Numerical modelling for rotational moulding with non-isothermal heating

Abstract

In the rotational moulding process, the internal air temperature has been widely recognised as a tool to predict an optimum cycle time. This paper presents a new numerical approach to predict the internal air temperature in a two-dimensional (2-D) static model without requiring the consideration of the tumbling motion of polymer powder. The initial non-isothermal heating of the static model is actually formed by two changeable plastic beds (stagnant and mixing beds), which represent the actual stagnant and mixing pools inside a rotating mould respectively. In the numerical approach, the lumped-parameter system and coincident node technique are proposed to incorporate with the Galerkin Finite Element Method in order to account for the complex thermal interaction of the internal air. It helps to overcome the difficulty of multidimensional static models in predicting an accurate internal air temperature during the heating stage of rotationally powdery plastic. Importantly, the predicted temperature profiles of the internal air, oven times for different part thicknesses and process conditions accord with the available experimental results.     

New approach to determination of maximum temperatures in welding using travelling heat sources

Abstract

The present paper considers methods of calculation of maximum temperatures in welding using a constant welding speed and a point heat energy source. A comparison of theoretically obtained times for the occurrence of maximum temperatures with those obtained experimentally shows slight deviations. The latter are dependent only on the accuracy of temperature measurements and the workpiece size (i.e. plate size in the present work). In the present case, the limitations refer only to the workpiece size. The advantage of this new approach is in the determination of a plane curve which is an envelope of all the points in the workpiece that are simultaneously at the maximum temperature in arc welding.     

Effect of torque loading on the pull-out response of a steel post inserted in a polyethylene cylinder

Posts are used in various implant designs to contribute to the short- and long-term fixation stability of artificial joints. This study was undertaken to investigate the effect of torque loading on the pull-out response of a steel post inserted into high-density polyethylene (HDPE) material. An experimental apparatus was designed and fabricated to perform mechanical characterization of a steel post embedded in a polymer cylinder with initial interference fit under pull-out, torque and combined torque/pull-out loadings. To analyze the effect of preload applied torque to the load transfer at the post-fixation interface under pull-out loading, we have chosen HDPE material with uniform mechanical and tribological properties. Under pull-out loading, the micro-slip initiation and propagation at the post-HDPE interface was found to be progressive and assuming Coulomb friction at the interface, the friction coefficient was calculated from the measured pull-out force. In the torque loading condition it was found that the torque dropped suddenly from the maximum value to an initial dynamic sliding torque value. The interface behaves like a chemically bonded one, and static and dynamic friction coefficients were determined. It has been found under combined torque/pull-out conditions that in addition to the reduction of the maximum pull-out force, the preload applied torque generates two instabilities in the pull-out behavior. The first one happens once the maximum pull-out force is reached where the load falls to a level required for the post extraction from the HDPE cylinder. The second instability takes place during the extraction process for a residual (critical) implantation length which depends on the preload applied torque value. This latter instability was marked by a sudden rotation of the HDPE cylinder against the steel post.     

New structure of automatic torch and new approach for controlling drop transfer in rotating spray welding arc

Abstract

Using computer aided design/computer aided manufacturing technology, an automatic welding system is designed and manufactured for high current, high speed, and high deposition metal arc active gas (MAG) welding. The welding torch structure is evaluated and optimised via the finite element method and practical welding technology experiments. Finally, the process of high deposition MAG welding is investigated, and it is highlighted that on application of a longitudinal magnetic field to the high deposition MAG welding process, a steady metal transfer and drop deposition condition can be achieved, which is suited to the demands of high current, high speed welding technology.     

Pendant droplet oscillation during GMAW

Abstract

This paper deals with pendant droplet oscillation in pulsed current gas metal arc welding (GMAW). Using a constant current power source, bead on plate welds were made on mild steel plates. The frequency of the pendant droplet oscillation was monitored visually (using a high speed video camera) and electronically (by analysis of the voltage signals). The results obtained are compared with the results predicted by a model based on a mass spring system. It was found that the oscillation frequency of small droplets revealed by both monitoring techniques matches that predicted by the model. For larger droplets, the oscillation frequency measured by voltage signal analysis tends to be higher than that revealed by video analysis and that predicted by the model. This implies that during droplet oscillation in (GMAW) the arc behaviour may change and as a result the arc voltage is not any more proportionally related to the arc length.     

Voltage change in the GMAW process due to the influence of a droplet travelling in the arc

Abstract

Experimental measurements have been made to investigate the meaning of the fluctuation or noise of electrical signals for the gas metal arc welding process through globular to spray transfer mode, with particular attention being paid to the so-called 'drop spray' transition mode. The results reveal that the welding arc voltage is significantly affected by the molten droplet travelling in the arc. A sharp drop in arc voltage appears just after the detachment of the droplet, reaching its minimum quickly and then recovering, according to the location of the droplet in the arc. Although the full explanation of the feature requires further study, we believe that an important influence is the geometrical effect. The existence of the droplet in the arc path significantly affects the geometrical shape of the arc and arc attachment at the anode, and hence the overall voltage of the arc.     

Simulation and application of dynamic heat transfer model for improvement of continuous casting process

Abstract

Numerical simulation is being increasingly used to improve the existing cooling systems. In order to attain highest quality strand, a two-dimensional dynamic mathematical heat transfer model of billet continuous casting of low carbon steel has been presented. This model can be used to compute the billet temperature distribution and shell thickness, especially it can be used to simulate the solidification process which is caused by frequently variational casting conditions. The fluctuation of measured temperature has been reduced to <10°C with thermal imaging system. The online model can monitor surface temperature and shell thickness in the casting process. So it provides the possibility for the online process control. For the validation of the dynamic model, a lot of billet surface temperature and shell thickness measurement were carried out on an actual casting machine. Finally, the dynamic model has been used for adjusting the operating parameters to improve the casting speed.     

Thermal performance of annealing line heating furnace

Abstract

In the present study, a stepwise model based on the enclosure concept has been applied to an annealing line heating furnace. The model has been satisfactorily tested using three industrial manufacturing data sets. As temperature measurement inside the furnace is difficult, the model could be used to improve control and to obtain the outlet temperature of the steel strip, the heat transfer rate loss and the strip heat transfer rate throughout the length of the furnace. Variations in the thermodynamic properties included in the model and in the operational conditions, which cannot be accurately known, have been tested to ascertain their effects on the evolution of the strip temperature. It is found that precise knowledge of the heat capacity and heating power introduced in the furnace are important to obtain good results in application of the model.     

DEVELOPMENT OF CALCULATING MODEL APPLICABLE FOR CYLINDER WALL DYNAMIC HEAT TRANSFER

In the calculation of submarine air conditioning load of the early stage, the obtained heat is regarded as cooling load. The confusion of the two words causing the cooling load figured out is abnormally high, and the change of air conditioning cooling load can not be indicated. In accordance with submarine structure and heat transfer characteristics of its inner components, Laplace transformation to heat conduction differential equation of cylinder wall is carried out. The dynamic calculation of submarine conditioning load based on this model is also conducted, and the results of calculation are compared with those of static cooling load calculation. It is concluded that the dynamic cooling load calculation methods can illustrate the change of submarine air conditioning cooling load more accurate than the static one.     

Modelling of the GMAW system in free flight and short circuiting transfer

Abstract

An efficient model of gas metal arc welding (GMAW) systems was developed. The model accounts for the short circuiting transfer with free flight transfer, with finite droplet height included. Furthermore, we have analytically solved the energy equation of the wire, including the contact resistance between contact tip and wire as a boundary condition of the energy equation. The model predicts the time variable properties of GMAW systems from short circuiting to free flight transfer mode. Model prediction results have been compared with experimental results, and have shown good agreement.