) will lead to the change of mechanical properties of molded fiber products (MFP). However, it is difficult to decouple the effects of temperature, humidity and MC on the mechanical properties of MFP, and predict the mechanical properties of MFP during the use. In this study, the laws and mechanism of mechanical properties of MFP with ambient temperature, humidity and MC were studied. The results showed that the direct effect of temperature (20−70 °C) on mechanical properties of MFP was insignificant, and the mechanical properties of MFP were mainly changed by MC. The MC was related to ambient temperature and humidity, and the relationship between the three could be described by the modified Guggenheim-Anderson-de Boer (GAB) model (20−70 °C and 30 %–90 % relative humidity). With the increase of MC, the elastic modulus and fracture strain was increased and decreased linearly, the yield strength and failure strength were presented GaussAmp laws, and the failure strain was presented asymptotic regressed distribution law. Two fracture modes of MFP, brittle fracture and ductile fracture, were revealed by the scanning electron microscopy of the mesoscopic fiber structure of sugarcane bagasse molded fiber products. The mathematical models and the changes of fiber structure were verified by wheat straw molded fiber products and waste paper molded fiber products. This study was contributed to understand the effects and mechanism of the change of temperature, humidity and MC on the mechanical properties of MFP.

基于Moldflow和SolidWorks的测温枪模流分析与模具设计

为降低模具设计的复杂程度,减少后期试模次数,基于Moldflow和SolidWorks软件对测温枪外壳进行模流分析与模具设计。根据测温枪外形分析与材料选择,设定合适的进浇位置和浇注方式,利用Moldflow软件仿真后得出模具的填充溶体流动分布图与注射压力时变图。利用SolidWorks软件完成模具模腔、模仁、推出机构、导向机构和冷却系统的设计计算。结果显示,测温枪外壳生产模具填充时间满足生产要求,注射压力在填充过程中变化幅度较小,有效避免了产品表面熔接痕的出现。

复合材料热压罐成型模具结构参数对温度场的影响

固化过程中热压罐内温度场分布的均匀性对于复合材料的结构性能、强度等方面具有重要影响。本文以目前航天制造业中广泛使用的用于制作复杂大型结构件的框架式模具作为研究对象,在COMSOL中建立了热压罐框架式模具在固化过程中的非定常三维温度场,研究了模具的不同筋板厚度和通风孔尺寸的情况以及型面下方半圆形散热孔存在与否的情况对模具型面温度场均匀性的影响。对比支撑筋板厚度分别为10,8,6和4 mm模具的温度标准差曲线发现:减小支撑筋板厚度可增加型面温度场均匀性,但模具整体稳定性随着筋板厚度变薄而迅速降低,更易发生受压失稳现象;对比纵向通风孔尺寸为130 mm×220 mm、150 mm×220 mm、170 mm×220 mm和190 mm×220 mm模具的温度标准差曲线发现:适当加宽筋板通风孔尺寸可提高型面温度场的均匀性,但宽度170 mm与190 mm的曲线基本重合,影响已不明显;同样型面下方散热孔的存在可以提高型面温度场的均匀性。

大规格泡沫铝锭成型模具轻量化设计

目的解决大规格泡沫铝模具(尺寸2800 mm×1000 mm×X mm)在成型过程中因极易发生塑性变形导致模具失效而造成模具寿命低、模具成本偏高的问题。方法基于数字模拟方法,实现模具轻量化设计。首先分析了泡沫铝模具的工作过程,确定了泡沫铝模具变形过程主要产生阶段;其次采用热弹塑性有限元法建立了泡沫铝模具冷却过程的有限元模型,得到了泡沫铝模具冷却过程,分别分析了冷却40、560、1330和4200 s时的温度场、最大塑性主应变场及位移场的分布情况和泡沫铝模具冷却过程中的变化规律,最后将模拟结果与实际的模具变形情况进行对比,泡沫铝模具变形方式与模拟情况基本吻合,再结合模拟轻量化模具结构,将模具用于实际验证。结果泡沫铝模具变形的主要原因为泡沫铝模具各部分冷却不均匀,导致其温度梯度大的地方产生了少量塑性变形。结论大规格泡沫铝锭成型模具可以通过优化模具结构、减轻模具重量来实现模具成本的降低。

一体化大型压铸模具温度控制方法

新能源汽车的发展使一体化压铸完成了从0到1的突破,给压铸模具发展带来了新的机遇。压铸模具是一个热交换器,压铸模具成型中的热平衡问题至关重要。以压铸模具的温度控制为基础,介绍了压铸模具的基本概况与压铸成型加工过程中热平衡的重要性,阐述了目前一体化压铸模模温控制所采用的模温控制岛、异形水路、红外线成像技术、模内传感器、一体化压铸模模温控制集成化等方案。模具温度的控制有利于提高铸件质量,延长模具使用寿命,提高压铸效率。