Melt Front Advancement Research Articles

Effects of size reduction on deformation, microstructure, and surface roughness of micro components for micro metal injection molding

The fabrication of 17-4PH micro spool mandrils by micro metal injection molding was described here. The effects of size reduction on deformation, microstructure and surface roughness were studied by comparing a ϕ500 μm micro post and a ϕ1.7 mm cylinder after debinding and sintering. Experimental results show that slumping of the micro posts occurred due to a dramatic increase in outlet vapor pressure initiated at the thermal degradation onset temperature and the moment of gravity. Asymmetrical stress distribution within the micro component formed during the cooling stage may cause warping. Prior solvent debinding and adjustment in a thermal debinding scheme were useful for preventing the deformation of the micro components. Smaller grain size and higher micro hardness due to impeded grain growth were observed for the micro posts compared with the ϕ1.7 mm cylinder. Surface roughness increased with distance from the gate of the micro spool mandril due to melt front advancement during mold filling and the ensuing pressure distribution. At each position, surface roughness was dictated by injection molding and increased slightly after sintering.

Study on the Fluid–Structure Interaction at Different Layout of Stacked Chip in Molded Packaging

This study investigates fluid–structure interaction (FSI) analysis of stacked chip in the encapsulation of molded underfill packaging using ANSYS Coupling Work bench with fluid and structural solvers. During encapsulation, FSI analysis is applied to a molded package with different layouts, namely cases 1–4 of stacked chip. An even ratio of inlet and outlet gate pressures is used to produce a regular melt front advancement. An experimental setup is fabricated to validate the simulation results in the FSI study. A digital camera is used to capture the melt front advancement and structural deformation. The interaction between structures (silicon chip) and epoxy molding compound (EMC) is displayed in the displacement profile. Maximum deformation is evaluated during the final stage of filling. The silicon die experiences von Mises stresses, which are monitored to observe the risk of die cracking. The results of this study showed that, the EMC flow front advancement was the fastest in case 4. The pressure distribution of each case was nearly identical, and the maximum von Mises stress was distributed unevenly at the middle of the stacked chip. The proposed analysis can serve as a reference and guide in designing and improving 3D integration packages in industry.

Mold filling simulation of low pressure injection molding (LPIM) of alumina: Effect of temperature and pressure

Filling stage is the most important stage in powder injection molding (PIM) process so that many defects may occur in this stage. In the present study, filling stage for low pressure injection molding (LPIM) of alumina was simulated by finite element method (FEM). Experimental tests were conducted on alumina feedstock with 60vol% powder. Feedstock properties such as rheological property, specific heat and thermal conductivity were measured. The simulation results were validated with experimental tests carried out at temperatures and pressures of 70–100°C and 0.1–0.6MPa respectively. In addition, injection molding experiments involving short shot tests were performed to track melt front advancement pattern. The melt front shape obtained from the simulation matched the experiments well in top view but was slightly different in side view. Comparison between simulation and experimental results showed good agreement with regard to the melt front locations. Simulation have been found to correctly describe trends such as an increase in the pressure required to fill molds as temperature decrease.

Model and simulation for melt flow in micro-injection molding based on the PTT model

Unsteady viscoelastic flows were studied using the finite element method in this work. The Phan-Thien–Tanner (PTT) model was used to represent the rheological behavior of viscoelastic fluids. To effectively describe the microscale effects, the slip boundary condition and surface tension were added to the mathematical model for melt flow in micro-injection molding. The new variational equation of pressure, including the viscoelastic parameters and slip boundary condition, was generalized using integration by parts. A computer code based on the finite element method and finite difference method was developed to solve the melt flow problem. Numerical simulation revealed that the melt viscoelasticity plays an important role in the prediction of melt pressure, temperature at the gate and the succeeding melt front advancement in the cavity. Using the viscoelastic model one can also control the rapid increase in simulated pressure, temperature, and reduce the filling difference among different cavities. The short shot experiments of micro-motor shaft showed that the predicted melt front from the viscoelastic model is in fair agreement with the corresponding experimental results.

Establishment of Gas Counter Pressure Technology and Its Application to Improve the Surface Quality of Microcellular Injection Molded Parts

Abstract The purpose of this study is to establish the Gas Counter Pressure (GCP) technology in combined with microcellular injection molding process. The application of gas into the mold cavity during the melt injection period provides a counter force against the melt front advancement, restricting the foaming process during the melt filling stage. Various gas counter pressures from 0 bar up to 300 bar and gas holding times of 0 to 10 s after filling completion were employed for investigating their relevant effect. It was found that when gas counter pressure is greater than 100 bar, foaming does not appear in the skin layer if no holding time is applied after melt-filling. As gas counter pressure increases, thickness of the solid skin layer without foaming also increases. Employment of holding time after melt-filling also assists foaming restriction in the melt core area. If a holding time of 10 s is applied combined with a counter pressure greater than 100 bar, foaming is completely restricted resulting in a transparent PS sample part. The part surface of those subjected to foaming-restriction in the skin layer also appears to be smooth and glossy. For the black PS parts molded via microcellular injection combined with gas counter pressure, it appears the same gloss quality as that achieved by conventional injection molding.

Core Deflection in Plastics Injection Molding: Direct Measurement, Flow Visualization and 3D Simulation

During plastics injection mold filling of long thin hollow parts, the melt may race ahead, asymmetrically, along one side of the core, and thus deflect the core. Several literatures have reported analytical or simulation results, but no experimental measurement has been done so far. The present study aimed to measure the developing core deflection during mold filling by flow visualization. A borescope behind a transparent quartz block was mounted upon a specially designed mold cavity to directly observe core deflection. The melt front advancement is also captured by a high-speed camera. The measured deflection closely resembles but falls below the simulated ones. The formation of the frozen layer may be suppressing core deflection, but more investigations are still needed.

Rheological characterization of polystyrene melts dissolved with supercritical nitrogen fluid during microcellular injection moulding

AbstractThere are several benefits of using the supercritical fluid microcellular injection moulding process. The part weight, melt temperature, viscosity, moulding pressure, shrink/warpage, and cooling/cycle time are all significantly reduced. The purpose of this study is to investigate the rheological behaviour of PS melt dissolved SCF of nitrogen during Microcellular Injection Moulding process applied with Gas Counter Pressure (GCP) technology. The application of gas into the mould cavity prior to the melt filling provides a counter force against the melt front advancement, restricting the foaming process during the melt filling stage. A slit cavity is designed to measure the pressure drop of polystyrene mixed with 0.4wt% supercritical nitrogen fluid under different mould temperatures (185°C, 195°C, and 205°C), injection speeds (5, 10, and 15 mm/s) as well as counter pressures (0, 150, 300 bars). It was found that melt viscosity is reduced by up to 30% when GCP is increased from 50 to 150 bar as compared to conventional injection moulding. The non-nucleation mixture melt obtained by using a GCP of 300 bar has 32~49% lower viscosity. In addition, the glass transition temperature, Tg, was found to be reduced from 96 °C to 50 °C when the applied GCP is 300 bar.

A Study on the Effect of Epoxy Molding Compound (EMC) Rheology During Encapsulation of Stacked-CHIP Scale Packages (S-CSP)

The numerical and experimental investigations of three-dimensional (3-D) mold filling during encapsulation process in stacked-chip scale package (S-CSP) are presented. The finite difference method (FDM) based on Navier—Stokes equations has been employed for the flow analysis in the mold cavity. The mold flow is assumed to be non-Newtonian and non-isothermal. The proposed models can take care of polymer rheology with cure effect (Castro—Macosko model) and without cure effect (Cross model). A package with five, six, and seven overhang stacking dies without wire bonds is considered for simulation. The epoxy molding compound (EMC) used is HITACHI CEL-9200. The effects of gap between die top and mold cap surface, and between adjacent dies on flow rheology are analyzed and presented. The flow retardation in the limitation region (gap region) and smooth flow in the free region of the package is being predicted. Higher initial conversion of EMC demonstrated higher viscosity and slower melt front advancement especially under the overhang area of same die stacking region and critical gap between the die and mold cap. The void mechanism occurred due to unbalanced mold flow and critical gap clearance. The simulation results are verified with those obtained from a typical electronic industry and found in good agreement. From the results; the Castro—Macosko model is found to be more stable and reliable on the flow rheology.

Study on micro-injection molding of light guiding plate by numerical simulation

Micro-injection molding is the most important technology in optical MEMS/NEMS industry for the necessary of scale, velocity and cost by the development of optical MEMS/NEMS industry and the innovation of optical MEMS/NEMS industry. It has the advantage of low cost, low interface and small volume in optical MEMS/NEMS industry. This paper emphasizes the analysis for light guiding plate of liquid crystal display on micro-injection molding. The finite element simulation in a three-dimensional inertia-free, incompressible flow is presented. A control volume scheme with a fixed finite element mesh is employed to predict melt front advancement. The plastic material of light guiding plate is used for PMMA material. The results show that the short shot situation on the filling stage of micro-injection molding. The results also indicate that the processing is very well for micro-injection molding by numerical simulation.

Analysis for microstructure of microlens arrays on micro-injection molding by numerical simulation

Micro-injection molding is the most important technology in MEMS/NEMS industry for the necessary of scale, velocity and cost by the development of MEMS/NEMS industry and the innovation of MEMS/NEMS industry. It has the advantage of low cost, low interface and small volume in MEMS/NEMS industry. This paper emphasizes the analysis for microstructure of microlens arrays on micro-injection molding. The finite element simulation in a three-dimensional inertia-free, incompressible flow is presented. A control volume scheme with a fixed finite element mesh is employed to predict melt front advancement. The plastic material of microlens arrays is used for PMMA material. The results show the short shot on the filling stage of micro-injection molding. The results also indicate that the melt front first advance into the thickness direction of microlens arrays, and then it goes into the microstructure of microlens arrays.

Simulation of Core Deflection in Powder Injection Molding

Abstract Core deflection in powder injection molding is one of the main causes of the structural defects in the molded part. Simulation of the core deflection in the injection molding process requires solution of the fluid-solid interaction problem, in which the fluid flow and solid deformation problems are coupled along the fluid-solid interface. A three-dimensional finite element program was developed in this work for a non-isothermal injection molding simulation including the fluid-solid interaction effects. With the assumption of small deformation in the solid structure, the fluid flow problem is formulated in an Eulerian frame-work, whereas the solid structure problem is formulated in a Lagrangian framework. Numerical simulation of the filling stage of injection molding for an airfoil-shaped part is presented and compared with the experimental data. The predicted melt-front advancement as well as the pressure is in good agreement with the corresponding experimental results. The predicted core deflection during the filling stage of the injection molding process is also compared with the measurements on a molded part.

Wire density in CAE analysis of high pin-count IC packages: Simulation and verification

Wire sweep has been recognized as one of the major defects in the encapsulation of microelectronic chips by the transfer molding process. As thinner and denser IC packages emerge, wire-sweep analysis becomes more challenging and troublesome. This paper studies the reactive flow in IC encapsulation by the CAE molding simulation and the wire-sweep phenomena. In fact, it presents a new methodology used to consider the effect of wire density (number of wire) by controlling the shape factor to simulate the flow resistance. The results show a better solution for melt-front advancement and wire-sweep prediction. Finally, one study case for high pin-count packages (BGA) is used to verify the research.

Flow front advancement of molten thermoplastic materials during filling stage of a mold cavity

In this study, an injection mold which allows images of the flow behavior in the mold cavity to be obtained was designed and manufactured. Under real processing conditions, HDPE and PP materials were forced into the mold cavity under five different injection pressures and six different injection velocities. Filling and flow behavior of molten HDPE and PP were recorded with a video camcorder capable of 25 frames/s. The records obtained were divided into frames and all of these frames were transformed to the ∗.jpg and ∗.bmp photo format with a photo capture program which was coded in Borland Delphi 6.0 by the authors. Under the same processing conditions, experimentally obtained melt front advancement results were compared with results from an independent injection molding analysis program, Moldflow 5.0, which employs a finite element method.

Comparison of numerical codes for simulation of powder injection moulding

In this study the powder injection moulding process was simulated with commercial analysis packages, C-Mold, Moldflow and ProCAST. The simulated melt front advancement, temperatures and pressures were compared with the experimental measurements. The three codes correctly predict the positions of weldlines. C-Mold performs the fastest analysis with good predictions of the temperatures and pressures within the cavity, but it is limited to two-dimensional models. Predictions of the in cavity pressure made by Moldflow are less accurate than those calculated by ProCAST and C-Mold. Predictions of the temperatures made by Moldflow and ProCAST are comparable with each other and agree reasonably well with the experimental observations. However, ProCAST is superior in terms of the number of different output data and visualising capabilities. The disadvantage of ProCAST is that it does not permit simulating the filling and post-fill ingstages of the powder injection moulding process in one simulation run.

Injection Moulding: Evaluation of Appropriate Melt Injection Points for the Gas Injection Technique

AbstractIn gas assisted injection moulding the melt front advancement has a considerable effect on the gas penetration. The evaluation of an appropriate melt filling is an important step to avoid instabilities in the process sequence. Taking a sample moulded part a procedure is presented that enables the part designer to evaluate required melt and gas injection points according to the gas injection technique. Using finite element simulations, different calculations for the melt front advancement lead to the correct gate location. Presentation of different degrees of filling for the optimised article geometry.magnified imagePresentation of different degrees of filling for the optimised article geometry.

Simulation of injection‐compression molding process, Part 3: Effect of process conditions on part birefringence

AbstractSimulations of the injection‐compression molding (ICM) process based on a Leonov viscoelastic fluid model has been employed to study the effects of processing conditions on the birefringence development and distribution in injection‐compression molded parts. A numerical algorithm combined with a modified control‐volume/finite‐element method is developed to predict the melt front advancement and the distributions of pressure, temperature, and flow velocity dynamically during the injection melt‐filling, compression melt‐filling, and postfilling stages of the entire process. Part birefringence was then calculated from residual stresses following the thermal‐mechanical history of the entire molding process. Simulations of a disk part under different process conditions including compression speed, switch time from injection to compression, compression stroke, packing pressure, and postfilling time were performed to understand their effects on birefringence variation. The simulated results were also compared with those required by conventional injection molding (CIM). It has been found that an ICM part shows a significant reduction of part birefringence near the gate area as compared with CIM parts. However, ICM parts exhibit higher birefringence values near the rim of the disk. The minimum birefringence occurs around the location where injection is switched over to compression. Although longer postfilling time and higher packing pressure result in higher birefringence values, their effects are not very significant. On the other hand, higher compression speed, larger compression stroke, and shorter switch time exhibit greater effects on the increase of part birefringence. Flow‐induced residual stress is the major origin of birefringence formation in the present case. The simulated birefringence for both ICM and CIM parts show good coincidence with those obtained from measurements by using a digital photoelasticity technique. © 2002 Wiley Periodicals, Inc. Adv Polym Techn 21: 177–187, 2002; Published online in Wiley Interscience (www.interscience.wiley.com). DOI 10.1002/adv.10024

Computer simulation of stress‐induced crystallization in injection molded thermoplastics

AbstractInjection molding of semicrystalline plastics was simulated with the proposed stress‐induced crystallization model. A pseudo‐concentration method was used to track the melt front advancement. Stress relaxation was considered using the WFL model. Simulations were carried out under different processing conditions to investigate the effect of processing parameters on the crystallinity of the final part. The simulation results reproduced most of the experimental results in the literature. Comparison is made between the slow‐crystallizing polymer (PET) and fast‐crystallizing polymer (PP) to demonstrate the effect of stress on the crystallization kinetics during the injection molding process for materials with different crystallization properties. The results show that for fast‐crystallizing plastics, stress has little effect on the final crystallinity in the injection molded parts.

A numerical simulation of the filling stage in injection molding based on a surface model

AbstractComputer simulation packages have had success in predicting filling behavior in extremely complicated geometries. However, most of the current numerical implementation is based on a hybrid finite‐element/finite‐difference solution with the middle‐plane model. This kind of model leads to some inconvenience in applications. Instead of using the middle‐plane as the datum plane, the surfaces of the cavity are used as datum planes in this article. Additional boundary conditions in the gapwise direction are employed to keep the flows in the surfaces at the same section coordinative. Comparisons with experimental results obtained with an instrumented test mold and C‐Mold results show good agreement in terms of pressure traces and melt‐front advancement. It is demonstrated that the present formulation is well suited to handle cavities generated directly by current CAD systems. © 2001 John Wiley & Sons, Inc. Adv Polym Techn 20: 125–131, 2001

Simulation of injection-compression-molding process. II. Influence of process characteristics on part shrinkage

A numerical algorithm is developed to simulate the injection–compression molding (ICM) process. A Hele–Shaw fluid-flow model combined with a modified control-volume/finite-element method is implemented to predict the melt-front advancement and the distributions of pressure, temperature, and flow velocity dynamically during the injection melt filling, compression melt filling, and postfilling stages of the entire process. Part volumetric shrinkage was then investigated by tracing the thermal–mechanical history of the polymer melt via a path display in the pressure–volume–temperature (PVT) diagram during the entire process. Influence of the process parameters including compression speed, switch time from injection to compression, compression stroke, and part thickness on part shrinkage were understood through simulations of a disk part. The simulated results were also compared with those required by conventional injection molding (CIM). It was found that ICM not only shows a significant effect on reducing part shrinkage but also provides much more uniform shrinkage within the whole part as compared with CIM. Although using a higher switch time, lower compression speed, and higher compression stroke may result in a lower molding pressure, however, they do not show an apparent effect on part shrinkage once the compression pressure is the same in the compression-holding stage. However, using a lower switch time, higher compression speed, and lower compression stroke under the same compression pressure in the postfilling stage will result in an improvement in shrinkage reduction due to the melt-temperature effect introduced in the end of the filling stage. © 2000 John Wiley & Sons, Inc. J Appl Polym Sci 75: 1640–1654, 2000

Simulations and Applications of Injection-Compression Molding

A numerical algorithm is developed to simulate the injection-compression molding (ICM) process and to predict melt front advancement, the distributions of pressure, temperature and flow velocity. Processing characteristics were understood by changing processing parameters including compression speed, switch time from injection to compression, compression stroke, as well as initial cavity thickness and part thickness. The simulated molding pressures were also compared with those required by conventional injection molding (CIM). It was found that the compression speed and compression stroke are the two factors affecting the molding pressure most significantly. Switch time also shows apparent effect on the pressure profiles. Using lower switch time, lower compression speed and higher compression stroke will result in lower cavity pressures. The percentage reduction in molding pressure is higher if part becomes thinner when using ICM. The melt velocity far from the gate was found to be higher than that near the gate during the compression stage contrary to that of CIM resulting in different part residual stress and melt temperature distribution. The simulated pressures for both ICM and CIM show good coincidence with those obtained from cavity pressure measurements.