Rectangular Mold Cavity Research Articles

Effect of Thermal Debinding Conditions on the Sintered Density of Low-Pressure Powder Injection Molded Iron Parts

Low-pressure powder injection molding (LPIM) is a cost-effective technology for producing intricate small metal parts at high, medium, and low production volumes in applications which, to date, have involved ceramics or spherical metal powders. Since the use of irregular metal powders represents a promising way to reduce overall production costs, this study aims to investigate the potential of manufacturing powder injection molded parts from irregular commercial iron powders using the LPIM approach. To this end, a low viscosity feedstock was injected into a rectangular mold cavity, thermally wick-debound using three different pre-sintering temperatures, and finally sintered using an identical sintering cycle. During debinding, an increase in pre-sintering temperature from 600 to 850 °C decreased the number of fine particles. This decreased the sintered density from 6.2 to 5.1 g/cm3, increased the average pore size from 9 to 14 μm, and decreased pore circularity from 67 to 59%.

Analysis of asymmetric morphology evolutions in iPP molded samples induced by uneven temperature field

Mold surface temperature has a strong effect on the amount of molecular orientation and morphology developed in a non‐isothermal flowing polymer melt. In this work, a well‐characterized isotactic polypropylene was injected in a rectangular mold cavity asymmetrically conditioned by a thin electric heater specifically designed. The cavity surface was heated at temperatures ranging from 80 to 160°C for different times (0.5, 8, and 18 s) after the first contact with the polymer. Asymmetrical thermal conditions have a strong influence on the melt flow, by changing its distribution along the cavity thickness, and final part deformation. The morphology distribution of the molded samples was found strongly asymmetric with complex and peculiar features. Optical and Electron microscopy confirmed the complete reorganization of the crystalline structures along the sample thickness. X‐rays analysis reveals that molecular orientation of the sample surface decreases with the mold temperature and the heating time. © 2016 American Institute of Chemical Engineers AIChE J, 62: 2699–2712, 2016

Effect of Mold Material on Boundary Heat Flux Transients during Gravity Die-Casting

Experiments were conducted to study the effect of mold material on boundary heat flux variation during gravity die-casting. Inverse method was used for determining heat flux on the inside and outside surface of the mold during casting of pure Aluminum and Al-B4C composites. Different chill materials were used as mold material on one side of the rectangular mold cavity. K- type thermocouples were used for measurement of mold temperature during casting solidification. The mold temperatures at various locations were recorded using a data logger. These measured temperatures were used as input by the inverse algorithm for the assessment of the surface heat flux as a function of time. It was observed that the temperature difference between the inner and outer surface of the copper is very less in comparison to the cast iron mold and stainless steel mold. The cooling curve of the insulation mold indicates that there is no heat transfer through the insulation mold. The boundary heat flux is much higher in the case of copper mold than in the cases of cast iron mold and stainless steel mold.

Transient Polymer Flow Rate in Injection Mold Filling

Abstract An analytical and experimental study of the transient flow behavior of polymer melt in injection molding is presented. The polymer volume flow rate into a rectangular mold cavity during the filling stage was determined using the Reynold's transport theory. The analysis, which neglects melt elasticity, shows that the flow rate is transient during the initial filling of the cavity. Moreover, the cavity flow rate was found to depend on material compressibility and expansivity, total volume upstream of the gate as well as time rate of change of melt pressure and temperature. Experimental results obtained through direct flow visualization are in good agreement with the theory. The effect of gate size on the cavity flow rate was also examined. A small gate gives rise to a greater degree of melt compressibility in the runner system. The prediction of actual flow rates into the cavity not only enhances determination of the interrelationship of flow rate, morphology, and properties in injection molding but also aids in the avoidance of molding defects. The results of this study can be used to improve the accuracy of CAE analysis.

Study on drag forces of the molding flow in IC packaging using optical fiber sensors

The use of optical fiber sensors to study the drag force in molding flow is described. Research has proved that this novel method can be used to study the drag force of the molding flow in IC packaging over a perfectly elastic circular cylinder. The result of this study is found to be useful for characterizing the behavior of the wire sweep in encapsulation of semiconductor chips. An inline fiber etalon (ILFE) as a sensor is designed, constructed, and implemented in the middle of the optical fiber for accurate strain measurement. This is done by first laying an optical fiber in the midplane of a simple rectangular mold cavity, and then measuring the strain at the midspan of the fiber when it is subjected to the flow of a homogeneous fluid. For a given flow field, several dragforce models have been tried to calculate the drag force on the optical fiber, and the resulting strain of the optical fiber has been calculated by finite-element modeling and compared with the experimental results. From the comparison, the Takaisi model to calculate the drag force exerted on the optical fiber by the flow of homogeneous fluid is modified according to the experimental data. This modified model can be applied to study the wire sweep in the encapsulation of semiconductor chips. It is concluded that this measurement system can be used to verify the dragforce model for a molding flow on a perfectly elastic circular cylinder. Even under the more complex conditions in actual molding flow, the proposed sensing system can be used to measure the axial strain accurately. In addition, the transient deformation of the optical fiber characterized by the ILFE sensor provides important information for developing a drag-force model that can be used in IC packaging.

Mathematical modeling of the wall effect on drag forces in molding flow using optical fiber sensing data

This paper studies the wall effect on drag forces in molding flow using optical fiber sensing data. The results indicate that the effect of the wall cavity is significant in the calculation of the drag forces and is required to be considered in molding. The study also shows the effectiveness of the approach in characterizing the behavior of the wire sweep in the encapsulation of semiconductor chips. An in-line fiber etalon (ILFE) sensor is designed and fused in the middle of the optical fiber for accurate strain measurement. This is done by first laying an optical fiber in the mid-plane of a simple rectangular mold cavity, and then measuring the strain at the mid-span of the optical fiber that is subjected to the flow of homogeneous fluid. For a given flow field, several drag force models have been used to calculate the drag forces over the optical fiber. The resulting strain of the optical fiber is estimated analytically first by FEM and then compared with experimental results. To study the wall effect, different sizes of mold cavities are employed for different b/a ratios and used in Takaisi’s model. Subsequently, Takaisi’s model is then modified to comply with the experimental data obtained by using various b/a ratios for comparison with the numerical drag force model computed by Han and Wang. The result indicates that a modified Takaisi model in compliance with the requirement of b/a within the range 20–200 is in good agreement with the numerical model proposed by Han and Wang who had taken effect of the wall cavity into consideration for drag-force calculation. The modified model based on a smaller b/a value is somewhat closer to the numerical model in a higher average cavity velocity. The wall effect of molding flow is concluded to be an important factor for various tests conducted at a greater average cavity velocity or a smaller value of b/a.

Characterization and modeling of race-tracking in liquidcomposite molding processes

Liquid composite molding (LCM) processes require the impregnation of a polymeric resin through a porous preform, usually composed of glass, carbon, or Kevlar fibers. Numerical mold-filling simulations are currently being developed to predict the flow through LCM mold cavities and are a powerful design tool. The accuracy of such simulations are very sensitive to the permeability components, input provided by the user to represent the resistance to flow provided by the porous preform. Air channels can be present within a mold cavity, either unintentionally formed, or intentionally placed to enhance the mold filling process. Such channels provide paths of relatively low flow resistance, and can drastically alter flow front advancement and injection pressures, this effect being commonly referred to as ‘race-tracking'. Race-tracking must be considered in numerical mold-filling simulations, one common approach being the application of ‘equivalent permeabilities' to finite elements within a model that physically represent the air channels present. A planar rectangular mold cavity is studied here, having a single air cavity of known dimensions running along one side of the mold. This geometry provides the simplest flow configuration, all Darcy velocity components being in-plane. In this paper, the equivalent permeability magnitudes are based upon steady-state, fully developed flow through a rectangular duct. Detailed flow visualization experiments have been completed, recording flow-front advancement and injection-pressure histories, for two different preform types, and a variety of mold cavity thicknesses, preform volume fractions air-channel widths. Significant effort was made to provide exact comparisons of both flow front advancement and injection pressure, between the experiments and simulations based on the equivalent permeability approach. The comparisons are very good, the magnitude of the deviations being close to what should be expected from natural permeability variations from preform to preform. Sources for experimental error have been examined, the major limitation of these experiments being caused by the complex interaction of the acrylic mold, and stiffening bars used. The equivalent permeability approach has been shown to model flow front advancement and injection pressures very well within a Darcy's law based mold filling simulation, for the volume fractions studied (0.50 to 0.16). This study has been limited to low volume fractions, and care should be taken when extending the equivalent permeability approach to woven and stitched preform styles. An alternative numerical method was investigated, modeling flow in the preform by using Darcy's law, and one-dimensional Stokes flow in the air channel, with a variable source term to model the transverse flow into the preform. The results from the two simulations are in excellent agreement, demonstrating that the equivalent permeability approach employing Darcy's law over the entire domain, models the transverse flow into the preform from the air channel accurately.

Modeling the packing stage in injection molding of thermoplastics

AbstractA model, for the packing stage In injection molding of thermoplastics is proposed. It allows one to calculate the time evolution of pressure and temperature fields and mass variations in simple geometries. The model holds for amorphous as well as for crystalline polymers if the kinetics of crystallization are known for temperatures far from the melting point, It applies after filling stage; initial temperature and pressure conditions are available from filling‐simulating software. The principles rest on finite difference schemes computing simultaneously temperature and velocity of compressible non‐Newtonian fluid in a filled cavity. The pressure field is determined from an equation of state at any time, owing to a mass balance in each gridmesh. The meaningful results of the simulation are local shrinkage and a good approximation of final weights of finished products. The algorithm is essentially checked for the influence of physical, thermal, and processing parameters on the cavity pressure for the injection of a polystyrene (Gedex 1541) into a rectangular mold cavity.

An experimental study on the injection molding of thermosetting polyester resin

AbstractAn experimental study was conducted on the injection molding of a thermosetting polyester resin. For the study, a general‐purpose unsaturated polyester resin was used, with benzoyl peroxide as initiator. A differential scanning calorimeter (DSC) was used for studying the curing kinetics, under isothermal curing conditions. A plunger‐type injection‐molding apparatus was constructed, and a rectangular mold cavity with glass windows on both sides was constructed, which permitted us to record on a film the changes in stress birefringence patterns in the mold cavity during the molding operation (i.e., during the isothermal cure, post cure, and subsequent cooling), using a crossed circular polariscope. The injection‐molded specimens were used to determine the distribution of the degree of cure at various positions in the flow direction, and to relate the degree of cure to the dynamic mechanical properties.

Studies on structural foam processing. IV. Bubble growth during mold filling

AbstractAn experimental and theoretical study was carried out to achieve a better understanding of bubble growth during the filling of gas‐charged molten polymers into a rectangular mold cavity. For the experimental study, a rectangular mold cavity (15.24 × 4.55 × 0.64 cm) was constructed, with glass windows on both sides to permit recording on a movie film of the growth of gas bubbles in the mold cavity as a molten polymer containing inert gas was injected into it. Sodium bicarbonate (generating carbon dioxide) was used as a chemical blowing agent, and the polymer used was a general purpose clear polystyrene. All experimental runs were made at isothermal molding conditions, and the injection rate was varied. It was found that, at and above a certain injection rate, little bubble formation was observed in the mold cavity during injection except at and near the moving melt front. For the theoretical study, the growth of a single gas bubble in a viscoelastic medium (represented by the DeWitt model), subjected to high injection rates, was considered by including the effects of diffusion from the liquid phase to the gas phase, interfacial tension between the liquid and the gas phases, and stress relaxation of the melt upon ejection. It was found that the level of stresses, built up in the met during injection, has a profound influence on the formation and growth of gas bubbles during the initial stage of mold filling. Also, a multichannel mold cavity was employed in order to observe the effect of processing variables on the cell size and its distribution in molded specimens. A uniform cell structure was obtained at higher injection pressures, at an optimum injection melt temperature, and with an optimum combination of blowing agent and nucleating agent concentrations.

Analysis of the packing stage in injection molding

AbstractAn unconditionally stable explicit finite difference numerical scheme is used to determine the pressure distribution during the packing stage of a rectangular mold cavity. Different initial conditions arising from both an isothermal and nonisothermal mold filling analysis are considered in relation to subsequent packing behavior. The packing stage is of short duration, may be assumed to be isothermal, and gives rise to a more uniform pressure distribution within the mold cavity.

Studies on structural foam processing II. Bubble dynamics in foam injection molding

AbstractAn experimental study was carried out to gain a better understanding of the dynamic behavior of gas bubbles during the structural foam injection molding operation. For the study, a rectangular mold cavity with glass windows on both sides was constructed, which permitted us to record on a movie film the dynamic behavior of gas bubbles in the mold cavity as a molten polymer containing inert gas was injected into it. The mold was designed so that either isothermal or nonisothermal injection molding could be carried out. Materials used were polystyrene, high‐density polyethylene, and polycarbonate. As chemical blowing agents, sodium bicarbonate (which generates carbon dioxide), a proprietary hydrazide and 5‐phenyl tetrazole, both generating nitrogen, were used. Injection pressure, injection melt temperature, and mold temperature were varied to investigate the kinetics of bubble growth (and collapse) during the foam injection molding operation. It was found that the processing variables (e.g., the mold temperature, the injection pressure, the concentration of blowing agent) have a profound influence on the nucleation and growth rates of gas bubbles during mold filling. Some specific observations made from the present study are as follows: an increase in melt temperature, blowing agent concentration, and mold temperature brings about an increase in bubble growth but more non‐uniform cell size and its distribution, whereas an increase in injection pressure (and hence injection speed) brings about a decrease in bubble growth but more uniform cell size and its distribution. Whereas almost all the theoretical studies published in the literature deal with the growth (or collapse) of a stationary single spherical gas bubble under isothermal conditions, in structural foam injection molding the shape of the bubble is not spherical because the fluid is in motion during mold filling. Moreover, a temperature gradient exists in the mold cavity and the cooling subsequent to mold filling influences bubble growth significantly. It is suggested that theoretical study be carried out on bubble growth in an imposed shear field under nonisothermal conditions.

Development of stress birefringence and flow patterns during mold fillin and cooling

AbstractAn experimental study was carried out to investigate the development of stress birefringence patterns of molten polymer during the mold filling and cooling operation. For this study, a rectangular mold cavity with glass windows on both sides was constructed, which permitted us to record on a movie film the changes in stress birefringence patterns in the mold cavity during the molding operation, using a circular polariscope. The mold was equipped with an automatic relay system which closes the shut‐off valve when the pressure in the mold cavity reaches a predetermined value. The mold was also equipped with both heating and cooling devices, so that either isothermal or non‐isothermal injection molding could be carried out. The mold temperature was controlled by thermistor regulated controllers. During the entire cycle of the molding operation, the mold cavity pressure was continuously recorded on a chart recorder, using a melt pressure transducer. The present study shows how molding conditions (namely, injection pressure, melt temperature, mold temperature) influence the distribution of stress birefringence patterns in a molten polymer while it is being injected into, and cooled in, a rectangular mold cavity.

Fluid mechanical analysis of injection mold filling

AbstractThe hydrodynamics of the filling of a rectangular mold cavity by a molten polymer is considered in terms of lubrication theory. Both isothermal and nonisothermal mold filling are analyzed. The relationship of the velocity field to the cavity geometry and temperature dependence of the rheological properties is predicted. Increasing the activation energy of viscous flow increases the tendency for channeling of melt through the center of the cavity. The results are compared to the experimental observations of our previous studies.