Sheet Temperature Distribution Research Articles

Transient thermal analysis of close pressure thermoforming process

Deformed configuration of sheet (a) numerically predicted and (b) photograph of the experimentally obtained deformed shape. Deformation of a PMMA sheet to the hemispherical dome of radius 55 mm is obtained by applying a pressure of 0.16 N/mm 2 in experiment, whereas the numerically predicted applied pressure is 0.18 N/mm 2 to produce the same hemispherical dome. • Studied pressure thermoforming process to find the required optimum cycle time. • Thermoforming process is analysed in three stages of heating, forming and cooling. • Validated numerically obtained sheet temperature with heating stage experiment. • Studied the effect of contact conditions on heating cycle by analyzing heat transfer rate. The processing of polymer sheets above the glass transition makes the thermoforming process temperature sensitive. The temperature distribution of the sheet has a significant impact on the mechanical behavior of the raw sheet during the process of transformation into the final product. It governs the aesthetic characteristics of the product formed, as well as its functionality. Thus, it is imperative to investigate the change in the thermal state of the sheet during processing. Various devices are available to monitor the temperature distribution of sheet with the requirement of direct access to the sheet surface, which not possible in the case of a closed thermoforming setup. Hence, in this study, the thermal state of the PMMA sheet during conversion into a hemispherical dome using a closed pressure thermoforming setup is investigated numerically. To incorporate the temperature dependence, temperature-dependent material properties, along with the necessary contact conditions, were used. It was found that radiation is the dominating mode of heat transfer, and for a most accurate result, realistic contact conditions must be used. Moreover, an experimental investigation was performed, and it was found that numerically predicted sheet temperature has good agreement with the measured sheet temperature. We proposed numerical simulation as a tool to monitor sheet temperature for the closed thermoforming setup where thermal monitoring is not possible without degrading the process output.

Experimental study on incremental sheet forming of magnesium alloy AZ31 with hot air heating

Products of tomorrow are characterized by environmental sustainability, individuality and economic feasibility. This trend also occurs in the field of sheet metal forming. Lightweight materials such as magnesium alloys are applied to reduce the weight and concluding the CO2 emissions of cars. They are characterized by a high specific strength and almost inexhaustible availability. Applying them in innovative forming technologies offers the chance for various new products. Flexible forming technologies such as single-point incremental forming (SPIF) have been newly developed to meet the requirements of decreasing lot sizes and a rising variety of products and design. The application of SPIF on magnesium alloys is considered to be an outstanding approach to obtain innovative sheet metal products. For this reason, SPIF of the magnesium alloy AZ31 with a hot air heating system is investigated experimentally. The mechanical properties are investigated as well as the initial microstructure and texture of the AZ31. An experimental setup is designed for SPIF using a hot air blower for heating of the AZ31 sheet. The setup is tested in order to obtain a homogeneous sheet temperature distribution. Forming trials are performed to determine the critical wall angle of a cone for temperatures up to 320 °C. During the experiments the occurring forming forces are investigated. Subsequently, an analysis is conducted of the sheet thickness distribution, the geometrical accuracy and the microstructure development.

Experimental and Numerical Determination of Hot Forming Limit Curve of Advanced High-Strength Steel

This paper studied the hot formability of the advanced high-strength steel B1500HS. The hot Nakazima tests were conducted to obtain the forming limit curve (FLC), and the sheet temperatures were recorded to analyze temperature distributions during deformation. Meanwhile, the numerical simulations of hot Nakazima tests were performed to compare with the experimental ones. By utilizing the commercial software, Abaqus, the punch force–displacement curve, sheet temperature distribution at the time of the maximum punch load and temperature path of the necked element were investigated from both of experiments and numerical simulations. The FLCs from experiment and numerical simulation showed a good agreement. The temperature path of the necked element on each FLC specimen was different due to the numerical stretching time and stress state. This study demonstrated the predictive capability of finite element simulation on hot stamping.

Investigation on the hot formability of TA15 titanium alloy sheet

The hot formability of TA15 titanium alloy sheet was studied experimentally and theoretically. The hot uniaxial tensile tests were conducted for further investigation on hot formability. Based on the finite element models in different thermal conditions, the punch displacement-force curves were compared to test data to investigate the influence of sheet temperature distribution on limit strain. The simulation results indicated that the hot punch was essential for keeping the temperature of sheet center region. Meanwhile, the non-uniform sheet temperature distribution that the temperature of central sheet was higher than the one at sheet edge, was positive for making the damage occur at an appropriate region due to the good deformability of central sheet with higher temperature. The theoretical FLCs of TA15 titanium alloy sheet were predicted by introducing Hill48 yield criterion and temperature path into M-K model. The results proved that the monotonic decreasing temperature path had no effect on sheet formability, and the Nakazima test results depended on the final temperature of region at where the fracture or necking occurred. The comparison results also indicated that the test data agreed well with the theoretical FLC at 810°C.

Considering thermal-viscous collapse of the Greenland ice sheet.

We explore potential changes in Greenland ice sheet form and flow associated with increasing ice temperatures and relaxing effective ice viscosities. We define “thermal‐viscous collapse” as a transition from the polythermal ice sheet temperature distribution characteristic of the Holocene to temperate ice at the pressure melting point and associated lower viscosities. The conceptual model of thermal‐viscous collapse we present is dependent on: (1) sufficient energy available in future meltwater runoff, (2) routing of meltwater to the bed of the ice sheet interior, and (3) efficient energy transfer from meltwater to the ice. Although we do not attempt to constrain the probability of thermal‐viscous collapse, it appears thermodynamically plausible to warm the deepest 15% of the ice sheet, where the majority of deformational shear occurs, to the pressure melting point within four centuries. First‐order numerical modeling of an end‐member scenario, in which prescribed ice temperatures are warmed at an imposed rate of 0.05 K/a, infers a decrease in ice sheet volume of 5 ± 2% within five centuries of initiating collapse. This is equivalent to a cumulative sea‐level rise contribution of 33 ± 18 cm. The vast majority of the sea‐level rise contribution associated with thermal‐viscous collapse, however, would likely be realized over subsequent millennia.

Study on temperature distribution of HSS in hot FLD test

In this paper, we study forming limit diagram (FLD) of high strength steel (HSS) under high temperature condition to simulate the hot stamping process. We investigate the sheet specimen temperature distribution in hot FLD test. First, we propose a simple model of heat transfer process in sheet specimen, in which the sheet specimen is separated into two sections due to different mechanism. In thermal radiation, the convection and thermal contact conductance are included in two partial differential equations (PDEs) to express the temperature distribution in these two sections (S1 and S2 sections). Second, hot FLD experiments (Nakazima test) are performed and temperature distributions of sheet specimen are investigated by thermal infrared imager. Furthermore, the experimental temperature distributions are compared with theoretical ones, and these two measurements make good agreement between each other. This conclusion validates the feasibility of proposed thermal model in hot stamping process. In addition, an uneven sheet temperature distribution is examined in expansion process experiment, we observe that the temperature of specimen increases from center to edge until it reaches the temperature of punch. The further theoretical analysis proves that this phenomenon is mainly caused by the punch through thermal contact conductance between punch and S1 section, which indicates that a hot punch is indispensable to sustain the temperature of the deforming sheet. Third, the theoretical temperature distribution on S1 section under different die height is calculated. The radiation heat flux of the sheet decreases as the die height is increasing, but the sheet temperature increment is small, which implies that the height of die affects the sheet temperature slightly.

Process Parameter Identification in Thin Film Flows Driven by a Stretching Surface

The flow of a thin liquid film over a heated stretching surface is considered in this study. Due to a potential nonuniform temperature distribution on the stretching sheet, a temperature gradient occurs in the fluid which produces surface tension gradient at the free surface of the thin film. As a result, the free surface deforms and these deformations are advected by the flow in the stretching direction. This work focuses on the inverse problem of reconstructing the sheet temperature distribution and the sheet stretch rate from observed free surface variations. This work builds on the analysis of Santra and Dandapat (2009) who, based on the long-wave expansion of the Navier-Stokes equations, formulate a partial differential equation which describes the evolution of the thickness of a film over a nonisothermal stretched surface. In this work, we show that after algebraic manipulation of a discrete form of the governing equations, it is possible to reconstruct either the unknown temperature field on the sheet and hence the resulting heat transfer or the stretching rate of the underlying surface. We illustrate the proposed methodology and test its applicability on a range of test problems.

Time-Dependent Temperature Distribution of Sheets in the Stage of Storing

Obtaining a uniform thickness of the final product using thermoforming is difficult, and the thickness distribution depends strongly on the distribution of the sheet temperature. In this paper, the time-dependent temperature distribution of the total sheets in the stage of storing was studied using the analysis code which has been verified based on the experimental data. It was found out that the time for storing should be managed for decreasing the temperature difference between the sheets. The time-dependent temperature distribution of the total sheets and the method for analysis in this study will be used to improve the quality of the final products.

Experimental study and numerical simulation of preform or sheet exposed to infrared radiative heating

Thermoplastic processing like the injection stretch blow moulding and thermoforming processes provide the heating stage with infrared oven. This is a critical stage of the process, as the final part thickness is strongly dependent on the preform or sheet temperature distribution prior to forming. Optimisation of the infrared oven is therefore necessary. Experiments have been conducted in order to characterise the heat source of the infrared emitter and the interaction between the heaters and a semi-transparent PET sheet. An 880 LW AGEMA infrared camera has been used to determine the surface distribution of the transmitted heat flux by measuring the temperature distribution on the surface of the thermoplastic sheet. In addition, numerical simulations of the temperature distribution using control-volume method have been carried out and compared with experimental data.

Three‐dimensional simulation of thermoforming process and its comparison with experiments

AbstractThree‐dimensional solid element analysis and the membrane approximated analysis employing the hyperelastic material model have been developed for the simulation of the thermoforming process. For the free inflation test of a rectangular sheet, these two analyses showed the same behavior when the sheet thickness was thin, and they deviated more and more as the sheet thickness increased. In this research, we made a guideline for the accuracy range of sheet thickness for the membrane analysis to be applied. The simulations were performed for both vacuum forming and the plug‐assisted forming process. To compare the simulation results with experiments, laboratory scale thermoforming experiments were performed with acrylonitrile‐butadiene‐styrene (ABS). The material parameters of the hyperelastic model were obtained by uni‐directional hot tensile tests, and the thickness distributions obtained from experiments corresponded well with the numerical results. Non‐isothermal analysis that took into account the sheet, temperature distribution measured directly from the experiments was also performed. It was found that the non‐isothermal analysis greatly improved the predictability of the numerical simulation, and it is important to take into account the sheet temperature distribution for a more reliable simulation of the thermoforming process.